Sonke Jan-Jakob, Zijp Lambert, Remeijer Peter, van Herk Marcel
Department of Radiation Oncology, The Netherlands Cancer Institute--Antoni van Leeuwenhoek Hospital, Plesmanlaan 121, 1066 CX Amsterdam, The Netherlands.
Med Phys. 2005 Apr;32(4):1176-86. doi: 10.1118/1.1869074.
A cone beam computed tomography (CBCT) scanner integrated with a linear accelerator is a powerful tool for image guided radiotherapy. Respiratory motion, however, induces artifacts in CBCT, while the respiratory correlated procedures, developed to reduce motion artifacts in axial and helical CT are not suitable for such CBCT scanners. We have developed an alternative respiratory correlated procedure for CBCT and evaluated its performance. This respiratory correlated CBCT procedure consists of retrospective sorting in projection space, yielding subsets of projections that each corresponds to a certain breathing phase. Subsequently, these subsets are reconstructed into a four-dimensional (4D) CBCT dataset. The breathing signal, required for respiratory correlation, was directly extracted from the 2D projection data, removing the need for an additional respiratory monitor system. Due to the reduced number of projections per phase, the contrast-to-noise ratio in a 4D scan reduced by a factor 2.6-3.7 compared to a 3D scan based on all projections. Projection data of a spherical phantom moving with a 3 and 5 s period with and without simulated breathing irregularities were acquired and reconstructed into 3D and 4D CBCT datasets. The positional deviations of the phantoms center of gravity between 4D CBCT and fluoroscopy were small: 0.13 +/- 0.09 mm for the regular motion and 0.39 +/- 0.24 mm for the irregular motion. Motion artifacts, clearly present in the 3D CBCT datasets, were substantially reduced in the 4D datasets, even in the presence of breathing irregularities, such that the shape of the moving structures could be identified more accurately. Moreover, the 4D CBCT dataset provided information on the 3D trajectory of the moving structures, absent in the 3D data. Considerable breathing irregularities, however, substantially reduces the image quality. Data presented for three different lung cancer patients were in line with the results obtained from the phantom study. In conclusion, we have successfully implemented a respiratory correlated CBCT procedure yielding a 4D dataset. With respiratory correlated CBCT on a linear accelerator, the mean position, trajectory, and shape of a moving tumor can be verified just prior to treatment. Such verification reduces respiration induced geometrical uncertainties, enabling safe delivery of 4D radiotherapy such as gated radiotherapy with small margins.
与直线加速器集成的锥形束计算机断层扫描（CBCT）扫描仪是图像引导放射治疗的有力工具。然而，呼吸运动会在CBCT中产生伪影，而用于减少轴向和螺旋CT中运动伪影的呼吸相关程序并不适用于此类CBCT扫描仪。我们开发了一种用于CBCT的替代呼吸相关程序并评估了其性能。这种呼吸相关CBCT程序包括在投影空间中进行回顾性排序，生成每个对应于特定呼吸阶段的投影子集。随后，将这些子集重建为四维（4D）CBCT数据集。呼吸相关所需的呼吸信号直接从二维投影数据中提取，无需额外的呼吸监测系统。由于每个阶段的投影数量减少，与基于所有投影的三维扫描相比，四维扫描中的对比度噪声比降低了2.6至3.7倍。获取了具有3秒和5秒周期且有和没有模拟呼吸不规则性的球形体模的投影数据，并将其重建为三维和四维CBCT数据集。四维CBCT与荧光透视之间体模重心的位置偏差很小：规则运动时为0.13±0.09毫米，不规则运动时为0.39±0.24毫米。在三维CBCT数据集中明显存在的运动伪影在四维数据集中大幅减少，即使存在呼吸不规则性，也能更准确地识别移动结构的形状。此外，四维CBCT数据集提供了三维数据中不存在的移动结构三维轨迹的信息。然而，相当大的呼吸不规则性会大幅降低图像质量。为三名不同肺癌患者提供的数据与体模研究结果一致。总之，我们成功实施了一种呼吸相关CBCT程序，生成了四维数据集。通过直线加速器上的呼吸相关CBCT，可以在治疗前验证移动肿瘤的平均位置、轨迹和形状。这种验证减少了呼吸引起的几何不确定性，能够安全地进行四维放射治疗，如小边缘的门控放射治疗。
