Dear Manuel,

Instead of ANCOVA, you should rather use 'proportional scaling', as is done is the reference you mention:
Normalization of global cerebral metabolism was performed using proportional scaling to a mean brain blood flow of 50 mL per 100 g per minute and an analysis threshold masking by 80% of mean brain blood flow.
The rest of the design looks fine otherwise.

Best,
Chris

===================================================
Christophe Phillips, Ir, PhD
FRS-FNRS. Research Associate
Adjunct assistant professor in applied sciences

Cyclotron Research Centre, B30
University of Liege, Sart Tilman
4000 Liege, Belgium
Tel:  +32 4 366 2316 (secr.)
        +32 4 366 2366
Fax: +32 4 366 2946
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web: http://www.cyclotron.ulg.ac.be
===================================================

Le 3/06/2013 22:06, Manuel Schütze a écrit :
[log in to unmask]" type="cite">
Hi there,

I would like to compare two 18-FDG PETs from the same patient, before and after chemoterapy, similar to what Hsieh and coleagues did (http://www.ncbi.nlm.nih.gov/pubmed/23320861).

I read some old e-mails on the group on paired t-test and just wanted to check the parameters.

On subject design I choose paired t-test and added 11 pairs, each with the pre and post treatment image. Grand mean scaling and ANCOVA = yes. Threshold masking = relative, 0.8. Overall grand mean scaling = yes, 50. Normalization = ANCOVA. The design matrix is attached.

On results I choose the t-contrasts -1 1 and 1 -1. FWE = 0.05. The first one is below:

 Inline image 3

The test didn't show any results, so I just wanted to check if everything is allright with the parameters or maybe I need a bigger group for some consistent results.

Best regards,

--
Manuel Schütze, MD
Post-graduate student
CIMol - Center for Molecular Imaging
Faculty of Medicine - Federal University of Minas Gerais
Belo Horizonte, Brazil