A detailed study of the performance of finite-difference time-domain (FDTD) and adaptive integral method (AIM) based simulators is presented for bioelectromagnetic (BIOEM) analysis in the UHF band. The comparison is complicated because modern simulators based on these methods can routinely perform high-fidelity BIOEM analysis with hundreds of millions of degrees of freedom. In this thesis, an empirical approach is adopted to investigate the accuracy-efficiency tradeoffs of an FDTD and an AIM based simulator. Specifically, comprehensive numerical experiments are performed using several benchmark multi-layered spherical phantoms. Scattering from these phantoms are computed by using increasingly finer resolution meshes and the results are compared to analytical solutions to investigate the accuracy as well as computational costs of the different methods. The results from the benchmark problems show that both FDTD and AIM based simulators achieve similar error levels for staircased voxel meshes but FDTD based simulation is less expensive, especially when the memory requirement and preprocessing cost are considered. The results also show that although both simulators can reduce errors by refining voxel meshes, AIM based simulators can significantly reduce errors by using CAD meshes instead of voxel ones without significant cost increase. / text

There is a need for improved understanding of interactions between electromagnetic fields and biological tissues. In this thesis, the effects of exposure to 50 Hz magnetic fields, associated with power generation and use, and 1800 MHz fields associated with mobile phones were investigated with particular focus on the plastic processes that are involved in cognitive function. After repeated, daily exposure of young adult C57Bl/6J mice to an 1800 MHz field at 3 W/kg, very subtle changes in expression of genes involved in synaptic plasticity were found (p < 0.05). Spatial memory as measured in the water maze was not significantly affected by exposure. Exposure at 0.3 W/kg did not significantly affect any of the endpoints (p > 0.05). Indications of a greater sensitivity to exposure at 3 W/kg were seen in a senescence accelerated prone mouse model (SAMP8) compared to a resistant strain (SAMR1). However, only subtle effects of exposure were seen. Exposure of young C57Bl/6J mice to a 50 Hz field at 100 or 300 μT induced small but significant changes in expression in synaptic plasticity related genes (p < 0.05). Furthermore, repeated exposure significantly increased microglial density in the dorsal hippocampus (p < 0.05) and slightly decreased proliferation in the dorsal hippocampus (100 μT, p < 0.05). Spatial memory was not significantly affected by exposure. Acute exposure to a 50 Hz magnetic field for 30 minutes at 300 or 580 μT did not affect the adrenal response to a nocturnal white or blue light shock, while exposure at 580 μT in the absence of light significantly decreased per1 expression in the adrenal glands (p < 0.05), but not in the liver or dorsal hippocampus. Exposure at 580 μT for 24 hours had only minor transient effects on the rhythmic expression of the core clock genes. In summary, exposure to 50 Hz or 1800 MHz fields caused subtle and transient changes to some molecular mechanisms and cells involved in cognitive function and circadian rhythm control.

In this work we explore two novel uses of microwave technology in biomedical applications. Introductory material on the electrical properties of biological tissues is presented to form the groundwork for the basic theory behind both techniques. First, we develop a technique that uses 60 GHz signals to detect changes in blood oxidation levels. Several atmospheric propagation models are adapted to predict oxygen resonance spectra near this frequency. We are able to predict and observe the changes in these levels as the blood ages up to 48 hours. Identical testing procedures performed using arterial blood gas (ABG) calibration samples with controlled oxygen levels show similar results to those obtained as bovine blood ages. We then discuss a potential application of this technique to the detection and diagnosis of skin cancer. The second application involves non-invasive measurement of internal body temperatures. Conventional methods of body temperature measurement provide a numerical value for a specific location on the body. This value is then applied to the remaining body systems as a whole. For example, a measurement of 37° C obtained orally can possibly lead to the erroneous conclusion that temperature is normal throughout the body. Temperature measurements made on specific internal organs can yield more information about the condition of the body, and can be invaluable as a tool for performing remote diagnostic evaluations. We explore the use of microwave radiometry in the low GHz spectrum to show that temperature information can be obtained directly and non-invasively for internal organs. We use the principles of black-body radiation theory combined with the reflection and transmission characteristics of biological tissues to predict the temperature delta that would be externally measured, given specific changes in the internal temperature. Data taken using a microwave radiometer and planar structures made with biological phantoms are compared to analytical results, showing that detection of internal temperature changes of can be performed externally in this manner.

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The efficiency of the conventional, explicit finite difference time domain (FDTD)method is constrained by the upper limit on the temporal discretization, imposed by the Courant–Friedrich–Lewy (CFL) stability condition. Therefore, there is a growing interest in overcoming this limitation by employing unconditionally stable FDTD methods for which time-step and space-step can be independently chosen. Unconditionally stable Crank Nicolson method has not been widely used in time domain electromagnetics despite its high accuracy and low anisotropy. There has been no work on the Crank Nicolson FDTD (CN–FDTD) method for frequency dependent medium. In this thesis a new three-dimensional frequency dependent CN–FDTD (FD–CN–FDTD) method is proposed. Frequency dependency of single–pole Debye materials is incorporated into the CN–FDTD method by means of an auxiliary differential formulation. In order to provide a convenient and straightforward algorithm, Mur’s first-order absorbing boundary conditions are used in the FD–CN–FDTD method. Numerical tests validate and confirm that the FD–CN–FDTD method is unconditionally stable beyond the CFL limit. The proposed method yields a sparse system of linear equations which can be solved by direct or iterative methods, but numerical experiments demonstrate that for large problems of practical importance iterative solvers are to be used. The FD–CN–FDTD sparse matrix is diagonally dominant when the time-stepis near the CFL limit but the diagonal dominance of the matrix deteriorates with the increase of the time-step, making the solution time longer. Selection of the matrix solver to handle the FD–CN–FDTD sparse system is crucial to fully harness the advantages of using larger time-step, because the computational costs associated with the solver must be kept as low as possible. Two best–known iterative solvers, Bi-Conjugate Gradient Stabilised (BiCGStab) and Generalised Minimal Residual (GMRES), are extensively studied in terms of the number of iteration requirements for convergence, CPU time and memory requirements. BiCGStab outperforms GMRES in every aspect. Many of these findings do not match with the existing literature on frequency–independent CN–FDTD method and the possible reasons for this are pointed out. The proposed method is coded in Fortran and major implementation techniques of the serial code as well as its parallel implementation in Open Multi-Processing (OpenMP) are presented. As an application, a simulation model of the human body is developed in the FD–CN–FDTD method and numerical simulation of the electromagnetic wave propagation inside the human head is shown. Finally, this thesis presents a new method modifying the frequency dependent alternating direction implicit FDTD (FD–ADI–FDTD) method. Although the ADI–FDTD method provides a computationally affordable approximation of the CN–FDTD method, it exhibits a loss of accuracy with respect to the CN-FDTD method which may become severe for some practical applications. The modified FD–ADI–FDTD method can improve the accuracy of the normal FD–ADI–FDTD method without significantly increasing the computational costs.