Osteoid osteoma: MR imaging revisited.

To assess and compare with computed tomography (CT) the performance of magnetic resonance (MR) imaging in the detection of osteoid osteoma, and determine the features of this lesion on MR imaging. The prospective MR imaging and CT diagnosis of osteoid osteoma was determined from original radiology reports. MR images were assessed retrospectively with regard to the location and signal intensity of the nidus and surrounding bone marrow and soft tissue edema. These findings were correlated with the age of the patient, duration of symptoms, and drug therapy. Ten patients with histologically proven osteoid osteoma who underwent MR imaging were reviewed. All 10 lesions were correctly diagnosed at the time of MR imaging. None of the lesions was intracortical. Nine lesions were intra-articular. Two out of five patients with extracortical lesions had false negative CT preceding the MR study. Signal intensity of the nidus, marrow, and soft tissue edema on MR imaging were variable. Perinidal edema was most pronounced in younger patients and had no apparent relation to drug therapy. MR imaging reliably demonstrates the nidus of an osteoid osteoma, which has a variable appearance related to its position relative to the cortex of the bone. A predominance of cancellous osteoid osteomas are encountered in patients referred for MR imaging. Marrow edema in the vicinity of the lesion improves the conspicuity of the nidus. CT may fail to diagnose osteoid osteoma when the nidus is in a cancellous location, due to the lack of perinidal density alteration.