Determining the minimal required radioactivity of ¹⁸F-FDG for reliable semi-quantification in PET-CT imaging: a phantom study.

Abstract

Purpose: To investigate the minimal required radioactivity and corresponding imaging time for reliable semi-quantification in PET-CT imaging to perform useful and comparable imaging studies in pursuit of as low as reasonably achievable (ALARA) in dose reduction. Materials and Methods: We performed ¹⁸F-FDG PET-CT study using a Jaszczak ECT phantom containing spheres of diameters (3.4, 2.1, 1.5, 1.2, 1.0 cm) filled with a fixed concentration of 165 kBq/ml and background of 23.3 kBq/ml at multiple time points over 20 hours of radioactive decay. The images were acquired for 10 minutes in a single bed position at each of 10 half-lives of decay using 3D list mode in a hybrid GE Discovery 690 PET-CT scanner. The images were reconstructed in 1, 2, 3, 4, 5, and 10 minutes per bed using ordered-subset expectation maximum (OSEM) algorithm with 24 subsets and 2 iterations with a gaussian 2-mm filter using an AW workstation (GE Healthcare) equipped with version 4.5 software. The maximum and average standardized uptake values (SUV) of each sphere were measured. Results: The minimal required activity concentration for precise SUV_max quantification in spheres (±10%) was determined to be 1.8 kBq/ml for 10 minutes, 3.7 kBq/ml for 3-5 minutes, 7.9 kBq/ml for 2 minutes, and 17.4 kBq/ml for 1 minute of acquisition per bed position. The minimal required value for the product of activity concentration and acquisition time per bed position was determined to be 10-15 kBq/ml*min for reproducible SUV measurement within the spheres without overestimation. Using the total radioactivity and count rate from the entire phantom, the minimal required values for the product with time per bed position was determined to be 17 MBq*min and 100 kcps*min, respectively. Conclusion: Our phantom study determined a threshold for minimal radioactivity and acquisition time for precise semi-quantification in FDG PET imaging that can serve as a guide in pursuit of achieving ALARA.