Is the
volume of the corpus callosum well defined, as opposed to its
cross-sectional area at the midline? I'm not sure how you define where it ends as it branches into the left and right hemispheres.
You could make your own mini-CC atlas by
(a) transforming all subject to Talairach coords
(b) draw the desired CC on some of them in Talairach coords (4..10?)
(c) create from (b) an average mask in Talairach coords
(d) for each subject, use 3dfractionize to inverse warp the CC mask from +tlrc back to +orig
(e) for each subject, use the inverse-warped mask as a starting point for drawing that person's actual CC
bob cox