Hi Karel,
These are the commands we would run to calculate thickness in an ROI volume:
mri_vol2surf mri_vol2label (please ignore misleading name, will also work on surfaces) mris_anatomicalstats
The wiki has documentation on all of these programs, but if you would like further details feel free to reach out! https://surfer.nmr.mgh.harvard.edu/fswiki -Matt
freesurfer@nmr.mgh.harvard.edu