Background : Although myocarditis has significant clinical consequences such as sudden death and dilated cardiomyopathy, its diverse clinical presentations and the lack of safe and accurate diagnostic tools have limited further progress in its evaluation and management. To address these issues, we developed an approach of cardiac magnetic resonance imaging (CMR) coupled with long-circulating magneto-fluorescent nanoparticles (MNP) and studied the feasibility and efficiency in detecting the status and assessing the grade of inflammation in experimental autoimmune myocarditis (EAM) rats. Methods and Results : EAM-induced Lewis rats were classified according to post-immunization days (days 11, 14, 18, and 20; n=9 in each group) and underwent CMR (pre-CMR). Afterwards, EAM and control rats were intravenously injected with a macrophage-avid MNP (Fe 12.5 mg/kg, n=36 in EAM rats, n=6 in control) and then taken repeated CMR 24 hour (24h-CMR) after injection. Control rats showed no difference in the contrast-to-noise ratio (CNR, myocardium versus pectoral muscle) after MNP injection between pre-CMR and 24h-CMR. However, 24h-CMR of EAM rats revealed increased CNR compared to MNP-injected controls (75짹12 vs 14짹8, p<0.001), which matched well with macrophage contents in immunohistochemical stainings of ex vivo specimens. In addition, the CNRs of 24h-CMR in each EAM rat correlated significantly with the fluorescent intensity of ex-vivo fluorescence reflectance imaging (p=0.002) and macrophage-immunostaining in confocal microscopy (p=0.01). The EAM rats with higher grade of inflammation in histology showed greater CNR than those with lesser grade (grades II, III vs I, II, 62짹20 vs 18짹12, p=0.001). Conclusions : This novel approach of CMR combined with MNP provides the feasibility and efficiency to noninvasively image, track, and quantify myocardial inflammation in EAM rats. These results support the potential of MNP-combined CMR as a valuable tool in the research and clinical applications of myocarditis.