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Background: Dynamic PET imaging studies provide accurate estimates of specific binding, but also measure the relative tracer delivery (R 1 ), which is a proxy for relative cerebral blood flow (rCBF). Recently, studies suggested that R 1 obtained from different tracers could be used interchangeably and is irrespective of target tissue. However, the similarities or differences of R 1 obtained from different PET tracers still require validation. Therefore, the goal of the current study was to compare R 1 estimates, derived from dynamic [ 18 F]florbetapir (amyloid) and [ 18 F]flortaucipir (tau) PET, in the same subjects with subjective cognitive decline (SCD)., Results: Voxel-wise analysis presented a small cluster (1.6% of the whole brain) with higher R 1 values for [ 18 F]flortaucipir compared to [ 18 F]florbetapir in the Aβ-negative group. These voxels were part of the hippocampus and the left middle occipital gyrus. In part of the thalamus, midbrain and cerebellum, voxels (2.5% of the whole brain) with higher R 1 values for [ 18 F]florbetapir were observed. In the Aβ-positive group, a cluster (0.2% of the whole brain) of higher R 1 values was observed in part of the hippocampus, right parahippocampal gyrus and in the left sagittal stratum for [ 18 F]flortaucipir compared to [ 18 F]florbetapir. Furthermore, in part of the thalamus, left amygdala, midbrain and right parahippocampal gyrus voxels (0.4% of the whole brain) with higher R 1 values for [ 18 F]florbetapir were observed. Despite these differences, [ 18 F]florbetapir R 1 had high correspondence with [ 18 F]flortaucipir R 1 across all regions of interest (ROIs) and subjects (Aβ-:r 2  = 0.79, slope = 0.85, ICC = 0.76; Aβ+: r 2  = 0.87, slope = 0.93, ICC = 0.77)., Conclusion: [ 18 F]flortaucipir and [ 18 F]florbetapir showed similar R 1 estimates in cortical regions. This finding, put together with previous studies, indicates that R 1 could be considered a surrogate for relative cerebral blood flow (rCBF) in the cortex and may be used interchangeably, but with caution, regardless of the choice of these two tracers., (© 2023. The Author(s).)

[ 11 C]UCB-J is a PET radioligand that binds to the presynaptic vesicle glycoprotein 2A. Therefore, [ 11 C]UCB-J PET may serve as an in vivo marker of synaptic integrity. The main objective of this study was to evaluate the quantitative accuracy and the 28-day test-retest repeatability (TRT) of various parametric quantitative methods for dynamic [ 11 C]UCB-J studies in Alzheimer's disease (AD) patients and healthy controls (HC). Eight HCs and seven AD patients underwent two 60-min dynamic [ 11 C]UCB-J PET scans with arterial sampling over a 28-day interval. Several plasma-input based and reference-region based parametric methods were used to generate parametric images using metabolite corrected plasma activity as input function or white matter semi-ovale as reference region. Different parametric outcomes were compared regionally with corresponding non-linear regression (NLR) estimates. Furthermore, the 28-day TRT was assessed for all parametric methods. Spectral analysis (SA) and Logan graphical analysis showed high correlations with NLR estimates. Receptor parametric mapping (RPM) and simplified reference tissue model 2 (SRTM2) BP ND , and reference Logan (RLogan) distribution volume ratio (DVR) regional estimates correlated well with plasma-input derived DVR and SRTM BP ND . Among the multilinear reference tissue model (MRTM) methods, MRTM1 had the best correspondence with DVR and SRTM BP ND . Among the parametric methods evaluated, spectral analysis (SA) and SRTM2 were the best plasma-input and reference tissue methods, respectively, to obtain quantitatively accurate and repeatable parametric images for dynamic [ 11 C]UCB-J PET., (© 2022. The Author(s).)

Background: Off-target binding in the choroid plexus (CP) may cause spill-in of the tau PET tracer [ 18 F] flortaucipir into the adjacent hippocampus region. The impact of this spill-in on hippocampal uptake was assessed using a novel partial volume correction method (PVC)., Methods: PVC was performed on 20 [ 18 F] flortaucipir dynamic PET scans (10 probable AD and 10 controls). Volumes of interest (VOIs) were defined for both hippocampus and CP. The correlation between hippocampal and CP distribution volume (V T ), with and without PVC, was determined. Both anatomically defined and eroded VOIs were used., Results: For controls, the correlation between hippocampal and CP V T was significantly reduced after using PVC along with an eroded VOI (r 2  = 0.59, slope = 0.80 versus r 2  = 0.15, slope = 0.15; difference: p < 0.05). The same was true for AD patients (p < 0.05)., Conclusion: PVC together with an optimized hippocampal VOI resulted in effective reduction of CP spill-in and improved accuracy of hippocampal V T .