  Filter-exchange imaging (FEXI) has already been utilized in several
biomedical studies for evaluating the permeability of cell membranes. The
method relies on suppressing the extracellular signal using strong diffusion
weighting (the mobility filter causing a reduction in the overall diffusivity)
and monitoring the subsequent diffusivity recovery. Using Monte Carlo (MC)
simulations, we demonstrate that FEXI is not uniquely sensitive to the
transcytolemmal exchange but also to the geometry of involved compartments:
Complicated geometry offers locations where spins remain unaffected by the
mobility filter; moving to other locations afterward, such spins contribute to
the diffusivity recovery without actually permeating any membrane. This
exchange mechanism challenges the interpretation of FEXI in complex media such
as brain gray matter and opens large room for investigation towards
crystallizing the genuine membrane permeation and characterizing the
compartment geometry.
