Parenchymal CSF fraction is a measure of brain glymphatic clearance and positively associated with amyloid beta deposition on PET

INTRODUCTION Mapping of microscopic changes in the perivascular space (PVS) of the cerebral cortex, beyond magnetic resonance‐visible PVS in white matter, may enhance our ability to diagnose Alzheimer's disease (AD) early. METHODS We used the cerebrospinal fluid (CSF) water fraction (CSFF), a magnetic resonance imaging–based biomarker, to characterize brain parenchymal CSF water, reflecting microscopic PVS in parenchyma. We measured CSFF and amyloid beta (Aβ) using 11C Pittsburgh compound B positron emission tomography to investigate their relationship at both the subject and voxel levels. RESULTS Our research has demonstrated a positive correlation between the parenchymal CSFF, a non‐invasive imaging biomarker indicative of parenchymal glymphatic clearance, and Aβ deposition, observed at both individual and voxel‐based assessments in the posterior cingulate cortex. DISCUSSION This study shows that an increased parenchymal CSFF is associated with Aβ deposition, suggesting that CSFF could serve as a biomarker for brain glymphatic clearance, which can be used to detect early fluid changes in PVS predisposing individuals to the development of AD. Highlights Cerebrospinal fluid fraction (CSFF) could be a biomarker of parenchymal perivascular space. CSFF is positively associated with amyloid beta (Aβ) deposition at subject level. CSFF in an Aβ+ region is higher than in an Aβ– region in the posterior cingulate cortex. Correspondence is found between Aβ deposition and glymphatic clearance deficits measured by CSFF.