Bismuth Breast Shielding during Thoracic CT: Dose Evaluation and Effect on Diagnostic Image Quality

Background and Objective: The bismuth breast shield has shown to be effective at reducing the superficial radiation exposure of the breasts during thoracic CT. The radiation dose reduction of ۲۶% to ۶۱% has been reported in both the phantom and clinical-based studies. However, some drawbacks such as introduction of the image noise, streak and beam-hardening artifacts and change in CT number (Hounsfield Units) of the images have been concerned. The first aim of this study was to quantitative assessment of the noise and CT number of the images following a ۰, ۱, ۲, and ۳-cm shield-to-phantom distance. The second aim was to assess dose reduction achieved by the shield and a qualitative assessment of image quality in the clinical study.Materials and Methods: A ۳۲-cm in diameter body phantom was scanned using adult thoracic CT protocol. Scanning was repeated after placing a ۰, ۱, ۲ and ۳-cm cotton as spacer between the shield and phantom surface and the noise and CT number of the images was measured. In the clinical phase of the study, ۱۸۰ female patients who were referred to thoracic CT recruited into the study. The left breast was shielded, whereas the right breast remained non-shielded. Dose measurements were performed using TLD. The qualitative image quality was assessed based on European guidelines.Findings: The shield with ۰-cm shield-to-phantom distance has significantly increased noise and CT number of the phantom images. Three-cm shield-to-phantom distance has effectively lowered image noise arising from the shield; however, the CT number was significantly increased in all shield-to-phantom distances (P<۰.۰۰۱). In the patient study, the average absorbed dose at the surface of shielded and non-shielded breasts was ۱۳.۶±۳.۱ mGy and ۲۴.۰۴±۴.۷ mGy, respectively that refers to ۴۳.۴۲% dose reduction. Qualitative image quality analysis showed no statistical difference between shielded and non-shielded sections of the thoracic images (P=۰.۹۹۷).Conclusion: Combining bismuth shield with ۳-cm shield-to-breast distance has significantly reduced radiation exposure without qualitative or quantitative deterioration of image quality in term of image noise and streak artifacts. However, increasing the CT number of the images remains controversial.

Keywords:

Bismuth Shield , Breast , Noise , CT number , Thoracic , Patient