Diffusion Tensor Imaging

Theory and Practice

Noam Eshkar, M.D. Chairman Director of Neuroradiology Department of Radiology JFK Medical Center Edison Radiology Group Edison, N.J.

Diffusion Tensor Imaging

• Method of Magnetic Resonance Imaging

• MRI sensitized to mobility of protons

– Differences in rate of motion mapped

– Each voxel represents rate of water diffusion at that location

• Derives from Brownian Motion

Brownian Motion Random motion of particles in a fluid as a result of continuous collisions with molecules of the surrounding medium

Restricted Diffusion

Isotropy / Anisotropy

Isotropy: Movement uniform in all directions

Anisotropy: Movement not uniform in all directions –directionally dependent

Diffusion - Normal Anatomy

CSF

White matter

Grey matter

Diffusion - Pathology

• Sensitive to changes in tissue microstructure

– Axonal damage

– Axonal loss

– Edema

– Demyelination

– Inflammation

– Necrosis

Diffusion - Pathology

• Increased diffusion restriction – Acute infarct – Abscess – Epidermoid

• Variable diffusion restriction – Hematoma – Tumor – Thrombus – Demyelination – Encephalitis – MS – CJD – Hypertensive encephalopathy – CPM

Increased Diffusion Restriction

Acute Infarct Abscess Epidermoid

Increased Diffusion Restriction

MS PRES CJD

Decreased Diffusion Restriction

Chronic infarct Arachnoid cyst GBM

Diffusion Tensor Imaging

• Adds directional information

Diffusion Tensor Imaging

DTI Fractional Anisotropy (FA) color map

Fractional Anisotropy color map

0.7 0.3 0.14

Tractography

• Linear assembly of voxels with similar maximum diffusion direction.

Intensity and direction

Tractography

Seed tracking

• Selection of voxels from which to build tracts – user selected

Tracts – Projection fibers

• Interconnect cortical areas to deep nuclei, brainstem, cerebellum, spinal cord

– Primary motor/premotor/supplementary

• Corticospinal

• Corticobulbar

• Corticopontine

– Geniculocalcarine

• (Optic radiations)

Corticospinal tracts

Non fused

Tracts – Commissural fibers

• Interconnect similar cortical areas between opposite hemispheres

– Corpus callosum

• Interhemispheric sensorimotor /auditory

– Anterior commissure

• Nociception/pain

Corpus callosum

Tracts – Association fibers

• Interconnect cortical areas within a hemisphere. – Cingulum

• Visceromotor/visuaspatial/memory

– Superior / Inferior occipitofrontal fasciculus • Spatial awareness/auditory/visual association

– Uncinate fasciculus • Auditory/verbal memory

– Arcuate fasciculus (part of superior longitudinal) • Auditory/Speech

– Occipitotemporal fasciculus (inferior longitudinal) • Visual emotion/memory

Arcuate fasciculus

Broca/Wernicke

Uncinate fasciculus

• Inferior frontal to anterior temporal

• Auditory verbal and declarative memory

Optic radiations

Tumor – Surgical Planning

• Minimizing damage to eloquent cortex and white matter tracts while maximizing tumor resection. – Motor, Sensory, Language

• Color maps/tractography as presurgical and intraoperative guidance in regions adjacent to functional tracts – Correlate with conventional MR .

– Intraoperative electrical stimulation as needed.

Tumor – Surgical Planning

• Accuracy - tracts can be over or underrepresented

– Preop registration accuracy 1-2 mm

• Vs gold standard intraop stimulation

– Decreased accuracy w/ intraop shift/swelling

– Artifacts - susceptibility, blood, edema, air

– Non standard language of tumor/tract interaction

• Disrupted, displaced, deviated, deformed, destroyed, degenerated, interrupted, infiltrated, splayed…

Low grade tumor

Tractography BOLD fusion (anatomic + functional)

finger toe

Tractography BOLD fusion

High grade tumor

Tumor progression - GBM

Mixed displacement /disruption

Trauma

• Diffuse Axonal Injury

– Shear strain injury with disruption of axonal membranes and cytoskeletal network.

– Detection of microstructural injury.

– Persistent injury after mild TBI.

• Most brain injury mild

• 30% with residual deficits/disability

• Large public health issue

• CT/MRI underestimate axonal injury

DTI – Axonal Injury

• Acute - Reduced Fractional Anisotropy (FA) – In vivo detection – early stages TBI < 24 hours.

• Most common: Internal capsule and corpus callosum

• Chronic - Marker for long term neurocognitive deficits – Reduced FA - poorer performance on cognitive tasks

• Currently limited utility for individual cases. – No universal threshold for standard for abnormality – Spurious low FA: crossing tracts, edema – Research limitations: Heterogeneity of trauma date, ratings,

mechanisms/ ages/ control groups/ cohorts/ magnets/sequences/ lesion localization/ study design/ data analysis/ outcome measures, etc.

– Longitudinal studies limited. Acute – subacute – chronic.

Axonal Injury

FLAIR Gradient Echo

< FA -Normal vs abnormal -Technical vs anatomic -?Significance -Group vs individual

Epilepsy

• Preoperative planning - avoiding eloquent structures

– Temporal lobectomy

– other resective surgeries

• Augmenting search for epileptogenic focus

– Conventional MRI sensitivity approx 50%

• Epilepsy protocol

– PET/SPECT/MEG

Epilepsy - preoperative

Meyer’s loop

Epilepsy – Diffusion - Acute

• Periictal / immediate postictal

– Restricted diffusion > normalization

• Similar to ischemia

• Normalizes in approximately 14 days

– Mostly grey matter changes

– May spread beyond epileptogenic focus

– Rare to capture clinically

• Differential diagnosis

Epilepsy – Diffusion - Chronic

• Chronic – interictal – Decreased FA. – May be more sensitive than conventional MR

• “Nonlesional” – negative MRI

– Subtle hippocamal sclerosis / cortical dysplasia • Decreased FA in normal appearing ipsilateral white matter

– Multilesional cases – epileptogenic lesion • Tuberous Sclerosis

– Corpus callosum - decreased FA in new epilepsy in children.

– Chronic refractory – synaptic reorganization/ altered connectivity

Multiple Sclerosis - DTI

• Decreased FA –normal appearing white matter – Most pronounced with chronic lesions – Peri -plaque FA abnormalities – May precede an acute lesion

• < FA lesion burden correlating w /disability – Global histogram vs regional ROI

• Decreased fiber tracts, local vs. distant. – Wallerian degeneration

• FA can be unreliable – fiber crossings – Edema – Anatomic variation – Technical

Multiple Sclerosis – DTI - FA

0.15

0.24

0.30

0.30

0.75

Dementia – cognitive impairment

• Mild Cognitive Impairment – early AD

– < FA in normal appearing white matter

– Decreased hippocampal FA may precede atrophy

– < FA in cingular bundle and other association tracts.

– FA abnormalities overlap with other dementias – nonspecific.

Psychiatric

• Schizophrenia

– Disconnectivity theory

• Alteration in fiber bundle connectivity

– < FA abnormalities in multiple areas – inconsistent

• Major Depression

– < FA in sup long/inf long/ inf front-occ fasciculi

Development

• Increasing FA in increasing age – Infancy to adulthoood – Conventional MR stabilizes at age 2

• Evaluating injuries that affect development • Hypoperfusion/hypoxia

• Autism – Prefrontal and temporal < FA – Connectivity disorder?

• Congenital anomalies – Corpus callosum and other tract malformations – Cortical dysplasias

• FA abnormalities beyond MR visible lesions • Decreased tracts serving cortical anomalies

Corpus Callosum Agenesis

Ischemia/Infarct

• DWI critical in acute stroke

• DTI progressively < FA with infarct age

• DTI abnormalities in Wallerian degeneration distant from infarct precedes visible MR abnormalities

– Role of local and distant FA abnormalities in prediction of clinical outcomes?

Brain stem/Spinal Cord

• Utility limited – Small size of cord – Breathing – Swallowing – CSF pulsation

• Multiple Sclerosis – < FA in lesions and in adjacent normal appearing cord

• Tumors – Ependymoma vs astrocytoma

• Areas of investigation – Spondylotic myelopathy/Transverse

myelitis/Trauma/Ischemia

Medulla tumor

Thank you!