Feasibility of Quantitative Mapping of Microscopic Cerebrospinal Fluid Motion Based on Q-space Imaging

Cerebrospinal fluid (CSF) in the intracranial space has a microscopic circulation called “bulk flow”. Since bulk flow is relating to clearance of wastes from neurons via glymphatic system, visualization of its flow velocity is necessary. In this work, we examined a quantitative visualization technique based on q-space imaging (QSI) obtained with proton MRI. A phantom with microscopic circulation of physiological saline through a silicon tube of 6 mm in inner diameter was placed in a vertical 9.4-T MRI. Flow rate of the pump was set at 0.1 ∼ 0.5 mL/min with 0.1 mL/min steps after calibrating at 0.5 or 1.0 mL/min. Strength of motion probing gradient (MPG) was changed from −43.4 to +43.4 mT/m with 6.2 mT/m steps. The direction of MPG was foot-head (FH) direction. This resulted in a q-space of 32 points at each voxel in an axial section. The q-space data was then Fourier-transformed into a probability density function (PDF) of proton displacement. The peak position of PDF indicated the water proton flow, while the width indicated diffusion. The spatial distribution of the flow velocity was then obtained by dividing the displacement by the MPG interval at all the voxels. The velocity distributions appeared to be laminar at all the flow rate settings. The resultant flow velocities were from 57.80 to $300.72\ \mu\mathrm{m}/\mathrm{s}$ , and were highly correlated ( $r=0.99,\ p ) with the velocities created by the pump. In conclusion, observation of microscopic flow of the order of several $10\ \mu \mathrm{m}/\mathrm{s}$ was sufficiently performed by the QSI technique, even when the self-diffusion exists.