Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire, USA.
Department of Radiation Oncology, Moffitt Cancer Center, Tampa, Florida, USA.
J Appl Clin Med Phys. 2021 Sep;22(9):252-261. doi: 10.1002/acm2.13377. Epub 2021 Aug 19.
Demonstrate a novel phantom design using a remote camera imaging method capable of concurrently measuring the position of the x-ray isocenter and the magnetic resonance imaging (MRI) isocenter on an MR-linac.
A conical frustum with distinct geometric features was machined out of plastic. The phantom was submerged in a small water tank, and aligned using room lasers on a MRIdian MR-linac (ViewRay Inc., Cleveland, OH). The phantom physical isocenter was visualized in the MR images and related to the DICOM coordinate isocenter. To view the x-ray isocenter, an intensified CMOS camera system (DoseOptics LLC., Hanover, NH) was placed at the foot of the treatment couch, and centered such that the optical axis of the camera was coincident with the central axis of the treatment bore. Two or four 8.3mm x 24.1cm beams irradiated the phantom from cardinal directions, producing an optical ring on the conical surface of the phantom. The diameter of the ring, measured at the peak intensity, was compared to the known diameter at the position of irradiation to determine the Z-direction offset of the beam. A star-shot method was employed on the front face of the frustum to determine X-Y alignment of the MV beam. Known shifts were applied to the phantom to establish the sensitivity of the method.
Couch translations, demonstrative of possible isocenter misalignments, on the order of 1mm were detectable for both the radiotherapy and MRI isocenters. Data acquired on the MR-linac demonstrated an average error of 0.28mm(N=10, R =0.997, σ=0.37mm) in established Z displacement, and 0.10mm(N=5, σ=0.34mm) in XY directions of the radiotherapy isocenter.
The phantom was capable of measuring both the MRI and radiotherapy treatment isocenters. This method has the potential to be of use in MR-linac commissioning, and could be streamlined to be valuable in daily constancy checks of isocenter coincidence.
展示一种使用远程相机成像方法的新型体模设计，该方法能够同时测量磁共振直线加速器上 X 射线等中心和磁共振成像（MRI）等中心的位置。
用塑料加工出一个具有明显几何特征的截顶圆锥体。将体模浸入小水箱中，使用 MRIdian MR 直线加速器（ViewRay Inc.，克利夫兰，俄亥俄州）上的房间激光进行对准。在 MR 图像中观察到体模的物理等中心，并将其与 DICOM 坐标等中心相关联。为了观察 X 射线等中心，将一个增强型 CMOS 相机系统（DoseOptics LLC.，汉诺威，新罕布什尔州）放置在治疗床脚，并使其居中，使得相机的光轴与治疗孔的中心轴重合。从四个方位用两个或四个 8.3mm x 24.1cm 的射束照射体模，在体模的圆锥形表面上产生一个光学环。在峰值强度处测量环的直径，并与照射位置的已知直径进行比较，以确定光束的 Z 方向偏移。在圆锥体的前表面采用星点法来确定 MV 光束的 X-Y 对准。对体模施加已知的偏移量，以确定该方法的灵敏度。
对于放射治疗和 MRI 等中心，都可以检测到约 1mm 的治疗床平移，这表明等中心可能存在错位。在 MR 直线加速器上获取的数据显示，在已建立的 Z 位移方面，平均误差为 0.28mm（N=10，R=0.997，σ=0.37mm），在放射治疗等中心的 XY 方向上的平均误差为 0.10mm（N=5，σ=0.34mm）。
该体模能够测量 MRI 和放射治疗等中心的位置。该方法有望在 MR 直线加速器的调试中得到应用，并可简化为在等中心重合的日常稳定性检查中具有价值。
