mri stroke
TRANSCRIPT
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EVALUATION OF HUMAN STROKE BY MR IMAGING
Ph.D Thesis by
Sverre Rosenbaum
Faculty of Medicine, University of Copenhagen, Denmark
and
Danish Research Center of Magnetic Resonance, Hvidovre Hospital
August 2000
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Preface
This Ph.D. project was carried out at the Danish Research Center of Magnetic Resonance,
Hvidovre Hospital, in cooperation with Division of Stroke, Medical Center, Hvidovre Hospital,
during my employment as a research assistent (1995-1999). It has been a unique opportunity for
me to acquire insight in the exciting fields of human stroke and Magnetic Resonance Imaging.
I wish to express gratitude to my three supervisors, Professor Olaf B. Paulson, Dr. Henrik B.W.
Larsson and Dr. Palle Petersen. Also I want to thank Professor Ole Henriksen for initiating the
project.
The project would not have been possible without the support of many people. Especially I want
to thank Egill Rostrup who developed the programs for post-processesing and with whom I have
had many inspiring discusions on the interpretation of the perfusion studies. Also special thanks
to Jacob Rørbech Marstrand for help with the recruitment of patients in the later part of the
study and for inspiring discussions. I wish to thank Helle Simonsen for assistance with the MRI
examinations, also at late nights, and for help with preparation of the illustrations for this thesis.
Thanks to Lars Hanson for his thorough readings of the manuscript, to Katja Krabbe helping
with evaluating the data for chapter 4, to Peter Born who helped with the lay-out of the thesis
and to Pia Olsen for assistance with the MRI examinations. Also thanks to the secretaries Lisa
Simonsen, Lotte Hansen, Marianne Rønn for all kinds of help and to the �Department of Clinical
Physiology and Nuclear Medicine� Bispebjerg Hospital for SPECT images used in figure 30 and
figure 40.
However, most I want to thank August, Sofie and Anne, for being patient with me never being at
home. Also thanks to my mother for taking care of August and Sofie when Anne and I were at
work.
Furthermore, I wish to thank ”Apotekerfonden af 1991”and ”H:S forskningspulje” for their financial
support.
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This thesis is written as a monograph although it incorporates a submitted paper
�Tissue Patterns Characterized with Perfusion- and Diffusion Weighted MRI in Acute Stroke�,
by Rosenbaum S., Rostrup E., Larsson H.B.W., Petersen P., corresponding to chapter 6.
The work also resulted in an abstract
�Tissue Patterns Characterized with Perfusion- and Diffusion Weighted MRI in Acute Stroke�,
by Rosenbaum S., Rostrup E., Larsson H.B.W., Petersen P., Proceedings of the International
Society for Magnetic Resonance in Medicine 7th Scientific Meeting and Exhibition, Philidelphia,
846 (1999).
And a published paper
�MR-visible Water in Acute Stroke� Gideon P., Rosenbaum S., Sperling B., Petersen P. published
in Magnetic Resonance Imaging , vol.17: no2, pp 301-304 (1999)
Sverre Rosenbaum
Copenhagen, August 2000
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Figures and Tables
Figure 1. The large arteries of the brain
Figure 2. Influence of collaterality
Figure 3. Thresholds of ischemia
Figure 4. Collateral flow and the Penumbra
Figure 5. Diffusion-Gaussian distribution
Figure 6. Diffusion detected in magnetic resonance
Figure 7. The Pulses Gradient Spin Echo experiment
Figure 8. Diffusion coefficient
Figure 9. Motion effects on net phase and the echo signal
Figure 10. Motion compensation by phase correction
Figure 11. Echo planar Imaging
Figure 12. Diffusion - signal attenuation
Figure 13. Tortuosity
Figure 14. Diffusion anisotropy
Figure 15. Vessel size and susceptibility effects
Figure 16. Contrast agent bolus tracking
Figure 17. Time-of-flight
Figure 18. MR angiography, Time-of-flight
Figure 19. Stages in hemorrhages
Figure 20. A case of acute hemorrhages
Figure 21. The sensitivity of EPI
Figure 22. Incidences of Hemorrhagic infarcts (HI) and BBB leakage
Figure 23. Hemorrhagic infarct - T2*W and T1W images
Figure 24. Hemorrhagic transformation and leaky BBB
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Figure 25. Incidences and temporal course
Figure 26. Ischemia ⇒ cytotoxic edema ⇒ decrease of ADC
Figure 27. ADC and signal on T2WI - time course during ischemia
Figure 28. MR Measures With Tissue Signature, Attribution and Their Significance
Figure 29. Human ischemic stroke
Figure 30. Perfusion thresholds of ADC changes
Figure 31. PWI>DWI mismatch �Stroke in progression�. Enlargement at follow up
Figure 32. Reperfusion
Figure 33. PWI>DWI mismatch corresponding to stenosis on left ICA/MCA
Figure 34. A patient suffering from and infarct in the left MCA
Figure 35. Dynamic susceptibility contrast MRI
Figure 36. Compensatory Vasodilation
Figure 37. Luxury Perfusion
Figure 38. Collateral and Ichemic patterns
Figure 39. Reperfusion with normalization of PWI
Figure 40. Incomplete Infarct
Table I MRI Findings in Acute Cerebral Ischemia
Table II The stages of Hemorrhages
Table III Patient data and examinations
Table IV Hemorrhagic infarctions (HI)-numbers of examinations
Table V Blood brain barrier-leakage - number of examinations
Table VI Patient data. Time for MR-examinations
Table VII Acute MR-findings
Table VIII MR-findings in the chronic phase
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Abbreviations
ACA Anterior Cerebral Artery
ADC Apparent Diffusion Coefficient
AICA Anterior Inferior Cerebellar Artery
AMPA Alpha-amino-3-hydroxy-55-methyl-4-isoxazole Propionic Acid
AUC Area under Curve
ATP Adenosine Triphosphate
BBB Blood Brain Barrier
BI Barthel Index
CBV Cerebral Blood Volume
CMRGl Cerebral Metabolic Rate Glucose
CMRO2 Cerebral Metabolic Rate Oxygen
CPP Cerebral Perfusion Pressure
CSF Cerebro-Spinal Fluid
CT Computer Assisted Tomography
CNR Contrast to Noise
DSC-MRI Dynamic Susceptibility Contrast Magnetic Resonance Imaging
DTPA Dithylen-Triamin-Penta-Acetiacid
DTPA-BMA DTPA-Bismethylamide
DW Diffusion Weighted
DWI Diffusion Weighted MR Imaging
DWI Diffusion Weighted Imaging
ECASS European Cooperative Acute Stroke Study
EEG Electric Encephalogram
EP Echo Planar
EPI Echo Planar Imaging
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EAA Excitatory Amino Acids
FLAIR Fluid Attenuate Inversion Recovery
FLASH Fast Low Angle Shot
FOV Field Of View
Gd Gadolinium
HASTE Half-Fourier Single-shot Turbo spin-Echo
HI Hemorrhagic Infarction
HMPAO 99mTc-hexa-methyl-propylene-amine-oxime
HT Hemorrhagic Transformation
ICA International Carotid Artery
ICH Intracerebral Hemorrhage
IEPI Interleaved Echo Planar Imaging
IIH Intra Infarct Hemorrhage
ITT Intention to Treat
LP Luxury Perfusion
MCA Middle Cerebral Artery
MIP Maximal Intensity Projection
MRI Magnetic Resonance Imaging
MTT Mean Transit Time
NAA N-Acetyl Aspartate
NIH National Institute of Health
NINDS National Institute of Neurological Disorders and Stroke rt-PA Stroke
Study Group
NMDA N-Methyl-D-Aspartate
OEF Oxygen Extraction Fraction
PCA Posterior Cerebral Artery
PD Proton Density
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PDW Proton Density Weighted
PET Positron Emission Tomography
PICA Posterior Anterior Cerebellar Artery
PWI Perfusion Weighted Imaging
rCBF regional Cerebral Blood Flow
rCBV regional Cerebral Blood Volume
RF Radio Frequency
RIND Reversible Ischemic Neurologic Deficit
RS Ranking Score
rt-PA recombinant Plasminogen Activator
SCA Superior Cerebellar Artery
SD Spreading Depression
SNR Signal to Noise
SPECT Single Photon Emission Tomography
T1W T1-Weighted
T1WI T1-Weighted Imaging
T2*W T2*-Weighted
T2*WI T2*-Weighted Imaging
T2W T2-Weighted
T2WI T2-Weighted Imaging
TE Echo Time
TIA Transient Ischemic Attack
TOF Time of Flight
TP Target Population
TR Repetition Time
TTP Time To Peak
WM White Matter
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Summary in Danish
Ph.d. projektet er gennemført i perioden 1995-1999, under min ansættelse som klinisk assistent
på Dansk Videncenter for Magnetisk Resonans, Hvidovre Hospital.
Gennem de sidste 10 år er der sket væsentlige fremskridt indenfor Magnetisk Resonans
billeddannelse (MRI), som har gjort det muligt at visualisere tidlige forandringer hos patienter
efter iskæmisk slagtilfælde. Iskæmisk hjernevæv kan visualiseres med de nye MR teknikker:
diffusionsvægtet MR billeddannelse (DWI) og perfusionssvægtet MR billeddannelse (PWI).
Studier af dyr og mennesker har indiceret at iskæmisk væv, som kan undgå udvikling til egentligt
infarkt hvis thrombolyse eller nervecelleskyttende behandling etableres i tide, kan påvises med
DWI og PWI. Specielt har en kombination af DWI og PWI vist sig interessant til påvisning af
væv som potentielt kan reddes.
Implementering af hurtige T2*-vægtede ekkoplanare billedoptagelser (EPI) har muliggjort tidlig
detektion af blødninger, indenfor de første timer efter symptomdebut.
Der er med disse fremskridt kommet fokus på, om MRI kan bruges til rationelt at stratifisere
patienter med akut slagtilfælde til behandling med trombolyse og/eller neuroprotektiv behandling
udfra en viden om vævets metaboliske status samt til udelukkelse af blødning.
Afhandlingen er udformet som en monografi. Den teoretiske baggund for de anvendte MR
teknikker og deres applikationer og fund på iskæmiske dyremodeller og patienter med slagtilfælde
gennemgås. I afhandlingen indgår desuden tre studier samt fire kausistikker:
T2*vægtet EPI synes lovende med henblik på at detektere blødninger i den akutte fase og som
præsernteret i en kausistik mere sensitive end andre MR-teknikker. Imidlertid er sammenlignende
studier af sensitiviteten af CT versus MR nødvendige, som illustreret i en kausistik, hvor en akut
blødning ikke blev detekteret.
Med ialt 92 undersøgelser over 6 måneder af 43 patienter med iskæmisk slagtilfælde blev det
undersøgt om T2* vægtet EPI havde større følsomhed i forhold til konventionel T1 vægtet MR
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billeddannelse., med henblik på at detektere blødninger som opstår sekundært i det afficerede
væv efter et iskæmisk slagtilfælde, såkaldt hæmoragisk infarkt (HI). Udviklingen af HI er også
forsøgt belyst i forhold til om blodhjerne barrieren er defekt eller ej. Påvisning af HI er væsentlig
ved etablering af trombolysebehandling - til vurdering af blødningsrisiko. Det blev fundet at T2*
vægtet EPI er langt dårligere end konventionel T1 vægtet MR billeddanelse til at detektere HI.
Dette fund skyldes formentlig at oxygeneringen ved HI formentlig er højere end ved
rumopfyldende blødninger.. Det blev ikke fundet, at en defekt blodhjerne barriere var associeret
til forekomsten af HI. Dog var forekomsten af HI associeret til defekt blodhjerne barriere hvis
HI blev påvist på både T2* EPI og T1 vægtet MR billeddannelse. Denne patientgruppe blev
tolket som havende specielt stor blødningsrisiko ved etablering af trombolyse eller
blodfortyndende behandling.
Ni patienter med iskæmisk slagtilfælde blev undersøgt serielt med DWI og PWI indenfor 24
timer til den kronsike fase 2 måneder efter symtomdebut, hvor slutinfarktvolumenet blev
bestemt.. Formålet var at undersøge om der med DWI og PWI i den akutte fase kunne
identificeres viabelt væv. Der blev identificeret fire typer: væv:
To af disse blev tolket som identificerende viabelt væv i de såkaldte mismatch områder mellem
DWI og PWI. En vævstype indikerede spontan reperfusion. Endelig blev en type tolket som
iskæmisk uden viabelt væv.
Identificering af disse typer i den akutte fase blev fundet værdifuld med henblik på stratifisering
af patienter til trombolytisk og /eller neuroprotektiv behandling.
Det blev det foreslået at kombinationen af DWI og PWI ville kunne bruges til at stratifisere
patienterne med henblik på forskellige mekanismer ved "stroke in progression".
Slutinfarktstørrelsen målt på T2 vægtede billeder bør tage højde for et varierende vandindhold,
som illustreret i en kausistik. Vandindholdet er maximalt een uge efter debut, som bestemt i et
studie.
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Summary in English
The Ph.d project was carried out between 1995-1999 during my employment as a research
assistant at the Danish Research Center for Magnetic Resonance, Hvidovre Denmark.
During the last 10 years, major advances have been made in Magnetic Resonance Imaging (MRI)
which have made it possible to visualize early changes in patients suffering from ischemic stroke.
Ischemic brain tissue can be visualized by new MR techniques: diffusion weighted MR imaging
(DWI) and perfusion weighted MR imaging (PWI). Studies of animals and humans have
indicated that ischemic tissue, that will survive if thrombolysis or neuroprotection is established
in time, can be depicted by DWI and PWI. The combination of DWI and PWI have shown
promising identifying ischemic areas potentially salvageable.
Implementation of fast T2* weighted echo planar imaging (EPI) has made it possible to detect
hemorrhages, within the first hours after onset of symptoms.
These advances have turned attention to the possible use of MRI for rational statification of
patients with stroke to thrombolytic or neuroprotective treament, from a knowledge of the
metabolic status and from the exclusion of hemorrhages.
The thesis is formed as a monography. The theoretical background to the applied MR techniques
and their applications and findings in animal and human studies on ischemic stroke are reviewed.
The thesis also consists of three studies and four case studies:
T2* weighted EPI seems promising detecting acute hemorrhages and it is more sensitive than
other MR techniques as demonstrated in a case study. However, further studies are needed to
compare the sensitivity to that of CT exemplified by a case of false negative detection of acute
hemorrhage.
In 43 patients suffering from ischemic stroke being examined 92 times during 6 months, it was
examined if T2* EPI was more sensitiv compared to T1 weigted imaging detecting secondary
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bleedings, hemorrhagic infarcts (HI). It was also examined if occurrence of HI was associated to
leakage of the the blod brain barrier. Detection of HI is important prior to thrombolytic therapy
� estimating the risk of inducing hemorrhages. It was found that T2* EPI detected HI with very
low sensitivity compared to conventional T1 weighted MRI. It was hypothesized that this was
caused by a higher oxygenation state than in space-occupying haemorrhages. It was not found
that leakage of the blod brain barrier was associated to development of HI. However HI and
leakage of the blood barin barrier was associated if HI was identified on both T2* EPI and T1
weighted imaging. This group of patients was hypothesized to have an increased risk of bleeding
after thrombolysis or anticoagulant treatment.
Nine patients suffering from ischemic stroke were examined serially by DWI and PWI from
within 24 hours to the chronic phase at 2 months after onset of symptoms, where the final infarct
was measured.. The purpose was to investigate if DWI and PWI could be used to identify
potentially salvageable ischemic tissue. Four tissue patterns were identified.
Two of these patterns were interpreted as identifying viable tissue. One pattern indicated
reperfusion. Finally one pattern was interpreted as identifying ischemic tissue without any
potential for tissue salvation.
The identification of these four patterns was found valuable stratifying acute stroke patients to
thrombolytic or neuroprotective treatment.
It was suggested that the combination of DWI and PWI can be used to stratify the patients
regarding different mechanisms related to �stroke in progression�.
The final infarct as measured on T2 weighted images should be interpreted with consideration of
the water content, as illustrated in a case study. The water content is at its peak within the first
week after onset of symptoms as measured in a study.
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Introduction During the past ten years advances in neuroimaging and magnetic resonance imaging (MRI) have
made it possible to image the brain during the period of ischemic evolution. Evolving ischemic
lesions can now be imaged with new MRI techniques including diffusion-weighted imaging
(DWI) (decreased diffusion is a marker of cellular injury in ischemia) and perfusion weighted
imaging (PWI) (for assessment of the perfusion in the tissue). Simultaneously with the advances
in MRI technology, there has been an increased clinical need for imaging of evolving focal
cerebral ischemia due to the developments in acute stroke therapy. The first therapy for acute
stroke, thrombolysis, has been approved in the USA since 1996. Imaging of stroke should be
used prior to thrombolysis to exclude hemorrhages and to stratify patients according to the risks
of the therapy and to guide treatment. Before the advances in MRI, only anatomical modalities
i.e. Computer Tomography (CT) or T2 and T1 weighted magnetic resonance imaging, were used.
However, these modalities do not reliably depict the evolving ischemic lesions within the first 24
hours. Functional imaging techniques such as positron emission tomography (PET) and single
photon emission computer tomography (SPECT) have drawbacks: PET is restricted to a few
centers and is logistically demanding. SPECT is more available but is insufficient for detecting
hemorrhages. MR imaging is clinically available and the new DWI and PWI techniques seem
promising for evaluating acute human stroke, for acquiring knowledge on the pathophysiological
state in acute human ischemic stroke, and for guiding pre-thrombolytic assessment. Most
recently, also detection of primary hyperacute intracerebral hemorrhages by T2* weighted echo
planar MRI (EPI) has become possible.
The aim of this study is to provide answers to the following questions:
Can MRI by echo planar imaging (EPI) improve the detection of intracerebral hemorrhages?
Can EPI improve detection of secondary bleedings, i.e. hemorrhagic transformations compared
to conventional MRI techniques?
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Is it possible from EPI, T1 weighted imaging and evaluation of the blood brain barrier by post
contrast T1 weighted imaging to provide information on pathophysiological aspects of
hemorrhagic transformation regarding the risk induced by thrombolysis or anticoagulant therapy?
Can the combination of DWI and PWI in acute human stroke identify ischemic tissue at risk of
infarction?
In Chapter 1, the background of human stroke is given. At the start of the project, the MRI
techniques for DWI and PWI as used in the project, were not available, and a considerable
amount of time was used establishing these techniques at the Danish Research Center of
Magnetic Resonance. These techniques are introduced in Chapter 2. Detection of hemorrhages
by MRI is reviewed in Chapter 3, and three cases of acute primary intracerebral hemorrhages
evaluated using EPI are presented. In Chapter 4, data is presented on detection of hemorrhagic
transformation by EPI in comparison to T1 weighted imaging. Data on the integrity of the blood
brain barrier in the context of hemorrhagic transformation is presented. In Chapter 5, the
literature on DWI and PWI in animal ischemic models and in humans are reviewed. Three case
studies on perfusion thresholds in DWI, on �stroke in progression� and on edema, respectively,
are presented. In Chapter 6, serial data on acute ischemic stroke on patients using DWI and PWI
are presented.
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Chapter 1. Background 1.1. Epidemiology of Stroke
In Denmark, there are 10.000 new cases of stroke per year (1). Approximately 20% of these
patients die within the first month (2). In the aging population, stroke is the leading cause of
disability and cognitive impairment and is one of the major causes of disability in people between
20-50. The number of fatal strokes has decreased in the last 50 years without any change in the
incidence of stroke, thereby increasing the number of persons with disability from stroke. The
incidence of stroke increases with age and with an increasing population of elderly people. In the
future, the number of persons with disability due to stroke will increase (3), i.e. stroke is a major
and important contributor to human suffering and it is an economic burden for the society.
Therefore, effective therapies in the acute phase and intensive rehabilitation are areas of
increasing importance. Development in different imaging modalities such as Computer
Tomography (CT), Single Photon Emission Computer Tomography (SPECT), Positron
Emission Tomography (PET) and Magnetic Resonance (MR) have improved diagnostic
possibilities giving more differentiated knowledge of which patients should receive acute medical
therapy such as thrombolytic or neuroprotective therapy.
1.2. Pathogenesis of Stroke
1.2.1. Classification
Stroke may result from localized cerebral ischemia, intracerebral hemorrhage (ICH),
subarachnoid hemorrhage or venous sinus thrombosis. If the stroke etiology is ischemia, it may
be caused by embolism or by thrombosis. The incidences differ between studies (4), (5). In one
autopsy series of 179 cases (4) the following incidences were found: Ischemic stroke 63%,
thrombotic infarcts 12%, embolic infarcts 32% (51% of the ischemic strokes), lacunes 18.5%,
ICH 15.5%, subarachnoid hemorrhages 4.5%, intermediate 9.5% and others 8%.
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From a clinical point of view, the temporal course of the localized stroke after cerebral ischemia
can be classified in different subtypes: 1. Transient Ischemic Attack (TIA) is disturbances of
localized neurologic function reversible within 24 hours. 2. Reversible Ischemic Neurologic
Deficit (RIND) is completely reversible within a few days up to a week. 3. Progressive ischemia,
neurological deficits increase in severity and extent within the first 48 hours from onset of
symptoms. 4. Completed infarction with stable neurological deficits lasting longer than 2-3 weeks
(6). ICH is characterized by demarcated bleedings from arteriolar eruptions with mass effect and
eventually blood breakthrough to the ventricles. ICH is the most serious stroke incidence with
the highest mortality rate (7). ICH clinically mimics the time course of progressive ischemia or
completed infarction and therefore should be confirmed by CT or Magnetic Resonance Imaging
(MRI).
1.2.2. Arteries of the brain
The internal carotid artery (ICA) splits into the anterior cerebral artery (ACA) and the middle
cerebral artery (MCA) (figure 1). The middle part of the frontal and parietal lobes are supplied
from ACA. The lateral part of the frontal, parietal and temporal lobes are supplied from MCA.
From the first part of the MCA, penetrating end-arteries supply the corpus striatum and part of
the internal capsule (the lenticostriate arteries).
The two vertebral arteries continue in the basilar artery that split into the two posterior cerebral
arteries (PCA). The PCAs supply the medial part of the temporal lobe and the whole occipital
lobe. They also supply the thalamus and the mesencephalon. The basilar artery supplies the
ventral part of the medulla, pons and mesencephalon. The dorsal part of the medulla, the pons,
the mesencephalon and the cerebellum is supplied by the cerebellar branches: The superior
cerebellar artery (SCA) and the anterior inferior cerebellar artery (AICA) both branches from the
basilar artery and the posterior inferior cerebellar artery (PICA), a branch from the vertebral
arteries.
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Figure 1. The large arteries of the b
1.2.3. Anastomoses and collaterals
To understand the etiology of ischemic st
essential. Collaterals form systems of physio
cerebral blood supply. Important anastomos
internal carotid siphon due to the facial-
Collaterals also exist between the vertebral
artery, the thyro-cervical trunk, leptomen
pharyngeal artery. The vertebral and carotid s
communicant arteries connecting PCA wit
ACAs by the anastomosing Circle of Will
between the ACAs and MCAs and intra-hem
and ACA territories known as watershed
arteries of MCA and ACA are poorly collater
Ophthalmic artery ACA
MCA ICA
PCA
Basilar artery Vertebral arteryrain (adapted from (6))
roke, knowledge of the collateral blood supply is
logical anastomoses constituting a safeguard to the
es are formed between the external carotid and the
, angular- and especially the ophthalmic arteries.
artery and the external carotid, the external occipital
ingeal branches and in some individuals via the
upply areas are connected through the two posterior
h ICA. The PCAs and ICAs are connected to the
is. Meningeal collaterals exist inter-hemispherically
ispherically between the supply areas of MCA, PCA
or border zone areas. The direct penetrating end-
alized, i.e. occlusion always leads to infarction.
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1.2.4. Arterial anatomy and infarct types
The degree and extent of the infarct zone is defined by the lesion site and the presence and state
of the collaterals. This is essential if the lesions are related to small non-collateralized penetrating
arteries (microangiopathies) or to large cerebral arteries (macroangiopathies) or both. Infarcts
may lie in the supply area of an artery (territorial infarcts) or in the border region between the
supply zones of different arteries (extraterritorial infarction). Macroangiopathies cause impaired
hemodynamics due to decreased cerebral perfusion pressure (CPP) through embolism or local
thrombosis. Hemodynamically produced infarcts are seen as �low-flow� infarcts in the
distribution area of the non-collateralized part of the MCA (the lenticostriate arteries) or as
border zone infarcts in the supply areas of the large vessels in the parasagittal border zones and
parieto-occipital border zones (extraterritorial infarcts). Territorial infarcts, caused by embolism
or thrombotic occlusion, develop in the supply territory of large superficial arteries and are often
wedge-shaped and limited to the supply territory of the artery. If the collaterals supply the
marginal zones sufficiently, only a central infarct will develop. Occlusion of the lenticostriate end-
arteries is a subgroup of territorial infarcts (lacunar infarct), (figure 2). Microangiopathies are
caused by local thrombosis in the small non-collateralized, penetrating end-arteries, e.g.
lenticostriate arteries, known as lacunar infarcts (8).
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Figure 2. Influence of collaterality
From left to right: Adequate collaterality with no infarction. Collaterally proximal branch occlusion; the additional
thrombosis leads to a lacunar infarct. Occlusion of a non-collaterally branch leading to territorial infarct (adapted
from (6))
1.2.5. Occlusion mechanisms
The various vascular territories affected in cerebral infarcts correlate to different etiologies (6).
A. ICA occlusions are most commonly caused by thrombosis in an arteriosclerotic vessel, often
in the carotid bifurcation. However, clinical symptoms are not obligatory and are dependent
on the circulation in the circle of Willis, the retrograde flow of the ophthalmic artery and the
flow in the meningeal collaterals.
B. MCA occlusions are supposed to be caused by emboli. When the MCA is occluded proximal
to the origin of the deep lenticulostriate end-arteries, the infarction will involve the
lenticostriate artery supply area and part of the cortical and the subcortical region of the
territory of MCA, depending of the collateral blood supply. If the occlusion is in the distal
part of the MCA, the infarct will only involve the cortical region and the underlying white
matter. The volume of infarcted cortex and underlying white matter will depend on the
sufficiency of the cortical meningeal collaterals (8)
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C. Infarction in the supply area of the ACA, supposed to be caused by embolic occlusion, is 20
to 30 times less frequent than infarcts in the MCA territory. Infarction in the supply area of
the ACA involves cortical and/or subcortical tissue and is most frequently caused by an
embolus. The mechanisms determining the infarct topography are similar in the ACA and
MCA territories � with deep end-arteries and superficial branches connected to meningeal
anastomoses.
D. The deep penetrating end-arteries of MCA and PCA can be occluded individually leading to
lacunar infarcts. These occlusions are due to local lipohyalinosis induced by hypertension or
thrombosis caused by arteriosclerosis. Because these arteries are end-arteries with no
collateral blood supply, occlusion will inevitable lead to infarction.
E. Occlusions in the vertebrobasilar arteries can be classified according to their locations. The
distal part of the basilar artery is the most frequent position often caused by an embolus.
Conclusions of the middle part of the basilar artery, in the transition region from the basilar
to the vertebral arteries, and large occlusions involving both arteries, are more often due to a
thrombus in the arteriosclerotic vessel. Cerebellum can be involved in brainstem infarctions.
Isolated cerebellar infarctions are caused by emboli in PICA in 50 % of the cases. Isolated
occlusions of SCA are often embolic while occlusions of AICA are thrombotic (9).
1.2.6. Reperfusion
The original pathological observations that proximal to the infarcts arterial occlusions were
missing, have been confirmed in studies showing spontaneous recanalization in serial
angiographies (9). The observed reperfusion rate depends on the time from onset of symptoms
to the first angiography and on the time between angiographies. The rate of spontaneous
reperfusion in the acute period relevant for induced thrombolysis appears to be less than 20%
(10). Spontaneous and induced thrombolysis is more effective to occlusions caused by an
embolus rather than a thrombus, because of the organization of the thrombus(8). This is not true
if the embolus is well organized as seen in rheumatic heart disease (8). It has been shown that
21
reperfusion is associated with better clinical outcome and smaller size of the infarction (11),(12).
Partial reperfusion or embolus migration is known to occur in human stroke. If a proximal MCA
occlusion migrates distally in the supply territory, a cortical area rescued from infarction by
meningeal collateral blood supply collaterals can become infarcted when the migrated occlusion
is situated distal to the collateral anastomoses (figure 2). Reperfusion often leads to �luxury
perfusion� as discovered by Lassen (13) in 1966. Luxury perfusion (LP) is characterized by
decreased oxygen extraction fraction (OEF) by definition. LP explains the reddish venous blood
following neurosurgery on cerebral arteries and the fast flow seen on conventional angiography
following reperfusion. LP is often accompanied with hyperperfusion, then classified as absolute
LP. In relative LP, perfusion is normal or even decreased. LP is ideally investigated with PET, by
which OEF and perfusion can be measured.
1.2.7. Hemorrhagic transformation
Hemorrhagic transformation (HT) occurs in an ischemic infarct with bleeding in the necrotizing
cerebral tissue. The definition of HT includes development of petechial hemorrhagic infarction
(HI) and secondary intra-infarct hematoma (IIH). In HI, bleeding is petechial with blood
interspersed with intact brain elements. This is a result from increased permeability of the
capillary bed, caused by disruptions of endothelial junctions, which is known as diapedesis. In
IIH, brain parenchyma is destroyed. IIH is probably caused by vascular rupture dependent on the
intensity and duration of ischemia, thus explaining the predilection of IIH in the lenticostriate
artery supply area (14). The latter is difficult to differentiate from primary ICH (14). In
pathological studies of ischemic stroke, the frequency of HT has been found to be about 30%
(14;15). It is of prognostic importance if HT is of petechial type or is an IIH, with severe clinical
disability associated with IIH, not demonstrated in HI (16).
It has been thought that HT is caused by embolus fragmentation and reperfusion followed by
bleeding from ischemic damaged capillaries or arterioles (4). HT, however, occurs in spite of a
permanent occlusion. In that case, blood is supplied via a collateral leptomeningeal vessel (17). It
22
has been hypothesized that reperfusion is neither necessary nor sufficient to cause HT, since the
incidence of HT is low in the human trials of thrombolysis with early recanalization (18). It was
suggested that early reperfusion actually prevents HT (18).
HT is most often seen within the two first weeks after debut of symptoms and is positively
correlated to the rate of reperfusion, collateral perfusion and infarct size. HT may be associated
with high blood pressure in the acute phase, age and anticoagulant therapy. Chronic arterial
hypertension does not seem to increase the incidence of HT (14;17).
HT due to IIH is the most feared complication of thrombolytic therapy in humans. The
important topic of identification and prediction of HT will be reviewed and discussed in relation
to MRI in section 3.2.8. and data will be presented in Chapter 4.
1.3. Pathophysiology of Ischemic Stroke
It has been known for several decades that brain tissue can remain viable during ischemia and
that functional deficits and cell death are not equivalent (19). This chapter reviews the
hemodynamics and cellular and biochemical events in focal cerebral ischemia.
1.3.1. Perfusion metabolism - coupling and uncoupling
Under physiological conditions, a coupling exist between the demand for oxygen and glucose by
the cells and the regional cerebral perfusion (rCBF) (20;21). During cerebral ischemia, the supply
of blood and therefore the supply of oxygen and glucose are decreased and energy demands may
not be met. The uncoupling process of rCBF and metabolism has been examined by PET in
several animal and human studies (22). The thresholds of rCBF and metabolism in ischemia are
reviewed in the following.
1.3.2. Thresholds of Ischemia
Ischemia of the brain is characterized by a series of thresholds. Each decrease of the rCBF relates
to essential pathological events (figure 3):
23
Normal Cerebral Blood Flow
The normal range of CBF (45-55 ml/100g/minute) is maintained by the cerebrovascular
autoregulation: The pre-capillary resistance vessels dilate thereby increasing the cerebral blood
volume (CBV) in response to a decrease in the cerebral perfusion pressure (CPP). This is seen
distal to an obstructive embolus or thrombus.
Oligemia
Beyond the limit of maximal dilation a further decrease of CPP will decrease rCBF and the
oxygen extraction fraction (OEF) will increase to maintain normal cerebral metabolic rate of
oxygen CMRO2 � this stage is called oligemia with rCBF in the range of 35-45 ml/100g/minute
(23;24).
Mild Ischemia
If rCBF is further reduced and compensation is no longer possible since the OEF is maximal,
CMRO2 will decrease. rCBF in the range of 20-35 ml/100g/minute is called mild ischemia. The
neurons posses electrical activity at this stage, and tissue lactate is increased due to anaerobic
glycolysis. The effects of lactate are complex and still uncertain. It has been thought that lactic
acid in severe ischemia is responsible for necrosis of the ischemic tissue (25) but in mild ischemia
on the contrary can protect ischemic brain tissue against harmful effects of Ca++-influx (26).
Lactate is supposed to be taken up by glia-cells from the extracellular space leading to a cellular
(�cytotoxic�) edema (27). The adenosine triphosphate (ATP) concentration is in the normal
range. The energy source is thereby sustained although a decrease of tissue phosphocreatin and
an increase of inorganic phosphate reflect an impaired energy capacity. This makes brain areas of
mild ischemia vulnerable to energy consuming events such as electrical DC charges (�spreading
depressions�), from the ischemic core (28). There is some evidence from animal models that
energy metabolism deteriorates although the mild ischemia is stable. This indicates that the rCBF
threshold for energy failure � at which ATP concentrations are decreased � increases with time in
mildly ischemic tissue (29).
24
Moderate Ischemia – ”Electric Failure”
At rCBF between 10-20ml/100g/minute OEF, is elevated and CMRO2 is further decreased. If
CMRO2 is below 1.4 ml/100g/minute, the tissue will become infarcted. Above this threshold, the
tissue outcome is uncertain, which is consistent with the concept of the penumbra (reviewed in
section 1.3.3) (22). In animal models, the electric encephalogram (EEG) is becoming isoelectric
corresponding to a cessation of neuronal electric activity and appearance of neurological deficits.
If rCBF is restored before energy failure occurs, the functional failure will reverse (30). The
energy source is sustained since the ATP content is in the normal range. The mechanism
underlying this potentially reversible functional failure is not fully understood, but it may reflect a
compromised neurotransmitter system (31). Associated with the threshold of moderate ischemia
and reversible functional electric failure, is extracellular release of the excitatory amino acids
(EAA) (glutamate and aspartate). Release of EAA induces intracellular Ca++ influx in channels
associated to the glutamate receptor N-methyl-D-aspartate (NMDA). If the intracellular energy
reserves are sufficient, this Ca++ increase will be buffered and no damage will occur. However, if
energy consuming events such as �spreading depression� occur, the limit of the energy reserve
may be exceeded leading to a decrease of ATP � i.e. energy failure � and to harmful effects of
Ca++ on the neurones (28).
Severe Ischemia
If rCBF is less than 10-12 ml/100g/minutes, the tissue will become infarcted and CMRO2 and
OEF will decrease. This severe ischemia leads to increased extracellular K+ and intracellular Ca++
caused by failure of ATP dependent membrane pumps, (Na+-/K+-, Ca++- pumps). Normally,
these systems maintain the electrochemical gradients. Increased extracellular K+ can lead to
irreversible cellular injury by
1. Energy dependent depolarization � �spreading depression� � of neurons, leading to a further
increase of intracellular Ca++ (28).
2. Stimulation of Na+/K+-ATPase of the endothelial cells of the vessels indirectly leading to
cytotoxic edema due to impaired Na+/K+ ATPase level in the cellular membranes (32).
25
3. Vasoconstriction impairing rCBF further (33).
Increased intracellular Ca++ at the threshold of severe ischemia can not be buffered due to energy
depletion and therefore numerous events with irreversible damage on the membranes,
mitochondria and enzymes take place leading to cell necrosis (34).
Figure 3. Thresholds of ischemia (adapted from (35))
1.3.3. The Ischemic Penumbra
In focal ischemia, a rCBF reduction is rarely totally absent distal to the occlusion. Due to
collateral supply, rCBF ranges from normal to severely reduced (figure 4).
Regions between the rCBF thresholds for electric and ionic failure are characterized by reversible
functional failure if perfusion is restored in time. If not, initially viable tissues will deteriorate into
infarction. These regions of electrical failure with reversible functional failure are called the
ischemic penumbra (figure 4) (36). Another more recent definition is given: The penumbra is a
region of constrained blood flow with preserved energy metabolism, which contrasts the
SEVERE
100 50
80 40
60 30
40 20
20 10
ISCHEMIA
MODERATE
MILD
OLIGEMIA
NORMALRANGE
CBFml/100g/minute%
Maintained by autoregulation;higher CBF in gray matter
Increased O2 extractionmay maintain normal CMRO2
? ↑ Glycolysis? ↓ Protein synthesisThreshold of electrical failureThe �Penumbra�
Threshold of ionic failureAnoxic depolarization(↑ ECF K+ & ↓ ECF Ca++)
26
ischemic core, being salvageable (37). A broader definition of the penumbra refers to the
ischemic tissue being salvageable by appropriate therapy (38). The latter definition is the one
adapted in the study presented in Chapter 6, where the potential of PWI and DWI predicting
salvageable tissue is investigated. The concept of the penumbra is essential for identifying the
potential of acute therapeutic intervention. The definition and pathophysiology of stroke and the
duration from onset of symptoms to treatment in human acute stroke are factors of investigation
and discussion (39). The existence of the ischemic penumbra has been documented by various
methods both in animal models of ischemia (36) and in human ischemic stroke, the latter by
different imaging modalities as reviewed in section 1.5-1.8. The rCBF range for the penumbra is
known to be around 10-20ml/100g/min in animal studies (36). The mechanisms by which the
ischemic penumbra deteriorates into infarct are not known. Studies have shown that the
penumbra deteriorates into infarction during stable low rCBF and that the rCBF threshold for
energy failure (ATP depletion) increases with time (40). This indicates that reduced rCBF is not
the only cause of infarct evolution in the penumbra. Other mechanisms in stable hypoperfused
tissues could be: a) exposure to the tissue of toxic plasma constituents due to early leakage of the
blood brain barrier, b) �spreading depressions� and c) interference of residual aerobic
metabolism by Ca++ and lactate (41).
Little is known about the anatomic extent, biochemical features and time course in the human
ischemic penumbra, although studies with imaging modalities such as SPECT, PET and
functional MRI recently have shed some light on the natural history of human ischemic
penumbra as discussed in section 1.5-1.8 and 5.2.
Figure 4. Collateral flow and the Penumbra (adapted from (35))
27
1.4. Medical Therapies for Ischemic Stroke
1.4.1. Thrombolytic Therapy
Efforts to treat acute stroke with thrombolysis are based on the observation that 80-90% of early
ischemic stroke is associated with thrombotic or embolic occlusion of a supplying artery (42).
Also the introduction of the ischemic penumbra has encouraged the prospect of a therapeutic
window for reperfusion therapy. Thrombolysis in acute ischemic stroke has been investigated in
several clinical trials using human recombinant plasminogen activator (rt-PA) (43;44) (45).
In the European Cooperative Acute Stroke study (ECASS), patients were randomized for rt-PA
or placebo within six hours from debut of symptoms. CT imaging exclusion criteria were �early
infarct signs�: hypointensity in one third of the supply area of the MCA, effacement of the sulci
due to edema or hemorrhage. Two groups of patients were chosen for evaluation. The �Intention
to treat� (ITT) group contained all patients included, whereas the �target population� (TP)
contained only patients in whom the protocol had not been broken. The most frequent cause of
breaking the protocol was inclusion of patients with �early infarct signs� on CT scan. In the TP, a
clinical effect of the treatment was seen. In the ITT group no clinical effect was seen. The
number of hemorrhages was significantly higher in both ITT and TP. Significantly higher
mortality occurred in the ITT population. The conclusion was that thrombolytic therapy was
effective in a selected population of patients, but because this selection was difficult using CT,
the treatment could not be recommended (43).
The National Institute of Neurological Disorders and Stroke rt-PA Stroke Study Group
(NINDS) examined the effect of rt-PA in stroke patients within three hours from debut of
symptoms. Patients with ICH on a CT scan were excluded. No significant effects were seen after
24 hours, but after three months, a significantly better outcome was seen in the rt-PA treated
group. A significantly higher incidence of symptomatic IIH was seen in the treated group but this
was out-weighted by the beneficial effects of treatment. It was concluded that, despite the higher
28
incidence of symptomatic hemorrhages, rt-PA treatment given within three hours is effective. In
1996, rt-PA was accepted by the Food and Drug Administration in the USA for treatment of
ischemic stroke within three hours (44).
ECASS II was designed with a lower dose of rt-PA compared to ECASS (similar to NINDS),
strict blood-pressure control matching the NINDS criteria, and more strict application of the CT
exclusion criteria. Patients with symptoms within 6 hours were randomized in the study. The
incidence of IIH was increased in the treated group. The study did not confirm a statistical
benefit of rt-PA within 6 hours from onset of symptoms although it was stated that thrombolysis
using rt-PA in selected patients within 3 hours might improve the clinical outcome (45). Whether
thrombolysis should be given within 3 hours is still a matter of controversy (46).
1.4.2. Neuroprotective Therapy
The knowledge of pathophysiological events during ischemia and reperfusion have led to the
development of numerous drugs for neuroprotection in human ischemic stroke (41)
• Substances hindering the damaging effects of excitatory amino acid release during ischemia
(such as glutamate) by 1) inhibition of glutamate synthesis, 2) blockade of glutamate release
and 3) postsynaptic receptor blockade (N-methyl-D-aspartate (NMDA)- and alpha-amino-3-
hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors) hindering the damaging
associated with Ca++ influx. This group of compounds antagonizing the effect of EEAs is by
far the largest.
In addition to hindering effects from excitatory amino acids, an increasing number of methods
for neuroprotection in ischemia have been considered. Calcium channel antagonists, free radical
scavengers, hypothermia, hyperpolarizing cells by the inhibitory neurotransmitter GABA-
agonists, serotonin agonists, neurotrophic growth factors, anti-adhesion molecules, nitric oxide
inhibitors and agents interfering with apoptosis (47). None of these neuroprotective
pharmaceuticals although having convincing effects in animal models, have shown clinical effects
in trials or have had severe side effects leading to cessation of initiated trials. Several authors have
29
tried to explain why drugs work in animal models but not in stroke patients. This has been
attributed to inherent limitations of the animal models in representing physiologic and tissue
characteristics in humans, the relation between the animal model and clinical trial design (for
instance how to measure outcome reliable in animal models) and limitations in the clinical
designs itself (48), (49), (50).
1.5. Imaging of Acute Stroke
In stroke patients considered for thrombolysis within 3-6 hours, an urgent brain scan is
mandatory excluding ICH, non-stroke pathologies and subarachnoid hemorrhage. However,
structural imaging modalities such as CT and conventional MRI do not reliably visualize the
evolving ischemic lesion within the first 6-24 hours. It is known from pathophysiological studies
that the ischemic lesion depends not only on the time from onset of symptoms but also on the
depth of the ischemia, the extent of collateral perfusion and the time of eventual reperfusion.
Ideally, imaging of ischemic brain tissue could reliably be used to predict tissue viability.
Reperfusion therapy could then be individualized with an extended time-window for some
patients (51). Conversely, in some patients, the ischemic lesion is completed early, i.e. before 3-6
hours. Reperfusion therapy should only be given to patients with: 1. An occluded artery, without
any hemorrhages 2. hypoperfused, viable tissue, that would otherwise complete to infarction.
Thrombolysis might for example be most valuable to patients with viable tissue in proximal MCA
occlusion and not to patients with reperfusion or lacunar infarction.
The potential of evaluating ischemic pathophysiology in acute stroke using the different imaging
techniques will be reviewed. In section 5.2, MRI and especially the possibilities of perfusion
weighted imaging (PWI) and diffusion weighted imaging (DWI) for assessment of acute stroke
will be reviewed.
30
1.6. CT in Acute Stroke
CT has been the method of choice in diagnostic evaluation of cerebrovascular diseases since its
introduction in the 1970s. CT image reconstruction relies on digital decoding of absorbed x-rays
measured in a slice. Images of the entire brain can be obtained in a few minutes. Due to artifacts
caused by contrast in the region of the part petrosa ossis temporalis, it is associated with major
difficulty to use CT for evaluation of infarcts in the brain stem (52).
CT images most often do not show the affected artery supply area or give information about the
degree of collateral blood supply. The localization or extent of the involved area or spontaneous
reperfusion can therefore not be determined using CT. Acute ischemic lesions are usually visible
after 24 hours. In 20% they are even visible as blurred, low intensity areas within the first eight
hours. CT identifies ischemic lesions, as hypointensities, 3mm in diameter at least in regions
above the part petrosa ossis temporalis. Hypointensity seen on CT is caused by an increased
water content, i.e., cytotoxic or vasogenic edema (53).
Special attention has been paid to the correlation of clinical outcome and �early CT findings�
within the first six hours, in thrombolytic trials. These findings are (54)
• Effacement of sulci due to swelling from edema.
• Loss of border between gray and white matter.
• Relative blurring of especially the lentiform nucleus.
Findings on CT obtained before thrombolytic therapy have indicated
• Early hypointensities indicate irreversible damage.
• Hypointensity in more than one third of the supply area of the middle cerebral artery MCA
indicates a high risk of developing secondary hemorrhage leading to disability and death.
In the ECASS (43) these �early findings� were used as exclusion criteria, but were difficult to use;
i.e. they were the most frequent reason for breaking the protocol (thrombolysis started in patients
with �early infarct signs�). Some studies have questioned the role of �early infarct signs� for
predicting infarction. In a study of CT and PET, no direct correlation was found between CT
31
signs and critically hypoperfused tissues (55). Although �early infarct signs� correlated with poor
outcome, some patients with �early signs� had good spontaneous outcome. Also some patients
with �early findings� did not develop infarcts. Incidence of HT after treatment by thrombolytic
therapy in the ECASS could, however, be predicted CT by �early infarct signs" using �(56).
However, CT is still much more widely available than MR and has been superior for detecting
hyperacute ICH within the first 6 hours, which is of paramount importance when stratifying
patients for thrombolytic therapies. CT is therefore still the method of choice for diagnosing
patients with stroke, although recent developments in MRI are very promising for detecting
hyperacute ICH as reviewed in section 3.2.3. (57;58).
Developments in CT include very fast acquisition using spiral techniques. CT therefore offers the
possibility of functional studies of the blood circulation using 133Xe as a tracer (59) although a bad
signal-to-noise ratio (SNR) and high sensitivity to patient movements hampers the clinical
assessment. Also, acquisition of CT images during a bolus injection of a contrast agent, has made
it possible to visualize the extra- and intracranial blood supplying vessels (60).
1.7. SPECT in Acute Stroke
Fast rotating multi detector systems have made the assessment of rCBF by SPECT possible. The
spatial resolution is reduced compared to PET and MRI. The tracers used for SPECT are
commercially available and instrumental and logistic demands are reduced compared to PET.
SPECT is more appropriate for clinical studies and clinical practice and has been applied to
human ischemic stroke studies and thrombolytic trials (61;62). SPECT is used for rCBF
measurements and receptor studies, of which only rCBF measurements in stroke studies will be
dealt with in this thesis. 133Xe is an ideal free diffusible tracer used in SPECT for absolute
quantification of the rCBF. The spatial resolution is low and requires a dedicated tomograph, why
only few studies of stroke patients have been performed. 99mTc-hexa-methyl-propylene-amine-
oxime (HMPAO) is a non-diffusible perfusion tracer. It is taken up and is fixed in brain tissue
32
having a high first-pass extraction fraction with a distribution proportional to rCBF in normal
brain, although not quantifiable. The affected supplying artery can be determined visualizing
rCBF by SPECT (63). SPECT rCBF measurements are also valuable for answering the question
of spontaneous reperfusion resulting in normal or increased perfusion. In the patients treated
with thrombolysis, reperfusion can be documented by serial SPECT measurements (64),
increasing knowledge of which occlusions are sensitive to thrombolysis. SPECT has been applied
in clinical trials of thrombolytic therapy within three to six hours using HMPAO as tracer (61;62).
In two studies, the perfusion reduction determined by HMPAO-SPECT prior to thrombolysis
was used to estimate the risk of hemorrhagic transformation (62). In several studies, the rCBF
ratio between the infarcted and the healthy hemisphere has been calculated showing correlation
to the clinical course and tissue survival (61;65). Also in one recent study of intra-arterial
thrombolysis, a rCBF-index threshold was identified for predicting infarction or recovery (66). It
seems therefore possible to estimate if there is a penumbra and if thrombolysis can be considered
safe using SPECT with HMPAO as a tracer (68). However, one serious drawback of SPECT is
the inability to detect hemorrhages.
1.8. PET in Acute Stroke
With PET it is possible to obtain quantitative maps of rCBF, CBV, CMRO2 and glucose
metabolism in the brain (CMRGl): The patophysiological findings in normal and ischemic brain
tissue were reviewed in section 1.3. The rCBF and CMRO2 allow calculation of the OEF. It is
possible to measure the pH in the brain tissue and to investigate numerous specific receptors in
the brain.
1.8.1. Findings of rCBF and metabolism in acute ischemic stroke with PET
Marchal et al. (67), (68) examined 30 patient with acute MCA stroke by PET and compared acute
findings of rCBF and CMRO2 to clinical outcome at 2 months. They identified three patterns of
33
PET changes within 5 to 18 hours from onset of symptoms, into which each patient could be
classified: Pattern 1: Profoundly decreased rCBF and CMRO2 suggesting beginning of necrosis.
All patients in this group had poor outcome. Pattern 2: Moderately to markedly reduced rCBF,
normal to little decreased CMRO2 and increased OEF. This pattern reflects ongoing ischemia
with still limited necrosis. The clinical outcome could not be predicted from pattern 2 ranging
from death to recovery. Pattern 3: Hyperperfusion, Luxury Perfusion, with normal or slightly
depressed CMRO2, reflecting early spontaneous reperfusion with only limited damage. All
patients in this group had a good clinical outcome. In conclusion: Pattern 1 and 3 were predictive
for clinical outcome. None of the two patterns can be used for thrombolytic therapy trials
because no positive effect would be obtained due to irreversible damage with increased risk of
HT in pattern 1 and spontaneous reperfusion in pattern 3. Effects from neuroprotection aimed
at avoiding reperfusion injury could be expected in the patients with pattern 3. The unpredictable
outcome of pattern 2 indicates that the ischemic tissue in this group only sometimes progressed
to infarction � in agreement with the concept of the penumbra. This group of patients may
benefit in therapeutic trials. The fact that this pattern was observed up to 18 hours after debut
indicates that the �time window� of three to six hours should be reconsidered and be
individualized.
PET is considered to be the imaging technique that most significantly has increased the
knowledge of the pathophysiology of acute human stroke (22). PET being available in few
centers only, is complex to perform and has no clinically relevance as a pre-thrombolytic imaging
modality.
In summary, CT has proven clinically useful, especially for excluding patients with ICH. A
drawback is the missing information of the metabolism in the ischemic tissue. SPECT is a
potentially clinically available technique, but it provides information on rCBF only. PET is the
superior supplier of pathophysiological information in ischemic tissue. MRI and especially the
newer MRI-modalities, DWI and PWI, are promising for characterizing ischemic tissue and
recently also for detecting acute ICH. MR is the only mentioned imaging technique not relying on
34
ionizing radiation. MR scanners have become widely available for clinical use in the last decade.
The new possibilities of MRI will be reviewed in chapter 3 and 5. But first the fundamentals of
MR and especially DWI and PWI will be reviewed.
35
Chapter 2. Magnetic Resonance Imaging
2.1. Physical Principals of Magnetic Resonance Imaging
MR represents a phenomenon where absorption of energy occurs when specific nuclei of atoms
within a magnetic field are exposed to radio frequency (RF) energy at a specific frequency. In
1946 Bloch (69) and Purcell (70) made the first experiments on solids and liquids. The observed
frequency of the MR signal was found to be dependent on the specific chemical environment of
the nuclei - the chemical shift - and MR proved to be a power-full tool for analytic chemistry and
biochemistry. The energies involved in MR are non-ionizing and the radiation easily penetrates
the human body. However, the clinical potential was limited due to the inability of getting spatial
information from the MR signal. In 1973, Lauterbur discovered that spatial localization of the
MR signal could be obtained by using magnetic field gradients and the first MR images were
acquired (71). The first human images were obtained in 1977 (72). Development of computers
and dedicated hardware made it possibly to design MR scanners for clinical use through the
1980�s.
Many significant technical innovations followed and today it is possible to acquire structural
information of tissues, as well as a variety of physiological information from in vivo studies in
animals and humans.
2.1.1. Basis of Magnetic Resonance
Nuclei, except those with an even number of protons and an even number of neutrons, possess
magnetic angular momentum or spin. This characterizes numerous nuclei e.g. 31P, 1H and 13C.
The spinning nuclei carry electric charges inducing a magnetic moment comparable to a tiny
magnetic dipole. When an external magnetic field B0 is applied, the magnetic dipoles will have a
tendency to line up parallel or anti-parallel to the magnetic field. For 1H with a spin quantum
number of ½ this means that the nuclei has two energy states corresponding to the spin being
directed parallel or anti parallel to the field. Orientation of the magnetic dipole moment parallel
36
to the external field represents a more favorable, i.e. lower energy state resulting in a tiny net
magnetization in an ensemble of spinning nuclei. The distribution between low and high, energy
states depends on the strength of B0, and the temperature. The distribution is given by the
Boltzmann equation:
where n) and n+ are numbers of nuclei in the two different states, k is the Boltzmann constant, )E
is the energy difference and T the temperature.
The magnetic moment of the nuclei will precess around the axis of the B0 at a frequency Τ given
by the Larmor equation:
where ( is the gyro magnetic ratio characteristic of the nucleus under consideration.
Transitions between the low and the high energy states occur when RF energy at the frequency of
the precessing magnetic moment (the Larmor frequency) is applied to the ensemble of nuclei
(aligned along B0). This transistion between spin-up and spin-down, occurs when the resonance
condition is fulfilled, referring to Τ and the frequency of the applied RF pulse being identical.
This is the reason for the magnetic resonance.
)( TkEnn /exp/ ∆÷=+÷ [ ]1.2
0B⋅= γω [ ]2.2
37
2.1.2. T1 and T2 relaxation
When a RF pulse is applied at the Larmor frequency, the nuclei absorb energy and will thereby be
excited to a higher energy state where the net-magnetization is no longer parallel to the external
magnetic field B0. When the RF excitation stops, the nuclei will return to a lower energy state
dissipating the excess energy to the surroundings � the so-called spin lattice relaxation
characterized by the time constant T1. After the nuclei are excited by RF radiation at the Larmor
frequency, they will all precess at the same frequency and at the same phase with the Larmor
frequency. In a homogeneous magnetic field, the transverse part of the net-magnetization will
decrease with time due to interactions between the magnetic moments. This process called
transversal relaxation, is also referred to as the spin-spin relaxation characterized by a time
constant T2. In practice the magnetic field is inhomogeneous due to hardware constraints and
local variation in the magnetic susceptibility. This causes the nuclei to precess at slightly different
frequencies. The resulting phase incoherence leads to a decrease of the transversal magnetization.
The time constant for signal loss caused by magnetic field inhomogenity and spin-spin interaction
is called apparent T2 or T2* relaxation. T2 and T2* relaxation differ due to different
environments in tissues and are therefore important factors influencing the contrast in MR
imaging as described in section 2.2.
2.1.3. Magnetic Resonance Imaging
MR images can be obtained by applying external magnetic field gradients in addition to the
homogeneous field B0. As seen by inspecting the Larmor equation, this will cause the magnetic
moments of the nuclei to precess at slightly different frequencies depending on position in space.
2D MR images can be acquired when magnetic field gradients are applied in two directions, the
�phase-encoding� and the �frequency-encoding� gradients. The slice thickness is adjusted by
matching a magnetic field gradient to the bandwidth of the slice-selective RF pulse. The concept
38
of k-space is a convenient mathematical tool describing the acquisition of MR images: In a
conventional MR experiment e.g. 256 data points are sampled in the time domain covering one
line in of k-space the frequency encoding direction. The following lines, e.g. a total of 256, are
acquired by increasing the phase encoding gradient stepwise. The set of 256x256 data points in k-
space are then transformed to the frequency domain by a 2D Fourier transformation resulting in
a 256x256 point image.
2.2. Anatomical MRI – Image Contrast
The contrasts of tissues in MRI are primarily due to differences in proton spin densities (PD),
longitudinal relaxation times (T1) and transverse relaxation times (T2). The PD contribution to
the MR signal varies up to 30% whereas T1 and T2 in the brain varies up to more than 100%
between gray and white matter compared to cerebrospinal fluid CSF (73). In pathologies these
changes of PD, T1 and T2 in tissues may be even greater. Tissue contrast derived from
differences in T1 relaxation is dependent on the strength of the applied field. T2 and PD
contrasts are less influenced by the field strength (73). Contrasts in conventional MR images are
based on combinations of PD and T1- and T2 relaxation. Changes of image contrasts can be
obtained by modifying the timing parameters TE and TR. TE is the echo time (the time between
the RF excitation and the refocusing of the spins) and TR is the repetition time of the MR
imaging sequence (time between two excitations of the same ensemble of spins).
2.3. Diffusion Weighted Imaging
Diffusion is a physical process important in many physiological functions. Transport of
metabolites such as oxygen and glucose from the capillaries through a liquid medium to the cells
relies on diffusion. Diffusive motion arises from intrinsic energy in the molecules leading to
random molecular motions. Diffusion is the flux of molecules along the concentration gradient,
as described by Ficks' law, which states that the flux is proportional to the concentration gradient.
39
The constant of proportionality is called the diffusion coefficient (74). With no concentration
gradient the molecules are still diffusing but no net transport occurs. Traditionally, diffusion is
measured by the introduction of a tracer in the medium. The concentration of the tracer is
monitored using chemical or radio- tracer techniques. Use of a tracer gives a spatial resolution of
millimeters. Methods such as infra red spectroscopy or Rayleigh scattering allows a spatial
resolution of micrometers. These techniques, however, are invasive and may therefore perturb
the system studied. MR is the only non-invasive method that provides information of diffusive
motions in the range of micrometers (74).
In 1950 Hahn (75) discovered that the MR signal was influenced by diffusion. Carr and Purcell in
1954 (76) demonstrated that the diffusion effects on the MR signal are reduced when applying
multiple spin echoes in the MR experiment. In 1965 Stejskal and Tanner (77) introduced the
�pulsed gradient spin echo� method (PGSE), which improved MR diffusion measurements. The
methods have been extended to DWI and through the last fifteen years DWI has proven to be a
valuable tool for in vivo studies. In 1984 DWI was used for detecting ischemic lesions (78) and
today changes in water diffusion measured by DWI is established as a sensitive early indication of
ischemia as first proved for cats in 1990 (79), (80). By measuring the diffusion asymmetry
(anisotropy) DWI provides visualization of white matter tracts (81).
2.4. Diffusion measurements based on Magnetic Resonance
Molecular diffusion is random motions of molecules caused by thermal energy. The random walk
of a molecule leads to a net displacement increasing with time. Looking at an ensemble of
molecules initially located at a particular position, the spatial distribution for homogeneous,
unrestricted diffusion will be Gaussian, with a mean displacement of zero, because the probability
of displacements in every direction is the same (figure 5).
The expected value of the squared distance of displacements in three dimensions is proportional
to the time t according to the Einstein equation:
40
D is termed the diffusion constant, and characterizes the mobility of molecules within a medium.
This means that the expected diffusion distance |r| increases with the square root of t.
The sensitivity of the MR signal to diffusion results from phase variations when the spins are
diffusing in an inhomogeneous magnetic field. A gradient pulse gives the spins a position
dependent labelling. The refocusing of the echo in the MR experiment will be incomplete due to
diffusion of the spins along the field gradient and the MR signal will be attenuated (figure 6). The
signal attenuation S/So for homogenous diffusion has an exponential dependence:
where b is a factor depending only on the magnetic field gradients and their timing, as given in
formula [2.9].
Figure 5. Diffusion - Gaussian distribution (adapted from (74))
)exp(/ 0 bDSS −=
[ ]4.2
Dtr 62 =⟩⟨ [ ]3.2
41
Figure 6. Diffusion detected in magnetic resonance (adapted from (82))
2.4.1. The Pulsed Gradient Spin Echo experiment
In 1965 the Pulsed Gradient Spin Echo (PGSE) experiment was introduced by Stejskal and
Tanner (76). In the PGSE experiment, two pulsed gradients of amplitude G and duration ∗ are
applied before and after the 180Ε RF pulse with a separation of ) (figure7). During the first
pulsed gradient the spins will dephase. The phase contribution for a spin at position z1 is:
The second pulsed gradient tends to refocus the magnetization but due to diffusion the spin have
moved to position z2 and the net phase Νnet therefore amounts to:
For stationary spins, Νnet is zero corresponding to z2 = z1. When diffusion occurs, z2 ≠ z1 and a
net phase has been acquired. The MR signal will be attenuated compared to the signal obtained
without pulsed gradients (G = 0). In the PGSE experiment, signal attenuation S/S0 caused by
diffusion is:
[ ]5.2∫ ⋅⋅⋅=⋅⋅⋅=t
zGdtzG0
111 δγγφ
( )12 zzGnet ÷⋅⋅⋅= δγφ [ ]6.2
Voxel
Random motion Phase disribution Echo attenuation
Gradient
42
where
The PGSE experiment offered several advantages to previous MR diffusion measurements: The
diffusion measurement time is known exactly and controllable in the PGSE experiment as ()!∗ /3)
which gives the possibility of studying time dependent diffusion, which is valuable in biological
systems (83). Also, because the diffusion encoding gradients are turned off during data collection,
the diffusion encoding gradients can be increased along with the diffusion sensitivity without
inducing an increased acquisition bandwidth i.e. increased noise.
Finally with large amplitude G and short duration, ∗ , of the pulsed gradients the residual B0 field
inhomogenities will be negligible thereby allowing measurements of very low diffusion
coefficients (74).
Figure 7. The Pulsed Gradient Spin Echo experiment
[ ]7.2( ) )3/exp(/ 2220 DGSS ⋅−∆⋅⋅⋅−= δδγ
[ ]8.2( )Db ⋅−= exp
[ ]9.2( )3/222 δδγ −∆= Gb
90 180 Echo
δδδδ δδδδ ∆∆∆∆
43
2.4.2. Diffusion attenuation- diffusion coefficient
As expressed in formula [2.8] and [2.9] the relative signal attenuation S/S0 depends on a known
parameter b, controlled by the timing and amplitudes of the gradients. The simplest way to
measure the diffusion coefficient, D, for homogeneous, unrestricted diffusion requires at least
measurements at two different b-values. The slope of a line fitted to lnS/So as a function of b is
!D (figure 8).
For diffusion to cause a measurable attenuation of the signal, the product bD must be near unity.
D values for water in the human brain is in the range of 1x10-3 mm2/s, and b should therefore be
in the range of 1x103 s/mm2. Estimating the diffusion coefficient from the slope in figure 8, the
highest b-value should be chosen so that the signal attenuation is above the noise level at a level
of good SNR. On the other hand, if the highest b-value is low the uncertainty of the measured
signal will significantly influence the slope and thereby the estimation of the diffusion coefficient
D. Also at very low b-values, the values of D can be influenced from perfusion in small vessels
mimicking diffusion. This is however a rather small contribution (84). For visual evaluation of
DWI, an optimum in contrast between normal and ischemic tissue, (decreased ADC), exist
between 800-1200 s/mm2 (1). Finally, the specificity of using only DWI in acute stroke will be
further discussed in Chapter 5.
44
Figure 8. Diffusion coefficient
2.4.3. Temperature effects
The diffusion coefficient D depends on the temperature T. Within the physiological range (T =
32-42 CΕ) the correlation of D and T is linear with an estimated 2.4% change in D for a 1%
change in the temperature (85).
2.4.4. Motion artifacts
Diffusion sensitive pulse sequences are highly sensitive to microscopic and macroscopic motions
from other sources than diffusion leading to severe artifacts in diffusion weighted in vivo studies.
Cardiac pulsation in the brain tissue and CSF, micro-circulation, breathing motions, head
translations and rotations with displacement in the order of micrometers all induce image
artifacts. Using phase contrast MR imaging, it has been shown that during the cardiac cycle, brain
tissue is displaced by 0.5 mm (86),(87). Cardiac gating should be used for acquiring DWI to
�
� �� � ��
� � � ��
��
�
�
0 200 400 600 800 1000b s/mm2
lnS
-D
45
compensate for movements induced by the cardiac cycle. The phase shifts due to motion results
in ghosting artifacts and leads to signal attenuation and therefore misinterpretation of the
calculated diffusion coefficients. Phase shifts are pronounced when diffusion gradients with large
amplitudes are applied in a MR sequence. However, the average phase shift due to diffusion is
zero (intravoxel phase incoherence) (figure 9 top). For linear bulk motion (coherent translation in
a given spatial direction) a phase shift develops depending on the applied gradient strength and
the velocity of the spins. No changes in the signal amplitude happen, (figure 9 middle). Finally,
nonlinear motions (e.g. rotations and incoherent motions) lead to both phase shifts and signal
loss in every acquisition (figure 9 bottom) (88). Three approaches have been applied in reducing
motion artifacts: bipolar diffusion gradients, navigation in spin-echo sequences and high speed
imaging as now discussed:
Figure 9. Motion effects on net phase and the echo signal (adapted from (88))
Diffusion Linear motion Nonlinear motion
46
2.4.5. Bipolar motion compensating gradients
Linear bulk motion e.g. flow effects are dependent on the velocity of the spins and on the used
gradients and can be cancelled out by adding bipolar motion compensating gradients in the
diffusion weighted pulse sequence. Such gradients should be placed on each side of the 180Ε RF-
pulse and should have identical Gt area. As an unfortunate side effect, the use of bipolar
gradients in diffusion weighted MR experiments with a fixed TE will reduce the effective
diffusion time and sensitivity compared to the unipolar diffusion weighting sequence (89).
2.4.6. Navigation
Different �navigator echo� techniques have been suggested for correction of linear velocity
motion in spin echo sequences (90) (91). These techniques use an additional echo acquired before
or after each spatially encoded echo. Any phase variation in the navigator echoes will be due to
motions along the gradient applied during navigator echo acquisition. Line-wise back rotation in
k-space can be performed using the individual navigator echoes. This retrospective modification
compensates for linear velocity of the head (figure 10). More advances techniques have been
developed, compensating for more complex motion patterns (92).
47
Figure 10. Motion compensation by phase correction (adapted from (88))
2.5. Diffusion Weighted Imaging sequences
It is described above how the diffusion coefficient in a homogenous media may be determined by
using a PGSE sequence. With the development of MR imaging systems, the methods for
determination of diffusion by PGSE was extended to the field of MR imaging. The aim of this
section is to describe the basic concepts of DWI.
Many DWI techniques have been proposed since DWI by combination of the PGSE experiment
and 2D Fourier transform MR imaging was developed (74). The most commonly used technique
has been the spin echo sequence because it gives sufficient diffusion weighting in high resolution
and is technically simple to implement. DWI using EPI techniques have the advance of speed.
Movement artifacts are effectively eliminated in the individual images, but other severe image
artifacts and low image resolution, are introduced. DWI of high spatial resolution and with few
artifacts can be rapidly acquired using the HASTE (half-Fourier single-shot turbo spin-echo)
sequence (93) and interleaved echo planar imaging (IEPI) (92).
2.5.1. Spin Echo Diffusion Imaging Methods
Most commonly, a couple of bipolar diffusion gradients are inserted in a conventional spin echo
imaging sequence for DWI of which an example was presented in figure 7. This approach
48
provides images of high resolution and good signal-to-noise ratio. The diffusion gradients are
applied on each side of the 180Ε refocusing RF pulse. Diffusion weighting increases by
increasing the delay ) between the two diffusion gradients but the TE will of course increase, thus
decreasing the signal in the images. Sufficient sensitivity can be achieved without strong diffusion
gradients that are not available on most MR scanners.
2.5.2. Fast Diffusion Imaging Methods
The fastest imaging sequence is EPI by which an image of 64x64 pixels can be acquired in 50-
100ms. The EPI sequence was proposed in 1978 (94) and has proven very suitable and is today
the method of choice for DWI. In single-shot EPI, an image is acquired after one excitation. The
traversal in k-space is shown in (figure 11). EPI can be strongly T2*-weighted and is then very
sensitive to susceptibility changes. This cause signal loss with increasing TE and cause distortions
of the images, particularly in the posterior fossa, the anterior and inferior temporal lobes and the
paranasal sinuses. Good homogeneity of the field is therefore needed and shimming before every
examination is performed. Severe chemical shift artifacts make good fat suppression mandatory.
Spin echo EPI has less image distortion due to susceptibility. Single shot EPI is not as good as
conventional MRI for visualizing anatomical details because of image distortions, low SNR, low
resolution and low contrast.
The high speed of EPI reduces motion artifacts and makes navigation and cardiac gating
unnecessary. As only one excitation is needed per image, saturation effects do not decrease the
SNR. The short image acquisition time allows DWI to be acquired using several b values or
increasing the number of measurements at every b value. The calculation of the diffusion
coefficient is then more precise (figure 8). A short acquisition time allows imaging of the whole
brain volume in only 3-4 seconds. This is in opposition to conventional MR imaging where only a
section of the brain can be covered using acquisition times of several minutes. The strong
gradients needed for EPI enables b values of 1000 s/mm2 as desired for brain tissue imaging. It is
hard to achieve b-factors this high on conventional MR systems.
49
Rapid switching of strong gradients is needed to perform single shot EPI, available on most new
whole-body MR scanners.
Some of the problems of distortions when doing fast imaging have been solved: Using an
interleaved EPI technique, IEPI, for DWI, susceptibility artifacts are reduced compared to single
shot EPI (95). Also image resolution and SNR are improved due to the increased image
acquisition time. IEPI is more sensitive to patient motion and cardiac gating and navigator echo
techniques are therefore needed. DWI covering 16 sections sufficient for calculation of the
diffusion coefficient can be obtained in few minutes. IEPI can be implemented on a
conventional scanner.
Another fast technique is diffusion weighted HASTE in which only half of the k-space is
traversed and the other half constructed by mirroring (93). HASTE is a non-EPI technique and
susceptibility artifacts are therefore reduced. Images covering the whole brain can be obtained in
one minute and it takes minutes to acquire data for calculation of the diffusion coefficient. This
technique can be implemented on most conventional MR systems.
Figure 11. Echo planar Imaging k-space trajectory
ky
kx
Echo-planar imaging
50
2.6. Diffusion in vivo
In this chapter, the application of diffusion weighted imaging in vivo is considered. The concept of
the apparent diffusion coefficient (ADC) will be presented, and diffusion measurements in
biological tissue will be discussed with emphasis on restriction, compartmentalization and
anisotropy. Finally, the implications for diffusion weighted imaging in vivo in terms of calculation
of the diffusion tensor and diffusion trace is given.
2.6.1. Apparent Diffusion Coefficient
Diffusion has until now been discussed for homogeneous systems were diffusion is �free� or
isotropic. In biological tissues, the assumption of free isotropic diffusion is not generally valid.
The diffusion is influenced by natural barriers such as the cell membranes, microstructures such
as organelles and microfilaments within the cells. Macromolecules in the different cellular
compartments change the viscosity and bind water thus reducing free diffusion. The measured
diffusion coefficient in a system where the diffusion is non-isotropic will depend on the MR
sequence and its settings of e.g. the diffusion time, the directions of gradients and the range and
numbers of b-values. The MR signal obtained in a particular pixel represents a mixture of
diffusion in different compartments with specific degrees of restriction. The term �apparent
diffusion coefficient� (ADC) has therefore been applied for the measured diffusion coefficients
obtained in in vivo studies (96).
2.6.2. Restricted Diffusion
Diffusion is restricted when barriers in the studied object prevent the molecules from moving
freely. Restriction is related to the diffusion time in the MR experiment. When the diffusion times
are very short, most molecules will move freely without reaching any boundaries within the
diffusion time. When the diffusion time is increased, the average diffusion distance will reach a
threshold when the diffusion distance is equal to the characteristic size of that compartment. The
51
diffusion coefficient is calculated as the slope when plotting the natural logarithm of the signal
attenuation versus the b values as expressed in formula [2.8] and illustrated in figure 8. The signal
attenuation depends on the diffusion distance obtained in the experiment. When the critical
diffusion distance for restriction has been reached, the signal attenuation will tend to level off as a
function of the b-factor as will the diffusion coefficient as shown in figure 12.
However, it has been shown that the measured diffusion coefficient in brain white matter is
independent of diffusion times longer than 20 ms (97). Several other studies indicate that there is
no effective hindrance to water exchange between the extra-and intracellular compartments (83),
(98).
This implies that restriction can not be the major mechanism explaining the acquisition-
dependence of the �apparent diffusion coefficient� measured in vivo.
Figure 12. Diffusion - signal attenuation isotropic (full line
2.6.3. Hindered Diffusion – tortuosity
Another mechanism to explain the concept of ADC is that of �to
with external tracers and non-MR techniques (99). The observed
path because of obstacles such as axons and macromolecules,
0 200 400 600 800 1000
0 -1
ln S
b
) in vivo (dotted line)
rtuosity� introduced in studies
molecule has to travel a longer
i.e. molecules have to diffuse
52
around structures impermeable to them. The expression for ADC when the diffusion is hindered
is:
D is the free diffusion and 8 the tortuosity.
Tortuosity can explain why the ADC is independent of diffusion times longer than 20 ms in
brain white matter: There is no real barrier in contrast to restriction, so that molecules diffuses
covering, long distances (figure13) (99).
Figure 13. Tortuosity: hinderage (adapted from (99))
2.6.4. Diffusion in multi compartment system
Water is distributed in four major compartments in the brain in vivo: the intracellular, the
extracellular space, the cerebrospinal fluid (CSF) and the vascular space. Only the intracellular
and extracellular compartments are considered in diffusion experiments because the vascular and
cerebrovascular spaces only account for a few percent of the total brain volume. The extra- and
intracellular compartments have different diffusion coefficients but are believed to exchange
2/ λDADC = [ ]10.2
A
B
53
water very fast in about 20-50ms (97). In DWI experiments, the diffusion time is essential for
determining which of two models describes diffusion signal attenuation. If the diffusion time is
short relative to the exchange time between the extra- and intracellular compartments, the
exchange should be considered slow. The expression for the diffusion attenuation then becomes
bi-exponential:
In most diffusion MR experiments the diffusion time is longer than the water exchange time and
the system observed is then a fast exchange system. The diffusion attenuation becomes a mono-
exponential function (83):
where fin and fex is the intracellular and extracellular volume fraction respectively.
2.6.5. Apparent restriction
Moonen et al. (97) has showed that at diffusion times longer than 20 ms, the diffusion coefficient
does not change as a function of diffusion time because water exchange is relatively fast and
appears unrestricted. In this case, the diffusion coefficient should be considered as a fast
exchange system and is an average of intra- and extracellular diffusion coefficients. From studies
of Norris et al. (95), (98) it appears that when diffusion times are set very short (< 2ms) the signal
attenuation is bi-exponential signifying slow exchange between the two compartments as in
formula [2.13]. When diffusion time is intermediary, the diffusion seems restricted as determined
by the exchange rates between the intra- and extracellular compartments. This is known as
�apparent restriction�. The transitions between the different stages are shown in figure 14.
)exp()exp( exexinin bDSbDSS −+−= [ ]11.2
( )( )exexinin DfDfbSS +−= exp0 [ ]12.2
C
54
Figure 14. Diffusion anisotropy dependence on diffusion time. A: Short diffusion time with isotropic
diffusion. B and C: intermediate diffusion time with anisotropic diffusion. D: long diffusion time with isotropic
diffusion (adapted from (83))
2.6.6. Anisotropy
The mobility of molecules depends on the direction (99). When diffusion is encoded
perpendicular to the axons, the measured diffusion coefficient is lower compared to
measurements parallel to the axons. This diffusion anisotropy results from myelin sheets in the
white matter and can be understood in the context of restriction, tortuosity and �apparent
restriction� {Norris & Niendorf 1995 #860}, (99). The diffusion anisotropy effects require
geometrical parameters to be taken into account when studying diffusion (100).
2.6.7. The Diffusion Tensor and the Diffusion Trace
The general, three dimensional, diffusion in vivo can not be described as a scalar due to anisotropy
effects but has to be considered as a tensor D represented by the matrix:
=
zz
yz
xz
zyzx
yyyx
xyxx
DDD
DDDDDD
D [ ]2.13
A B
D C
55
If the media observed is isotropic then Dxx=Dyy=Dzz and the diffusion coefficient can be
expressed as a scalar and the off-diagonal elements in the tensor are zero. If one of the axis in the
reference coordinate system used, is chosen parallel to the axons the diffusion tensor D is
described only by the principal diffusivities (Dxx , Dyy , Dzz), with all off-diagonal components of
the matrix being zero. In practice, the diffusion measurements are made in a laboratory reference
frame with axis along the applied diffusion gradients. These are not necessarily aligned to the
principal axis of the diffusion tensor and the D, of the laboratory reference, frame is related to
the diagonal D by a rotation matrix. The matrix D is symmetric, Dzx=Dxz , Dyx=Dxy , Dzy=Dyz , so
D can be calculated by applying only six different diffusion encoding directions instead of nine.
In recent years, the diffusion tensor has been shown to be measurable in vivo by diffusion tensor
imaging (101). A number of properties of the diffusion tensor are independent of the reference
frame and the rotation of the acquired tensor. If the purpose is to measure the average diffusion
coefficient without effects from orientation within the magnet, the most commonly the rotation
in-variant, is the diffusion trace, that is the sum of the diagonal components of the acquired D,
the directional mean of the diffusion coefficient. For imaging purposes in vivo, diffusion encoding
in only three directions are needed instead of the six needed to determine the entire tensor (102).
2.6.8. Values of ADC
In normal brain tissue, approximately 20% of the brain water is distributed in the extracellular
space and 80% in the intracellular space. The intracellular diffusion coefficient is lower than in
the extracellular space due to intracellular barriers as organelles, membranes and macromolecules.
In one study, van Zijl et al. (103) measured the diffusion coefficient in cell cultures and
determined the intra- and extracellular diffusion coefficients to Din = 1.43 x 10-4 mm2/sec and
Dex =3.25 x 10-3 mm2/sec, respectively. Assuming free water exchange Benveniste (104)
proposed a model to calculate the macroscopic ADC in vivo. For use in formula [2.12], the value
(0.8 x Dintracellular + 0.2 x Dextracellular) is 0.8 x 10-3mm2/sec. In the rat brain, the ADC is somewhat
56
lower than calculated for cell suspensions: 0.60-0.75 x 10-3 mm2/s. The range of values in
different studies depend on the region of interest, on the diffusion time and on the direction of
the diffusion encoding (105). In early human studies, the calculated ADC varied from 0.3-1.2 x
10-3 mm2/s in human brain white matter depending on the direction of the diffusion encoding
due to the effects of anisotropy (102), (27). Ulug et al. calculated the trace of the diffusion tensor
in healthy subjects and found less inter individual variation in ADC than the individual ADC
values calculated in the three orthogonal directions (102). They found small differences between
white and gray matter. The trace was 0.83 ∀ 0.06 x 10-3mm2/s and 0.92∀ 0.15 x 10-3 mm2/s,
respectively. It was concluded that the ADC should be calculated as the trace of the diffusion
tensor. Also it was concluded that for stroke patients, the ratios of the ADCs in the infarct and in
the contralateral region should be reported thereby reducing the variations between reported
values from centers using different parameters for DWI, which influence the ADC values.
2.7. Perfusion Weighted Imaging
Several techniques have been developed for determination of perfusion. Both PET and SPECT
are relevant for clinical assessment and often use diffusible tracers, which are ideal for measuring
perfusion. Both techniques have drawbacks being invasive, using radioactively labeled tracers and
having relative low spatial and temporal resolution. For measuring perfusion, MRI has the
advantages of high temporal and spatial resolution using no radioactivity, being almost non-
invasive and offering combination studies using other MR techniques (diffusion, spectroscopy,
angiography, structural imaging).
MR perfusion imaging is divided into categories: (1) susceptibility based techniques either (a)
using intravenous bolus injection of a paramagnetic contrast agent or (b) detecting changes in the
endogenous paramagnetic substance deoxyhemoglobin (blood oxygen level dependent, BOLD),
and (2) arterial spin labeling of protons in blood.
57
The most commonly used of these techniques is (1a): contrast agent bolus tracking also known as
dynamic susceptibility contrast magnetic resonance imaging (DSC-MRI). DSC-MRI was applied
in our studies and will be further described.
2.7.1. Contrast agents
Opposite to radioactive tracers, MR contrast agents are considered safe. Their effect is due to
paramagnetic properties causing shortening in T1 and T2 relaxation (further described below).
MR contrast agents containing gadolinium-chelates are approved for use in humans. Gadolinium
is a lanthanide metal being paramagnetic property due to seven unpaired electrons. Due to
toxicity gadolinium is chelated to different compounds: Gadolinium-DTPA (Dithylen-Triamin-
Penta-Acetiacid) is known as gadopentate and sold as Magnevist® (Schering), DTPA-BMA
(DTPA-bismethylamide) known as gadodiamide and sold as Omniscan®.
2.7.2. Vascular effects
The effect of MR contrast agents on T1 relaxation is caused by so-called dipole-dipole
interactions i.e. on direct interaction of protons in the water molecules with the dipole moment
of the unpaired electrons of the paramagnetic contrast agent. Gadolinium-compounds in the
brain are intravascular when the blood brain barrier is intact. T1 shortening produces signal
enhancement in the blood volume, that contributes about 5% of the total brain volume.
Susceptibility is a measure of the ability of a compound to become magnetized when subject to a
magnetic field thus increasing or decreasing the applied external magnetic field.
The intravascular paramagnetic contrast agents cause susceptibility changes and increase the local
external magnetic field. A magnetic field gradient between the lumen of the vessel and the
surrounding tissue is induced (figure15), thereby increasing the T2* relaxation rate and dephasing.
This effect on T2* relaxation extends beyond the blood vessel, thus generating relative large and
distant signal changes. The method is therefore more suitable for MRI perfusion measurements,
than the dipole-dipole effect changing T1 relaxation in the vessel only (106), (107). The signal
58
change caused by a paramagnetic compound also depends on the distance that the water
molecules diffuse compared to the field inhomogenity created by the paramagnetic agent, i.e.,
how many different magnetic fields the spins experience during the duration of the pulse
sequence (figure 15). Around a larger vessel, water molecules will only experience the same
"static" magnetic field because the distance travel led by diffusion is small in relation to the
characteristic distance of field inhomogenity. Dephasing due to static inhomogenity leading to
signal loss will occur in a gradient-echo experiment but not in a spin-echo experiment. Around
smaller vessels, the water molecules will experience varying magnetic field gradients due to
diffusion (around the vessel and around the neighboring vessels), this will lead to dephasing and
signal loss in a gradient-echo as well as in a spin-echo experiment. Using gradient-echo
sequences, the T2* effects from both large and small vessels will therefore be detected. Using
spin-echo sequence the T2* effects from the large vessels will be suppressed. However, gradient-
echo is often used in studies due to the better SNR, (greater signal loss than with spin-echo
sequences), when using the same amount of MR contrast agent. The susceptibility effect will
decrease when the BBB is damaged because local field inhomogenity decreases. The dipole-
dipole effects shortening T1 will cause a greater signal increase in the tissue. The MRI sequence
can be optimized so T1 effects are decreased and post-processing of the signal time curve has
also been proposed (108), (109). Use of paramagnetic contrast compounds causing small T1
effects, e.g. dysprosium, can minimize effects from leakage of the BBB, although not available for
human studies (110).
Perfusion is confined to the capillary bed and using a spin echo sequence will select the relevant
compartment for a perfusion measurement, (111). Spin echo images have better contrast-to-noise
(CNR) than gradient echo images and are without severe susceptibility artifacts in the inferior
frontal and temporal lobes. On the other hand, information on hemodynamics in the large
vessels is abolished. Also, more contrast agent is needed in bolus tracking using a spin echo
sequence because )R2* is smaller than )R2.
59
Figure 15. Vessel size and susceptibility effects
From left to right: small vessel, intermediate vessel, large vessel
2.7.3. Contrast agent bolus tracking
When tracking a bolus injection of paramagnetic contrast agent by fast imaging, the MRI signal
will drop transiently, due to the susceptibility effect when the gadolinium-compound passes
through the detecting plane (figure16). The bolus of 0.1-0.2mmol/kg is injected via the
antecubital vein (3-5ml/sec), using a power injector. With the availability of EPI, it has become
possible in the last five years to obtain the passage with high temporal (1-2 seconds) and high
spatial resolution (~1mm) by multislice imaging (6-11slices). 10-40 frames of baseline images
should be acquired before the bolus is injected increasing SNR ratio when calculating )R2* (see
formula [2.20]) (111). The TE should be long (60-75 ms) increasing the susceptibility sensitivity
(111).
60
Figure 16. Contrast agent bolus tracking signal-time curve
2.7.4. Tracer kinetics
Quantification of perfusion is done using the central volume theorem (112) based on certain
assumptions describing a systems response to an injection of a tracer: Flow is constant during
observation and the tracer is inert with no metabolism or retention.
The central volume theorem states that:
CBF is the perfusion, CBV is the blood volume and MTT is the mean transit time.
The central volume theorem is a general equation for all kind of tracers (diffusible, intravascular
or intermediate).
The model used when calculating perfusion in DSC-MRI is based on tracer kinetics for non-
diffusible intravascular tracers (113).
When a bolus of contrast agent is injected, the concentration Cvoi(t) of the tracer in a volume
(VOI) can be described in terms of
CBV/MTT CBF = [ ]14.2
Seconds
Signal
61
• h(t), the frequency distribution function, describing the distribution of transit times through the
VOI following an ideal instantaneous bolus.
• R(t), the residue impulse response function i.e., the fraction of the bolus still present in the VOI at
time t following a ideal instantaneous bolus. R(t) and h(t) are related by:
[2.16], represents the accumulated fraction of the bolus that has left the VOI. Formula [2.15]
state that after a instantaneous bolus, R(t) is the fraction of the bolus in the VOI at time - t.
• AIF, the arterial input function, Ca(t), the concentration of contrast agent in the supplying vessel
to the VOI.
From these definitions Cvoi(t) is calculated as:
where
- CBFvoi is the perfusion in VOI,
- ∆ is the density of the tissue,
- kh is correcting for differences in hemotocrit in capillaries and large vessels.
∫=t
dhtH0
)()( ττ
[ ] ( )tHdhtRt
−=−= ∫ 1)(1)(0
ττ [ ]15.2
[ ]16.2
ρ t R t Ca CBFvoi k t Cvoi h )) ( ) ( ( / ) ( ⊗ ⋅ ⋅ = [ ]17.2
τ τ τ ρ d t R Ca CBFvoi k t
h ) ( ) ( / 0
− ⋅ ⋅ ⋅ = ∫ [ ]18.2
62
Formula [2.17] states that Cvoi can be expressed as a convolution of the residue impulse
response function, R(t), and the arterial input function, Ca(t). To calculate CBFvoi, the arterial
input function Ca(t) and the Cvoi(t) therefore must be deconvolved determining the residue
response function R(t) and CBFvoi. Deconvolution is theoretically demanding with inherent
uncertainties in the estimated R(t), because the signal time curve in the artery and the tissue is
almost identical when using intravascular tracers. However, attempts of calculating the rCBF
have shown good agreement with values from PET measurements (114), (115). An accurate
estimate of the arterial input function is necessary but can be difficult to obtain due to partial
volume effects and bad SNR in EPI. Also it is assumed in the model that the bolus curve in the
VOI is not dispersed or delayed compared to the AIF, which is certainly not valid in ischemic
regions (116).
The CBV is expressed as:
Relative CBV can be estimated without knowledge of the AIF, assuming the same AIF to all
parts of the tissue. This approach has therefore often been applied in human studies.
For intravascular tracers used in DSC-MRI, the time concentration curve obtained in the VOI,
also depend on the vascular architecture. The first moment of the tissue concentration curve
differs from the true MTT (117). Calculating CBF by the central volume theorem [2.14], with
MTT calculated as the first moment of the tissue curve, is therefore wrong.
2.7.5. Concentration dependency
The concentration in the VOI Cvoi used in the calculation of the hemodynamics is related to the
change in T2* relaxation:
∫∫⋅=
dttCa
dttCvoikCBV h
)(
)(
ρ
Cvoik*R2*2T
1 ⋅==
∆∆
[ ]19.2
[ ]20.2
63
k is the relaxivity constant.
Assuming the linear relationship [2.20] between the concentration of the gadolinium-compound,
Cvoi, and the change in R2*. )R2* is determined from the baseline signal S0, and SVOI(t), the
signal in the VOI at time t (118), (107), (119). TE is the chosen echo time:
[2.21], is valid for both the tissue and arterial input function, (however k may differ).
2.7.6. Perfusion parameter maps
With the technical and practical difficulty of absolute quantification of perfusion using DSC-
MRI, various parameters derived from the )R2* time curve can been calculated. The time to R2*
peak, (TTP), the bolus arrival time (BAT), the peak height, the area under the )R2* curve (AUC)
(which is related the CBV) and the first moment of the R2* time curve used as an approximation
for the MTT. The interpretation, except for AUC = CBV is not straightforward because there is
no simple relation between these parameters and the rCBF. Also the parameters should be
normalized, because they vary depending on the vascular architecture, cardiac output, injection
rate and dose. In ischemia, the normal contralateral side will often be used for normalization of
the different parametric maps.
The effects of re-circulation can be eliminated either by considering only the first part of the
signal-time curve or by fitting the curve to an assumed model function, most often a gamma
variate function (120). The primary aim of fitting a gamma variate function to the data is to
estimate the area under the first pass curve and to improve the SNR of the TTP estimate.
⋅−=⋅=
0SS(t)ln
TEk∆R2*kCvoi(t) [ ]21.2
64
Due to the technical difficulties of deconvolution we used TTP and AUC maps in the study of
acute stroke presented in Chapter 6. However, TTP maps have some inherent differences
compared to MTT and CBF maps in ischemic volumes. For instance, when considering an
ischemic area caused by a stenosis or an occlusion, the CBV and CBF are decreased in the
ischemic area itself, and the "true" calculated MTT will be unchanged. However, TTP in the case
of stenosis will be prolonged in the whole area distal to the stenosis. If collateral blood supply is
involved, the TTP will be prolonged in the collateral vessels, because blood is by-passed from the
occluded vessel through the collaterals. In both cases TTP will overestimate the ischemic area.
However, as discussed in Chapter 6 information on the collateral blood supply can be visualized
on TTP maps. Generally, it can be shown that TTP does not depend on the perfusion but only
on the actual input function and configuration of the residue impulse response function.
However, it is not known from empirical data, if these parameters in ischemic tissue actually have
configurations giving changes in TTP occurring parallel to changes in CBF.
2.8. Magnetic Resonance Angiography
Magnetic resonance angiography has proven to be a powerful technique with an ability to
visualize abnormalities in the cerebrovascular system and the technique is increasingly used in
clinical examinations. In contrast to conventional angiography, magnetic resonance angiography
does not normally require injection of contrast agents. Two types of flow related processes have
been recognized (121): The observation that spins moving into a slice appear bright compared to
the stationary surrounding tissue has been developed into the MR angiographic technique called
�time-of-flight� (TOF). The correlation between the velocity of the blood flow and the phase of
moving spins is exploited for �phase-contrast MR angiography�. TOF-MR angiography is used in
the present thesis and is discussed further below.
65
2.8.1. Time-of-flight MR angiography
After several consecutive RF excitations using a gradient-echo sequence, an equilibrium
magnetization is established in stationary tissues decreasing the MR signal to a low steady state
value due to saturation. Spins that flow into the imaging slice during acquisition will not be
saturated compared to the stationary spins because they have not been influenced by previous RF
pulses and will therefore contribute with a much larger signal. Vascular structures are therefore
visible because inflow increases their signal intensity compared to stationary tissue (figure 17).
Time of flight MR angiography is sensitive to slow flow of the venous system. To visualize the
vascular tree, the brightest pixels are projected, �maximum intensity projection� (MIP) (figure
18). One limitation of the MIP reconstruction method is that low-intensity-pixels are not
visualized. Pixels of false low intensity occur when blood velocity (flow void) or turbulence is
high. The lumen of the vessel is therefore underestimated and vascular stenosis overestimated
(121). In the brain, extra- and intracranial vessels can be visualized with identification of
aneurysms, occlusions and vascular malformations. Turbulent flow and slow flow in veins and
small vessels distal in the vascular tree are difficult to visualize.
Figure 17. Time-of-flight velocity and saturation in the detection plane (from (121))
V = 0
Saturated signalUnsaturated signal ∼∼∼∼ maximum signal
66
Figure 18. MR angiography, Time of flight
67
Chapter 3. Magnetic Resonance Imaging in Acute Stroke 3.1. Structural MRI in Acute Stroke
The detection of ischemic stroke by structural MRI (proton density weighted (PDW), T1-
weighted (T1W) and T2-weighted (T2W) imaging) depends on the following features: 1) Vascular
flow, 2) Mass effects, 3) Parenchymal signal changes, and 4) Parenchymal signal changes after
administration of MR contrast agent. MR findings are listed in Table I, (122).
Clinically, conve
tissue contrast a
sensitivity of MR
In normal arteri
called flow-void
seen on T1W im
artery. Arterial e
due to shortenin
after onset of sy
MECHANISM MRI FIN Flow Kinetics Absent flow Arterial enha Biophysiologic T1morpholo T2 signal cha T1 signal cha Combination Delayed pare Early exagge
Table I. MRI Findings In Acute Cerebral Ischemia
ntional MR imaging has proven superior to CT in many aspects as it offers good
nd higher sensitivity for detecting early ischemic abnormalities. In one study the
I (PDW, T2W) was 82% versus 58% for CT within the first 24 hours (123).
es, high-velocity turbulent flow is seen as absence of signal in the arteries, so-
on T2W- and T1W imaging. Absence of flow-void due to slowing of flow can be
ages as isointensity or hyperintensity within the distribution area of the affected
nhancement refers to the same phenomenon accentuated by MR-contrast agents
g of the relaxation times. These changes in vascular flow are identified shortly
mptoms (122).
DINGS POSSIBLE CAUSES ESTIMATED TIME (h)
Slow flow; occlusion Early ncement Accentuation of flow derangement Early gic change Cytotoxic edema (free water) 2-4 nge Blood-brain barrier break down; vasogenic edema; 8
macromolecular binding nge Blood-brain barrier break down; vasogenic edema; 16-24
macromolecular binding nchymal enhancement Impaired delivery of significant contrast agent >24 rated enhancement Intact delivery of contrast agent; blood-brain barrier leakage 2-4
focal hyperemia
68
On T1W images, brain swelling is seen within the first 2-4 hours. This represents cellular,
(cytotoxic), edema characterized by accumulation of intracellular water. The total water amount is
increased by 3-5% (122). Because this water is supposedly free i.e. not bound to macromolecules,
the relaxation times will be relatively long causing no signal changes compared to normal tissue.
Swelling can be identified on good anatomical images e.g. T1W images, due to mass effects from
the edema (122). The morphologic T1W changes were seen in 66% of the studies, within the first
24 hours and in approximately 50% of studies performed within 2-6 hours. It was especially seen
in cortical lesions (80%), probably due to good contrast between CSF and cortex. Final infarct or
final prognosis was not reported (122).
Hyperintensity occurs after 6-8 hours on T2W images and hypointensity is seen on T1W images
after 16-24 hours. This is due to vasogenic edema, i.e. extracellular edema associated with
breakdown of the BBB. This leads to a relative signal increase on T2W images and a relative
signal decrease on T1W images compared to the normal tissue. The signal changes are identified
later on T1W images due to limited contrast between the ischemic and the surrounding tissues.
Hyperintensity on T2W images caused by vasogenic edema is believed to reflect infarcted tissue
and is used for measuring final infarct volume in MR studies (122).
When the BBB disrupts at approximately 6 hours, the normally strictly intravascular MR contrast
agent will leak into the infarcted tissue. Relaxation times will decrease, which can be seen as
hyperintensity on T1W images. This hyperintensity occurs when the blood supply has been
reestablished through neo-vascularity after 5-7 days. Enhancement is also seen after reperfusion
or incomplete or non-occlusive ischemia earlier after 6 hours, but not in complete ischemia at
early time points (124).
In conclusion, structural MRI has proven superior to CT for ischemic stroke detection within 24
hours. In the first hours after onset of symptoms, signal changes are only seen in supplying
arteries to ischemic tissue and structural MRI is therefore insufficient for characterizing ischemic
tissue in the initial phase most relevant for pharmacological intervention.
69
3.2. Hemorrhages in MRI
From the 1970's, CT has allowed precise delineation of site, size and space occupying effects of
ICH with a sensitivity of almost 100% even at the earliest time points (125), (126), (127).
If MR should be used to stratify acute stroke patients to thrombolytic therapy then it is of great
importance that ICH can be detected reliably. In this section, the principles for MRI detection of
hemorrhages will be presented. The ability to detect hyperacute ICH and HT will be discussed
and finally three cases of acute ICH will be presented (128), (129).
After a bleeding, blood goes through several transitions regarding magnetic properties, intra- and
extracellular distribution and content of proteins and water (129).
3.2.1. Effects from Protein and Water
MR visible water, if bound to macromolecules such as proteins present in hemorrhages, will
shorten T1 and T2 relaxation. On the other hand, increased free water will tend to increase T1
and T2 relaxation. Protein rich fluids will therefore not cause signal changes.
3.2.2. Paramagnetic Effects
Oxyhemoglobin is degraded to deoxyhemoglobin and further to methemoglobin, ferritin and
hemosiderin. These molecules can be characterized by their magnetic susceptibility effect.
Magnetic susceptibility properties the ability of different molecules or tissues to increase or
decrease the applied magnetic field. All degradation products are paramagnetic
(deoxyhemoglobin, methemoglobin) or even superparamagnetic (ferritin, hemosiderin).
Oxyhemoglobin is diamagnetic with no practical influence on the magnetic field. Paramagnetic
substances cause a local change of the magnetic field. Changes to the magnetic field cause a local
shortening of T1 and T2 due to direct interactions between water molecules and (super)
paramagnetic molecules, and a shortening of T2* due to static inhomogenity. Severe signal loss
caused by T2* effects will only occur when the (super) paramagnetic degradation products are
70
confined in small compartments. For instance, (hyper) acute degradation to intracellular
deoxyhemoglobin will cause signal decrease due to static inhomogenity. When red blood cells
lysis, the effect from static inhomogenity will decrease, and only the direct effect on adjacent
water molecules from deoxyhemoglobin will be present. Spin-echo sequences used for structural
MR imaging are only sensitive to the diffusion effects. Gradient-echo MR-imaging is sensitive to
both diffusion and static inhomogenity effects. Drawbacks of gradient-echo imaging are
compromised anatomic details and susceptibility artifacts in regions with varying susceptibility
e.g. between the air-containing sinuses and brain and especially between hemorrhages and normal
tissue leading to overestimation of the volume of hemorrhages.
Most studies have been done at a low field strength (0.5T) (27). Susceptibility effects increase
when the magnetic field increases. Today, most scanners for clinical use have a field of 1.5T. The
recent availability of gradient-echo EPI, which is highly T2*-weighted, together with the
development and distribution of high field scanners, seems to increase the detection rate of
hemorrhages with MR, especially within the first hours.
The different stages are illustrated in figure 19 and Table II (129).
Figure 19. Stages in hemorrha
OXYHEMOGLOBIN
DEOXYHEM BIN
OGLO
ges (adapted from (129)
71
Table II. The stages of Hemorrhages
Metabolic phase Distribution Relaxivity Dephasing
Relaxation process effects
Oxygenation (oxyhemoglobin) - within RBC ÷ ÷ Deoxygenation (deoxyhemoglobin) - within RBC ÷ + Oxidation (methemoglobin) - within RBC + + - extracellular + ÷ Iron storage (hemosiderin) - within macrophages ÷ + and glia cells
72
3.2.3 Hyperacute Hemorrhage
Immediately after an ICH the mass will consist of a protein rich fluid that contains
oxyhemoglobin. On spin echo sequences, this will appear dark to slightly hyperintense on T1W-
and T2W images and the differentiation between ischemic and normal tissue is therefore difficult.
Two studies have reported susceptibility changes in hyperacute ICH (57), (58). The patients (5
and 9 respectively) were examined from 2-6 hours from onset of symptoms. The T2*W EPI
detected all hemorrhages in both studies; also smaller thalamic hemorrhages were seen. T2*W
EPI was as sensitive as CT in the study where MRI was performed immediately after CT (130).
In one of the studies (58) it was seen that T2*W EPI overestimated the volume compared to CT.
Actually in this study it was reported that all MR sequences in their standard �Stroke-MR-
protocol� detected all hemorrhages, DWI having the best correlation of volume compared to CT.
These studies indicated that MRI � especially T2*W EPI- at 1.5T is very promising for detecting
hyperacute ICH. In one recent study (131) 5 patients with ICH verified by CT, were examined
within two hours (23 minutes to 120 minutes) using an acute MRI protocol. It was concluded
that T2*W EPI was the most sensitive MRI modality in detecting hyperacute ICH. The lesions
could be divided into three regions: an isointense to hyperintense center on T2*W EPI,
appearing hypointense on T1W MRI, and probably reflecting edema or proteinaceous solution.
In the periphery, an area with signal loss was seen on T2*W EPI enlarging over time. The
surrounding outer rim was hyperintense on T2W and hypointense on T1W, reflecting vasogenic
edema. The segmentation of the ICH was in agreement to findings in an animal study of ICH
within one hour, where signal loss on T2*W MRI was seen in the periphery of the hemorrhage
corresponding to histology with transformation of oxyhemoglobin to deoxyhemoglobin from the
periphery of the hemorrhage. In conclusion, the authors suggest that MRI may be of diagnostic
value for hyperacute ICH; but larger prospective studies are needed (131).
The matter of sensitivity of MRI for hyperacute ICH has been of increased importance after
thrombolysis by rt-PA has been approved in the United States for ischemic stroke within 3 hours.
73
At the moment only 19 patients have been reported examined within 6 hours by T2*W EPI as
discussed above. One of the cases examined 23 minutes after onset of symptoms has raised some
doubt whether T2*W EPI is 100% sensitive to ICH. The method is therefore not ready to be
implemented in pre-thrombolytic imaging excluding ICH instead of CT. In section 3.2.7, a case
of acute ICH is presented questioning the sensitivity of T2*W EPI to hyperacute ICH.
3.2.4. Acute Hemorrhage
The transition of oxyhemoglobin to deoxyhemoglobin and further to methemoglobin, inside the
red blood cells, contributes to static inhomogenity and is therefore detectable on T2*W imaging.
3.2.5. Subacute Hemorrhage
When lysis of red blood cells occurs within several days to weeks, the effect from static
inhomogenity will decrease and dipole-dipole effects will dominate with hyperintensity on T1W
images and to less extent, hyperintensity on the T2W images. The latter is caused by edema in the
mass.
3.2.6. Chronic Hemorrhage
After some weeks phagotic cells will catabolize the hemoglobin degradation products to the
superparamagnetic ferritin and hemosiderin. Intracellular superparamagnetic molecules will add
to the static inhomogenity which can be detected with T2*W imaging, in particular.
74
3.2.7. Cases of Acute Hemorrhage
We examined three patients with stroke 5, 6 and 12 hours after onset of symptoms respectively,
all confirmed on CT. The patients were examined on a Vision Siemens whole body scanner
operating at 1.5 T. using a double spin-echo sequence for PDW- and T2W imaging (TR=2500
ms, TE=20, 80 ms, FOV=230 mm, matrix = 256 x 256) or TurboT2 imaging (TR=3520 ms, TE
= 115 ms, FOV 230 mm, matrix = 512 x 512) and spin-echo sequence for T1W imaging
(TR=614 ms, TE=14 ms, FOV=230 mm, matrix = 256 x 256). T2*W imaging was performed
using a FLASH (fast low angle shot) sequence (TR=600 ms, TE =15 ms, FOV = 230 mm,
matrix = 256 x 256) and T2*W EPI (TR= 0.96 ms, TE=66 ms, FOV 230 mm, matrix 128 x128).
Two of the cases are illustrated:
Figure 20. A case of acute hemorrhage 5 hours after onset of symptoms. On the top the T2W
images with only subtle changes in the right thalamus are seen. The T2*W EPI at the bottom
clearly shows decreased signal in the right thalamus due to paramagnetic effects probably from
increased deoxyhemoglobin. This finding was confirmed on CT as a hyperintense lesion. T1W
images showed no signal changes.
T2W
T2*W EPI
75
It is illustrated how susceptibility artifacts in the boundaries between sinuses containing air and
brain � here in the inferior frontal lobe � hampers the T2*W image due to susceptibility effects.
These artifacts could be misinterpreted as an ICH excluded from the CT in the actual case.
T2*W MRI has the drawback of artifactual signal loss due to susceptibility changes between
tissues with different magnetic properties. Susceptibility artifacts are seen especially in the inferior
frontal and inferior temporal lobes. Therefore in these regions, excluding ICH is difficult
especially if the patient symptoms originate from these regions.
76
In figure 21 the sensitivity of EPI is illustrated. The patient was exami d 12 hours from onset
of symptoms with sudden onset of psychic and behavioral changes. op row: T2W images.
Second row: T2*W images acquired by FLASH. Third row: the T2*W
row: the T1W images. Fifth row: the CT.
The lesion identified as hemorrhage on CT is seen on all MRI modal
images. It should be noticed that the hemorrhage is segmented in an
ne
T
images by EPI. Fourth
ies except on the T1W
yperintense area in the
it
h
77
posterior part and a significantly more hyperintense area in the anterior part. On the T2*W EPI it
is seen that an area in the posterior part of the pathology shows severe signal loss probably due to
the content of deoxyhemoglobin. This finding correspond to findings on the T2W MRI and
FLASH MRI where areas of signal loss indicates hemorrhage. In the anterior part, an area, as
hyperintense as the CSF, is seen on T2*W EPI, T2W and FLASH and seen as a subtle
hypointense area on T1W. On CT, this very hyperintense area is confirmed to be a hemorrhage.
So this case illustrates an acute ICH where two bleedings occur: The primary bleeding giving the
patient symptoms 12 hours earlier and a re-bleeding not observed clinically. The case is in
accordance with the findings in a case examined within 23 minutes after debut of symptoms
where the center of the hemorrhage was hyperintense on the T2*W EPI (edema or proteinaceous
solution (129)); but also surrounded by a significant rim of signal loss reflecting
deoxyhemoglobin (131). The re-bleeding in the present study could surely have been recognized
as an ischemic area and not a bleeding, due to the hyperintensity and it represents a case of false
negative detection on T2*W EPI. Looking very carefully it is however retrospectively possible to
see a tiny outer rim of signal loss i.e. early stage of transition from oxyhemoglobin to
deoxyhemoglobin. Follow-up MR examination did not found suspicion of tumor or any AV-
malformation, why the primary diagnosis of ICH was confirmed.
3.2.8. Hemorrhagic Transformation
MRI is considered superior for detecting HT in the subacute and chronic phase compared to CT.
In a study (132) it was observed that signal decrease on T2W images in the acute phase were
smaller than would be expected for ICH. This was hypothesized to be caused by reperfusion
leading to increased oxyhemoglobin abolishing the paramagnetic effect from deoxyhemoglobin.
Data on detecting HT using T1W- and EPI-T2*W imaging will be presented in chapter 4 with
further discussion.
78
Finally, the discussion of hemorrhage, detected by MRI, is summarized. Paramagnetic effects
from intra- and extracellular degradation products of hemoglobin were discussed in relation to
hemorrhages. The recent reports on T2*W EPI on high field scanners (1.5T) being superior for
detecting ICH in the hyperacute phase compared to previous MRI, were reviewed. Three cases of
T2*W EPI were presented, questioning susceptibility artifacts mimicking ICH and in one of the
cases the early sensitivity of T2*W imaging to ICH compared to CT. Both are essential topics, for
only performing MRI in pre-thrombolytic evaluation.
79
Chapter 4. Detection of Hemorrhagic Transformation in Stroke. Comparison of EPI-T2* weighted and T1 weighted Magnetic Resonance Imaging 4.1. Introduction
HT defined in section 1.2.7. i.e., HI and IIH is a matter of concern in management of stroke. If
HT is present, the patient should not receive thrombolytic therapy (43). Development of
techniques for detection of HT is therefore of clinical interest. Detection of HT has primarily
been done in CT studies although MRI has been reported more sensitive using conventional MRI
(T1 and T2 weighted MRI) (123), (133), (134). Furthermore, the use of a T2*W FLASH sequence
is thought to detect microbleeds with high sensitivity (135), (136).
Along with EPI, heavily T2*W sequences have been introduced on clinical MR scanners. In
stroke, EPI seems promising for detecting hyperacute ICH due to the high sensitivity to
paramagnetic degradation products from hemoglobin. EPI has reduced the time after which ICH
can be detected by MRI (57), (58), (131), (reviewed in section 3.2.3). In the present study, the aim
was to investigate if T2*W EPI can improve detection of HT in stroke patients at different time
points compared to conventional T1W imaging. T1W imaging is considered a standard method
for detecting HI with MRI (137).
The etiology of HT is not clarified as discussed in section 1.2.7. In the present study, our second
aim was to see if leakage of the BBB was associated to development of HT, on which only few
studies have been reported (138).
4.2. Subject and Methods
A total of 92 MRI examinations in 43 stroke patients, (29 men 14 women, age 66.4∀ 11.3 years
(mean ∀ 1SD), range 42-86 years), were performed. All patients had completed stroke or TIA.
The examinations were performed serially: First examination within 48 hours from onset of
symptoms, (�acute�, n=39, 19.2∀ 11.4 hours (mean ∀ 1SD), range (5-47 hours)). Again within 7
80
days, (�early subacute�, n=15, 4.4∀ 2.1 days (mean ∀ 1SD), range (2-7 days)) and 21 days
respectively, (�late subacute�, n=5, 15.0∀ 4.4 days (mean ∀ 1SD), range (9-21days)). After 2-3
months, (�early chronically�, n=27, 63.4∀ 10.1 days (mean ∀ 1SD), range (52-92 days)) and finally
after 6-7 months, (�late chronically� (n=6)). All patients gave written informed consent for
participating in the study. The study was approved by the Danish Ethics Committee.
The patient data, examination schedule and findings are listed in Table III.
4.2.1. MRI
MR imaging was performed on a 1.5T Siemens Magnetom Vision whole body scanner (Siemens
AG, Erlangen, Germany) capable of EPI. Except for the second T1W imaging and MRA, all
sequences where applied before injection of Gd-DTPA. Our MRI protocol consisted of T1W
imaging, (TR = 510 ms, TE = 14 ms, 16-17 sections of 5 mm, FOV 230 mm, matrix 256 x 256,
voxel size of 4.1 mm3). The sequence was applied before and 5 minutes after injection of Gd-
DTPA, Magnevist® (0.1mmol/kg). T2*W images were acquired using EPI, (TR=0.8ms,
TE=66ms, in 6 sections of 5mm, FOV 230 mm, matrix 128x128, voxel size of 6.7 mm3). T2W
imaging was done using a double spin echo sequence, (TR=2800 ms, TE=80msec, whole brain
5mm sections, FOV 230 mm, matrix 256x256). DW imaging was performed using a se-EPI
sequence, (TR =0.8 ms, TE=101 ms, FOV 230 mm, matrix 128x128) covering the whole brain.
One reference image (b ~ 0 s/mm2) and three diffusion weighted images (b = 1000 s/mm2) in
three orthogonal directions were obtained, whereby the acute ischemic lesion could be identified.
PWI was performed using a multislice T2* (susceptibility) weighted EPI sequence (TR=1sec, TE
=66 msec, FOV 230 mm, matrix 128x128 ), 6 contiguous slices of 5 mm in a series of 128
measurement covering the lesion identified on DWI or T2W images. EPI was performed during
a bolus injection of Gd-DTPA in approximately 5 seconds using a SPECTRIS� power injector,
MEDRAD®. The postprocessing is described in section 6.2.3. Parametric maps of rCBV and
TTP were generated. All MR images covered identical slice positions.
81
MRA was performed using a time of flight 3D sequence, (TR=39 msec, TE=65msec, FOV 300
mm, matrix 256x512, slab thickness 100mm, effective thickness 1.47mm), covering the Circle of
Willis. Reconstruction of the MRA was performed using software supplied by the scanner
manufacturer.
4.2.2. Evaluation of MRI
The MR images and MRA were evaluated by a radiologist blinded to patient data and to clinical
findings.
The EPI images and the pre- and post Gd-DTPA T1W images were evaluated visually, and was
scored as isointense, hypointense or hyperintense, when compared to the contralateral site.
Hypointensity on the T2*W EPI images or hyperintensity on pre contrast T1W images were
interpreted as HT. Differentiation of HI and IIH was defined as stated in the ECASS study
where IIH is defined only when the bleeding is associated with space-occupying effect (43).
Principles for specific signal changes caused by (super) paramagnetic degradation product from
hemoglobin were reviewed in section 3.2.2. Leakage through the BBB was identified visually as
signal increase on T1W images comparing pre-and post contrast studies.
MRA was evaluated for occlusions/stenosis in the proximal branches of the MCA, PCA and
ACA. Pathological findings on MRA were interpreted as relevant or irrelevant to the actual stroke
accident. PWI at the �acute� examination was visually evaluated for reperfusion identified as
shortened TTP and increased CBV.
4.3. Results
We made 92 examinations of 43 patients (table III). In this study, all cases of HT were
interpreted as petechial HT (HI), and none as secondary IIH.
Of the 43 patients HI was identified in 19 (44%) and 11 (26%) had leakage of the BBB.
When pooling all data, HI was seen in 33 (36%) of the 92 examinations and in 16 (17%) the BBB
was leaky (Table IV, V, figure 22):
82
• I. In 27 (29%) of 92 examinations HI was identified only on pre-Gd-DTPA T1W imaging,
T2*W imaging being normal (Table IV, figure 22). In 4 (15%) of these 27 examinations the
BBB was leaky (Table VI, figure 22, figure 23).
• II. None of the examinations showed a pattern with HI detected on T2*W images alone
(Table IV, figure 22), (Table V, figure 23).
• III. HI was identified on both T2*W imaging and pre-Gd-DTPA T1W imaging in 6 (7%) of
the 92 examinations (Table IV, figure 22). In 5 (83%) of the 6 examinations, leakage of the
BBB was seen (Table V, figure 22, figure 24). It was tested if a leaky BBB was more
commonly associated to HI if identified on both T1W- and T2*W images, compared to if HI
was only identified on T1W images. This was found to be significant using Fishers exact test
(p= 0.0028).
• IV. Finally in 59 (64%) of 92 examinations, HI was not identified (Table IV, figure 22).
Leakage of the BBB was seen in 7 (12%) of the 59 examinations without HI (Table V, figure
22).
It was tested if groups with and without HI differed in frequency of leaky BBB. No differences
between groups were found, using Fishers exact test. HI was therefore not associated to leakage
of the BBB when pooling all data.
The examinations are stratified by time from onset of symptoms (Table IV, V and figure 25):
• In 39 �acute� examinations, HI was seen in 4 (10%) and a leaky BBB in 4 (10%).
• In the 15 �early subacute� examinations, HI was identified in 5(33%), and leaky BBB in 4
(27%).
• HI was seen in 2 (40%) of 5 �late subacute� examinations and a leaky BBB in 1 (20%).
• In the 27 �early chronical� examinations, 18 (67%) HI were seen and a leaky BBB was seen in
6 (22%).
83
• Finally HI was identified in 4 (67%) of 6 �late chronical� examinations and a leaky BBB in 1
(17%).
As seen in table IV, V and figure 25 the incidences of BBB leakage and HI were parallel at the
�acute� and �early subacute� examination. At later times, the incidence of HI continued to
increase while the incidence of leaky BBB decreased slightly. No statistical significance using
Fisher�s exact test (p= 0.6159) was found when testing if HI was associated to a leaky BBB in the
�acute� and in the �early subacute� phase.
When evaluating the individual patients serially, it was seen that:
• In 11 (26%) of all the 43 patients, a leaky BBB was found. HI was identified in 9 (82%) of
these 11 patients. In 8 patients, of the 11 with leaky BBB, at least one follow-up examination
was performed. HI developed afterwards at follow up in 3 (38%) of these 8 patients.
• Reperfusion identified by acute PWI or serial MRA was seen in 5 (13%) of the 43 patients of
whom 3 (60%) (of the 5) developed leaky BBB and 4 (80%) developed HI.
Table III. Patient data and examinations
Pt no./
Age/sex
Lesion
Location
Examination
Hours(h), Days(d),
Months(m)
HT at
T2*W
+/!
HT at T1W
+/!
BBB
Leakage
+/!
PWI
Reperfusion
+/!
MRA
Occlusion
+/!
1/74/f Deep WM 19h,4d,15d,58d ! ! ! ! + + + + ! ! ! ! ! ! ! ! !
2/58/m Parietal cortex 9h,9d ! ! ! ! + + ! ! !
3/77/m Watershed 6h ! ! ! ! +
4/81/m Deep WM 20h, 3d, 59d ! ! ! ! ! ! ! ! ! ! ! ! !
5/60/m TIA 18h ! ! !
6/75/f Parietal cortical 65d + + + ! !
7/82/m Pons 5h,52d ! ! ! ! ! ! ! +
8/57/m Occipital 9h,47h,6d,54d ! ! ! ! ! ! ! ! ! ! ! ! !
9/63/m Deep WM 52d + + + ! +
10/57/f Deep WM 7d,61d ! ! + + ! ! ! !
84
11/75/m Watershed 60d ! + + ! +
12/60/m FT 11h,2d ! ! ! ! ! ! + !
13/66/m Occipital 36h ! + ! !
14/49/m Deep WM 20h ! ! ! ! !
15/82/f Watershed 40h,11d ! ! ! ! ! ! ! ! ! ! + + +
16/76/f Occipital 35h, 4d,63d + + + + + + + + + !
17/82/f FT 12h ! ! ! !
18/77/m Deep WM 16h,3d,58d ! ! ! ! ! + ! ! ! !
19/76/m Occipital 6h, 61d ! + ! + ! ! ! + !
20/75/m Deep WM 9h,63d ! ! ! + ! ! ! ! !
21/73/m TIA 4d, 66d ! ! + + ! ! ! ! !
22/76/f Watershed 36h ! ! ! ! +
23/64/f Deep WM 8h,7d,57d ! ! ! + + + ! ! ! ! ! ! !
24/76/m Deep WM 40h,3d,63d ! ! ! ! ! ! ! ! ! ! ! ! !
25/42/m Temporal 25h,7m ! ! ! ! ! ! ! ! !
26/76/m Parietal 23h ! ! ! !
27/56/f Temporal 61d ! ! ! ! +
28/58/m Parietal 5h,43h ! ! ! ! + ! ! + +
29/50/m Deep WM 22h,3d,62d,6m ! ! ! ! ! ! + + ! ! + + ! + !
30/57/f WM 24h,6m ! ! ! ! ! ! ! ! !
31/72/m Parietal 14h,6d,60d,6m ! ! ! ! ! ! + + ! + ! ! ! + ! ! !
32/62/m Occipital 18h,2d,63d ! ! ! ! ! + ! + ! ! ! ! !
33/76/m Parietal 10h,5d,18d,90d ! ! ! ! ! ! + + + + ! ! + ! ! ! !
34/57/f WM 19h,16d,81d ! ! ! ! ! ! ! ! ! ! + + +
35/62/m Parietal 11h,58d ! ! ! ! ! ! + +
36/59/m FT 22h,92d ! ! ! + ! ! ! !
37/56/m Temporal 70d, 6m ! ! + + ! ! ! ! !
38/50/m TIA 20h,62d ! ! ! ! ! ! ! + +
39/73/m DeepWM 24h,2d,57d ! ! ! ! ! ! ! ! ! ! ! ! !
40/46/f TIA 15h ! ! ! ! !
41/70/m Parietal 7h,45h,58d,6m ! ! ! ! ! ! + + ! ! + ! ! + + + +
42/86f/ Parietal 20h ! ! !
43/ 57/f TIA 8h ! ! ! ! +
Abbreviations:(WM)-white matter, (TIA)-transitory ischemic attack, (FT)-fronto-temporal
85
Table IV. Hemorrhagic infarctions (HI)- numbers of examinations
Hemorrhagic Infarct
visible on:
T2*/T1
Numbers
Examinations
Acute
<48hours
Early subacute
<7days
Late Subacute
<21 days
Chronic
2-3 months
Late chronic
6 months
+/+ 6 1 1 0 4 0
+/! 0 0 0 0 0 0
!/+ 27 3 4 2 14 4
!/! 59 35 10 3 9 2
HI(%) 35% 10% 33% 40% 67% 67%
Table V. Blood brain barrier – leakage - number of examinations
Hemorrhagic
Infarction
Visible on:
T2*/T1
BBB leakage Acute
<48h
Early subacute
<7dg
Late subacute
<21dg
Chronic
2months
Late chronic
6months
+ 1 1 3 +/+
6 ! 1
+ +/!
0 !
+ 3 1 !/+
27 ! 3 4 2 11 3
+ 3 3 1 !/!
60 ! 32 7 2 9 2
BBB
leakage(%)
10% 27% 20% 22% 17%
86
4.4. Discussion
HI and IIH are feared events that may follow focal brain ischemia especially seen in relation to
cardio-embolism (139), (140), (16). In particular, although the newly developed thrombolytic
agents may be effective in stroke therapy, they promote HT (43). It is therefore critical to
understand the real incidence of HT after ischemic stroke, and to try to identify the underlying
mechanisms. Also improved sensitivity to HI could narrow or prolong the time window of three
months in which it is recommended not performing thrombolysis after a previous stroke (43).
The differentiation of HT into IIH and HI is important due to the different influence on the
prognosis as reviewed in 1.2.7. (16). As discussed in 1.2.7. HI may be caused by increased
permeability of the capillary bed due to disruptions of the endothelial tight junction. The etiology
of IIH is controversial (139), (7), (141). IIH is probably due to a vascular rupture secondary to
major capillary ischemia with necrosis of the endothelium depending on the degree and duration
of ischemia (18). The dependence of the degree of ischemia explains the predilection of IIH in
the territory of the lenticulostriate arteries characterised by absence of collateral anastomoses.
This was also confirmed from one human study finding that IIH developed in tissue with low
perfusion (62). IIH is thought in some instances to be an aggravation of HI when the petechial
bleeding becomes confluent. This mechanism has been emphasised by several authors (141),
(139), (7). Therefore improved ability to detect new or old HIs may improve prediction of IIH in
relation to thrombolysis. In other instances IIH have been proposed related to vasospasms with
intimal lesioning (139). Studies have reported that IIH occurs significantly earlier than HI peaking
respectively within the first days and first weeks. This indicates differences in pathogenesis, as
now discussed (16).
Fisher and Adams (4) observed in autopsy studies that HT developed only when the embolus
had migrated to the distal arterial branches or if recanalization occurred, (the "migration-
recanalization theory"). Other CT and autopsy studies found that HT developed distal to the
87
occluding embolus and it was hypothesised that HT develops due to reopening of the pial
collaterals in the subacute phase when the vasogenic oedema decreases (17), (142), (18).
Supporting the latter theory of the influence of collateral flow, it was shown in an experimental
study that HT did not occur when sources of collateral flow were blocked simultaneously with
the acute embolization (143). The theory of collateral blood flow being important in the
pathogenesis of transformation was also indicated in our study, in that HI in some cases
developed in spite of persisting occlusion of the MCA identified on MRA. Thus it appears that
persistent collateral flow via the anterior and posterior cerebral arteries is essential for
development of HI after MCA occlusion. The discussion of pathogenesis of HT is most
important in relation to the safety of thrombolytic treatment in ischemic stroke in that the
"migration-recanalization theory" would question the safety of thrombolysis. In the ECASS it
was found that patients treated with rt-PA had more IHH and fewer HIs than those who
received placebo. This gives some impact to the "migration-recanalization" theory when focusing
on the IIH after trombolysis. However, one third of the IIHs occurred, areas distant from the
ischemic areas, within presumably normal brain tissue. Lyden and Zivin (18) reviewed a series of
smaller thrombolytic trials. They concluded that using an early treatment time-window of 60-90
minutes the incidences of HT was remarkably low suggesting that early reperfusion prevents
development of HT.
The occurrence of HI versus IIH could therefore depend on the degree of ischemia, the presence
or absence of reperfusion, the delay in the starting of thrombolysis, and the existence or efficacy
of collateral circulation.
Microbleeds associated to cerebral amyolid angiopathy, seems to be a direct marker of vascular
fragility (144), (136). MR evidence of microbleeds, most likely comparable to HI regarding size,
may therefore be a direct indicator of patients with increased risk of secondary hemorrhage most
relevant when stratifying stroke patients for thrombolytic therapy. This was indicated from the
ECASS study in which it was found that one third of the ICHs developing after rt-PA
administration occurred in brain tissue distant from the cerebral infarct appearing normal on CT
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(56). It was hypothesized that this was associated to cerebral amyolid angiopathy, which is
associated to microbleeds (144), (135) and shown related to the incidence of ICH in prospective
studies (145), (14). Also in a trial of anticoagulants the occurrence of bleedings were thought to
be associated with cerebral amyolid angiopathy. Actually this trial were terminated prematurely
due to the increased occurrence of ICH (146). Increasing the sensitivity of imaging techniques to
microbleeds in a broad sense seems therefore relevant in thrombolytic and anticoagulant trials,
preventing ICH. However, it has been argued that correlation of cerebral amyolid angiopathy and
ICH is overestimated and rather should be understood in relation to infarction (7).
In the present study, the sensitivity of EPI T2*W imaging was compared to T1W imaging for
detection of HI in stroke patients at different times after the stroke. EPI T2*W imaging was by
far less sensitive than T1W imaging: No cases of HT was identified on only EPI T2*W images, and not on
T1W images. Comparisons between the sensitivity of EPI T2*W imaging and of T1W imaging in
detecting HT have not been performed earlier.
In the present study, no statistical significance of a leaky BBB in HI could be found. However,
when HI was identified on both MR modalities this was statistically associated to a leaky BBB.
4.4.1. Sensitivity of EPI T2*WI and T1WI for detecting Hemorrhagic Infarct
In an autopsy study, association between microbleeds and hypointensity on T2*W images were
seen post mortem (135), using a gradient-echo FLASH sequence. In the present study, T2*W
imaging using a gradient-echo EPI sequence was compared to conventional spin echo T1
weighted imaging. Conventional MRI (T1 weighted imaging and T2 weighted imaging) has been
shown to be more sensitive than CT in several studies (123), (133). T1 weighted images is
considered a standard in detection of HT (137). In one study (123) increased signal was seen on
T1 weighted images also when no signal decrease was seen on T2 weighted or T2* weighted
images even at the first day of ischemia. In CT studies incidences of HIs ranges from 15-26%
during the first two weeks and 43% after the first month (16), (18). In one study comparing the
sensitivity of conventional MRI and CT, the frequency of HT was 80% versus 43% respectively,
89
indicating that previous CT studies underestimates the incidence of HT (133). In the present
study, the frequencies of HI at different time points corresponded to the findings in other
studies, especially, the finding of incidence increasing after some weeks (Table IV and figure 25).
However only few patients had MRI at all time-points and caution should be taken in interpreting
the development in HI frequencies over time.
The optimism regarding EPI T2*W imaging based on the high sensitivity for identifying
hyperacute ICH (57), (58), (131) was not confirmed in the present study aimed at detecting HI at
different times after onset of symptoms. One technical explanation for this is the smaller spatial
resolution of EPI having a voxel volume of 6.7 mm3 compared to 4.1 mm3 on the T1W images.
To make pathophysiological conclusions from a comparison of the two sequences, the voxel
volume might have to be of the same size. However, the T2* (susceptibility) effect decreases
when the voxel size is reduced. In the autopsy study mentioned above, non-EPI gradient-echo
T2*W imaging was performed with high spatial resolution (135), (136). In the present study the
TE used in the EPI sequence was optimised to detect small signal changes of 2-5% in effort to
detect even small bleedings. The optimal TE depends however, on the T2* in the actual
pathology. The T2* is governed by a combination of the paramagnetic strength and the amount
of the molecule studied. It is expected that the T2* relaxation is more rapid i.e. is associated with
a larger signal decrease for methemoglobin than for deoxyhemoglobin. In qualitative study such
as the present where signal loss due to paramagnetic compounds is detected, the TE should be as
long as possible with SNR in the normal tissue and susceptibility artefacts setting the upper limit
for TE. When interpreting the results in a pathophysiological context, we know that the
relaxation changes in intracerebral hemorrhages (reviewed in section 3.2): Hyperintensity on T1W
images is seen in the subacute phase after several days to several weeks. This signal increase is
due to the dipole-dipole interaction between water molecules and the degradation products of
hemoglobin, i.e. deoxyhemoglobin and especially methemoglobin. Methemoglobin also results in
shortening of T2* relaxation, leading to a signal decrease on T2*W images, primarily due to
susceptibility variation when methemoglobin is still within the red cells. The effect of
90
susceptibility variation disappears when the cells are lysed and methemoglobin is distributed
evenly within the tissue. The dipole-dipole effect on T2* (T2) remains, but it is known to be
smaller than the corresponding T1 effect which causes signal increase on T1W images. For
explaining the superiority of T1W imaging found in the present study, it could be hypothesised
that, due to a higher oxygenation in petechial HI compared to ICH, the content of
deoxyhemoglobin is relatively low and that degradation by oxygenation to extracellular
methemoglobin is relatively increased. This was indicated in the study mentioned above with
early superiority of T1W imaging in detecting HI (123). Methemoglobin is known to cause
stronger paramagnetic effects than deoxyhemoglobin as shown in Table II section 3.2.2. since
methemoglobin increases the differences between dipole-dipole effects influencing T1 and T2*
(128). This mechanism (relatively reduced transition of deoxyhemoglobin to methemoglobin in
HI) was indicated in two studies were T2 and T2* changes in HI caused smaller signal decrease
than expected from ICH studies (132). It was speculated that reperfusion leading to higher
oxygenation decreases the amount of deoxyhemoglobin in acute HI. Reperfusion, evaluated from
initial PWI and serial MRA, was only seen in 5 patients in the present study and reperfusion
therefore do not seem to explain the findings in the present study. However, in the present study,
the earliest MRA was performed at a time, where reperfusion often already have occurred.
Oxygenation is still most likely to be higher in HI than in ICH (123).
Most studies of HI using T2*W imaging, have been done in the chronic phase where
paramagnetic effects from static inhomogenity due to intracellular accumulation of hemosiderin
and ferritin is the major mechanism (147), (136), (148). In the present study, examinations were
performed at different times ranging from the acute to the late chronic phase and therefore also
at times where dipole-dipole effects (highly detectable on T1W images) were more pronounced
than effects of static inhomogenity (primarily detectable on T2*W images).
91
4.4.2. BBB leakage and HI
Several studies have investigated if clinical or radiological predictors of HT could be suggested
(also reviewed in 1.2.7); but have not been well defined or confirmed. Parameters such as
occlusion mechanism, (4), (16) collateral circulation, (143), (18), reperfusion, (14), use of
anticoagulants, (146), thrombolytics, (149) have been suggested (all discussed above). In a SPECT
study, a perfusion index threshold was identified to predict HT (62). Infarct size has been
emphasized (16), (150). Early hypointensity on CT predicted HT (149), (56), (142), and is a
contraindication if thrombolysis is considered. Ogata et al. (17) found that increased blood
pressure in the acute phase might predict HT. In an animal study using MRI, contrast
enhancement in the infarct (reflecting leakage of the BBB) predicted HT after reperfusion at a
time where all other MR modalities used did not show any signs of bleeding, which also was
confirmed by histology (151). In a prospective study, delayed signal hyperintensity (reflecting
leaky BBB) was seen using CT in seven patients, after injection of contrast agent. In four out of
the seven patients delayed hyperintensity predicted HT. In examinations of patients without
delayed contrast enhancement, HT did not develop on follow-up examination (138). One case
report using T1W MRI predicted HT (152).
In the present study, only half of the total number of examinations with HT had a leaky BBB.
No statistical significance was found between development of HI and leaky BBB when pooling
all data. The percentages of HI and leaky BBB were equal at the �acute� and �early subacute�
examinations (Table IV, V and figure 25). However, no statistical significance indicated
association between HI and a leaky BBB. At later time points, the percentage of HI continued to
increase, because HI does not reverse (Table IV and figure 25). The incidence of BBB leakage
increased within the first week. It then decreased slightly at �late chronic follow-up�, probably
reflecting a transition from neo-vascularization with leaky BBB to tightening of the BBB in the
chronic phase. The reason that HI can be seen without a leaky BBB, contrary to the referred CT
study, can be due to opening and closing of the BBB at time points where patients were not
92
examined. Also the sensitivity of T1W images 5 minutes after administration of Gd-DTPA could
be questioned. The sensitivity to a leaky BBB could be improved by delaying the recording of
post-Gd-DTPA T1W images or applying MR techniques more sensitive to BBB disruption (153).
Also a higher dose of contrast agent could be administered. From studies of multiple sclerosis, a
triple dose of paramagnetic contrast agent should be preferred (154). Also evaluating BBB
disruption visually is difficult, especially in cases where pre-contrast T1W images show areas with
hyperintensity. Therefore MR techniques with an ability of quantifying the disruption of the BBB
should be preferred (153), (155), (156).
In 83% of the examinations where HI was detected on both T2*W images and T1W images, the
BBB was leaky as well, (statistically significant compared to HI only detected on T1W images).
This indicates that a routine MR protocol for stroke and HI including both modalities could be
used to stratify patients with severe disruption of the BBB and these patients probably have an
increased risk of developing IIH after reperfusion therapy or therapy with anticoagulant.
Leakage of the BBB often should precede development of HT although only little evidence exists
in the literature (151),(138). Therefore patophysiological variables predicting leakage of the BBB
i.e. size of infarct, localization, degree of ischemia and time of reperfusion (157), (158) could also
be considered as prediction parameters for HT.
4.4.3. Conclusion
From the present study, it seems that EPI T2*W imaging is far less sensitive to petechial HT i.e.
HI than T1W imaging. HI identified on both MR modalities is speculated to reflect a severely
disrupted BBB identifying a subgroup of patients with a higher risk of secondary hemorrhage.
Further studies analysing the pathophysiological evolution of petechial HI in the light of changes
on T1, T2 and T2* relaxation, and improval of MR techniques in detecting disrupted BBB are
needed to optimize MR strategies for detection of HI and to investigates if BBB leakage predicts
HT.
93
Figure 22. Incidences of Hemorrhagic infarcts (HI) and BBB leakage
Figure 23. Hemorrhagic infarct – T2*W and T1W images (61 days)
Hemorrhagic infarct only visible on T1W images
0
10
20
30
40
50
60
BBB intact BBB leakageI
BBB intact BBB leakageII
BBB intact BBB leakageIII
BBB intact BBB leakageIV
Num
bers
of e
xam
inat
ions
HI only on T1W
HI on T2*Wand T1W
no HI
HI only on T2*W(none)
94
Figure 24. Hemorrhagic transformation and leaky BBB (63 days)
From left to right: T2*W images, pre-contrast T1W images (HI), post-contrast
T1W images (leaky BBB)
Figure 25. Incidences and temporal course
0
10
20
30
40
50
60
70
80
90
Exam
inat
ions
(%)
HI, BBB intactHI, BBB leakageNo HI, BBB intactNo HI BBB leakage
No patients with BBB leakage*
* * *Acute Subacute Late subacute Early chronic Late chronic
95
Chapter 5. Functional Magnetic Resonance Imaging in Acute Stroke In the last decade, advances in functional MR imaging using DWI and PWI have occurred. These
techniques have potential clinical applications. A review of animal and human studies are given
below.
5.1. Animal Studies
DW imaging provides information about disturbances of cellular water homeostasis, which is one
of the first elements of the pathophysiological cascade leading to ischemic injury. In the early
1990s several groups found using experimental stroke models, that hyperintensity on DWI
corresponding to a decreased ADC, occurred very early after ischemia was induced (80), (79),
(105). Findings in animal models on changes in DWI are reviewed.
5.1.1. Mechanism of ADC decrease in ischemia
The hyperacute decrease in ADC in ischemic stroke reflects a cytotoxic edema. However, the
relative contributions of various pathophysiological mechanisms, explaining the decrease in the
ADC, are not known. Various mechanisms have been proposed: a shift of water from the
extracellular compartment to the intracellular compartment, decreased permeability of the
membranes in the cell, and temperature decrease. The different mechanisms responsible for
decreased ADC are discussed below in section 5.1.2.
Initially in ischemia, a cellular or cytotoxic edema develops. A cytotoxic edema is caused by
breakdown of the energy dependent Na+/K+ pump causing a massive influx of Na+ into the
intracellular compartment and efflux of K+ into the extracellular compartment. Anaerobic
glycolysis, causing an accumulation of lactate and other osmotically active products intracellular,
also contributes to the cytotoxic edema (159).
96
• In a study by Benveniste et al. (104) a cytotoxic edema was induced by oubain that is a
substance blocking the Na+/K+ pump leading to cyotoxic edema without ischemia. Oubain
causes an ADC decrease similar to that seen in ischemia. It was hypothesized that decreased
ADC reflects a shift of water from the extracellular compartment with a high ADC to the
intracellular compartment with a low ADC. It was shown that the model could explain a 40%
decrease of the ADC. In an in vitro study, the ADC was measured in a suspension of red
blood cells while varying the extracellular space. The ADC decreased by 45% after reducing
the extracellular volume from a fraction of 0.2 to 0.1, corresponding to values in ischemia
(160). Norris et al. have proposed a model of �apparent restricted diffusion� which takes into
account changes in the extracellular volume and exchange times of water between the intra-
and extracellular compartments (83), (161) (figure 26).
• Several studies have indicated decreased intracellular ADC (162), (163), (164), (165).
Decreased diffusion of intracellular metabolites have been reported during ischemia (163),
(164). We performed measurements of the ADC of the intracellular metabolite, N-acetyl-
aspartate (NAA), a neuronal marker, during 6 hours of ischemia in an animal model of focal
ischemia (165) (see Appendix I). To ensure that NAA was intracellular during ischemia, we
performed microdialyses measuring the extracellular NAA. It was found that the ADC of
water decreased by approximately 30% and the ADC of NAA by approximately 10%. The
extracellular content of NAA, measured by microdialyses, was low through the whole
experiment (less than 2%). A histiological examination was not performed on the sacrified
animals. The study confirms that the intracellular ADC decreases during ischemia, probably
due to increased intracellular viscosity. We also measured the ADC of NAA in 17 stroke
patients. However, the average ADC of NAA was increased, probably relecting development
of vasogenic edema developed at the relatively late inclusion time of 48 hours. When
evaluated only in the patients with decreased ADC of water, the ADC of NAA was decreased
confirming the results from animal studies (166) (see Appendix II).
97
• Changes in membrane permeability have been proposed based on a simulation study by
Helpern et al. (167). However, in another simulation study, the model used by Helpern et al.
was criticized and no influence of membrane permeability was indicated (168). Also in an
animal study on ischemia, the anisotropy did not change during ischemia indicating
unchanged membrane permeability (98). Changes in permeability are not thought to influence
the ADC. However, experimentel data are missing.
• In one study, the temperature was measured to drop by 1-1.5 CΕ in the ischemic tissue (169),
which can not explain a 50% decrease of ADC.
Figure 26. Ischemia ΨΨΨΨ cytotoxic edema ΨΨΨΨ decrease of ADC
For typical MRI the spatial resolution is down to approximately 1mm3. While this is also true for
DWI, it nevertheless reflect motions in the range of Φm. Also the bi-exponentiality of the MR
signal as a function of the b-factor reflects compartmentalization (intra- versus extracellular).
These examples illustrate that MR provides information on structures and dynamics on length
98
scales much smaller than the spatial resolution. The definite lower resolution in MRI is limited by
physiological noise from brain- and head movements, diffusion and T2 effects leading to signal
decrease before the outer k-space regions, defined by the spatial resolution, have been traversed.
5.1.2. ADC changes in focal ischemia
In 1990, Moseley et al. (80), (79) showed that ischemic lesions can be detected as hyperintensity a
few minutes after occlusion in cat brains using DWI. T2W images did not show any changes. The
signal increase on DWI reflects a decline in ADC that can be detected within a few minutes after
onset of ischemia as found in several studies (105). T1 and T2 relaxation time changes occur after
3 hours. The ADC progressively decreases the first 24-48 hours of ischemia to a minimum of 50-
60% of normal tissue values (170). In permanent focal ischemia, the ADC will return normal
values after 24-48 hours. This is known as pseudo-normalization with histopathologic correlation
to vasogenic edema with eosinophilic neurons and beginning of necrosis (170). At the time of
pseudo-normalization, a signal increase will be present on T2W images allowing distinction of
tissues with normal ADC. In the chronic phase, ADC will increase reflecting disintegration of
cellular membranes and necrosis in the transition from edema to cystic formation (170), (171)
(figure 27). In the study by Welch et al. (170) it was hypothesized that the combination of ADC
and T2W imaging can be used for predicting if the tissue will transform into infarct or not, as
summarized in figure 27 and figure 28.
Figure 27. ADC and signal on T2WI – time course during ischemia (from (170))
99
5.1.3. Evolution of the Ischemic Volume in Focal Ischemia
The evolution of the infarct volume evaluated using DWI varies: In one study, the ischemic
volume reached the final infarct size within 2 hours. After 7 hours, the volumes with changes in
T1 and T2 relaxation became congruent with the initially larger volume with decreased ADC
(172). In other animal models, the final infarct size on DWI was reached within 7 respectively 24
hours (27).
5.1.4. Correlation of ADC to Perfusion
A perfusion threshold correlating to changes in ADC in ischemia has not been established.
However, in one study of global ischemia (173), it was found that DWI changes appeared at
ischemic levels corresponding to perfusion thresholds for production of lactate (CBF
<30ml/100g/min). A sharp signal increase was seen on DWI at CBF levels at 15-20
ml/100g/min corresponding to the perfusion level of cytotoxic edema. The study indicates that
DWI changes can be seen at CBF levels above thresholds for infarction.
5.1.5. Correlation of ADC to Metabolism
In an animal study of permanent focal ischemia (174), the outer rim of the DWI changes
corresponded to a 10% reduction of the ADC. In this zone, lactate was increased but the ATP
Figure 28MR Measures With Tissue SignatureAttribution and Their Significance
ADCw T2 Prediction/Mark Signature (Refers to fig 27)
Normal Normal Normal ALow Normal ? Predicts recovery, ? Predicts necrosis BLow High ? Predicts necrosis CNormal High Transition to necrosis DHigh High Cell necrosis EHigh Normal Cell necrosis F
100
level stayed within the normal range. The perfusion corresponded to mild ischemia in this area.
The center of the region with hyperintense DWI had a 23 % reduction of ADC. In this region,
ATP was severely decreased and lactate was increased. The perfusion corresponded to severe
ischemia. After seven hours, the volume of decreased ATP had enlarged into the total volume
with initial DWI changes corresponding to the final infarct in the histological examination. It was
hypothesized that the area with initial DWI changes (decreased ADC), with lactate and with a
normal level of ATP, may represent the penumbra. Decreased ADCs have been seen in areas
with only slightly impaired metabolism, which are potentially reversible from ischemia (e.g.
increased lactate but normal or slightly decreased ATP) (80), (175). Transiently decreased ADCs
have been found in peri-infarct tissues identified as spreading depressions (SD) in several animal
studies (176), (177). These studies also show decreased ADC in tissues that do not become
infarcted.
5.1.6. Reperfusion Studies
In reperfusion studies, it has been shown that DWI changes can be reversed in focal ischemia
within 33-60min (178). In several studies, reperfusion prevented further enlargement of the
infarction (27). No ADC threshold usable for predicting infarction has been established. Even
maximal ADC decreases of short duration, can be reversed by reperfusion (179). It seems that
reversibility of ADC decrease, and thereby prevention of infarction, depends both on the degree
and the duration of the ADC decrease (180).
In conclusion, it appears from animal studies that all or part of the lesion on DWI is potentially
reversible. It also seems that infarction can be predicted from a combination of depth and
duration of the ADC decrease, and that penumbral tissue can be identified, by mapping of
perfusion and biochemistry.
101
5.2. Human Studies of DWI and PWI
The therapeutic window for thrombolytic therapy in acute ischemic stroke is generally believed to
be quite narrow, at maximum 6 hours (37), although it is likely that the tissue viability depends on
the collateral circulation and metabolic status, which can be heterogenic inter-individually even in
the same vascular territory (51), (181). Based on PET studies, it has been suggested that up to 17
or even up to 48 hours after onset of symptoms, ischemic tissue is present in which CBF, OEF
and CMRO2 are in the penumbral range (discussed in section 1.3 and 1.8) (182), (183), (184). In a
SPECT study, the therapeutic window varied between individuals up to 12 hours (185).
The advance in neuroimaging by DWI and PWI has proven valuable, also for clinical assessment.
These techniques can provide information on the ischemic tissue viability, also at the earliest
stages.
5.2.1. Diffusion Weighted Imaging
DWI detects decreased water movement in acute ischemic tissue due to failure of the Na+/K+-
ATP pump, as discussed in section 5.1. In the patients suffering from ischemic stroke, changes
have been seen within 39 minutes (186). The first studies in humans were performed in 1992 by
Warach et al. (187) confirming the findings from animal studies, that hyperintensity on DWI,
reflecting decreased ADC, could be identified in ischemic stroke patients at the earliest stages
(figure 29). DWI has been reported to have high sensitivity and specificity for assessment of
human stroke: In one study of 24 patients studied within 6 hours after onset, sensitivity and
specificity were both 100% (188). In another series of 122 patients examined within 48 hours,
sensitivity and specificity were 81% and 100% respectively (27). In a recent study (189), 782
patients with suspected stroke were examined using DWI. In 27 patients, DWI was normal
despite of stroke-like deficits. Ten of the 27 patients had other conditions than ischemia
including migraine, seizures functional disorders transient global amnesia and brain tumor. The
remaining 17 patients without changes on DWI were considered to have had a cerebral ischemic
event: TIA, RIND or prolonged infarct. In 6 of the 17 patients, an infarct showed up at a follow-
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up examination. In the 2 patients with prolonged stroke, initial PWI showed decreased perfusion.
This study demonstrates a very high sensitivity of DWI for ischemic stroke (755 of 772 stroke
patients had changes on DWI). Also it was concluded that normal DWI of patients with in
stroke-like deficits should stimulate a search for other etiologies. False negative DWI in
hyperacute stroke was also reported in a previous study (190).
The time course of the ADC in humans is controversial, since different groups have different
findings. This may be due to the use of different and possibly insufficient techniques. Ulug et al.
(102) therefore concludes that the trace of the diffusion tensor (as discussed in section 2.6.8)
should be measured. It seems that the ADC decreases for about 96 hours to a minimum of
approximately 50% of normal values (27). The ADC tends to normalize after one week (pseudo-
normalization) and will be increased above the normal level in the chronic phase (191). As a
consequence, DWI and ADC maps can, opposed to T2W images be used to, differentiate
between new and older infarcts, having opposite signal changes in the acute and in the chronic
phase. However, at the time of pseudo-normalization, DWI should always be interpreted along
with a T2W image.
An important question; is if a DWI signal increase (ADC decrease) is indicative of irreversibly
injured tissue. In animal studies (as reviewed in section 5.1.) ADC reversibility has been
demonstrated in several reperfusion studies. Decreased ADCs have also been demonstrated at
perfusion levels where the tissue is viable (CBF<30ml/100g/min) (173). In human studies, only
few cases have until now been reported where DWI signal changes (decreased ADC) have
reversed (192), (193). DWI changes have therefore been thought to reflect irreversibly injured
tissue. This assumption is not valid however: In one recent study of TIA, it was seen that DWI
signal changes (decreased ADC) were reversed at a follow-up examination in those patients
where the duration of the symptoms was shortest (194). This indicates that DWI changes have
the potential to depict penumbral tissue. Also recently it was reported that four patients receiving
thrombolytic therapy within 6 hours from symptom onset had significant reduction in DWI
lesion size. Therefore, at least within the first hours, DWI changes can not be interpreted as
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irreversible and can not be considered as a predictor of infarction. Future studies should
characterize DWI changes in the acute phase to differentiate between reversible and irreversible
changes. Attempts at characterizing the heterogeneity of ADC and T2 changes in ischemic
lesions identified on DWI have been done in previous studies (170), (195).
Figure 29. Human ischemic stroke
5.2.2. Case study of ADC and perfusion changes in human ischemia
A 52 years old man had several TIAs from the right hemisphere within a couple of months. A
perfusion SPECT measurement using Xenon as a tracer was followed by a MR examination
(figure 30). At both examinations, the patient did not have any symptoms. The SPECT perfusion
measurement showed ischemia in the right hemisphere. The perfusion in the ischemic
hemisphere was as low as 18 ml/100g/min (∀ 2ml/100g/min (SD)) to 33ml/100g/min (∀
8ml/100g/min (SD)) as seen in figure 30. Although the perfusion range also includes values of
�moderate ischemia� (10-20 ml /100g/minute), being in the range of penumbral tissue, in
humans as found in PET studies and illustrated in figure 3 in section 1.3. no changes were seen
on DWI. No statistically significantly ADC changes were found in the ischemic hemisphere (in
regions with decreased perfusion), when compared to the non-ischemic hemisphere. This
T2W DWI ADC
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indicates that changes in ADC in humans do not necessarily happen at perfusion values
corresponding to �moderate ischemia� (penumbral range). This is in contrast to what has been
found in animal studies where changes have been found in �mild ischemia� (20-35
ml/100g/minute), reviewed above in section 5.1. The case indicates the possibly different
thresholds of ischemia and ADC changes between humans and animal models.
Figure 30. Perfusion thresholds of ADC changes
From left to right: normal DWI and low perfusion (xenon-SPECT) 18 ml/100g/min (∀ 2ml/100g/min (SD)) to 33ml/100g/min
(∀ 8ml/100g/min (SD)). Patient with ICA-stenosis
5.2.3. Perfusion weighted imaging
PWI provides information on reduction of blood flow, reflecting the primary cause underlying
the pathophysiology of acute ischemia. As reviewed in section 2.7. PWI by DSC-MRI is primarily
used in calculation of parameters derived from the signal curve. MTT-, TTP-, AUC- and BAT-
maps are constructed.
20 ml/ 100g/ min
25 ml/ 100g/ min
33 ml/ 100g/ min
25 ml/ 100g/ min
18 ml/ 100g/ min
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5.2.4. Combined DWI/PWI studies
The combination of DWI and PWI has been shown to provide useful information for
management of acute ischemic stroke. The volumes of ischemic tissue in the acute phase,
measured before 24 hours or even before 6.5 hours after onset of symptoms, using both DWI
and PWI are reported to be highly correlated with clinical severity and clinical outcome on
different stroke scales (190), (196), (197). DWI and PWI can consequently play a prognostic role
in acute ischemic stroke, valuable for the management of acute stroke patients.
A mismatch between DWI and PWI in acute examinations has been reported in several studies.
Sorensen et al. (192) found that the volume of prolonged MTT was larger than the volume with
changes in CBV and DWI in the patients examined within 10 hours. In another study (198),
patients studied serially in the acute phase had enlargement of the lesion, when comparing
changes on the acute DWI and follow-up T2WI, up to 53 hours after onset of symptoms. In
cases where PWI was acquired, enlargement only occurred when initial PWI depicted an area
larger than on DWI. When abnormal areas on PWI and DWI were of the same size, enlargement
did not occur. It was hypothesized that the initial PWI/DWI mismatch could represent
penumbral tissue (figure 31). In one preliminary study (27), four different patterns of DWI/PWI
volumes seen within 24 hours from onset of symptoms, were identified and interpreted: Type I:
PWI>DWI (70%), type II: PWI=DWI (10%), type III: PWI<DWI (10%) and finally (type IV)
normal PWI and abnormal DWI (10%). This distribution of PWI/DWI patterns has been
confirmed in several studies of acute ischemic stroke (196), (190), (197), (199). It was suggested
that type III and IV reflect partial or complete reperfusion (figure 32). Enlargement of lesions
were only seen in type I, further confirming the hypothesis, that PWI >DWI mismatch
represents penumbral tissue (figure 31). In patients having early resolution of the PWI lesion (for
instance after thrombolysis) the DWI lesion usually do not enlarge, enlarge less than in a group
with no reperfusion (200), or is even diminished (201). These patients often experience early
clinical improvement (202), (200), (201). Therefore patients with PWI>DWI may respond
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favorable to thrombolytic therapy. In a recent study (203) 35 patient suffering from acute stroke
were examined within 90-690 minutes using PWI, DWI and MRA. In 21/35 patients, a
PWI>DWI mismatch were identified, all with occlusion of the MCA identified on the MRA. The
14/35 patients without mismatch did not have major vessel occlusion. 11/21 patients received
thrombolytic therapy. In 8/21 patients with a mismatch, follow-up MRA showed reperfusion. In
the other 13/21 the vessel remained occluded. Follow-up MRI showed significant smaller
infarcts at day 2 and day 5 in the recanalised group than in the non-recanalised group. Clinically,
the reperfused group had better clinical outcome after 30 days.
Figure 31. PWI>DWI mismatch. “Stoke in progression”. Enlargement at
follow-up
Figure 32. Reperfusion. TTP shortened and CBV increased in left MCA-supply territory
DWI TTP rCBV
acute
DWI
CBV
TTP
Acute
Follow-up
DWI
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5.2.5. Case study of “stroke in progression”
Figure 31 illustrates a case first examined 40 hours after onset of symptoms. The patient had mild
non-fluid (expressive) aphasia, which, however, alternated with fluid (global) aphasia. No paresis
was seen at this time. The symptoms worsened in the following days. At follow-up examination
11 days after onset, the aphasia was now permanently fluid (global) and a right hemiparesis had
evolved.
Acute DW imaging showed a very little hyperintense area in the left fronto-temporal cortical area
corresponding to Brocas' area of speech. PWI was also performed and TTP- and CBV maps
were constructed. TTP was prolonged in the whole left hemisphere corresponding to stenosis of
the ICA, demonstrated in a Doppler examination. It is seen that TTP is most prolonged in the
border zones (red color), including the fronto-temporal area (Brocas' area of speech), the parietal-
occipital area (Werniches' area of speech) and the area of the left internal capsular. The CBV is
increased in the left hemisphere, probably due to compensatory vasodilatation induced by a
decreased CPP.
This case illustrates that the initial DW images only showed very sparse changes although the
hyperintense area corresponded to the symptoms. However, the fluctuation of the aphasia in the
acute phase indicating involvement of both Brocas' and Werniches' speech areas, are better
illustrated by the acute TTP map with severe prolongation in these areas. Also acute TTP depicts
the half-moon shaped area corresponding to the evolving symptoms and infarct at follow-up
examination. The case clearly illustrates that the combination of DWI and PWI is valuable and
can visualize tissue at risk - penumbral tissue. On DW images the ischemia was almost invisible
but it was depicted on PWI. The present cases also indicate that the combination of DWI and
PWI may be valuable in selection of patients in risk of getting a worsening of the symptoms, so
called �stroke in progression�. This is a very common phenomenon, and has been shown to
occur in every third of all stroke patients (38). The causes are not clear and many theories about
the underlying patophysiology exits. Most likely, patients with stroke in progression are not a
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homogeneous population but have different combinations of underlying causes, which have to
be differentiated and dealt with separately (38). The current treatment is anti-coagulative after
having excluded a hemorrhage. This might be a rational treatment for some of the patients, but
not necessarily for all of them. The CPP should also be considered, especially for patients with
ischemia in the border zone areas where the CPP is known to play a role in the genesis. It could
be speculated that the present patient would have benefited from increasing or stabilizing the
blood pressure. Actually the patient got worse at nights where the blood pressure is lowered
during sleep. In other patients suffering from �stroke in progression� we did not see a
�hemodynamic� pattern in DWI and PWI indicating alternative mechanisms (edema,
hemorrhages). It therefore seems that DWI and PWI can give important information on the
mechanisms of �stroke in progression�.
5.2.6. Combined DWI/PWI/MRA studies
In a study of acute ischemia by Rordorf et al. (199) in which PWI, DWI and MRA were
performed. It was possible to classify the cases into two groups: Group 1, patients with MCA
stem occlusion had a mismatch of PWI>DWI with infarct developing at follow-up into the
region of initial abnormal PWI. Group 2, the MRA was normal and PWI=DWI with no
enlargement of the injured tissue at the follow-up examination. Also it was found that patients
with occlusion of the MRA had larger regional CBF abnormalities and larger final stroke sizes
than patients without occlusion on the MRA. This classification may be valuable in selecting
patients who could benefit from thrombolytic or neuroprotective therapy. The finding has been
confirmed in another study (204) (figure 33). However, (199), (204) it was found that 30-35% of
patients with MCA occlusion did not have lesion enlargement. This is probably due to the
adequacy of the collateral circulation, which can not be determined by MRA only.
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Figure 33. PWI>DWI mismatch corresponding to stenosis on left ICA/MCA
5.2.7. Predictive value of DWI and PWI
In the study of Rordorf et al. (199) it was shown that the volumes depicted on CBV maps were
larger than the volumes depicted on DWI in the patients examined within 12 hours from onset
of symptoms. It was also shown that CBV better predicted the final infarct on follow-up
examination. In studies (197), (205) it has been found that initial CBF, MTT or TTP
overestimates the final infarct even when evaluated at the follow-up examination (197). This is
not surprising, because MTT/TTP depicts altered hemodynamics in tissue that may receive
sufficient collateral flow from more circuitous routes. Rother et al. (116) suggested that it is
possible to use CBV maps to differentiate between degrees of ischemia. In one recent study
(206), maps of CBV, MTT, CBF and DWI were evaluated simultaneously in patients with onset
of symptoms within 12 hours, to see if additional information could be obtained regarding the
transition to infarction on follow-up examinations. It was found that the lesion size on CBV map
and DWI correlated best with the final infarct size compared to CBF and MTT maps. It was seen
that especially acute MTT and also acute CBF overestimated final infarct since it depicts also
oligemic and mild ischemia not in any critical range of ischemia. Another important finding was
that CBV in some cases was increased probably reflecting a compensatory hemodynamic
TTP
rCBV
DWI
MRA
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response, which may delay transition to infarction. It was concluded that no predictor is
sufficient to determine final infarct size alone. It was hypothesized that a combination of
different maps into a tissue characterization model could better predict what tissue is salvageable.
Further studies should be made in metabolic characterization of the mismatch regions for
instance using fast spectroscopic imaging methods generating lactate maps or as stated in a recent
study (207) to correlate perfusion parameters to oxygen metabolism.
In my experience perfusion deficits are easy to identify and delineate with TTP maps, although
this parameter as discussed in section 2.7.6. is only an indirect measure of tissue perfusion,
probable due to the uniform TTP in gray and white matter. In chapter 6 a study of prediction of
infarction from initial findings on DWI and TTP- and CBV maps is presented.
5.2.8. Quantitative approaches of DWI and PWI
Several studies have measured absolute CBF and CBV in healthy subjects with determination of
the arterial input function as described in section 2.7. (114), (115). However, in stroke studies
only volumes and not values have been reported (206), (208). In stroke patients, delay of the
bolus and dispersion may occur in the supplying arteries, leading to an underestimation of CBF
of about 30%. Technical improvements have been reported but they are not validated using
clinical data yet (209). At the moment, PWI should be regarded as a semi-quantitative technique
when applied in ischemia.
In a recent study of ischemic stroke within 24 hours (205), TTP-maps were evaluated semi-
quantitatively by constructing maps with TTP delays of 2, 4, 6, 8 and 10 seconds respectively.
The volumes of the different TTP delay maps were compared to the acute DWI volume, the final
infarct volume and the clinical score. It was found that only when the TTP delay exceeded 6
seconds, lesion enlargement was seen. The volume of regions with TTP delay larger than 4
seconds was correlated to the acute clinical score, indicating that 4 seconds might be the
threshold for functional impairment of brain tissue (penumbral tissue). In one study, a semi-
quantitative CBF index (CBV/MTT) was assessed. Boundaries, that defined the lowest CBF-
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index of non-infarcted penumbra and CBF-index of infarcted tissue, were 0.58 and 0.38
respectively (210). In an another study of acute stroke within 24 hours, the acute DWI-PWI
mismatch zone with transition to infarct on follow-up examination was defined as the penumbra.
Relative values were found from maps of CBF and CBV may allow identification of penumbral
tissue (211).
5.2.9. Conclusion
In conclusion, there are indications that DWI/PWI in acute human stroke indicate that
DWI/PWI may potentially yield information that can be used to predict tissue viability and
facilitate the clinical decision of which patients should receive thrombolytic therapy. It seems that
PWI>DWI mismatch identifies a clinically useful MR pattern indicating potential tissue viability.
Future approaches should aim at characterizing the actual ischemic core and penumbra maybe by
identifying a combination of time-based ADC and perfusion thresholds (180). However, the
recent observations of early ADC reversal in human stroke after thrombolysis, indicating the
presence of penumbral tissue, needs further investigation. In relation to thrombolysis this means
that tissue with "match" between PWI and DWI (DWI=PWI) lesions could reverse from
ischemia if reperfusion (thrombolysis) is established in time.
5.2.10. Vasogenic oedema
In most studies of acute stroke, follow-up T2W images are acquired within the first week, where
vasogenic edema will be at a maximum leading to overestimation of the infarcted volume. We
measured the water content serially in stroke patients (212) and used MR-spectroscopy to
estimate brain water content during the course of cerebral infarction (213) (see Appendix III).
Measurements were performed serially in the acute, subacute, and chronic phase of infarction.
Fourteen patients with acute cerebral infarction were examined as well as 9 healthy controls. CBF
SPECT-scanning using 99mTc-HMPAO as a flow tracer was performed as well. The mean water
content (SD) in the infarct area was 37.7 (5.1); 41.8 (4.8), 35.2 (5.4), and 39.3 (5.1) mol x [kg wet
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weight]-1 at 0-3, 4-7, 8-21, and >180 days after stroke, respectively. The water content increased
between day 0-3 and day 4-7 (p = 0.034) and decreased from day 0-3 to day 8-21 (p = 0.028).
The water content at day 4-7 was significantly higher than in controls (p < 0.05). At the same
time intervals, mean CBF (SD) was 76 (23), 94 (31), 106 (35), and 64 (26)%, respectively. There
was a significant increase in CBF from day 0-3 to day 4-7 (p = 0.050) and from day 0-3 to day 8-
21 (p = 0.028). No correlation between CBF and water content was found. The water content in
ischemic brain tissue increased significantly between day 4-7 after stroke. This study of the water
content at different times after onset of symptoms, illustrates that the infarct volume may differ
due to changes in water content depending on the timing of the follow-up examination as shown
in figure 34.
Figure 34. From the left to the right are seen acute TTP (A), CBV (B) and DWI (C) at 5 hours after onset
of symptoms. Two follow-up studies acquired after 2 days (D) and after 2 months (E) respectively, identifying
different final infarct volume, due to presence of vasogenic edema in the earlier examination
A B C
D E
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The case shown in figure 34 illustrates a major discrepancy depending on the time at which the
final infarct is evaluated: The conclusion after 2 days would be that the acute TTP map at 5 hours
give the final infarct size. After 2 months, it seems that the initial CBV was a better predictor of
final infarct size.
The ideal time for follow-up examination is difficult to estimate, due to vasogenic edema in the
subacute phase and due to shrinkage of the infarct in the chronic phase. When evaluating
enlargements of the affected areas on the DW images serially within the first days, the vasogenic
edema will tend to cause an overestimation of the lesion due to a T2 effect - the so-called �T2-
shine through� effect (214). Also old infarcts could be misinterpreted as new infarcts. ADC maps
do not have this problem. ADC maps should therefore be evaluated to insure that ADC is
decreased as a marker of a recent (<10days) ischemic insult.
5.2.11. Clinical assessment of DWI and PWI in acute stroke
DWI provides the earliest clinically practical method for detecting acute ischemic stroke lesions
directly. The main potential of DWI and PWI are within the first 24-48 hours where ischemic
injury is most likely to develop. DWI is becoming increasingly available and has become a part of
the clinical routine approved by the American Food and Drug Administration for investigation of
stroke (27).
A multi-modal MR protocol could be used to investigate if an arterial occlusion is still present
and if reperfusion has occurred, to measure the severity of perfusion deficits (PWI), to identify
viable and non-viable tissue (DWI/PWI) and finally to detect ICH by T2*W EPI. Incorporation
of EPI, DWI and PWI into an acute stroke protocol that includes rapid MRA, T1W-, T2W-,
PDW-, T2*W- and fluid-attenuated inversion recovery (FLAIR) sequences can take down to 20
to 25 minutes to perform. Using only EPI DWI and PWI, the protocol can be performed within
5-10 minutes (27).
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The issues that must be clarified regarding the utility of DWI and PWI in acute stroke, especially
in pre-thrombolytic evaluation, are: a) improvement of the time required for performing an MR
examination, b) determination of the sensitivity and specificity of DWI for detecting ischemic
lesions, c) the accuracy of the detection of ICH by T2*W imaging, d) determination of the clinical
value of DWI and PWI for detecting salvageable tissue, e) development of software for fast
evaluation of PWI (27).
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Chapter 6. Tissues Patterns Characterized with Perfusion Weighted Imaging and Diffusion Weighted Imaging in Acute Stroke 6.1. Introduction
Major efforts are dedicated to trials in which pharmacological compounds are investigated in
acute human ischemic stroke. Until now, only i.v. thrombolysis with rt-PA effectuated within 3
hours after the onset of symptoms, has shown clinical effect in subgroups of stroke patients (41).
Trials investigating different kinds of neuroprotective agents in acute human stroke have so far
been negative although positive results were found in animal studies (2).
The criterion for inclusion in most stroke studies concerns the time from onset of symptoms,
although the time window for therapeutic intervention in acute human stroke has not been
established. The variation in human stroke regarding location, collateral flow, reperfusion, degree
of flow reduction and metabolic status implies that the presence of potentially salvageable
ischemic brain tissue is a highly individualized phenomenon, which should be considered before
including stroke patients in clinical pharmacological trials (46).
Newer MR technologies such as DWI and PWI can visualize pathophysiological changes at a
very early stage in experimental cerebral ischemia and human ischemic stroke (163), (186), (168).
The ADC measured by DWI is measurably lower in areas of ischemia compared to non-ischemic
areas, thought to reflect a cellular edema (cytotoxic edema). These changes can be detected after a
few minutes of ischemia in animal models (74), (75) and within the first hour in human ischemic
stroke (163), (187). In animal studies, decreased ADC normalizes after early reperfusion
(discussed further in 5.1.4) (154). In human studies, however, only a few cases with reversible
ischemia have been identified on DWI (168), (169). It has been suggested that normal DWI in
the acute phase might be used for predicting clinical recovery (24). This implies that DWI at an
early stage after onset of symptoms can be used for predicting infarction in human stroke.
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On T2W images, hyperintensity can be seen after six hours reflecting a vasogenic edema. These
changes are considered irreversible and a sign of infarction (114).
PWI obtained by bolus injection of the paramagnetic MR contrast agent, Gd-DPTA, delineates
hypoperfused areas in animal ischemic models as in human ischemic stroke (168), (172).
In studies of hyperacute human ischemic stroke, PWI in most cases depicted an area larger than
the hyperintense region seen on DWI (168), (172). It was also seen on follow-up MR or CT
examinations that abnormalities had spread into the initial hypoperfused area. Examination with
DWI and PWI in the acute phase of ischemic stroke may therefore identify regions with
salvageable brain tissue (168), (173).
When a bolus of a paramagnetic substance is given, i.e. Gd-DTPA, the area under the dephasing-
rate (∆R2*), versus time curve, the �area under curve� (AUC), correlates to the cerebral blood
volume (CBV) as reviewed in section 2.7.
The time to the ∆R2*-peak, TTP, gives information of the bolus peak arrival time in every pixel
and may thus be related to the degree of arterial occlusion or the presence of collateral perfusion.
In ischemic tissue, changes in CBV and TTP are expected (figure 35).
In the present study, PWI and DWI were performed serially on patients with clinical acute stroke.
When evaluating PWI, maps of TTP and CBV were constructed. We investigated whether if the
combination of CBV maps, TTP maps and DWI in the acute phase could be used for predicting
the region of infarcted tissue on MRI at follow-up examinations.
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Figure 35. Dynamic susceptibility contrast MRI. Signal-time curve in (A) ischemic tissue showing
decreased CBV and prolonged TTP and (B) contralateral tissue. (ROI is drawn on a TTP map)
6.2. Subjects and Methods
Nine patients (7 men and two vomen, median age 61 years, range 42 to 76 years) with acute
stroke had an MR-examination within 24 hours after the onset of symptoms, in the following
called �acute�, within 2-6 days (n=8) in the following called �early subacute�, within three weeks
(n=6) in the following called �late subacute�, within 2 months (n=7) in the following called �early
chronic� and finally an examination after 6 months (n=5) in the following called �late chronic�.
The inclusion criterion was sudden onset of neurological deficits from the supply area of the
MCA. Exclusion criteria was evidence of intracerebral hemorrhage, previous stroke or cerebral
tumor. Patients participating in other clinical trials were not included. The study was approved by
the local Ethics Committee. Informed consent was obtained in all cases.
Patient data are seen in Table VI.
6.2.1. Magnetic Resonance Imaging Protocol
All studies were performed on a 1.5 Tesla Siemens Magnetom Vision (Siemens Medical Systems,
Erlangen, Germany) whole body scanner, capable of EPI.
Head movements were minimized by using a vacuum pillow. Studies were registered so that
identical slice positions were used for all MR modalities. The entire MR protocol lasted about 60
B
A A B
118
minutes and included a double-spin-echo sequence (TR 2500 msec, TE 20, 80 msec, matrix 256x
256, FOV 300 mm, twenty-one 5 mm sections). A screening was performed using one of the
DWI sequences covering the whole head and lasting about 10 minutes (further description of
the DWI and postprocessing is given below). T1W images were acquired before and after bolus
tracking of Gd-DTPA (TR 510ms, TE 14ms, matrix 256x256, FOV 300 mm, seventeen 5 mm
slices) in order to differentiate between HT and injured BBB. The latter is only visible only on
T1W images after Gd-DTPA enhancement. To obtain PWI, a bolus injection of Gd-DTPA
(Magnevist®; Schering AG Farma, Germany), was given while acquiring susceptibility-weighted
EPI (further description of the sequence and postprocessing is given below). Finally, MRA was
performed using a time of flight 3D sequence (TR 39 msec, TE 65msec, FOV 300, matrix
256x512, slab thickness 100mm, effective thickness 1.47mm) covering the circle of Willis.
Reconstruction of the MRA was performed using software supplied by the manufacturer.
The MR protocol was repeated at every MR-examination. Identical positioning was ensured by
careful comparison with images from the first MR examination. The patients continued in the
study for at least two follow-up MR examinations.
6.2.2. Diffusion Weighted Imaging
The DWI sequence used, was a spin-echo diffusion-sensitive pulse sequence (TR~ 2000msec,
RR x 2, TE=95 msec, FOV 300 mm, matrix 228 x 256, five axial 5mm slices). Motion artifacts
were minimized by using velocity-compensating bipolar diffusion-encoded gradients and cardiac
triggering. Postprocessing with phase correction and navigator echoes were used (82) (principals
and drawbacks described in section 2.4.4-2.4.6). We acquired diffusion-encoding images in three
orthogonal directions (b = 275 x 10-6 s/m2 ) and one non encoded set of images (b=0). The
duration of the single gradient pulse was δ= 17 ms. The distance between the leading edges of
the bipolar gradient pulse was ∆= 18 ms, with an amplitude of the diffusion-encoding gradients
of 23mT/m. The DW images were evaluated after phase correction and were only evaluated
further, if motion artifacts did not affect the image quality, i.e. if the visibility of ischemic areas
119
and anatomical details were satisfactory. The DW images for the three orthogonal directions were
added for drawing of regions of interest. ADCtrace maps were calculated and a decreased ADC
ratio (between the suspected ischemic area and the contralateral area) together with an visually
inspection of the ADC map was used to select the acute ischemic area on the added DW images.
6.2.3. Perfusion Weighted Imaging
PWI was performed using a multislice susceptibility-weighted EPI sequence ( TR=1sec, TE=62
msec, FOV=300 mm, matrix 64 x 128, five contiguous slices of 5 mm in a series of 128
measurements). Gd-DTPA was given manually via a catheter placed in the antecubital vein as a
bolus injection of 0.1 mmol/kilogram bodyweight. The injection was given in approximately 5
seconds after the first 15 measurements. The signal intensity during the bolus passage from a
region of interest was displayed in a MATLAB display program to evaluate the quality of the
study with respect to patient movements before further postprocessing. To prevent trasient
effects on the initial signal change caused by to saturation effects, the first five measurements
were excluded. ∆R2* was calculated pixel by pixel in every measurement assuming an exponential
relationship between the signal intensity change and ∆R2* according to the formula:
S0 is the mean of the first 15 baseline images and S(t) is the signal at time t. A gammavariate
function was fitted to the ∆R2* curve at every pixel thereby minimizing of recirculation effects
and ensuring return to the baseline (yielding several different parametric maps). According to
kinetic principles for intravascular tracers, the area under the ∆R2* versus time curve (AUC) is
proportional to the CBV, assuming ∆R2* is proportional to the concentration of Gd-DTPA in
the vessels (see section 2.7.4). Maps of CBV and time to maximum ∆R2 *, i.e. time to peak
(TTP), were constructed. When post Gd-DTPA T1W images showed signal increase as a sign of
( ) ( )( ) ( ) ( )002 TEStStR −−= /lnln*∆ [ ]1.6
120
a leaky BBB, the assumptions for intravascular tracers where not valid and the PWI scan were
not further evaluated (see section 2.7.2 for further discussion).
6.2.4. Regions of Interest
Regions of interest were identified visually on a DW image (calculated by addition of the three
orthogonal DW images) as hyperintense regions, and visually on CBV- and TTP maps as
differences between ipsi- and contralateral regions. The volumes were calculated.
The ratio of CBV (CBV-index) and differences of TTP (∆TTP) between the ischemic and the
non-ischemic contralateral regions were calculated (table VII and table VIII). Infarction was
defined as volume in which T2W changes persisted after 2, or in case 7 and 8 (table VI) 6
months. At this time, the vasogenic edema has decreased (182).
A statistical comparison between volumes of abnormality, was performed using a paired student t
-test, with significant difference between groups when p < 0.05.
6.3. Results
Clinical, MR-angiographic findings and the MR-imaging schedule are summarized in table VI.
Generally, volumes of altered hemodynamics were delineated on DWI, CBV and TTP maps.
There was a clear tendency for the volumes affected on TTP maps (and to less degree CBV
maps) to be larger than those seen on DWI, in the acute phase. This was only significant
comparing abnormalities on TTP maps with DWI (p < 0.001), (table VII). At chronic
examinations, significant differences were found between volumes affected on TTP maps and on
TW2 images (p < 0.025), (table VIII). Volumes of abnormalities on TTP maps did not decrease
significantly from the acute to the chronic examinations (table VII and VIII). An increase of the
affected volumes from the acute DWI to the T2W images on chronic examinations were found,
although not statistical significant ( p < 0.069) (table VII and VIII). All lesions present on the
acute DWI persisted at follow-up examinations.
121
Discrepances were noted not only in the extent but also in the location of the abnormal regions
seen in the different modalities. In some cases, abnormalities on CBV maps were covering the
regions of abnormalities seen on the other modalities (figure 36). TTP abnormalities were in
some cases distant from the ischemic region identified on DW images as illustrated in figure 37.
Our MR-tissue findings could be divided into the following four groups. It should be emphasized
that patients often had more than one tissue pattern within a region of ischemia:
Group1. Tissue with prolonged TTP and no abnormalities on CBV maps or DWI in the acute
phase. The tissue became infarcted or recovered as evaluated on chronic T2W images.
Group2. Tissue with shortened TTP and increased CBV, both regions corresponding to hyper-
intensity on DWI. At the time of chronic examination, infarction had developed in the area.
Group3. Tissue with prolonged TTP, decreased CBV and hyperintense signal on DWI in
corresponding regions in the acute phase. The tissue was infarcted at the time of chronic
examination.
Group 4. Tissue with prolonged TTP and increased CBV with DWI hyperintensity in a smaller
region. Infarction had developed at chronic examination in the region depicted on the initial
DWI. No enlargement of the infarct into the region of prolonged TTP or into the region of
increased CBV from the acute examination, was seen.
The MR-tissue patterns described for group 1. Group 1, is characterized by prolonged TTP, normal
CBV maps and normal DWI. At chronic MR examination, infarction had developed in some of
the cases. This pattern was seen for patient 1, 3, 4 and 6.
For patient 1 at the �early subacute� examination, the abnormalities on DW images and T2W
images had spread into the region that initially showed prolonged TTP and normal CBV,
probably reflecting vasogenic edema. On T2W images from the �late subacute� examination, the
infarct was reduced to the size of the hyperintensity seen on the initial DW images as illustrated
in, figure 38. This difference probably reflects decreased vasogenic edema. The case was
presented as a case study in section 5.2.11.
122
In patient 3 and 4, the initial mismatch between the sizes of abnormalities on TTP maps and
DWI had disappeared or was decreased because of the spread of the infarction seen on �early
subacute�, �late subacute� and �early chronic� examination T2W images.
The MR-tissue patterns described for group 1 is likely to represent �Collateral Perfusion� or
partial stenosis identified as a mismatch between the prolonged TTP region and the
hyperintensity on DWI.
The MR-tissue pattern for group 2. This group is characterized by shortened TTP, hyperintensity on
DWI and increased CBV in similar regions that were infarcted at the time of chronic examination
(figure 37). The pattern was seen in the patients 4 and 5. Both regions of shortened TTP became
infarcted at chronic examinations, although there was a tendency that the initial region with
shortened TTP was larger than the final infarct. At �late subacute� examinations, prolonged TTP
and decreased CBV were seen in the regions formerly having shortened TTP. Acute MRA was
acquired in the examination of patient 4 showing no occlusion of the supplying vessel. MRAs at
later examinations were normal in both patient 4 and 5.
At the acute examination of patient 5, a small region of decreased CBV was identified in the
center of the region of increased CBV and shortened TTP, probably caused by edema
compressing the vessels. MR-tissue group 2 is likely to represent luxury perfusion as described by
Lassen (9).
The MR-tissue pattern of group 3. This group includes patients with regions of prolonged TTP,
decreased CBV and hyperintensity on DWI in similar regions. This pattern led to infarction in
the chronic examination. The pattern was seen in patients 1 and 6 (figure 38).
In patient 6, at the �late subacute� and "early chronic� examinations, post Gd-DTPA T1W
images showed enhancement, indicating disrupted blood brain barrier in the anterior part of the
infarct, and no enhancement in the posterior part, reflecting cystic encephalomalacia. The CBV at
this time was increased in the anterior part and decreased in the posterior part. After 6 months,
the entire infarct became cystic as seen on T1W and T2W images corresponding to the intial
abnormalities seen. The CBV at this time was decreased in the whole region.
123
MR-tissue group 3 is likely to represent an �ischemic pattern�, without any potentially salvageable
ischemic regions depicted in the acute examination.
The MR-tissue pattern described in group 4. This group includes patients with regions of prolonged
TTP and increased CBV. DWI showed hyperintensity in a smaller region. No enlargement of the
infarct was seen on chronic examination compared to initial DW images. The pattern was seen in
patients 2, 5, 7 having larger abnormal regions and 9, 10 having smaller abnormal regions.
Patient 2 represented a case of subcortical ischemia due to a MCA stem occlusion seen on the
MRA. DWI showed a lesion in the supply area of the deep penetrating vessels of MCA, whereas
the TTP changes also involved the overlying cortical regions. At the �acute� examinations,
increased CBV on the cerebral surface over the ischemic region was seen, probably reflecting
dilated leptomeningeal vessels. At the �late subacute� examination, MRA showed reperfusion of
MCA and showed that the region of prolonged TTP and increased CBV was now normalized.
Clinically, the symptoms had remitted (figure 36 and 39).
In the acute examination of patient 5, prolonged TTP and increased CBV was identified
posterior to the ischemic region on DWI. The pattern of abnormalities on PWI was still present
at the �early subacute� examination, but had vanished at the �late subacute� examination. No
spread of the infarct into the region of prolonged TTP was seen between the examinations
(figure 37).
In patient 9, small hyperintense regions in the left occipito-parietal (borderzone) area were visible
on DW images, but TTP showed increased values in the entire supply area of the left ICA. CBV
was increased at the overlying cortex. The reason seemed to be occlusion of the left ICA judged
from MRA. In the chronic examinations, the large area of TTP prolongation persisted, as did the
ICA occlusion, and there was no evolution of the lesions seen on the DW images.
MR-tissue group 4 is likely to represent �Compensatory Hyperemia� caused by dilation of
collaterals seen as increased rCBV, and serving to maintain sufficient perfusion in the ischemic
area.
124
Table VI
Table VII
125
Figure 36. Compensatory vasodilatation. Acute: (A) TTP, (B) CBV, (C) DWI, (D) MRA
Table VIII
MR-findings in the chronic phase (2-6months). Volumes on TTP- and CBV-mapsand T2W images.∆TTP(difference) and CBV ratio.Patient no. TTP
cm3
CBV
cm3
T2W
cm3
∆TTP (difference)Infarct ! contralateral Seconds
CBV ratio Infarct/contralateral
1 29.80 13.22 13.11 2.58 0.61
2 3.68 3.62 1.60 1.03 1.27
3 14.10 13.55 13.75 8.50 0.51
4 24.47 22.60 12.40 3.38 0.34
5 6.04 4.39 4.06 0.35 0.17
6 26.96 21.72 20.55 4.48 0.35
7 1.48 - 1.16 1.24 1.05
8 9.11 - 0.93 1.91 0.98
9 31.76 20.46 0.38 3.19 1.33
Mean 16.37 11.06 7.55
Std. 11.95 9.29 7.46
A B
C D
126
Figure 37. Luxury Perfusion.
Acute: (A) TTP, (B) CBV, (C) DWI. Early chronic: (D) T2W imaging.
Figure 38. Collateral and Ischemic pattern.
Acute: (A) prolonged TTP, (B) decreased CBV, (C) DWI. (D) early chronic: T2W imaging
A B
C D
A B
C D
127
Figure 39. Reperfusion with normalization of PWI.
(A) TTP, (B) CBV, (C) T2W imaging, (D) MRA
6.4. Discussion
In the present study, PWI was evaluated by constructing AUC and TTP maps. The maps were
compared to the ischemic region delineated by DWI in the acute phase, and to the findings in
chronic MR examinations. Infarct was identified as hyperintensity on T2W images after 2
months, and in two cases, 6 months. Four tissue groups were identified in the acute phase within
24 hours:
MR-tissue group 1, could represent �Collateral Perfusion� or partial stenosis of a supplying vessel
without vasodilatation from autoregulation. The pattern is identified as prolonged TTP, normal
CBV and normal DWI. The tissue is potentially salvageable.
MR-tissue group 2, may represent �Luxury Perfusion� identified as shortened TTP. The acute DWI
shows abnormality in the region, and the tissue becomes infarcted.
A B
C D
128
MR-tissue group 3, could represent an �Ischemic Pattern� with no collateral perfusion or
vasodilatation available. The pattern is identified as prolonged TTP, decreased CBV and
hyperintensity on DWI in the same region. The tissue becomes infarcted.
MR-tissue group 4, may represent �Compensatory Hyperemia� caused by vasodilatation of
collateral vessels seen as increased CBV in regions of prolonged TTP and normal DWI. The
tissue is potentially salvageable.
Several animal and human studies have shown that cerebral ischemia can be seen very early using
DWI to detect decreased diffusion possibly due to cytotoxic edema (74), (163). In animal studies,
DW signal changes have been shown to depend on the degree and duration of ischemia (156). As
the changes can be reversed, this may identify the penumbra (154). In human studies, however,
reversible DWI changes have only rarely been reported (168), (169). In humans, DWI may
therefore identify irreversible ischemic changes after onset of a cerebral ischemic event (24). This
finding was confirmed in the present study with no reversal of DWI changes. However in the
present study the earliest examination was performed 5 hours after onset of symptoms. As
indicated from a recent study, DWI lesions can reverse at least within the first hours of acute
ischemic stroke {Kidwell, Saver, et al. 2000 #1460}.
Estimating the cerebral perfusion using a bolus injection of a paramagnetic intravascular tracer is
not straightforward. Different parameters derived from the ∆R2*-time curve can be used as
indicators of cerebral perfusion (PWI). The area under the ∆R2* is proportional to CBV,
according to tracer kinetics (107). A prolonged time to maximal ∆R2* and increased width of the
curve reflects an overall prolonged transit time of the contrast agent through the affected tissue
and the supplying vascular system. According to the central volume theorem (107), it is known
that for a given volume of distribution of the tracer, the inverse mean transit time, MTT, is
proportional to perfusion as discussed in section 2.7.
PWI has been used to delineate hypoperfused areas in animal models (188) and human ischemic
stroke (168), (172). In one study where PWI was used for acute stroke during the first 6 hours,
129
three different patterns for the passage of the bolus were identified (110): 1. In the infarct core,
total absence of the bolus passage was found and identified. 2. A prolonged bolus with decreased
peak intensity and increased width represented the potentially salvageable tissue. 3. Finally a
minimal bolus delay with a slightly reduced peak intensity marked tissue that recovered at follow
up. In a study of hyperacute human ischemic stroke within 10 hours, decreased CBV was seen in
a larger area than the ischemic area depicted on DWI (168). On follow-up MR or CT
examinations, the infarction had spread into the region depicted on the initial CBV map. In
another study, PWI was evaluated by calculating MTT and a mismatch was found between
abnormality on PWI and DWI. On follow-up examination, DWI changes had spread into the
area initially only depicted on PWI (172). Tong et al. (166) found that TTP was more sensitive to
ischemia than CBV and DWI: In one of the cases, TTP depicted ischemia that was not depicted
on rCBV maps or DWI, and that had become infarct at follow-up examination. Examination
with both DWI and PWI in the acute phase of ischemic stroke may therefore identify regions
with salvageable brain tissue. This is supported by several studies in which enlargement of the
infarct identified on DWI only occurred in cases where volumes of ischemia identified on PWI
were larger than seen on DWI in the acute phase (173). The different aspects of the PWI/DWI
mismatch were reviewed in section 5.2.4-5.2.10.
Evaluating PWI by constructing maps of TTP and CBV, we have the possibility to go further
into characterizing the hemodynamics in acute stroke by MRI, especially regarding the mismatch
between PWI and DWI. We compared the different ischemic patterns found, to see whether
infarction developed.
“Collateral Perfusion” group (MR-tissue group 1), (see figure 38).
It can be hypothesized that, prolonged TTP seen in specific regions, in this group, is caused by
perfusion through collaterals or perfusion through a stenosis of the supplying vessel. It can also
be hypothesized that CBV reflects the degree of compensatory vasodilatation in ischemic regions.
The tissue pattern of group 1 probably reflects two different situations: Either the perfusion
through collaterals or through a stenosis of the supplying vessel is sufficient without need for
130
compensatory vasodilatation. This tissue is probable not at risk of infarction and may not even
not be hypoperfused because TTP depicts altered hemodynamics in tissue receiving sufficient
collateral perfusion from more circuitous routes. Also the volume of abnormal acute and even
chronic MTT/TTP tends to be larger than the final infarct as found in the present study where
the volumes depicted on TTP acutely and chronically were significantly larger than the final
infarct. This finding confirms a previous study (172). Alternatively, the tissue pattern could reflect
perfusion through collaterals or through a stenosis of the supplying vessel in tissue suffering
from severe ischemia but without compensatory vasodilatation. Such tissue will become infarcted
if reperfusion does not occur. The latter has been described in human PET studies with
normalization of CBV in severe ischemic tissue caused by constriction of the vasculature due to
metabolic derangement (189).
“Luxury Perfusion” group (MR-tissue group 2), (see figure 37).
Shortening of the TTP and increased CBV in the area depicted as ischemic on DWI, probably
reflects the luxury perfusion syndrome with increased perfusion due to abnormal CBF after
spontaneous reopening of the occluded vessel as described by Lassen (9). In the present study,
the final infarct volume was equal to that of initial TTP-, CBV-and DWI changes. The cases
therefore most probably correspond to non-nutritional LP (190).
“Ischemic Pattern” group (MR-tissue group 3), (see figure 38)).
The regions of prolonged TTP and reduced rCBV at the first examination corresponded to those
of ischemia depicted on DWI and later infarction. This contradicts the findings in the study of
Sorensen et al. (168), in which an area with decreased CBV in the acute phase did not always
become infarcted. The reason for this could be that patients were examined within 10 hours after
onset of symptoms, in contrast to the time inclusion of 24 hours in the present study. Decreased
CBV occurring after the hyperacute phase could therefore reflect irreversibly damaged tissue as
seen in the present study.
“Compensatory Hyperemia” (MR-tissue group 4), (see figure 36 and figure 39).
131
The increased CBV could reflect compensatory vasodilatation, maintaining sufficient perfusion
of the tissue and hindering development of infarction. These findings are consistent with PET
studies showing that compensatory vasodilatation occurs in mildly to moderately ischemic brain
tissue (189), (19). The increased CBV could also indicate hyperemia due to reperfusion (luxury
perfusion). In patient 2, the acute MRA showed occlusion of the MCA. In that case, increased
CBV must therefore reflect active vasodilatation (figure 36). Also, if increased CBV reflects
hyperemia due to reperfusion, TTP should be short or normal and not prolonged as seen in this
study. In a recent study of ischemic stroke examined within 12 hours from onset, increased CBV
was also suggested to represent a compensatory hemodynamic response (178). In the "early
subacute" examination, patient 2 had recanalisation as seen on the MRA, CBV was increased,
probably due to LP, and TTP was normalized. The symptoms were almost totally remitted
corresponding to the dissolution of the lesion on TTP. Three such cases with remission of
clinical symptoms corresponding to normalization of PWI at follow-up were previously reported
(172). Also Baird et al. in a preliminary study found correlation of clinical remission and
differences between the volume of tissue depicted acutely and the final infarct (24). The patient in
our study had the acute examination 22 hours after onset of symptoms and may represent a case
of prolonged penumbral tissue. This was also indicated by a benzodiazepine receptor
examination, using Iomazenil® in SPECT, that the patient had after some months. The
Iomazenil® study showed cortically selective neuronal death (incomplete infarction) (191) only in
the region depicted on the initial PWI maps (figure 40). Incomplete infarction has been
speculated to be associated with the transition of penumbral tissue to infarct (penumbra Ψ
incomplete infarct Ψ infarct) (191).
In conclusion, it was found that PWI using Gd-DTPA as an intravascular tracer for measuring
both CBV and TTP maps in combination with DWI improves evaluation of ischemic tissue in
the acute phase.
132
In two of the recognized patterns, potentially salvageable ischemic tissue was identified. In one
group, there was no potentially salvageable tissue. Finally one group had luxury perfusion due to
reperfusion hyperperfusion. Identifying different DWI-PWI patterns in acute stroke may be
useful in more accurately selecting which patients have salvageable tissues, thereby helping in
deciding which patients should receive thrombolytic or neuroprotective therapy.
Figure 40. Incomplete Infarct.
(A) TTP, (B) CBV, (C) T2W imaging, (D) Iomazenil-SPECT
A B
C D
133
Conclusions The following conclusions can be drawn regarding the questions raised in the introduction:
T2* weighted EPI seems very promising for the detection of ICH and it is more sensitive than
other MR techniques as demonstrated in the case study. However, further studies are needed to
compare the sensitivity to that of CT exemplified by a case of false negative detection of ICH.
Also the limitations of T2* weighted EPI due to susceptibility artefacts, have to be addressed in
future studies.
T2* weighted imaging was far less sensitive for detection of HI compared to T1 weighted
imaging. It was speculated that this is due to low image resolution. However, pathophysiological
explanations are suggested based on the relative high oxygenation in the tissue influencing the
magnetic properties. Further studies measuring the T1- and T2* relaxation for a range of image
resolutions are needed for optimizing the T2* weighted sequences for detecting HI and for
providing more information on the pathophysiological evolution of HI.
It was found that HI identified on both MR modalities (in contrast to HI identified on T1
weighted images only) was significantly associated to disrupted blood brain barrier, thereby
probably identifying a group of patients with increased risk of secondary hemorrhage.
Four tissue patterns in acute stroke patients were identified combining DWI and PWI. Two of
the patterns identified potentially salvageable tissue �collateral pattern� and �compensatory
vasodilatation�, which might be candidates for thrombolytic or neuroprotective therapy. A third
pattern identified �luxury perfusion� due to reperfusion with no indication for thrombolytic
therapy (but maybe indication of neuroprotection to prevent reperfusion injury). A fourth
�ischemic pattern� depicted ischemic injured tissue probably without potential for salvation.
134
Based on a case study of �stroke in progression� it was hypothesised that the combination of
DWI and PWI can be used to stratify the patients regarding different mechanisms related to
�stroke in progression� i.e. hemodynamics, edema. Further serial studies are needed to clarify if
�stroke in progression� can be predicted from acute DWI and PWI.
The final infarct as measured on T2 weighted images should be interpreted with consideration of
the water content, as illustrated in a case study. The water content is at its peak within the first
week after onset of symptoms as measured in a published study of ours.
In one patient with ischemia, no changes on DWI were identified although the perfusion range
had values in the human penumbra range. This case is in accordance with the experience that
DWI changes in humans are rarely reversed in contrast to findings in animal studies, although
this matter has been challenged in recent reports as discussed and needs further studying.
135
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