Regional impact of time-to-equilibrium on indirect Patlak whole-body parametric imaging: a multi-tissue class analysis at the entire body level

Abstract

Purpose

Patlak parametric imaging is widely employed for kinetic modeling due to its simplicity and robustness. The time-to-equilibrium (t*), which must be defined to estimate kinetic parameters, is currently set empirically and uniformly across the entire body. In this study, we evaluate the regional impact of varying t* values on kinetic parameter estimates using a multi-tissue segmentation approach at the whole-body level.

Methods

Data from 53 patients who underwent one-hour dynamic 18 F-FDG PET/CT scans were retrospectively analyzed. Parametric maps of the net influx rate (K_i) and blood distribution volume (dv) were calculated for four t* values (10, 20, 30, and 45 min) using in-house software (PET KinetiX). Voxel-wise K_i and dv values were extracted from 10 predefined tissue structures through automated segmentation. Using t* = 30 min as the widely accepted reference, relative mean errors and relative absolute mean errors of K_i and dv estimated at t*shifts = 10, 20 and 45 min were calculated for each tissue. Pearson correlation coefficients between K_i or dv reference values and those estimated at t* shifts = 10, 20, and 45 min were also computed.

Results

Compared to the reference t*30, K_i estimates ranged from − 21.4% (liver) to 7.3% (SAT) at t*10, and from − 13.8% (lungs) to 2.4% (brain) at t*20. Median absolute bias was 12.8% at t*10 (6.5% brain to > 25% liver) and 8.6% at t*20 (3.2% brain to > 15% lungs and liver). At t*45, K_i was consistently overestimated, with a median bias of 19.4% (2.7% brain to > 33% lungs and liver) and median absolute bias of 19.8% (5.5% brain to > 33% lungs and liver). For dv, biases ranged from − 25.2% (brain) to 8.6% (spleen) at t*10; − 13.7% (brain) to 5.7% (lungs) at t*20; − 15.5% (liver) to 8.8% (brain) at t*45. Median absolute biases were 14.0% at t*10 (9.8% heart to 25.2% brain), 9.4% at t*20 (7.7% heart to 14.1% brain), and 15% at t*45 (12.4% skeletal muscle to 18.5% brain). Regardless of t*, K_i values exhibited strong linear correlations (r > 0.7) across all organs, whereas dv correlations showed greater variability, falling below 0.7 in 80% of organs at t*45.

Conclusion

Kinetic parameter sensitivity to time-to-equilibrium (t*) varies across organs in Patlak whole-body parametric imaging, underscoring the necessity of adopting flexible or adaptive t* values at the whole-body level.