Dear Thomas,

Looking at the file rh.sphere.asc, it seems that the problem comes from the spherical registration that generates a spherical surface with, as you said, small triangles slightly overlapping.

However, these distortions should be extremely small. If you really need to get rid of them, one hacky solution would be to detect these (small) regions and to smooth them untill the overlapping edges(faces) disappear. Since these regions are small, this should not generate large metric distortions.

But maybe Bruce has some other ideas.

Florent

------------------------------------- Florent Segonne PhD Candidate Stata Center 32-D430 CSAIL MIT 1 617 253 2986 http://people.csail.mit.edu/~fsegonne -------------------------------------

On Thu, 10 Nov 2005, Thomas, Adam (NIH/NIMH) wrote:

I recently noticed that some of my subject's spherical surfaces appears to have several small, pyramid-like points on them. One example is shown here in tksurfer:

http://fmrif.nimh.nih.gov/~adamt/sphere.jpg

Looking at these points in more detail in suma shows that several triangles are folder on top of each other so that all the vertices are on the spheres surface, but some triangles are entirely overlapped by others.

The spherical surface is here:

http://fmrif.nimh.nih.gov/~adamt/rh.sphere.asc (11MB)

And this is a close up of near node 141619 in suma:

http://fmrif.nimh.nih.gov/~adamt/sphere_suma_zoom.jpg

These distortions are causing us problems with subject averaging. Is there something we can do to fix or avoid them?

Thanks, -Adam

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Adam Thomas adamt@nih.gov Functional MRI Facility, NIMH/NIH/DHHS 10 Center Dr, Room B1D708A Bethesda MD. 20892-1148 Phone:301-402-6351 Fax: 301-402-1370

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