Hi all SPM users,
I am doing a VBM analysis, using the CAT12 toolbox. Gray matter images are modulated. I've used the ROI analysis to extract the gray matter volume within ROIs for a single subject, which my values are decimal like 0.36. Now my question is, what do this values reflect in modulated gray matter images؟ (In comparison with fslstats which we have volume in mm3.)
Thank you so much in advance for your time and help!
Best wishes,
My understanding is that they calculated from the voxel intensities and they are not a physical representation of "volume", which is something you can calculate from FSL or FreeSurfer.
Read the original VBM paper by Ashburner and Friston and see if that helps. Good luck!
thank you for your answering.
I want to have regional differences in concentration (density) (not in absolute volume ). So, can I use CAT12 for this purpose?
Because I noticed that in CAT12 partial volume estimation is used, where the voxel values represent the fractional volume for one tissue type.
In VBM the voxel value in a modulated image is defined by the volume change during non-linear transformation. So, modulated images only exist in a certain template space after diffeomorphic transformation.
The pve-images are indeed representations of the partial volume estimation, it will only tell you, to what degree a voxel belongs to a certain tissue. Of course, these images can be transformed into template space, too, but won't tell you anything about volume anymore. (-> normalization removes volume differences)
I'm not sure, whether concentrations of MRI images are ideal for comparison. Perhaps intra-subject it could work, but between subjects you will surely need something for intensity normalization, as MRI does not produce absolute values in regards to the used measurement parameters (as e.g. CT will do with it's Hounsfield units).
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