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-
Freesurfer mailing list Freesurfer@nmr.mgh.harvard.edu https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer