Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Medical Biophysics

Supervisor

Chronik, Blaine A

Abstract

Magnetic resonance imaging utilizes electromagnets to produce anatomical images in both clinical and research settings. In the race towards increasing performance head-optimized scanners have begun playing a significant role in providing high quality imaging of the head. However, they are implemented using smaller geometries and as such fail to allow entrance of the patient past their shoulders. This is overcome by designing asymmetric gradient coils which have their imaging region located towards one end of the gradient coil, as opposed to the geometric center, allowing brain imaging. There exists interest in compact configurations which allow imaging further into the cervical spine which is unfeasible using current asymmetric gradients. This work seeks to explore the design of asymmetric gradient coils with shoulder cut-outs to enable neck imaging by allowing the patient to enter further into the gradient coil while maintaining the small inner radius of a head-only platform.

First, the relative trade-offs in designing an asymmetric shoulder cut-out gradient coil are explored and extended by rotating the transverse gradient axes to produce gradient coils which compensate for some of the electromagnetic burden due to the loss of conducting surfaces on the sides. Next, a complementary set of spherical harmonic active shims are designed and explored for implementation within this gradient coil configuration. From there the design of a cylindrical radiofrequency coil using gradient design techniques is investigated as preliminary work towards implementing these low-frequency design techniques which have had success designing gradient coils towards the design of radiofrequency coils.

Finally, motivated by the complexity of the induced eddy currents in the surrounding conductive structures due to asymmetric gradient coils the final project explores the design of a multi-coil matrix array aimed at fitting within the compact gradient housing to dynamically compensate eddy currents during imaging.

This work ultimately demonstrates the feasibility of implementing an asymmetric shoulder cut-out gradient coil with rotated transverse gradient axes to enable neck imaging in a compact MRI scanner while providing potential solutions to handle the increased eddy current complexity associated with a setup such as this.

Summary for Lay Audience

Magnetic resonance imaging (MRI) uses a series of nested electromagnets to produce anatomical images of the body. This thesis explores the design of three of these electromagnets: the gradient coil, the radiofrequency (RF) coil, and a set of shim coils. Many clinical MRI scans are performed of the head and as such development of “head-optimized” scanners have gained popularity. These are smaller than those used for full-body imaging and have limited patient entrance due to their small radius. In these systems the entrance into the MRI scanner is stopped by the shoulders limiting imaging to the brain region. This thesis asks the question “can we design an MRI gradient coil for compact head-optimized geometries which allows imaging of the cervical spine.”

The first chapter of this thesis goes into the necessary background information required to understand the work outlined here. The next two chapters in this thesis explore the design of a gradient coil with portions of the sides removed to accommodate the patient’s shoulders. This allows further entrance into the MRI scanner while maintaining geometries appropriate for a head-optimized system. This would allow the neck to slide into the region in the MRI scanner where the imaging takes places. I investigated the trade-offs in performance and propose designs which fit inside an experimental MRI scanner. This is extended in the fourth chapter by designing of a complimentary set of shim coils to improve imaging performance by dynamically shimming the MRI imaging environment. Radiofrequency coil design is explored in Chapter 5 where I design an RF coil using gradient coil design techniques, construct the coil and perform experimental validation. The constructed coil had field artefacts compared to the ideal design which I attribute to a violation of an assumption made in the design. Next, the sixth chapter investigates the design of a special type of coil to help improve imaging by actively compensating for eddy currents which produce parasitic magnetic fields of their own that degrade image quality. Finally, in Chapter 7 I summarize the work of the preceding chapters and outline some next steps to expand on this thesis.

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