Hi Aaron,
this is probably due to the partial volume correction we do when
computing the volumes. Basically we assume that each voxel is a linear
combination of the tissue at that voxel and the next most likely label that
neighbors that voxel. The weighting is computed as mixture weight for the
local class mean for each class that explains the voxel intensity. Our
internal studies show that this improves test-retest reliability and
presumably also accuracy.
cheers,
Bruce
On Thu, 7 Sep 2006, Goldman, Aaron (NIH/NIMH) [C]
wrote:
Hi,
I had a question regarding the total volumes of aseg labels listed in
aseg.stats. We've noticed slight discrepencies between the values
reported in the text file, and values obtained by converting aseg.mgz to
analyze (using mri_convert) and adding up the same structures ourselves
in matlab. The differences ranged from 0-2%, and varied in
directionality.
To give a more specific idea of what we did, we added up all of the wm
labels in the stat file to get one value, and all the gm to get another.
We then created binary masks for the same sets of labels, and calculated
total volume of these images in matlab.
I was wondering if the stats in aseg.stats were simple voxel counts (as
I'd previously assumed since they're all integers), or if there were
additional nuances to the calculation (in which case these discrepencies
would presumably be expected).
Thanks,
-Aaron-
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