Dear Justine,
> I have read that VBM generates a map of differences/correlations with voxel
> intensities. These voxel intensities represent volume of GM/WM, or
> probability of being in GM/WM.
>
> But this is confusing because volume of GM/WM seems like a very different
> thing from the probability of being in GM/WM. So which is it? If it is
> both how do they mean the same thing?
During tissue class segmentation, the intensity assigned to a voxel
reflects its probability of belonging to a tissue class. So, after
segmentation, voxel values lie between 0-1.
The voxel intensities on which the statistics are run depend on the
image pre-processing that has been done. Typically for gray matter
images, this will be either unmodulated ("preserve concentration") or
modulated ("preserve total"). An unmodulated analysis can generally
be thought of as looking at gray matter probability/concentration, and
a modulated analysis volume. Though sometimes different authors use
different terms (or synonyms), which can get confusing.
If you have access to it, the following article may also be of some
use (e.g., the section "Concentration or Volume Differences?"):
Mechelli A, Price CJ, Friston KJ, Ashburner J (2005) Voxel-based
morphometry of the human brain: Methods and applications. Current
Medical Imaging Reviews 1:105-113.
Hope this helps!
Best regards,
Jonathan
--
Dr. Jonathan Peelle
Department of Neurology
University of Pennsylvania
3 West Gates
3400 Spruce Street
Philadelphia, PA 19104
USA
http://jonathanpeelle.net/
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