[Magnetic resonance imaging in the diagnosis of myositis ossificans circumscripta]

A retrospective study of 5 cases, 4 of myositis ossificans circumscripta (MOC) and 1 of non-ossificans myositis, is presented. The clinical presentation was a painful soft-tissue swelling, and the final diagnosis was established by biopsy in 3 cases and clinical evolution in 2 cases. Different types of imaging techniques were performed: 4 MRI, 2 arteriographies, 3 Tc 99m scintigraphies, 3 US, 3 CT and plain film radiographs for all patients. MOC is a benign process (as opposed to myositis ossificans progressiva, which is an inherited pathology with a fatal prognosis) with 3 phases of evolution: an acute or pseudo-inflammatory phase, a sub-acute or pseudo-tumoral phase and a chronic phase with a spontaneous healing. The radiologic diagnostic findings are dependent of the phase of the disease. The calcifications are seen earlier with scintigraphy than plain films, with US being less helpful. A heterogenous mass and calcifications are seen with CT. MRI allows the characterization of oedema during the acute phase and sometimes can exclude a malignant process. MRI is the best method for an early diagnosis, the differential diagnosis and to follow the evolution.