Osteoid osteoma: value of MR imaging

Osteoid osteoma (OO) is a frequently encountered benign bone tumor, seen in your adults with male predominance. Nine patients complaining of nonspecific extremity pain underwent MRI examination. The sequences obtained were T1 and T2 weighted spin-echo and T2 weighted gradient echo. A CT scan examination followed in all cases, exploring the region of the abnormal signal seen on MRI. The results of both examinations were compared. In six of the nine patients (66.6 %) MRI showed evidence suggestive of osteoid osteoma, comparable that seen on CT scan. In three patients (33.3 %), MRI showed a nonspecific and ill-defined bone marrow signal abnormally. CT cuts focused on those areas of signal abnormality showed the nidus. MRI is more sensitive than CT scan in detecting soft tissue and bone marrow abnormalities adjacent to an osteoid osteoma. This may produce a misleading aggressive appearance on MR images. CT scanning is more specific that MRI, by showing the nidus. In three patients studied, the nidus was only seen by CT, the other six osteoid osteomas were equally seen by CT and by MRI. In our study, MRI revealed abnormalities in all the cases. It was also highly specific for osteoid osteoma in 66.6 %. MRI is very sensitive in detecting bone marrow and soft tissue abnormalities, and can suggest the diagnosis of OO in a good number of patients. In the remainder cases MRI guides the CT-scan. CT is more accurate and remains the definite examination for the diagnosis of OO, by showing the nidus. (authors)