Arthur W. Guy

Therapeutic applications of electromagnetic power

The use of electromagnetic (EM) power for therapeutic applications has existed since EM sources have been available to man. Physical medicine has been a major user of both shortwave (27.33 MHz) and microwave (2450 MHz) diathermy over the decades in which the EM power has been used to heat deep tissues for stimulating various medically beneficial physiologic responses in the relief of certain pathological conditions. Experimental and clinical research indicates that these responses will occur as a result of elevating the tissue temperatures in the range 41° to 45°C requiring absorbed power densities from 50 to 170 W/kg in the deep tissues where treatment is desired. The combination of pain responses and a large reserve of blood cooling capacity seems to be sufficient for limiting the heating to safe, but therapeutic levels in vasculated and innervated tissue. Recent research has shown that the use of the industrial, scientific, and medical (ISM) frequency of 915 MHz is more efficient than the currently used 2450-MHz microwave frequency in terms of maximum power transfer to deep tissues. The results also show that in addition to thermal applications, microwave energy can be used for the controlled transcutaneous stimulation of nerve action potentials via implanted miniature microwave diodes.

Effect of 2450-Mhz Radiation on the Rabbit Eye

The cataractogenic effects of near-zone 2450-MHz radiation in rabbits are presented. The power deposition pattern inside the eyes and head of rabbits has been determined using a thermocouple technique. It was found that a peak absorption of 0.92 W/kg occurred between the lens of the eye and the retina for each milliwatt/square centimeter incident. Time and power-density studies indicated a cataractogenic threshold of a 150-mW/cm2 incident, or 138-W/kg peak absorption behind the lens for 100 min. The threshold time decreased with increasing power density. Agreement between in vivo intraocular temperature measurements and finite-element computer predictions reinforces the suggestion of a thermal mechanism for microwave-induced lens opacities.

Power Deposition in a Spherical Model of Man Exposed to I-20-MHz Electromagnetic Fields

The induced fields and the associated power deposition in mail exposed to HF electromagnetic (EM) fields have been investigated theoretically using spherical models. The induced electric fields inside the model exposed to either plane wave or near fields can be described adequately by a combination of quasi-static electric and magnetic induction solutions. It is shown that for field impedances less than 1200/spl pi//spl Omega/ the magnetically induced energy absorption predominates. Therefore, H fields must be measured to obtain any estimate of the hazards due to HF exposure. For a 70-kg model of man exposed to a plane wave field, the theory indicates that the time-average power absorption per unit volume is less than 2.5x10/sup -3/ mW/g for each milliwatt per square centimeter incident at 20 MHz and below. This suggests that the thermal safe-exposure levels for the HF band are many orders of magnitude in excess of the 10-mW/cm/sup 2/ level recommended for the microwave region.

Auditory perception of radio‐frequency electromagnetic fields

Absorption of pulsed microwave energy can produce an auditory sensation in human beings with normal hearing. The phenomenon manifests itself as a clicking, buzzing, or hissing sound depending on the modulatory characteristics of the microwaves. While the energy absorbed (∠10 μJ/g) and the resulting increment of temperature (∠10−6 °C) per pulse at the threshold of perception are small, most investigators of the phenomenon believe that it is caused by thermoelastic expansion. That is, one hears sound because a miniscule wave of pressure is set up within the head and is detected at the cochlea when the absorbed microwave pulse is converted to thermal energy. In this paper, we review literature that describes psychological, behavioral, and physiological observations as well as physical measurements pertinent to the microwave‐hearing phenomenon.

Low-level microwave irradiation and central cholinergic systems

Our previous research showed that 45 min of exposure to low-level, pulsed microwaves (2450-MHz, 2-microseconds pulses, 500 pps, whole-body average specific absorption rate 0.6 W/kg) decreased sodium-dependent high-affinity choline uptake in the frontal cortex and hippocampus of the rat. The effects of microwaves on central cholinergic systems were further investigated in this study. Increases in choline uptake activity in the frontal cortex, hippocampus, and hypothalamus were observed after 20 min of acute microwave exposure, and tolerance to the effect of microwaves developed in the hypothalamus, but not in the frontal cortex and hippocampus, of rats subjected to ten daily 20-min exposure sessions. Furthermore, the effects of acute microwave irradiation on central choline uptake could be blocked by pretreating the animals before exposure with the narcotic antagonist naltrexone. In another series of experiments, rats were exposed to microwaves in ten daily sessions of either 20 or 45 min, and muscarinic cholinergic receptors in different regions of the brain were studied by 3H-QNB binding assay. Decreases in concentration of receptors occurred in the frontal cortex and hippocampus of rats subjected to ten 20-min microwave exposure sessions, whereas increase in receptor concentration occurred in the hippocampus of animals exposed to ten 45-min sessions. This study also investigated the effects of microwave exposure on learning in the radial-arm maze. Rats were trained in the maze to obtain food reinforcements immediately after 20 or 45 min of microwave exposure.(ABSTRACT TRUNCATED AT 250 WORDS)

Microwave Heating of Simulated Human Limbs by Aperture Sources

Microwave heating of phantom models of human limbs by aperture sources is investigated theoretically and experimentally. These phantom models consist of triple-layered circular lossy dielectric cylinders. The three layers of dielectric materials simulate human tissues of fat, muscle, and bone. In the theoretical investigation, apertures operating in the frequency range of 433 to 2450 MHz are used as microwave sources for heating the dielectric materials. The theoretical investigation makes use of the technique of summation of cylindrical waves. A high-speed computer is used to calculate the numerical results. For the experimental investigation, an aperture is designed and built to operate at the frequency of 918 MHz. The resulting temperature patterns in the phantom models are detected by the use of a thermograph camera. The theoretical results are shown to be in agreement with the experimental results. The technique and results of this investigation may be applied towards the design of applicators for therapeutic heating of human tissues.

Determination of Power Absorption in Man Exposed to High Frequency Electromagnetic Fields by Thermographic Measurements on Scale Models

When the body of man, small compared to a wavelength, is exposed to high frequency (HF) electromagnetic (EM) fields, the absorbed power density patterns and total absorbed power may be approximated by the simple superposition of the internal electric fields obtained from the quasistatic coupling characteristics of the electric and magnetic field components determined independently. These characteristics were obtained for full scale man by thermographic studies of power absorption in scale models of man exposed to fields at frequencies scaled up inversely proportional to the model size. A VHF resonant cavity was used to provide the necessary field strengths for producing measurable power absorption patterns under simulated HF exposure conditions. The results indicate that peak power absorption densities as high as 5.63 W/kg can be produced in man exposed to 10 mW/cm2 31 MHz radiation fields. The results show that the absorption decreases as the square of the frequency as predicted by theory for frequencies below 31 MHz.

A quarter century of in vitro research: a new look at exposure methods.

The specific absorption rate (SAR) distributions in radio frequency–exposed solutions containing suspended or plated cells in vessels used for in vitro research were calculated by the finite-difference-time-domain method, graphed in color, and statistically analyzed in terms of uniformity for application to research on safety of wireless devices. The uniformity of SAR was quantified by visual inspection of colored plots, histograms, means, standard deviations, and maximums for the cell suspensions exposed in test tubes, Petri dishes, and rectangular flasks. Exposure sources included plane waves, transverse electromagnetic (TEM) cells, and striplines used at frequencies of 837, 2450, or 3,000 MHz. The results demonstrated that the most nonuniform SARs for plated or suspended cells in solution occurred for exposures of test tubes and rectangular flasks with plane waves, polarized for maximal absorption. The most uniform SARs for a layer of cells occurred for exposure of Petri dishes oriented for weakest coupling to the fields in a TEM cell. Additional improvement in uniformity was found to be possible by restricting the edge of the layer of cells from being too near the edges of the dish. It was not possible to achieve satisfactory uniformity in the SAR in cell suspensions exposed in standard vessels to any of the sources. The best but not satisfactory SAR uniformity was observed for cells suspended in the lowest 1-ml volume of the liquid contained in a test tube exposed at the bottom in a TEM cell. Experimental measurements of average SAR by temperature change for this case varied from 18% higher to 26% lower than finite difference time domain–derived values. The most uniform SAR distribution for cell suspensions in nonstandard containers was found for a rectangular slab configuration exposed in a stripline with the plates separated from the media by a thin layer of insulation. It is possible to experimentally implement this model by placing a fluid-filled thin-wall rectangular container tightly between the plates of a stripline. Bioelectromagnetics 20:21–39, 1999.

Development of a 915-MHz Direct-Contact Applicator for Therapeutic Heating of Tissues

The design of a 915-MHz diathermy dielectric-loaded applicator with a TE/sub 10/-mode aperture field distribution is described. The lightweight porous dielectric used for loading the applicator allows for the transmission of refrigerated air through the cavity to provide surface cooling so therapeutic temperature can be produced in deep tissues without excessive heating of surface tissues. The design is based on theoretical calculations previously developed by the authors which predict optimal size of the aperture and field distribution that would provide the best heating patterns in deep layers of tissue. Experimental evaluations of the heating of tissues of models and human beings are discussed.

Calculations Of Therapeutic Heat Generated by Ultrasound in Fat-Muscle-Bone Layers

An analytical approach to obtain the relative heating pattern due to the propagation of ultrasonic waves in a fat-muscle-bone layered tissue system is presented. The effects on the relative heating pattern due to the change of incident angle and the change of frequency are discussed in this paper. The contribution to heating due to mode conversion at the bone surface is also discussed.