afni.nimh.nih.gov

Hi,
I have an image composed of 146 sub-bricks and would like to calculate voxel-by-voxel mean intensity across sub-bricks. I used 3dTstat with the -mean option but didn't work (I still have an output composed of 146 sub-bricks). Where is the problem?
Another question, how can I calculate the degrees of freedom in an AFNI file?

Thanks,
giovanni