Hello Listmembers,
I have previously used SPM2 sucessfully, but I am using SPM5 for the first
time to perform a VBM analysis. It is also the first time that I am using
3T scanner data. By comparison to our earlier 1.5T data the gray
matter/white matter contrast is lower. Inhomogeneity has also been a
problem with this data. Reducing the spatial frequency of the bias
correction in the Segment function has helped, based on visual comparison
of segmented data to the original scans.
Compared to my SPM2/1.5T data results, I have noticed that the voxel
posterior probabilites in the gray matter segments are lower. My SPM2 gray
matter segments contained mostly probabilities near 1 or 0 (which I expect,
if I understand some of the published or SPM list discussions of
segmentation). The cortical regions at the edges of the image, where the
inhomogeneity has been a problem, have values more in the range of .75-.85
(perhaps less). I can't fully correct the inhomogeneity problem, it seems,
without inaccurately reclassifying gray matter as white matter. Does
SPM5's Unified Segmentation contribute to the greater variability in the
probability values? More importantly, can I still expect valid estimates
of group effects?
Thanks.
Ken
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