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Radiofrequency Radiation
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Radiofrequency Radiation
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Except for Section 7.2.6, this chapter was written by S. J. Allen, then Project Scientist, U.S. Air Force School of Aerospace Medicine. He adapted Section 7.1 from material written by Arthur W. Guy, Ph.D., Professor of Bioengineering, University of Washington. Dr. Guy's material was later published in an IEEE reprint volume (Osepchuk, 1983).
Early dosimetric techniques were developed when electromagnetics were first used in physical medicine. Attempts to quantify exposures consisted of measuring the radiofrequency output current to the electrode or coil applicators of short-wave diathermy equipment. This technique proved to be inadequate, and Mittleman et al. (1941) conceived and conducted the first true RF dosimetry by quantifying the temperature rise in exposed tissue in terms of volume-normalized rate of absorbed energy, expressed in watts per 1,000 cm³. Little use was made of this work by personnel involved either in microwave therapy or later in bioeffects research. Virtually all reported biologic effects were related to incident-power density, making it difficult, if not impossible, to correlate data from animal experiments to those expected in man.
In the early 1950s Schwan (1948, 1953, 1954), Schwan and Piersol (1953), Schwan and Li (1957), and Cook (1951, 1952) developed the foundation for later analytical work by characterizing the properties of biological tissues. This allowed Schwan and co-workers to use simple models consisting of plane layers of simulated muscle, fat, and skin to analyze microwave fields and their associated patterns of heating in exposed tissues (Schwan and Piersol, 1953; Schwan and Li, 1956; Schwan, 1956a, 1958, 1960). Such predictions were confirmed by Lehmann et al. (1962, 1965) in the early 1960s.
During the period of triservices-supported research, beginning in the mid-1950s and continuing through 1960, preliminary experimental approaches to microwave dosimetry were made. Schwan (1959) and his colleagues described methods that could be used to fabricate tissue-equivalent phantom models and portions of the human anatomy and to experimentally measure energy absorption during exposure to microwave radiation (Salati and Schwan, 1959). Mermagen (1960) reported using tissue-equivalent phantoms to study energy-absorption characteristics as a function of animal position in a near-field exposure. He recommended measurement of watts per cubic centimeter in an absorber whose dielectric constant is similar to that of tissue and whose volume would represent a finite attenuation of microwave beams, again similar to that of tissue.
Franke (1961) made the first analyses of biologic models that revealed frequency dependencies in the coupling of electromagnetic fields to electrically small bodies. These analyses were discussed later in Pressman's book (1970). The models Franke used varied from circular cylinders to prolate spheroids of homogeneous muscle-equivalent dielectric. Volume-normalized energy absorption rate and heat produced in prolate spheroidal models of man were determined at lower RF frequencies, using quasi-static mathematical solutions. In other theoretical studies, Schwan's group used spherical models to determine relative absorption cross sections as a function of frequency of the incident RF field (Anne et al., 1960). The absorptive cross section varied widely with frequency, reaching a maximum at the model's resonant frequency. From the mid to late 1960s, Guy and his colleagues (Guy and Lehmann, 1966; Guy et al., 1968) developed and used more realistic tissue simulating models for experimental measurements of field coupling.
In the late 1960s Justesen and King (1969) introduced mass-normalized RF dosimetry using phantom models in laboratory bioeffects research. During the same time period Guy and colleagues (Guy et al., 1968; Guy, 1971a), through the technique of thermography, developed distributive dosimetry measurement of the anatomical distribution of local SARs in biological models and in the bodies of small mammals. Mumford (1969) pointed out the importance of considering environmental factors such as temperature and humidity rather than focusing only on dosimetric quantities.
The 1970s saw a significant increase in the development of experimental and theoretical approaches to definition of SAR in man and animals. Gandhi et al. (Gandhi, 1975a; Gandhi and Hagmann, 1977a; Gandhi et al., 1977) initiated the first in a succession of analytical and empirical studies of electrical and geometrical constraints on SAR. An animal's orientation with respect to the vectors of an incident planewave proved to be a powerful controlling influence on the quantity of energy absorbed in an RF field.
During the 1970s numerous new techniques and exposure devices were developed. The twin-cell calorimeter developed by Hunt et al. (1980) and Phillips et al. (1975) allowed accurate measurement of whole-body SAR, as did the continuous integration of momentary energy absorption rates via differentiation of transmitted and reflected power in a special environmentally controlled waveguide system developed by Ho et al. (1973). A transverse electromagnetic (TEM) mode chamber--developed by Mitchell (1970) at the U.S. Air Force School of Aerospace Medicine (USAFSAM)--allowed, for the first time, measurement of SAR in large animals (monkeys, pigs, and dogs) in uniform and well-defined fields, thus letting SAR be compared with incident-power density. Guy and Chou (1975) and Guy et al. (1979) developed a circularly polarized cylindrical-waveguide exposure system that enabled precise control of SARs and virtually continuous irradiation of small animals that are fed and watered and have their excrement accumulated with minimal disturbance of the field.
Theoretical work and the development of instrumentation for quantifying electromagnetic-field interactions with biological materials increased substantially in the 1970s, with many important developments. Interactions of planewave sources with layered tissues were studied (Guy, 1971; Ho et al., 1971; Ho, 1975b, 1977). New and novel instruments provided better characterization of exposure fields (Aslan, 1971; Bowman, 1972; Hopfer, 1972; Hassan and Herman, 1977; Hassan, 1977). More sophisticated mathematical and physical models of biological tissues and anatomical structures generated a much better understanding of absorptive characteristics of tissues of differing composition and geometry.
Theoretical human-head models that consisted of a "brain" and spherical shells to simulate the skull and scalp led to Shapiro's observation that electrical "hot spots" (localized regions of intensified energy absorption) could occur deep within the brain at frequencies associated with resonance (Shapiro et al., 1971). Due to the high dielectric constant and the spherical shape of the head, focusing of the field caused considerably less RF-energy absorption at the surface of the head. The theoretical results were expanded and verified experimentally to predict energy absorption by animal and human bodies of a wide range of sizes and exposed at various frequencies (Johnson and Guy, 1972; Ho and Guy, 1975; Ho and Youmans, 1975; Kritikos and Schwan, 1975; Lin et al., 1973b; Weil, 1975; Joines and Spiegel, 1974). Better and more detailed analyses were developed for various geometric objects that simulate bodies of man and animals; i.e., cylinders (Massoudi et al., 1979aHo, 1975a, 1976; Wu, 1977), prolate spheroids (Durney et al., 1975; Johnson et al., 1975; Barber, 1977a), and ellipsoids (Massoudi et al., 1977a, 1977b, 1977c).
Theoretical analyses were verified experimentally via thermography (Johnson and Guy, 1972; Guy et al., 1974a, 1974b), newly developed calorimetric techniques (McRee, 1974; Allis et al., 1977; Blackman and Black, 1977), special temperature-sensing probes composed of microwave-transparent materials such as fiber-optics guides (Rozzell et al., 1974; Gandhi and Rozzell, 1975; Johnson and Rozzell, 1975; Cetas, 1976) and high-resistance leads (Bowman 1976; Larsen et al., 1979), and differential power-measurement technique: (Allen, 1975; Allen et al., 1975, 1976). Cetas (1977) has reviewed and discussed the relative merits of these different techniques. Also, probes were developed to directly measure electric fields within exposed tissues (Bassen et al., 1977b; Cheung, 1977). The microwave transparent materials used for new dosimetry instrumentation were also used in recording physiological signals from live laboratory animals under microwave exposure (Chou et al., 1975; Tyazhelov et al., 1977; Chou and Guy, 1977b).
Numerical studies in conjunction with high-speed computers were developed along with sophisticated programs for calculating the electromagnetic fields and the associated heating patterns in arbitrarily shaped bodies (Chen and Guru, 1977a, 1977b, 1977c; Neuder, 1977; Gandhi and Hagmann, 1977a; Hagmann et al., 1977, 1978, 1979a). Mathematical models included the effects of convective cooling via blood flow in calculating steady-state temperatures for various parts of the body, including critical organs such as the eyes and the brain (Chan et al., 1973; Emery et al., 1975, 1976a). The continued development and use of phantom models has contributed significantly to our understanding of energy absorption in biological bodies (Guy, 1971a; Guy et al., 1974a, 1974b, 1977; Allen, 1975; Allen et al., 1975, 1976; Cheung and Koopman, 1976; Chou and Guy, 1977a, 1977b; Balzano et al., 1978a, 1978b). For a brief summary of the theoretical methods and experimental work found in the literature, see Sections 5.3 and 7.3.
When simultaneously applied in various laboratories, these new developments of the 1970a allowed the microwave hearing, or "Frey effect," to be quantified and understood as a complex thermal acoustic phenomenon--which for nonbiological materials had been quantified by the physicists more than a decade earlier (Chou et al., 1975; Guy et al., 1975b; Foster and Finch, 1974; White, 1963; Gournay, 1966).
The collective advances in RF radiation dosimetry culminated in publication of the first dosimetry handbook for RF radiation by C. C. Johnson and colleagues of the University of Utah and USAFSAM (1976). The collective results were empirically cast in a succinct form by Durney et al. (1979). Subsequent editions of the handbook were supported and published by USAFSAM to maintain documentation of advancements in the overall state of knowledge in RF dosimetry (Durney et al., 1978, 1980).
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Last modified: June 14, 1997
© October 1986, USAF School of Aerospace Medicine,
Aerospace Medical Division (AFSC), Brooks Air Force Base,
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