Indium In-111 pentetate

After intrathecal administration, the radiopharmaceutical is absorbed from the subarachnoid space as described below, 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 radiopharmaceutical 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, the radiopharmaceutical does not normally enter the cerebral ventricles. Although the primary absorption of cerebrospinal fluid (CSF) into the blood stream occurs at the arachnoid villi, there is some evidence that a significant fraction of CSF is also absorbed across both the cerebral and spinal leptomeninges. Lesser quantities may also be absorbed across the ventricular ependyma. It is also generally held that these alternate routes of CSF absorption may assume primary importance when the major routes of the flow are pathologically obstructed