Dose optimization in ¹⁸F-fluorodeoxyglucose positron emission tomography based on NECR measurement and image quality assessment

Abstract

Rationale: The present study aims to optimize injected dose of ¹⁸F-FDG in whole body PET/CT scan and assess its effect on Noise Equivalent count rate (NECR) and visual image quality (IQ) assessment. Methods: Patients scheduled to undergo ¹⁸F-FDG PET/CT were prospectively recruited in the study from January to December 2019, regardless of the indication/underlying disease. Patients were divided into four groups and injected different amounts of ¹⁸F-FDG radioactivity/kg body weight (1.85 MBq/kg, 3.7 MBq/kg, 5.5 MBq/kg and 7.4 MBq/kg). All patients underwent ¹⁸F-FDG PET/CT studies and NECRlocal was calculated by noting the trues rate, total prompts and randoms rate for each bed position. Whole body NECRglobal was calculated as average of NECR for all bed positions. Qualitative IQ assessment was done for each bed-position (IQlocal) and for whole-body PET (IQglobal) by two readers using 5-point scores based on prevalence of noise, contrast and lesion detectability. Values of NECR and IQ in all four activity groups were compared. Patients were also subdivided into four body mass index (BMI) 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 <0.05 was considered significant. Results: A total of 109 patients underwent ¹⁸F-FDG PET/CT studies after injecting different amounts of ¹⁸F-FDG radioactivity and a mean uptake time of 62.32 minutes. Mean value of NECRglobal and IQglobal for each group were found to be significantly different from other groups (p<0.05), with higher NECR and IQ values in high-activity groups in comparison to low-activity groups. The overall image quality was acceptable in all patients, even in lowest-activity group (1.84 MBq/Kg). The mean values of NECRglobal and IQglobal were found to be significantly different in all the four BMI groups (P < 0.05) except between group II and III (p>0.05). NECRlocal and IQlocal were moderately correlated (r=0.64). Conclusion: Optimisation of injected ¹⁸F-FDG radioactivity from 7.4 MBq/kg (200 µCi/kg) body weight to 1.85 MBq/kg (50 µCi/kg) body weight resulted in acceptable image quality, despite reduction in NECR.