25 Shortening the scan duration INTRODUCTION Dynamic positron emission tomography (PET) scan protocols allow for accurate quantitative measures [1, 2] of specific binding of PET tracers. Moreover, dynamic scan protocols yield additional information about functional measures such as perfusion [3]. Semi-quantitative measures from static scans are usually sufficient for clinical application, but accurate quantification of tracer uptake is of major importance in the context of early-stage pathology, clinical trials [1] and longitudinal studies. Some PET tracers like the tau tracer [18F]flortaucipir require a long acquisition period because of the slow tracer kinetics. This can be challenging, especially when working with a vulnerable population (like patients with Alzheimer’s disease (AD)). In vivo quantification of tau pathology is important because intracellular accumulation of hyperphosphorylated tau proteins into neurofibrillary tangles (NFTs) is one of the pathological hallmarks of AD [4]. Indeed, histopathological studies have shown that the amount of NFTs correlate well with the severity of their cognitive symptoms during life [5, 6]. [18F]Flortaucipir is worldwide the most widely used PET tracer for detecting and quantifying these NFTs. For the analysis of [18F]flortaucipir scans, most studies prefer semi-quantitative measures due to their practical applicability and computational simplicity [7–9]. However, studies involving dynamic imaging provided more accurate and precise pharmacokinetic parameters and provide estimates for relative tracer delivery (R1) or relative cerebral blood flow (rCBF) [2, 10–15], which is important for monitoring flow changes. For instance, a study by van Berckel et al. [16] observed that longitudinal changes in [11C]PIB standardized uptake value ratio (SUVr) do not reflect changes in specific [11C]PIB binding but rather are secondary to changes in blood flow during the natural course of AD. Our group has performed dynamic acquisition of [18F]flortaucipir scans, using a 130-min dual-time-window dynamic scan protocol including a 20-min break (after the first 60 min of acquisition) [17–21]. Several aspects are of importance to obtain a reliable protocol with reduced overall scanning time. Firstly, the scan must include the wash-in of the tracer and tissue peak activity to be able to assess the tracer influx into the tissue. In addition, tracer efflux information is also necessary to be able to estimate the tracer efflux back to plasma and the specific binding compartment. The second part ideally has to contain the 80–100 min interval to calculate SUVr, since this is the internationally conventional SUVr interval for [18F]flortaucipir [22]. So, the new scanning protocol needs to include an early part of the tracer kinetics and also at least 80–100 min post-injection (p.i.), implying that a dual-time-window protocol should be used. Scanning time can be shortened by increasing the gap of the dual-time-window. Interpolation is needed to fill this gap in the time activity curve (TAC) of the reference region to be able to perform reference tissue model–based 2
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