A Finite Difference Time Domain Study on the Design of Microwave Catheters

.An investigation of the design aspects along with proposed improvements in the construction of microwave ablation catheters are reported in this thesis. The computational methods used to carry out this research include an in-house created cylindrical coordinate rotationally symmetric Finite Difference Time Domain (FDTD) scheme. Firstly, a systematic means of modelling and designing microwave catheters is proposed. The method capitalizes on the rotationally symmetric nature of the microwave catheter and reduces the design from three dimensions to a two-dimensional problem. Secondly issues related to resonant frequency and leaky waves, an inherent property of microwave ablation, are investigated and subsequent solutions are proposed. For the issue of resonant frequency, the addition of a terminating cap halves the catheter’s resonant frequency allowing for acceptable return loss, less than -10 dB, at a resonant frequency of 2.7 GHz without a sleeve choke and 2.45 GHz with a choke. Several designs are investigated in order to eliminate the power coupled into waves travelling along the coaxial feedline’s exterior. The proposed catheter design with the sleeve choke is successful at eliminating surface waves whilst attaining a return loss of -14.61 dB at resonance. The internally matched catheter is equally as effective and attains a return loss of -49.39 dB at resonance while the catheter with a floating sleeve only partially reduces the amplitude of surface waves whilst achieving a return loss of -39.08 dB at resonance. The effectiveness of adding a dielectric cylinder around the monopole in order to improve return loss, bandwidth and overall Specific Absorption Rate (SAR) distribution is also investigated. Near to far field transformations are implemented and the far field pattern of the catheter is shown to be that of a dipole, at resonance. Furthermore, a dispersive FDTD algorithm is developed to incorporate a metamaterial plug. The effects of this are shown to be highly dependent on the dielectric properties of the metamaterial and act to lower the resonant frequency allowing for overall length reductions. Finally, the bioheat equation is investigated and is implemented in the context of microwave catheters by analyzing temperature rise at varying radial distances from the catheter

Microwave Non‐Destructive Testing of Non‐Dispersive and Dispersive Media Using High‐Resolution Methods

This chapter discusses the principle and application of two model‐based algorithms for processing non‐dispersive and dispersive ground penetrating radar (GPR) data over layered medium under monostatic antenna configuration. Both algorithms have been selected for their super‐time resolution capability and reduced computational burden; they allow GPR to measure a layer thickness smaller than the fraction of the dominant wavelength. For non‐dispersive data, the ESPRIT algorithm is generalized to handle different kinds of data models encountered in experiments and in the literature. For dispersive data, the proposed adaptation of the MPM algorithm allows recovering the full‐time resolution and jointly estimating the time delays and quality factors of a layered medium with reduced bias. Both processing techniques are applied to probe‐layered roadways for NDT&E purposes

The EMCC / DARPA Massively Parallel Electromagnetic Scattering Project

The Electromagnetic Code Consortium (EMCC) was sponsored by the Advanced Research Program Agency (ARPA) to demonstrate the effectiveness of massively parallel computing in large scale radar signature predictions. The EMCC/ARPA project consisted of three parts

A Finite Difference Time Domain Study on the Design of Microwave Catheters

.An investigation of the design aspects along with proposed improvements in the construction of microwave ablation catheters are reported in this thesis. The computational methods used to carry out this research include an in-house created cylindrical coordinate rotationally symmetric Finite Difference Time Domain (FDTD) scheme. Firstly, a systematic means of modelling and designing microwave catheters is proposed. The method capitalizes on the rotationally symmetric nature of the microwave catheter and reduces the design from three dimensions to a two-dimensional problem. Secondly issues related to resonant frequency and leaky waves, an inherent property of microwave ablation, are investigated and subsequent solutions are proposed. For the issue of resonant frequency, the addition of a terminating cap halves the catheter’s resonant frequency allowing for acceptable return loss, less than -10 dB, at a resonant frequency of 2.7 GHz without a sleeve choke and 2.45 GHz with a choke. Several designs are investigated in order to eliminate the power coupled into waves travelling along the coaxial feedline’s exterior. The proposed catheter design with the sleeve choke is successful at eliminating surface waves whilst attaining a return loss of -14.61 dB at resonance. The internally matched catheter is equally as effective and attains a return loss of -49.39 dB at resonance while the catheter with a floating sleeve only partially reduces the amplitude of surface waves whilst achieving a return loss of -39.08 dB at resonance. The effectiveness of adding a dielectric cylinder around the monopole in order to improve return loss, bandwidth and overall Specific Absorption Rate (SAR) distribution is also investigated. Near to far field transformations are implemented and the far field pattern of the catheter is shown to be that of a dipole, at resonance. Furthermore, a dispersive FDTD algorithm is developed to incorporate a metamaterial plug. The effects of this are shown to be highly dependent on the dielectric properties of the metamaterial and act to lower the resonant frequency allowing for overall length reductions. Finally, the bioheat equation is investigated and is implemented in the context of microwave catheters by analyzing temperature rise at varying radial distances from the catheter

Extraction of frequency-dependent electrical characteristics of biological tissues using ultra-wideband electromagnetic pulse

There have been many important contributions to imaging for biomedical applications. The most popular methods include X-ray mammography, magnetic resonance imaging (MRI), ultrasound, and most recently, microwave imaging. While the first three of these have been used for biomedical applications for over three decades, microwave imaging has seen many developments over the last few years. This is primarily due to the large contrast in electrical parameters between different body tissues (including differences between healthy and diseased tissues) at microwave frequencies. There are also vast improvements possible for the comfort of the patient undergoing such imaging as compared to mammography. However, there has been no relevant work to date on extraction of the electrical characteristics of tissues within a living patient. Rather, all of the work in the field of microwave imaging has focused on utilizing the vast contrast in electrical parameters to create an image of internal body structures. The electrical properties of human body tissues can be considered as non-magnetic, lossy, frequency-dependent dielectrics in the general case. All that is needed to fully describe these tissues is the frequency-dependent complex relative permittivity. The present work focuses on a unique application of Ultra-Wideband (UWB) radar to extract the frequency-dependent electrical properties of tissues modeled as multiple layers of dielectric regions. By applying an incident pulse to this series of dielectric regions, and by analyzing the reflected signals, the electrical characteristics can be extracted. The results can be expressed in terms of frequency-dependent relative permittivity and conductivity. This work focuses on the time-domain processing to determine the thickness of dielectric regions. Also, a calibration method is proposed to remove interference from the outer dielectric region. Finally, a generalized methodology is proposed to extract the electrical parameters of multiple dielectric regions in the frequency-domain. In all cases, excellent agreement is found between extracted and expected results