Hi Jürgen,
I guesss you used grand mean scaling for your 2004 data.
After segmentation, what you will get are probability maps
and you can scale them to whatever values you want.
I did a little calculation. 253 divided by 30 is about 8.43
and if you multipy 41 abd 13 with 8.43, you will get 346 and 110 respectively,
very close to 348 and 107.
Cheers,
Yong
On 7/22/05, Cyril Pernet <[log in to unmask]> wrote:
> 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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