Hi Eric,
I've got short questions on your post. I guess I've missed a step in the
VBM procedure...
>Jurgen,
>
>VBM of Ashburner and Friston uses as the dependent variable not the signal
>from the MRIs per se but rather the posterior probability of a voxel being
>gray as an operationalization of gray matter concentration. So, the
>difference in scaling of your MR images would seem to be completely
>irrelevant to the validity of comparing the images via VBM.
>
>
I agree with that ... it does not matter for tissue classification. Now,
as almost everybody, I suppose Jurgen will modulate his images and thus,
will take into account initial values 'whatever' nomalization
transformations, right ? If you have big scalling differences as in
Jurgen's images, does these differences also be in the modulated
normalized images?
>Also, as Yong
>just pointed out in his reply, the ratios of signal intensities seem to be
>preserved across the images, supporting that the difference in absolute
>signal intensity is not indicative of a failed MR pulse sequence. In
>summary, if your MRIs look valid, then VBM should be valid, regardless of
>overall differences in scaling.
>
>Eric
>
If I'm right hereabove, the 'huge' scalling factor will mask 'small'
differences that are usually observeb (or maybe I'm completely wrong >:o
). In this case it needs to be modelled (?)
Best
- cyril
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