Dear Paul,
It sounds as though you have checked the raw global intensities for outliers
etc, which one could do with something like:
P = spm_get(inf,'img', 'Select all the images in the analysis');
V = spm_vol(P);
for i = 1:length(V)
g(i) = spm_global(V(i));
end
g
The compression effect you are seeing implies that there is a much greater
range in the patient's globals than for the controls.
Have you checked whether the globals correlate with the design - for the
depressed patients for example?
In PET, higher global counts may be associated with better signal to noise - I
guess this might explain some of what you are seeing.
See you,
Matthew
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|