Multiphase Patlak Plot Enabled by High Temporal Resolution Total-body Dynamic PET

Objectives: The Patlak graphical method is widely used in dynamic PET for modeling irreversible radiotracer kinetics. Its common use is to extract the late-time linear phase of the graphical plot for characterization of the tracer net uptake rate. The uEXPLORER total-body PET has enabled high-temporal resolution (HTR) dynamic imaging. We propose to combine the benefit of HTR with the Patlak method. The objective of this paper is to report our findings in identifying two additional approximately linear phases in the early period of the Patlak plot achieved by HTR dynamic imaging in healthy human subject scans.

Methods: Four healthy subjects’ dynamic FDG-PET data were included in this study. Each subject was scanned on the uEXPLORER total-body PET/CT for one hour following injection. The dynamic data were rebinned and reconstructed into 120 HTR frames (60 x 1s, 30 x 2s, 6 x 10s, 6 x 30s, 12 x 120s, 6 x 300s), and then resampled into 30 frames of standard temporal resolution (6 x 10s, 2 x 30s, 4 x 60s, 12 x 120s, 6 x 300s). The time activity curves (TACs) were extracted from multiple regions of interest (ROIs). For each ROI, the Patlak graphical plot was generated from both the 1-hour data with standard temporal resolution and the 1-hour HTR data. Linear or linear phases were identified. The slopes and intercepts were estimated using linear fitting. The multiphase linear plots were also implemented pixel-wise to generate parametric images corresponding to each linear phase.

Results: The Patlak plot with standard temporal resolution commonly shows one late linear phase (Figure 1A). The early-time period (0-2 minutes) did not provide enough points for reliable linear fitting in the plot of standard temporal resolution (Figure 1B). With HTR imaging, we were able to identify two additional approximately linear phases: one is around 20-30s (first-pass) and the other is around 1-2 minutes (early-time) (Figures 1C and 1D). The multiphase linearities were observed in different ROIs in all four subjects. Total-body parametric images of the slopes of the first-pass, early-time and standard late-time linear phases were successfully generated (Figure 2) and demonstrated different spatial patterns. Correlations between regional of the two new phases and the standard one was low.

Conclusions: With improved temporal resolution by using the uEXPLORER total-body scanner, the Patlak graphical plot demonstrates two additional approximately linear phases: first-pass (20s - 30s) and early-time (1min - 2min). The slope from the first-pass and early-time phases are different from the standard Patlak slope, indicating that multiparametric information may be extracted from Patlak plot with HTR data. We speculate that the first-pass represents blood flow and the early-time represents the overall FDG influx rate from the interstitial space to the tissue cell space. Our future work will pursue validation and investigation of the physiological meanings of these new parameters.

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