60 Chapter 3 Figure 5. Percentage bias in SUVr relative to the true DVR values as a function of SUVr time intervals for simulated flow condition for (a) SCD (almost no binding condition) (b) AD with low binding condition (c) AD with medium binding condition and (d) AD with high binding condition. Legend in the figure illustrates different flow conditions (increase or decrease in flow 0 – 25%). DVR = distribution volume ratio. SUVr = standardized uptake value ratio. SCD = subjective cognitive decline. AD = Alzheimer’s disease. DISCUSSION We compared changes in 18F-flortaucipir specific binding using SUVr and DVR. In a 2-y longitudinal study, changes in 18F-flortaucipir DVR and SUVr were comparable in all patient groups. Only small changes in R1 occurred during this period, but these most likely contributed to the lack of difference between DVR and SUVr. However, simulations demonstrated marked differences between DVR and SUVr when large(r) changes in R1 were introduced. In addition, these differences between DVR and SUVr were shown to be dependent on the underlying level of tau pathology. The most important finding in this study was the lack of major differences in the percentage change between 18F-flortaucipir DVR and SUVr in a 2-y observational study. Congruently, sample size calculations based on these data to inform future trials showed negligible differences between methods. Unlike a previous study using 11C-PiB (6), this finding indicates that 18F-flortaucipir SUVr provides an accurate esti-
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