Design, implementation and construction of an eight channel RF TEM array and its use in MR-EPT

Ider, Yusuf Ziya
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Bilkent University
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Magnetic Resonance - Electrical Properties Tomography, aiming at reconstructing the electrical properties (EPs) at radio frequencies, has a continuously increasing importance in terms of identifying the cancerous tissues and distinguishing between ischemic and hemorrhagic stroke. The presently prominent MR-EPT method \Convection{Reaction Equation Based MR-EPT" is still not clinically used due to the presence of image artifacts. In this regard, the objective of this thesis is to eliminate the low convective eld (LCF) artifact, which refers to abrupt and point-wise image perturbations on the conductivity and permittivity reconstructions of cr-MREPT method. Since the proposed methods involve the use of parallel RF transmission, a multichannel transceiver array is designed by carefully scrutinizing the original TEM resonator, proposed by J.Thomas Vaughan in 1994. Finite Element Method (FEM) based simulations of that structure, which includes the use of coaxial line elements (transmission lines), are done in Comsol Multiphysics. For better practical feasibility, a microstrip transmission line based eight{channel TEM array was designed, simulated and constructed. Each of the eight ports of this array is matched to 50 with re ection coe cients as low as -40 dB at 123.2 MHz. Worst decoupling between the ports is measured as -14 dB. With the use of quadrature excitation, clear MRI images of experimental phantoms and highly homogeneous B+ 1 maps are obtained. Using simulations, a method to eliminate the LCF artifact from the EP reconstructions is proposed. This method involves the use of the TEM array in two di erent excitation con gurations. In the rst excitation, the conventional quadrature drive is used. The second excitation, on the other hand, uses magnitude and phase optimized RF sinusoids to produce a proper transmit eld (B+ 1 ) within the object. This intentionally adjusted (B+ 1 ) eld, which comprises high eld and low eld regions with a transition in the middle, shifts the LCF artifact towards a non-central location. Finally, data from both drive experiments are simultaneously used to reconstruct EP's. It has been further shown that the method can be applied to di erent patients without requiring patient-speci c B+ 1 optimizations. Experimentally implementing the proposed method, another novel algorithm to extract the phase of the transmit eld ( B+ 1 ) in a non-quadrature excitation is proposed. In this algorithm, the receive phases of individual channels, being common for quadrature and non-quadrature experiments are found from an additional quadrature drive experiment with the use of transceive phase assumption. Then, the transmit phase of non-quadrature drive is extracted by subtracting the receive phases from the transceive phase distributions. Strong consensus between the simulated and experimentally estimated transmit phases is observed. In conclusion, the conductivity reconstructions of an experimental phantom, with the use of the developed methods, is provided. It has been shown that the LCF artifact is alleviated and better experimental setups are required to fully eliminate it.

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RF Coil, RF Transceiver Array, TEM Coil, MR-EPT, B1 Shimming
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