Longitudinal PiB-PET and MRI Brain Imaging

All participants underwent simultaneous three-dimensional [¹¹C] PiB-PET and three-dimensional T1-weighted MRI using a 3.0T Biograph mMR (PET-MR) scanner (Siemens) at baseline and at a 2-year follow-up visit. The details of PiB-PET image acquisition and preprocessing were previously described.^{28 (link)} An automatic anatomic labeling algorithm^{29 (link)} and a region combining method^{30 (link)} were applied to determine regions of interests (ROIs) to characterize the PiB retention level in the frontal, lateral parietal, posterior cingulate-precuneus, and lateral temporal regions. A global Aβ retention value was the mean standardized uptake value ratio (SUVR) for all voxels of the four ROIs, calculated by dividing the mean uptake value of a reference region. Given the aims of the study to investigate longitudinal changes, for all PiB-PET scans (both baseline and follow-up), the SUVRs were calculated using a reference region that included the inferior cerebellar gray matter of the Spatially Unbiased Infratentorial Template for the cerebellum atlas,^{31 (link)} cerebellar white matter (thresholded at 50%),^{32 (link)} pons, and cerebrum white matter (threshold at 95% and eroded by three voxels).^{33 (link),34 (link)} Extracted SUVRs were used to derive the annualized change of Aβ retention.