Will you may get a quick telegraphic reply on this, these are each really separate and quite complex questions and you will likely get many answers
For subcortical volumes, while dividing by TICV has become a common solution and we have done it, it is not the optimal method. Introduces fixed error for all structures in a subject. See footnote in Buckner paper on this. If you are doing between group analysis, analysis of covariance is preferable. Proc glm or mixed in SAS, easy in other packages This can handle as covariates ICV, and depending on your population, you can add handedness, gender, IQ or the kitchen sink.
Another plus for covariance is that you get an interpretable group mean, a real number that can be compared to the literature instead of some weird fraction.
For surface, we have used the (third root of TICV), squared, based on suggestions by Bruce Fishl, I don't see why covariance with this as the term would not be slightly preferable to diving for the same reasons as above.
For thickness, two schools of thought. Some do no correction. Others use mean thickness over the whole brain of the individual or a subregion. It has been convincingly argued that what you use and whether you use it it depends again on the question you are actually asking. There are some very good and smart threads on this if you search. Mike Harms I seem to recall
Carl Schwartz
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hi miro,
as per me, i would suggest to divide ICV with TotalGrayVol and other aseg.stats files' global measures to remove the invariability across these measures. divide ICV to totalgrayvol,corticalwhitemattervolume,supratenorialvol,subcorticalgrayvol...this will remove any in-variability within those measures across ICV......
hope that helps
kk
----- Original Message ----- From: "Miro Drahos" miroslav.drahos@ucsf.edu To: freesurfer@nmr.mgh.harvard.edu Sent: Thursday, June 21, 2012 2:21:13 PM Subject: [Freesurfer] correcting for normal inter-cranial variability
Hi all, When analyzing output measures from FreeSurfer, what is the proper way to correct for normal anatomical variability before comparing between subjects? Which variable should be used to calculate normalized measures?
volumetric measures -- total intra-cranial volume? total brain volume? surface measures -- total surface area? thickness measures -- mean thickness?
Thank you! Miro Drahos
Hi all, When analyzing output measures from FreeSurfer, what is the proper way to correct for normal anatomical variability before comparing between subjects? Which variable should be used to calculate normalized measures?
volumetric measures -- total intra-cranial volume? total brain volume? surface measures -- total surface area? thickness measures -- mean thickness?
Thank you! Miro Drahos