Dear Greg,
the jy*-images are jacobian determinant maps of the deformation vector field. That is, they represent volumetric changes at the voxel-level, where voxel-values above 1 represent shrinkage and voxel-values below 1 represent expansion of the voxel in the warped image to match the template. (Smoothed) jacobian-maps can be directly used to assess volumetric differences between groups of subjects, irrespective of the tissue classes these differences may belong to (tensor-based morphometry). See for example: Teipel et al., 2007 (http://www.ncbi.nlm.nih.gov/pubmed/17827035) or Lee et al., 2007 (http://www.ncbi.nlm.nih.gov/pubmed/17161622).
They are also often used to "modulate" tissue-segments (via multiplication) for voxel-based morphometry. An integration of high-dimensional image warping into VBM (like in the DARTEL-VBM approach) may therefore be referred to as "TBM-like VBM" (Ashburner J, Friston KJ, 2001. Morphometry. In
Human Brain Function, 2nd edition, Frackowiack RSJ, Friston K and
Frith C, eds. (Elsevier LTD, Oxford): 707-725.)
Hth,
Michel
Date: Mon, 27 Sep 2010 13:02:18 -0400
From: [log in to unmask]
Subject: [SPM] high dimensional warping
To: [log in to unmask]
I've
successfully used HDW to warp some images to a template and then
extract what I need, so everything is working out perfectly.
My question is what do the intensity values represent in the jy_
images that are produced during warping? They appear to be the
magnitude of warping occurring at the particular voxel. Greater
than 1 for positive movement, less than 1 for negative movement?
-Greg
