Head-to-head comparison of relative cerebral blood flow derived from dynamic [ 18 F]florbetapir and [ 18 F]flortaucipir PET in subjects with subjective cognitive decline

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.