Metal Artifact Reduction (MAR) in Image-Guided Radiation Therapy (IGRT)

X射线计算机断层扫描（CT）中的图像质量经常受到由金属植入物引起的伪影的影响。例如，这些伪影可能是由于假体、由于牙齿填充物、由于固定物、夹子或标记物。金属伪影可能会损害诊断，或者可能会损害图像的定量评估，例如放射治疗计划所需的图像。金属伪影的类型和强度取决于金属植入物的类型、大小、密度、位置和数量，以及CT扫描的扫描参数。其他因素，如预期或非预期的患者运动也起着重要作用。这尤其适用于图像引导放射治疗（IGRT），其中使用平面探测器锥束CT技术在自由呼吸下采集患者数据，该技术比诊断CT系统更容易产生伪影。目前的方法不能减少或消除这种移动的金属伪影，因为它们出现在IGRT中。其中一些方法用一些替代信息代替穿过金属对象的射线的投影值。另一些则不是基于这样的修复技术，而是使用金属阴影中的信息和测量过程的物理模型来得出校正的原始数据。在大多数情况下，MAR专用于临床（诊断）CT。较少的工作已经发表专门的平面探测器CT系统，虽然这些是更敏感的散射辐射，因此，金属伪影。除了一种手动方法之外，还不知道专门用于科普患者运动的图像引导放射治疗的方法，尽管IGRT必须满足特殊要求，以便能够使治疗计划适应当前患者情况。一方面，图像质量应该高，以便能够可靠地描绘肿瘤边缘。在前列腺的情况下，例如，周围的髋关节假体可能产生金属伪影，其延伸到目标体积中并且可能阻止治疗计划的准确调整。在其他情况下，肿瘤用也产生金属伪影的金属标记物标记。另一方面，患者自由呼吸是这些扫描的典型特征。然而，患者运动会改变或放大金属伪影并使金属物体模糊。这种模糊，然而，可能会阻止一个可靠的分割植入物，但这种分割是一个先决条件，金属artifact reduction.This project aims at developing a dedicated MAR algorithm for imaging guided radiation therapy.一方面，我们要考虑平面探测器的特殊属性，另一方面，我们要开发一种专用的运动补偿方法，使MAR也能应用于移动金属的情况。应使用模拟、体模测量和患者测量证明可行性。

Image quality in x-ray computed tomography (CT) often suffers from artifacts arising from metal implants. For example these artifacts can be due to prostheses, due to dental fillings, due to fixations, clips or markers. The metal artifacts may impair diagnosis or they may impair quantitative evaluations of the images such as it is required for radiation treatment planning. The type and intensity of the metal artifacts depends on the type, size, density, position and number of the metal implants, and on the scan parameters of the CT scan. Additional factors such as intended or unintended patient motion play a significant role, too. This particularly applies for image-guided radiation therapy (IGRT) where patient data are acquired under free breathing using flat detector cone-beam CT technology which is much more prone to artifacts than diagnostic CT systems. Current methods are not able to reduce or remove such moving metal artifacts, as they arise in IGRT.Several methods for metal artifact reduction (MAR) are described in the literature. Some of them replace the projection values of rays that run through a metal object by some surrogate information. Others are not based on such inpainting techniques but rather use the information in the metal shadow and a physical model of the measurement process to come up with corrected rawdata. In most cases MAR is dedicated to clinical (diagnostic) CT. Less work has been published dedicated to flat detector CT systems, although those are much more sensitive to scattered radiation and therefore to metal artifacts. Approaches dedicated to image-guided radiation therapy that cope with patient motion are not known, apart from one manual method, although IGRT has to fulfill special requirements to be able to adapt the treatment plan to the current patient situation. On the one hand image quality should be high to be able to reliably delineate the tumor margins. In case of the prostate, for example, surrounding hip prostheses may generate metal artifacts that extend into the target volume and that potentially prevent an accurate adaptation of the treatment plan. In other cases tumors are marked with metal markers that also generate metal artifacts. On the other hand free patient breathing is typical for those scans. Patient motion, however, changes or amplifies the metal artifacts and blurs the metal object. This blurring, however, may prevent a reliable segmentation of the implants but this segmentation is a prerequisite for metal artifact reduction.This project aims at developing a dedicated MAR algorithm for image-guided radiation therapy. We want to consider the particular properties of flat detectors on the one hand, and want to develop a dedicated motion compensation approach that allows to apply MAR also to the case of moving metal. The feasibility shall be demonstrated using simulations, phantom measurements, and patient measurements.