We acquried a number of participants and discovered that field maps were not acquired correctly (1 out of 2 is missing for all participants).
Now, a year later, we are back collecting data.
We have the possibility of collecting field maps but my question is whether it would be OK to group participants who were preprocessed with field maps with the older ones whithout field maps. Would it be better to process them all without field maps?
I would use the same processing technique for all participants, as SDC can lead to substantial geometric corrections that will impact functional localization across subjects. That being said, if you do not have field maps for everyone, you can try using a fieldmap-less distortion correction method (e.g. SynBOLD-DisCo or --use-syn-sdc in fMRIPrep) on everyone instead.