Dear SPM users,
I would like to ask some questions about VBM. We have some discussion
amongst colleges about the differences between modulated and unmodulated
images that are calculated during the segmentation step in SPM5 and VBM in
general. I would be great if you can help us discuss these topics because
reading the VBM posts didn't completely resolve our issues. The questions
are as follows:
1. Am I correct in assuming that theoretically there shouldn't be any
structural difference between the modulated and unmodulated images, only
intensity differences? When I subtract the binarized modulated from the
binarized unmodulated images all that seems to remain are some voxels around
the edges of the brain and ventricles.
2. When you calculate the modulated and unmodulated images in the same
segmentation step I interpret them as being complementary to each other. So
if I for example find significant differences in the modulated images
between two groups this might result from a volumetric difference between
those two groups in that region. But the difference might also stem from an
initial difference in concentration that was already present before
modulation. So if I would then have a look at my unmodulated images and I
find no differences in that specific area I can be a bit more confident in
assuming that the differences I found in the modulated comparison were due
to volumetric differences. But if I find differences in the unmodulated
images as well things become a bit more tricky. Does this sound reasonable
or am I missing something?
3. There has also been some discussion in the SPM mailing list about using
total gray matter as a covariate or total intracranial volume (and there are
a few more options). When you compare two groups (patient vs control) and
you find a lot of regional differences that are all in the same direction
(for example always patient > control) this will also create a global
difference in gray matter. When you subsequently use total gray matter as a
covariate you will mask all interesting regional differences. Therefor I am
more inclined to use some measure of total intracranial volume as opposed to
total gray matter, despite of the difficulties in determining CSF. Does
anyone have any suggestions regarding this topic?
Thanks very much!
Harma Meffert
University Medical Center Groningen
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