Arterial fraction of cerebral blood volume in humans measured by positron emission tomography

In quantitative functional neuroimaging with positron emission tomography ( PET) and magnetic resonance imaging (MRI), cerebral blood volume (CBV) and its three components, arterial, capillary, and venous blood volumes are imp ortant factors. The arterial fraction for systemic circulation of the whole body has been reported to be 20-30%, but there is no report of this fracti on in the brain. In the present study, we estimated the arterial fraction o f CBV with PET in the living human brain. (CO)-O-15 and dynamic (H2O)-O-15 PET studies were performed in each of seven healthy subjects to determine t he CBV and arterial blood volume (V-a), respectively. A two-compartment mod el (influx: K-1, efflux: k(2)) that takes V-a into account was applied to d escribe the regional time-activity curve of dynamic (H2O)-O-15 PET. K-1. k( 2) and V-a were calculated by a non-linear least squares fitting procedure. The V-a and CBV values were 0.011 +/- 0.004 ml/ml and 0.031 +/- 0.003 ml/m l(mean +/- SD), respectively, for cerebral cortices. The arterial fraction of CBV was 37%. Considering the limited first-pass extraction fraction of ( H2O)-O-15, the true arterial fraction of CBV is estimated to be about 30%. The estimated arterial fraction of CBV was quite similar to that of the sys temic circulation, whereas it was greater than that (16%) widely used for t he measurement of cerebral metabolic rate of oxygen (CMRO2) using PET. The venous plus capillary fraction of CBV was 63-70% which is a important facto r for the measurement of CMRO2 with MRI.