Analysis of current induction on thin conductors inside the body during MRI scan
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The aim of this thesis is to develop a method to analyze currents on thin conductor structures inside the body during Magnetic Resonance Imaging (MRI) scan based on Modified Transmission Line Method (MoTLiM). In this thesis, first, Active Implantable Medical Devices (AIMDs) are modeled and the tissue heating problem, which is a result of coupling between AIMD and incident Radio Frequency (RF) fields, is examined. Then, usage of MoTLiM to analyze the currents on the guidewires is shown by solving currents on guidewire when a toroidal transmit receive coil is used with guidewire. At first, a method to measure MoTLiM parameters of leads using a network analyzer is shown. Then, IPG case and electrode are modeled with a voltage source and impedance. Values of these parameters are found using the Modi- fied Transmission Line Method (MoTLiM) and the Methods of Moments (MoM) simulations. Once the parameter values of an electrode/IPG case model are determined, they can be connected to any lead, and tip heating can be analyzed. To validate these models, both MoM simulations and MR experiments are used. The induced currents on the leads with the IPG case or electrode connections are solved using the proposed models and MoTLiM. These results are compared with the MoM simulations. In addition, an electrode is connected to a lead via an inductor. The dissipated power on the electrode is calculated using MoTLiM by changing the inductance and the results are compared with the specific absorption rate results that are obtained using MoM. Then, MRI experiments are conducted to test the IPG case and the electrode models. To test the IPG case, a bare lead is connected to the case and placed inside a uniform phantom. During a MRI scan the temperature rise at the lead is measured by changing the lead length. The power at the lead tip for the same scenario is also calculated using the IPG case model and MoTLiM. Then an electrode is connected to a lead via an inductor and placed inside a uniform phantom. During a MRI scan the temperature rise at the electrode is measured by changing the inductance and compared with the dissipated power on the electrode resistance. Second, based on the similarity between currents on guidewires and transmission lines, currents on the catheter are solved with MoTLiM. Current distributions on an insulated guidewire are solved and B1 distribution along the catheter is calculated. Effect of stripping the tip on the tip visibility is analyzed. It is shown that there is an increase in the B1 at the insulation and bare guidewire boundary. Then, a characteristic impedance is defined for the guidewires and impedance seen at the point where guidewire is inserted into the body is calculated. It is shown with EM simulations that if the impedance converges to the characteristic impedance of the guidewire, tip visibility of the guidewire is lost. At last, a new method to measure electrical properties of a phantom material is proposed. This method is used for validation of the coaxial transmission line measurement (CTLM) fixture, which is designed for measurement of electrical properties of viscous phantom materials at MRI frequencies, and which is previously presented by our group. The new method depends on the phenomena of the lead tip heating inside a phantom during MRI scan. Electrical properties of a phantom are influential on the relationship between tip temperature increase and the lead length. MoTLiM is used and the relationship between the lead length and the tip temperature increase is formulated as a function of conductivity and permittivity of the phantom. By changing the lead length, the tip temperature increase is measured and the MoTLiM formulation is fitted to these data to find the electrical properties of the phantom. Afterwards the electrical properties of the phantom are measured with the CTLM fixture and the results that are obtained with both methods are compared for an error analysis. To sum, electrical models for the IPG case and electrode are suggested, and the method is proposed to determine the parameter values. The effect of the IPG case and electrode on tip heating can be predicted using the proposed theory. An analytical analysis of guidewire with toroidal transceiver is shown. This analysis is helpful for better usage and improvements of toroidal transceiver. Also, MoTLiM analysis can be extended to other MRI guidewire antennas.