Summary:We measured the rate of washout of 150_ labeled water generated from labeled oxygen accumu lated in brain after bolus [150]02 inhalation, and com pared the washout with that of labeled water measured with H2150. Contrary to the original expectation, the ra dioactive water generated from labeled oxygen failed to leave the brain tissue at the rate predicted by exogenous water. Therefore, the use of a separately measured value for exogenous water clearance led to an error in the cal culation of oxygen consumption. A new method pre sented in this paper eliminated the error by yielding ox ygen consumption in a single oxygen study. We used Measurements of the oxygen consumption of in dividual brain regions have been possible only after the advent of positron emission tomography (PET). Two methods have been reported, both intended to yield the unidirectional clearance of the very short lived oxygen isotope, oxygen-15. The equilibrium method (Frackowiak et aI., 1980) yielded the oxy gen clearance from the steady-state balance be tween oxygen influx, radioactive oxygen decay, and radioactive water efflux. The autoradiographic method (Mintun et aI., 1984) yielded the clearance from a transient (non-steady-state) analysis of the uptake. Both methods required separately obtained information about blood flow and blood volume and assumed, therefore, that the physiological state of the subject, and its position in the scanner, re mained unchanged for 30-60 min, the time required Abbreviation used: CBV, cerebral blood volume; CMROz, ce rebral metabolic rate for oxygen; FWHM, full width at half max imum; OEF, oxygen extraction fraction; PET, positron emission tomography; RMS, root mean squared.
179time-weighted integration to estimate three parameters, including the unidirectional clearance from blood to brain (K? 2 ), the fractional clearance of the distribution volume in brain (k�2), and the vascular volume correction (yo) ' We showed that the clearance of oxygen from blood to brain can be estimated with acceptable precision by this new approach, and that the new method yields a reliable measure of oxygen consumption. Key Words: Positron emission tomography-Oxygen consumption-U nidirec tional clearance of oxygen-Single oxygen study-Time weighted integration method.to perform three separate studies by positron emis sion tomography. In addition, both methods de pended on several simplifying assumptions which may cast doubt on the results. However, the uncer tainty is not easy to resolve because of the absence of independently obtained regional values that might serve as the ultimate standard.To examine the consequences of the specific an alytic method for the calculated oxygen consump tion values, we modified the experimental approach to yield values of oxygen consumption from a single short (3 min) bolus e50]02 inhalation study, elimi nating some assumptions but introducing others.