afni.nimh.nih.gov

You can interpolate your FMRI data to a finer mesh, but this won't increase the resolution of the functional maps -- the data in those smaller voxels will still contain information from nearby non-PAG regions that 'leaks' into them. That is, signal changes in nearby non-PAG regions that look like your stimulus time series will appear in the PAG ROI, no matter how cleverly drawn it is.