The George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA.
Med Phys. 2011 Nov;38(11):6027-38. doi: 10.1118/1.3651619.
Applications of volumetric CT (VCT) are hampered by shading and streaking artifacts in the reconstructed images. These artifacts are mainly due to strong x-ray scatter signals accompanied with the large illumination area within one projection, which lead to CT number inaccuracy, image contrast loss and spatial nonuniformity. Although different scatter correction algorithms have been proposed in literature, a standard solution still remains unclear. Measurement-based methods use a beam blocker to acquire scatter samples. These techniques have unrivaled advantages over other existing algorithms in that they are simple and efficient, and achieve high scatter estimation accuracy without prior knowledge of the imaged object. Nevertheless, primary signal loss is inevitable in the scatter measurement, and multiple scans or moving the beam blocker during data acquisition are typically employed to compensate for the missing primary data. In this paper, we propose a new measurement-based scatter correction algorithm without primary compensation for full-fan VCT. An accurate reconstruction is obtained with one single-scan and a stationary x-ray beam blocker, two seemingly incompatible features which enable simple and efficient scatter correction without increase of scan time or patient dose.
Based on the CT reconstruction theory, we distribute the blocked data over the projection area where primary signals are considered approximately redundant in a full scan, such that the CT image quality is not degraded even with primary loss. Scatter is then accurately estimated by interpolation and scatter-corrected CT images are obtained using an FDK-based reconstruction algorithm.
The proposed method is evaluated using two phantom studies on a tabletop CBCT system. On the Catphan©600 phantom, our approach reduces the reconstruction error from 207 Hounsfield unit (HU) to 9 HU in the selected region of interest, and improves the image contrast by a factor of 2.0 in the high-contrast regions. On an anthropomorphic head phantom, the reconstruction error is reduced from 97 HU to 6 HU in the soft tissue region and image spatial nonuniformity decreases from 27% to 5% after correction.
Our method inherits the main advantages of measurement-based methods while avoiding their shortcomings. It has the potential to become a practical scatter correction solution widely implementable on different VCT systems.
容积 CT（VCT）的应用受到重建图像中阴影和条纹伪影的阻碍。这些伪影主要是由于大照射野内的强 X 射线散射信号引起的，这导致 CT 数不准确、图像对比度降低和空间不均匀性。尽管文献中已经提出了不同的散射校正算法，但仍然没有标准的解决方案。基于测量的方法使用射线束阻挡器来获取散射样本。与其他现有算法相比，这些技术具有无与伦比的优势，因为它们简单高效，可以在不了解成像物体的先验知识的情况下实现高精度的散射估计。然而，在散射测量中不可避免地会有主要信号丢失，并且通常采用多次扫描或在数据采集过程中移动射线束阻挡器来补偿丢失的主要数据。在本文中，我们提出了一种新的基于测量的散射校正算法，无需对全扇区 VCT 进行主要补偿。通过单次扫描和固定的 X 射线束阻挡器获得准确的重建，这两个看似不兼容的特性使得散射校正简单高效，而不会增加扫描时间或患者剂量。
基于 CT 重建理论，我们将被阻挡的数据分布在投影区域中，在全扫描中，主要信号被认为在该区域是近似冗余的，因此即使存在主要信号丢失，图像质量也不会降低。然后通过插值准确估计散射，并使用基于 FDK 的重建算法获得散射校正后的 CT 图像。
使用在台式 CBCT 系统上进行的两项体模研究对所提出的方法进行了评估。在 Catphan©600 体模上，我们的方法将选定感兴趣区域的重建误差从 207 个亨氏单位（HU）降低到 9 HU，并将高对比度区域的图像对比度提高了 2.0 倍。在人体头部体模上，软组织区域的重建误差从 97 HU 降低到 6 HU，校正后图像空间不均匀性从 27%降低到 5%。
我们的方法继承了基于测量的方法的主要优点，同时避免了它们的缺点。它有可能成为一种实用的散射校正解决方案，可广泛应用于不同的 VCT 系统。
