Hi Katie,

FreeSurfer operates mostly in "FreeSurfer Space", which is voxel resolution of 1mm^3 and sagittal orientation.  

You can compare orig.mgz to the original input to see what the difference between native space and FreeSurfer Space would be.

eTIV is computed using the transformation matrix from FreeSurfer Space to MNI305 space.  There is more information here about it here:
https://surfer.nmr.mgh.harvard.edu/fswiki/eTIV

-Paul

From: freesurfer-bounces@nmr.mgh.harvard.edu <freesurfer-bounces@nmr.mgh.harvard.edu> on behalf of Katie Rose Jobson <katie.jobson@temple.edu>
Sent: Thursday, June 23, 2022 6:08 PM
To: freesurfer@nmr.mgh.harvard.edu <freesurfer@nmr.mgh.harvard.edu>
Subject: [Freesurfer] aseg.stats - eTIV volume space
 

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Hi FreeSurfer experts,

I apologize if this question has been addressed, but I was unable to find a definitive answer elsewhere.

I am using the aseg.stats file to extract the estimated Total Intracranial Volume (eTIV) value to be used as a correction to my anatomical data. I just want to confirm what space this data is in. I had previously assumed this data was in native anatomical space. But I read on the wiki that this statistic is calculated using the Talairach transform file (talairach.xfm), which is the conversion from MNI305 to Talairach. I just want to ensure I am correct in the assumption that this data is represented in native anatomical space.

Thank you so much for your help in this matter.

Best Wishes,
Katie

--
Katie Jobson, M.S.
Department of Psychology and Neuroscience | Temple University