Improved Dose Regimen in Pediatric PET

PET image quality depends strongly on patient weight and habitus, decreasing for increasing weight and body mass index. Common adult injection rules prescribe either a dose proportional to weight or a fixed dose. In light patients, image quality may improve for decreasing weight more than by inverse proportion. If better quality than in average-adult studies does not justify the associated dose burden, attractive options are to reduce scan time, reduce dose, or any combination of the 2. The objective of this study was to determine quantitative injection rules for pediatric PET allowing clinical implementation of these trade-offs. Methods: Literature methods combining phantom with clinical data were followed to derive patient-specific noise-equivalent count rate density (NECRD) curves as a function of injected dose. From these, it was possible to estimate retrospectively for each patient the scan time that would have been sufficient for the same NECRD as in a 70-kg reference adult; the reduced dose sufficient for constant NECRD and scan time; and a general relationship among scan time, dose, and NECRD. Correlation to the patient statistic giving highest correlation, which was found to be weight, provided rules applicable prospectively. Data from 73 patients (weight, 11.5–91.4 kg; mean, 45.4 kg) were acquired and analyzed. Results: Following the clinical injection rule, which was proportional to weight, the NECRD increased linearly with decreasing weight. The expression exp[0.019 × (weight [kg] − 70)] for the time reduction possible with the current dose at constant NECRD correlated well with data (R2 = 0.86). The dose (in MBq) necessary for constant NECRD that should be injected 60 min before imaging is predicted well by 14.8 × exp[0.046 × weight (kg)] (R2 = 0.88) with the current scan time. A more complex expression to convert NECRD in whole or part to both dose and time savings was also derived. Comparison to common pediatric injection rules showed reasonable agreement with Clark's rule, albeit not at all weights. Conclusion: Results suggest that pediatric PET of constant image quality (in an NECRD sense) can be performed with time or dose savings, up to 50% for the lightest patients (10–20 kg).