Dear Yang,
Based on the before.jpg in https://www.jiscmail.ac.uk/cgi-bin/webadmin?A2=spm;595652bd.1604 you encounter large signal loss in ventral regions / OFC. You can't recover that signal post-hoc (BTW, a fieldmap would only correct for geometric deformations, but not for signal drop).
In principle you could adjust the intensity threshold so that these voxels remain in the analysis, but you shouldn't do so, as this would lead to incorrect conclusions (it's not that there are no BOLD signal changes in OFC, it's that you are not able to detect any such signal at all). Thus, if these regions are critical then you have to turn to a different sequence / scanning parameters. Also make sure that subjects are positioned in a similar way inside the scanner and that the FoV of the sequence is placed consistently across subjects, as otherwise, there might be drop-out in region 1 for sub 1 and drop-out in region 2 for another.
Best
Helmut
|