Rit Simon, Wolthaus Jochem W H, van Herk Marcel, Sonke Jan-Jakob
Department of Radiation Oncology, The Netherlands Cancer Institute-Antoni van Leeuwenhoek Hospital, Plesmanlaan 121, 1066 CX Amsterdam, The Netherlands.
Med Phys. 2009 Jun;36(6):2283-96. doi: 10.1118/1.3115691.
Respiratory motion causes artifacts in cone-beam (CB) CT images acquired on slow rotating scanners integrated with linear accelerators. Respiration-correlated CBCT has been proposed to correct for the respiratory motion but only a subset of the CB projections is used to reconstruct each frame of the 4D CBCT image and, therefore, adequate image quality requires long acquisition times. In this article, the authors develop an on-the-fly solution to estimate and compensate for the respiratory motion in the reconstruction of a 3D CBCT image from all the CB projections. An a priori motion model of the patient respiratory cycle is estimated from the 4D planning CT. During the acquisition, the model is correlated with the respiration using a respiratory signal extracted from the CB projections. The estimated motion is next compensated for in an optimized reconstruction algorithm. The motion compensated for is forced to be null on average over the acquisition time to ensure that the compensation results in a CBCT image which describes the mean position of each organ, even if the a priori motion model is inaccurate. Results were assessed on simulated, phantom, and patient data. In all experiments, blur was visually reduced by motion-compensated CBCT. Simulations showed robustness to inaccuracies of the motion model observed on patient data such as amplitude variations, phase shifts, and setup errors, thus proving the efficiency of the compensation using an a priori motion model. Noise and view-aliasing artifacts were lower on motion-compensated CBCT images with 1 min scan than on respiration-correlated CBCT images with 4 min scan. Finally, on-the-fly motion estimation and motion-compensated reconstruction were within the acquisition time of the CB projections and the CBCT image available a few seconds after the end of the acquisition. In conclusion, the authors developed and implemented a method for correcting the respiratory motion during the treatment fractions which can replace respiration-correlated CBCT for improving image quality while decreasing acquisition time.
呼吸运动会在与直线加速器集成的慢速旋转扫描仪上采集的锥束（CB）CT图像中产生伪影。已经提出了呼吸相关CBCT来校正呼吸运动，但仅使用CB投影的一个子集来重建4D CBCT图像的每一帧，因此，要获得足够的图像质量需要较长的采集时间。在本文中，作者开发了一种实时解决方案，用于在从所有CB投影重建3D CBCT图像时估计和补偿呼吸运动。从4D计划CT估计患者呼吸周期的先验运动模型。在采集过程中，使用从CB投影中提取的呼吸信号将该模型与呼吸相关联。接下来，在优化的重建算法中对估计的运动进行补偿。在采集时间内，补偿后的运动被强制平均为零，以确保即使先验运动模型不准确，补偿也能产生描述每个器官平均位置的CBCT图像。在模拟、体模和患者数据上评估了结果。在所有实验中，运动补偿CBCT在视觉上减少了模糊。模拟显示对在患者数据上观察到的运动模型不准确具有鲁棒性，如幅度变化、相位偏移和设置误差，从而证明了使用先验运动模型进行补偿的效率。与4分钟扫描的呼吸相关CBCT图像相比，1分钟扫描的运动补偿CBCT图像上的噪声和视图混叠伪影更低。最后，实时运动估计和运动补偿重建在CB投影的采集时间内完成，并且在采集结束后几秒钟即可获得CBCT图像。总之，作者开发并实施了一种在治疗分次期间校正呼吸运动的方法，该方法可以替代呼吸相关CBCT来提高图像质量，同时减少采集时间。
