Department of Radiology, UZ Gasthuisberg, Herestraat 49, B-3000 Leuven, Belgium.
Med Phys. 2012 Jun;39(6):3167-80. doi: 10.1118/1.4711754.
Four different practical methodologies of quantifying scattered radiation for two different digital mammographic systems are compared. The study considered both grid in and grid out geometries for two different antiscatter grid types, a typical linear grid and a cellular grid design. The aim was to find quick and reproducible methods that could be used in place of the beam stop technique.
The scatter to primary ratio (SPR) and the scatter fraction (SF) were used to quantify scattered radiation as a function of poly(methyl methacrylate) (PMMA) thickness, grid position, and beam quality. The four scatter estimation methods applied were (1) the beam stop method, (2) a hybrid method that combined measured detector (scatter-free) modulation transfer function (MTF) data and a Monte Carlo simulation of the scatter point spread function, (3) from the low frequency drop data taken from the system MTF, and (4) from the edge spread function (ESF) measured in the presence of PMMA. Repeatability error was assessed for all methods.
SPR results acquired with the beam stop method ranged from 0.052 to 0.187 for the system with linear grid and from 0.012 to 0.064 for the cellular grid system, as PMMA thickness was increased from 20 to 80 mm. With the grid removed, beam stop SPR was similar for both systems, ranging between 0.268 and 1.124, for corresponding MTF thicknesses. The direct MTF method had a maximum difference of 24% from the beam stop SPR and SF data for all conditions except the cellular grid in geometry, where maximum difference in SPR was 0.044 (164%). The ESF technique gave large differences from the beam stops for both grid geometries but agreement was within 21% for the grid out geometry. Repeatability error with beam stops was between 1% and 5% for the grid out geometries, while for the grid in cases it was 13% and 87% for the linear and cellular grids, respectively. Repeatability error for the direct MTF method applied to both systems and grid geometries ranged between 3% and 12%.
All three alternative methods to the beam stop technique gave reasonable estimates of SPR without grid, with a maximum difference of 24% (mean difference 8%). For the grid in geometry, the direct MTF method gave a maximum difference of 24% for the linear grid system, while maximum percentage difference was 119% (absolute difference of 0.042) for the system with the cellular grid, where SPR values were low. Except for cases where the SPR is very low, the direct MTF method offers a quick and reproducible alternative to the beam stop technique.
比较两种不同数字乳腺摄影系统中四种不同的散射辐射定量实用方法。该研究考虑了两种不同的防散射栅类型的栅内和栅外几何形状，一种是典型的线性栅格，另一种是蜂窝状栅格设计。目的是找到快速且可重复的方法，可以替代束流限束器技术。
散射与原发比（SPR）和散射分数（SF）用于定量散射辐射，作为聚甲基丙烯酸甲酯（PMMA）厚度、栅格位置和射束质量的函数。应用的四种散射估计方法是：（1）束流限束器方法；（2）结合了测量探测器（无散射）调制传递函数（MTF）数据和散射点扩散函数蒙特卡罗模拟的混合方法；（3）从系统 MTF 中获取的低频下降数据；（4）在存在 PMMA 的情况下测量的边缘扩展函数（ESF）。对所有方法进行了重复性误差评估。
使用束流限束器方法获得的 SPR 结果，在线性栅格系统中，随着 PMMA 厚度从 20 毫米增加到 80 毫米，范围从 0.052 到 0.187；在蜂窝状栅格系统中，范围从 0.012 到 0.064。当栅格移除时，两种系统的束流限 SPR 相似，对应于 MTF 厚度，范围在 0.268 到 1.124 之间。直接 MTF 方法在所有条件下，除了栅格内的蜂窝状系统外，与束流限 SPR 和 SF 数据的最大差异为 24%，在 SPR 中最大差异为 0.044（164%）。ESF 技术对于两种栅格几何形状，与束流限器的差异都很大，但对于栅格外的几何形状，一致性在 21%以内。对于栅格外的几何形状，束流限器的重复性误差在 1%到 5%之间，而对于栅格内的情况，线性栅格和蜂窝状栅格的重复性误差分别为 13%和 87%。直接 MTF 方法应用于两种系统和栅格几何形状的重复性误差在 3%到 12%之间。
束流限束器技术的三种替代方法都可以合理估计无栅格的 SPR，最大差异为 24%（平均差异为 8%）。对于栅格内的几何形状，直接 MTF 方法在线性栅格系统中的最大差异为 24%，而对于具有蜂窝状栅格的系统，最大百分比差异为 119%（绝对差异为 0.042），其中 SPR 值较低。除了 SPR 值非常低的情况外，直接 MTF 方法提供了一种快速且可重复的替代束流限束器技术的方法。
