Feasibility of filter-exchange imaging (FEXI) in measuring different exchange processes in human brain

Transmembrane water exchange, including intra-to-extravascular and intra-to-extracellular ones, are potential biomarkers in the diagnosis and understanding of cancers, brain disorders, and other diseases. Filter-exchange imaging (FEXI), a special case of diffusion exchange spectroscopy (DEXSY) adapted for clinical applications, has the potential to reveal different physiological water exchange processes using the same MRI sequence. In this study, we aim to explore the feasibility of FEXI in measuring different water exchange processes by modulating the diffusion filter (b(f)) and detection blocks in FEXI. Two FEXI protocols were implemented on a 3T MRI clinical scanner and reveal distinct apparent exchange rate (AXR) contrast in brain tissues in seven healthy volunteers. AXR estimated from a FEXI protocol with b(f) = 250 s/mm(2), which is expected to filter out the vascular water specifically, are significantly larger than those of a FEXI protocol with b(f) = 900 s/mm(2). Besides, the filter efficiency of FEXI with b(f) = 250 s/mm(2) shows a strong correlation with vascular density, a metric estimated as the fraction of water exhibiting intravoxel incoherent motion (IVIM). AXR of FEXI with b(f) = 250 s/mm(2) agrees with the vascular water efflux rate constants reported by other independent measurements, although the physiological basis of the AXR of FEXI with b(f) = 900 s/mm(2) is not clear yet. Collectively, our current results demonstrate the feasibility of FEXI in measuring different water exchange processes in vivo, and that FEXI targeting the vascular water could help characterize the intra-to-extravascular water exchange process.