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Dear Freesurfer experts,
I am going to do convert the preprocessed surface functional data to volume in fsaverage space so that I can used the processed data in SPM for further analysis, saying, beta series analysis. Because the trial nubmer is not same for each subject in my experiment, I think freesurfer can not deal with this condition (please tell me if I am wrong). I tried mri_surf2vol, it worked. But it is time consuming. It takes around 5 minutes per 10 volumes. In addition, the result volume is too big (67MB/ volume for 256*256*256 matrix, 1mm resolution). The code is  as follows: 

mri_surf2vol --o ${Dir_Subj_fMRI}/$isub/bold/${irun}/Ribbon/${iVolume} --subject fsaverage --so ${SUBJECTS_DIR}/fsaverage/surf/lh.white ${Dir_Subj_fMRI}/$isub/bold/${irun}/LH/${iVolume} --so ${SUBJECTS_DIR}/fsaverage/surf/rh.white ${Dir_Subj_fMRI}/$isub/bold/${irun}/RH/${iVolume} 

Are there better ways to accomplish my goal?

Best,
Zhiwei