driving, electromagnetic compatibility (EMC) problems with prostheses such as pacemakers, and interference with safety critical medical and control equipment.

This chapter considers how guidelines for human exposure to radio frequency (RF) are derived, known interactions with human tissue and their measurement, and the evidence for the existence of health effects.

26.2 Radio Frequency Effects in Biological Tissues

26.2.1 General Tissue Properties

The velocity of propagation of electromagnetic waves through tissue is decreased compared with that of free space and this can be regarded as a result of increased capacitance. Additionally, the impedance of tissue compared with that of free space is low compared with the 377 Ω of free space. This has three effects on a wave meeting a tissue. First, there is an impedance mismatch and some energy is reflected as it would in a change of coaxial line impedance; second, the wavelength of the field in the tissue is decreased; and third, the rate of attenuation is increased. The parameters that define these changes are the real and imaginary components of the relative dielectric constant ε. At microwave frequencies the values are determined largely by the water content; and because much of the content of human tissues consists of water (up to 85%), the properties of different tissues can largely be described by their water content. However, there are major differences in dielectric constant so that the value of ε is highest in blood, liver, brain, and muscle, and lower in bone, skin, and fat (in decreasing order). There are also, for example, differences between white (nerve axons) and gray (nerve cells) matter of brain, where ε is about 39 and 56, respectively, at about 1 GHz, reflecting the different fat content associated with myelin sheaths over the nerves. There is little evidence of resonances occurring in biological tissues (unlike pure water or ice) below about 100 GHz, but relaxational effects can be observed. These are degenerated resonances arising because of the sluggish nature or viscosity of the water on which the structural proteins of cells reside. These have been described by Schwan1 as three zones labeled α, β, and γ. Figure 26.1 shows how the complex permittivity (or dielectric constant) relates to frequency for a typical tissue (muscle) and decreases in a nonlinear fashion because of the degraded resonances.

FIGURE 26.1 Complex dispersion of a typical biological tissue with frequency illustrating the very large frequency dependence.

© 2002 by CRC Press LLC

In interpreting these relationships it becomes possible to relate the particular zones to the physical structure or characteristic of the tissue, and to some extent at low frequencies this reflects the physiological properties of the tissue. As stated previously, these zones are characterized by three regions, described as follows:

1.α region — This is dominated by counterion relaxation and electrophoretic relaxation. This characteristic is largely of live cells with intact membranes able to maintain a potential difference resulting from selective secretion of ions across the cell membrane.

2.β region — This results from inhomogeneous structures (Maxwell–Wagner effect, or interfacial polarization where an inhomogeneous structure shows frequency-dependent dielectric and conductive properties that differ from those of the constituents of the mixture). These properties characterize living or dead tissue that has not undergone significant autolysis (i.e., there is still structure present).

3.γ region — This is defined here by the behavior of free water and extends from a few MHz to about 20 GHz. There is a contribution from the rotational motion of amino acids, and in the region up to 2 GHz the effects of the presence of larger proteins increase the dielectric constant.

Large differences in tissue dielectric values range from ε = 5-15 for fat, ε = 49 for muscle, to ε = 56 for brain at about 1 GHz. The most important effects are (1) the contraction of wavelength in the tissue by ε, so that, for example, the length of a resonant antenna in muscle is n 49 of the free-space size, and (2) the losses determined by conductivity σ at lower frequencies, or defined by dielectric relaxational losses at higher frequencies. Conductivity increases with increasing frequency, which results in more limited penetration with increasing frequency (the rate of energy deposition increases).

26.2.2 Limitations of Animal Models

At 1 GHz the penetration in muscle is about 3.0 cm for plane wave state (far field), and less for nonplane wave (near field). This leads to the argument that experimental animal research at a particular frequency cannot mimic human exposure. A good example is the behavioral study in mice or rats where animals are exposed to 900-MHz, cell-phone-type irradiation. In a human, this irradiates perhaps 1 to 2 cm of the brain cortex and at 10W/kg would be dispersed by the blood flow into the large heat sink of the body, whereas in the rodent this would result in whole body thermal stress. Scaling frequency to the animal is beset with other problems because the interaction with tissue also changes with frequency as described earlier. Therefore, animal research relating to RF exposure needs to be treated with caution, particularly where this involves complex physiological interactions such as for cognitive studies. Also the question of thermal stress is very different in man and animals.

26.2.3 Measurement Techniques

Measurement in plane wave (far-field) conditions are relatively straightforward even in conditions of high attenuation rate as occurs in tissue, but if the source is close to the target volume (e.g., cell phone close to the head) there are additional problems. Not only are the wave fronts curved and varying spatially along the source, but near-field measurement is difficult because of the arbitrary phase relationship of the three components of each of the fields. It is usual (in air) to infer equivalent plane wave power density D from

	D =				E		2
		377									(26.1)
or	D =	377							H			2

In high dielectric media such as water (ε = 80) or muscle (ε = 49), the apparent wavelength contraction of by ε as described in Section 26.2.1 suggests that the effective distance for near-field conditions of 0.5 to 1 λ usually enables plane wave conditions to be assumed.

© 2002 by CRC Press LLC

26.2.4 Measurement Probes for Human Exposure Assessment

1.Radiation hazard meters are available for different frequency ranges, and for most applications an isotropic probe is preferable to cope with unknown polarization for near-field measurements, and where there may be multipath reflections. An H-field probe is needed for near-field conditions, in particular. Below 100 MHz, induced currents and contact currents are important and need to be considered in addition to spatially averaged E- and H-field measurements. However, at high RF and microwave frequencies it is convenient and usually accurate to measure either the E- or H-field component and to relate these on the basis of the medium in which measurements are to be made, whether this is air or tissue.

2.Probes should be specific to the field parameter (e.g., E-field probes should not respond to H- fields). This can be achieved by ensuring that there are no conducting paths between the E-field plates and that the electronics are not sensitive to induced pickup; or conversely, in B-field probes there are no open-circuit components susceptible to electric fields.

3.Probes need to be small in dimension compared with the wavelength in the medium at the highest frequency of interest. At ELF this is no problem, but if the probe is large compared with the wavelength, it becomes very sensitive to both position and orientation and underestimates the field.

4.Probes should be isotropic — this can be achieved with three orthogonal dipoles or loops. An alternative arrangement is to use a monopole set at 270° that can be rotated in the field to detect radiation in x-, y-, and z-axes.2

5.Probes need to be nonperturbing — this is attainable by use of resistive leads of small physical cross section, such as carbon-loaded plastic, or carbon monofilament of similar conductivity to the tissue or medium.

6.Probes should also be reasonably accurate — an error of <3 dB is adequate for protection purposes; although for calibration, 10% would probably be as good as could be reasonably achieved. Calibration must be done in the medium in which the probe is to be used; and although this can pose a problem, there are techniques using a loaded waveguide with a window dividing the air section from that loaded with medium in which the transition point can be used to compare in-air and in-medium sensitivities.

E-field probes have been reviewed by Chou et al.3 and Stuchly4 and these fall into the following different groups:

1.Simple diode-based probes are effective up to 1 Wcm–2 over the range 400 kHz to 12 GHz but have a complex power/output relationship because of the square law behavior. At lower levels the output voltage is proportional to [E]2 or [H]2, but at higher levels is proportional to E and H

directly. Diodes also have a high-temperature coefficient (0.05 dB °C–1). The use of resistive, nonperturbing leads typically with static impedance of about 100 KΩ/m to 1 MΩ/m gives a high

time constant leading to difficulty in measurement of short pulses, particularly where these are high with respect to the average power. Diode probes need to be calibrated at the frequency and intensity of interest, but are relatively resistant to damage by overload.

2.Thermocouples can be linked to form a linear resistive dipole and these offer good linearity with a square law characteristic but again are slow in response.

3.Electro-optical sensors with internal or external modulation offer fast response and recording of both phase and amplitude and have been reviewed by Stohr et al.5

4.Multiple arrays of all these devices are possible, but particularly of diodes, to give specific absorption rate (SAR) surface mapping on real or simulated human bodies (phantoms). A typical example

is an array made by Szentpali et al.6 who used a probe fabricated by thick film technology on a 125-µm polyester substrate on which were GaAs planar doped zero bias diodes connected to silver-

printed diodes. A similar construction of Schottky barrier diodes was used by Kaatee and Van Rhoon7 to carry out their study of temperature rise and SAR induced by mobile phones.

© 2002 by CRC Press LLC

The SAR is the rate of absorption or dissipation of energy (W) in unit mass (M):

SAR =	d dW				=	d		dW		(26.2)
	dt dm					dt		ρdV

where ρ is specific density.

Because the absorption or dissipation of energy results in heat, it is also possible to measure SAR as a temperature change by knowledge of the rate of temperature rise and the thermal capacity (specific heat) of the material or tissue.

26.2.5 Practical Measurements of Radio Frequency Absorption

These methods are always invasive if they are not to be restricted to surface exposure only and are only applicable to simulation techniques or phantoms. Currently, there is no easy method of measuring RF energy disposition in real bodies, although the use of functional magnetic resonance imaging (MRI) system at 2- to 3-tesla (T) field strength makes a measurement of a relatively large SAR feasible, based on the relative shift in T1 with temperature. Different material (e.g., fat and water) decay amplitudes have different resonant frequencies and therefore different temperature coefficients, and the combination of these can result in highly variable results. This is because the coefficients for fat and water have not only different amplitudes, but also different signs. The change observed with temperature is therefore not defined for different tissues, but MRI can be used for comparative measurements of single tissue entities.

Unfortunately, MRI is the only practical noninvasive measurement method and therefore has its uses in safety research. In practice, the very lowest temperature rise that can claim to be detected is about 0.3oC (personal study and as demonstrated by Yablonskiy et al. on a 1.5 T seimens (S) MRI),8 which is a similar temperature rise as that resulting from prolonged exposure at the current guideline.

In practice, it is necessary to use a phantom that is constructed from tissue equivalent materials (TEM) to simulate the real (ε′) and imaginary (ε″) dielectric components at the frequency of interest. In addition, the phantom can be constructed to be complex to represent the tissue layers, or simply canonical in design. A simple phantom may consist of a plastic cube or sphere containing 1.5-kg saline (0.9% NaCl in water), or may be improved by use of a sucrose/saline or ethandiol/water brain TEM. This can be further improved by a two layer skin simulation, or by including skin, fat, muscle, bone, eye, etc. to give an anatomically correct phantom that can be imaged either for direct temperature change, or to provide a basis for modeling. Dielectric values for phantom construction can be obtained from References 9 and 10.

26.2.6 European Committee for Electrotechnical Standardization Phantom

One problem with the previously described approach is that no TEM has ever been developed that can simulate electrical properties of tissue and that is stable. Solid dielectrics can be high loss or high permittivity, but not both, in the way human tissue presents itself. Even when constructed as a gel, such TEMs contain mobile liquid that diffuses from the layers. The use of nonpermeable layers to constrain liquid movement is not possible because this introduces more interfaces. This poses a problem for international standards because no two measurement phantoms would be the same if constructed from unstable materials with a short life. The new European Committee for Electrotechnical Standardization (CENELEC) phantom uses a different approach and has designed a specific anthropomorphic mannequin (SAM) for SAR measurements consisting of a thin low-loss, low-permittivity shell (ε = <5, i.e., loss tangent <0.05) containing a uniform liquid. This is dielectrically a compromise with values lowered (e.g., ε = 42, σ = 0.99 S/m at 900 MHz) to take account of the skin, fat, and bone between the source and the brain. Because this is liquid, then scanning of the probe is possible allowing the SAR in 1 or 10 g of tissue to be calculated for different regions as is required in the guidelines described later in Section 26.5. This is likely to be the standard for all areas except perhaps the United States.

© 2002 by CRC Press LLC