Muscle imaging in myositis: MRI, US, and PET

Imaging is an important tool in the evaluation of idiopathic in-flammatory myopathies. It plays a role in diagnosis, assessment of disease activity and follow-up, and as a non-invasive biomarker. Among the different modalities, nuclear magnetic resonance imaging (MRI), ultrasound (US), and positron emission tomog-raphy (PET) may have the most clinical utility in myositis. MRI is currently the best modality to evaluate skeletal muscle and pro-vides excellent characterization of muscle edema and fat replacement through the use of T1-weighted and T2-weighted fat suppressed/STIR sequences. Although MRI can be read qualita-tively for the presence of abnormalities, a more quantitative approach using Dixon sequences and the generation of water T2 parametric maps would be preferable for follow-up. Newer pro-tocols such as diffusion-weighted imaging, functional imaging measures, and spectroscopy may be of interest to provide further insights into myositis. Despite the advantages of MRI, image acquisition is relatively time-consuming, expensive, and not accessible to all patients. The use of US to evaluate skeletal muscle in myositis is gaining interest, especially in chronic dis-ease, where fat replacement and fibrosis are detected readily by this modality. Although easily deployed at the bedside, it is heavily dependent on operator experience to recognize disease states. Further, systematic characterization of muscle edema by US is still needed. PET provides valuable information on muscle function at a cellular level. Fluorodeoxyglucose (FDG-PET) has been the most common application in myositis to detect patho-logic uptake indicative of inflammation. The use of neurodegen-erative markers is now also being utilized for inclusion body myositis. These different modalities may prove to be comple-mentary methods for myositis evaluation.(c) 2022 Elsevier Ltd. All rights reserved.