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Dear Freesurfer experts, I have multiple PET volumetric images in MNI standard space. I can average (or get the Tmean) of these images using fslmerge then fslmaths. I would like to do something similar on surfaces images. I registered the images from native space to Freesurfer space using spmregister as follows: spmregister ---s $subject -mov PET.nii.gz --reg reg.dat --o PET_T1.mgh

Then I used mris_preproc with the flag --mean to register the volumetric images to fsaverage, then concatenate the surfaces and average it as follows: mris_preproc --target fsaverage --hemi lh --projfrac 0.5 --iv PET_T1_subj1 reg_subj1.dat --iv PET_T1_subj2 reg_subj2.dat . ...... .--iv PET_T1_subjN reg_subjN.dat --mean --o lh.mgh

the final output "lh.mgh" is an empty image. I am not sure what I am doing wrong here. Any guidance to average the surfaces images in fsaverage would be highly appreciated.

john