Department of Radiation Oncology, University of Michigan, Ann Arbor, Michigan 48109-5010, USA.
Med Phys. 2013 Aug;40(8):081711. doi: 10.1118/1.4813910.
To present and characterize a measurement technique to quantify the calibration accuracy of an electromagnetic tracking system to radiation isocenter.
This technique was developed as a quality assurance method for electromagnetic tracking systems used in a multi-institutional clinical hypofractionated prostate study. In this technique, the electromagnetic tracking system is calibrated to isocenter with the manufacturers recommended technique, using laser-based alignment. A test patient is created with a transponder at isocenter whose position is measured electromagnetically. Four portal images of the transponder are taken with collimator rotations of 45° 135°, 225°, and 315°, at each of four gantry angles (0°, 90°, 180°, 270°) using a 3×6 cm2 radiation field. In each image, the center of the copper-wrapped iron core of the transponder is determined. All measurements are made relative to this transponder position to remove gantry and imager sag effects. For each of the 16 images, the 50% collimation edges are identified and used to find a ray representing the rotational axis of each collimation edge. The 16 collimator rotation rays from four gantry angles pass through and bound the radiation isocenter volume. The center of the bounded region, relative to the transponder, is calculated and then transformed to tracking system coordinates using the transponder position, allowing the tracking system's calibration offset from radiation isocenter to be found. All image analysis and calculations are automated with inhouse software for user-independent accuracy. Three different tracking systems at two different sites were evaluated for this study.
The magnitude of the calibration offset was always less than the manufacturer's stated accuracy of 0.2 cm using their standard clinical calibration procedure, and ranged from 0.014 to 0.175 cm. On three systems in clinical use, the magnitude of the offset was found to be 0.053±0.036, 0.121±0.023, and 0.093±0.013 cm.
The method presented here provides an independent technique to verify the calibration of an electromagnetic tracking system to radiation isocenter. The calibration accuracy of the system was better than the 0.2 cm accuracy stated by the manufacturer. However, it should not be assumed to be zero, especially for stereotactic radiation therapy treatments where planning target volume margins are very small.
介绍并描述一种用于量化电磁跟踪系统到辐射等中心校准精度的测量技术。
该技术是作为多机构临床分割前列腺研究中使用的电磁跟踪系统的质量保证方法而开发的。在该技术中，使用基于激光的对准方式，按照制造商推荐的技术将电磁跟踪系统校准到等中心。创建一个带有位于等中心的转发器的测试患者，其位置通过电磁方式进行测量。在每个机架角度（0°，90°，180°，270°）的四个位置，使用 3×6 cm2 辐射野，以 45°、135°、225°和 315°的准直器旋转角拍摄四个转发器的透视图像。在每个图像中，确定转发器的铜包铁芯的中心。所有测量均相对于该转发器位置进行，以消除机架和成像仪下垂的影响。对于每个 16 个图像，识别 50%准直边缘，并使用该边缘找到代表每个准直边缘旋转轴的射线。来自四个机架角度的 16 个准直器旋转射线穿过并限定辐射等中心体积。相对于转发器计算限定区域的中心，然后使用转发器位置将其转换为跟踪系统坐标，从而找到跟踪系统相对于辐射等中心的校准偏移。使用内部软件实现所有图像分析和计算，以实现用户独立的准确性。这项研究评估了两个不同地点的三个不同的跟踪系统。
使用制造商标准临床校准程序，校准偏移量始终小于制造商规定的 0.2 cm 的精度，范围为 0.014 至 0.175 cm。在三个临床使用的系统中，偏移量的大小分别为 0.053±0.036、0.121±0.023 和 0.093±0.013 cm。
本文提出的方法提供了一种独立的技术，用于验证电磁跟踪系统到辐射等中心的校准。系统的校准精度优于制造商规定的 0.2 cm 精度。但是，特别是在计划靶区体积边界非常小的立体定向放射治疗治疗中，不应假定为零。
