St Aubin J, Steciw S, Carlone M, Fallone B G
University of Alberta, Edmonton, AB.
Cross Cancer Institute, Edmonton, AB.
Med Phys. 2008 Jul;35(7Part3):3411. doi: 10.1118/1.2965971.
The coupling of a 0.2T bi-planar Magnetic Resonance Imager and medical linear accelerator (linac) is proposed to provide real-time Image Guided Radiotherapy. This coupling necessitates the linac to be within the fringe fields of the bi-planar magnets causing magnetic interference. The design and optimization of the minimum required shielding is necessary to reduce the fringe field magnitudes to a point where a clinically useful radiation beam is produced.
A first step to designing shielding is the full 3D radio-frequency modeling of the linac waveguide using the Finite Element Method. Various optimizations were performed on the linac model in order to achieve a desired resonant frequency, π/2 phase shift per cavity and other desired properties. An accelerating cavity (AC) and coupling cavity (CC) was first optimized in 3D to have identical resonant frequencies before the full 3D model was generated.
In order to increase the capture efficiency of the injected electrons, the electric field in the first AC was reduced by shifting the first CC towards the gun end of the linac. The input waveguide AC dimensions were adjusted to account of the additional coupling iris and the last full AC had its gap length decreased.
This work is the first step to determining the minimum magnetic shielding required to produce a clinically useful radiation beam from a coupled MR-Linac system. The fully optimized 3D model more accurately calculates the electric and magnetic field values since it includes the effects of coupling.
提出将0.2T双平面磁共振成像仪与医用直线加速器（直线加速器）耦合，以提供实时图像引导放射治疗。这种耦合要求直线加速器处于双平面磁体的边缘场范围内，从而产生磁干扰。设计和优化所需的最小屏蔽，以将边缘场强度降低到能产生临床可用辐射束的程度是必要的。
设计屏蔽的第一步是使用有限元法对直线加速器波导进行完整的三维射频建模。对直线加速器模型进行了各种优化，以实现所需的谐振频率、每个腔的π/2相移和其他所需特性。在生成完整的三维模型之前，首先对加速腔（AC）和耦合腔（CC）进行三维优化，使其具有相同的谐振频率。
为了提高注入电子的捕获效率，通过将第一个CC向直线加速器的枪端移动来降低第一个AC中的电场。调整输入波导AC的尺寸以考虑额外的耦合虹膜，最后一个完整的AC的间隙长度减小。
这项工作是确定从耦合的磁共振直线加速器系统产生临床可用辐射束所需的最小磁屏蔽的第一步。完全优化的三维模型更准确地计算电场和磁场值，因为它包括了耦合效应。
