Microstrip Radio-Frequency Coil and Array Design for Magnetic Resonance Imaging

This dissertation, "Microstrip Radio-frequency Coil and Array Design for Magnetic Resonance Imaging" by Bing, Wu, 吳冰, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled Microstrip Radio-frequency Coil and Array Design for Magnetic Resonance Imaging submitted by Wu Bing For the degree of Doctor of Philosophy at The University of Hong Kong in September 2006 In the past five years, microstrip RF coils have been developed and widely applied for MR applications at high magnetic fields. In contrast to conventional surface coils, the microstrip coil uses the microstrip transmission line which in its simplest form consists of a thin strip conductor and a ground plane separated by a low-loss dielectric substrate. The semi-open, unbalanced structure microstrip generates a unique B field distribution mainly on one side of the coil where a sample is located. This results in several unique features: reduced losses, higher Q- factor, reduced coupling among multiple microstrip coils. In addition, the microstrip RF coil's unbalanced nature obviates the need of a matching balun. This dissertation presents the author's investigates in microstrip RF coil and array designs at the fields higher than 1.5 Tesla. Firstly, a novel tunable loop microstrip (TLM) RF coil based on the ring resonant circuit has been presented. SNR comparison between the TLM coil and conventional surface coil has been performed at various magnetic field strengths from 1.5 Tesla to 11.1 Tesla. Our study has demonstrated that utilization of the TLM coil can substantially reduce radiation loss and deliver better SNR performance than a conventional coil at ultra high fields. Results also indicate a trend to superior performance of the TLM coil as the field increases. Secondly, several current decoupling techniques have been utilized to the TLM planar array, which consists of two identical TLM coils elements. Simulation, bench test and MRI experiments have been carried out to provide a quantitative analysis of those decoupling schemes. Thirdly, the commonly used capacitive decoupling method for microstrip arrays has been analyzed. It appears that the decoupling capacitance is usually quite small at ultra-high fields and difficult to finely tune. A capacitively decoupled TLM array has been proposed, fabricated and tested at 7 Tesla. Using the TLM array, the capacitive decoupling method can be easily applied with reasonable decoupling capacitance. Bench test and MRI experiments at 7 Tesla show that excellent isolations (-37 dB) between the adjacent elements can be achieved and this TLM array is appropriate for SENSE imaging. Lastly, a new inductive decoupling approach for microstrip arrays has been presented at fields higher than 7 Tesla. In contrast to the capacitive decoupling methods, the decoupling inductance is independent of the resonant frequency, making this method much easier to be implemented. An inductively decoupled eight- channel microstrip array has been implemented and tested at 9.4 Tesla. This decoupling approach should enable more elements to be packed into microstrip arrays for the purpose of parallel imaging at ultrahigh fields. Number of words: 425 Signature: DOI: 10.5353/th_b3704672 Subjects: Magnetic resonance imaging