MAGNETIC-RESONANCE-IMAGING AND COMPUTED-TOMOGRAPHY OF THE INTERNAL AUDITORY-CANAL AND CEREBELLOPONTINE ANGLE

Computed tomography (CT) and magnetic resonance imaging (MRI) assessment of internal auditory canal lesions focuses on acoustic neurinomas (Schwannomas), a small percentage of which are entirely intracanalicular. MR is now the imaging method of choice and allows for the detection of intracanalicular tumors as small as 3 mm. Acoustic neurinomas are isointense relative to the pons on MR T1-weighted images, mildly hyperintense on MR T2-weighted images, and enhance intensely after i.v. administration of gadolinium-DTPA. The radiologic evaluation of the cerebellopontine angle first addresses lesions of the angle itself, other than acoustic neurinomas. On CT, meningiomas show calcifications in 25% of cases and homogeneous enhancement in 90%; on MR they demonstrate homogeneous gadolinium-DTPA enhancement. Epidermoids do not enhance on MR. Cholesterol granulomas are strongly hyperintense on MR T1- and T2-weighted images. Rare vascular lesions may mimic neoplasm in the posterior fossa and in the cerebellopontine angle: vertebral basilar dolichoectasia, vascular loop or aneurysm of the anterior inferior cerebellar artery. CT and MR characteristics of lesions extending into the cerebellopontine angle cistem are then reviewed: nonacoustic posterior fossa schwannomas, which have the same MR signal characteristics as the acoustic schwarmoma; jugular fossa lesions, the most often encountered being the glomus jugular tumor; and rare intraxial posterior fossa tumors that extend into the cerebellopontine angle.