Hi Rick,
As suggested some time ago, I used 3dresample with -master option, but yet I got an error message (Input and mask datasets are not same dimensions!). When I checked the resampled mask, # of voxels in each direction was 61 73 61, while the input image had 74 74 40 (both images had the same voxel size, 3mm). What else could I do to force the grids to match?
Thanks,
Sophia