MR and CT correlation of cholesterol cysts of the petrous bone.

Four patients with expansile cystic lesions of the petrous bone had correlative CT and MR scans. Characteristic findings were noted on MR scans obtained with T1- (TR = 400-800, TE = 25-32) and T2- (TR = 2000-3000, TE = 64-80) weighted images. These findings include an expansile cystic lesion centered in the petrous apex with high signal intensity on both the T1- and T2-weighted images, compatible with subacute or chronic hemorrhage. This signal pattern is distinct from the typical intradural epidermoid tumor, which has signal intensities similar to CSF with low signal on T1-weighted images and high signal on T2-weighted images. Surgical exploration yielded similar findings of a cyst containing free-flowing, brown watery fluid. Histologically, an inflammatory response was present as well as a variable number of cholesterol crystals. There was little identifiable capsular tissue but abundant evidence of subacute or chronic hematoma. The nomenclature of the etiology in these four cases is currently in some controversy, with some authors classifying these lesions as epidermoid or primary cholesteatomas while others call them cholesterol granulomas or giant cholesterol cysts. Whatever they are named, the MR image pattern is consistent and is dominated by findings indicative of hemoglobin breakdown products.