Department of Radiology, Stanford University, Stanford, California 94305, USA.
Med Phys. 2010 Feb;37(2):934-46. doi: 10.1118/1.3298014.
Scatter correction is a major challenge in x-ray imaging using large area detectors. Recently, the authors proposed a promising scatter correction method for x-ray computed tomography (CT) using primary modulation. Proof of concept was previously illustrated by Monte Carlo simulations and physical experiments on a small phantom with a simple geometry. In this work, the authors provide a quantitative evaluation of the primary modulation technique and demonstrate its performance in applications where scatter correction is more challenging.
The authors first analyze the potential errors of the estimated scatter in the primary modulation method. On two tabletop CT systems, the method is investigated using three phantoms: A Catphan 600 phantom, an anthropomorphic chest phantom, and the Catphan 600 phantom with two annuli. Two different primary modulators are also designed to show the impact of the modulator parameters on the scatter correction efficiency. The first is an aluminum modulator with a weak modulation and a low modulation frequency, and the second is a copper modulator with a strong modulation and a high modulation frequency.
On the Catphan 600 phantom in the first study, the method reduces the error of the CT number in the selected regions of interest (ROIs) from 371.4 to 21.9 Hounsfield units (HU); the contrast to noise ratio also increases from 10.9 to 19.2. On the anthropomorphic chest phantom in the second study, which represents a more difficult case due to the high scatter signals and object heterogeneity, the method reduces the error of the CT number from 327 to 19 HU in the selected ROIs and from 31.4% to 5.7% on the overall average. The third study is to investigate the impact of object size on the efficiency of our method. The scatter-to-primary ratio estimation error on the Catphan 600 phantom without any annulus (20 cm in diameter) is at the level of 0.04, it rises to 0.07 and 0.1 on the phantom with an elliptical annulus (30 cm in the minor axis and 38 cm in the major axis) and with a circular annulus (38 cm in diameter).
On the three phantom studies, good scatter correction performance of the proposed method has been demonstrated using both image comparisons and quantitative analysis. The theory and experiments demonstrate that a strong primary modulation that possesses a low transmission factor and a high modulation frequency is preferred for high scatter correction accuracy.
在使用大面积探测器的 X 射线成象中，散射校正仍是一个主要挑战。最近，作者提出了一种使用初步调制的有前途的 X 射线计算机断层扫描（CT）散射校正方法。概念验证先前通过蒙特卡罗模拟和具有简单几何形状的小体模的物理实验进行了说明。在这项工作中，作者对初步调制技术进行了定量评估，并证明了它在散射校正更具挑战性的应用中的性能。
作者首先分析了初步调制方法中估计散射的潜在误差。在两个台式 CT 系统上，使用三个体模：Catphan 600 体模、人体胸部体模和带有两个环的 Catphan 600 体模，对该方法进行了研究。还设计了两种不同的初步调制器，以显示调制器参数对散射校正效率的影响。第一个是调制较弱且调制频率较低的铝调制器，第二个是调制较强且调制频率较高的铜调制器。
在第一项研究中，Catphan 600 体模上，该方法将选定感兴趣区域（ROI）的 CT 数误差从 371.4 减少到 21.9 亨氏单位（HU）；对比度噪声比也从 10.9 增加到 19.2。在第二项研究中的人体胸部体模中，由于散射信号和物体异质性较高，情况更为复杂，该方法将选定 ROI 中的 CT 数误差从 327 减少到 19 HU，整体平均误差从 31.4%减少到 5.7%。第三项研究是为了研究物体尺寸对我们方法效率的影响。无环（直径 20 厘米）的 Catphan 600 体模上的散射与原始射线比值估计误差处于 0.04 的水平，在具有椭圆形环（短轴 30 厘米，长轴 38 厘米）和具有圆形环（直径 38 厘米）的体模上，该误差上升到 0.07 和 0.1。
在三个体模研究中，使用图像比较和定量分析证明了所提出方法的良好散射校正性能。理论和实验证明，优先使用具有低透射因子和高调制频率的强初步调制来实现高散射校正精度。
