[Freesurfer] Cortical thickness average

Hello FreeSurfer community,

I calculated cortical thickness for 100 controls following the instructions described here: https://surfer.nmr.mgh.harvard.edu/fswiki/VolumeRoiCorticalThickness

So I have 100 lh.thickness.fsaverage.mgh, one for each subject. I was wondering if there is any way to average the thickness over these 100 lh.thickness.fsaverage.mgh files to get one final lh.thickness.fsaverage.mgh

I am interested in calculating cortical thickness for 15 previously defined ROIs, but I do not want to calculate 15x100 text files using mri_segstats i.e. 15 text files for each subject and then average over those but I would rather prefer to average over 100 lh.thickness.fsaverage.mgh files first and then calculate thickness for each ROI to get 11 text files.

Any help would be really appreciated.

Thanks, Sahil