Hi Jenna,
It is very difficult to say, as it can depend not only on motion and the like, but the scanner and its coils. Some scanners show higher TSNR near the edges of the brain, some a little deeper. I am used to seeing somewhat similar values between WM and GM, though lower at the outer cortex.
What is the average censored motion in this subject? The more residual motion in the time series, the worse it is for GM.
- rick