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. In particular, the change in venous blood volume induced by neural activity is one critical component of the blood oxygenation level-dependent (BOLD) signal 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 α-chlorolase-anesthetized rats under hypercapnia were measured by novel diffusion-weighted ¹⁹F NMR following an i.v. administration of intravascular tracer, perfluorocarbons, and continuous arterial spin labeling methods, respectively. The relationship between rCBF and total rCBV during hypercapnia was rCBV(total) = rCBF^0.40, which is consistent with previous PET measurement in monkeys. This relationship can be linearized in a CBF range of 50-130 ml/100 g/min as ΔrCBV(total)/ΔrCBF = 0.31 where ΔrCBV and ΔrCBF represent rCBV and rCBF changes. The average arterial volume fraction was 0.25 at a basal condition with CBF of ∼60 ml/100 g/min and increased up to 0.4 during hypercapnia. The change in venous rCBV was 2-fold smaller than that of total rCBV (ΔrCBV(vein)/ΔrCBF = 0.15), while the arterial rCBV change was 2.5 times larger than that of total rCBV (ΔrCBV(artery)/ΔrCBF = 0.79). These 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 provide a quantitative physiological model of BOLD contrast.