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Dear Manuel and Christophe, We've generally used ANCOVA rather than proportional scaling, in order to avoid scaling the noise with the signal (for application in paired PET studies analysed with SPM, see e.g. PubMed identifiers PMID 18809501, PMID 17301080). I don't know whether your FDG PETs were absolutely quantified - probably not, and you may have the then typical inter-subject coefficients of variation for global values of ~30%. You also seem to have two outliers with unusually high values; it's always worth checking those for any anomalies. I wonder whether it is then a good idea to scale the two groups separately; maybe you could rather try the following design matrix (extract): Hope this helps, All the best, Alexander ----------------------------- Alexander Hammers, MD PhD Chair in Functional Neuroimaging Neurodis Foundation http://www.fondation-neurodis.org/ Postal Address: CERMEP – Imagerie du Vivant Hôpital Neurologique Pierre Wertheimer 59 Boulevard Pinel, 69003 Lyon, France Telephone +33-(0)4-72 68 86 34 Fax +33-(0)4-72 68 86 10 Email [log in to unmask];[log in to unmask] --------------------------------- Other affiliations: Visiting Reader; Honorary Consultant Neurologist Division of Neuroscience and Mental Health, Faculty of Medicine Imperial College London, UK --------------------------------- Honorary Reader in Neurology; Honorary Consultant Neurologist Department of Clinical and Experimental Epilepsy National Hospital for Neurology and Neurosurgery/ Institute of Neurology, University College London, UK On 4 juin 2013, at 09:14, Christophe Phillips wrote: > > 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 > email: [log in to unmask] > web: http://www.cyclotron.ulg.ac.be > =================================================== > > Le 3/06/2013 22:06, Manuel Schütze a écrit : >> 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: >> >> <Mail Attachment.jpeg> >> >> 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 >