Evaluation of Direct FID-Based Dielectric Parameter Retrieval in MRI

Generalized Signal Models

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This thesis research focuses on MRI technology and more specifically the evaluation of the generalized signal model (full-wave signal). The research aims to investigate how dielectric properties influence the magnetic fields in MRI scans, by using a more complex signal model that takes into account scattered fields caused by dielectric properties of human tissue. Optimizing the Signal-to-Noise ratio (SNR) has been a main research focus since the invention of MRI machines. Higher-quality images, result in better diagnosis and therefore help the investigations of patient conditions. One way to improve the SNR is to operate MRIs in higher frequencies. Operating an MRI in higher frequencies, affects the measured signal at the receiving coil, as dielectric properties of biological tissue, induce currents due to magnetization. By having a full wave signal model, one can model the effect of the dielectric properties and thus take them into account when reconstructing an MRI image. The first step was to investigate whether a magnetic dipole can be simulated as a single emitting loop. The results from the simulation and the analytical solution have been compared. There’s a high correlation be-
tween the two and thus it is concluded that it can be used. Using a single emitting loop, the effect of varying conductivity was simulated for a dielectric sphere placed in free space, with the single emitting loop submerged into the solution. The simulations showed that by increasing the conductivity, the measured voltage at the receiver coil reduces. Note that this simulation was performed twice, for relative
permittivity values εr = 1 and εr = 80. The reason for choosing these two values was to first eliminate the effect of relative permittivity εr = 1 and to simulate a water-based solution εr = 80. Simulations verified that for εr = 1, the measured voltage at the receiver location is lower than εr = 80. After verifying the full-wave model and observing the effect of varying the electrical conductivity, two more simulations were performed to prepare for the experiment. In the first simulation, instead of a single loop, a series of loops was used to simulate the beam excitation of an MRI. As it is shown in the results, there is not a linear correlation between increasing the conductivity and the induced voltage at the receiver coil. Secondly, the experimental setup was simulated. In addition to the previous simulation, a plastic layer was added to the outside of the ball, to model the plastic sphere used during the experiment. In chapter 4, the results of the experiment are introduced. The results from the experiment could not justify the simulations due to reasons that are explained in the conclusions.