What Does FEXI Measure?

Abstract

Filter-EXchange Imaging (FEXI) evaluates the permeability of cell membranes, which is already used in clinical research. It relies on suppressing the extracellular signal using strong diffusion weighting (mobility filter causing a reduction in the overall diffusivity) and monitoring the subsequent diffusivity recovery. Using simulations, we demonstrate the presence of a spurious exchange: In compartments with complex geometry, there are locations where spins remain unaffected by the mobility filter. Moving to other locations afterward contributes to the diffusivity recovery, thus mimicking exchange. The discovered phenomenon opens large room for investigation towards characterizing the compartment geometry and crystallizing the genuine membrane permeation.