Hi Stephanie
By using the average/std within region, then averaging those, you are weighting
small regions more. I would just use the average/std across cortex.
Cheers
Bruce
From: freesurfer-boun...@nmr.mgh.harvard.edu
<freesurfer-boun...@nmr.mgh.harvard.edu> On Behalf Of Stephanie K
Sent: Wednesday, November 18, 2020 12:24 PM
To: freesurfer@nmr.mgh.harvard.edu
Subject: [Freesurfer] Mean thickness estimation
External Email - Use Caution
Hi,
I want to estimate the mean cortical thickness. For this I have summed the
thickness across all 34 regions mapped to the Desikan-Killiany atlas. However,
I also have the average mean thickness of left and right hemispheres (direct
output variables of Freesurfer). As there is no visual inspection of the
imaging in the particular cohort, I remove measures that are 3 standard
deviations above or below the mean. Hence, I may expect more outliers to be
removed when I take the average across the regions. I am using these brain
measures as outcomes in association analyses with the genetic score as the
exposure. For the mean thickness (averaged across the left and right hemisphere
thickness variables of freesurfer after removing outliers), the regression
coefficients have a smaller standard deviation than with thickness averaged
across the 34 regions. I’m not sure which one to use - which one is more
accurate? When I look at the mean thickness (which I derived using 34 regions)
and it’s standard deviation, it is similar to that of the average mean
thickness across the two hemispheres as well as the standard deviation of that.
Can you suggest what is most accurate please and what the difference is between
the mean thickness across the two hemispheres obtained from freesurfer and
those calculated across the regions? Why does one result in more precision than
in the other?
Thank you!
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