Department of Radiation Oncology, University of California San Diego, La Jolla, California 92093, USA.
Med Phys. 2010 Jun;37(6):2855-61. doi: 10.1118/1.3432615.
Four-dimensional computed tomography (4DCT) has enhanced images of the thorax and upper abdomen during respiration, but intraphase residual motion artifacts will persist in cine-mode scanning. In this study, the source and magnitude of projection artifacts due to intraphase target motion is investigated.
A theoretical model of geometric uncertainty due to partial projection artifacts in cine-mode 4DCT was derived based on ideal periodic motion. Predicted artifacts were compared to measured errors with a rigid lung phantom attached to a programmable motion platform. Ideal periodic motion and actual patient breathing patterns were used as input for phantom motion. Reconstructed target dimensions were measured along the direction of motion and compared to the actual, known dimensions.
Artifacts due to intraphase residual motion in cine-mode 4DCT range from a few mm up to a few cm on a given scanner, and can be predicted based on target motion and CT gantry rotation time. Errors in ITV and GTV dimensions were accurately characterized by the theoretical uncertainty at all phases when sinusoidal motion was considered, and in 96% of 300 measurements when patient breathing patterns were used as motion input. When peak-to-peak motion of 1.5 cm is combined with a breathing period of 4 s and gantry rotation time of 1 s, errors due to partial projection artifacts can be greater than 1 cm near midventilation and are a few mm in the inhale and exhale phases. Incorporation of such uncertainty into margin design should be considered in addition to other uncertainties.
Artifacts due to intraphase residual motion exist in 4DCT, even for ideal breathing motions (e.g., sine waves). It was determined that these motion artifacts depend on patient-specific tumor motion and CT gantry rotation speed. Thus, if the patient-specific motion parameters are known (i.e., amplitude and period), a patient-specific margin can and should be designed to compensate for this uncertainty.
四维计算机断层扫描（4DCT）增强了呼吸过程中胸部和上腹部的图像，但在电影模式扫描中仍会存在同相位残留运动伪影。本研究旨在探讨同相位目标运动导致投影伪影的来源和幅度。
基于理想周期性运动，推导出电影模式 4DCT 中部分投影伪影引起的几何不确定性的理论模型。使用刚性肺体模连接到可编程运动平台，将预测的伪影与测量误差进行比较。理想周期性运动和实际患者呼吸模式被用作体模运动的输入。在运动方向上测量重建目标的尺寸，并将其与实际已知尺寸进行比较。
电影模式 4DCT 中的同相位残留运动引起的伪影在给定的扫描仪上可达几毫米至几厘米，并且可以根据目标运动和 CT 机架旋转时间进行预测。当考虑正弦运动时，理论不确定性可以准确地描述 ITV 和 GTV 尺寸的误差，而当使用患者呼吸模式作为运动输入时，在 300 次测量中有 96%的情况可以准确地描述误差。当峰值到峰值运动为 1.5 厘米，呼吸周期为 4 秒，机架旋转时间为 1 秒时，在中部呼吸时，由于部分投影伪影引起的误差可能大于 1 厘米，在吸气和呼气阶段则为几毫米。除了其他不确定性之外，还应考虑将这种不确定性纳入到边缘设计中。
即使对于理想的呼吸运动（例如正弦波），4DCT 中也存在同相位残留运动伪影。结果表明，这些运动伪影取决于患者特定的肿瘤运动和 CT 机架旋转速度。因此，如果患者特定的运动参数是已知的（即幅度和周期），则可以并且应该设计特定于患者的边缘来补偿这种不确定性。
