CEREBRAL [O-15]WATER CLEARANCE IN HUMANS DETERMINED BY PET .1. THEORYAND NORMAL VALUES

When used to measure blood flow in the brain, water leaves a residue i n the vascular bed that influences the estimation of blood flow by cur rent methods. To assess the magnitude of this influence, we developed a two-compartment model of blood flow with separate parameters for tra nsport and vascular distribution of brain water. Maps of the water cle arance, K-1 into brain tissue, separated from the circulation by a mea surably resistant blood-brain barrier (BBB), were generated by time-we ighted integration. Depending on the validity of the assumptions under lying the two-compartment model presented here, the maps revealed a si gnificant overestimation of the clearance of water when the vascular r esidue was ignored. Maps of V-o, the estimate of the apparent vascular distribution volume of tracer (H2O)-O-15, clearly revealed major cere bral arteries. Thus, we claim that the accumulation of radioactive wat er in brain tissue also reflects the volume of the arterial vascular b ed of the brain.