Positron Emission Tomographic Measurement of Cerebral Blood Flow and Permeability—Surface Area Product of Water Using [15O]Water and [11C]Butanol

Abstract

We have previously adapted Kety's tissue autoradiographic method for measuring regional CBF in laboratory animals to the measurement of CBF in humans with positron emission tomography (PET) and H215O. Because this model assumes diffusion equilibrium between tissue and venous blood, the use of a diffusion-limited tracer, such as H215O, may lead to an underestimation of CBF. We therefore validated the use of [11C]butanol as an alternative freely diffusible tracer for PET. We then used it in humans to determine the underestimation of CBF that occurs with H215O, and thereby were able to calculate the extraction Ew and permeability–surface area product PSw of H215O. Measurements of the permeability of rhesus monkey brain to [11C]butanol, obtained by means of an intracarotid injection, external detection technique, demonstrated that this tracer is freely diffusible up to a CBF of at least 170 ml/min-100 g. CBF measured in baboons with the PET autoradiographic method and [11C]butanol was then compared with CBF measured in the same animals with a standard residue detection method. An excellent correspondence was obtained between both of these measurements. Finally, paired PET measurements of CBF were made with both H215O and [11C]butanol in 17 normal human subjects. Average global CBF was significantly greater when measured with [11C]butanol (53.1 ml/min-100 g) than with H215O (44.4 ml/min-100 g). Average global Ew was 0.84 and global PSw was 104 ml/min-100 g. Regional measurements showed a linear relationship between local PSw and CBF, while Ew was relatively uniform throughout the brain. Simulations were used to determine the potential error associated with the use of an incorrect value for the brain–blood partition coefficient for [11C]butanol and to calculate the effect of tissue heterogeneity and errors in flow measurement on the calculation of PSw.