title: Normalization in PET group comparison studies--the importance of a valid reference region.
abstract: BACKGROUND
  In positron emission tomography (PET) studies of cerebral blood flow (CBF) and metabolism, the large interindividual variation commonly is minimized by normalization to the global mean prior to statistical analysis. This approach requires that no between-group or between-state differences exist in the normalization region. Given the variability typical of global CBF and the practical limit on sample size, small group differences in global mean easily elude detection, but still bias the comparison, with profound consequences for the physiological interpretation of the results.
  
  
  METHODS
  Quantitative [15O]H2O PET recordings of CBF were obtained in 45 healthy subjects (21-81 years) and 14 patients with hepatic encephalopathy (HE). With volume-of-interest (VOI) and voxel-based statistics, we conducted regression analyses of CBF as function of age in the healthy group, and compared the HE group to a subset of the controls. We compared absolute CBF values, and CBF normalized to the gray matter (GM) and white matter (WM) means. In additional simulation experiments, we manipulated the cortical values of 12 healthy subjects and compared these to unaltered control data.
  
  
  RESULTS
  In healthy aging, CBF was shown to be unchanged in WM and central regions. In contrast, with normalization to the GM mean, CBF displayed positive correlation with age in the central regions. Very similar artifactual increases were seen in the HE comparison and also in the simulation experiment.
  
  
  CONCLUSIONS
  Ratio normalization to the global mean readily elevates CBF in unchanged regions when a systematic between-group difference exists in gCBF, also when this difference is below the detection threshold. We suggest that the routine normalization to the global mean in earlier studies resulted in spurious interpretations of perturbed CBF. Normalization to central WM yields less biased results in aging and HE and could potentially serve as a normalization reference region in other disorders as well.
authors.affiliation: PET center, Aarhus University Hospitals, Denmark; Center of Functionally Integrative Neuroscience (CFIN), Aarhus University, Denmark. Electronic address: per@pet.auh.dk.
authors.forename: Per
authors.initials: P
authors.lastname: Borghammer
authors.affiliation: Center of Functionally Integrative Neuroscience (CFIN), Aarhus University, Denmark.
authors.forename: Kristjana Yr
authors.initials: KY
authors.lastname: Jonsdottir
authors.affiliation: Center of Functionally Integrative Neuroscience (CFIN), Aarhus University, Denmark.
authors.forename: Paul
authors.initials: P
authors.lastname: Cumming
authors.affiliation: Department of Neurology, Aarhus University Hospitals, Denmark.
authors.forename: Karen
authors.initials: K
authors.lastname: Ostergaard
authors.affiliation: PET center, Aarhus University Hospitals, Denmark.
authors.forename: Kim
authors.initials: K
authors.lastname: Vang
authors.affiliation: Center of Functionally Integrative Neuroscience (CFIN), Aarhus University, Denmark.
authors.forename: Mahmoud
authors.initials: M
authors.lastname: Ashkanian
authors.affiliation: Center of Functionally Integrative Neuroscience (CFIN), Aarhus University, Denmark.
authors.forename: Manoucher
authors.initials: M
authors.lastname: Vafaee
authors.affiliation: PET center, Aarhus University Hospitals, Denmark; Department of Internal Medicine (V), Aarhus University Hospitals, Denmark.
authors.forename: Peter
authors.initials: P
authors.lastname: Iversen
authors.affiliation: PET center, Aarhus University Hospitals, Denmark; Center of Functionally Integrative Neuroscience (CFIN), Aarhus University, Denmark.
authors.forename: Albert
authors.initials: A
authors.lastname: Gjedde
mesh_terms: D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D002560:Cerebrovascular Circulation; D005260:Female; D006501:Hepatic Encephalopathy / Q000503:physiopathology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D049268:Positron-Emission Tomography