# 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

| LGE Pattern | Location | Differential Diagnosis |
|------------|----------|----------------------|
| Subendocardial/transmural | Coronary territory | Ischemic cardiomyopathy (MI) |
| Mid-wall | Septal or free wall | Dilated cardiomyopathy, myocarditis, HCM |
| Epicardial | Inferolateral wall | Myocarditis, sarcoidosis |
| Global subendocardial | Circumferential | Amyloidosis |
| Patchy, multifocal | Non-coronary distribution | Sarcoidosis, Anderson-Fabry |
| RV insertion point | Anterior/inferior RV-septal junction | HCM, pulmonary hypertension |

**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.

![Cardiac MRI late gadolinium enhancement showing subendocardial infarction in the LAD territory](images/cardiac-mri-lge-infarct.png)

### 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.

![Cardiac MRI T2 map showing elevated T2 values consistent with myocardial edema in acute myocarditis](images/cardiac-mri-t2-map-myocarditis.png)

### 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.

![Cardiac MRI showing fibrofatty replacement of the RV free wall in ARVC](images/cardiac-mri-arvc-rv.png)

## 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.