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