Radioisotope scanning in posterior fossa lesions.

It has been alleged that radioisotope scanning of posterior fossa lesions is of little value (1-8) or at best, difficult (9,10) and frequently unrewarding (11). Some clinics even report the virtual abandonment of posterior fossa scanning (12,13). On the other hand, there are those who maintain that scanning of the posterior fossa is a worthwhile adjunct in the evaluation of lesions in that re gion (14-19). Two of the studies (17, 18) employed multifocal probe scanning as opposed to conventional rectilinear scanning. Much of the prejudice directed against posterior fossa scanning is of long standing and is felt to have originated in the period when brain scanning was a relatively new technique. It was during this era when iodinated human serum albumin was the scanning medium commonly employed, and when less sophis ticated equipment was available for use. Both of these factors contributed to poor definition of lesions in an area where anatomical factors compounded the difficulties of interpretation. In addition to the confluence of the sinuses at the torcular, the transverse or lateral sinuses traverse the posterior fossa adding con fusing vascular pickup. The statistical contribution of these vascular structures must be evaluated in the study of posterior fossa disease. With improved scanning media, the mercurials and more recently numTc, the vascular structures are more sharply delineated. Their delineation is enhanced by the improved technical depiction afforded by today's modern scanning equipment. A normal lateral scan, with the posterior fossa landmarks indicated, is shown in Fig. 1. Inherent in the diagnosis of a posterior fossa lesion is its demonstration on at least two projections. In addition to the appropriate lateral view, a posterior scan should be obtained. Posterior fossa disease will often be obscured or missed on the anterior scan.