[Magnetic resonance imaging in the study of acoustic neurinoma]

Exploration of acoustic neurinoma, the most frequent benign tumor of posterior fossa, now includes magnetic resonance imaging, and an C.G.R. supraconductor apparatus (0.5 Tesla) has been in use since March 1986. This equipment provides high resolution frontal sections, not only of posterior fossa but of the two pontocerebellar angles and even the contents of the two IAC (thin, contiguous 4 mm sections in 256 x 256 matrix). A total of 43 patients were examined, 37 with surgically confirmed acoustic neurinoma and 2 with tumor recurrence. The neurinoma appears on the first echo as a hyposignal in relation to brain stem in sequences predominant in T1 and as a hypersignal in relation to peripontic CSF in sequences predominant in T2. Short sequences predominant in Tl (partial saturation or short ET and RT spin-echos) appear to be most effective for detection of acoustic neurinoma. Comparison of MRI and CT scan imaging showed that firstly, all cases detected by conventional scanography were correctly analyzed by MRI, with superiority for MRI for preoperative screening due to the frontal sections obtained without interference and, secondly, the non-invasive confirmation of diagnosis in doubtful or negative cases requiring computerized gaz meatocisternography. No false negative or false positive results were reported in our series or those documented in the literature using this new imaging technique, but high performance apparatus is necessary (thin sections ans powerful principal magnetic field of at least 0.5 resla). Improvements in this method could be obtained by the availability of more rapid programmes, the use of surface coils and the injection of paramagnetic substances (galodinium chelates) presently under study. NMR imaging is a reliable and non-invasive imaging method capable of replacing the scanner for study of acoustic neurinoma as soon as more apparatuses become available.