High resolution T1-weighted structural MRI was performed at 1.5 Tesla (n=63, Siemens Vision, Erlangen, Germany) or 3T (n=111, Siemens TIM Trio) using a magnetization-prepared rapid gradient echo (MPRAGE) sequence as previously described19 (link), 20 (link). Pittsburgh compound B (PIB) PET21 (link) was used as the imaging biomarker for β-amyloid. Participants underwent a 60-minute dynamic PET scan following injection of ~10 mCi PIB22 (link). The PIB PET imaging has been described in detail17 (link), 22 (link) and was conducted with a Siemens 962 HR+ ECAT PET scanner (Siemens/CTI, Knoxville KY) or on a Siemens Biograph 40 scanner. Structural MRIs were parcellated using FreeSurfer23 (link) (Martinos Center, Boston, MA) and, within each region, partial volume correction was applied24 (link). Standard uptake value ratios (SUVR) were calculated from regions of interest (ROI), with PIB-positivity defined as the mean cortical SUVR (from prefrontal, parietal and temporal ROIs) >1.42, a value commensurate with a mean cortical binding potential of 0.18 defined previously for a similar cohort17 (link), 22 (link). Cerebellar cortex, a region negative for amyloid in AD, was used as the reference region. Since the difference in spatial resolution between the two scanners used in this study is small (~0.01 SUVR unit), the same PIB cut-off was used for the entire dataset.