Dear SPM'ers
I'm studying effects of general anesthesia on rCBF using [15O]H2O and PET.
I've made a substraction analysis (n=8) of conditions "concentration1" -
"awake". Results show a large (>16000 voxels) highly significant cluster.
Problem is that the cluster is situated partly outside brain tissue; around
hypophysis. I suspect this is due vascular effect/ artefact. Furthermore,
the artefact-part of the cluster seems to be 'cut' or partly masked out from
fronto-inferior side.
I've used FullMonty model with
proportional global scaling (mean to 50),
no covariates or nuisance variables,
proportional treshold masking (0.8 as treshold)
no implicit mask,
masked images explicitly with brainmask.img
and - global mean calculation using fullmean/8 mask
We figured that brainmask.img could be manually altered to exclude this
artefact area from the analysis. Using result rendering I've visualised the
cluster on brainmask.img. Although the shape of cluster resembles reasonably
well that of the mask, the outlines do not match exactly. This made me
suspicious; are there other factors/procedures contributing to restrict the
area of analysis, which should be delt with before/rather than tamper with
explicitly used mask?
Should I proceed with the initial idea of manually altering the
brainmask.img with, say, MRIcro? If not, I would most appreciate if someone
could suggest an alternative solution?
Yours,
Kaike
Kaike Kaisti
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TUCH/ Dept of Anesthesiology and Intensive Care
P.O.box 52 ; 20521 TURKU ; Finland
tel+358-2-2611611 ; fax+358-2-2613960
www.utu.fi/med/pet/anesthes.htm
www.utu.fi/med/pet/index.html .
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Laikkakuja 14 B ; 20660 Littoinen ; Finland
tel+358-2-2365446 ; GSM:040-7686871
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