Exact hr tomograph

Participants in the Aβ chronicity subsample underwent T1‐weighted magnetic resonance imaging (MRI) for anatomical delineation and [C‐11]PiB‐PET for the quantification of cerebral Aβ. Details regarding radioligand synthesis, image acquisition, processing, and analysis of MRI and PiB‐PET images have been described previously.²⁷ Amyloid burden was quantified as the average cortical PiB distribution volume ratio (DVR; Logan graphical analysis, cerebellum gray matter reference region) using dynamic PiB data acquired 0‐70 minutes post‐injection with either a Siemens Biograph Horizon PET/CT or Siemens EXACT HR+ tomograph. MRI and PET image processing and quality control were performed using a pipeline that uses MATLAB (The Mathworks, Inc., Natick, MA) and SPM12 (www.fil.ion.ucl.ac.uk/spm). The cut‐point for PiB positivity was a global DVR of 1.19.²⁸

Participants underwent 70‐minute dynamic [11C] (PiB) PET imaging acquired on either a Siemens Biograph Horizon mCT or a Siemens EXACT HR+ tomograph. Amyloid burden was quantified as the mean distribution volume ratio (DVR) across eight bilateral regions of interest defined from T1‐weighted magnetic resonance imagin using Logan graphical analysis with the cerebellum gray matter as a reference region.¹⁷ Because PiB‐PET scans and blood samples were not temporally aligned, we used group‐based trajectory modeling (GBTM) and piecewise regression on a larger dataset of 179 participants with longitudinal PiB data to produce model‐based estimates of global PiB DVR (mDVR) at the time of blood sample collection.¹⁸ In this method, the age of PiB⁺ was first estimated for individual cases referencing their last available PiB scan as previously described. Piecewise regression was then applied to model the relationship between PiB DVR versus PiB⁺ chronicity (age at PiB scan – age PiB⁺. Finally, PiB mDVR was estimated for each participant by calculating PiB⁺ chronicity at the time of blood sampling (age at last PiB scan – age at blood sampling + PiB⁺ chronicity at last PiB scan) and entering this value into the piecewise equation. For categorical analyses, the threshold for PET Aβ positivity was observed (i.e., not modeled) DVR > 1.19 at the time of measurement.