Dear Doug
I haven't use SPM for PET studies yet, but since that for fMRI it can detect
sometimes less than 0.5% changes, it would surprise me that 5% would not be
detected even in 18 single subject scans.
> One of the major issues in the use of any analysis technique is its
> "real-life" ability to detect change.
> We wanted to get an idea of the degree of change (i.e. % change) between
> two
> groups that can be detected using SPM and the usual number of subjects.
>
> To achieve this we took [18F]-setoperone scans (measuring cortical
> 5-HT2
> receptors) of 18 normal subjects. We randomly assigned them to two
> groups.
> We observed that SPM did not report any group differences at baseline
> We then added 5%, 15%, 35% and 50% increase
> in one of the groups in a bilateral frontal region in each original
> image of
> the group member. We compared the two groups using single subject, no
> covariates and
> age as a nuisance since there is a strong decline with age for [18F]
> Setoperone. Scans were
> normalized with a ligand specific template and smoothed at 12 mm. Since
> these were parametric
> images no global scaling was used.
I may be miss understand what you mean by 'parametric', but here is perhaps one
key to your problem. Gobal scaling remove the variance part due to the global
amount of radioactivity recorded during each scan (I supposed that this was the
part you wanted to remove with age as nuisance variable). If you haven't use
this as a possible confound value in your model than it may be that this
important part of undesirable variance in PET data mask the effect that you
injected in your scans. A first suggestion would be to run your stat again
using glob scaling.
> Much to our surprise, SPM did not detect these changes using its
> conventional levels of significance. Only the 50% increase showed p<0.01
> at a corrected
> cluster level. Please see attachment
>
> This raises some interesting issues regarding the power of SPM to detect
>
> these changes. Looking at the result outputs however, it suggests that
> the activation had actually started appearing in the
> uncorrected images, much ahead of the false positives. Could it be that
> the conventional criteria (corrected values for
> K and voxel) are far too conservative for this iteration.
>
> Thanks in advance for any comments
>
> Doug Hussey
> PET Centre
> Centre for Addiction and Mental Health
> Toronto Canada
Best regards
Jack_
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