Sample size estimates for well-powered cross-sectional cortical thickness studies.
BACKGROUND
Cortical thickness mapping is a widely used method for the analysis of neuroanatomical differences between subject groups. We applied power analysis methods over a range of image processing parameters to derive a model that allows researchers to calculate the number of subjects required to ensure a well-powered cross-sectional cortical thickness study.


METHODS
0.9-mm isotropic T1 -weighted 3D MPRAGE MRI scans from 98 controls (53 females, age 29.1 ± 9.7 years) were processed using Freesurfer 5.0. Power analyses were carried out using vertex-wise variance estimates from the coregistered cortical thickness maps, systematically varying processing parameters. A genetic programming approach was used to derive a model describing the relationship between sample size and processing parameters. The model was validated on four Alzheimer's Disease Neuroimaging Initiative control datasets (mean 126.5 subjects/site, age 76.6 ± 5.0 years).


RESULTS
Approximately 50 subjects per group are required to detect a 0.25-mm thickness difference; less than 10 subjects per group are required for differences of 1 mm (two-sided test, 10 mm smoothing, α = 0.05). Sample size estimates were heterogeneous over the cortical surface. The model yielded sample size predictions within 2-6% of that determined experimentally using independent data from four other datasets. Fitting parameters of the model to data from each site reduced the estimation error to less than 2%.


CONCLUSIONS
The derived model provides a simple tool for researchers to calculate how many subjects should be included in a well-powered cortical thickness analysis.
Brain Research Institute, Florey Neuroscience Institutes, Melbourne Brain Centre, Austin Hospital, Heidelberg, Victoria, Australia; Department of Medicine, The University of Melbourne, Victoria, Australia.
Heath R
HR
Pardoe
David F
DF
Abbott
Graeme D
GD
Jackson
D000328:Adult; D019412:Anatomy, Cross-Sectional / Q000379:methods*; D001931:Brain Mapping / Q000379:methods; D002540:Cerebral Cortex / Q000033:anatomy & histology*; D015331:Cohort Studies; D003627:Data Interpretation, Statistical; D005260:Female; D055614:Genetic Phenomena; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008279:Magnetic Resonance Imaging; D008297:Male; D008959:Models, Neurological; D015203:Reproducibility of Results; D018401:Sample Size; D055815:Young Adult