Hello,
We are using VBM8 in SPM8 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 VBM8. 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 the SPM VMB8 manual 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
However 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"
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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