Estimation of human brain fluid clearance using intrathecal imaging contrast agents

BACKGROUND: Extra-vascular brain fluid dynamics, including exchange through glymphatic and meningeal lymphatic pathways, has been proposed essential for waste clearance and solute transport in the central nervous system. Although animal studies provide evidence for these pathways' role in clearing neurotoxic proteins and distributing solutes delivered to cerebrospinal fluid (CSF), translation to humans is limited. Quantitative, in vivo measures of inter-individual variability in brain fluid clearance in clinical populations are largely lacking. This study investigated the feasibility of estimating brain fluid clearance indices using intrathecal imaging contrast agents. METHODS: In a prospective and observational study, iodixanol (Visipaque, GE Healthcare, USA, 270 mg I/mL, 3 mL) was administered intrathecally as a CSF tracer and its clearance from CSF to blood was quantified using population pharmacokinetic modeling. Intracranial tracer enrichment in CSF was assessed by its radiodensity (Hounsfield units) within predefined regions of interest (cisterna magna, Sylvian fissure and vertex) on cranial computed tomography (CT) performed 2-, 4- and 24-hours post-injection. These results were compared with brain fluid clearance indices derived from a separate population pharmacokinetic model using intrathecal gadobutrol (Gadovist, Bayer AG, GE, 1.0 mmol/mL, 0.50 mL), with its enrichment in CSF defined as gadobutrol-induced T1 signal increase from baseline within corresponding subarachnoid spaces on magnetic resonance imaging (MRI). RESULTS: The study included 52 subjects receiving intrathecal iodixanol and 45 individuals receiving intrathecal gadobutrol. Marked inter-individual variability was observed in brain fluid clearance to blood for both contrast agents, as well as their distribution in CSF as assessed by CT and MRI. Brain fluid clearance from iodixanol and gadobutrol were significantly correlated. Notably, clearance indices showed stronger correlations with intracranial distribution in CSF for gadobutrol-derived MRI metrics than for iodixanol-derived CT metrics. CONCLUSIONS: These findings demonstrate the feasibility of estimating brain fluid clearance in humans using intrathecal imaging contrast agents as CSF tracers. Brain fluid clearance capacity exhibits pronounced inter-individual variability and correlates with the degree of contrast enrichment in intracranial subarachnoid spaces, particularly when assessed by MRI. The results underscore the importance of individualized assessment of brain fluid clearance, with implications for studies on pathogenetic mechanisms of neurodegeneration and optimization of intrathecal drug delivery.