Rapid Tissue-CSF Water Exchange in the Human Brain Revealed by Magnetization Transfer Indirect Spin Labeling.

Purpose: To apply the Magnetization Transfer Indirect Spin Labeling (MISL) MRI technique for quantifying tissue-CSF water exchange in the human brain, and to investigate its utility in 1) visualizing perivascular spaces (PVS), and 2) characterizing altered tissue-CSF dynamics in pathologic conditions. Methods: MISL was implemented on a 3T MRI using off-resonance MT to label parenchymal water and blood. The resulting exchange with CSF was captured via long-TE 3D-TSE readout to suppress parenchymal/blood signals. CSF-region-specific quantification was achieved by atlas-based segmentation. Studies were conducted in healthy subjects across age groups and in patient with metastatic brain tumor. Results: MISL revealed widespread and regionally heterogeneous tissue-CSF exchange, with the strongest signals observed in the PVS and areas adjacent to the choroid plexus. MISL signals were typically 2-3% in the ventricles and subarachnoid space, and reached 3% in the cerebellar regions, suggesting tissue-to-CSF flow rates in the range of 100-300 mL/100□mL/min. The high MISL signals observed in the PVS (∼8.4%) provided enhanced sensitivity for visualizing PVS compared to T2-weighted images.Significant age-dependent decreases in water exchange were observed across most brain regions, except for the third and fourth ventricles. In tumor patients, MISL detected elevated exchange even in areas without overt T2 hyperintensity, suggesting sensitivity to early edema formation. Conclusion: MISL enables robust, non-invasive mapping of tissue-CSF exchange with high sensitivity and spatial resolution. It offers a unique window into tissue-CSF dynamics, with potential applications in PVS visualization, which reflects glymphatic function.