Microanatomy of the cerebellopontine angle and internal auditory canal: study with new magnetic resonance imaging technique using three-dimensional fast spin echo.

Abstract

We report a new magnetic resonance imaging technique that uses three-dimensional fast spin echo and the minimum intensity projection method. Using this technique, detailed images of the cerebellopontine angle (CPA) and internal auditory canal (IAC) were obtained in normal volunteers and in patients with acoustic neuromas or hemifacial spasm. Ten normal volunteers, 44 patients with acoustic neuromas, and 31 patients with hemifacial spasm were studied using the three-dimensional fast spin echo magnetic resonance imaging protocol. The CPA and IAC were scanned by using a 1-mm slice thickness in the axial and parasagittal planes. Normal anatomy was as follows. 1) The vestibulocochlear nerve was ovoid near the brain stem and changed to a slightly crescentic configuration (C shape) as it traveled laterally. 2) Separation of the cochlear and vestibular nerves was observed near the central part of the IAC. 3) Discrimination between the superior and inferior vestibular nerves was also possible near the fundus of the IAC. 4) The facial nerve was easily identifiable as a discrete nerve at the anterior aspect of the vestibulocochlear nerve. 5) The meatal loop of the cerebellar artery was located medial to the porus in 44% of 95 CPAs and reached the porus or protruded into the porus in 56%. Acoustic neuromas were as follows. 1) In a patient with a very small intracanalicular tumor, the nerve on which the tumor was located could be identified. 2) In 22 of 44 acoustic neuromas, cerebrospinal fluid was present between the tumor and the fundus of the IAC. Hemifacial spasm was as follows. The relationship between the responsible artery and the facial nerve could be precisely observed. The three-dimensional fast spin echo method offers ultrahigh-resolution images, which are extremely useful in understanding the surgical anatomy of the CPA and IAC.