This is a fairly complicated topic really. Regarding the specific questions of atrophy measures, there are a few possibilities:
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CSF probability. Using probabilistic segmentation from a program like 3dSeg or similar, you can generate probabilities of different tissue classes. Gray matter probability is another option here.
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Deformation to a normal template with Jacobian to get a compression/expansion type metric.
https://afni.nimh.nih.gov/afni/community/board/read.php?1,165536,165558#msg-165558
I have more thoughts on this, but it’s a bit much for a message board post.