Hi All,
I've processed a set of 40 subjects with freesurfer-Linux-centos4_x86_64-stable-pub-v3.0.5 and am now working on a series of morphometric analyses. I am comparing gray matter volumes found in: 1) aparc.stats from recon-all, (same as output of "mris_anatomical_stats" run from the command line) 2) "mris_volume" run from the command line for each hemisphere. I'm seeing slightly different results for the same subject and was wondering why.
** From aparc.stats, I see an initial value of 197405 mm3. This includes a column of data labeled corpus callosum that I assume is not gray matter. Subtracting that, I get 196549 mm3 for a single subject. I'm assuming that the corpus callosum value found in aparc.stats, though small, ought to be removed when calculating total gray volume per hemisphere? ** Running the command line option "mris_volume rh.pial" , followed by "mris_volume rh.white" and subtracting the two values I get 193215 mm3. This is 4190 mm3 smaller than the initial values from aparc.stats for that subject.
The question: Are these differences expected? Which value is better to use, and why? Would the general cause of these differences in cortical gray matter volume quantification be consistent for all subjects?
Thanks very much, Paul