To analyze the change in muscle damage under different imaging sequences using muscle MRI in patients of necrotizing myopathy, and to analyze the distribution of muscle damage. Methods: We recruited 16 patients with immune-mediated necrotic myopathy (IMNM) who were confirmed positive for SRP antibodies, 14 patients with polymyositis, and 18 healthy individuals of similar age and sex with no underlying illness. All subjects underwent thigh muscle MRI scans using T2 mapping and iterative decomposition of water and fat with echo asymmetric and least-squares estimation (IDEAL) to quantitatively measure values in regions of edema in muscles of both thighs. The values of T2 mapping and IDEAL were compared between the IMNM, polymyositis, and healthy control groups and between the muscles within each group. Results: The T2 values of the IMNM and polymyositis groups were higher compared with that of the control group (P=0.021, 0.012). The IDEAL value of the IMNM group was higher than that of the control group (P=0.039); the values showed no significant difference when compared between other groups (all P>0.05). When comparing the T2 mapping of individual muscle groups between the IMNM and control groups, statistical difference was found in the vastus lateralis, vastus intermedius, long head of the biceps femoris, and semimembranosus; comparing the polymyositis and control groups, statistical difference was found in the vastus lateralis, vastus intermedius, vastus medialis, rectus femoris, and semimembranosus. When comparing the IDEAL of individual muscle groups between the IMNM and control groups, statistical difference was found in the vastus lateralis, vastus medialis, and semimembranosus. Conclusion: This study demonstrates the use of the non-invasive MR sequence to quantitatively evaluate the characteristics and distribution of muscle involvement in IMNM and provides a possible approach for the identification of other myopathy types.