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An inverse method for designing high-frequency RF coils in MRI

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While, PT (2007) An inverse method for designing high-frequency RF coils in MRI. PhD thesis, University of Tasmania.

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Abstract

In this thesis an inverse method is developed for the theoretical design of radio-frequency
(RF) coils for use in magnetic resonance imaging (MRI) at high-frequency.
New design techniques are required for RF coils at high-field, where
the wavelength of operation becomes comparable to the dimensions of the
coils. In addition, the effects of sample loading on the homogeneity of induced
fields become more pronounced as the frequency of operation increases and
can result in severe distortion to the acquired image. The general aim of the
design method presented is to find a current density solution on the coil that
will induce some desired homogeneous magnetic field upon a spherical target
region within. This is an ill-conditioned inverse problem that can be solved
using regularisation, in which the error between induced and target magnetic
fields is minimised along with some additional constraint of the designer's
choosing. This general technique is demonstrated and adapted throughout
the thesis for a wide variety of RF coil types.
The method is initially applied to the design of unloaded, unshielded RF
volume coils, with asymmetrically located target regions. Time-harmonic
methods involving retarded potential Green's functions are used to obtain
field expressions, with the geometry of the coil accounted for using a Fourier
series representation of the current density components. The resulting integral
equation is solved using a hybrid regularisation penalty function that allows
a trade-off between coil winding simplicity and field homogeneity. The important
case of RF shielding is considered in a straightforward extension of
the design method, to induce a null field exterior to the coils. In addition,
the method is reworked to account for sample loading and its corresponding
effects at high-field. This requires a three-layer model which is solved using
Fourier-Bessel series expansions, along with appropriate boundary conditions
on the surfaces of the load and RF coils. Finally, the inverse method is adapted
further for the design of RF phased arrays, which afford a number of advantages
over RF volume coils, including a high signal-to-noise ratio (SNR) over
a large field of view. A robust design method is presented for this coil type,
which allows complete freedom over array size, field focussing and polarisation
considerations.

Item Type: Thesis (PhD)
Keywords: Magnetic resonance imaging, Image processing
Copyright Information:

Copyright 2007 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Date Deposited: 07 Apr 2008 15:27
Last Modified: 21 Feb 2017 21:24
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