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.