Measurement of Regional Blood Flow in Cerebral Cortex of Man through Intact Skull Regional blood flow in the cerebral cortex has been measured in man by Lassen and Ingvar (1961) and Ingvar and Lassen (1961) using the krypton-85 clearance method developed by them. Krypton-85, dissolved in saline, is injected over several minutes into the carotid artery. The gas diffuses almost equally between the blood and the brain. On cessation of the injection the krypton diffuses back into the blood, and the rate at which it is cleared from the brain tissue will depend on the quantity of blood perfusing the brain. There is no significant recirculation of the gas, as it is almost completely excreted in the lungs. Since krypton-85 emits mainly beta particles with a range in tissue averaging 0.7 mm., this method can be used only on the exposed brain cortex, and so its clinical applications could only be limited. Ingvar and Lassen (1962) have suggested that this disadvantage could be