In computed tomography (CT), some superficial organs which have increased sensitivity to radiation, receive doses that are significant enough to be matter of concern. Therefore, in this study, the effects of using shields on the amount of dose reduction and image quality was investigated for pediatric imaging. Absorbed doses of breasts, eyes, thyroid and testes of a series of pediatric phantoms without and with different thickness of bismuth and lead were calculated by Monte Carlo simulation. Appropriate thicknesses of shields were chosen based on their weights, X-ray spectrum, and the amount of dose reduction. In addition, the effect of lead shield on image quality of a simple phantom was assessed quantitatively using region of interest (ROI) measurements. Considering the maximum reduction in absorbed doses and X-ray spectrum, using a lead shield with a maximum thickness of 0.4 mm would be appropriate for testes and thyroid and two other organs (which are exposed directly) should be protected with thinner shields. Moreover, the image quality assessment showed that lead was associated with significant increases in both noise and CT attenuation values, especially in the anterior of the phantom. Overall, the results suggested that shielding is a useful optimization tool in CT.
V/Q SPECT should not always be preferred for pregnant patients suspected of PE. This finding is in contrast with the guidance to choose the preferred modality based on the maternal effective dose. The reason of this issue was discussed in this paper based on chord length distributions (CLDs). The importance of considering fetal organs separately in MC calculations was also highlighted.
In order to establish an organ and effective dose database for Iranian children undergoing computed tomography (CT) examinations, in the first step, two Iranian 11-year-old phantoms were constructed from image series obtained from CT and magnetic resonance imaging (MRI). Organ and effective doses for these phantoms were calculated for head, chest, abdomen–pelvis and chest–abdomen–pelvis (CAP) scans at tube voltages of 80, 100 and 120 kVp, and then they were compared with those of the University of Florida (UF) 11-year-old male phantom. Depth distributions of the organs and the mass of the surrounding tissues located in the beam path, which shield the internal organs, were determined for all phantoms. From the results, it was determined that the main organs of the UF phantom receive smaller doses than the two Iranian phantoms, except for the urinary bladder of the Iranian girl phantom. In addition, the relationship between the anatomical differences and the size of the dose delivered was also investigated and the discrepancies between the results were examined and justified.
-The global increase in the number of computed tomography (CT) examinations have enhanced concerns regarding stochastic radiation risks to patients, especially for children. Considering that cancer risk is cumulative over a lifetime and each CT examination contributes to the lifetime exposure, there is a need for a better understanding of radiation-induced cancer incidence and mortality, and better dose estimates. Accordingly, some authors estimated organ and effective dose in reference phantoms, but still there is a critical need to expand these data to larger groups of non-reference children. As an initial step to address this issue, in this study organ and effective doses were calculated in common CT procedures in non-reference pediatric phantoms and were compared with those of reference phantoms with the similar ages. Thirteen pediatric phantoms, BABY CHILD, five voxel-based UF pediatric phantoms (B-series) and six phantoms developed at The Foundation for Research on Information Technologies in Society (IT'IS) were implemented into MCNP. According to the results, there were no consistent differences between the doses of organs exposed indirectly and effective doses of these three phantom types, but it was observed that for organs located in the scan region, there was a relation between absorbed doses and pediatric age group, as expected. Generally, using the results of this study one can estimate the absorbed doses more accurately. But it should be noted that these low expansion data are not comprehensive enough for finding a reasonable relationship between phantom size and effective dose except in chest-abdomen-pelvis (CAP) imaging.Keywords: Pediatric computed tomography / reference and non-reference phantoms / age and size dependent absorbed dose / effective dose estimation
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