Cerebral blood volume changes in arterial and post-arterial compartments and their relationship with cerebral blood flow alteration during brief breath-holding and visual stimulation in human brain

Introduction: Arterial and post-arterial cerebral blood volumes (CBV) have different physiological and functional roles in the brain. Arterial vessels are the major active regulator of cerebral blood flow (CBF), while post-arterial (capillary, venous) vessels mostly dilate passively to accommodate changes in CBF. A change in CBV is one of the primary factors affecting the stimulus evoked BOLD signal change. As the extravascular tissue R2 has a value between arterial and post-arterial blood R2s (1), an alteration in arterial CBV (CBVa) or post-arterial CBV (CBVpa) alone would affect the overall BOLD signal, even without any change in the blood oxygenation level. In many previous studies (2), CBVa change was assumed to have negligible influence on BOLD signal, mainly because CBVa occupies only around 20% of total CBV at baseline. Nevertheless, recent MRI and optical imaging studies in animal brains revealed that the largest relative vasodilation during hypercapnia (3) and neural activation (4,5) occurs in arterioles. Therefore it is essential to measure CBVa and CBVpa responses separately. Detailed investigations of CBVa and CBVpa changes in human brain under global hypercapnic and focal neuronal stimulations remain scarce. In this study, we employ vascular-space-occupancy (VASO) (6) and inflow VASO (iVASO) (7) MRI to detect total CBV and CBVa responses, respectively, in human visual cortex during short breath-hold or visual stimulation, and investigate their relationships with CBF alterations.