Longitudinal tau PET using [¹⁸F]flortaucipir: Comparison of (semi)quantitative parameters

Abstract

Purpose: Semi-quantitative PET measures can be affected by changes in blood flow, whereas quantitative measures are not. The aim of the study was to compare semi-quantitative(SUVr) and quantitative(R1, BPND) parameters of longitudinal tau PET scans with [18F]flortaucipir, with respect to changes in blood flow. Methods: Subjects with subjective cognitive decline(SCD; n=38) and Alzheimer′s disease(AD) patients(n=24) underwent baseline(BL) and 2-year follow-up(FU) dynamic [18F]flortaucipir PET scans. BPND and R1 were estimated using RPM and SUVr(80-100min) was calculated (cerebellar gray as reference). For each region-of-interest ((trans)entorhinal, limbic and neocortical) and parameter, %change was calculated. Regional SUVrs were compared to corresponding DVR (= BPND+1) using paired T-tests. Additionally, simulations were performed to model effects of flow changes on BPND and SUVr in different binding categories. Thereafter, %bias for SUVr with respect to underlying binding and flow were evaluated. Results: In SCD, there was a difference between %change in the (trans)entorhinal ROI (DVR 2.56% vs SUVr 1.85%) only. In AD, a difference was found in the limbic ROI (DVR 6.61% vs SUVr 7.52%) only. R1 changes were small(+0.7% in SCD and -1.6% in AD). Simulations illustrated with increasing flow a decreased %bias for SUVr in low binding conditions, whereas a slightly increased bias was observed in high binding conditions. Conclusion: SUVr provided an accurate estimate of specific binding for [18F]flortaucipir over a two-year follow-up. However, simulations showed that flow changes can affect [18F]flortaucipir SUVr, hence DVR/BPND should be preferred in more advanced disease stages and/or conditions that could induce significant flow changes like pharmacotherapeutic interventions.

Precision estimates of relative and absolute cerebral blood flow in Alzheimer’s disease and cognitively normal individuals

“…A secondary aim was to assess whether repeatability of relative CBF proxies was stable across diagnostic groups for [ 18 F]florbetapir and [ 18 F]flortaucipir. 9 , 21 …”