Muscle imaging in inflammatory myopathies.

Myositides are characterized by perivascular and intrafascicular inflammatory infiltrates and often by fiber de- and regeneration. In chronic disease, muscle size decreases, and replacement of muscle parenchyma by adipose and fibrous tissue (and, in childhood myositis, calcifications of the muscles and subcutaneous fat) occurs. Muscle imaging techniques such as ultrasonography and magnetic resonance (MR) imaging facilitate the assessment of the type (edema, inflammation, fat, fibrosis, and calcifications), degree, and localization of these alterations. Both methods allow evaluation of the activity, chronicity, and severity of myositis and assist in directing the biopsy site. However, MR imaging is more sensitive in the detection of muscle edema, which is common in the early stages of inflammatory myopathies. In general, MR tomographic features provide more information regarding the differential diagnosis than do creatine kinase activity and even electromyography. Muscle imaging techniques should be considered in the diagnostic evaluation and to assist in treatment of inflammatory myopathies. This paper reviews the value and limitations of muscle imaging in inflammatory myopathies.