Hi Liesel,

First, let me be sure what you mean (the "NOT brain volume" does not
quite clarify it to me).

Do you mean everything within the skull? If so, computing the volume
of an anatomical mask might be reasonable. However those are not meant
to be precise. To be precise might mean measuring the volume contained
within the pial surface.

Do you mean only Grey matter? If so, the most accurate measurement
might come from FreeSurfer (though Ziad might have to chime in for where).
SurfaceMeasures has a volume computation that is applied between the pial
and smoothwm surfaces as well.

Is there a different computation you are looking for, such as literally
within the skull?

- rick