Hi Ben
> 1. Is normalizing to mni-Space reasonable in Schizophrenic patients (given that recent Meta-analyses indicate Morphologic differences between Schizophrenic & normal subjects)?
I wouldn't expect this to be a problem. The template produced by
DARTEL, which is used to estimate the transformation to MNI space,
will include both people with schizophrenia and controls, and I
suspect will be similar enough that you would get a reasonable
normalization to MNI space. In general people seem to have success
with all sorts of populations who may differ from the MNI-space
template (dementia patients, older adults, children...), so it should
be fine. If anyone has had difficulty with this step, though, it
would be good to hear about it...
> 2. Regarding the statical analysis: Is it valid to restrict the group-comparison of Gray matter differences to the Gray matter by restricting (masking) the Analysis to Grey matter?
Presumably for this analysis, the images you are putting in are the
gray matter segmentations. Thus, any results you get are already in
some sense restricted to gray matter. But because the segmentations
aren't always perfect, there are things that crop up outside, and so
some sort of masking is usually a good idea. It seems that absolute
or proportional thresholds are both quite common, although you could
also use a gray matter mask, as you suggest.
> 3. Most Studies use DARTEL/VBM to investigate Gray matter differences - should it be used to investigate White matter abnormalities?
I don't think there is any principled reason not to do this. I
suspect that the reason you don't see more VBM for white matter is
there is that people my feel that DTI or some similar method may
provide a better measure of tissue integrity (e.g. fractional
anisotropy) than what you would get with the volume measurements
provided by VBM. But I imagine there are useful things that can still
be gleaned from VBM in white matter. Although, perhaps others have
more experience with this?
Hope this helps!
Best regards,
Jonathan
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