Defining optimal tracer activities in pediatric oncologic whole-body 18F-FDG-PET/MRI

Abstract

Purpose: To explore the feasibility of reducing administered tracer activities and to assess optimal activities for combined ¹⁸F-FDG-PET/MRI in pediatric oncology.

Methods: 30 ¹⁸F-FDG-PET/MRI examinations were performed on 24 patients with known or suspected solid tumors (10 girls, 14 boys, age 12 ± 5.6 [1-18] years; PET scan duration: 4 min per bed position). Low-activity PET images were retrospectively simulated from the originally acquired data sets using randomized undersampling of list mode data. PET data of different simulated administered activities (0.25-2.5 MBq/kg body weight) were reconstructed with or without point spread function (PSF) modeling. Mean and maximum standardized uptake values (SUV_mean and SUV_max) as well as SUV variation (SUV_var) were measured in physiologic organs and focal FDG-avid lesions. Detectability of organ structures and of focal ¹⁸F-FDG-avid lesions as well as the occurrence of false-positive PET lesions were assessed at different simulated tracer activities.

Results: Subjective image quality steadily declined with decreasing tracer activities. Compared to the originally acquired data sets, mean relative deviations of SUV_mean and SUV_max were below 5 % at ¹⁸F-FDG activities of 1.5 MBq/kg or higher. Over 95 % of anatomic structures and all pathologic focal lesions were detectable at 1.5 MBq/kg ¹⁸F-FDG. Detectability of anatomic structures and focal lesions was significantly improved using PSF. No false-positive focal lesions were observed at tracer activities of 1 MBq/kg ¹⁸F-FDG or higher. Administration of ¹⁸F-FDG activities of 1.5 MBq/kg is, thus, feasible without obvious diagnostic shortcomings, which is equivalent to a dose reduction of more than 50 % compared to current recommendations.

Conclusion: Significant reduction in administered ¹⁸F-FDG tracer activities is feasible in pediatric oncologic PET/MRI. Appropriate activities of ¹⁸F-FDG or other tracers for specific clinical questions have to be further established in selected patient populations.

Keywords: Children; FDG; PET/MRI; Radiation dose; Tracer dose.