Abstract
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Objectives: This work focused on the creation of enhanced computational models of the brain and eye for incorporation into the UF/NCI series of reference pediatric phantoms. The new brain models support dose assessments for those radiopharmaceuticals that non-uniformly distribute within the brain subregions. The eye model - initially developed for use in ocular dosimetry for external x-ray and proton therapy fields - is also included in the revised phantom series. These new models are intended to support risk/image-quality optimization studies of 18FDG PET imaging in pediatric patient populations.
Methods: The brain model features 43 compartments segmented from high-resolution CT and MR images of normal adult male and female patients. Each model was converted into a polygon mesh structure and then scaled to proper size prior to insertion within the UF/NCI newborn, 1-year, 5-year, 10-year, and 15-year male and female hybrid phantoms. For each model, a scalable and deformable adult eye model, containing 11 ocular substructures, was also inserted within the phantom series. Monoenergetic photon, electron, and alpha particle sources were then simulated using the PHITS Monte Carlo radiation transport code. Specific absorbed fractions were then combined with the radionuclide decay scheme data from ICRP Publication 107 to develop a new library of pediatric radionuclide S values for both Tc-99m and F-18.
Results: An updated series of pediatric male and female reference phantoms was created, allowing independent modeling of non-uniform uptake in brain subregions for pediatric nuclear medicine imaging studies. Each brain model allows consideration of 43 different neutral tissues, as both source region and target region, for dose assessment. Advanced models of the eye were additionally included to allow for dose assessment to the eye lens and other ocular targets following annihilation photon emissions following 18FDG decay.
Conclusions: The phantoms created in this study allow more detailed dosimetry to be performed for a variety of radionuclides that may have differential uptake in brain substructures. Additionally, the updated eye model allows for dose assessment to the radiosensitive tissues of the eye lens that might be relevant in the CT portion of hybrid SPECT/CT or PET/CT imaging systems. Acknowledgments: This work was support by NIH Grant R01 EB013558 from the National Institute for Biomedical Imaging and Bioengineering.