Regional blood flow in the normal cerebral hemisphere.

Measurement of the rate of clearance of 133Xenon from the brain after injection of this radioisotope into the internal carotid artery has become an accepted method for estimating cerebral blood flow (H0edt-Rasmussen, 1965, 1967; Ingvar, Cronqvist, Ekberg, Risberg, and H0edt-Rasmussen, 1965). Resolution of the clearance curves into two components, fast and slow, may be carried out (H0edtRasmussen, 1965,1967; Ingvar etal., 1965; Wollman, Alexander, Cohen, Stephen, and Zeiger, 1965; Mallett and Veall, 1965). Many workers feel confident that in normal circumstances, at least, the fast component refers to the clearance of 133Xenon from the grey matter, and the slow component refers to its clearance from the white matter (Kety, 1965). If this hypothesis is correct, it means that from a single clearance curve obtained by a gamma-ray detector monitoring cerebral tissue one may ascertain: (1) the blood flow rate in the grey matter; (2) the blood flow rate in the white matter; (3) the relative weights of the perfused grey and white matter in the field of observation of the detector-for example, the cerebral tissue in a particular field of observation might consist of 56% grey matter and 44% white matter; (4) an average flow rate for the cerebral tissue in the field of observation, knowing the perfusion rates and the relative weights of the two component tissues; this is known as the weighted mean flow rate. Recently the gradient of the initial slope of the clearance curve when plotted logarithmically has been used as an index of flow (H0edt-Rasmussen, Skinkoj, Paulson, Ewald, Bjerrum, Fahrenkrug, and Lassen, 1967; Paulson, 1968) and its value as a rapid assessment of flow in the resting state, and during short-lived physiological test situations, has been stressed. In this report the clearance-of 133Xenon from the cerebral hemisphere after a single intracarotid injection has been observed by 16 regional detectors