Evaluation of silicon and 10% bismuth shield with variable thickness compared with constant thickness on the dose reduction and image quality during chest CT examination

Abstract

One of the best methods to reduce the breast radiation dose in chest CT scan exam is using the bismuth shield. Due to the fact that, in CT scan tests, the breast dose is higher in the central axis of body than other sides, so the bismuth shield with variable thickness (outer half 1 mm thick and inner half 2 mm thick) is designed in this study. The objective of this study is to investigate in effectiveness of silicon and 10% bismuth composite shied with variable thickness on reducing radiation dose and image quality parameters in chest CT scan test compare to 1 mm and 2 mm of fixed thickness shield. Physical chest phantom underwent chest CT scan without or with bismuth shields with thickness of 1 mm, 2 mm, and variable in 90, 120 and 140 kVp in inactive TCM mode. Dosimetry was performed by using TLD, and image quality was evaluated quantitatively (by drawing the ROI in the identical images like image j, calculation of noise, CT number, SNR and CNR) and qualitatively (by two expert radiologists). Designed bismuth shield with unsteady thickness in inner and outer side compare to 1 mm and 2 mm thickness shields is presented in 120 kVp, has a significant difference in the amount of breast dose reduction (19% reduction), as well as in 140 kVp, all these three bismuth shields resulted in a significant dose reduction almost in the same similarity. At 120 kVp, the bismuth shield with variable thickness lead to a significant change in CT numbers in the heart and lungs, but it does not have a remarkable effect on other image quality parameters. The bismuth shield with variable thickness can lead to a better effectiveness in reducing breast dose without any side effect on image quality at 120 kVp, which requires further studies in this field.