Abstract
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Objectives: One of the major issues regarding radiation protection is the optimization of therapeutic exposure, and the International Committee for Radiological Protection and the International Atomic Energy Agency recommend establishing diagnostic reference levels as tools for dose optimization[1, 2]. Particularly, radiation exposure that is “as low as reasonably achievable” is important in pediatric diagnostic imaging, and positron emission tomography (PET) is also required to have the lowest absorbed dose consistent with adequate imaging quality. Pediatric imaging with PET using 18F-fluorodeoxyglucose (18F-FDG) has been initiated to diagnose tumors and epilepsy. Some of the guidelines advocated the reduction of 18F-FDG PET dose for children[3-6]. The recommended doses for children are substantially from one-half to one-third of those recommended for adults, which are extremely low doses in comparison. In this study, we analyzed the relationship between age and radioactivity distribution in the pediatric brain[7]. Further, the optimum scan duration in children to ensure image quality equivalent to those obtained in adults was examined.
Methods: Volume and radioactivity analyses of the brains of children (14 boys and 32 girls; range, 0-15 years; mean, 9 years) and adults (12 men and 12 women; range, 18-64 years; mean, 42 years) were performed. The uptake rate of injection dose and radioactivity concentration per ml in the brain were compared between children and adults. A Hoffman phantom and a cylinder were filled with 25 MBq of 18F-FDG. These phantom images were reconstructed using an order subset expectation maximization algorithm with time-of-flight. The images were reconstructed with nine scan durations (range, 10-33 min), which were obtained from the regression equation, assuming the following ages: 0.5, 1, 1.5, 2, 3, 6, 9, and 13 years and adults. The physical parameters were assessed using noise equivalent count (NECphantom), %contrast, and coefficient of variance (CV). Statistical analyses were conducted on the uptake rate and radioactivity concentration in the brain using linear regression and analysis of variance. In addition, visual evaluation was performed using the Scheffe’s method of paired comparisons (Nakaya variation) at a significance level of 0.05.
Results: The uptake rate in the brain of children aged ≤3 years increased with aging (r = 0.56), of children aged >3 years decreased with aging (r = −0.82), and of adults gradually decreased with aging (r = −0.70). A significant difference was noted in the uptake rate in the brain among three generations (0-3 and 4-15 years old and adults) (p < 0.001). With respect to the radioactive concentration, a significant difference was observed between children and adults (p < 0.001), although no difference was observed between children aged ≤3 years and those aged >3 years. The NECphantom, %contrast, and CV of both phantoms were almost constant and not dependent on the scan duration at each age obtained from the regression equation. No significant differences were observed with respect to the scan duration in the visual evaluation.
Conclusions: Brain PET in children aged >3 years can sufficiently ensure image quality at 1.5 times the scan duration of adults. However, we recommend twice the scan durations of adults for children aged ≤3 years.