SUMMARY Regional cerebral blood flow (rCBF) was simultaneously measured at seven locations in each hemisphere by the Obrist '"xenon inhalation method. In each of 35 healthy righthanded subjects two rest measurements were performed on consecutive days. The data analysis comprises the two-compartmentally derived parameters for flow (f,), relative tissue weight (w,), and fractional flow (FF,) respectively of the first compartment, and in addition the initial slope index (ISI). At each detector location the coefficient of variation (C.V.) of the change from first to second measurement was on average 10.4% (ISI), 14.2% (f,), 7.2% (w,), and 2.9% (FF,) respectively. However, when each regional measurement was expressed as a percentage of its hemispheric mean, the C.V. of the intermeasurement change was on average 4.4% (ISI), 7.0% (f,), 7.7% (w,), and 1.9% (FF,) respectively; that of the hemispheric means of ISI, f,, and FF, was found to be distinctly larger, whereas that of w, was about equal in size. The interhemispheric coefficient of variation for the change of the hemispheric means from first to second measurement was only 2.3% (ISI), 4.4% (f,), 1.6% (w,), and 1.1% (FF,) respectively.The findings suggest that (1) the variability of rCBF from subject -to subject and in consecutive measurements in the same subject is to a substantial degree of physiological origin, and that (2) there are two determinants of rCBF which may operate independently: a determinant of the hemispheric mean level, probably a single determinant for both hemispheres, and a set of determinants for each separate region superimposed on the hemispheric mean level.

This paper introduces a new index for the assessment of regional cerebral blood flow. The index is proportional to total flow, and is obtained from the ratio of regional count rate to arterial indicator input to a region. This index is a more sensitive indicator of impaired perfusion than the traditional flow rate indices which express flow per unit mass of tissue per minute. It accounts for brain tissue partly or totally deprived of its blood supply. Examples of clinical application are reported. A good correlation with the findings of computer-assisted tomography has been found.

SUMMARY When the133 Xe inhalation method is employed for measuring regional cerebral blood flow, the arterial 133 Xe concentration is usually approximated by the end-tidal air concentration. However, this approximation may be invalid in the presence of certain lung pathologies or when the breathing pattern is irregular. Jaggi and Obrist, using an intravenous injection of Xe, suggested that the counts detected by an external lung probe could provide an alternative estimate for arterial blood concentration once the noise produced by Xe in superficial tissues is removed from the signal. A mathematical model, based on hypotheses similar to theirs is presented here together with a new computational procedure for removing the noise. Results from normal rest studies on ten healthy young males indicate that the approximations for arterial blood concentration obtained from end-tidal air and from corrected lung counts are not equivalent when l33 Xe is administered by inhalation. The concentration-time curves have different shapes, and these differences are reflected in blood flow values computed by head channel. However, there is no effect on comparisons between homologous regions of the left and right hemispheres.