Browsing by Subject "MR-EPT"

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Open Access
B1 + phase retrieval for non-quadrature radio frequency excitation and its preliminary application in MR-EPT
(Institute of Physics and Engineering in Medicine, 2019-01) Arıtürk, Gökhan; İder, Yusuf Ziya
Non-quadrature radio frequency (RF) excitation has been widely studied in the fields of RF shimming, local SAR estimation, and MR-EPT with the use of multi-channel transceiver arrays. These studies generally require the retrieval of the complex transmit field ( ), which can be accomplished by acquiring its magnitude and phase in different steps. Magnitude of the transmit field is acquired with the conventional methods which give accurate results for both quadrature and non-quadrature excitations. On the other hand, there is no straightforward method to acquire the absolute phase of the transmit field and generally approximations in MRI experiments are made in order to get it. However, many of these approximations fail in non-quadrature excitation and/or in ultra high fields. In this study, we propose a simple method to acquire the absolute transmit phase in non-quadrature excitation with an eight channel transceiver TEM array for 3 T. The proposed method requires the application of a single additional quadrature drive in order to get the receive phases of the individual channels of the transceiver coil. These receive phases are then subtracted from the transceive phase of the non-quadrature drive experiment to acquire its transmit phase. The developed ideas are tested in the framework of simulations and MRI experiments with the use of four different non-quadrature drive configurations. It has been observed that the simulated and experimentally acquired transmit phase distributions tend to have a strong consensus which supports the validity of the proposed method. Finally, the estimated transmit phase distribution of non-quadrature drive is used in the standard MR-EPT study to get the conductivity reconstructions in order for the validation of its eligibility in MR-EPT studies.
Open Access
Design, implementation and construction of an eight channel RF TEM array and its use in MR-EPT
(2018-07) Arıtürk, Gökhan
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.