| Summary: | <b>(1) Background:</b> Compared to acute myocarditis in the initial phase, detection of subacute myocarditis with cardiac magnetic resonance (CMR) parameters can be challenging due to a lower degree of myocardial inflammation compared to the acute phase. <b>(2) Objectives:</b> To systematically evaluate non-invasive CMR imaging parameters in acute and subacute myocarditis. <b>(3) Methods:</b> 48 patients (age 37 (IQR 28–55) years; 52% female) with clinically suspected myocarditis were consecutively included. Patients with onset of symptoms ≤2 weeks prior to 1.5T CMR were assigned to the acute group (<i>n</i> = 25, 52%), patients with symptom duration >2 to 6 weeks were assigned to the subacute group (<i>n</i> = 23, 48%). CMR protocol comprised morphology, function, 3D-strain, late gadolinium enhancement (LGE) imaging and mapping (T<sub>1</sub>, ECV, T<sub>2</sub>). <b>(4) Results:</b> Highest diagnostic performance in the detection of subacute myocarditis was achieved by ECV evaluation either as single parameter or in combination with T<sub>1</sub> mapping (applying a segmental or global increase of native T<sub>1</sub> > 1015 ms and ECV > 28%), sensitivity 96% and accuracy 91%. Compared to subacute myocarditis, acute myocarditis demonstrated higher prevalence and extent of LGE (AUC 0.76) and increased T<sub>2</sub> (AUC 0.66). <b>(5) Conclusions:</b> A comprehensive CMR approach allows reliable diagnosis of clinically suspected subacute myocarditis. Thereby, ECV alone or in combination with native T<sub>1</sub> mapping indicated the best performance for diagnosing subacute myocarditis. Acute vs. subacute myocarditis is difficult to discriminate by CMR alone, due to chronological connection and overlap of pathologic findings.
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