As the title suggests, I’m curious about the reason for limiting the estimation of susceptibility distortions to the phase encoding direction (as described here https://fmriprep.readthedocs.io/en/stable/api/index.html#phase-encoding-polarity-pepolar-techniques).
In my experience (from inspecting fieldmaps estimated through FSL topup) the directions of susceptibility distortions in a region seem to be a product of interactions between phase encoding direction and subject anatomy (e.g. volume of air-filled cavities). For example, with PE-direction A–>P the frontal lobes will mostly be compressed, but some parts will also show slight stretching in the anterior direction. (see https://github.com/poldracklab/fmriprep/issues/1547 for an extreme example).
Are there any obvious negative effects of allowing the displacement to be estimated in both directions of the PE axis?
And a related question: does anyone know if topup estimated fieldmaps (in Hz) can be used as input to the “Direct B0 mapping”-workflow for susceptibility distortion correction? (i.e.: https://fmriprep.readthedocs.io/en/stable/api/index.html#direct-b0-mapping-sequences)
If so, maybe this could be a workaround (unless, of course, displacement estimation is limited also in this workflow).