I have a data set of 84 subjs with 2 DTI scans each in which I would like to perform TBSS on between two groups. On a large subset of the subjects (say 75%), GRE fieldmaps were also scanned, most of the time just before the two DTI scans. 

My question is would it be kosher to use the fieldmaps to perform the distortion correction for the 75% of subjects who do have the EPI scans? And the for the ~25% of subjects who do not, just don't perform and fieldmap based distortion correction? 

My intuition says that this should be OK and would only help, but I wanted to double check to make sure that this would not bias in favor of subjects with the fieldmap scans. 

Thanks!

-Ricky