Diffusion filters, T2-filters, diffusion tensor component diffusivities, and compartmental diffusivities. What is happening in FEXI?

We explored how compartmental anisotropy affects filter-exchange imaging (FEXI) data in white matter by applying different types of tissue water magnetization filters, including multi-orientation and spherical tensor encoded diffusion filters, as well as a T2 filter. The results show that, depending on the mutual orientation of the filter and detection gradient pairs relative to fiber orientation, filtering efficiencies can be positive or negative. We conclude that the fast compartment reduced during diffusional filtering need not be extracellular, but can also reflect intra-axonal signal losses with effect size determined by the diffusion tensor components perpendicular and parallel to the fibers.