Hello,
Just to be sure, you are not using standard mesh surfaces for this? They allow for comparison across subjects.
3dVol2Surf allows for incomplete output, so that one can make small ROI files. Consequently, it is often good to add options:
-oob_value 0.0 \
-oom_value 0.0 \
That should make the output datasets complete in node coverage. Otherwise, if a mask or volume does not fully cover a surface, the result would be missing nodes.
- rick