使用单次扫描和固定阻挡器的锥束CT散射修正方法研究

Applications of cone-beam CT (CBCT) 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 and image contrast loss. 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 proposal, we propose a new measurement-based scatter correction algorithm without primary compensation for full-fan CBCT. 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 will be evaluated using two phantom studies on a tabletop CBCT system and clinical on-board imaging system.On the phantom studies, our approach is expected to reduce the reconstruction error to be around 5 HU in the selected region of interest, and improve the image contrast by a factor of 2.0 in the high-contrast regions. This approach will be further evaluated in patient studies after the thorough evaluation and optimization using phantom experiments. 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 large-volumn CT systems.

基于大面积探测器技术的锥束CT广泛应用于放射治疗领域，但其应用受到大体积辐照中散射污染导致的严重杯状与条纹伪影困扰。基于测量的散射修正技术将x射线阻挡器放置在x射线源与物体之间，获取散射抽样数据，具有简单有效且无需预知被测物体信息的优点。然而为了补偿散射抽样过程中源信号的损失，需要扫描多次或在扫描时移动阻挡器，限制了该方法的广泛应用。本项目基于CT重建理论，提出了全新的无需源补偿的散射修正方法：采用新型"叉指"形阻挡器，阻挡区分布于投影域中源信号冗余处，散射分布通过插值估计，修正后的CT图像用基于滤波反投影算法精确重建。新方法通过固定的"叉指"形阻挡器，仅用单次扫描得到的数据，简洁有效的估计散射并获得精确的CT图像。本项目减少了高精度锥束CT扫描时间和剂量，不仅可以提供更精确的病患解剖结构信息用于肿瘤诊断，还能为新一代自适应放射治疗提供定量计算基础，进一步提升放射治疗的效果。

X射线散射光子严重影响锥束CT的图像质量，本项目提出的“指交叉”阻挡器根据散射信号的空间低频分布特性，对散射信号进行空间采样，进而通过插值估计出散射分布；在布置阻挡铅条时充分考虑CT重建过程的数据完整性，单次扫描可得到高质量的锥束CT图像。本项目取得以下研究成果：（1）安装“指交叉”型阻挡器在瓦里Trilogy机载锥束CT千伏级射线源前，借助图像分割方法建立阻挡器投影因悬臂等中心偏移与机架震动引起的摆动数学模型，成功将阻挡器应用于临床锥束CT散射修正；（2）优化射束阻挡器几何结构，对阻挡器在图像域造成的数据缺失进行数学优化建模，定量评估阻挡器对重建图像的影响，借助网格自适应直接搜索法求解阻挡器的几何最优结构，为基于阻挡器测量的散射修正方法建立了坚实的理论基础，进一步揭示阻挡器设计对临床锥束CT散射修正的重要性；（3）在进行了细致的模体评估后，课题组经浙江大学附属邵逸夫医院伦理委员会批准，将该项技术应用于临床患者，进一步发掘了阻挡器的临床实用价值；（4）设计强度修正的快速对称Demons算法对治疗前后的CT图像配准，以治疗前后肿瘤收缩程度测量为例，验证方法的临床意义；（5）为了提高该算法的临床应用价值，实时快速地得出重建结果，本项目开发了基于高性能图形工作站的医学图像移动处理算法和软件；（6）为了进一步提升图像质量，课题组设计了一种图像域阴影伪影迭代抑制算法，对残留伪影进行消除。课题组在临床放疗机载的锥束CT系统上，通过Catphan©500模体对提出的方法进行定量评价，在选择的感兴趣区域中，CT值误差由115 HU降到了6 HU，对比度平均提高了1.45倍。在临床病人头部选择相近组织的感兴趣区域，重建CT值误差由124 HU降到了8 HU，图像非均匀度有12%降到了7%。课题组实现了采用“指交叉”型阻挡器在阻挡区域采集散射抽样，在未阻挡区域通过改进的滤波反投影算法完成单次扫描的图像精确重建并实现其临床应用。

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