Imaging of myositis: An overview and update

Idiopathic inflammatory myopathies (IIMs) are heterogeneous autoimmune disorders that affect skeletal muscle and may involve the skin, lungs, heart, gastrointestinal tract, or be associated with malignancy. Imaging supports phenotyping, biopsy targeting, assessment of disease burden, and monitoring. Magnetic resonance imaging (MRI) remains the principal modality for muscle assessment, depicting edema, fascial and subcutaneous inflammation, fatty replacement, and atrophy without ionizing radiation. Whole-body MRI maps disease distribution, whereas quantitative techniques, including Dixon fat-fraction imaging and T2 mapping, provide objective measures of activity and chronic damage. Ultrasound offers a portable, inexpensive adjunct, although operator dependence and limited standardization restrict comprehensive assessment. For nuclear medicine, 18F-fluorodeoxyglucose (FDG) PET/CT provides a complementary whole-body measure of metabolism, potentially identifying muscle inflammation, interstitial lung disease, other systemic manifestations, and occult malignancy in selected patients. Physiologic muscle uptake and heterogeneous acquisition and quantification protocols limit specificity and prevent universal PET thresholds. Hybrid PET/MRI combines metabolic and structural characterization, but evidence supporting routine use remains limited. Fibroblast activation protein inhibitor (FAPI) PET is an emerging pathway-targeted approach with preliminary evidence in muscle and myositis-associated lung disease. This review summarizes imaging patterns across major myositis phenotypes, contrasts established and investigational uses of MRI, ultrasound, FDG PET/CT, PET/MRI, and emerging tracers, illustrates important mimics and pitfalls, and proposes a practical multimodality imaging approach.