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Optimization of a Non-axial Magnet Design for a Hybrid Radiation Treatment and MR Imaging System Open Access


Other title
Longitudinal Linac-MR
Magnet Design
Type of item
Degree grantor
University of Alberta
Author or creator
Yaghoobpour Tari, Shima
Supervisor and department
Fallone, B. Gino (Oncology - Medical Physics Division)
Wachowicz, Keith (Oncology - Medical Physics Division)
Examining committee member and department
St. Aubin, Joel (Oncology - Medical Physics Division)
DeZanche, Nicola (Oncology - Medical Physics Division)
Warkentin, Brad (Oncology - Medical Physics Division) - Chair
Larocque, Matt (Oncology - Medical Physics Division)
Department of Oncology
Medical Physics
Date accepted
Graduation date
2017-06:Spring 2017
Master of Science
Degree level
A prototype rotating hybrid magnetic resonance (MR) imaging system and linac has been developed to allow for simultaneous imaging and radiation delivery parallel to B_0. However, the design of a compact magnet capable of rotation in a small vault with sufficient patient access and a typical clinical source-to-axis distance (SAD) is challenging. This work presents a novel superconducting magnet design that allows for a reduced SAD and ample patient access by moving the superconducting coils to the side of the yoke. The yoke and pole-plate structures are shaped to direct the magnetic flux appropriately. A closed symmetrical system with different pole plate structures was studied to find the most suitable optimization algorithm and pole plate structure. Then, the outer surface of the pole plate for a non-axial design was optimized subject to the minimization of a cost function, which evaluates the uniformity of the magnetic field over an ellipsoid. This non-axial design is reminiscent of a C-core transformer. Magnetic field calculations were performed with the 3D finite element method (FEM) software package Opera-3D. Each tentative design strategy was virtually modeled in this software package, which is externally controlled by MATLAB, with its key geometries defined as variables. The optimization variables were the thickness of the pole plate at control points distributed over the pole plate surface. Optimized magnet assemblies that generate homogenous 0.2T and 0.5T magnetic fields over an ellipsoid with a large axis of 60 cm and small axes of 40 cm were obtained. The distinct features of this model are the minimal distance between the yoke's top and the isocentre, which allows for a minimal SAD, and the improved patient access. Additionally, obtaining field homogeneity over a large field-of-view leads to a unit with enhanced imaging flexibility.
This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for the purpose of private, scholarly or scientific research. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.
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