Last edit: 05-08-29 Graham Wideman
|FreeSurfer Area and Thickness Files
Article created: 2003-02-24
This page contains a few more details on the FreeSurfer Area and Thickness files.
This file type uses an Int2 value-per-vertex format with every vertex included. This corresponds to the "old curvature" format.
The meaning of the value at each vertex is, loosely speaking, the thickness at that vertex, between the grey-white boundary mesh and the pila mesh. However, this is not merely the distance between the corresponding (same-IDed) vertices in each mesh. Bruce Fischl explains (Feb 14 2003):
"The thickness isn't actually computed along the normal at each vertex, as that can give strange results if the surfaces are not parallel (which they aren't in general). Instead, we compute the nearest point on the each surface to the other (white and pial), then set the thickness to the average of the two. There are other more sophisticated (e.g. laplace equation) methods, but this works well enough."
This file again provides one value for each vertex, but in this case it's a 4-byte float. At first glance it corresponds to the "new curvature format", but looking at actual area files indicates only a 6-byte header, which does not quite agree with the new curvature format (or my docs on that are wrong).
The meaning of the value at each vertex is described by Bruce Fischl (Feb 11 2003):
"It's the average area of all the triangles the vertex is associated with."