Abstract
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Introduction: The long axial field of view PET scanners such as Siemens Biograph Quadra and uEXPLORER have been installed worldwide gradually. They can cover multiple organs simultaneously with a higher temporal resolution (90-200 time frames) in a single-bed position. When adopting Patlak method for dynamic total-body PET imaging on such scanners, more time points (much greater than 2) can be employed to fit the linear line and the optimal fit time interval for diverse tissues is yet to be determined. Some recent studies show that the dynamic PET FDG scan duration can be shortened to 30 min by presetting truncated analysis windows in the early or late time frames. The objectives of this study are to assess the performance of all the possible time intervals with the fixed duration for an FDG PET study and propose a most appropriate time interval for multiple organs to enhance the quantitation of dynamic total-body imaging on uEXPLORER PET scanner.
Methods: A 60-min dynamic scan for a lung adenocarcinoma cancer patient was performed immediately after the administration of 18F-FDG. It was conducted on the uEXPLORER with the approval of the Ethics committee of Henan Provincial People's Hospital and reconstructed as following sequences: 50x2s, 20x10s, 10x30s, 10x60s, 8x300s. A series of regions of interest (ROIs) in diverse organs/tissues (liver, spleen, kidney, lung, bladder, bone, tumor, myocardium and brain) were drawn manually in PMOD. The input function is obtained from the descending aorta. All the possible time windows with a duration of 30 min are employed to implement Patlak analysis. The influx (Ki) for the above ROIs and parametric images corresponding to each analysis time interval are derived. The choice of the time interval is based on the R squared, which is a common metric to quantify the goodness of linear fit and closer to 1 indicating a better fit. Hence the optimal fit interval is determined by the maximum R squared.
Results: R squared across all possible time windows between 5 and 60 min is shown and the optimal time interval for each ROI based on this metric is also indicated (Fig. 1). Results show 15-45 min should be a reasonable analysis window for most of the organs, except the kidney. Comparisons of the linear fitting among three representative windows: early frames (10–40 min), intermediate frames (20–50 min), and late frames (30– 60 min) of acquisition and associated Ki values are presented (Fig. 1, Table 1). The fit time interval has a considerable impact on some specific organs like kidney and bladder. Parametric images of Ki show the visual differences in kidney and liver with various analysis windows (Fig. 2)
Conclusions: The optimal fit time interval of Patlak model for most of the organs is 15-45 min based on the current single patient study. The kidney, liver and bladder are sensitive to the varying analysis windows. These findings may have a potential application in the short-scan studies. Further investigations in a patient cohort and various durations are ongoing.