If you make use of the estimated Total Intracranial Volume (eTIV) calculation found in the aseg.stats file of your subject data, then you should be aware that we have recently found a more accurate way to make this calculation. We have found that using the talairach.xfm transform instead of the talairach_with_skull.lta transform, from which the determinant is extracted and multiplied by a scale factor, results in lower variability of the eTIV calc in our test set.
This new way of calculating eTIV will appear in the next release of Freesurfer.
If you want to make use of this new calculation right away, you can download a new version of mri_segstats from here:
ftp://surfer.nmr.mgh.harvard.edu/pub/dist/freesurfer/fixes
and run it using this command:
mri_segstats --subject subjid --etiv-only
and it will output the new eTIV.
Nick