Measurement of the mean transit time of cerebral circulation by external detection of an intravenously injected radioisotope.

A technique is described in which the mean circulation (transit) time of each hemisphere is determined by means of an intravenous radioisotope technique. A sharp bolus of a rapidly excreted I131 labeled compound is released from an antecubital vein. The time of entrance into the brain blood pool of the densest portion of this bolus is determined by plotting the first derivative of the amount of isotope seen in the brain by a paired external collimation-detection system. The time of exit of the densest portion of the bolus is also similarly determined. The interval between the entrance and exit represents, we believe, the mean cerebral transit time. In 15 normal males transit times were obtained between 6.5 and 10 seconds. The test is virtually painless, repeatable, and exposes the patient to minimal radiation.