DBM usually refers to looking at volume changes using the determinant of the Jacobian matrix field, referred to as “the Jacobian” in SPM. This is calculated during spatial normalization steps, and saved with the “j_*.nii” prefix.
The Jacobian is analyzed like any other map.
From J. Ashburner and K.J. Friston. Morphometry. In R.S.J. Frackowiak, K.J. Friston, C. Frith, R. Dolan, K.J. Friston, C.J. Price, S. Zeki, J. Ashburner, and W.D. Penny, editors, Human Brain Function. Academic Press, 2nd edition, 2003. [PDF] Keyword(s): introduction. [bibtex-entry]
(pdf at bottom of page)
“The field obtained by taking the determinants at each point gives a map of structural volumes relative to those of a reference image [14, 18, 11, 10].”
Thus, the Jacobian as termed in SPM has at each voxel a scale (ratio) relative to the template volume. Thus, a value of 1.1 would be a voxel with 10% higher volume (native > template), whereas a value of 0.95 would be 5% lower volume (native < template).
DBM using field
More sophisticated DBM would use the actual field:
