This study investigates the functional anatomy of the human subarachnoid space using a cerebrospinal fluid (CSF) tracer (gadobutrol) and consecutive magnetic resonance imaging (MRI) in 75 patients. The results show that the subarachnoid space is compartmentalized by a perivascular subarachnoid space (PVSAS), which facilitates antegrade tracer transport along major cerebral arteries (anterior, middle, and posterior cerebral arteries). The tracer enrichment around these arteries precedes enrichment in the surrounding subarachnoid space and cerebral cortex. This suggests that the PVSAS acts as a functional barrier, enabling directional transport of solutes from the subarachnoid space to brain tissue. The study also found that impaired intracranial pressure-volume reserve capacity and idiopathic normal pressure hydrocephalus (iNPH) are associated with reduced periarterial tracer transport and enlarged PVSAS. Additionally, the time of first tracer enrichment around arteries correlates with early enrichment in the cerebral cortex. The findings indicate that the PVSAS is a semipermeable membrane that allows for directional transport of solutes along arteries, which is crucial for the glymphatic circulation and brain clearance of metabolic waste. The study further shows that the PVSAS communicates directly with the basal cisterns, and that the tracer can pass freely through the Liliequist membrane. The results suggest that the PVSAS is a key component of the subarachnoid space, enabling efficient transport of solutes and potentially playing a role in the glymphatic system. The study also highlights the importance of the PVSAS in CSF flow and its potential impact on brain clearance and neuroimmune interactions at the meninges. The findings have implications for understanding the pathophysiology of diseases such as iNPH and other CSF-related disorders.This study investigates the functional anatomy of the human subarachnoid space using a cerebrospinal fluid (CSF) tracer (gadobutrol) and consecutive magnetic resonance imaging (MRI) in 75 patients. The results show that the subarachnoid space is compartmentalized by a perivascular subarachnoid space (PVSAS), which facilitates antegrade tracer transport along major cerebral arteries (anterior, middle, and posterior cerebral arteries). The tracer enrichment around these arteries precedes enrichment in the surrounding subarachnoid space and cerebral cortex. This suggests that the PVSAS acts as a functional barrier, enabling directional transport of solutes from the subarachnoid space to brain tissue. The study also found that impaired intracranial pressure-volume reserve capacity and idiopathic normal pressure hydrocephalus (iNPH) are associated with reduced periarterial tracer transport and enlarged PVSAS. Additionally, the time of first tracer enrichment around arteries correlates with early enrichment in the cerebral cortex. The findings indicate that the PVSAS is a semipermeable membrane that allows for directional transport of solutes along arteries, which is crucial for the glymphatic circulation and brain clearance of metabolic waste. The study further shows that the PVSAS communicates directly with the basal cisterns, and that the tracer can pass freely through the Liliequist membrane. The results suggest that the PVSAS is a key component of the subarachnoid space, enabling efficient transport of solutes and potentially playing a role in the glymphatic system. The study also highlights the importance of the PVSAS in CSF flow and its potential impact on brain clearance and neuroimmune interactions at the meninges. The findings have implications for understanding the pathophysiology of diseases such as iNPH and other CSF-related disorders.