Institute of Medical Physics, University of Erlangen-Nürnberg, 91052 Erlangen, Germany.
Med Phys. 2013 Sep;40(9):091915. doi: 10.1118/1.4818061.
To avoid motion artifacts in medical imaging or to minimize the exposure of healthy tissues in radiation therapy, medical devices are often synchronized with the patient's respiratory motion. Today's respiratory motion monitors require additional effort to prepare the patients, e.g., mounting a motion belt or placing an optical reflector on the patient's breast. Furthermore, they are not able to measure internal organ motion without implanting markers. An interesting alternative to assess the patient's organ motion is continuous wave radar. The aim of this work is to design, implement, and evaluate such a radar system focusing on application in CT.
The authors designed a radar system operating in the 860 MHz band to monitor the patient motion. In the intended application of the radar system, the antennas are located close to the patient's body inside the table of a CT system. One receive and four transmitting antennas are used to avoid the requirement of exact patient positioning. The radar waves propagate into the patient's body and are reflected at tissue boundaries, for example at the borderline between muscle and adipose tissue, or at the boundaries of organs. At present, the authors focus on the detection of respiratory motion. The radar system consists of the hardware mentioned above as well as of dedicated signal processing software to extract the desired information from the radar signal. The system was evaluated using simulations and measurements. To simulate the radar system, a simulation model based on radar and wave field equations was designed and 4D respiratory-gated CT data sets were used as input. The simulated radar signals and the measured data were processed in the same way. The radar system hardware and the signal processing algorithms were tested with data from ten volunteers. As a reference, the respiratory motion signal was recorded using a breast belt simultaneously with the radar measurements.
Concerning the measurements of the test persons, there is a very good correlation (ρ = 0.917) between the respiratory motion phases received by the radar system and the external motion monitor. Our concept of using an array of transmitting antennas turned out to be widely insensitive to the positioning of the test persons. A time shift between the respiratory motion curves recorded with the radar system and the motion curves from the external respiratory monitor was observed which indicates a slight difference between internal organ motion and motion detected by the external respiratory monitor. The simulations were in good accordance with the measurements.
A continuous wave radar operating in the near field of the antennas can be used to determine the respiratory motion of humans accurately. In contrast to trigger systems used today, the radar system is able to measure motion inside the body. If such a monitor was routinely available in clinical CT, it would be possible optimizing the scan start with respect to the respiratory state of the patient. Breathing commands would potentially widely be avoided, and as far as uncooperative patients or children are concerned, less sedation might be necessary. Further applications of the radar system could be in radiation therapy or interventional imaging for instance.
为了避免医学成像中的运动伪影或尽量减少放射治疗中健康组织的辐射暴露，医疗设备通常与患者的呼吸运动同步。如今的呼吸运动监测器需要额外的努力来准备患者，例如，安装运动带或在患者的乳房上放置光学反射器。此外，如果不植入标记物，它们就无法测量内部器官运动。评估患者器官运动的一个有趣的替代方法是连续波雷达。这项工作的目的是设计、实现和评估这种雷达系统，重点是在 CT 中的应用。
作者设计了一种在 860 MHz 频段运行的雷达系统，用于监测患者的运动。在雷达系统的预期应用中，天线位于 CT 系统工作台内靠近患者身体的位置。一个接收天线和四个发射天线用于避免对患者位置的精确要求。雷达波在患者体内传播，在组织边界处反射，例如在肌肉和脂肪组织之间的边界处，或者在器官的边界处。目前，作者专注于检测呼吸运动。该雷达系统由上述硬件以及专用的信号处理软件组成，用于从雷达信号中提取所需信息。该系统使用模拟和测量进行了评估。为了模拟雷达系统，设计了一个基于雷达和波场方程的仿真模型，并使用 4D 呼吸门控 CT 数据集作为输入。模拟雷达信号和测量数据以相同的方式进行处理。雷达系统硬件和信号处理算法已使用来自十个志愿者的数据进行了测试。作为参考，使用乳房带同时与雷达测量一起记录呼吸运动信号。
就测试人员的测量而言，雷达系统接收的呼吸运动相位与外部运动监测器之间存在非常好的相关性（ρ=0.917）。我们使用发射天线阵列的概念被证明对测试人员的定位具有广泛的不敏感性。在使用雷达系统记录的呼吸运动曲线和来自外部呼吸监测器的运动曲线之间观察到时间偏移，这表明内部器官运动和外部呼吸监测器检测到的运动之间存在轻微差异。模拟与测量结果非常吻合。
可以使用工作在天线近场的连续波雷达准确地确定人体的呼吸运动。与当今使用的触发系统相比，雷达系统能够测量体内运动。如果这种监视器在临床 CT 中常规使用，则可以根据患者的呼吸状态优化扫描开始。可能会广泛避免呼吸指令，并且对于不合规的患者或儿童，可能需要更少的镇静剂。该雷达系统的其他应用可能在放射治疗或介入成像等领域。
