[MINC-users] determinant of non-linear local deformation fields (fwd)

[Andrew Janke]

2008/8/8 Marc BOUFFARD <bouffard at bic.mni.mcgill.ca>:

> I am trying to apply the method used in SPM VBM but with the minc tools.
> Assuming we have the gray matter classified images linearly transformed
> then in "standard VBM" we would just apply the linear model to that and
> solve for a contrast of interest with glim_image, e.g., group 1 - group 2.
> Now, if the data is additionally non-linearly transformed another step
> called modulation can be applied:
>
> "in order to preserve the actual amounts of gray matter within each
> structure, a further processing step that multiplies (modulates) the
> images by the relative voxel volumes can be incorporated.  These relative
> volumes are simply the Jacobian determinants of the deformation field."
> (Ashburner and Friston, Neuroimage 11 805-821)
>
> So according to that we just need to multiply by the Jacobian determinants
> but apparently the deformation fields calculated in SPM are relative to
> the target and not the source like with nlpfit (does that make sense?).
> So, logically if the deformation fields are relative to the source to the
> target(as you mention above) we should multiply by the voxel-wise inverse
> of the Jacobian determinant to get the same effect as with SPM?

Correct.  (hows that for a short answer to a long question! :)



-- 
Andrew Janke - andrew.janke at anu.edu.au
Department of Geriatric Medicine, ANU
(a.janke at gmail.com || http://a.janke.googlepages.com/)
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