Carl H. Durney

Extending the two-dimensional FDTD method to hybrid electromagnetic systems with active and passive lumped elements

The finite-difference-time-domain (FDTD) method is extended to include distributed electromagnetic systems with lumped elements (a hybrid system) and voltage and current sources. FDTD equations that include nonlinear elements like diodes and transistors are derived. Calculation of driving-point impedance is described. Comparison of FDTD calculated results with analytical results for several two-dimensional transmission-line configurations illustrates the accuracy of the method. FDTD results for a transistor model and a diode are compared with SPICE calculations. The extended FDTD method should prove useful in the design and analysis of complicated distributed systems with various active, passive, linear, and nonlinear lumped electrical components. >

Numerical Calculation of Electromagnetic Energy Deposition for a Realistic Model of Man

Numerical calculations of absorbed energy deposition have been made for a block model of man that is defined with careful attention given to the biometric and anatomical features of a human being. CalcuIated post-resonant absorption and distribution of energy deposition through the body have better agreement with experimental results than previous calculations made using less realistic models.

Electromagnetic dosimetry for models of humans and animals: A review of theoretical and numerical techniques

Theoretical dosimetry, i.e., calculation of the electromagnetic (EM) energy absorbed by humans in radiation fields, has become increasingly important as the use of EM devices in our society has increased. Since such calculations are difficult and complicated, a number of EM analysis techniques have been used by researchers in the field to obtain dosimetric data and understanding of absorption characteristics. Both analytical and numerical EM techniques have been applied extensively. In this paper, important techniques are reviewed and significant dosimetric data are summarized. Some important areas of future research in EM dosimetry are discussed.

A mathematical model of diamagnetic line broadening in lung tissue and similar heterogeneous systems: Calculations and measurements

Abstract In order to explain recently observed internal inhomogeneous broadening in lung tissue, we constructed and calculated diamagnetic field shifts for models consisting of a spherical shell of water and a hexagonally packed array of spherical air bubbles in water. For the spherical-shell model, the field equations were solved exactly for an arbitrary number of concentric shells. In contrast, in the hexagonal model, a Monte Carlo algorithm was used to pick points within the specimen at which the field was calculated to first order by adding together contributions from all the spheres. The linewidth calculated for the spherical-shell model agrees well with the results of our experimental measurements. Furthermore, the hexagonal model accurately predicts the observed linewidth in rat lung. These models can be used in correlating the NMR linewidth with the state of inflation or injury of the lung. They also may have application in a broad class of heterogeneous systems (e.g., slurries, bone).

A new procedure for improving the solution stability and extending the frequency range of the EBCM

The extended boundary condition method (EBCM) has been frequently used to obtain the absorption and scattering characteristics of axisymmetric dielectric objects. For applications involving relatively high-loss dielectric objects, however, the method was usable only at frequencies below resonance. In this paper a new procedure for improving the stability and extending the frequency range of the EBCM is presented. This new procedure has two main features: 1) it is iterative, since it starts with a known solution that approximates the scattering problem, and 2) it involves separate field expansions in each of the overlapping subregions which describe the total interior volume of the object. For example, for high-loss dielectric objects, such as the biological models of humans and animals, the first step in the procedure is to replace the lossy dielectric object with a perfectly conducting one of the same shape and solving the scattering problem to determine the current density on the surface of the conductor. This surface current is then used to calculate the induced field expansions inside the dielectric object. It is shown that the numerical stability of the solution is further improved by dividing the interior region of the object into overlapping subregions, in each of which a separate field expansion is assumed. The electric and magnetic surface currents so obtained from the solution of the internal problem are then used to improve the initial estimate of the current density on the surface of the object. The iterative procedure continues until convergent values of the surface currents and the fields are obtained. Numerical results illustrating the improved stability of the iterative EBCM (IEBCM) solution at higher frequencies as well as its accuracy in calculating the absorption characteristics of a spheroidal model of man in the resonance and the postresonance frequency range are presented.

Long Wavelength Analysis of Plane Wave Irradiation of a Prolate Spheroid Model of Man

An electromagnetic (EM) field perturbation technique is used to find internal electrical fields and the absorbed power of a prolate spheroid being irradiated by a plane wave when the waveIength is long compared to the dimensions of the spheroid. The results show significant differences in the power absorption patterns with changes in the orientation of the spheroid with respect to the incident EM fields. Calculations of the power absorbed by a prolate spheroid model of man are given.

Long-Wavelength Electromagnetic Power Absorption in Prolate Spheroidal Models of Man and Animals

A previously developed electromagnetic (EM) field perturbation analysis is used to calculate the electric fields in tissue prolate spheroids irradiated by plane waves with long wavelength compared to the spheroid dimensions. This theory is applied to prolate spheroid models of man and animals to obtain internal electric field strength, absorbed power distribution, and total absorbed power. These data are of value in estimating tissue EM power absorption in experimental animals and man. The theory may be used to help extrapolate animal biological effects data to man, and as a guide to establishing an EM radiation safety standard.

A Procedure for Calculating Fields Inside Arbitrarily Shaped, Inhomogeneous Dielectric Bodies Using Linear Basis Functions with the Moment Method

A moment method for calculating the internal field distributions of arbitrarily shaped, inhomogeneous dielectric bodies is presented. A free-space Greens function integral equation is used with 3-D linear basis functions to describe the field variation within cells. Polyhedral volume elements are used to model the scatterers curvature realistically without an excessive number of unknowns. A new testing procednre, called the modified Galerkins method, is developed and used to obtain the matrix equations with less CPU time but greater accuracy. Calculated internal field distributions of dielectric spheres, spheroids, and a composite model of a rat are compared with other calculations and experimental data. The agreement is generally good.

Optical temperature probe

A temperature probe measures temperature changes of biological tissue whilehe tissue is being irradiated with microwaves. The temperature probe is completely nonmetallic to minimize perturbation of the microwave field, and the structure of the probe includes a plurality of optic fibers which carry light to and from a reflective liquid crystal film. The liquid crystal is enclosed in a housing at the tip of the probe which is in contact with the biological tissue, and the liquid crystal undergoes changes in absolute reflectance proportional to the temperature changes of the tissue. The optic fibers are gathered in a bundle with a first portion of the bundle transmitting light from a light generation device to the liquid crystal and the remainder of the bundle transmitting reflected light from the crystal back to a photo transistor. The reflected light received by the photo transistor is converted into an electrical signal and displayed as a representation of temperature.

Calculation and interpretation of inhomogeneous line broadening in models of lungs and other heterogeneous structures

Abstract To extend previous calculations of internal inhomogeneous line broadening in models of lung, we derived a surface integral expression for the magnetic field shift produced when a structure composed of magnetic material with a magnetic susceptibility of a few parts per million is placed in an otherwise uniform magnetic field. The expression is integrated exactly for rectangular surfaces. NMR lineshapes for simple structures such as spheres, spherical shells, cubes, cubical shells, and rectangular parallelepipeds are calculated and presented. A physical interpretation of the results is given in terms of “thick” and “thin” magnetic material. Application of the results to inhomogeneous line broadening in models of the lung and correlation of NMR lineshapes with lung properties are discussed.