Effect of filter on average glandular dose and image quality in digital mammography

Abstract

The purpose of this study is to determine the average glandular dose, AGD and the image quality in phantom and patient for different target-filters (W/Rh and W/Ag) of the digital mammography system. The ACR mammography phantom and BR 12 phantom were used to determine the AGD and the image quality, the contrast to noise ratio (CNR) and the signal to noise ratio (SNR) of various target filters in different phantom thickness. A retrospective mammography study includes 200 patients which 100 patients were examined using the W/Rh target-filter and 100 different patients using W/Ag target-filter. The compressed breast thickness (CBT, mm), compression force (lbs), average glandular dose (AGD, mGy), peak kilovoltage (kVp) and tube current time (mAs) were recorded and compared between W/Rh and W/Ag target. The result in the phantom study, at the same thickness, the CNR and SNR values were decreased with increasing tube voltages in AEC technique. At the manual technique, when the tube voltage was constant and tube current time was increasing, CNR and SNR were increased. This was agreeable for both W/Rh and W/Ag target-filters. In patient study, the mean AGD, CBT, CF, kVp, mAs of W/Rh were 1.26 mGy, 54.18 mm, 13.24 lbs., 29.68 and 106.75 respectively in CC view. The mean AGD, CBT, CF, kVp, mAs of W/Ag was 1.79 mGy, 81 mm, 14.15 lbs., 31.79 and 113.34 respectively. At 40 mm CBT, the optimal tube output for W/Rh range from 27 to 29 kVp and mAs range from 58 to 129, the AGD range from 0.82 to 1.15 mGy. The optimal tube output for W/Ag range from 30 to 35 kVp and mAs range from 145 to 200, the AGD range from 1.28 to 2.97 mGy. For compress breast thickness range from 40 mm to 60 mm, the W/Rh target-filter should be chosen and the CBT greater than 70 mm, the W/Ag target-filter is appropriate in order to compensation between good image quality and dose reduction. Conclusion in patient study, the determination of patient dose in King Chulalongkorn Memorial Hospital in 2016-2017 revealed that the mean AGD was 1.26 mGy, the mean CBT was 54.18 mm for W/Rh. The mean AGD was 1.79 mGy, the mean CBT was 81 mm for W/Ag. In our study, 2.5% of the patients received the average glandular dose over the diagnostic reference level of 3 mGy per view.