Due to fetal radiosensitivity, the estimation of internal dose received by fetus from radiopharmaceuticals incorporated to the mother is often important in nuclear medicine. A new 9-month pregnant phantom based on magnetic resonance (MR) images tied to the International Commission on Radiological Protection (ICRP) reference voxel phantom was developed. Maternal and fetal organs were segmented from a set of pelvic MR images of a 9-month pregnant subject using 3D-DOCTORTM and then imported to the 3D modeling software package RhinocerosTM for combining with adult female ICRP voxel phantom and further modeling. Then, the phantom organs were rescaled to match with reference masses introduced in ICRP Publications. Internal anatomy of previous pregnant models was limited only to a model consists of the fetal brain and skeleton while the fetus model developed in this study contains 20 different organs. The current reference phantom is developed to be applied for comprehensive dosimetric study in nuclear medicine. The internal dosimetry calculations were performed for thyroid agents by using the Monte Carlo transport method. Biokinetic data during pregnancy for these radiopharmaceuticals were used to estimate cumulated activity and maternal and fetal organ doses at seven different maximum thyroid uptake levels. Calculating the dose distribution was also presented in sagittal view of pregnant model utilizing mesh tally function. The comparisons showed, in general, an overestimation of absorbed dose to the fetus and an underestimated fetal thyroid dose of the previous studies compared to the values based on the current hybrid phantom.

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