Hi Carl,
it is a bit of a problem that your covariate (filter) and your group (no
healthy control scanned with filter 1) are confounded. You could still
try to test the effect of including the covariate using an F-test, and
quantify how much variance it explains. If this is "not much" (however
defined), you may be able to argue that the covariate is irrelevant and
that you can go without. Actually, I would expect the effect to be very
small indeed for structural data in any case, while it could be more of
an issue when using fMRI or DTI sequences.
But that is just my 2 cents.
Cheers,
Marko
Carl Johan Ekman wrote:
> Dear SPM (VBM) users! We have scanned a large number of patients and
> healthy control subjects on the same scanner over a long period of
> time using the same camera settings. During this time a software
> filter in the camera has been updated two times, the second time the
> camera was also moved to a different location. There are no visible
> differences betweeen images acquired using the different filters but
> I suspect that there may be slight differences in intensity
> distribution. The images are used for VBM analyses, segmented using
> DARTEL in VBM8. When checking images using "Sample Homogeneity"
> images from different filters does not group and are scattered
> "randomly" in the boxplot. I believe I probably still have to correct
> for "Filter setting" in my analyses so my question is how do I do
> this?
>
> Is it possible to use a categorical variable (Filter1, 2, 3) as a
> nuisance variable in regression analyses?
>
> If comparing, for example, Patients vs. Controls, should I use a 3x2
> ANOVA? Then my problem is that no healthy controls has been scanned
> using filter 1.
>
> Any other ideas? Any other ways to check if I need to correct for
> filter setting in my analyses?
>
> I would be grateful for all the help I can get!
>
> / Carl Johan Ekman, MD
>
--
____________________________________________________
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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