Techniques of Application of Electromagnetic Fields
Electric field. A typical laboratory arrangement for the application of an electric field to a test subject is depicted in figure 4.4. If the linear dimensions of the plates exceed the distance between them by greater than a factor of 2-3, then the electric field, measured in volts (v) or kilovolts (kv) per meter (m), is relatively homogeneous and, at least in the absence of the test subject, it is given by V/d. Reliable methods for measuring the electric field have recently been developed (61). An exposure assembly that is suitable for use with small animals at low frequencies is shown in figure 4.5.
Fig. 4.4. Arrangement for application of electric fields.
Fig. 4.5. Our small-animal exposure assembly. The stalls consist of metal plates sandwiched between layers of wood. By appropriately energizing or grounding the plates, adjacent electric-field and control regions can be produced.
In theory, the dielectric constant and the conductivity of the subject determine the strength and distribution of the electric field that penetrates into its tissue. However, because of the structural complexity of biological tissue, both constants are point functions that vary with location over cellular (and smaller) dimensions, and reliable methods for their functional determination under physiologically realistic conditions do not exist.
If the structural organization of tissue is not considered, it is possible to measure a sort of average tissue constant-the dielectric constant of a 1-cm. thick plate of brain tissue, for example. Such data can be used to analyze what might be called the physical reactions of biological tissue- reactions such as heat production and induced current that do not depend on whether the tissue is alive or dead. But even the average-value tissue constants are difficult to measure, and, as a result, the reported values for specific tissues vary over several orders of magnitude (62-64).
The absence of reliable tissue-constant data prevents the calculation of unique, meaningful internal electric fields. We considered, for example, several of the common physical models for biological systems, including the sphere, sphere-in-a-sphere, ellipsoid, and rectangular solid (65-68). Using typical average-value tissue constants, we found that the calculated value of the internal fields varied by a factor of 2-100 depending upon the assumed values of the tissue constants and the particular model. When the analysis was broadened to include the transient response, or when more complicated geometric models were considered, the range of arbitrariness bracketed by the calculations was even larger. Efforts to take into consideration the point-to-point variations of the constants would result in even further uncertainty.
Because the internal fields cannot be quantified, the actual "dose" of electric field received by test subjects in electric-field studies is not well defined. This poses no problem with regard to the characterization of the dependent parameter in any particular experiment, because it is always possible to specify the strength of the applied electric field-the field that exists before the presence of the subject. Moreover, the applied field is an appropriate measure by which to compare different experiments on the same species. There is a problem, however, in relating electric-field studies that involve different species. Because of shape and structural differences, the amount that various animals perturb the applied electric field, and the dose they receive from it, vary greatly-a factor of 5-10 would not be surprising. Thus, for example, a particular biological effect observed in rats following exposure to I kv/m would not necessarily be expected to occur in monkeys at that field strength.
Magnetic field. Figure 4.6 depicts a typical laboratory arrangement for the application of a magnetic field to a test subject. The current through the coils gives rise to a magnetic field that is reasonably uniform near the common axis of the coils; the strength of the field, measured in gauss, can be calculated from the knowledge of the coil current and geometry, and it can be measured by means of a small calibrated induction coil. A coil-exposure system suitable for use with human subjects is shown in figure 4.7 (69).
Fig. 4.6. Arrangement for application of magnetic field.
Since tissue is magnetically transparent, an applied magnetic field completely penetrates the subject's tissues, where it induces an electric field that is proportional to its rate of change. The induced electric field does not depend on the tissues' electrical constants, but it does depend on the shape of the test subject and on its position within the subject (68). For these reasons we again find that the concept of dose is not well defined, and that the applied field, therefore, is the appropriate dependent parameter in magnetic field studies.
Fig. 4.7. The large-coil exposure system at the Naval Aerospace Medical Research Laboratory. (Reproduced, by permission, from D. E. Beischer, et al., Exposure of man to magnetic fields alternating at extremely low frequency, NAMRL 1180, Naval Aerospace Medical Research Laboratory, Pensacola, Florida, 1973.)
Electromagnetic radiation. The electric and magnetic fields depicted in figures 4.4 and 4.6 are, in a sense, bound to their respective sources: the electric field is associated with the voltage on the metal plates, and the magnetic field arises from the current in the coil. Electromagnetic radiation is a propagating physical entity consisting of inseparable electric and magnetic fields. It has its origins in electronic transitions in a source-typically an antenna-with which it has no physical link once it is liberated. The plane wave, in which the electric and magnetic fields are orthogonal to one another and to the direction of propagation of the wave, is the simplest and most important type of electromagnetic radiation. The power density, P, of a plane wave is measured in terms of power per unit of area traversed. We shall express it in microwatts per square centimeter (µW/cm2). The relation between P and the strength of the electric field, E, (in v/m) is:
Fig. 4.8. Arrangement for application of electromagnetic radiation.
A typical laboratory arrangement for the application of electromagnetic radiation to test subjects is shown in figure 4.8. The power density can be measured in the wave-guide or in the space near the subject. An exposure system used for the exposure of chick embryos is shown in figure 4.9 (70).
Fig. 4.9. System for the exposure of eggs to electromagnetic plane waves. (Reproduced, by permission, from D. L McRee et al., Ann. N.Y. Acad. Sci. 247, p. 377.)
Although there are many studies dealing with the penetration of electromagnetic radiation into a plethora of mathematical, metallic, and saline models of living systems, little more is known about penetration into actual tissue than was known in 1888 when radiation was discovered by Hertz.