What is the recommended imaging modality for diagnosing myocarditis?

Imaging for Myocarditis

Cardiac MRI (CMR) is the recommended imaging modality for diagnosing myocarditis, as it provides superior tissue characterization to detect myocardial inflammation, edema, and necrosis with high diagnostic accuracy. 1

Initial Imaging Approach

Transthoracic echocardiography (TTE) should be performed first in all patients with suspected myocarditis to assess:

• Ventricular systolic and diastolic function
• Regional wall motion abnormalities
• Pericardial effusion
• Valvular abnormalities
• Differentiation between fulminant myocarditis (severe dysfunction with thickened walls), acute non-fulminant myocarditis, or dilated cardiomyopathy 1, 2

However, echocardiography lacks the sensitivity and specificity to definitively diagnose myocardial inflammation, making it insufficient as a standalone diagnostic test. 1

Cardiac MRI as the Definitive Diagnostic Test

Why CMR is Superior

CMR evaluates the tissue characteristics of myocardial inflammation through multiple sequences that detect:

• Myocardial edema (T2-weighted imaging or T2-mapping)
• Hyperemia and capillary leak (early gadolinium enhancement)
• Myocardial necrosis or scarring (late gadolinium enhancement with characteristic subepicardial or midwall pattern) 1, 3

CMR impacts clinical decision-making in >50% of patients with suspected myocarditis and provides a new diagnosis in 11% of patients. 1, 3

Diagnostic Criteria: Lake Louise Criteria

The updated 2018 Lake Louise Criteria require:

• At least one T2-based marker (T2-weighted imaging OR T2-mapping) showing myocardial edema
• At least one T1-based marker (abnormal T1-mapping, extracellular volume [ECV], OR late gadolinium enhancement) showing myocardial injury 1, 3

The original 2009 Lake Louise Criteria (requiring any 2 of 3 findings: T2 edema, early gadolinium enhancement, or late gadolinium enhancement) remain valid and should continue to be used in centers with good experience in their application, as they provide good overall diagnostic performance with 82% sensitivity in pediatric populations. 1, 4

Diagnostic Performance

Native T1-mapping demonstrates the highest accuracy with 90% sensitivity, 91% specificity, and 91% accuracy—superior to T2-weighted imaging and late gadolinium enhancement alone. 1, 3

The combination approach (any 2 of 3 criteria positive) yields:

• 76% sensitivity
• 95.5% specificity
• 85% diagnostic accuracy 4

Critical Timing Considerations

CMR has significantly higher sensitivity in acute myocarditis (within 14 days of symptom onset) with 81% diagnostic accuracy, compared to only 45% accuracy in chronic myocarditis (>14 days from symptom onset). 3, 5

For chronic myocarditis, increased global relative enhancement and edema ratio remain common findings that correlate with immunohistologic inflammation, but late gadolinium enhancement has low sensitivity (27%) and accuracy (49%). 5

Clinical Utility Beyond Diagnosis

CMR provides critical prognostic information:

• Late gadolinium enhancement in biopsy-proven viral myocarditis predicts subsequent risk of ventricular arrhythmias and cardiovascular death 3
• CMR can localize inflammatory changes to guide endomyocardial biopsy when needed, reducing sampling errors 3
• CMR differentiates ischemic from non-ischemic causes by the pattern of enhancement (epicardial/midwall in myocarditis vs. endocardial in ischemic disease) 1, 3

Alternative and Adjunctive Imaging

FDG-PET/CT may be useful specifically for cardiac sarcoidosis evaluation in acute presentations, but is not commonly used for routine myocarditis diagnosis. 1

Cardiac CT can show focal or multifocal enhancement correlating with MRI findings, but lacks the tissue characterization capabilities of CMR. 1

Advanced echocardiographic techniques (speckle tracking with global longitudinal strain, myocardial contrast echocardiography) can detect inflammation but cannot differentiate inflammation from other causes of strain reduction and have limited direct visualization of inflammation compared to CMR. 2

Important Caveats

• In patients with severe renal dysfunction (eGFR <30 mL/min/1.73 m²), non-contrast CMR is reasonable for describing location, size, and hemodynamic effects, but may be less useful for tissue characterization. 1
• T2-mapping is more specific for acute inflammation, while T1-mapping is also sensitive to chronic changes like scarring or expanded extracellular space. 3
• Removing early gadolinium enhancement from the original Lake Louise Criteria does not significantly reduce diagnostic accuracy, though it may slightly lower the positive likelihood ratio. 1