A new simpler image quality index based on body size for FDG-PET/CT.

Noise equivalent count density (NEC density ) is often used to evaluate the image quality of whole-body fluorodeoxyglucose tomography tests. However, this index is calculated using the patient volume, which is difficult to obtain at every facility. In this study, we proposed new image quality indices that can be evaluated at all facilities. In total, 94 patients were enrolled in the study. The correlations of patients' body weight and BMI with volume were examined. New image quality indices normalized by body weight and BMI were defined as NEC bw and NEC bmi , respectively. Correlations between NEC bw , NEC bmi , and NEC density were examined. Further, the correlations between these two new indices and visual scores were evaluated. Good correlations were observed between volume and body weight (r = 0.861, P  < 0.001) and between volume and BMI (r = 0.728, P  < 0.001). NEC bw and NEC bmi correlated well with NEC density (r = 0.954 for NEC bw and r = 0.897 for NEC bmi , P  < 0.001). These correlations improved when the examined bed positions were set to the same number. Additionally, the correlations of visual scores with NEC bw and NEC bmi were similar to those between the visual score and NEC density . Our investigation indicated that the newly proposed image quality metrics, NEC bw and NEC bmi , were easily calculated and as useful as NEC density for evaluating image quality when subjects had similar physiques.

Optimization of injection dose in 18F-FDG PET/CT based on the 2020 national diagnostic reference levels for nuclear medicine in Japan.

OBJECTIVE: Recently, the national diagnostic reference levels (DRLs) in Japan were revised as the DRLs 2020, wherein the body weight-based injection dose optimization in positron emission tomography/computed tomography using 18F-fluoro-2-deoxy-D-glucose (18F-FDG PET/CT) was first proposed. We retrospectively investigated the usefulness of this optimization method in improving image quality and reducing radiation dose. METHODS: A total of 1,231 patients were enrolled in this study. A fixed injection dose of 240 MBq was administered to 624 patients, and a dose adjusted to 3.7 MBq/kg body weight was given to 607 patients. The patients with body weight-based injection doses were further divided according to body weight: group 1 (≤ 49 kg), group 2 (50-59 kg), group 3 (60-69 kg), and group 4 (≥ 70 kg). The effective radiation dose of FDG PET was calculated using the conversion factor of 0.019 mSv/MBq, per the International Commission on Radiological Protection publication 106. Image quality was assessed using noise equivalent count density (NECdensity), which was calculated by excluding the counts of the brain and bladder. The usefulness of the injection dose optimization in terms of radiation dose and image quality was analyzed. RESULTS: The body weight-based injection dose optimization significantly decreased the effective dose by 11%, from 4.54 ± 0.1 mSv to 4.05 ± 0.8 mSv (p < 0.001). Image quality evaluated by NECdensity was also significantly improved by 10%, from 0.39 ± 0.1 to 0.43 ± 0.2 (p < 0.001). In no case did NECdensity deteriorate when the effective dose was decreased. In group 1, the dose decreased by 32%, while there was no significant deterioration in NECdensity (p = 0.054). In group 2, the dose decreased by 17%, and the NECdensity increased significantly (p < 0.01). In group 3, the dose decreased by 3%, and the NECdensity increased significantly (p < 0.01). In group 4, the dose increased by 14%, but there was no significant change in the NECdensity (p = 0.766). CONCLUSION: Body weight-based FDG injection dose optimization contributed to not only the reduction of effective dose but also the improvement of image quality in patients weighing between 50 and 69 kg.