Sci-Fri AM: YIS-09: The effect of magnetic interference on a coupled MR-linac system: Optimization of 3D FEM linac model.

Abstract

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.