The brain’s arterial flow and flow territories have been well investigated, while little is known about the venous system. There have been only occasional reports in literature that assessed cerebral blood flow variation in trajectories of the venous system¹. Similarly, studies investigating the venous oxygenation (Y_v) are scarce due to a lack of tools for in vivo quantification of this parameter². Therefore the purpose of this study was to investigate the distributions of venous flow and oxygenation in the brain, and understand how they might change under challenged state. For this purpose, we measured the blood flow and oxygenation at the major veins in the brain’s drainage system under both baseline and physiological challenge conditions with hypercapnia and hyperoxia.

Study design:

Eight subjects (4 males, 24-37yrs) were studied on a Philips 3T scanner. The study contained two parts: 1) Measurement of venous blood flow at major veins, specifically superior sagittal sinus (SSS), straight sinus (SS) and internal jugular veins (IJVs), using phase-contrast (PC) MRI during both room air and hypercapnia (5% CO₂) breathing; 2) Measurement of venous oxygenation in these veins using T2-relaxation-under-spin-tagging (TRUST) MRI³ under both room air and hyperoxia (95%O₂) breathing.

Venous flow measurements:

For each subject, a 3D time-of-flight angiogram and venogram was performed first to visualize the brain’s feeding arteries and major draining veins. For reference, whole-brain arterial blood flow was calculated as the sum of flux measured at the four feeding arteries, left and right internal carotid arteries (ICAs) and left and right vertebral arteries (VAs). Venous flow was measured in SSS, SS and IJVs. The PC-MRI imaging parameters used those optimized in a recent study⁴. Manual ROI drawing was used to quantify the blood flux (in ml/min) in each vessel.

Venous oxygenation measurements:

TRUST MRI is based on the principle that T₂ relaxation time of the blood has a well-known and calibratable relationship with its oxygenation. The scan duration of TRUST MRI was 1 min 12 sec and its imaging protocol and analysis followed that of a recent multi-site trial⁵.

Venous flow distribution:

Results of the blood flow measurements are shown in Table 1. The venous drainage distribution was illustrated in Figure 1a. We found that the venous blood drained from the cortex (i.e., flow through SSS) and that drained from deep brain structures (i.e., flow through SS) comprised around 62% of the whole-brain blood (i.e., total arterial inflow). Since the IJVs drain about 76% of the whole-brain blood, it means the cerebellar veins and other small veins joining the venous sinuses after the sinus confluence contribute about 14% of the total blood. Moreover, about 24% of cerebral blood was not drained by the IJVs, but via venous plexuses which consequently drain into the cervical veins and external jugular veins. The drainage distribution did not change significantly during hypercapnia.

Venous oxygenation distribution:

The measured Y_v at baseline was 61±4%, 64±4%, and 62±4% in SSS, SS, and IJVs respectively (Figure 1b). The SS-Y_v was significantly higher than SSS-Y_v (p=0.0003, Figure 2a). Since the blood in the IJV is originated predominantly from SSS and SS, not surprisingly the IJV-Y_v value is between that of SSS and SS. There was significant correlation between SSS-Y_v and SS-Y_v (R²=0.88, Figure 2b), and between SSS-Y_v and IJV-Y_v (R²=0.65). The Y_v at hyperoxia was 70±3%, 71±5%, and 68±5% in SSS, SS, and IJVs respectively. No significant difference in hyperoxia-induced Y_v change was detected among the major veins.

In this work we evaluated the distribution of flow and oxygenation in the venous drainage system of the brain. Remarkably, about 24% of the venous blood is not drained via the commonly assumed pathway, i.e. the cerebral venous sinuses. We also found that although the oxygenation is slightly higher in SS which drains deep brain structure, the oxygenation in IJVs, which reflects whole brain venous oxygenation, is correlated and mainly driven by cortical venous oxygenation. Therefore, if one wants to know the whole-brain oxygenation consumption, Y_v measurements on the SSS are sufficient. Caution should be taken in case of disease conditions in which oxygen consumption in the deep gray matter or cerebellum differs considerably from the rest of the brain, e.g. ischemia within these areas. In this case, additional Y_v measurements on the SS and IJVs by TRUST MRI can be considered.