Dear Emmanuel,
Be careful if you get any result in the ventricles (or in white matter)
although you made an analysis of gray matter. This effect is presumably
caused due to very low values of gray matter and therefore the probability
of gray matter is much lower than white matter or CSF. You should use an
absolute threshold for masking around 0.1 (in subsequent analyses you can
increase this value to 0.2). This prevents that you get (opposite) results
in white matter or CSF although you only made an analysis for gray matter.
This effect is due to the negative correlation of gray and white matter (or
CSF) intensity values around the border between different tissues. If you
choose an absolute threshold masking you will only compute your statistic in
those areas that are above this threshold.
Best regards,
Christian
____________________________________________________________________________
Christian Gaser, Ph.D.
Department of Psychiatry
Friedrich-Schiller-University of Jena
Philosophenweg 3, D-07743 Jena, Germany
Tel: ++49-3641-935805 Fax: ++49-3641-935280
e-mail: [log in to unmask]
http://dbm.neuro.uni-jena.de
>Hi all,
>
>In a VBM analysis of a patient group vs. a control group I get
>significant differences in the patient group which correspond to
>enlarged ventricles. I don't know how to report this. Should I just
>accept that there should have been some tissue there, enlarged
>ventricles means atrophy and report it as such, or should I utilise more
>nonlinear parameters in the normalisation stage to bring the ventricles
>back to normal and consequently report differences in grey/white matter
>concentrations that will hopefully arise?
>
>Any opinions, references would be greatly appreciated.
>Many thanks,
>Emmanuel
>
>________________________________________________________________________
>Emmanuel A Stamatakis PhD
>Department of Experimental Psychology
>University of Cambridge
>Downing Street
>Cambridge CB2 3EB
>U.K. http://csl.psychol.cam.ac.uk
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