INTERACTION OF ELECTROMAGNETIC ENERGY WITH ABSORPTIVE MATERIAL BY THERMALLY INDUCING ELASTIC STRESS WAVES
Several central nervous system (CNS) alterations have been shown to occur as a result of low power, nonionizing electromagnetic radiation. This report covers an experimental investigation to document and explore one such effect: blood-brain barrier (BBB) permeability changes due to microwave exposure. In the course of this investigation, it was discovered, for the first time, that low power microwave exposure can increase the local cerebral blood flow of rats. This report also contains a theoretical investigation into a physical mechanism, the conversion of electromagnetic energy into elastic stress waves, which may account for several of the recently discovered, microwave induced, biological effects. In the experimental portion, rats were exposed to 1.3 or 2.8 gHz, pulsed or continuous wave (CW) microwave energy. Exposure time, incident power, and microwave pulse characteristics were varied. Local cerebral blood flow, blood volume, and blood-brain barrier permeability experiments were performed with several different, quantitative, radioactive isotope techniques with measurement by brain homogenization and liquid scintillation counting. The first experiments confirmed that low power microwaves could alter the uptake of small molecular weight saccharides in the blood-brain barrier of rats. It was further learned that the permeability increases occurred for small but not large molecular weight saccharides. The rises returned to normal after 24 hours and were of greatest magnitude in the medulla, cerebellum, and hypothalamus. The relationship between the magnitude of the increase and the pulse characteristics of the microwaves were determined. The absolute level of permeability could not be quantified because in the next set of experiments it was discovered that low power microwaves alter the local cerebral blood flow of rats, and all techniques capable of measuring small differences in permeability uptake rely on constant blood flow during the experiment. The theoretical portion investigated the conversion of electromagnetic energy to elastic stress waves by thermal expansion in biological media. This phenomenon is caused by the microwaves heating, in depth, the surface of an object, and thus creating a time varying thermal expansion which generates elastic stress waves in the media. The magnitude and characteristics of the elastic stress waves were determined for several different models of an animal's skull. First, an electromagnetic wave equation was solved to determine the magnitude of the electric field created in a material from a known electromagnetic field impinging upon it from air. A knowledge of this electric field along with the properties of the media gave the induced temperature as a function of time and position in the material. Second, with the induced temperature rise, a heat conduction equation was solved to give the overall temperature distribution in the material. Finally, with this temperature distribution, an elastic wave equation was solved to give the generated elastic (acoustic) wave. Expressions for both particle displacement and pressure waves were found for different boundary conditions. The solutions were found to consist of both a stationary part, whose effect is important only in the immediate region of the incident electromagnetic wave, and a traveling part which propagates through the material. The stress gradients thus generated by thermal expansion were then compared to those which would be induced by radiation pressure or electrostriction. It was found that thermal expansion was many times more effective than electrostriction or radiation pressure in converting electromagnetic energy to acoustic energy. It was also found that microwaves of average power density less than the current US safety standard could cause acoustic pressure pulses sufficient to explain several of the low power microwave effects on the CNS.