Background:Clinically,it is difficult to differentiate multiple acyl-CoA dehydrogenase deficiency (MADD) from immune-mediated necrotizing myopathy (IMNM) because they display similar symptoms.This study aimed to determine whether muscle magnetic resonance imaging (MRI) could be used for differential diagnosis between MADD and IMNM.Methods:The study evaluated 25 MADD patients,confirmed by muscle biopsy and ETFDH gene testing,and 30 IMNM patients,confirmed by muscle biopsy.Muscles were assessed for edema and fatty replacement using thigh MRI (tMRI).Degrees and distribution patts of fatty infiltration and edema in gluteus maximus and thigh muscles were compared.Results:Total fatty infiltration and edema scores (median,[Q 1,Q3]) were 4.00 (1.00,15.00) and 0 (0,4.00) in MADD and 14.50 (8.00,20.75)and 22.00 (16.75,32.00) in IMNM,respectively,which were significantly more severe in IMNM than that in MADD (P =0.000 and P =0.004,respectively).Edema scores for gluteus maximus,long head of biceps femoris,and semimembranosus were significantly higher in IMNM than in MADD (all P =0.000).Fatty infiltration scores for anterior and medial compartments were significantly more severe in IMNM than that in MADD (all P =0.000).Conclusion:Different patts of muscle involvement on tMRI can contribute to differential diagnosis between MADD and IMNM when clinical suspicions alone are insufficient,thereby reducing the need for muscle biopsy.