Numerical design of resonator coils for high field magnetic resonance imaging
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Date
2005-08
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Publisher
The Ohio State University
Abstract
In the past, design of transmit/receive coils for magnetic resonance imaging
was performed using simple lumped circuit models; however, as developments in
technology have allowed for increasingly high magnetic fields, the frequency at
which the imaging must be performed has increased to the hundreds of megahertz,
causing technical difficulties to arise. This project seeks to overcome the problem of
inhomogeneous magnetic excitation through a redesign of the resonator coil based in
electromagnetic theory. Several designs were attempted, with the primary
modifications being in the dielectric filling and length of the resonator.
Since the lumped circuit model is no longer valid at today’s high fields, it
must be replaced by a full-wave electromagnetic solution. Therefore, all simulations
for this project were performed with a FDTD program previously developed at Ohio
State for this purpose.
The simulations have shown that a significant increase in homogeneity can be
achieved by loading the resonator with a high permittivity dielectric. This in turn
lowers the resonant frequency, so the cavity length must be shortened to raise the
frequency back to normal values. The best case design found here is with a 10 cm
resonator filled with relative dielectric of 60. A better design can likely be found with
a resonator of approximately 8 cm in length, but this was unable to be tested due to
technical difficulties and time constraints. The project also confirms that dielectric
loading of a certain region will cause the field peak to shift towards the load. This
could be useful for locating the field peak at a certain point in the head in order to
obtain an MRI image in just that area.
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Keywords
electromagnetics, fdtd, tem resonator, mri, numerical methods