An Unbalanced Electric Applicator for RF Hyperthermia

An applicator of the electric type for the RF heating of living tissues is described. It consists essentially of an unbalanced capacitor where the RF current return is provided by a large groundplane electrode. A water layer is an integral part of the electric design, offering a relative independence from variations in the position of the electrodes with respect to the body being treated. The water also eleminates the detrimental effects of the high field concentrations at the edges of the "hot" electrode, and is additionally used for surface cooling.     

Focused Array Hyperthermia Applicator: Theory and Experiment

The design and analysis of a focused linear array at 2450 MHz for microwave hyperthermia research is described. The array, which was submerged in deionized water to reduce its size and to provide a better impedance match to a high dielectric medium representative of human tissues, consisted of four titanium dioxide loaded horn antennas with apertures of 2.0 x 1.4 cm and a feed network with weighted phase shifts, Power radiation pattern measurements were made in planes ranging from 7.6 to 10.2 cm from the array to determine the focusing characteristics and beam spot size. Due to high attenuation in the medium, planes beyond 10.2 cm were not considered. The half-power beamwidth (HPBW) measured at the focal point was approximately 1.3 cm. The measured patterns were found to be in close agreement with theoretical predictions.     

A Direct-Contact Microwave Lens Applicator with a Microcomputer-Controlled Heating System for Local Hyperthermia

A direct-contact lens applicator for local microwave hyperthermia is proposed and developed with a computer-controlled microwave heating system. The applicator is a practical one that can converge the radiated electromagnetic field to deposit its energy deep in human tissues. The experimental results, which agree well with the theoretical ones, show that the applicator which operated at 2450 MHz could heat at twice the depth at which a simple and conventional waveguide applicator could heat. The experimental results using a developed computer system that supplies microwave energy and circulated cooling water to the developed applicator show that the fluctuations of temperature at the heating location in the human tissue model were maintained within +-0.3° C of the set temperature. The results of the phantom model and the animal experiment using the system with the applicator show that the maximum depth of noninvasive heating was more than 30 mm below the surface. These results are available for the clinical hyperthermia treatment of cancer.     

A Hyperthermia System Using a New Type of Inductive Applicator

A hyperthermia system working at frequencies around 150 MHz is described. It is useful for a penetration depth of 3-4 cm in tumor and muscle tissue. The system includes a new type of inductive applicator giving a beam-like distribution in the tissue, a controllable surface cooling system, and multipoint temperature measurements. A phase I clinical study shows the efficacy of the system, but also the importance of temperature measurements at many points since, in general, the temperature gradients are large.     

Analysis of the Radiation Leakage for a Four-Aperture Phased-Array Applicator in Hyperthermia Therapy

We develop a criterion for the justified neglect of the field leaking from the regions between the horn apertures in a phased-array applicator. The condition is (b/delta)psi <<1, where delta is the skin depth of the conductive material, b is the radius of the torso or limb, and psi is the wedge angle between the horn apertures.     

Low-Frequency Electrical Characteristics of RF Shielding Materials

Values of initial magnetic permeability relative to free space have been obtained by means of novel techniques which permit measurements on material in the form of a flat sheet, a box, or any other configuration containing at least a six-inch square flat surface. As a by-product of the technique, electrical conductivity values may also be obtained.     

Experimental Investigation of a Retro-Focusing Microwave Hyperthermia Applicator: Conjugate-Field Matching Scheme

A seven-element array of dielectric-loaded open-ended waveguides totally immersed in a water tank is tested as a possible hyperthermia applicator. Experimental results show a substantial increase in focusing ability of the array if a conjugate matching scheme is used to adjust the phase of each element excitation. This scheme could offer a practical procedure for operating a focused hyperthermia applicator in a living patient.     

A 432-MHz Local Hyperthermia System Using an Indirectly Cooled, Water-Loaded Waveguide Applicator

A water-loaded, indirectly cooled wavegnide antenna with a small aperture size (2 x 5.5 cm/sup 2/) has been developed for local hyperthermia applications. The operation frequency is 432 MHz. The indirect cooling is achieved by circulating water in an external chamber having common walls with the waveguide, and good thermal and electrical stability is achieved for radiation powers up to 100 W. The water loading provides a good coupling of the radiated power into tissue. The observed variations in standing wave ratios and the concentration of the specific absorption rates (SAR's) are found to be satisfactory for localized superficial hyperthermia treatments. The matching of the water-loaded antenna into the power generator requires special attention because of two possible matting conditions corresponding to high- and low-power dissipations inside the antenna cavity. SAR distributions inside tissues have been computed and compared with thermographic measurements performed in phantoms. Preliminary results of trials in Wistar rats transplanted with Walker sarcoma are also reported.     

Development and Testing of a 2450-MHz Lens Applicator for Localized Microwave Hyperthermia (Short Paper)

A new type of applicator with a convergent lens for localized microwave hyperthermia is developed. A lens applicator of direct contact type was designed to conduct actual and progressive experiments with phantoms of simulated fat and muscle tissues heated at 2450 MHz. The experimental results showed that the heating power penetration depth increased 40 percent with this applicator as compared to a simple rectangular waveguide applicator with the same size aperture that had generally been used for microwave hyperthermia. Our applicator had a concave-shaped aperture and was designed to contact well with the heating medium whose shape was cylindrical like a human body.     

Clinical RF Hyperthermia by Magnetic-Loop Induction: A New Approach to Human Cancer Therapy

There has been mounting laboratory evidence that temperatures of /spl ges/ 42°C (108°F) are tumoricidal in tumor cell cultures and animal models. Localized heat by electromagnetic waves appears to be the most practical means for producing hyperthermia and has been shown to be potentially effective against human surface tumors. However, attempts to treat deep internal human cancers with available techniques have been either ineffective or dangerous because of injury to surface tissue. A fundamentally new approach magnetrode magnetic-loop induction hyperthermia, which was cooperatively developed by electrical engineers and oncologists for the treatment of deep-seated human tumors, is the subject of this report. The concept, rationale, design, and performance of this applicator in phantoms, animals, and humans is presented, as are the early results of clinical cancer trials. The data suggest reason for enthusiasm for the future treatment of advanced human tumors.     

A Differentiator for Low-Frequency Signals

An electronic differentiator specifically for low-frequency signals is described. The difference between a signal and its staircase approximation is computed; the peaks of this function approximate the time derivative of the input signal. By regulating the width of the steps in the staircase function, very slow signals can be differentiated.     

All-MOS ASK Demodulator for Low-Frequency Applications

A miniature amplitude-shift-keying (ASK) demodulator without any passive elements, i.e., R or C, for low-frequency applications is presented in the paper. The noise margin of the envelope detector in the proposed ASK demodulator is enlarged such that any Schmitt trigger or current limiting resistor is no longer needed. It results in the number of transistors required for the ASK demodulator circuit is reduced to 12, while the area is merely 0.003025 mm² using 0.35-mum 2P4M CMOS process. The power consumption is found to be 1.01 mW by physical measurement on silicon. The data rate is measured to be 250 kbps for 2-MHz carrier frequency and 27% modulation index.     

Low-Frequency Model for Rectangular Semi-Anechoic Chambers

An efficient and accurate method to compute the electromagnetic field inside a rectangular semi-anechoic chamber from 30 to 100 MHz is provided. A modal analysis method is used to develop the homogeneous and inhomogeneous solution. They are then combined with a generalized circuital analysis to take into account the effect of the several layers of ferrite and dielectrics on the walls. Several numerical results of the normalized site attenuation (NSA) of a semi-anechoic chamber are provided and compared with measurements, showing a very good agreement     

A Diathermy Applicator for the Irradiation of Rat Brain

A direct contact microwave applicator, constructed of dielectric-loaded J-band waveguide, was designed for local irradiation of the rat head with 2450 MHz CW microwaves. A stereotaxic method was used to achieve precise and reproducible orientation of the Applicator with the animal's head. Calorimetric measurements of specific bsorption rate, watts/kg, for the whole head indicated that approximately 62 percent of the net power applied to the applicator is absorbed in the head, and that a reproducible exposure condition could be achieved from one animal to the next. Thermographic pictures were taken of the bisected rat head immediately after exposure, and showed that the maximum rate of energy absorption took place within the brain, in the thalamus and mesencephalon.     

A Waveguide Applicator for Sheet Materials (Letters)

A partially loaded waveguide applicator for uniform heating of high-attenuation sheet materials is described. Experimental results are presented to exhibit its performance.     

Interstitial Equal-Phased Arrays for EM Hyperthermia

Microwave radiating antenna arrays frequently have been inserted directly into cancerous tomors to heat directly and destroy the tumor cells. Example heating patterns are shown with a simple antenna design. An improved design is also described and power absorption patterns are calculated by a three-dimensional numerical model. This model is used to show effects of frequency and applicator design on the power pattern. The numerical model is also applied to show an improved power pattern with a synchronous (equal-phase) compared to a nonsynchronous array. The model shows that an increase in tip capacitance increases the heating at the tip.     

P-I-N Diodes for Low-Frequency High-Power Switching Applications

The development of high-power low-frequency diodes, conditions for their operation, and results measured in actual circuits are described. Harmonic distortion at 500 kHz and 2 MHz has been found to decrease with increasing diode lifetime and forward-bias current. Large reverse bias voltages are necessary at low frequencies to keep the RF voltage swing from penetrating the forward conduction region. The improvement of p-i-n diode lifetimes with thicker I-layers or with planar construction has been studied and the performance of these diodes in a routing switch is reported.     

Mini-Annular Phased Array for Limb Hyperthermia

The design concepts of a thin-shell cylindrical or annular phased-array of conformal strip radiators is shown to be suitable for deep heating of phantoms simulating the limbs of the human body. The Mini-Annular Phased Array (MAPA) is presently under clinical investigation for treatment of cancer of the limbs at a limited number of institutions. The effect of frequency and tissue conductivity is shown under simulated conditions for a few sample power-density patterns illustrating the deep focusing of the radiated EM fields. These are compared to a numerical model pattern which can be used to estimate the patterns in varying diameter limbs. The effect of offsetting the limb within the array to steer pattern of heat deposition has also been shown to produce desirable asymmetrical patterns.     

High Efficiency Mode Converter for Low-Frequency Gyrotron

A high efficiency quasi-optical (QO) mode converter for high-power, low-frequency gyrotron have been designed and tested. For low-frequency gyrotrons, the scales of the mode converter are comparatively small on the wavelength scale, thus causing significant diffraction losses. Over-1 MW power gyrotron with TE_8,3 cavity at 28 GHz have been developed, which has a high efficiency mode converter designed by the use of numerical methods for launcher optimization. This calculation is sufficiently optimized to maximize the fractional Gaussian content of the far field. The total transmission efficiency from the mode converter to output window is 94.7%. For the experimental result of first tube, the output power of more than 1 MW has been obtained with about 40% efficiency and output burn pattern agrees fairly with the calculated profiles, which imply the design appropriateness. Besides, the frequency dependence for diffraction loss is discussed, and these results give the guiding design principle of the mode converter for high-power, low-frequency and long-pulse gyrotrons.     

Low-Frequency Characteristic Modes for Aperture Coupling Problems

In this paper, a recently suggested general procedure which leads to an eigenvalue equation for aperture problems is specialized to the range of frequencies for which the maximum linear dimension of the aperture l is much smaller than the wavelengths, known as the Rayleigh region. As kl asymptotically approaches zero, we arrive at a set of two eigenvalue equations which aided by the edge condition, constitutes an alternative set of equations for a derivation of the quasi-static distributions characterizing the aperture.