Import fieldmap from earlier session rather than syn-correction?

We have a dataset where most of BOLD data have a corresponding fieldmap to run sdc with fmriprep. I have noticed that syn-sdc is much much worse than fieldmap-sdc. So I am thinking to use a field map acquired from the same subject for another fMRI scan (during the same session, or on different sessions) to correct for distortions rather than using syn-sdc. Has anybody tried this before? Is it ok to borrow a field map from the same session (or even from another session from 1 year ago) to run sdc in fmriprep?

My limited understanding is that fieldmaps are acquired before each fMRI scan to have the exact head position that will be in fMRI, but after all the head can move during fMRI itself, so I am not sure to what degree a field map obtained from the same subject at another time can be used for the current scan.

A more extreme idea is the use of a field map from another subject, how bad would that idea be?

Using another subject’s fieldmap is probably not a great idea. But using another fieldmap from the same subject is worth a try, IMO.

If you have data where SyN-SDC is performing significantly worse than fieldmaps, and are able to share the data, consider opening an issue on, where further development of SyN-SDC will happen.

Thank you for the feedback. Can’t share the data just like that but can probably share the .svg pics to show the difference. You can still get the data though, but through a data use agreement with my institution which on our side is straightforward.

Will post the svg on github when the fmriprep processing frenzy is over. Thanks