Jurgen,
VBM of Ashburner and Friston uses as the dependent variable not the signal
from the MRIs per se but rather the posterior probability of a voxel being
gray as an operationalization of gray matter concentration. So, the
difference in scaling of your MR images would seem to be completely
irrelevant to the validity of comparing the images via VBM. Also, as Yong
just pointed out in his reply, the ratios of signal intensities seem to be
preserved across the images, supporting that the difference in absolute
signal intensity is not indicative of a failed MR pulse sequence. In
summary, if your MRIs look valid, then VBM should be valid, regardless of
overall differences in scaling.
Eric
----- Original Message -----
From: "Jürgen Hänggi" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Friday, July 22, 2005 9:10 AM
Subject: [SPM] Mean value from segmentation
> 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
>
> Is it possible and scientifically valid to compare segments with such
> different
> voxel intensities using VBM?
>
> Thanks a lot in advance
> Best regards
> Juergen
>
>
> ----------------------------------------------
> Juergen Haenggi, Ph.D. student
> Psychiatric University Hospital
> Division of Psychiatry Research
> University of Zurich, Switzerland
> P.O. Box 1931
> Lenggstrasse 31, 8032 Zurich
> 0041 44 384 26 10 (office phone)
> 0041 76 445 86 84 (mobile phone)
> 0041 44 384 26 86 (fax)
> H 115 (office room number)
> [log in to unmask] (division email)
> http://www.dpr.unizh.ch/ (division website)
> http://www.juergenhaenggi.ch (private website)
> ----------------------------------------------
>
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