Hello Domenico,
> I'm conducting a multiple regression study on a population of
> subjects. The aim of the study is to find a correlation between the
> GM density maps and some neuro-psycological scores. To do this I've
> used the toolbox "vbm8" to segment the images and the multiple
> regression analysis of spm. For the normalization step I used Dartel.
So far, so good :)
> I intentionally removed the modultaion step because the images
> segmented with modulation were hyper-intense in the subcortical
> regions. Without modulation the GM images seem to be right.
Generally, you should make your choice re: modulation (and which way of
modulation, but I won't go there :) depending on your question. I am
therefore not sure I would agree with your rationale: modulation
incorporates the volume changes occurring during spatial normalization
into the tissue maps. If the images indeed look wrong following
modulation, then something went wrong during spatial normalization. Yes,
this will not be as obvious if you look at tissue density instead, but
it will still have gone wrong, and should worry you. Having said that,
there was a recent mail to the list to which Christian replied yesterday
which may give you some clues to the effect that this may not be "wrong"
after all.
> segmentation step I've applied a smoothing kernel of 10x10x10. For
> the multiple regression analysis I introduced the psycological scores
> as covariates and the smoothed GM maps as volumes.
You mean the sum of voxel values from the smoothed maps? Since you are
not modulating, this should not be necessary (see another all-time
favorite mail of mine from Christian on September 15, 2005).
Hope this helps,
Marko
--
____________________________________________________
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
[log in to unmask]
http://www.medizin.uni-tuebingen.de/kinder/epn
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