Browsing by Author "Dr. Gianluca Lazzi, Committee Chair"

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Antenna Design for Ultra Wide Band Communications and Frequency Selective Surfaces
(2008-09-30) Rajagopalan, Ajit; Dr. Gianluca Lazzi, Committee Chair
Computation of recruitment volumes in the human body due to external electric or magnetic stimulation using ADI-FDTD method
(2009-11-30) Kwatra, Nitin; Dr. J. K. Townsend, Committee Member; Dr. Griff. L. Bilbro, Committee Member; Dr. Gianluca Lazzi, Committee Chair
The Alternating-Direction Implicit Finite-Difference Time-Domain (ADI-FDTD) method is a computational electromagnetic method which, unlike the traditional explicit FDTD method, is unconditionally stable and allows arbitrarily large time steps. The decrease in the simulation time is achieved at the cost of accuracy. However, in computational problems where the results are averaging based, the effects of such local errors are largely minimized. For example, using ADI-FDTD in bioelectromagnetics, quantities like Specific Absorption Rates (SAR) and total induced currents can be computed over models with fine geometrical resolutions without decreasing the time step proportionally. In this work, the volume of neurons excited (recruitment volume) inside a human body due to external electric or magnetic stimulation is computed using the D-H formulation of ADI-FDTD method. The electric stimulation considered is through current injection by contact electrodes of a Human Electro-Muscular Incapacitative (HEMI) device. For magnetic stimulation, two high frequency current pulses flowing in opposite directions in circular coils are considered. A neuron can be excited if the electric field or the gradient of the electric field along its length exceeds the nerve threshold value. These modes of excitation are termed as `end mode' and `center mode' respectively. The nerve excitation threshold values are decided based on experimental investigations on laboratory animals found in the literature. Memory and simulation time requirements are reduced by employing expanding grid techniques and DFT averaging. The uniform grid of a 1 mm resolution model is logarithmically expanded to 5 mm in region far from source such as head, lower legs and arms. This reduces the computational size of the model by 90\%. For HEMI current source stimulations, Discrete Fourier Transform (DFT) is used to find the induced electric fields due to the dominant frequencies in the current source. By using quasi-static assumptions, the DFT is evaluated for duration substantially lesser than the time period of the different frequencies. The field values are then obtained as the ratios to the fields at the source and then scaled depending upon the magnitude of the source. This study builds upon the efficient use of use of ADI-FDTD method for the solution of the low frequency bioelectromagnetics problems by employing expanding grid techniques and DFT averaging. Recruitment volumes due to a HEMI current source device are computed. A novel stimulation technique of magnetic stimulation is investigated.
Finite-Difference Time-Domain Analysis of Currents from a Human Electro-Muscular Incapacitation Device
(2010-04-27) Mayhew, Rebecca; Dr. Leda Lunardi, Committee Member; Dr. H Troy Nagle, Committee Member; Dr. Gianluca Lazzi, Committee Chair
This research studies the currents generated within the human body when subjected to a Human Electro-Muscular Incapacitation (HEMI) device â€“ commonly known as a Stun Gun. The currents are calculated using the grid based Finite-Difference Time-Domain (FDTD) method of computational electromagnetics. The FDTD technique consists of first defining the Computational Domain - in this case the model of a human torso and a HEMI device. Then the electromagnetic source is defined and the FDTD algorithm and boundary logic is applied to the computational domain to calculate the magnetic and electric fields within the model. In this research, changes were made to the model of the electromagnetic source and to the model of the human torso in order to determine the changesâ€™ effects on the peak currents observed at key observation points within the body. The electromagnetic source was modeled as both a gap current source and a magnetic frill source. The input signal waveform and the length of wires connecting the HEMI device to the subject were changed. In addition, the probe penetration depth, probe separation width, and probe contact locations were all changed. Finally, changes to the human torso model were made, including modeling the skin in wet and dry conditions, as well as adding clothing of various thickness and electromagnetic properties. This research shows that modeling the source as a gap source or magnetic frill are equivalent and the length of the wires connecting the HEMI device to the subject has no impact on the currents within the body. It also shows that probe penetration and probe separation increase the current penetration into the body. Wet skin, or any slightly conductive layer, reduces current penetration to a greater degree than clothing with no conductive properties. However, the greatest impact on the currents at set observation points within the body was the location of the contact probes. This suggests that when a person is shot with a HEMI device, the location of impact of the probes has more of an effect on the currents that a person is subject to than the depth or separation of the probes, or whether the person was wet or wearing clothes.
Finite-Difference Time-Domain Methods for Electromagnetic Problems Involving Biological Bodies
(2006-03-13) Schmidt, Stefan; Dr. Zhilin Li, Committee Member; Dr. Gregory T. Byrd, Committee Member; Dr. Gianluca Lazzi, Committee Chair; Dr. Brian L. Hughes, Committee Member
As more applications of wireless devices in the personal space are emerging, the analysis of interactions between electromagnetic energy and the human body will become increasingly important. Due to the risk of adverse health effects caused by the use of wireless devices adjacent to or implanted into the human body, it is important to minimize their electromagnetic interaction with biological objects. Efficient numerical methods may play an integral role in the design and analysis of wireless telemetry in implanted biomedical devices, as well as computation and minimization of the specific absorption rate (SAR) associated with wireless devices as an alternative to repetitive design prototyping and measurements. Research presented in this dissertation addresses the need to develop efficient numerical methods for the solution of such bio-electromagnetic problems. Bio-electromagnetic problems involving inhomogeneous dispersive media are traditionally solved using the Finite-Difference Time-Domain (FDTD) method. In this class of problems, the spatial discretization is often dominated by very fine geometric details rather than the smallest wavelength of interest. For an explicit FDTD scheme, these fine details dictate a small time-step due to the Courant-Friedrichs-Lewy (CFL) stability bound, which in turn leads to a large number of computational steps. In this dissertation, numerical methods are considered that overcome the CFL stability bound for particular bio-electromagnetic problems. One such method is to incorporate a thin wire sub-cell model into the explicit FDTD method for the computation of inductive coupling. The sub-cell model allows the use of larger FDTD cells, hence relaxing the CFL stability bound. A novel stability bound for the method is derived. Furthermore, an extension to the Thin-Strut FDTD method is proposed for the modeling of thin wire elements in lossy dielectric materials. Numerical results obtained by the Thin-Strut FTDT method were compared with measurements. Furthermore, the Partial Inductance Method (PIM) was implemented using arbitrarily oriented cylindrical wire elements to obtain an analytical approximation of inductive coupling and to verify the Thin-Strut FDTD method. The PIM method was also shown to be a very efficient tool for the approximation of free-space or low frequency inductive coupling problems for biomedical applications. The Alternating-Direction-Implicit (ADI) FDTD method is another method considered in this dissertation. Due to its unconditional stability, the ADI FDTD method alleviates the CFL stability bound. The objective is to apply the ADI method to the simulation of bio-electromagnetic problems and the computation of the SAR. For large time-steps, the ADI method has larger dispersion and phase errors than the explicit FDTD method, but it is still useful for the computation of SAR where those errors are tolerable. An improved anisotropic-material Perfectly-Matched-Layer (PML) Absorbing-Boundary-Condition(ABC) is presented for the ADI FDTD method. The material independent D-H-field formulation of the PML ABC leads to an efficient and simple implementation and allows the truncation of dispersive material models. Furthermore, this formulation is easily extended to n[superscript th]-order dispersive materials. Numerical results for reflection errors associated with the PML and their dependence on parameters like PML conductivity and time-step size are investigated. Furthermore, uniform and expanding grid implementations of the ADI FDTD method are used to compute the Specific Absorption Rate (SAR) distribution inside spherical objects representative of bio-electromagnetic problem. Different grid implementation sizes are considered, and errors associated with the ADI FDTD method are investigated by comparing numerical results to those obtained using the explicit FDTD method.
Implantable Devices for a Retinal Prosthesis: Design and Electromagnetic and Thermal Effects
(2009-12-16) Singh, Vinit; Dr. Gianluca Lazzi, Committee Chair; Dr. Zhilin Li, Committee Member; Dr. Griff Bilbro, Committee Member; Dr. Mehmet Ozturk, Committee Member
A retinal prosthesis, wherein electrical stimulation is provided to the retina of a person inflicted with outer retinal degenerative diseases such as Retinitis Pigmentosa and Age-related Macular degeneration, has been clinically tested and has succeeded in providing limited vision; such as shape recognition. It is hoped that an increased electrode count will improve visual acuity. The retinal prosthesis considered in this work is a dual-unit system with an external (outside the human body) unit and an internal (inside the body) unit with a wireless link for power and data transfer between them. Such a system poses possible health risks due to the incident electromagnetic energy of the wireless link and the power dissipated by the internal components, particularly the processing chip which drives the electrodes responsible for eliciting a neural response from the retina. Tissue damage via heating is one the primary concerns for such a system making it necessary to obtain via simulation and in-vivo and in-vitro experiments, accurate estimates of thermal elevation due to the operation of the such devices. In this work, numerical methods have been developed to compute temperature increases and electromagnetic effects due to the prosthesis components in anatomically correct human head models. The explicit and the Alternating-Direction Implicit (ADI) Finite-Difference Time-Domain (FDTD) have been used. Further, a hybrid explicit-ADI method was developed for the heat equation which provided simulation speedup of more than 10x over conventional methods for the models considered. FDTD methods were employed to compute the induced current densities and Specific Absorption Rate (SAR) in the human head due the inductive link comprising the primary coil (external) and a secondary coil (internal). Different orientations of the primary coil were considered in a frequency range of 1 MHz-20 MHz to provide guidelines for choosing eventual frequency and power parameters to conform to international safety standards. A novel displacement field excitation method was used for the spiral primary coil and verified with analytical results. In an effort to reduce the size of the internal unit and to allow integration of a patch antenna (for a separate data link), and the active devices on a single substrate, a 3-D trench inductor geometry was investigated. To enable patterning of structured surface, a custom experimental setup was designed and maintained to process a positive tone PEPR2400 electro-depositable photoresist.
Investigation of Vector Antennas and their Applications
(2008-08-28) Gupta, Gaurav; Dr. Gianluca Lazzi, Committee Chair; Dr. Brian L. Hughes, Committee Co-Chair; Dr. Robert T. Buche, Committee Member; Dr. J. Keith Townsend, Committee Member
An Investigation on Vector Antennas
(2006-04-11) Konanur, Anand Satyanarayana; Dr. Gianluca Lazzi, Committee Chair; Dr. Brian. L. Hughes, Committee Member; Dr. J.Keith Townsend, Committee Member; Dr. Jean-Pierre Fouque, Committee Member
Wireless networks consisting of compact antennas find applications in diverse areas such as communication systems, direction of arrival estimation, sensor networks and imaging. The effectiveness of many of these systems depends on maximizing the reception of RF power and extracting maximum information from the incident electromagnetic wave. Traditionally, this has been achieved through MIMO systems employing a spatial array of antennas, and results in enhanced channel capacity. We show that similar increases in capacity can be obtained through the use of vector antennas consisting of co-located loops and dipoles that can respond to more than one component of the incident electromagnetic field. Such a system is constructed and its performance measured in a rich scattering environment. It is shown that the system, with three and four element vector antennas at both the transmitter and receiver, supports three and four times more information, respectively, as compared to conventional systems consisting of sensors with single antennas. Alternate geometries evolving out of the study of co-located antennas, consisting of closely spaced elements with optimized mutual coupling and envelope correlation characteristics are discussed. The design and implementation of filter networks that act as conjugate matching networks and enable better absorption efficiency is described. The enhancement in the Expected Mutual Information of systems employing such filters in conjunction with vector antennas is quantified for the designed antennas. Preliminary results indicating possible applications of vector antennas to medical imaging are also included.
Modeling of RF Field effects due to MRI Fields in Patients with a Retinal Implant
(2007-11-18) Jasti, Srinivas; Dr. Gianluca Lazzi, Committee Chair; Dr. Kevin Gard, Committee Member; Dr. Douglas Barlage, Committee Member
Novel Compact Antennas for Biomedical Implants and Wireless Applications
(2005-08-09) Gosalia, Keyoor Chetan; Dr. Robert J. Trew, Committee Member; Dr. Zhilin Li, Committee Member; Dr. Gianluca Lazzi, Committee Chair; Dr. Brian Hughes, Committee Member
Novel design methodologies and implementation techniques for antennas with an extremely small form factor (kr < 1; k is the wavenumber and r is the radius of enclosing spherical volume) are presented. These size reduction techniques are applied to design antennas for two emerging fields: Short (or long) range wireless connectivity and human body implants (prosthetic devices). The first test bed describes compact microstrip patch antennas employing polarization diversity for optimizing the available channel bandwidth in conventional wireless communications. Extremely small antennas (for implantation in an eye ball) operating at microwave frequencies for a visual prosthesis are designed and implemented for the second test bed. The visual prosthesis under consideration is an implantable prosthetic device which attempts to restore partial vision in the blind (patients suffering from retinal degeneration) by artificial stimulation of the retinal cells. Mutually exclusive power and data transfer via a wireless link with the implanted device is proposed where inductive coil coupling transfers power at low frequencies while data communication is performed using extraocular and intraocular antennas at microwave frequencies. The microwave data telemetry link is characterized computationally (using Finite Difference Time Domain-FDTD) and experimentally with appropriately sized external and implanted antennas. It is observed that the head and eye tissues act as a form of dielectric lens and improve the coupling performance between the two antennas (with intraocular antenna embedded in the eye ball) as compared to coupling in free space. The data telemetry link is characterized with novel small microstrip patch and planar wire dipole as intraocular antennas. An electromagnetic and thermal analysis of the operation of such a visual prosthesis is performed. Electromagnetic power deposition in the head is evaluated in terms of Specific Absorption Rate (SAR). Temperature rise in the tissues is characterized by computationally discretizing and implementing the bio-heat equation in three dimensions in an anatomically accurate head model.
Using the ADI-FDTD method to compute currents induced in the human body by HEMI devices at low frequencies
(2009-09-30) Ajeet; Dr. Gianluca Lazzi, Committee Chair; Dr. J. K. Townsend, Committee Member; Dr. W. Rhett Davis, Committee Member
The traditional explicit Finite-Difference Time-Domain (FDTD) is a condition- ally stable method where the time step must adhere to the Courant-Friedrichs-Lewy (CFL) limit. In problems such as those encountered in the bioelectromagnetics and VLSI circuits, the spatial resolution is dictated by the geometric detail rather than the resolution of the smallest wavelength. Thus, severe limitations are often imposed on the time step, leading to long time-domain simulations. This is particularly true for the low-frequency problems, which would require prohibitively large number of time steps with the explicit method. The Alternating-Direction Implicit Finite Difference Time Domain (ADI-FDTD) is a theoretically unconditionally stable method, which allows the use of an arbitrarily large time step for the simulations. Research presented in this thesis aims to compute the induced electric field and current densities in the human body due to the contact electrodes of a Human Electro-Muscular Incapacitation (HEMI) device at frequencies below 200 KHz using the ADI-FDTD method in a D-H formulation. In order to reduce the memory and simulation time requirements, logarithmic expanding grid technique has been used for modeling the human body. Computational model resolution of 1 mm has been used for most of the human body model, including regions proximal to the current contact points, while a progressively coarser resolution up to 5 mm is utilized according to an expanding grid scheme for body regions distant from the source, such as the lower extremities. Discrete Fourier Transform (DFT) of the electric field has been computed at the dominant frequencies present in the source signal to find out the electric field distribution in the model due to the application of the HEMI pulse. Using quasi-static assumptions, computation of the DFT values have been done for time durations much shorter than the time periods of the different frequencies. The field values induced in the human body were then obtained as the ratios of the DFT magnitudes with respect to the source, which can be scaled depending on the magnitude of the electric field at source. This study suggests that the ADI-FDTD method can be effectively used for the solution of low frequency bioelectromagnetic problems. When paired with quasi-static assumptions and Fourier series decomposition for the considered problem, this can lead to simulations that are four orders of magnitude faster than the computational time required with the use of a traditional FDTD method.