Mapping subarachnoid cerebrospinal fluid circulation in the human brain

The subarachnoid space is critical to cerebrospinal fluid (CSF) circulation and waste clearance, yet its flow organization in humans remains poorly characterized due to the lack of methods for direct, quantitative measurement of ultra-slow flow. Here we introduce slow-flow-sensitized phase-contrast imaging (SOPHI), which enables noninvasive quantification of ultra-slow flow velocities (e.g., ∼100 μm/s) and directionality at high spatiotemporal resolution, allowing whole-brain mapping of complex CSF circulation in the human subarachnoid space. We found brain-wide, spatiotemporally coherent flow patterns characterized by strong cardiac-driven oscillations. Flow dynamics were closely coupled with vascular anatomy, exhibiting higher velocities and earlier responses in subarachnoid spaces near major arteries and a spatiotemporal propagative pattern to distal spaces. We further identified localized flow pathways associated with potential CSF efflux in ventricles and key subarachnoid regions. Together, SOPHI reveals a previously undercharacterized, highly organized CSF flow system, providing a macroscopic view of human CSF circulation and a framework for investigating brain fluid transport.