Scatter Correction for Cone-Beam CT to Improve Radiation Therapy Treatments

描述（由申请人提供）：图像引导放射治疗（IGRT）提供准确的肿瘤放射传递，通常用于各种癌症部位。过去10-15年见证了图像引导技术的爆炸式发展。机载x射线成像从二维发展到三维，锥束计算机断层扫描（CBCT）越来越多地应用于放射治疗机。然而，目前CBCT的使用受到了很大的阻碍，主要是由于分散的伪影造成的图像质量差，并且临床应用仅限于治疗设置。具有高HU精度的定量CBCT图像对于剂量验证和适应性放射治疗尤其重要，但仍然无法实现。x射线散射污染随辐照体积的增大而增大。锥束投影中的大散射信号导致CT图像对比度降低和严重的阴影/条纹伪影，这被认为是CBCT图像质量的根本限制。散射校正是近二十年来一个非常活跃的研究领域。由于人体成像的散射比（SPR）大，成像对象具有高度的非均匀性，这是一个具有挑战性的问题。标准的解决方案仍不明确。为了促进CBCT在放射治疗中的应用，我们根据放射治疗的独特特点，提出了一种新的散射校正方法。诊断型多探测器CT （MDCT）固有的散射信号小得多，可以获得准确的CT图像，通常用于治疗计划。利用MDCT图像作为患者的“自由”先验信息，我们使用简单有效的图像处理技术估计和校正CBCT投影中的散点。我们的方法不同于其他CBCT校准方法，这些方法完全依赖于MDCT图像来提供患者的解剖细节。我们从MDCT图像中使用有限的患者信息，因此保留了基于cbct的治疗监测的优点。我们在一个特性良好的桌面系统上的初步幻影研究表明，所提出的散射校正将CT数误差从~350 HU降低到~5 HU，并大大提高了低对比度物体的可见度。本研究项目的总体目标是在桌面系统上扩展和优化我们的方法，并证明我们可以在临床CBCT系统上对患者实现20 HU的CT数精度，并使用改进的CBCT进行准确的肿瘤描绘和剂量计算。我们的方法有可能成为一项颠覆性的技术，为适应性放射治疗的高性能IGRT和精确剂量监测消除障碍。

DESCRIPTION (provided by applicant): Image-guided radiation therapy (IGRT) provides accurate radiation delivery to the tumor and is being commonly used for various cancer sites. The last 10-15 years have witnessed an explosion of techniques for image guidance. On-board x-ray imaging evolves from two-dimensional to three-dimensional, and cone-beam computed tomography (CBCT) is being increasingly implemented on radiation therapy machines. Nonetheless, the current use of CBCT is greatly hindered by its poor image quality mainly due to scatter artifacts, and the clinical applications are limited to treatment setup. Quantitative CBCT images with high HU accuracy, which are particularly important for dose verification and adaptive radiation therapy, are still not achievable. The x-ray scatter contamination increases as the size of the irradiated volume increases. The large scatter signals in cone-beam projections lead to a contrast reduction and severe shading/streak artifacts in the CT images, which is considered the fundamental limitation of CBCT image quality. Scatter correction has been a very active research area in the last twenty years. The problem is challenging in human imaging since the scatter- to-primary ratio (SPR) is large and the imaged object has high heterogeneity. A standard solution still remains unclear. To facilitate the applications of CBCT in radiation therapy, we propose a new scatter correction method based on a unique feature of radiation therapy. With much smaller inherent scatter signals, diagnostic multi-detector CT (MDCT) obtains accurate CT images and is routinely used for treatment planning. Using the MDCT images as the "free" prior information of the patient, we estimate and correct for scatter in CBCT projections using simple and efficient image processing techniques. Our approach is distinct from other CBCT calibration methods, which totally rely on the MDCT image for providing the patient anatomic details. We use limited patient information from the MDCT image, and the merit of CBCT-based treatment monitoring is therefore retained. Our preliminary phantom study on a well-characterized tabletop system has shown that the proposed scatter correction reduces the CT number error from ~350 HU to ~5 HU and substantially increases the visibility of low-contrast objects. The overall goal of this research program is to extend and optimize our approach on the tabletop system, and to demonstrate that we can achieve a CT number accuracy of 20 HU on patients on a clinical CBCT system and use the improved CBCT for accurate tumor delineation and dose calculation. Our method has a potential to be a disruptive technology that removes hurdles toward high- performance IGRT and accurate dose monitoring for adaptive radiation therapy. PUBLIC HEALTH RELEVANCE: The project aims to provide a correction solution to one of the fundamental physical processes, x-ray scatter, which limits the quality of cone-beam Computed Tomography (CBCT) imaging and its applications of image guidance in the current radiation therapy. If our method proves to be effective, it can remove the shading and streak artifacts in a CBCT image and make small and low-contrast tumors more visible. This project is very significant in that it could lead to a breakthrough toward accurate monitoring of delivered radiation dose on the tumor, which makes implementations of many new radiation therapy techniques possible.