Relaxation-exchange magnetic resonance imaging (REXI): a non-invasive imaging method for evaluating trans-barrier water exchange in the choroid plexus

Abstract

Background: The choroid plexus (CP) plays a crucial role in cerebrospinal fluid (CSF) production and brain homeostasis. However, non-invasive imaging techniques to assess its function remain limited. This study was conducted to develop a novel, contrast-agent-free MRI technique, termed relaxation-exchange magnetic resonance imaging (REXI), for evaluating CP-CSF water transport, a potential biomarker of CP function.

Methods: REXI utilizes the inherent and large difference in magnetic resonance transverse relaxation times (T₂s) between CP tissue (e.g., blood vessels and epithelial cells) and CSF. It uses a filter block to remove most CP tissue magnetization (shorter T₂), a mixing block for CP-CSF water exchange with mixing time t_m, and a detection block with multi-echo acquisition to determine the CP/CSF component fraction after exchange. The REXI pulse sequence was implemented on a 9.4 T preclinical MRI scanner. For validation of REXI's ability to measure exchange, we conducted preliminary tests on urea-water proton-exchange phantoms with various pH levels. We measured the steady-state water efflux rate from CP to CSF in rats and tested the sensitivity of REXI in detecting CP dysfunction induced by the carbonic anhydrase inhibitor acetazolamide.

Results: REXI pulse sequence successfully captured changes in the proton exchange rate (from short-T₂ component to long-T₂ component [i.e., k_sl]) of urea-water phantoms at varying pH, demonstrating its sensitivity to exchange processes. In rat CP, REXI significantly suppressed the CP tissue signal, reducing the short-T₂ fraction (f_short) from 0.44 to 0.23 (p < 0.0001), with significant recovery to 0.28 after a mixing time of 400 ms (p = 0.014). The changes in f_short at various mixing times can be accurately described by a two-site exchange model, yielding a steady-state water efflux rate from CP to CSF (i.e., k_bc) of 0.49 s^-1. A scan-rescan experiment demonstrated that REXI had excellent reproducibility in measuring k_bc (intraclass correlation coefficient = 0.90). Notably, acetazolamide-induced CSF reduction resulted in a 66% decrease in k_bc within rat CP.

Conclusions: This proof-of-concept study demonstrates the feasibility of REXI for measuring trans-barrier water exchange in the CP, offering a promising biomarker for future assessments of CP function.

Keywords: Blood cerebrospinal fluid barrier; Choroid plexus; Contrast-agent-free; Magnetic resonance imaging; Relaxation exchange.

Fig. 1

Principles of REXI in the measurement of CP function. A Pulse sequence diagram of REXI, comprising a filter block with fixed echo time (TE_f), a mixing block with varying mixing time (t_m), and a detection block with multi-echo acquisition (TE). Paired crusher gradients (G_c) for coherence pathway selection are added before the second 90° pulse and after the third 90° pulse. A spoiler gradient (G_s) is applied to eliminate unwanted transverse magnetization in the mixing block. Pulses in the filter and detection blocks are non-selective and selective, respectively. B Illustration of REXI in the measurement of water transport from CP to CSF. When the filter is applied (gray shade), water molecules in blood become largely undetectable by MRI (filled dots), whereas those in CSF remain largely unfiltered, leading to a reduction in the short-T₂ component (i.e., CP) from f_short^eq to f_short(t_m = 0 ms) (dashed line). After a specific mixing time, water exchange between blood and CSF results in the recovery of f_short from f_short(t_m = 0 ms) to f_short^eq, and the recovery rate can be described by the water exchange rate k and Eq. (1)

Fig. 2

Raw images and fitting results of REXI in measuring proton exchange within urea-water phantoms. A MSME and REXI images with minimum TE = 7 ms. B Signal decays of MSME and REXI at different t_ms and C the f_short-t_m curve from one representative phantom at pH = 7.0. In (B), filled blue circles represent MSME and REXI data, and curves are the results of model fitting to Eq. (4). In (C), the first circle at t_m < 0 denotes the f_short^eq estimated from MSME data; other circles represent the f_short(t_m) values estimated from REXI data, and dashed curves are the results of model fitting to Eq. (1). D Statistical comparison of k_sl between the two groups at different pH values using Mann–Whitney tests. Data points (black dots) are overlaid on the corresponding box plots. Bar height and error bar width represent the mean and standard error of the mean (s.e.m.), respectively (same for subsequent figures). ** p < 0.01, * p ≤ 0.05

Fig. 3

Raw images and fitting results of REXI in measuring water exchange within the rat CP. A MSME and REXI images with minimum TE = 7 ms at each t_m, B T₂w anatomical image and corresponding ROI of the CP, and C signal decays of MSME and REXI at different t_ms from a representative rat. D Statistical comparison between f_short^eq, f_short(t_m = 25 ms) and f_short(t_m = 400 ms) using the Friedman test. * p < 0.05, **** p < 0.0001. In (D), open blue circles represent MSME and REXI data, and dashed red curves are the results of model fitting. E The f_short-t_m curve from a representative rat. bi-exp. fit, bi-exponential fitting; mono-exp. fit, mono-exponential fitting

Fig. 4

Scan-rescan reproducibility of k_bc, f_short^eq, T_{2, long}, and T_{2, short} depicted via BA plots. The horizontal axis represents the average of two measurements, whereas the vertical axis represents the difference between the two measurements. Solid line and dashed lines in BA plots denote mean difference and mean difference ± 1.96 * standard deviation, respectively

Fig. 5

REXI detection of pharmacologically induced downregulation of CSF secretion via k_bc alteration in rats. A The f_short-t_m curves of representative rats in the treatment and control groups. B Statistical comparison of k_bc, f_short^eq, T_{2, short}, and T_{2, long} in the rat CP between treatment and control groups. AZE: treatment group with i.v. delivery of acetazolamide, CON: control group with i.v. delivery of vehicle. Mann–Whitney tests. ** p < 0.01