Demonstration of Small Acoustic Neuromas, using Negative Contrast Medium with Tomography

The diagnosis of acoustic neuroma usually does not present a major problem to the radiologist. The bony changes produced in the region of the internal acoustic meatus are well known and can usually be delineated in films of the petrous bone, either in the standard projections or with the aid of tomography (1, 3). Further information may be gained by fractional pneumoencephalography, which also helps to increase the number of cases in which a positive diagnosis can be made with certainty (6). There still remains, however, a small but significant group of patients in whom the above investigations fail to provide a clear-cut answer as to the presence or absence of a tumor, in most instances because of its small size. This report describes an additional method of examining the region of the internal acoustic meatus. It has been used in effective delineation of small acoustic neuromas and so thoroughly visualizes the contents of the cerebellopontine cistern and acoustic meatus that a negative study effectively excludes the presence of a mass. Method The examination is carried out on the Massiot-Philips tomographic table, using the hypocycloid movement. A spinal tap is performed in the usual way, and the patient is then placed prone on the table with a small support under each shoulder to allow for full flexion of the head. The head is flexed until the canthomeatal line is perpendicular to the table-top, with the forehead resting on the table, and then rotated 15–20 degrees toward the side to be examined in such a way that the corresponding cerebellopontine angle cistern is slightly elevated (Fig. 1). The optimal degree of rotation varies; in general we have found that the shorter the internal auditory canal, the less the required degree of rotation. Once the head is positioned correctly it must be fixed by a clamp or some other means so that the position remains the same throughout the study. The tomographic table is tilted head-up to bring the posterior fossa to a slightly higher level than the cervical and upper thoracic spine. Tomographic scout films are taken to establish the plane of the internal auditory canal. Seven milliliters of air is then injected into the subarachnoid space. The air will rise into the cerebellopontine cistern on the side being examined and will remain trapped there. A minute or two after the injection, tomographic films are taken to cover a depth of approximately 10 mm on either side of the internal auditory meatus. If the patient is correctly positioned the cerebellopontine angle cistern, internal auditory meatus and canal, and the lateral aspect of the belly of the pons should be clearly demonstrated (Fig. 2).