Volumetric muscle composition analysis in sporadic inclusion body myositis using fat‐referenced magnetic resonance imaging: Disease pattern, repeatability, and natural progression

Abstract Introduction/Aims Fat‐referenced magnetic resonance imaging (MRI) has emerged as a promising volumetric technique for measuring muscular volume and fat in neuromuscular disorders, but the experience in inflammatory myopathies remains limited. Therefore, this work aimed at describing how sporadic inclusion body myositis (sIBM) manifests on standardized volumetric fat‐referenced MRI muscle measurements, including within‐scanner repeatability, natural progression rate, and relationship to clinical parameters. Methods Ten sIBM patients underwent whole‐leg Dixon MRI at baseline (test–retest) and after 12 months. The lean muscle volume (LMV), muscle fat fraction (MFF), and muscle fat infiltration (MFI) of the quadriceps, hamstrings, adductors, medial gastrocnemius, and tibialis anterior were computed. Clinical assessments of IBM Functional Rating Scale (IBMFRS) and knee extension strength were also performed. The baseline test–retest MRI measurements were used to estimate the within‐subject standard deviation (s w ). 12‐month changes were derived for all parameters. Results The MRI measurements showed high repeatability in all muscles; s w ranged from 2.7 to 18.0 mL for LMV, 0.7–1.3 percentage points (pp) for MFF, and 0.2–0.7 pp for MFI. Over 12 months, average LMV decreased by 7.4% while MFF and MFI increased by 3.8 pp and 1.8 pp, respectively. Mean IBMFRS decreased by 2.4 and mean knee extension strength decreased by 32.8 N. Discussion The MRI measurements showed high repeatability and 12‐month changes consistent with muscle atrophy and fat replacement as well as a decrease in both muscle strength and IBMFRS. Our findings suggest that fat‐referenced MRI measurements are suitable for assessing disease progression and treatment response in inflammatory myopathies.