Normalization in PET group comparison studies--the importance of a valid reference region.
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.
Borghammer
Per
P
PET center, Aarhus University Hospitals, Denmark; Center of Functionally Integrative Neuroscience (CFIN), Aarhus University, Denmark. Electronic address: per@pet.auh.dk.
Jonsdottir
Kristjana Yr
KY
Center of Functionally Integrative Neuroscience (CFIN), Aarhus University, Denmark.
Cumming
Paul
P
Center of Functionally Integrative Neuroscience (CFIN), Aarhus University, Denmark.
Ostergaard
Karen
K
Department of Neurology, Aarhus University Hospitals, Denmark.
Vang
Kim
K
PET center, Aarhus University Hospitals, Denmark.
Ashkanian
Mahmoud
M
Center of Functionally Integrative Neuroscience (CFIN), Aarhus University, Denmark.
Vafaee
Manoucher
M
Center of Functionally Integrative Neuroscience (CFIN), Aarhus University, Denmark.
Iversen
Peter
P
PET center, Aarhus University Hospitals, Denmark; Department of Internal Medicine (V), Aarhus University Hospitals, Denmark.
Gjedde
Albert
A
PET center, Aarhus University Hospitals, Denmark; Center of Functionally Integrative Neuroscience (CFIN), Aarhus University, Denmark.
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