Tan Jun, Li H Harold, Klein Eric, Li Hua, Parikh Parag, Yang Deshan
Department of Radiation Oncology, School of Medicine Washington University in St. Louis, St. Louis, Missouri 63110, USA.
Med Phys. 2012 Aug;39(8):4695-704. doi: 10.1118/1.4736535.
Onboard cone-beam computed tomography (CBCT) connected to radiotherapy linear accelerators suffers CT number consistency and uniformity problems in addition to limited longitudinal coverage. Such problems have prevented CBCT from being fully utilized in many quantitative applications including tumor response evaluation and daily radiation dose computation. This paper presents a feasibility study on the helical CBCT scan with exact reconstruction that could be a potential solution.
A Varian TrueBeam treatment machine was programmed in the research mode to accomplish helical scans that required synchronized gantry circular rotation and couch table linear motion. Two physical phantoms were scanned in both 360° and 720° helical trajectories. A Katsevich exact reconstruction algorithm was implemented and tested with digital phantom simulations. It was further optimized to account for mechanical instabilities of both gantry rotation and couch table motion from the physical phantom measurements. Preprocessing was employed to correct photon scattering, beam hardening, and bowtie filtration. The reconstructed images were compared to those reconstructed from the FDK approximate reconstruction algorithm using the same phantom projections. Comparisons have also been made with the clinical circular CBCT images and the diagnostic helical CT images of the same physical phantoms.
Satisfactory reconstruction results were obtained for the Katsevich algorithm in digital phantom study. Physical phantom results demonstrated that a 360° helical scan could provide up to 19 cm longitudinal coverage, which could be increased to 54 cm with a 720° helical scan. Image spatial resolution and soft tissue contrast were sufficient. The Q-value, which combined the spatial frequency response (modulation transfer function) and the image noise, was calculated, and suggested that the Katsevich algorithm was superior to the FDK algorithm.
A helical CBCT scan is useful to extend the longitudinal coverage. The Katsevich exact reconstruction algorithm could provide additional advantages in image qualities over the traditional FDK approximate algorithm. The combination of helical CBCT scan with exact reconstruction was proved feasible and would render CBCT more useful in image-guided radiation therapy.
与放射治疗直线加速器相连的机载锥形束计算机断层扫描（CBCT）除了纵向覆盖范围有限外，还存在CT值一致性和均匀性问题。这些问题阻碍了CBCT在包括肿瘤反应评估和每日放射剂量计算在内的许多定量应用中的充分利用。本文提出了一项关于螺旋CBCT扫描及精确重建的可行性研究，这可能是一种潜在的解决方案。
在研究模式下对瓦里安TrueBeam治疗机进行编程，以完成需要同步机架旋转和治疗床直线运动的螺旋扫描。在360°和720°螺旋轨迹上对两个物理体模进行扫描。实施了卡采维奇精确重建算法并用数字体模模拟进行测试。根据物理体模测量结果，对该算法进一步优化以考虑机架旋转和治疗床运动的机械不稳定性。采用预处理来校正光子散射、束硬化和蝴蝶结滤过。将重建图像与使用相同体模投影通过FDK近似重建算法重建的图像进行比较。还与相同物理体模的临床圆形CBCT图像和诊断螺旋CT图像进行了比较。
在数字体模研究中，卡采维奇算法获得了令人满意的重建结果。物理体模结果表明，360°螺旋扫描可提供高达19 cm的纵向覆盖范围，720°螺旋扫描时可增加到54 cm。图像空间分辨率和软组织对比度足够。计算了结合空间频率响应（调制传递函数）和图像噪声的Q值，结果表明卡采维奇算法优于FDK算法。
螺旋CBCT扫描有助于扩大纵向覆盖范围。与传统的FDK近似算法相比，卡采维奇精确重建算法在图像质量方面具有额外优势。螺旋CBCT扫描与精确重建相结合被证明是可行的，这将使CBCT在图像引导放射治疗中更有用。
