PET imaging using the 18F-labeled Aβ tracer florbetapir was performed for participants enrolled in ADNI-GO or ADNI-2. Participants were administered a bolus injection of approximately 370 MBq florbetapir intravenously. Fifty minutes later, a 20-minute continuous cranial PET scan was initiated. Images were reconstructed immediately following the scan using iterative algorithms, and repeat scans were acquired if motion artifact was detected. Preprocessing of the scans was performed as previously described.13 Briefly, image frames were averaged, aligned to a standard space (AC-PC), resampled to a standard image and voxel size, smoothed to a uniform resolution, and normalized to an atlas-based bilateral and symmetric cerebellar reference region. This cerebellar reference region consisted largely but not exclusively of grey matter and was expected to exhibit nonspecific binding, ultimately resulting in standardized uptake value ratio (SUVR) images. These preprocessed scans were downloaded from the ADNI database (http://adni.loni.ucla.edu/) for 621 participants. For each scan, mean regional SUVR values were extracted for the frontal, parietal, temporal, limbic, and occipital lobes using the MarsBaR toolbox implemented in the Statistical Parametric Mapping 8 (SPM8) software (http://www.fil.ion.ucl.ac.uk/spm/software/spm8/). The average SUVR
