The reduction of dose to breast tissue during multi-slice CT examinations using bismuth breast shields and dose modulation technologies

摘要

With the rapid increase in the number of CT scans being performed worldwide, the importance of dose reduction to the radiosensitive breast tissue during these procedures is becoming a critical factor in ensuring that the induction of breast cancer in this modality is minimised. This study utilised a novel geometric phantom that allowed rapid modification of the size of the breasts to analyse the effectiveness of various dose reduction techniques under a variety of different circumstances from paediatric patients to larger breasted patients. Doses were accurately measured throughout the entire breast and chest area using over 300 individual TLD crystals, providing extremely comprehensive dose contour maps for CT scanning of the chest area. The efficiency of bismuth breast shields was investigated and it was found that an average dose reduction of 21% for small sized breasts and up to 44% for larger breasts could be achieved. A maximum dose reduction of 57% was found, which does confirm the manufacturer’s claims. The breast shields did create additional quantitative noise in the diagnostic regions of interest, as well as qualitative noise in the form of streaking or beam hardening artefacts. The study found that lower breast doses could be achieved for a consistent noise value by simply manually reducing the tube mA. Dose modulation on the Philips 64 CT scanner was also found to be an effective way to reduce the breast dose with reductions of between 40% and 50% depending on the type of dose modulation used. Dose modulation coupled with the breast shield created an even greater dose reduction, with certain techniques providing almost an 80% reduction in breast dose, though this was coupled with a significant increase in noise. Based on the findings in this study, it is believed that further analysis on acceptable levels of noise is needed before a definitive position can be taken on the clinical use of bismuth breast shields. Other methods are available to the operators that provide equivalent levels of dose reduction to the breast tissue, whilst providing lower levels of noise in the clinical regions of interest.