Hi Jürgen,
>Dear SPM users
>
>After segmentation one can see the means of the GM, WM, CSF segments
>in the graphic window of SPM2. I think that this value is the mean voxel
>intensity of the corresponding segment.
>I have logitudinal data from 4 different years and intend to do VBM.
>But I have seen that the mean voxel intensities of my segments differ
>extremely.
>e.g. GM / WM / CSF segments from 2004: 253 / 348 / 107
>e.g. GM / WM / CSF segments from 2003: 30 / 41 / 13
>
>
Well, it looks strange :-\
Are these data processed with the same matlab/spm?
Did you change something in the MRI? different machine or different
acquisition procedure? If it is the case, then enter that as a
categorical covariable (01) in your design to take into account this effect.
>Is it possible and scientifically valid to compare segments with such
>different voxel intensities using VBM?
>
>
If you have the same 'strange' effect for each subject, I would say that
it's OK as long as you work with covariables. You will then just look at
relative differences.
Hope it helps you, a bit...
Good luck
- cyril
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