Age-related brain-region-specific changes in diffusion anisotropy in subarachnoid space of young adults.

The glymphatic system theory postulates that brain waste is removed through the cerebrospinal fluid (CSF) flow. According to this theory, CSF in the subarachnoid space (SAS) moves to the perivascular space around the penetrating arteries, flows into parenchyma to mix with interstitial fluid and brain waste, and then moves to the perivenous space to be flushed out of the brain. Despite the controversies about the glymphatic theory, it is clear that SAS plays a key role in waste clearance. For instance, the SAS around the middle cerebral artery is known to be highly involved in glymphatic influx. While diffusion tensor imaging has been used for studying the glymphatic system, there has been limited exploration of age-related changes in diffusion anisotropy within SAS and their regional variations. Given the narrow and heterogeneous morphology of SAS, the fractional anisotropy (FA) in diverse brain regions may be more relevant to glymphatic transport than mean diffusivity (MD). The goal of this study was to investigate FA in SAS to observe age-related changes across different brain regions and to interpret the results based on the glymphatic transport. We segmented SAS in the whole brain of 83 young adults and divided SAS into four cortical lobes. We demonstrated regional variations in FA and MD within SAS and an age-related decline in FA among young adults, indicating that diffusion within SAS becomes more isotropic with aging. These findings raise new questions about the factors influencing diffusion anisotropy within SAS, which are relevant to glymphatic transport.