This thesis addresses the problem of radiofrequency probe design for Ultra High Frequency Magnetic Resonance Imaging (7T). The signal-to-noise ratio available in Magnetic Resonance Imaging (MRI) is determined by the static magnetic field strength, causing a continued drive toward higher fields to enable faster image acquisition at finer spatial resolution. The resonant frequency increases linearly with static field strength. At 7T the proton resonant frequency is 300MHz, with a wavelength of approximately 13cm in tissue. As this is smaller than the dimensions of the human head, the phase of the radiofrequency (RF) signal varies considerably across the sample, producing field cancellation due to interference. A full wave electromagnetic simulator, using the Transmission Line Matrix (TLM) method, was developed to investigate RF probes at high frequency. A Birdcage probe operating at 64, 128 and 300MHz (corresponding to 1.5, 3 and 7T) was simulated, loaded with an anatomically detailed human head model. A half-wave microstrip was investigated for use as a high frequency probe element. Magnetic and electric fields produced by a single microstrip were simulated, and the strip dimensions varied to investigate the effect on field penetration into the head and Specific Absorption Rate (SAR). A transmit-receive array probe using four microstrip elements was then developed. Bloch simulations were run, using TLM generated magnetic fields, to investigate imaging at short wavelength. Parallel receive probes are demonstrated to offer considerable advantage over volume probes, as signals from receive elements can be combined without interference. There is no transmit equivalent to parallel reception; simultaneous excitation of independent probe elements causes interference in exactly the same manner as a volume probe. A new imaging sequence was developed using a Burst-like encoding to allow sequential excitation of probe elements, without interference, which can be recalled in a single readout. An improvement in image homogeneity was demonstrated, and SENSE acceleration of the new imaging sequence is shown. The sequence was implemented at 3T using a purpose built four element microstrip probe. An RF multiplexer was also built to enable transmit element switching during the imaging sequence. It was demonstrated that images due to different RF excitations, acquired in a single EPI readout, can be separated.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:442298 |
| Date | January 2007 |
| Creators | Magill, Arthur W. |
| Publisher | University of Nottingham |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://eprints.nottingham.ac.uk/10740/ |
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