Hi Jeff,
> Dear FSL_VBM experts,
> Im a bit confused about what the VBM analysis shows. It
> looks to me
> like it investigates focal density differences in GM/WM
> however the
> website http://www.fmrib.ox.ac.uk/fsl/fslvbm/index.html says
> that it
> "investigates voxel-wise changes in the grey matter
> volume/topography". If someone could clarify what VBM
> results mean and
> what the difference between my interpretation of the
> results and the
> websites.
According to Good et al., 2001, the optimised protocol including the Jacobian modulation step leads to results in grey matter "absolute amount" or "volume", while the standard protocol with no modulation leads to (not interpretable) results of "concentration" or "density" (NOT recommended anyway).
> Also is there a way to use VBM to get specific regional
> volume
> differences with actual numbers (say in mm^3)?
Yes, just multiply the volume of the ROI (e.g., whole brain, your significant results etc.) by the mean of the values in the smoothed registered GM inside the ROI.
For instance:
fslmaths mysign_results -thr 0.95 -bin mask_mysign_results
then fslstats mask_mysign_results -V will give you the volume (take the one in mm2) of the ROI, and finally:
fslmeants -i GM_mod_merg_s3 -m mask_mysign_results -o mean_GM_volume.txt will give you the mean GM values for each subject that you'll have to multiply by the volume of the ROI.
Cheers,
Gwenaelle
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Gwenaëlle Douaud, PhD
FMRIB Centre, University of Oxford
John Radcliffe Hospital, Headington OX3 9DU Oxford UK
Tel: +44 (0) 1865 222 523 Fax: +44 (0) 1865 222 717
www.fmrib.ox.ac.uk/~douaud
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