Dear Bryn,
It is arbitrary which way around the Jacobian is calculated -
that is, from reference to original space, or from the original
to the reference space.
Hence the difference between FSL and SPM is simply a
different choice of convention and reflects nothing more than
that. The principle is exactly the same.
All the best,
Mark
On 30 Jun 2011, at 20:46, Bryn Levitan wrote:
> Hello,
>
> We are using FSL-VBM for voxel-based morphometry analysis and hope to identify the volumetric changes between patients and normal controls. I have a few questions regarding GM modulation.
>
> 1. The first question is with regards to the Jacobian determinant, which is an output from VBM. The Jacobian determinent represents the relative volume between the subject and template. I am unsure about the directionality. If |Jacobian determinant| > 1, does it mean that the subject has a larger volume than the template? If |Jacobian derminant| is less than 1, does it mean that the subject has a smaller volume than the template?
>
> 2. In FSL, "the registered partial volume images were then modulated (to correct for local expansion or contraction) by dividing by the Jacobian of the warp field."
> However in the SPM VMB8 manual, it says the modulated GM is calculated as: "Affine+non-linear produces tissue class images in alignment with the template, but multiplies ("modulates") the voxel values by the Jacobian determinant derived from the spatial normalization.
>
> Why is modulation done differently in SPM and FSL (multiply vs divide)?
>
> I would really appreciate anyone's help in clarifying these matters.
>
> Thank you!
> Bryn
>
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