Department of Radiation Oncology, University of Texas Southwestern Medical Center, Dallas, TX 75235, USA.
Institute of Image Processing and Pattern Recognition, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China.
Med Phys. 2017 Sep;44(9):e215-e229. doi: 10.1002/mp.12326.
X-ray scatter is a significant barrier to image quality improvements in cone-beam computed tomography (CBCT). A moving blocker-based strategy was previously proposed to simultaneously estimate scatter and reconstruct the complete volume within the field of view (FOV) from a single CBCT scan. A blocker consisting of lead stripes is inserted between the X-ray source and the imaging object, and moves back and forth along the rotation axis during gantry rotation. While promising results were obtained in our previous studies, the geometric design and moving speed of the blocker were set empirically. The goal of this work is to optimize the geometry and speed of the moving block system.
Performance of the blocker was examined through Monte Carlo (MC) simulation and experimental studies with various geometry designs and moving speeds. All hypothetical designs employed an anthropomorphic pelvic phantom. The scatter estimation accuracy was quantified by using lead stripes ranging from 5 to 100 pixels on the detector plane. An iterative reconstruction based on total variation minimization was used to reconstruct CBCT images from unblocked projection data after scatter correction. The reconstructed image was evaluated under various combinations of lead strip width and interspace (ranging from 10 to 60 pixels) and different moving speed (ranging from 1 to 30 pixels per projection).
MC simulation showed that the scatter estimation error varied from 0.8% to 5.8%. Phantom experiment showed that CT number error in the reconstructed CBCT images varied from 13 to 35. Highest reconstruction accuracy was achieved when the strip width was 20 pixels and interspace was 60 pixels and the moving speed was 15 pixels per projection.
Scatter estimation can be achieved in a large range of lead strip width and interspace combinations. The moving speed does not have a very strong effect on reconstruction result if it is above 5 pixels per projection. Geometry design of the blocker affected image reconstruction accuracy more. The optimal geometry of the blocker has a strip width of 20 pixels and an interspace three times the strip width, which means 25% detector is covered by the blocker, while the optimal moving speed is 15 pixels per projection.
X 射线散射是锥形束计算机断层扫描（CBCT）图像质量提高的一个重大障碍。之前提出了一种基于移动阻挡器的策略，旨在从单次 CBCT 扫描中同时估计散射并重建整个视场（FOV）内的体积。阻挡器由铅条组成，插入在 X 射线源和成像物体之间，并在旋转架旋转期间沿旋转轴前后移动。虽然在之前的研究中获得了有希望的结果，但阻挡器的几何设计和移动速度是经验性设置的。这项工作的目标是优化移动阻挡系统的几何形状和速度。
通过蒙特卡罗（MC）模拟和具有不同几何设计和移动速度的实验研究来检查阻挡器的性能。所有假设的设计都采用了人体骨盆模型。通过在探测器平面上使用 5 到 100 个像素的铅条来量化散射估计的准确性。使用基于全变差最小化的迭代重建来从散射校正后的未阻挡投影数据重建 CBCT 图像。在不同的铅条宽度和间隔（10 到 60 像素）以及不同的移动速度（1 到 30 像素/投影）组合下，对重建图像进行了评估。
MC 模拟表明，散射估计误差在 0.8%到 5.8%之间。体模实验表明，重建 CBCT 图像中的 CT 数误差在 13 到 35 之间。当条带宽度为 20 像素且间隔为 60 像素，移动速度为 15 像素/投影时，实现了最高的重建精度。
在较大的铅条宽度和间隔组合范围内可以实现散射估计。如果移动速度超过 5 像素/投影，则对重建结果的影响不是很强。阻挡器的几何设计对图像重建精度的影响更大。阻挡器的最佳几何形状具有 20 像素的条带宽度和条带宽度三倍的间隔，这意味着 25%的探测器被阻挡器覆盖，而最佳的移动速度为 15 像素/投影。
