Hi all,
I am doing a VBM analysis, using the VBM8 toolbox. Gray matter images are modulated. I've used the get_totals script to extract the gray matter volume within an ROI for a single subject, which was 0.36. Now my question is, what do this value reflect in modulated gray matter images? Is this value plausible, and how do I interpret it?
All comments are appreciated!
Best,
Lasse
If you're using the modulated output (m0), then the output is the "absolute amount of tissue corrected for individual brain sizes". The value is plausible.
If my memory serves, I would interpret the 0.36 is the 36% grey matter in that area after correcting for differences in brain size. Sadly it's not as easy as GM + WM = Total Brain Volume, because the toolbox uses partial volume estimation, meaning that the combination of GM, WM, and CSF can be greater than 1.0 (100%) due to mixed classes.
More info can be found on pages 27-29 in the VBM8 manual. The bit on the Partial Volume Estimation was posted by the VBM8 authors on the SPM mailing list years ago.
Thank you very Peter!
I found the partial volume estimation part of the VBM manual very helpful.
Indeed, modulated output takes into account the size and shape of the brain. Therefore, I think it represents both volume and density. Have a look at the attached paper.
http://www.sciencedirect.com/science/article/pii/S1053811901907864
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