MR-IMAGING OF MYOSITIS-OSSIFICANS - VARIABLE PATTERNS AT DIFFERENT STAGES

Five patients with a palpable mass at presentation underwent magnetic resonance (MR) imaging. The final diagnosis was myositis ossificans (M O). MR imaging features, particularly after injection of gadopentetate dimeglumine, mimicked those of an inflammatory mass or neoplasm. The lesions were excised in three patients, and the images were correlated with histologic findings. Three different appearances were noted on M R images, corresponding to the stages of maturation of MO. Two cases i nvolved early-stage lesions, and T1-weighted MR images showed a mass w ith homogeneous intermediate signal intensity. Both lesions showed rim enhancement after contrast agent injection and high signal intensity on T2-weighted images. Pathologic specimens demonstrated stroma with m asses of spindle cells in which osteoid production was interspersed. T he enhanced rim of the lesion mimicked the expected MR appearance of a n abscess or necrotic tumor. Areas of enhancement in adjacent muscle w ere also seen on postcontrast T1-weighted images. Intermediate-stage M O was present in one case; there was evidence of a thin rim of calcifi cation on plain radiographs and fatty changes in the lesion on T1-weig hted images, corresponding with histologic findings. One case of a mat ure lesion showed a considerable degree of peripheral calcification bo th on MR images and at histology. MR imaging is nonspecific in the dia gnosis of early-stage MO.