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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 cistern are then reviewed: nonacoustic posterior fossa schwannomas, which have the same MR signal characteristics as the acoustic schwannoma; jugular fossa lesions, the most often encountered being the glomus jugular tumor; and rare intraxial posterior fossa tumors that extend into the cerebellopontine angle.