Residency · Residency · Diagnostic Radiology

Cardiac MRI: Structure, Function, and Tissue Characterization

Introduction

Cardiac MRI (CMR) is the gold standard for assessing ventricular volumes, mass, and ejection fraction, and it offers unparalleled tissue characterization capabilities. Its ability to detect myocardial edema, fibrosis, iron overload, and infiltrative disease makes it indispensable in modern cardiovascular imaging.

Technical Foundations

ECG Gating

Retrospective gating acquires data continuously and retrospectively sorts it to cardiac phases, making it the method used for cine imaging. Prospective triggering acquires data at a specific phase of the R-R interval and is used for static images. Arrhythmias (especially atrial fibrillation) degrade image quality, and real-time or arrhythmia rejection techniques may be necessary.

Core Pulse Sequences

Steady-state free precession (SSFP) is the workhorse for cine imaging, providing excellent blood-myocardium contrast as a bright blood technique. Spin echo (dark blood) is used for morphologic assessment of cardiac and vascular structures. Phase-contrast velocity mapping quantifies flow velocity and volume across valves and vessels, functioning analogously to Doppler echocardiography. Short tau inversion recovery (STIR) is a T2-weighted fat-suppressed sequence sensitive to myocardial edema.

Functional Assessment

Cine Imaging

SSFP cine images are acquired in a short-axis stack from base to apex and in standard long-axis views (2-chamber, 3-chamber, 4-chamber). Endocardial and epicardial contours are traced at end-diastole and end-systole for volumetric analysis. Key parameters include LV end-diastolic volume (EDV), end-systolic volume (ESV), stroke volume (SV), ejection fraction (EF), and myocardial mass. Normal LVEF is 55-70% and normal RVEF is 45-65%. CMR is the reference standard for RV volumes and function, which is crucial in congenital heart disease and pulmonary hypertension.

Valvular Assessment

Phase-contrast imaging quantifies regurgitant fraction and peak velocity across stenotic valves. Planimetry of valve area on cine images is particularly useful for aortic stenosis. CMR is preferred when echocardiographic assessment is discordant or technically limited.

Tissue Characterization

Late Gadolinium Enhancement (LGE)

LGE images are acquired 10-15 minutes after gadolinium-based contrast agent (GBCA) administration. Gadolinium accumulates in the expanded extracellular space (fibrosis, necrosis, edema). Inversion recovery sequences null the normal myocardium, making areas of enhancement appear bright. The TI (inversion time) must be optimized to null normal myocardium, typically at 250-350 ms.

LGE Patterns and Differential Diagnosis

Subendocardial or transmural enhancement following a coronary artery territory indicates ischemic cardiomyopathy (myocardial infarction), and the transmural extent predicts viability -- less than 50% transmural enhancement suggests viable myocardium likely to recover with revascularization. Mid-wall enhancement is seen in dilated cardiomyopathy, myocarditis (active or healed), and hypertrophic cardiomyopathy (at RV insertion points). Epicardial enhancement is characteristic of myocarditis (especially the inferolateral wall) and sarcoidosis. Global subendocardial enhancement suggests amyloidosis, often accompanied by difficulty nulling the myocardium. Patchy, multifocal enhancement is seen in sarcoidosis and Anderson-Fabry disease.

T1 and T2 Mapping

Native T1 mapping provides quantitative measurement of myocardial T1 values without contrast. Elevated T1 is seen in edema, fibrosis, amyloidosis, and inflammation, while reduced T1 occurs in iron overload, Anderson-Fabry disease, and lipomatous metaplasia. The extracellular volume (ECV) fraction is calculated from pre- and post-contrast T1 maps with hematocrit correction; elevated ECV indicates diffuse fibrosis, amyloidosis, or edema, with a normal value of approximately 25%. T2 mapping detects myocardial edema through elevated T2 values and is useful in acute myocarditis and myocardial infarction (area at risk).

T2* Mapping

T2 mapping is used specifically for myocardial iron overload assessment in hemochromatosis and thalassemia. A T2 less than 20 ms indicates iron overload, while less than 10 ms indicates severe overload with a high risk of heart failure. Serial monitoring guides chelation therapy.

Specific Clinical Applications

Ischemic Heart Disease

CMR stress perfusion (adenosine or regadenoson) identifies inducible ischemia as perfusion defects on first-pass imaging. LGE defines infarct size and transmurality to guide revascularization decisions. Microvascular obstruction (MVO) appears as a dark core within an area of LGE on early post-contrast images and indicates severe microvascular injury with worse prognosis.

Myocarditis

The Lake Louise criteria (updated 2018) base diagnosis on at least one T1-based criterion (elevated T1, elevated ECV, or LGE) plus at least one T2-based criterion (elevated T2 or T2 mapping). The classic LGE pattern is epicardial or mid-wall enhancement, typically in the inferolateral wall. The acute phase shows myocardial edema on T2-weighted/STIR sequences.

Hypertrophic Cardiomyopathy (HCM)

HCM features asymmetric septal hypertrophy (septal-to-lateral wall ratio >1.3). LGE at the RV insertion points and within hypertrophied segments correlates with fibrosis and arrhythmia risk. CMR quantifies maximum wall thickness, which is important for ICD decision-making since a thickness greater than 30 mm is a risk factor for sudden cardiac death.

Cardiac Amyloidosis

The characteristic finding is diffuse subendocardial or transmural LGE with difficulty nulling the myocardium, accompanied by markedly elevated native T1 and ECV values and concentrically thickened ventricles with biatrial enlargement.

Arrhythmogenic Right Ventricular Cardiomyopathy (ARVC)

ARVC presents with RV dilation, reduced RVEF, and regional wall motion abnormalities on cine imaging. Fibrofatty replacement of the RV myocardium is best seen on fat-suppressed sequences. CMR is part of the revised Task Force criteria for ARVC diagnosis.

Key Clinical Pearls

The pattern of LGE is the most powerful tissue characterization tool: subendocardial or transmural enhancement in a coronary territory means ischemic, while mid-wall or epicardial enhancement means non-ischemic. LGE transmural extent less than 50% predicts functional recovery after revascularization. **T2 mapping* is the reference standard for myocardial iron quantification in transfusion-dependent anemias. The updated Lake Louise criteria for myocarditis require at least one T1-based and one T2-based abnormality.

References

1. Standardized CMR Interpretation and Reporting. J Cardiovasc Magn Reson. 2020;22(1):1-22.
2. Ferreira VM, et al. Cardiovascular Magnetic Resonance in Nonischemic Myocardial Inflammation: Expert Recommendations. J Am Coll Cardiol. 2018;72(24):3158-3176.
3. Clinical Indications for Cardiac MRI: A Comprehensive Review. Radiographics. 2019;39(4):978-999.
4. Tissue Characterization by Cardiac MRI. Eur Heart J. 2021;42(3):303-316.

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