Relative changes of cerebral arterial and venous blood volumes during increased cerebral blood flow: Implications for BOLD fMRI

Measurement of cerebral arterial and venous blood volumes during increased cerebral blood flow can provide important information regarding hemodynamic regulation under normal, pathological, and neuronally active conditions. I n particular, the change in venous blood volume induced by neural activity is one critical component of the blood oxygenation level-dependent (BOLD) s ignal because BOLD contrast is dependent only on venous blood, not arterial blood. Thus, relative venous and arterial blood volume (rCBV) and cerebral blood flow (rCBF) in alpha -chlorolase-anesthetized rats under hypercapnia were measured by novel diffusion-weighted F-19 NMR following an i.v. admin istration of intravascular tracer, perfluorocarbons, and continuous arteria l spin labeling methods, respectively. The relationship between rCBF and to tal rCBV during hypercapnia was rCBV(total) = rCBF(0.40), which is consiste nt with previous PET measurement in monkeys. This relationship can be linea rized in a CBF range of 50-130 ml/100 g/min as Delta rCBV(total)l Delta rCB F = 0.31 where Delta rCBV and Delta rCBF represent rCBV and rCBF changes, T he average arterial volume fraction was 0.25 at a basal condition with CBF of similar to 60 ml/100 g/min and increased up to 0.4 during hypercapnia. T he change in venous rCBV was P-fold smaller than that of total rCBV (Delta rCBV(vein)Delta rCBF = 0.15), while the arterial rCBV change was 2.5 times larger than that of total rCBV (Delta rCBV/(artery)/Delta rCBF = 0.79), The se NMR results were confirmed by vessel diameter measurements with in vivo videomicroscopy, The absolute venous blood volume change contributes up to 36% of the total blood volume change during hypercapnia, Our findings provi de a quantitative physiological model of BOLD contrast. (C) 2001 Wiley-Liss , Inc.