Janosch,
it indeed looks like the GM maps from your second timepoint are somehow
systematically bigger than the ones from the first. This may of course
well be the case if you look at the developing brains of younger
children, but the effect should not be that obvious when normalizing to
a common stereotaxic space. Whether this is good or bad news depends on
how you normalized to what priors and what maps you investigated and so
on, so it is not easy to say for sure how to fix it. There is of course
clear evidence for increasing white matter volume in this age range as
well as GM (if you are in the, say, 6-9 years of age range), so the "GM
decrease" in the inner GM boundary may indeed be a white matter
increase, which of course you could test. Also, the results look fairly,
but not suspiciously symmetrical as there are some obvious
discrepancies, so I don't think there's erroneous flipping involved.
Perhaps Christian could elude some more but I think he may still be in
China.
Cheers,
Marko
Janosch Linkersdörfer wrote:
> Dear list,
>
> we are doing a longitudinal analysis with elementary school children
> with one year in between (so far) 2 measurement points.
>
> We used the standard longitudinal workflow of the VBM toolbox. For
> testing purposes, we just looked at GM changes over the two measurement
> points (using the statistical model for one group as described in the
> VBM-Toolbox manual on page 19).
>
> We find results that we can´t explain: The contrast testing for growth
> of GM (year2-year1) results in significant voxels on the outside border
> of the mask, while the contrast testing for reduction of GM
> (year1-year2) finds those only on the inside border. Also, these effects
> appear to be very symmetrically. We now wonder whether the effects
> really represent GM growth/reduction patterns or are due to some
> error(s) in the image processing (like systematic registration error
> differences between the images of a participant from the first and
> second year).
>
> To display the effects more clearly, we attached overlays with a very
> high threshold (p=0.1)
>
> It would be really great, if you could help us in interpreting these
> results and what might have gone wrong!
>
> Thanks,
>
> Janosch
--
____________________________________________________
PD Dr. med. Marko Wilke
Facharzt für Kinder- und Jugendmedizin
Leiter, Experimentelle Pädiatrische Neurobildgebung
Universitäts-Kinderklinik
Abt. III (Neuropädiatrie)
Marko Wilke, MD, PhD
Pediatrician
Head, Experimental Pediatric Neuroimaging
University Children's Hospital
Dept. III (Pediatric Neurology)
Hoppe-Seyler-Str. 1
D - 72076 Tübingen, Germany
Tel. +49 7071 29-83416
Fax +49 7071 29-5473
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http://www.medizin.uni-tuebingen.de/kinder/epn/
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