The idiopathic inflammatory myopathies: spectrum of MR imaging findings.

Magnetic resonance (MR) imaging is useful for demonstrating the soft-tissue and musculature changes seen in patients with idiopathic inflammatory myopathies (IIMs). These changes include edema within and around muscle, subcutaneous reticulation, muscle calcification, and fatty infiltration of muscle. Muscle edema is visible as areas of hyperintensity on short inversion time inversion recovery (STIR) images. Abnormal reticulation of the subcutaneous tissue can be due to subcutaneous edema or an infiltrating process; edema from inflammation appears as areas of low signal intensity on T1-weighted images and as areas of high signal intensity on STIR images. Intramuscular calcium deposition appears as hypointense areas with all pulse sequences. On T1-weighted images, fatty infiltration appears as areas of high signal intensity within muscles. Because of the improved visualization of muscle inflammation provided by STIR imaging and because MR imaging is noninvasive, it has become a useful modality for evaluating patients with IIMs.