Zhu Lei, Bennett N Robert, Fahrig Rebecca
Department of Electrical Engineering, Stanford University, Stanford, CA 94305, USA.
IEEE Trans Med Imaging. 2006 Dec;25(12):1573-87. doi: 10.1109/tmi.2006.884636.
An X-ray system with a large area detector has high scatter-to-primary ratios (SPRs), which result in severe artifacts in reconstructed computed tomography (CT) images. A scatter correction algorithm is introduced that provides effective scatter correction but does not require additional patient exposure. The key hypothesis of the algorithm is that the high-frequency components of the X-ray spatial distribution do not result in strong high-frequency signals in the scatter. A calibration sheet with a checkerboard pattern of semitransparent blockers (a "primary modulator") is inserted between the X-ray source and the object. The primary distribution is partially modulated by a high-frequency function, while the scatter distribution still has dominant low-frequency components, based on the hypothesis. Filtering and demodulation techniques suffice to extract the low-frequency components of the primary and hence obtain the scatter estimation. The hypothesis was validated using Monte Carlo (MC) simulation, and the algorithm was evaluated by both MC simulations and physical experiments. Reconstructions of a software humanoid phantom suggested system parameters in the physical implementation and showed that the proposed method reduced the relative mean square error of the reconstructed image in the central region of interest from 74.2% to below 1%. In preliminary physical experiments on the standard evaluation phantom, this error was reduced from 31.8% to 2.3%, and it was also demonstrated that the algorithm has no noticeable impact on the resolution of the reconstructed image in spite of the filter-based approach. Although the proposed scatter correction technique was implemented for X-ray CT, it can also be used in other X-ray imaging applications, as long as a primary modulator can be inserted between the X-ray source and the imaged object.
具有大面积探测器的X射线系统具有高散射与原发射线比率（SPRs），这会在重建的计算机断层扫描（CT）图像中导致严重伪影。引入了一种散射校正算法，该算法可提供有效的散射校正，但无需增加患者的辐射剂量。该算法的关键假设是，X射线空间分布的高频分量不会在散射中产生强烈的高频信号。在X射线源和物体之间插入一张带有半透明阻挡物棋盘图案的校准片（“原发射线调制器”）。基于该假设，原发射线分布由高频函数部分调制，而散射分布仍具有占主导地位的低频分量。滤波和解调技术足以提取原发射线的低频分量，从而获得散射估计值。使用蒙特卡罗（MC）模拟验证了该假设，并通过MC模拟和物理实验对该算法进行了评估。对软件人形体模的重建给出了物理实现中的系统参数，并表明所提出的方法将感兴趣中心区域重建图像的相对均方误差从74.2%降低到了1%以下。在标准评估体模上进行的初步物理实验中，该误差从31.8%降低到了2.3%，并且还证明了尽管采用基于滤波的方法，但该算法对重建图像的分辨率没有明显影响。尽管所提出的散射校正技术是针对X射线CT实现的，但只要可以在X射线源和成像物体之间插入原发射线调制器，它也可用于其他X射线成像应用。
