41 Shortening the scan duration was also not possible, since we want to be able to compare with the internationally mostly used 80–100-min SUVr time interval. The shortened dual-time-window (0–30/80–100 min) acquisition protocol provided almost identical TRT values when compared to the observed TRT values for the original scan protocol (0–60/80–130) (Figure 3). This implies that the shortened dual-time-window protocol not only provides quantitatively acceptable estimates but also result in high repeatability, suggesting that it can be reliably used for longitudinal and treatment-monitoring studies. However, the benefits of using a dynamic scanning protocol over static scanning protocol in a longitudinal setup for [18F]Flortaucipir need further validation, even though it has been presented by van Berckel et al. [16] that SUVr does get affected by changes in flow and under-/overestimates the underlying specific binding in case of [11C]PIB. Making use of a dual-time-window protocol also has some challenges to consider. Even if the total acquisition time is reduced with 60 min, it still takes longer than a single static acquisition protocol used for SUVr. Another disadvantage of using a dual-time-window protocol is the added CT attenuation scan for the second part of the scan, which is still present in this new shortened scanning protocol. Furthermore, the pre-processing of the PET images are more demanding compared to simplified methods, for instance, due to a required additional co-registration of the first part to the second part of the scan. Yet, the main advantage is that, apart from obtaining quantitative information on BPND, this protocol also generates parametric R1 data that may be used as a surrogate for flow and/or FDG uptake [15] and this protocol could therefore obviate the need to make a separate FDG scan, when clinically required. CONCLUSION The current study demonstrated that quantitatively acceptable [18F]flortaucipir kinetic parameters can be obtained using just 50 min of total scanning time by implementing a dual-time-window protocol (0–30/80–100 min). The best method to interpolate the gap in the reference tissue is 2T4k_VB tracer kinetic model with population-averaged metabolite-corrected plasma input function. Reducing the dual-time-window protocol enables interleaved scanning, reduces patient burden and enables efficient use of tracer batches and cost-effectiveness. 2
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