University of Western Ontario - Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Physics

Supervisor

Dr. Blaine A. Chronik

Delay of Publication

1

Abstract

This thesis addresses several aspects of gradient and shim coil design and fabrication. New design techniques are coupled with experimental construction methods to expand small animal insert gradient and shim technology. The design techniques are also applied to other areas of magnetic resonance hardware.

A custom 2-axis gradient insert coil is designed and fabricated for the purpose of eddy current characterization. The construction tolerances were examined via bench top inductance measurements and eddy currents measurement inside a 7.0 T head-only MR system. A great deal of freedom is available when positioning shielding coils with respect to their corresponding primary coils in small animal inserts before eddy currents become prohibitive for imaging.

A new method for actively shielding electromagnets is presented. The minimum energy method for designing shielding coils of any geometry is developed and validated against historical methods. Several shielded gradient insert coils are designed, including a cylindrical gradient set with rectangular shields, which demonstrates the versatility of this new method. The performance of the shielded insert coils is reported.

A high power custom shim insert coil is designed and optimized for dynamic shimming applications. This 10-axis shim insert coil is designed to operate at currents higher than any previously existing shim sets. Several experimental fabrication methods are tested during the construction of the insert coil. Inductance, resistance and cooling measurements are conducted and compared to design specifications. Field measurements are taken using a 3-axis field transducer and the shim efficiencies are calculated. Finally mutual inductance measurements are taken between strongly coupled axes to verify active shielding performance.

Lastly, the minimum energy method for active shielding is applied to several MR fringe field type problems. Shields are designed to conform to rooms within an imaging facility for the purpose of controlling the magnetic footprint of an MR system. The MR room itself it designed to house an active shield, along with rooms adjacent to the MR room and a small equipment cabinet located inside the MR room is also fitted with a shield. The performance of the shields is calculated, and the feasibility of such shields is discussed.

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