Dear SPMers,
in a VBM analysis I compared 60 psychiatric patients and 70 healthy controls, looking for GM volume differences in SPM5/Matlab 7.1 on Windows.
To my slight surprise, GM volume differences appear to be entirely outside the brain. In the attached image you see areas of GM volume reduction in patients overlayed on an unsmoothed normalised GM segment mwc1*.img. If I overlay the results on the SPM5/canonical/single_subj_T1.nii, it's the same: GM volume reductions do not at all overlap with the GM.
If I overlay the results on the mask.img, results are all inside the mask, right at its edge.
For my analyses I used the smwc1*.img and the following design:
PET / two-sample t-test
Independence: yes
Variance: unequal
Grand mean scaling: No
Ancova: No
Absolute threshold 0.05
Implicit mask: yes
Explicit mask: no
Global values: user-defined GM-volume integrals (from the smwc1*.img)
Overall grand mean scaling: No
Normalisation: proportional scaling
I am aware that VBM-GM results may appear to extend GM, see John's reply to a similar problem in VBM/SPM2b, 7 May 2003:
http://www.jiscmail.ac.uk/cgi-bin/webadmin?A2=ind03&L=SPM&P=R166060&I=-3
"* There are likely to be some non-brain regions misclassified as GM,
which could appear outside the glass brain.
* The spatial normalisation is not 100% exact (cortical surface is
usually registered within about 1cm).
* Smoothing will spread signal outside the glass brain."
However, I am puzzled that these GM-results seem to be ENTIRELY outside the brain (and only towards the CSF, not towards WM), and do not even overlap a bit with the GM in the template or the normalised GM segments. It just looks more like a CSF result ;)
I tried to check for obvious errors. But the segmentation seems to have gone well for all images (that means, the first reason given by John is unlikely in this case if I am not mistaken). Looking with Checkreg at the smwc1*.img and mwc1*.img, they seem to be all in the same space, orientation, voxel-size etc.
I would be very grateful for your opinion:
Is this a true result (given John's explanations 2 and 3 above)?
Or has something gone wrong? If so, any ideas where, when and how??
Many thanks for any suggestions,
Nicolas
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