962 hr ecat pet scanner

The PiB PET assessment has been previously described in detail (Su et al., 2013 (link), 2014 ). Imaging was conducted on a Siemens 962 HR+ ECAT PET scanner or a Siemens Biograph 40 scanner. PET data were analyzed using previously developed methods (Su et al., 2013 (link), 2015 (link)). FreeSurfer segmentation (Fischl et al., 2002 (link)) (http://freesurfer.net/) was used as the basis for quantitative analysis to obtain regional SUVR with cerebellar gray matter serving as the reference region. Partial volume correction was also performed using a regional spread function technique (Rousset et al., 1998 (link); Su et al., 2015 (link)).
PiB positivity was defined using the MC SUVR across the precuneus, prefrontal, gyrus rectus, and temporal FreeSurfer regions of interest (ROIs) (Morris et al., 2010 (link)). A cutoff value of 1.42 was used which is comparable to an MC binding potential of 0.18 that was previously defined (Mintun et al., 2006 (link); Su et al., 2013 (link)). Characteristic SUVR images are presented in Fig. 1. The equivalence for an MC SUVR of 1.42 and MC-BP of 0.18 was previously determined using regression in an independent sample (unpublished data). Because of the criticality of this cutoff for subsequent analyses, primary results are also replicated using an alternative cutoff determined from this sample (Supplementary Material).

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 described^{19 (link), 20 (link)}. Pittsburgh compound B (PIB) PET^{21 (link)} was used as the imaging biomarker for β-amyloid. Participants underwent a 60-minute dynamic PET scan following injection of ~10 mCi PIB^{22 (link)}. The PIB PET imaging has been described in detail^{17 (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 FreeSurfer^{23 (link)} (Martinos Center, Boston, MA) and, within each region, partial volume correction was applied^{24 (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 cohort^{17 (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.