Shrestha Suman, Vedantham Srinivasan, Karellas Andrew
Department of Radiology, University of Massachusetts Medical School, Worcester, MA 01655, United States of America.
Phys Med Biol. 2017 Mar 7;62(5):1969-1993. doi: 10.1088/1361-6560/aa58c8. Epub 2017 Jan 11.
In digital breast tomosynthesis and digital mammography, the x-ray beam filter material and thickness vary between systems. Replacing K-edge filters with Al was investigated with the intent to reduce exposure duration and to simplify system design. Tungsten target x-ray spectra were simulated with K-edge filters (50 µm Rh; 50 µm Ag) and Al filters of varying thickness. Monte Carlo simulations were conducted to quantify the x-ray scatter from various filters alone, scatter-to-primary ratio (SPR) with compressed breasts, and to determine the radiation dose to the breast. These data were used to analytically compute the signal-difference-to-noise ratio (SDNR) at unit (1 mGy) mean glandular dose (MGD) for W/Rh and W/Ag spectra. At SDNR matched between K-edge and Al filtered spectra, the reductions in exposure duration and MGD were quantified for three strategies: (i) fixed Al thickness and matched tube potential in kilovolts (kV); (ii) fixed Al thickness and varying the kV to match the half-value layer (HVL) between Al and K-edge filtered spectra; and, (iii) matched kV and varying the Al thickness to match the HVL between Al and K-edge filtered spectra. Monte Carlo simulations indicate that the SPR with and without the breast were not different between Al and K-edge filters. Modelling for fixed Al thickness (700 µm) and kV matched to K-edge filtered spectra, identical SDNR was achieved with 37-57% reduction in exposure duration and with 2-20% reduction in MGD, depending on breast thickness. Modelling for fixed Al thickness (700 µm) and HVL matched by increasing the kV over (0,4) range, identical SDNR was achieved with 62-65% decrease in exposure duration and with 2-24% reduction in MGD, depending on breast thickness. For kV and HVL matched to K-edge filtered spectra by varying Al filter thickness over (700, 880) µm range, identical SDNR was achieved with 23-56% reduction in exposure duration and 2-20% reduction in MGD, depending on breast thickness. These simulations indicate that increased fluence with Al filter of fixed or variable thickness substantially decreases exposure duration while providing for similar image quality with moderate reduction in MGD.
在数字乳腺断层合成和数字乳腺摄影中，不同系统的X射线束滤过材料和厚度各不相同。研究了用铝替代K边滤波器，目的是减少曝光时间并简化系统设计。使用K边滤波器（50μm铑；50μm银）和不同厚度的铝滤波器模拟了钨靶X射线光谱。进行了蒙特卡罗模拟，以量化单独各种滤波器的X射线散射、压缩乳房时的散射与原发射线比（SPR），并确定乳房的辐射剂量。这些数据用于分析计算钨/铑和钨/银光谱在单位（1 mGy）平均腺体剂量（MGD）下的信号差与噪声比（SDNR）。在K边滤波器和铝滤波器光谱的SDNR匹配时，针对三种策略量化了曝光时间和MGD的减少量：（i）固定铝厚度并匹配千伏（kV）的管电压；（ii）固定铝厚度并改变kV以匹配铝和K边滤波器光谱之间的半价层（HVL）；以及（iii）匹配kV并改变铝厚度以匹配铝和K边滤波器光谱之间的HVL。蒙特卡罗模拟表明，铝滤波器和K边滤波器在有无乳房情况下的SPR没有差异。对于固定铝厚度（700μm）和与K边滤波器光谱匹配的kV进行建模，根据乳房厚度，曝光时间减少37 - 57%，MGD减少2 - 20%，可实现相同的SDNR。对于固定铝厚度（700μm）并通过在（0,4）范围内增加kV来匹配HVL进行建模，根据乳房厚度，曝光时间减少62 - 65%，MGD减少2 - 24%，可实现相同的SDNR。对于通过在（700, 880）μm范围内改变铝滤波器厚度来使kV和HVL与K边滤波器光谱匹配，根据乳房厚度，曝光时间减少23 - 56%，MGD减少2 - 20%，可实现相同的SDNR。这些模拟表明，固定或可变厚度的铝滤波器增加注量可显著减少曝光时间，同时在MGD适度降低的情况下提供相似的图像质量。
