Hi Qi - Use eTIV as a covariate for any analyses on volume, thickness, or surface area measures. 

In a situation where a regional FA difference is high enough to also cause a whole brain FA difference, regressing out the latter could wipe out the former. Not sure why you'd want to do that, unless there's a specific question you want to ask that requires it.

Anastasia. 
From: freesurfer-bounces@nmr.mgh.harvard.edu <freesurfer-bounces@nmr.mgh.harvard.edu> on behalf of Zeng, Qi <qi.zeng@icahn.mssm.edu>
Sent: Monday, August 31, 2020 10:03 PM
To: Freesurfer support list <freesurfer@nmr.mgh.harvard.edu>
Subject: Re: [Freesurfer] group level analysis
 

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Hi Anastasia, 

Thank you for your reply. So for segmentation.stats, should I divide the regional areas by eTIV or treat it as a covariate for a group comparison? Is it the same with Total cerebral white matter volume for wmparc.stats. For Diffusion measurement, for example, FA, if a subject has lower FA in general in the brain, should I adjust the whole brain FA or average FA for that matter?

Best, 
Qi 

On Mon, Aug 31, 2020 at 6:53 PM Yendiki, Anastasia <AYENDIKI@mgh.harvard.edu> wrote:
Hi Qi - Correcting for overall brain size is important when you are comparing measures of length/area/volume. As in, you want to know if a region specifically is bigger in population A vs. B, and not just because the whole brain is bigger. In that case, use eTIV (estimated total intracranial volume) from the freesurfer segmentation stats.

FA is not measuring size of a region, so correcting for brain size is less of an issue there. It's a possibility perhaps that for someone with a substantially smaller brain there may be more partial voluming affecting FA, so it can't hurt to check for an effect before including it in your analysis.

Anastasia.
From: freesurfer-bounces@nmr.mgh.harvard.edu <freesurfer-bounces@nmr.mgh.harvard.edu> on behalf of Zeng, Qi <qi.zeng@icahn.mssm.edu>
Sent: Monday, August 31, 2020 11:44 AM
To: Freesurfer support list <freesurfer@nmr.mgh.harvard.edu>
Subject: [Freesurfer] group level analysis
 

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Hi, 

When conducting group-level analysis, for example comparing volumetric differences or tractography FA across subjects between groups. How we correct for the size of the brain when comparing volumetric differences or correct for the whole brain FA?
Thank you so much!

Best, 
Qi 


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Ph.D. candidate 
Icahn School of Medicine at Mount Sinai