Head‐to‐head comparison of Relative Cerebral Blood Flow derived from [¹⁸F]florbetapir and [¹⁸F]flortaucipir in subjects with Subjective Cognitive Decline

Abstract Background Several studies validated R 1 as a marker for relative cerebral blood flow (rCBF). However, the similarities or differences of R 1 estimates obtained from different PET tracers has never been investigated. Aim The aim of the current study was head‐to‐head comparison of R 1 estimates, derived from dynamic [ 18 F]florbetapir and [ 18 F]flortaucipir PET scans. Method Fifty subjects with subjective cognitive decline (Aβ‐;N = 31, Aβ+;N = 19) underwent both [ 18 F]florbetapir and [ 18 F]flortaucipir PET (interval: 50.6 ± 34.7 days). Receptor parametric mapping (RPM) was used to generate R 1 maps with cerebellar grey matter as reference. For voxel‐wise analyses, Statistical Parametric Mapping software (SPM12) was used. Furthermore, different regions‐of‐interest were assessed (in subject‐space). Paired t‐tests, coefficients of determination (r 2 ) and ICC were used to compare R 1 estimates from [ 18 F]florbetapir and [ 18 F]flortaucipir PET. Results Voxel‐wise analysis revealed only a few small significant clusters in part of the thalamus, insular gyri, midbrain and part of the cerebellum with higher R 1 estimates for [ 18 F]florbetapir and higher R 1 estimates in the hippocampus, vermis, midbrain and calcarine sulcus for [ 18 F]flortaucipir in Aβ‐ subjects (Figure 1a & 1b). In Aβ+ subjects, [ 18 F]florbetapir showed higher R 1 estimates only in part of the thalamus and [ 18 F]flortaucipir showed higher R 1 estimates in the hippocampus and striatum (Figure 1c & 1d). Despite the small amount of significant voxels, [ 18 F]florbetapir R 1 showed good correlation and excellent ICC with [ 18 F]flortaucipir for whole brain (Aβ‐:r 2 = 0.76, ICC:0.92; Aβ+:r 2 = 0.60,ICC = 0.87). Conclusion To conclude, [ 18 F]flortaucipir and [ 18 F]florbetapir showed similar R 1 estimates in cortical regions, illustrating that R 1 is a robust marker for flow independent of tracer of interest.