This study investigates the functional anatomy of the human perivascular subarachnoid space using a cerebrospinal fluid (CSF) tracer (gadobutrol) and consecutive magnetic resonance imaging (MRI) in 75 patients. The findings suggest that the subarachnoid space is compartmentalized by a perivascular membrane, facilitating antegrade tracer transport along major cerebral artery trunks (anterior, middle, and posterior cerebral arteries). The correlation between the time of first enrichment around arteries and early enrichment in nearby cerebral cortex is significant, indicating that the perivascular transport is directional and facilitates CSF and intrathecal drug passage towards brain tissue. Impairment of periarterial transport is observed in subjects with reduced intracranial pressure-volume reserve capacity and in patients with idiopathic normal pressure hydrocephalus (iNPH), characterized by enlarged perivascular spaces and impaired periarterial tracer propagation. The study also explores the association between age and periaxial subarachnoid transport, finding that with increasing age, the first-time appearance of tracer in perivascular spaces is delayed, and tracer enrichment in the cerebral cortex is reduced. These findings provide new insights into the functional implications of subarachnoid space anatomy and its potential role in CSF dynamics and brain clearance.This study investigates the functional anatomy of the human perivascular subarachnoid space using a cerebrospinal fluid (CSF) tracer (gadobutrol) and consecutive magnetic resonance imaging (MRI) in 75 patients. The findings suggest that the subarachnoid space is compartmentalized by a perivascular membrane, facilitating antegrade tracer transport along major cerebral artery trunks (anterior, middle, and posterior cerebral arteries). The correlation between the time of first enrichment around arteries and early enrichment in nearby cerebral cortex is significant, indicating that the perivascular transport is directional and facilitates CSF and intrathecal drug passage towards brain tissue. Impairment of periarterial transport is observed in subjects with reduced intracranial pressure-volume reserve capacity and in patients with idiopathic normal pressure hydrocephalus (iNPH), characterized by enlarged perivascular spaces and impaired periarterial tracer propagation. The study also explores the association between age and periaxial subarachnoid transport, finding that with increasing age, the first-time appearance of tracer in perivascular spaces is delayed, and tracer enrichment in the cerebral cortex is reduced. These findings provide new insights into the functional implications of subarachnoid space anatomy and its potential role in CSF dynamics and brain clearance.