2020 年 37 巻 2 号 p. 34-40
Volumetric breast density (VBD) is considered to be the most reliable measure among those that evaluate breast density (BD) using mammographic apparatus, but its theory relies solely on primary x-rays that penetrate breast tissue. Thus the purpose of this study is to evaluate scatter x-ray influence on VBD, since the amount of scatter x-rays can not be ignored in usual mammographic exposures. Using breast equivalent phantoms whose BD is 0%, 50% and 100% respectively, we first measured scatter-to-primary ratio (SPR) with the lead disk method by changing kVp, phantom thickness, grid (+–) and BD. The true SPR was derived from 3 mm diameter lead disk data by utilizing extrapolation towards 0 mm diameter. Through the measurement, we found that SPR slightly increased as BD increased, while it vastly depended on other factors. Based on these measurements, we calculated the error that was caused by the scatter x-rays on the BD estimated. As a result, the error stayed within 1% for the most cases, though it reached as large as 10.7% at the extreme case of 8 cm phantom thickness (100% BD) without anti-scatter grid. These results are expected to contribute to efforts in improving the accuracy of VBD.