After intrathecal administration, the In-111 DTPA is absorbed from the subarachnoid space and the remainder flows superiorly to the basal cisterns within 2 to 4 hours and subsequently will be apparent in the Sylvian cisterns, the interhemispheric cisterns, and over the cerebral convexities. In normal individuals, the it will have ascended to the parasagittal region within 24 hours with simultaneous partial or complete clearance of activity from the basal cisterns and Sylvian regions. In contrast to air, In-111 DTPA does not normally enter the cerebral ventricles