Hi Philipp

I'll leave the volume question to Doug. The curvatures are estimated by fitting a second order polynomial to the local height function in the tangent bundle then computing the Hessian using the white (gray/white boundary) surface. We also have some discrete measures of curvature that uses the Gauss-Bonnet theorem, but those aren't run by default.

cheers
Bruce


On Fri, 6 Sep 2019, Loske, Philipp wrote:


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Hello FreeSurfer Developers,


I have a question about references and descriptions to all the morphological
features available in aparc.stats, extracted from structural MRI. I am using
all the different features (SurfArea, GrayVol, ThickAvg, ThickStd, MeanCurv,
GausCurv, FoldInd, CurvInd) for the 68 cortical regions available from the
Desikan Kiliany atlas. I have been trying to find references to how these
features are measured or derived, but couldn't find a proper list of
references to each of the features. I a lot of references
here https://surfer.nmr.mgh.harvard.edu/fswiki/FreeSurferMethodsCitation,
with detailed descriptions on surface reconstruction and how cortical
thickness is measured, but couldn't find a detailed explanation on the other
features, especially curvature values.

I think I have been able to find out what each of the measurements mean and
how they are calculated in principle, through searching the internet a lot,
but with no direct link to FreeSurfer. For instance, I know you can calculate
the curvature values from the principle curvatures, but how does FS estimate
those form the reconstructed Surface? And which one of the two surfaces does
it use? White-pial or pial-csf surface? I found https://brainder.org/ really
helpful, but it says for example that a new way of measuring cortical volume
was implemented in FS 6.0.0, so does that mean for previous versions FS uses
the Thickness x SurfaceArea method? Would be great if you can help me with
this, or point me to the correct website I might just have missed during my
search.


Best wishes

Philipp




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