Purpose To evaluate multiparametric cardiac MRI for assessing myocardial inflammation, fibrosis, and spatiotemporal distribution in autoimmune myocarditis, with histologic reference. Materials and Methods This study (December 2022 to January 2024) included 84 BALB/c mice (42 with experimental autoimmune myocarditis, 42 controls). Multiparametric cardiac MRI, including T1 mapping, T2 mapping, and extracellular volume fraction (ECV) mapping, was performed at 14 days, 28 days, and 3 months after immunization using a 9.4-T scanner. Cardiac tissues were collected at each time point for histopathologic assessment of inflammation (inflammatory index) and fibrosis (collagen volume fraction). Cardiac MRI parameters were analyzed at global, regional, and segmental levels to evaluate temporal and spatial changes. Group comparisons were performed using one-way analysis of variance with Tukey post hoc tests or Student t tests, and associations were assessed using Pearson or Spearman correlation coefficients, as appropriate. Results Among 81 mice analyzed, experimental autoimmune myocarditis (n = 39) demonstrated progression from inflammation to fibrosis. T2 values were elevated on days 14 and 28 (P < .001) and normalized at 3 months. Precontrast T1 and ECV increased progressively. T2 correlated with the inflammatory index (rho = 0.72, P < .001), and ECV correlated with collagen volume fraction (r ≥ 0.70, P < .01). T2 mapping showed the highest diagnostic performance on day 14 (area under the receiver operating characteristic curve [AUC], 0.89; 95% CI: 0.72, 0.99; P < .001) and day 28 (AUC, 0.81; 95% CI: 0.60, 0.97; P = .01), whereas ECV performed best at 3 months (AUC, 0.85; 95% CI: 0.69, 0.99; P < .001). Spatial distribution of abnormalities was heterogeneous across disease stages. Conclusion Multiparametric cardiac MRI characterizes myocardial inflammation and fibrosis and demonstrates stage-dependent spatial patterns in autoimmune myocarditis. Keywords: Animal Studies, MR-Imaging, Cardiac, Myocardium, Tissue Characterization Supplemental material is available for this article. © RSNA, 2026.