Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Physics

Supervisor

Dr. Blaine Chronik

Abstract

High performance gradient and shim coils are highly interested for high-field magnetic resonance imaging and spectroscopy to correct for large B0 inhomogeneities created by the magnetic susceptibility differences between tissues, bone, and air. In chapter two, complete sets of high-performance gradient and shim coils are designed using two different methods: the minimum inductance and the minimum power target field methods. A quantitative comparison of shim performance in terms of merit of inductance, ML, and merit of resistance, MR, is made for shim coils designed using the minimum inductance and the minimum power design algorithms. The coils designed using the target field method are not controlled over the length of the coil. In order to produce realistic coils for use in human or small-animal studies, direct control over the length of the coils is necessary. Therefore in chapter three, an extended Fourier series method for the design of shim coils with predetermined length is presented. This simple method is based on a truncated Fourier series expansion of the current density to allow for explicit control over the coil length. This method is mathematically simple, easy to implement and computationally fast. Also a quantitative comparison of figures of merit for inductance and resistance is made as a function of shim coil length. Coils of 40 cm diameter are designed with lengths of 50 cm, 60 cm, 80 cm, and 100 cm.

Pushing the boundaries of shim design in MRI, we designed a region specific, custom shim coil to correct for large field inhomogeneities that are consistent among subjects. In chapter four, we have designed a custom shim coil for the medial temporal lobe of the human head to correct for the significant field inhomogeneities caused by magnetic susceptibility differences at air/tissue interfaces. The custom coil was designed using the boundary element method. This method is capable of designing coils wound on arbitrarily shaped surfaces so as to produce specific field shapes. We propose that, the addition of this custom coil to the MRI systems can improve the field inhomogeneities significantly. A systematic displacement of head within the custom coil is also presented in this chapter as a method of investigating the sensitivity of the customized shim coil to small differences in subject positioning.


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