William T. Joines

The measured electrical properties of normal and malignant human tissues from 50 to 900 MHz

The electrical conductivity and relative permittivity of malignant and normal human tissues were measured at frequencies from 50 to 900 MHz. The measurements were made between 23 and 25 degrees C using a network analyzer connected to a flat-ended coaxial probe that was pressed against the freshly excised tissue samples. The malignant tissues were of the following normal tissue origin: bladder, colon, kidney, liver, lung, lymph nodes, mammary gland, spleen, and testes. The normal tissues included: colon, kidney, liver, lung, mammary gland, and muscle. Normal tissue samples of bladder, lymph, spleen, and testes were not available. In general, at all frequencies tested, both conductivity and relative permittivity were greater in malignant tissue than in normal tissue of the same type. For tissues of the same type, the differences in electrical properties from normal to malignant were the least for kidney (about 6% and 4% average differences over the frequency range in permittivity and conductivity, respectively), and these differences were the greatest for mammary gland (about 233% and 577% average differences in permittivity and conductivity, respectively). To illustrate a potential use of these data in hyperthermia applications, frequency-selective heating of malignant tissue (modeled as a sphere) surrounded by host normal tissue is calculated from the measured electrical properties for certain tissues.

Effects of ELF Fields on Calcium-Ion Efflux from Brain Tissue in Vitro

It has been previously demonstrated that carrier waves of 50 and 147 MHz, when sinusoidally amplitude modulated at 16 Hz (ELF), can cause enhanced efflux of radiolabeled calcium ions from chick brain tissue in vitro. This phenomenon occurs only when the samples are exposed to specific intensity ranges of the carrier wave. Unmodulated carrier waves do not affect the ion efflux. Since the ELF signal must be demodulated from the carrier wave to be effective, a study of the efflux ehnancement due to the ELF signal alone may lead to an identification of the site of demodulation, as well as provide clues to the underlying mechanism. We report here that 16-Hz sinusoidal fields in the absence of a carrier wave can alter the efflux rate of calcium ions. The results show a frequency-dependent, field-induced enhancement of calcium-ion efflux within the ranges 5 to 7.5 V/m and 35 to 50 V/m (peak-to-peak incident field in air) with no enhancement within the ranges 1 to 2, 10 to 30, and 60 to 70 V/m.

Design of Yagi-Uda antennas using genetic algorithms

A method of using genetic algorithms to optimize the element spacing and lengths of Yagi-Uda antennas is presented. A method of moments code, NEC2, performs the task of evaluating each of the antenna designs generated by the genetic algorithm (GA) during the optimization process. To illustrate the capabilities of the method, the length and spacing of several Yagi-Uda antennas are optimized for various performance characteristics. The results are compared to published results from other optimization techniques and to well-designed equally spaced arrays.

Active microwave imaging. I. 2-D forward and inverse scattering methods

Active microwave imaging (MWI) for the detection of breast tumors is an emerging technique to complement existing X-ray mammography. The potential advantages of MWI arise mainly from the high contrast of electrical properties between tumors and normal breast tissue. However, this high contrast also increases the difficulty of forming an accurate image because of increased multiple scattering. To address this issue, we develop fast forward methods based on the combination of the extended Born approximation, conjugate- and biconjugate-gradient methods, and the fast Fourier transform. We propose two nonlinear MWI algorithms to improve the resolution for the high-contrast media encountered in microwave breast-tumor detection. Numerical results show that our algorithms can accurately model and invert for the high-contrast media in breast tissue. The outcome of the inversion algorithms is a high-resolution digital image containing the physical properties of the tissue and potential tumors.

Electromagnetic time-reversal imaging of a target in a cluttered environment

Electromagnetic time-reversal imaging is addressed for a target situated in a cluttered background. We first investigate the theory of electromagnetic time-reversal imaging, followed by an experimental demonstration. A transmitter-receiver antenna array is connected to a network analyzer and applied to transmit wideband waveforms for detecting a target within a cluttered environment. We assume the cluttered background is fixed, thus the target signature is extracted by observing changes manifested by the introduction of a target. A numerical algorithm is required for computation of the Greens function employed within the time-reversal imager, with this implemented here via ray tracing. Example time-reversal images of different cluttered backgrounds and different targets are presented using measured data, with comparisons to a traditional radar imaging technique. Results show that the time-reversal imagery yields good focusing at the target, significantly better than when the background is not accounted for.

Air-gap reluctance and inductance calculations for magnetic circuits using a Schwarz-Christoffel transformation

A method for calculating the reluctance of air gaps in magnetic circuits making use of a Schwarz-Christoffel transformation is described. The method is applied to the calculation of the inductance of example inductor and transformer configurations and its validity established through comparisons with calculations based on the finite-element method of numerical analysis. >

Microwave power absorption differences between normal and malignant tissue

Abstract An open-ended coaxial probe that produced a fringing field in the termination tissue was used to measure the fractional power absorption for malignant tumors and normal adjacent tissue in female W/Fu isogeneic rats over the 30 to 2000 MHz range. The fractional power absorption was determined as the difference in measured incident and reflected power divided by the incident power for SMT-2A tumors, and for normal adjacent muscle and mammary tissue. For the coaxial probe, the ratio of power absorbed in malignant to that in normal adjacent tissue peaked at 180 MHz with values of 4.28 and 1.40 for malignant/mammary and malignant/ muscle, respectively. By fitting the measured data to an equation for power absorption because of the fringing-field probe, tissue conductivity and dielectric constant versus frequency were determined. These tissue properties were used to determine the ratio of power absorbed in malignant to normal tissue versus frequency for a propagating plane wave. For plane waves, peak differential absorption occurs at 300 MHz; 4.81 and 1.46 for malignant/ mammary and malignant/muscle, respectively. Since direct-contact applicators for microwave hyperthermia have both fringing-field and plane-wave characteristics, our results indicate that any operating frequency from 180 to 300 MHz would provide a near optimum differential power absorption between malignant and normal tissue.

Active Microwave Imaging II: 3-D System Prototype and Image Reconstruction From Experimental Data

A 3-D microwave imaging system prototype and an inverse scattering algorithm are developed to demonstrate the feasibility of 3-D microwave imaging for medical applications such as breast cancer detection with measured data. In this experimental prototype, the transmitting and receiving antennas are placed in a rectangular tub containing a fluid. The microwave scattering data are acquired by mechanically scanning a single transmit antenna and a single receive antenna, thus avoiding the mutual coupling that occurs when an array is used. Careful design and construction of the system has yielded accurate measurements of scattered fields so that even the weak scattered signals at S21 = -90 dB (or 30 dB below the background fields) can be measured accurately. Measurements are performed in the frequency domain at several discrete frequencies. The collected 3-D experimental data in fluid are processed by a 3-D nonlinear inverse scattering algorithm to unravel the complicated multiple scattering effects and produce high-resolution 3-D digital images of the dielectric constant and conductivity of the imaging domain. Dielectric objects as small as 5 mm in size have been imaged effectively at 1.74 GHz.

EFFECTS OF LOW‐INTENSITY MICROWAVES ON ISOLATED NEURONS*

Surprisingly few studies of the effects of microwaves directly on nerve cells have been undertaken. Instead, most .of the evidence for or against a presumed neuronal effect has been extrapolated from studies of whole animals or from hypothetic models. With whole animal studies, considerable uncertainty exists as to dosimetry and specificity of effect a t the neuronal level. For modeling studies, serious questions often arise about the validity of the assumptions upon which the model is based. It seems to us, therefore, that isolated neuron preparations offer a useful approach in this area of investigation. In our laboratory, we have been utilizing ganglia from the marine gastropod Aplysia in studies of neuronal function for several years, and we are now using the same preparation to study the effects of microwaves on isolated nerve cells. In addition to the usual advantages of these ganglia for neural studies,’ they are much smaller in size ( I or 2 mm in diameter) than even the shortest length microwave we employ. This factor is extremely useful with respect to dosimetry and instrumentation. Our experimental approach consists of placing an Aplysiu ganglion within a microwave stripline and employing intracellular glass microelectrodes to record the electric activity of individual neurons before, during, and after the ganglion is irradiated. In addition to monitoring the incident and reflected microwave power levels, we have carefully recorded, and run controls for, ganglionic temperature. We have noted definite effects on the firing patterns of Aplysiu neurons at absorbed microwave power levels that are below what human brain cells would be exposed to a t the accepted American “safety” level (10 mW/cc). In large part, these effects are attributable to slight ganglionic warming, but in some cases, we have also found effects that are not accompanied by, or not reproduced by, ganglionic warming.

Resonance Absorption of Microwaves by the Human Skull

Resonance absorption of microwaves by the human skull is examined by making computerized calculations of theoretical models of the skull. The calculated relative absorption versus frequency is plotted and compared for homogeneous and inhomogeneous skull models. At a frequency of maximum power absorption, the spatial distribution of intracranial field intensity (based upon the theoretical model) is also calculated and plotted.