Cardiac MRI

History of MRI• Magnetic resonance imaging (MRI), nuclear magnetic

resonance imaging (NMRI), or magnetic resonance tomography (MRT)

• Raymond Damadian, an Armenian-American physician, scientist - worlds first MRI machine 1972

• Paul Lauterbur - technique to generate images from MRI

• Peter Mansfield - mathematical technique to generate images from MRI

• Nobel Prize in Physiology or Medicine for their "discoveries concerning magnetic resonance imaging"

Paul Lauterbur Raymond Damadian Peter Mansfield

• MR System Components

• Main magnet coils

• Static magnetic field B0

• 3 gradient coils

• integral RF transmitter coil

• Application of RF pulse tilts net magnetisation vector - flip angle

• two independent relaxation processes return the net magnetization vector to its thermal equilibrium

• Longitudinal relaxation results from the transfer of energy from the excited protons to surrounding molecules in the local environment.

• T1 - time to recover 63% of original energy

• Transverse or spin-spin relaxation, describes the decay of the magnetization vector in the transverse plane

• T2 - time to lose 63% of transverse momentum

• T1 and T2 properties of tissue

• Advantages of CMR over other cardiac imaging modalities

• Lack of ionizing radiation

• Free choice of imaging planes

• Capability for tissue characterisation

• Qualitative and quantitative evaluation of the motion of both the blood and the myocardium

• Assessment of regional perfusion

MRI Safety• Powerful magnetic field

• 1.5 T approx 30,000 times earth’s magnetic field

• Objects near entry - pulled into machine

• objects within machine - experience torque

• Rapid change in field with distance from magnet

• Heating effects

• RF magnetic fields can cause heating if concentrated in small areas - ECG leads, thermistor leads, may damage device or cause burns

• Claustrophobia

• 4% of patients

Cardiac gating• Essential component of cardiac MRI to

overcome blurring of images caused by myocardial contraction and flow effects from pulsatile blood

• Data collected over same point in ECG over successive heart beats

• Good lead placement, prominent R wave

• Avoid loop formation

• Prospective gating

• Data acquisition over at start of cardiac cycle over several cycles

• Better temporal resolution

• Retrospective gating

• Continuous data acquisition and then post processing for images at certain time points

• Whole cardiac cycle can be imaged

• Less sensitive to arrhythmias

• Useful for cine images

• Better for regional and global wall motion assessment

• Temporal blurring

Respiratory Gating• Maximum motion craniocaudal

• Some degree of anteroposterior and transverse motion

• Approx 1 cm movement craniocaudally

• Breath holding - Acquisition times 10 sec

• Varies within cycles

• Adequate respiratory reserve

• Imaging with breath holding at end tidal position

• Diaphragmatic tracking - by respiratory navigator

• Respiratory monitoring using bellows

Pulse Sequence Structure

• An individual pulse sequence is a combination of radiofrequency pulses, magnetic gradient field switches, and timed data acquisitions, all applied in a precise order, that results in either accentuation or suppression of specific biological

• Imaging engine

• provides the spatial relationship of objects i.e. the image

• Modifier

• Optional components that add specific information or speed the image

Examination sequence

• Scouting

• Function/Volumes

• Perfusion at stress/rest

• Viability and Infarction

• Additional

• Morphology

• Flow/Velocity

• T2 Weighted Edema imaging

• Scouting

• First procedure

• To establish long and short axis of heart with respect to scanner coordinates

• Single shot

• Steady state free precession (SSFP), Half Fourier single-shot TSE (HASTE)

Function and volumes

• CineMRI using Gradient Recall Echo(GRE) or SSFP

• Captures movie of beating heart

• 20-25 frames per cycle, 35-45 ms per frame

• Single shot/segmented

• Core examination

• Short-axis stack from mitral valve plane to apex

• Two, Three and Four Chamber long axis views

• Slices - 5-6mm

Perfusion at Stress and Rest

• Fasting growing use

• Movie of the transit of contrast media (typically gadolinium-based) with the blood during its initial pass through the left ventricular (LV) myocardium (first-pass contrast enhancement).

• 4 to 5 short-axis views are obtained every heartbeat

• Total of 40 to 60 heartbeats consisting of the entire first-pass

• Patient then partially pulled out

• Adenosine 140 mcg/kg/min

• Patient returned after 2 minutes

• Gadolinium contrast administered (0.075-0.10 mmol/kg) 4ml/s

• After contrast clears from LV myocardium adenosine stopped (total 3 - 3.5 mins)

• 15 minutes for contrast to wash out from blood pool

• Rest perfusion scan

• Additional gadolinium contrast (0.075-0.10 mmol/kg) 4ml/s

• Same imaging

• Delayed enhancement imaging

• After 5 minutes

• Total duration ~ 45 minutes

Viability and Infarction

• Delayed enhancement MRI (DE-MRI), Late Gadolinium enhancement CMR, delayed hyperenhancement imaging

• Images with high contrast between abnormal myocardial tissue, which generally accumulates excess gadolinium (after intravenous administration), and normal tissue, in which gadolinium concentration is low.

• Gadolinium cannot penetrate intact sarcolemmal membrane

• Injured myocytes take up gadolinium and increased tissue concentration

• Chronic infarction, interstitial space is increased

• High tissue concentrations of gadolinium leads to shortened T1 relaxation times

• Infarct - bright/hyperenhanced

• Viable - black/nulled

• Morphology scan

• Usually cardiac and proximal vascular structures are imaged in the core examination

• If additional information required

• congenital heart disease, cardiac masses, aortic root dilation

• Bread loaf - parallel slices, axial, sagittal or coronal

• Single shot using

• SSFP - blood bright (bright blood technique)

• TSE - flowing blood dark (black blood technique)

Flow/velocity• Velocity encoded cine imaging (VENC-MRI), phase

contrast velocity mapping

• Signal from moving blood or tissue will undergo a phase shift relative to stationary tissue if a magnetic field gradient is applied in the direction of motion

• Cine loop across cardiac cycle - pixel intensity proportional to blood velocity

• Grayscale - White maximum in one direction, black maximum in the other direction

• Segmented GRE during breath hold

• versus doppler echocardiography

• flow through an orifice is directly measured on an enface image of the orifice with through-plane velocity encoding

T2 weighted edema imaging• Necrotic myocardium - tissue water content

increases markedly

• longer intrinsic T2 for infarcted myocardium (60-65 ms) compared with that of normal (45-50 ms)

• Uses

• Chronic lesions from those of recent onset

• Possible role in identifying myocardium at risk

Contrast agents• Only gadolinium based contrast agents(GBCA)

are used at present

• Following iv injection

• 15 to 30 seconds for a first pass through the cardiac chambers and blood vessels (first-pass phase)

• 10 to 15 minutes transient plateau of GBCA concentration (equilibrium between contrast washing in to the extracellular space and washing out to the blood pool)

• Myocardial perfusion CMR and most types of magnetic resonance angiography (MRA) are performed during the first-pass phase,

• Late gadolinium enhancement (LGE) images are obtained during the equilibrium phase

• Mild allergic reactions 0.01% to 0.07%

• Serious adverse effects rare

Nephrogenic Systemic Fibrosis

• Fibrosing disorder seen only in renal failure patients

• Thickening and hardening of the skin overlying the extremities and trunk

• Marked expansion and fibrosis of the dermis in association with CD34-positive fibrocytes

• Excess exposure to free Gd3+ in patients with kidney disease leads to tissue damage

• Skin

• symmetrical, bilateral fibrotic indurated papules, plaques, or subcutaneous nodules

• ankles, lower legs, feet, and hands

• Systemic

• Muscle induration

• Lung fibrosis, diaphragm, myocardium, pericardium, pleura and dura mater

• Chronic unremittant course

• Review - 28% no improvement, 20% modest, 28% death

• Prevention - avoid Gadolinium

Assessment of Coronary Artery Disease

Imaging of myocardial infarction

• Late Gadolinium enhancement

• Currently the most precise and accurate noninvasive method to quantify infarct size and morphology

• correlates with serum CK, time to treatment and ST resolution

• In both acute and chronic infarcts sensitivity 99% and specificity of 94%

• First 5 mins of LGE detects microvascular obstruction (no reflow) as dense hypoenhanced area within the core of a bright region of infarction

• Myocardial hemorrhage marker of reperfusion injury

• High sensitivity in detecting infarcted tissue - upto few grams

• Significance of LGE

• Marker of unrecognised MI - independent and strong predictor of cardiac death

• Diabetics without clinical/ECG evidence of MI, LGE consistent with MI has 3.6 fold elevated hazard of death

• Cheong and colleagues has reported that LGE scar transmurality index is a strong independent predictor of death or cardiac transplantation at a median follow-up of 4.4 years; this provides complementary prognostic information to the left ventricular (LV) ejection fraction

• Tissue inhomogeneity in LGE tissues may identify arrhythmogenic substrates

• Schmidt and colleagues have developed methods for quantifying the heterogeneous peri-infarct zone

• Roes and coworkers have found that peri-infarct zone is the strongest predictor of spontaneous ventricular arrhythmias that required appropriate ICD therapy

• Future studies needed to establish standard to quantify peri-infarct zone

Assessment of myocardial viability

• Myocardial viability defined as preservation of cellular function without any irreversible cellular damage

• Augmentation of regional function in response to low dose dobutamine (5-10 mcg/kg/min)

• Contractile reserve defined as increase in systolic thickening by 2 mm (sensitivity 89%, specificity 94%) in predicting segmental viability

• Transmural extent of LGE scar predicts a progressive stepwise decrease in the likelihood of function recovery accurately, despite successful coronary revascularization. (Kim and associates)

• Especially with resting akinesia/dyskinesia

• 88% of segments with less than 25% transmural extent of LGE improved contractile function, whereas only 4% of segments with more than 50% transmural extent of LGE

• LGE

• easy to perform and interpret

• 50% cutoff sensitive in predicting contractile segmental recovery

• Low dose dobutamine

• physiologic assessment of the midmyocardial and sub- epicardial contractile reserve, particularly in segments with subendocardial infarction involving less than 50% of the transmural extent.

• Wellnhofer and coworkers assessed 29 patients

before and 3 months after coronary revascularization with both low-dose dobutamine cine imaging and LGE imaging, and reported better prediction of segmental contractile recovery by dobutamine cine imaging

• low-dose dobutamine cine CMR can be complementary in assessing myocardial viability early after acute MI when tissue edema is prominent or when there is a need to assess the benefit of bypass surgery in patients at high preoperative risk

• Detection and differentiation of ACS from non coronary causes

• qualitative assessment of T2-weighted imaging and LGE yielded a 96% specificity in differentiating acute from chronic MI.

• Patients with acute chest pain presenting with ECG and enzyme negative, adding T2-weighted imaging and LV wall thickness to cine and LGE imaging increases the specificity from 84% to 96% without any loss of sensitivity

Detecting and Quantifying Myocardial

Ischemia• Qualitative assessment of vasodilator stress CMR myocardial perfusion is rapid and accurate in detecting CAD

• Combined multicomponent CMR provides higher accuracy than a single component alone

• MRA, LGE, and cine CMR reached an excellent sensitivity of 96% while maintaining a high specificity of 83% in detecting coronary stenosis(Plein et all)

• CMR stress perfusion versus radionuclide perfusion imaging

• Not limited by attenuation artefacts

• No need for ionizing radiation

• Three- to fourfold higher spatial resolution than SPECT.

• Stress CMR - 30-45 mins, SPECT - 2 hours

• Can characterise dynamic blood flow, not limited by plateau effect as in nuclear tracers

• MR-IMPACT

• CMR perfusion better than SPECT

• Especially patients with multi vessel stenoses

• Perfusion can be assessed by quantitative methods like signal intensity versus time curves derived from LV myocardial segments

• Absolute blood flow in ml/gm/min

• Quantitative methods minimised reader bias, also maps of perfusion reserve can be used in testing novel therapies

Dobutamine Stress CMR

• Sensitivity 83-86%

• Specificity 83-86%

• Superior to dobutamine stress echocardiography when echo windows poor

• Real time cine CMR imaging eliminates requirement for breath holding or ECG gating during dobutamine stress

• Strong prognostic value

• Ingkanisorn et all

• Adenosine stress CMR - sensitivity 100% and specificity 93% for clinical events at 1 year

• Jahnke et al

• CMR with dobutamine stress cine and adenosine perfusion normal - 99.2% negative 3 year event rate for cardiac death/acute MI

Imaging of atherosclerotic plaques

• Carotid artery and Descending aorta

• Most comprehensive non invasive method to assess plaque structure and activity

• Carotid bifurcation is relatively immobile, large, and superficial to the skin surface, and it shows the full spectrum of atherosclerotic lesion types.

• Contrast weighted sequences

• carotid plaque fibrous cap, hemorrhage, calcifications, and loose matrix.

• Contrast enhanced dynamic MRI - plaque neovascularisation

• USPIO (Ultrasmall super-paramagnetic particles of iron oxide)

• target macrophage activity at high affinity based on histologic and electron microscopic analyses of atherosclerotic plaques.

• 24 to 36 hours after USPIO injection carotid macrophage plaque activity can be measured

• MRI of aortic plaques

• Difficult due to increased signal-noise ratio in achieving submillimeter spatial resolution and blood flow artifacts.

• Intravascular MR coils recent advancement

• Coronary artery

• Plaque imaging difficult

• cardiac and respiratory motion and small vessel size

• Targeted contrast - fibrin-binding contrast agent EP-2104R (EPIX Pharmaceuticals) - coronary thrombus

• Intravascular coils

• High field CMR

Cardiomyopathies

Hypertrophic Cardiomyopathy• CMR cine imaging of LV structure, function and tissue characterisation

helpful

• Pathological and physiological LVH

• end-diastolic wall thickness–to–cavity volume ratio less than 0.15 mm/mL/m2 and lack of LGE of the ventricular myocardium can provide accurate differentiation between physiologic and pathologic LVH

• Helpful in patients with poor echocardiographic windows

• Echo underestimates hypertrophy in basal anterolateral wall by 20%

• Extreme hypertrophy (>30 mm) by 10%

• Apical hypertrophy in apical HCM not detected by echo

• Where discrepancy between ECG and echocardiography

• Can assess septal thickness after surgical myomectomy/Alcohol septal ablation

• especially severe septal hypertrophy and symptomatic dynamic LV outflow obstruction

• markedly elevated LV mass index (men > 91 g/m2; women > 69 g/ m2) sensitive (100%), maximal wall thickness of more than 30 mm specific (91%) for cardiac deaths

• RVH or myocardial edema by T2W imaging

Arrhythmogenic Right Ventricular

Cardiomyopathy• Predisposition to ventricular arrhythmias that precede overt morphologic abnormalities, histologic substrate and by diverse phenotypic manifestation

• Quantitative and volumetric assessment of cardiac function

• Characterization of myocardial fibrofatty tissue

• CMR had a sensitivity of 96% and a specificity of 78% in detecting ARVC

Myocarditis• Myocardial edema by T2-weighted imaging, regional

hyperemia and capillary leak by early gadolinium enhancement ratio (EGEr), and myocardial necrosis or fibrosis by LGE imaging.

• In cases with high index of clinical suspicion but negative CMR tissue findings, a repeat study in a few weeks may be necessary for diagnosis because inflammation may be focal and difficult to detect in the first few days of disease.

• Early evidence has indicated that a persistence of LGE 4 weeks after symptom onset is predictive of adverse functional and clinical outcomes

Cardiac Sarcoidosis• Detects disease through successive histological stages

• Tissue edema, non- caseating granulomatous infiltration, and patchy myocardial fibrosis

• DE-MRI 2 fold increase in detection as compared to Japanese Ministry of Health Criteria

• LGE positive patients 9 fold increase in death

• Can guide endomyocardial biopsy

• Can monitor disease progression accurately

• Limited to monitoring tool for progression

Cardiac amyloidosis

• DE-MRI shows diffuse LV hyper enhancement

• Subendocardium preferentially involved not limited to arterial territory

• Difficult to detect mild amylodosis

Iron overload cardiomyopathies

• Hemolytic anemias/iron overload pathologies

• T2 mapping to exclude cardiac siderosis

• T2 CMR enables amount of myocardial iron to be estimated

• T2 value < 20 ms highly suggestive of cardiac siderosis

• T2 < 10 ms prone for heart failure

• Used to assess response to chelation therapy

Pericardial Disease• Constrictive Pericarditis

• > 4 mm abnormal

• <2 mm normal

• TSE morphology and SSFP

• Realtime CineMRI

• increased ventricular interdependence, a hemodynamic hallmark of pericardial constriction.

• Pericardial effusion

• Loculated and circumferential effusions

• Simple transudative effusions typically appear bright and homogenous on T2-weighted images and dark on T1-weighted images. On SSFP cineMRI, which exhibits T2/T1 weighing, simple effusions appear bright with the same or even higher image intensity than epicardial fat.

• Complex effusions may appear heterogeneous and darker on T2 and SSFP imaging. Additionally, unlike simple effusions, complex effusions may demonstrate increased image intensity on T1-weighted imaging after administration of gadolinium contrast media

Hemodynamics• VENC-MRI

• ASD

• en face imaging of ASD

• rim of tissue separating the ASD from the base of the aorta (retroaortic rim), tricuspid valve, vena cavae, and coronary sinus can be viewed from a single image plane

• Qp/Qs measurement

• additional information multiple ASDs, extent of rim tissue, presence of sinus venosus defects with anomalous pulmonary vein

• Aortic stenosis

• Planimetry by cineMRI correlates well with other modalities

• Peak velocity also correlates with Doppler echocardiography

• Correct plane along peak velocity

Cardiac masses• Protocol sequence to assess morphology, motion, perfusion, and

delayed enhancement, in addition to inherent differences in T1 and T2 and abnormal physiology

• benign masses - atrial myxoma, rhabdomyoma, fibroma, and endocardial fibroelastoma

• Atrial myxoma

• left atrium (75%), right atrium (20%), or ventricles or mixed chambers (5%)

• inhomogeneous brightness in the center on cine SSFP imaging because of its gelatinous content

• may have a pedunculated attachment to the fossa ovalis.

• Metastatic malignancy 20 times more common than primary malignancy

• direct invasion (lung and breast), lymphatic spread (lymphomas and melanomas), and hematogenous spread (renal cell carcinoma)

• Primary

• children/young adults

• angiosarcoma,fibrosarcoma, rhabdomyosarcoma, and liposarcoma.

• LV thrombus

• LGE imaging can detect thrombus with a higher sensitivity than echocardiography by depicting high contrast between the dark thrombus and its adjacent structures and by imaging in three dimensions.

• Mural thrombus does not enhance on first-pass perfusion and often has a characteristic etched appearance (black border surrounding a bright center) on LGE imaging, thus providing higher diagnostic specificity than anatomic information alone

• Microvascular obstruction from MI can be confused with mural thrombus, but it is usually confined within the infarcted myocardium characterized by LGE.