PurposeTo explore the effect of half 18 F-FDG dose on image quality of pediatric oncological patients using totalbody PET/CT. and investigate the ultra-low dose of administered tracer activity. Methods 100 pediatric oncological patients who underwent total-body PET/CT using the uEXPLORER scanner with half 18 F-FDG dose (1.85 MBq/kg) were retrospectively enrolled. The PET images were rstly reconstructed using all the 600s data, and then split into 300s, 180s, 60s, 40s and 20s duration groups. The subjective analysis method was assessed using a 5-point Likert scales. Objective quantitative metrics included the maximum standard uptake value (SUV max ), SUV mean , standard deviation (SD), signal to noise ratio (SNR) and SNR norm of the backgrounds. The variability in lesion SUV mean , SUV max , and tumor-to-background ratio (TBR) were also calculated. ResultsThe overall image quality scores in Group 600s, 300s, 180s and 60s were 4.9 ± 0.2, 4.9 ± 0.3, 4.4±0.5 and 3.5±0.5 respectively. All the lesions identi ed in half-dose images were localised down to G60s (1/20-dose) images; while 56% of the lesion could be clearly identi ed in G20s (1/60-dose) images. With stimulated reduced doses, SUV max and SD of backgrounds were gradually increased, while TBR values showed no statistically signi cant deviation among all the groups (all p > 0.1). Using the half-dose images as reference, the variability of lesion SUV max gradually increased from G180s to G20s, while lesion SUVmean remained stable for all the age groups. SNR norm was highly correlated with the age in a negative direction. ConclusionsThe total-body PET/CT with half 18 F-FDG dose achieved good performance in pediatric patients considering the su cient image quality and lesion conspicuity. Su cient image quality and lesion conspicuity could be maintained with an administered dose reduced down to 1/20-dose (0.185 MBq/kg, , estimated effective dose: 0.18-0.26 mSv).

Background The axial length of a conventional PET/CT scanner is about 15–30 cm. However, uEXPLORER Total-Body PET/CT has an ultra-long axial field of view of 194 cm. By taking full use of all the scintillation photons, uEXPLORER has a 40 times higher sensitivity for photon detection relative to the conventional PET/CT. Ordered subset expectation maximization (OSEM) is a commonly used iterative algorithm in PET, however, it has a limitation that the image noise will increase when large number iteration is selected. A new penalized-likelihood iterative PET reconstruction, termed HYPER Iterative, was invented and now is available on the uEXPLORER Total-Body PET/CT. To date, its impact in lesion conspicuity in the patients with full injected dose or half injected dose was unclear. The goal of this study is to determine a proper protocol for routine 18F-FDG uEXPLORER Total-Body PET/CT scan. Results The quality of the 5 min PET image was excellent (score 5) for all the dose and reconstructed methods. Using the HYPER iterative method, PET image reached the excellent quality at 1 min with full-dose, and at 2 min with half-dose. While PET image reached a similar excellent quality at 2 min with full-dose and 3 min with half-dose using OSEM. The noise in OSEM reconstruction was higher than that by HYPER Iterative. Compared to OSEM, HYPER Iterative had slightly higher SUVmax and TBR of the lesions for large positive lesions (≥ 2cm) (SUVmax: up to 9% higher in full-dose and up to 13% higher in half-dose; TBR: up to 9% higher in full-dose and up to 23% higher in half-dose). For small positive lesions(≤ 10mm), HYPER Iterative had obviously higher SUVmax and TBR of the lesions (SUVmax: up to 45% higher in full-dose and up to 75% higher in half-dose; TBR: up to 45% higher in full-dose and up to 94% higher in half-dose). Conclusions Our study demonstrates that 1min scan with full dose and 2 min with half dose is proper for clinical diagnosis using HYPER Iterative, and 2 to 3 min scan for OSEM reconstruction. For detection of the small lesions, HYPER Iterative reconstruction is preferred.

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BackgroundConventional short‐axis PET typically utilizes multi‐bed multi‐pass acquisition to produce quantitative whole‐body dynamic images and cannot record all the uptake information simultaneously, resulting in errors when fitting the time‐activity curves (TACs) and calculating radiation doses.PurposeThe aim of this study is to evaluate the 13N‐ammonia biodistribution and the internal radiation doses using a 194 cm long total‐body PET/CT scanner (uEXPLORER), and make a comparison with the previous short‐axis PET results.MethodsTen subjects (age 40–74 years) received 13N‐NH3 injection (418.1‐670.81 MBq) and were under a dynamic scan for about 60 min with using a 3‐dimensional whole‐body protocol. ROIs were drawn visually on 11 major organs (brain, thyroid, gallbladder, heart wall, kidneys, liver, pancreas, spleen, lungs, bone marrow, and urinary bladder content) for each subject. TACs were generated using Pmod and the absorbed radiation doses were calculated using Olinda 2.2. To compare with the conventional PET/CT, five points were sampled on uEXPLORER's TACs to mimic the result of a short‐axis PET/CT (15 cm axial FOV, consisted of 9 or 10 bed positions). Then the TACs were obtained using the multi‐exponential fitting method, and the residence time and radiation dose were also calculated and compared with uEXPLORER.ResultsThe highest absorbed organ doses were the pancreas, thyroid, spleen, heart wall, and kidneys for the male. For the female, the first five highest absorbed organ dose coefficients were the pancreas, heart wall, spleen, lungs, and kidneys. The lowest absorbed dose was found in red marrow both for male and female. The simulated short‐axis PET can fit TACs well for the gradually‐changed uptake organs but typically underestimated for the rapid‐uptake organs during the first‐10 min, resulting in errors in the calculated radiation dose.ConclusionuEXPLORER PET/CT can measure 13N‐ammonia's TACs simultaneously in all organs of the whole body, which can provide more accurate biodistribution and radiation dose estimation compared with the conventional short‐axis scanners.

Purpose To characterize the diagnostic performance of [18F]FDG dose de-escalation with shortened acquisition times using total-body PET/CT in pediatric tumor imaging in terms of the subjective image quality and quantification of tracer uptake. Methods In this single-center prospective study, 31 pediatric oncology patients under 14 years old were enrolled and underwent total-body PET/CT using the uEXPLORER PET/CT scanner. All patients were randomly assigned to one of four [18F]FDG dose groups: full-dose (3.7 MBq/kg), 1/2-dose (1.9 MBq/kg), 1/3-dose (1.2 MBq/kg), and 1/4-dose (0.9 MBq/kg). Images with a shortened acquisition time frame (20 min, 12 min, 10 min, 8 min, 6 min, 5 min, 4 min, 3 min, 2 min, 1.5 min, 1 min, and 0.5 min) were reconstructed for each study, with the 20-min images as controls for each dose group. Semiquantitative uptake metrics were assessed using region-of-interest (ROI) analysis of healthy liver and suspected lesions. The subjective analysis was performed using 5-point Likert scales. Suspected major lesions and microlesions were recorded, while 3-point Likert scales were used for diagnostic confidence. Results With shortened acquisition times, the liver maximum standard uptake value (SUVmax) and standard deviation (SD) increased in each dose group. The signal-to-noise ratio (SNR) was significantly reduced with shortened acquisition time, while the lesion SUVmax and tumor-to-background ratio (TBR) showed no significant deviation. A decent subjective image quality score could be achieved in the full-, 1/2-, 1/3-, and 1/4-dose groups with at least 2-min, 4-min, 6-min, and 8-min acquisitions, respectively, where great overall image quality and brain delineation (scored 5.0) and superior organ boundaries and image noise (scored over 4.0) could be achieved, and all suspicious lesions found in 20-min images were detectable with high diagnostic confidence. Conclusions The regimen of full-dose [18F] FDG with a 2-min scan, 1/2-dose with a 4-min scan, 1/3-dose with a 6-min scan, and 1/4-dose with an 8-min scan using total-body PET/CT can provide great image qualities, can maintain a desired diagnostic performance and is feasible for pediatric oncological clinical applications. Trial registration: ChiCTR2000036334. Registered 22 August 2020.