Browsing by Subject "Finite Element Method"
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Item Open Access Development of image reconstruction algorithms for three dimensional magnetic resonance-electrical impedance tomography(2003) Onart, SerkanThe electrical resistivity of biological tissues differ among various tissue types. Human body has a large resistivity contrast between a wide range of its tissues. The aim of this study is to reconstruct conductivity images of three dimensional objects with higher resolution and better accuracy than existing conductivity imaging techniques. In order to achieve our goal, we proposed a technique named as Magnetic Resonance - Electrical Impedance Tomography (MR-EIT) which combines the peripheral voltage measurements of classical Electrical Impedance Tomography (EIT) technique with magnetic flux density measurements acquired using a Magnetic Resonance Imaging (MRI) scanner. Five reconstruction algorithms are proposed and computer simulations are made. The proposed algorithms fall in two categories those that utilize current density data and those that utilize magnetic flux density data directly. The first group of algorithms get the current density data from magnetic flux density by Ampere’s law. For calculation of current density with Ampere’s law, we need to all three components of magnetic flux density but that is not possible to get all of them in one measurement phase. Total of three measurement phases are needed for getting all of them but this is not practical because, for measurement of each component the object has to be rotated appropriately in the MRI scanner. The algorithms in the second group suggest an exit to this difficulty and achieve the conductivity reconstruction by using only the data which was acquired in one measurement phase. As can be seen in the results, conductivity reconstruction of three dimensional objects on tomographic planes are made successfully with all of the algorithms. They also work fine against to the measurement noise up to an acceptable level.Item Open Access Finite element method based simulation, desing, and resonant mode analysis of radio frequency birdcage coils used in magnetic resonance imaging(2012) Gürler, NecipRadio Frequency (RF) birdcage coils are widely used in Magnetic Resonance Imaging (MRI) since they can generate very homogeneous RF magnetic field inside the coil and have high signal-to-noise ratio (SNR). In practice, designing a birdcage coil is a time-consuming and difficult task. Calculating the capacitance value, which is necessary for the coil to resonate at the desired frequency, is the starting point of the design process. Additionally, it is also important to know the complete resonance frequency spectrum (or resonant modes) of the birdcage coil that helps the coil designers to be sure that working mode is far away from the other modes and so that tuning and matching procedures of the coil can be done without interfering with the other modes. For this purpose, several studies have been presented in the literature to calculate the capacitance value and the resonant modes of the birdcage coil. Among these studies, lumped circuit element model is the most used technique in capacitance and resonant modes calculations. However, this method heavily depends on the inductance calculations which are made under quasi-static assumptions. As a consequence of this assumption, error in the calculations increases as the frequency increases to a point at which the wavelengths are comparable with the coil dimensions. Additionally, modeling the birdcage coil in a 3D simulation environment and making electromagnetic analysis in the volume of interest is also important in terms of observing the electromagnetic field distributions inside the coil. In this thesis, we have proposed three different Finite Element Method (FEM) based simulation methods which are performed using the developed low-pass and high-pass birdcage coil models in COMSOL Multiphysics. One of these methods is the FEM based optimization method in which magnitude of the port impedance or variance of H+ is used as the objective function and the capacitance value is used as the control variable. This is a new method proposed for calculating the capacitance value of the birdcage coils. The other method is the eigenfrequency analysis which is used to determine not only the resonant modes of the birdcage coil but also the electromagnetic fields distributions inside the coil at these resonant modes. To the best of our knowledge, FEM based eigenfrequency analysis of a birdcage coil is also a new study in the field of MRI. The last method is the frequency domain analysis which is used solve for the electromagnetic fields of a birdcage coil for the specified frequency (or frequencies). One can also use this method to estimate Specific Absorption Rate (SAR) at any object inside the coil. To make these three simulation methods easily and according to the user-specified parameters, we have developed two software tools using MATLAB which have also graphical user interface (GUI). In order to compare the results of the proposed methods and the results of the methods that use lumped circuit element model with the experimental results, we have constructed two handmade birdcage coils and made measurements for different capacitance values. Then, we have compared the measured resonant modes with the calculated resonant modes; used capacitance values with the calculated capacitance values. For the worst case (in which the frequency is the highest), proposed FEM based eigenfrequency analysis method calculates the resonant modes with a maximum of 10% error; proposed FEM based optimization method calculates the necessary capacitance values with 20-25% error. Methods which use lumped circuit element model, on the other hand, calculate the resonant modes and capacitance values with 50-55% error for the worst case.Item Open Access Finite element method based simulations of low frequency magnetic field in seawater(2013) Şimşek, Fatih EmrePropagation properties of the electromagnetic waves in seawater are different than in air (vacuum) due to electrical conductivity (σ) and high relative permittivity (εr) of the seawater. Numerically it is hard to solve the electromagnetic waves in seawater for the complex geometries. With the help of the advances in the Finite Element Method (FEM) tools as well as the personal computers, we have chance to analyze magnetic field of the complicated and complex geometries of physical systems in seawater. In this thesis; an air-cored multilayer transmitting coil is designed. Then the low frequency magnetic flux density of this coil in different studies in seawater in COMSOL Multiphysics is solved. In the first study; the magnetic flux density of the coil in air and in seawater for different frequencies on different observation points is solved. In the second study; the shielding effect of the material of the case of the coil as well as the thickness of the case is analyzed. Specific materials as well as thickness for the case are proposed. In the third study; the perturbation of the magnetic flux density of the coil due to a metal plate is analyzed. The material of the metal plate is taken iron and copper. Iron has high relative permeability ( r) and high electrical conductivity (σ). Copper has unity permeability ( 0) and high electrical conductivity (σ). Effect of the high electrical conductivity on the perturbation of the magnetic flux density on the observation point is analyzed. Effect of high relative permeability on the phase shift of the field on the observation point is observed. A detection region for the plate and coil geometries according to the attenuation of the secondary fields caused by the eddy currents on the metal plate is proposed. In the last study; perturbation of ambient Earth magnetic field due to a submarine is solved and how this perturbation can be imitated by an underwater system, which tows a DC current carrying wire is analyzed. These underwater systems are used to test detection performance of magnetic anomaly detector (MAD) equipped aircrafts.