I am trying to average out all the time frames for my pet images by using mri_concat. However, I am getting an error (see below). How can I resolve this issue? Thanks. 

Best, 

Paul 

My command = mri_concat frame1.nii.gz frame2.nii.gz frame 3.nii.gz frame4.nii.gz --o Aveframe.nii.gz --mean 

FreeSurfer error: 

ninputs = 4

Checking inputs

niiRead(): error opening file /User/shared/Paul/library/PET/AGPA1F/PET-AV45/subject1/subject_frame1.nii.gz

ERROR: reading subject_frame1.nii.gz