Was the pepolar-style SDC using 3dQwarp in old LTS fmriprep 20.2.x responsive to total readout time parameters in the json metadata?

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Back in version 20.2.x of fMRIPrep (release notes for subsequent year’s version, 21.0.0), pepolar-style susceptibility distortion correction was done with AFNI’s 3dQwarp. From what I can tell, 3dQwarp is not sensitive to total readout time (not confident about this, so would welcome corrections). When applying the resulting warp to a bold image that has the same total readout time (as defined by FSL), that seems fine, but there are situations in which that time can differ between the fmap epis and the bold images (e.g., the fmaps have a longer echo time to maximize distortion, but then the bold has something smaller). Did older versions of fMRIPrep adjust the estimated distortion according to differences in total readout time?

Sorry to ask about the details of such an old version!

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From talking with @effigies and @oesteban, it sounds like this was correct; the expectation was that the total readout time in the blipup blipdown images and bold matched.

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Just to put the information here for searchability, if the BOLD has a shorter TRT than the input EPIs, then the correction in 20.2.x will be exaggerated, while a longer BOLD TRT will result in understated correction.

This is because the result of 3dQwarp are displacement fields, and displacement is the field (in Hz), multiplied by the TRT (in s) to get voxel displacements. If the TRT increases/decreases, then the expected distortion increases/decreases but there is no mechanism to scale the displacements by TRT_BOLD / TRT_EPI.

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