Dose Optimization in ¹⁸F-FDG PET Based on Noise-Equivalent Count Rate Measurement and Image Quality Assessment

The present study aimed to optimize the injected dose of ¹⁸F-FDG in whole-body PET/CT scans and assess its effect on noise-equivalent count rate (NECR) and visual image quality (IQ). Methods: Patients scheduled to undergo ¹⁸F-FDG PET/CT were prospectively recruited in the study from January to December 2019, regardless of the indication or underlying disease. Patients were divided into 4 groups and injected with different amounts of ¹⁸F-FDG radioactivity per kilogram of body weight (1.85, 3.7, 5.5, and 7.4 MBq/kg). All patients underwent ¹⁸F-FDG PET/CT studies, and NECR_local was calculated by noting the trues rate, total prompts, and randoms rate for each bed position. Whole-body NECR_global was calculated as the average NECR for all bed positions. IQ was qualitatively assessed for each bed position (IQ_local) and for whole-body PET (IQ_global) by 2 readers using 5-point scores based on prevalence of noise, contrast, and lesion detectability. NECR and IQ were compared among all 4 activity groups. Patients were also subdivided into 4 body-mass-index groups (group I, 15-20 kg/m²; group II, 20.1-25 kg/m²; group III, 25.1-30 kg/m²; and group IV, 30.1-35 kg/m²) for comparison. A P value of less than 0.05 was considered significant. Results: In total, 109 patients underwent ¹⁸F-FDG PET/CT studies after injection of different amounts of ¹⁸F-FDG radioactivity and a mean uptake time of 62.32 min. The mean NECR_global and IQ_global for each group were significantly different from other groups (P < 0.05), with NECR and IQ being higher in high-activity groups than in low-activity groups. The overall IQ was acceptable in all patients, even in the lowest-activity group (1.84 MBq/kg). The mean NECR_global and IQ_global were significantly different in all 4 body-mass-index groups (P < 0.05), except between groups II and III (P > 0.05). NECR_local and IQ_local correlated moderately (r = 0.64). Conclusion: Optimization of injected ¹⁸F-FDG radioactivity from 7.4 MBq/kg (200 μCi/kg) to 1.85 MBq/kg (50 μCi/kg) resulted in acceptable IQ, despite a reduction in NECR.