In a patient with suspected autoimmune cardiac involvement such as myocarditis, pericarditis, cardiac sarcoidosis, or vasculitis, which imaging modality—fluorodeoxyglucose positron emission tomography (FDG‑PET) or cardiac computed tomography (CT)—is preferred to identify active myocardial inflammation?

PET vs CT for Autoimmune Cardiac Manifestations

For suspected autoimmune cardiac involvement, cardiac MRI is the preferred first-line imaging modality, not CT or PET. 1, 2 However, when MRI is contraindicated or unavailable, FDG-PET/CT is superior to standard cardiac CT for detecting active myocardial inflammation, particularly in cardiac sarcoidosis. 1, 3

Primary Recommendation: Cardiac MRI First

Cardiac MRI with tissue characterization (T1/T2 mapping, late gadolinium enhancement, and extracellular volume assessment) should be the initial advanced imaging study for all suspected autoimmune cardiac disease. 1, 2

Why MRI is Superior

• Comprehensive tissue characterization: MRI simultaneously detects myocardial inflammation (edema on T2-weighted imaging, elevated native T1 values), fibrosis (late gadolinium enhancement in characteristic mid-wall/subepicardial patterns), and functional abnormalities in a single examination. 1, 2

• High diagnostic accuracy: In autoimmune myocarditis, MRI demonstrates 91% sensitivity and 91% specificity using native T1 mapping, superior to T2-weighted imaging or LGE alone. 1

• Prognostic value: The presence of LGE predicts higher risk of adverse outcomes including ventricular assist device implantation, cardiac transplantation, or death in autoimmune myocarditis. 2

• Treatment monitoring: MRI quantitatively tracks response to immunosuppression, with measurable reductions in T2 values (e.g., from ~70 ms to ~59 ms) following treatment. 2

• Pattern recognition: MRI distinguishes autoimmune from ischemic disease by characteristic distribution—mid-wall/subepicardial LGE in autoimmune conditions versus subendocardial/transmural patterns in ischemic disease. 1

When to Use FDG-PET/CT Instead of Standard CT

FDG-PET/CT Indications

FDG-PET/CT is particularly useful for cardiac sarcoidosis and should be obtained when:

• Cardiac sarcoidosis is specifically suspected based on systemic disease or high-risk features (ventricular tachycardia, conduction abnormalities, unexplained cardiomyopathy). 1, 3

• MRI is contraindicated (implanted devices, severe claustrophobia, renal dysfunction precluding gadolinium). 1

• Monitoring treatment response in established cardiac sarcoidosis, as FDG uptake reflects active inflammation and guides immunosuppression titration. 3

FDG-PET/CT Advantages Over Standard CT

• Detects active inflammation: FDG uptake identifies metabolically active inflammatory foci, which standard CT cannot visualize. 1, 3

• High-risk stratification: Cardiac inflammation on FDG-PET is a high-risk feature in sarcoidosis that mandates aggressive immunosuppression. 3

• Treatment guidance: Serial FDG-PET studies demonstrate reduction in inflammatory activity with effective therapy. 3

Standard Cardiac CT Has Limited Role

Standard cardiac CT (without FDG) provides only anatomic information and cannot identify active myocardial inflammation. 1

When CT May Be Appropriate

• Coronary artery assessment: CT coronary angiography excludes obstructive coronary disease when ischemia remains a consideration despite clinical suspicion for autoimmune disease. 1

• Anatomic evaluation only: CT can assess cardiac chamber size, wall thickness, and pericardial abnormalities when echocardiography is inadequate and MRI is contraindicated. 1

• Pericardial calcification: CT is superior to MRI for detecting pericardial calcification in constrictive pericarditis. 1

However, CT cannot differentiate active inflammation from chronic fibrosis and provides no tissue characterization for autoimmune myocardial disease. 1

Practical Algorithm for Imaging Selection

Step 1: Order Cardiac MRI with Inflammation Protocol

• Request specific sequences: T2-weighted STIR imaging, native T1 mapping, extracellular volume quantification, and late gadolinium enhancement—not just a "routine cardiac MRI." 1, 2

• Timing matters: Perform MRI early in the disease course, as inflammatory changes may resolve or progress to irreversible fibrosis. 1

Step 2: Add FDG-PET/CT If Sarcoidosis Suspected

• Dual imaging strategy: In suspected cardiac sarcoidosis, obtain both MRI (for fibrosis/scar burden) and FDG-PET (for active inflammation), as they provide complementary information. 1, 3

• Patient preparation critical: Require prolonged fasting (12-18 hours) and high-fat, low-carbohydrate diet before FDG-PET to suppress physiologic myocardial glucose uptake and maximize sensitivity for inflammation. 3

Step 3: Use CT Only When MRI/PET Unavailable

• CT with contrast can show focal or multifocal myocardial enhancement correlating with MRI findings, but lacks the sensitivity and specificity of MRI for autoimmune disease. 1

• CT coronary angiography is appropriate to exclude atherosclerotic disease when clinical presentation is ambiguous. 1

Critical Pitfalls to Avoid

Do Not Rely on Echocardiography and ECG Alone

Echocardiography and ECG frequently miss subclinical myocardial involvement that MRI readily identifies. 2, 4 In systemic autoimmune disease, 46-55% of patients have focal LGE on MRI despite normal or nonspecific echocardiographic findings. 2

Do Not Order "Routine" Cardiac MRI

Standard cardiac MRI protocols without dedicated inflammation sequences (T2-weighted imaging, native T1 mapping, ECV) will miss active autoimmune myocardial involvement. 1, 2 Explicitly request an "inflammation protocol" or "myocarditis protocol."

Do Not Assume FDG-PET Is Specific for Sarcoidosis

FDG uptake can occur in other inflammatory conditions including viral myocarditis, tuberculous myocarditis, and other autoimmune diseases. 5, 6 Clinical context and pattern of uptake are essential for interpretation.

Do Not Delay Imaging in High-Risk Presentations

In patients with new heart failure, ventricular arrhythmias, or high-degree AV block in the setting of known autoimmune disease, obtain cardiac MRI or FDG-PET urgently (within 24-48 hours) as these presentations may represent life-threatening cardiac involvement requiring immediate immunosuppression. 1, 3, 4

Do Not Use CT as a Substitute for Functional/Inflammatory Assessment

Standard cardiac CT provides only anatomic data and cannot guide immunosuppressive therapy decisions. 1 If MRI is contraindicated, FDG-PET/CT—not CT alone—is the appropriate alternative for detecting active inflammation. 1, 3