Optimizing Reconstruction Parameters Allows Dose Reduction in Pediatric Oncology Patients with Delayed Total-Body FDG PET/CT

Introduction: Given the increased signal collection efficiency, total-body (TB) PET/CT allows for protocol flexibility including dose reduction which is of particular interest in pediatric patients undergoing multiple PET/CT examinations. Therefore, the aim of the study was to define a lower limit of reduced injected dose in delayed [¹⁸F]FDG TB PET/CT without loss of diagnostic image quality in pediatric oncology patients.

Methods: In this single-center study, 19 children (12 y/o (5-17y), 54.53±19.75kg) were scanned for 20 min with TB PET/CT, 120 min after administration of 4.07±0.49 MBq/kg [¹⁸F]FDG intravenously. Five low-count reconstructions without replacement were generated using 1/4^th, 1/8^th, 1/16^th, 1/32^nd of the counts of the randomly resampled full-dose list-mode reference standard acquisition (20 min), to simulate dose reduction. Follow-up [¹⁸F]FDG TB PET/CT from 12/19 patients were used to select the optimal reconstruction parameters for the simulated low-dose protocols. Low-count reconstruction protocols with varying number of iterations (4-6) and smoothing filters (46 mm FWHM Gaussian or non-local means (NLM)) were evaluated and compared to the full-dose dataset reconstructed with 4 iterations (20 subsets), 256x256 matrix, and with no smoothing, which is the clinical standard at our institution. All reconstructions were simultaneously displayed and independently rated from 1 (best) to 4 (worst) for interpretation/diagnosis by three nuclear medicine physicians.

Optimal reconstruction parameters selected for each low-dose protocol were used with the initial staging scans of the 19 patients to define the lower limit of dose reduction for pediatrics. Background uptake was measured on the low-count reconstructions and compared to the clinical reference standard image, with volumes-of-interest (VOI) placed on the ascending aorta, right liver lobe, and lumbar vertebra body three (L3). Tumor lesions were segmented using a 40% iso-contour VOI approach. Coefficient-of-variation (COV), tumor-to-background ratio (TBR), and contrast-to-noise ratio (CNR) were calculated. Three physicians identified malignant lesions independently and assessed the image quality using a 5-point Likert scale.

Results: In total, 113 malignant lesions were identified in the full-dose PET/CT in 18/19 patients, who met the inclusion criteria. 87.6% of the lesions were quantifiable. Lowest COV was detected in the 20 min reference standard. All low-count reconstructions obtained significantly higher COV (p<0.0001). Tumor uptake (SUV_max), TBR, and total lesion count were significantly lower in the reconstructions with 1/16^th and 1/32^nd counts of the reference standard (p<0.05). CNR and clinical image quality were significantly lower in all low-count reconstructions compared to the reference standard. Additionally, readers selected the reference reconstruction parameters for the 1/4^th and 1/8^th dose reduction (4 iterations, 20 subsets, no filter), but preferred additional smoothing by applying either Gaussian or NLM filters on 1/16^th and 1/32^nd respectively to compensate for the higher COV.

Conclusions: Dose reduction with delayed [¹⁸F]FDG TB PET/CT imaging in children is possible without loss of subjective image quality or lesion conspicuity after optimizing TB PET/CT reconstruction parameters. However, our results indicate that PET centers should not reduce the dose below 0.5 MBq/kg (1/8^th of 4.07MBq/kg) for [¹⁸F]FDG pediatric imaging at 120-min p.i. to maintain diagnostic image quality and provide conclusive images to the pediatric oncologists.

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