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A Fast Magnetic Field Mapping Method for An MR-Guided Radiotherapy Environment

T Stanescu

T Stanescu1*, D Constantin2 , I Dayarian1 , D Jaffray1 , (1) Princess Margaret Cancer Centre, Toronto, ON, (2) Varian Medical Systems, Palo Alto, CA


TU-L-GePD-J(A)-2 (Tuesday, August 1, 2017) 1:15 PM - 1:45 PM Room: Joint Imaging-Therapy ePoster Lounge - A

Purpose: To develop an efficient and accurate method for the mapping of magnetic fringe fields generated by an MRI scanner placed in a complex MR-guided radiotherapy environment.

Methods: The field mapping was framed as a boundary value problem (BVP) for which experimental data was collected only on the 3D perimeter of the domain of interest to generate 2D/3D boundary conditions (BCs). The BVP was defined as an inner Dirichlet problem with shape functions as BCs and was solved using a finite element method in COMSOL Multiphysics. Sparse data was collected with a Hall magnetic field sensor at pre-defined locations on 2D perimeters of quadrilateral faces bounding cuboidal domains. Additional profiles were measured inside the mapped domains for validation purposes. The 2D BCs were generated from 1D polynomial fitting functions applied to the experimental data. The Dirichlet problem was solved first in 2D for each individual face of the cuboidal space. The 2D solutions were then used as 2D BCs for the 3D Dirichlet problem solved for the entire space. Validation of the numerical simulations was performed for both the 2D and 3D solutions using measured data.

Results: The numerical simulations were found to be in good agreement with the experimental data. The maximum deviation was within 1G, which was the resolution of the Hall probe measuring device. The technique was used to characterize the magnetic fields in the case of an MR-guide EBRT system for multiple scenarios involving various degrees of magnetic coupling between a linac and an MR-on-rails.

Conclusion: A new magnetic field mapping technique, based on a hybrid approach using experimental measurements and numerical simulations, was developed and validated for an MR-guided RT environment.

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