Persistent Focal Myocardial 18F-FDG Uptake Discordant with CMR Oedema on Hybrid PET/MRI Six Months After Acute Myocarditis

Background: Hybrid positron emission tomography/magnetic resonance imaging (PET/MRI) enables simultaneous assessment of myocardial metabolism and CMR tissue characteristics. We investigated whether integrated 18F-FDG PET/MRI could identify persistent metabolic abnormalities approximately 6 months after acute myocarditis, including abnormalities discordant with concomitant CMR oedema markers. Methods: In this retrospective single-centre study, 20 consecutive patients with a previous episode of acute myocarditis underwent integrated cardiac 18F-FDG PET/MRI approximately 6 months after the acute event. Focal myocardial FDG uptake, myocardial SUVmax and SUVmean, target-to-background ratios, T2 mapping, late gadolinium enhancement (LGE), and ventricular function were assessed within the hybrid examination. Results: Hybrid PET/MRI was performed at a median of 202 days (interquartile range [IQR], 173–221) after the acute episode. Four patients (20%) had non-diagnostic PET because of inadequate myocardial glucose suppression. Among 16 evaluable patients, four (25%) remained PET-positive with focal myocardial FDG uptake. Their myocardial SUVmax was 2.85 (IQR, 2.58–3.25) versus 1.65 (IQR, 1.38–1.80) in PET-negative patients (p = 0.001), and the myocardium-to-blood-pool ratio was 1.03 (IQR, 1.00–1.19) versus 0.82 (IQR, 0.75–0.86), respectively (p = 0.003). None of the four PET-positive patients had myocardial oedema on concomitant T2 mapping, whereas all had persistent LGE. One PET-negative patient showed persistent CMR oedema. Conclusions: Integrated PET/MRI identified persistent focal metabolic abnormalities in 25% of evaluable patients approximately 6 months after acute myocarditis, despite absent concomitant CMR oedema. Although the cohort is small and contemporary longitudinal clinical characterisation is limited, these hybrid-imaging findings support further investigation of PET/MRI as a tool for phenotyping the post-acute phase of myocarditis.