[Freesurfer] Making mid-gray surfaces without mris_expand?

I have seen that the official recommendation for making a mid-gray surface is to use mris_expand. This seems to be a very slow procedure on my machine. I have also tried just loading the ?h.white and ?h.pial surfaces into matlab, averaging their vertices, and exporting the result. This seems to result in a nearly identical surface as mris_expand. On a test subject, only 48 out of 127,998 (0.0375%) vertices differed by more than 0.1mm, and these were almost entirely around the edges of the corpus callosum.

Is there a reason this alternative is a bad idea? I imagine mris_expand does additional topology checks along the way, but if the vertices vary so little, can that be a major problem?

I'm inclined to go with the faster solution, but since I'm relatively new to this procedure, I wanted to see what the community's experience might be.

Thanks! Keith

_______________________________ Keith Jamison Graduate Research Assistant

Department of Biomedical Engineering University of Minnesota 7-105 NHH, 312 Church St. SE Minneapolis, MN 55455

Office: 6-112 Nils Hasselmo Hall Mobile: 607-227-0696 kjamison@umn.edu