Physical Aspects of Radiofrequency Radiation Dosimetry

6 Physical Aspects of Radiofrequency Radiation Dosimetry 6.1 6.2 6.3

Introduction ........................................................... 119 Applications of RF Radiation Sources ............... 120 Physical Characterization of RF Fields .............. 121 Electric and Magnetic Field Vectors • Emitted and Received Power • Modulation • Polarization • Radiation Pattern • Fading

6.4 6.5 6.6

RF Exposure Parameters ......................................123 RF Field Zones........................................................123 RF Exposure Measurement ..................................125 Electric and Magnetic Field Strength Meters • Spectrum Analyzers • Power and Power Density Meters • Induced Current Meters • Contact Current Meters

6.7 6.8

Marko S. Andjelković and Goran S. Ristić University of Niš

Uncertainty in RF Measurement........................ 129 RF Dosimetry .........................................................130 SAR Measurement • Current Density Measurement • Experimental Dosimetry • Analytical Dosimetry

6.9 Biological Effects of RF Radiation...................... 134 6.10 Exposure Standards...............................................135 6.11 Conclusion ............................................................. 136

6.1 Introduction During the past two decades, environmental radiofrequency (RF) exposure has increased tremendously due to the rapid development and widespread use of wireless communication technologies such as mobile telephony, wireless networking, radar systems, TV and radio broadcasting, microwave ovens, and so on. As a consequence of this increased RF radiation exposure, concerns regarding possible adverse health effects 119

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Dosimetry in Bioelectromagnetics

associated with exposure to RF radiation have become a serious issue and a motivation for extensive scientific research [1–3]. However, RF radiation has many useful applications in medicine. It has been shown through numerous research studies that specific RF radiation exposure can be used under controlled conditions for diagnosis and treatment of various diseases [4–7]. Use of RF radiation in medicine is noninvasive and safe, and is therefore often regarded as an alternative to conventional medicine. For this reason, substantial research is being devoted to the exploration of medical uses of nonionizing radiation sources. To ensure maximum protection from the adverse health effects of electromagnetic radiation exposure, the national and international regulatory bodies have introduced reference levels for general public exposure and occupational exposure [8]. It is therefore necessary to conduct systematic monitoring of RF radiation exposure not only to check compliance with the prescribed limits but also to investigate the effects of RF exposure on human health. In this chapter, the fundamental physical aspects of RF radiation, which are essential for conducting practical RF dosimetric measurements and understanding the effects of RF radiation exposure, are presented.

6.2 Applications of RF Radiation Sources Among the sources of nonionizing radiation, RF radiation sources have attracted significant interest in the past few decades because of the rapid proliferation of electronic devices operating in the RF range. In the electromagnetic spectrum, RF radiation covers the frequency range up to 300 GHz and is divided into a number of subranges that are utilized for various commercial and special purpose applications [8–14]. RF radiation is employed in a wide range of applications. Typical applications can be divided into four major groups [13]: 1. 2. 3. 4.

General applications Safety applications Industrial applications Medical applications

General applications are related to the use of RF radiation in everyday life, and typical examples include: 1. 2. 3. 4. 5.

Mobile communication services (mobile telephony) Wireless communication services (Bluetooth, WLAN) Household devices (microwave ovens) Radio and TV broadcasting services Satellite communications

Safety applications are related to the use of RF radiation for safety purposes, and typical examples are: 1. RFID (radiofrequency identification) systems used in the identification and tracking of objects.

Physical Aspects of RF Dosimetry

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2. Electronic article surveillance for prevention of theft in establishments such as shops. 3. Military radar for controlling/tracking airborne objects. 4. Radar used in air traffic control for guidance and surveillance of civil planes. 5. Weather radar used in weather forecasting to identify precipitation data. Industrial applications are related to the use of RF radiation sources in manufacturing processes such as: 1. Induction heating: RF induction heaters are used extensively in industries for various applications such as surface hardening, zone hardening, and brazing. 2. Dielectric heating: Machines for dielectric heating are widely used in industries for welding. 3. Plasma discharge equipment: Plasma etchers are used in various stages of the semiconductor manufacturing process to break down polymer etch-resistant coatings. Medical applications refer to the use of RF radiation for diagnostic and therapeutic purposes in medicine. Some typical examples are: 1. Magnetic resonance imaging (MRI): A technique used in radiology to image the anatomy and the physiological processes of the body in both health and disease. MRI scanners use strong magnetic fields, radio waves, and field gradients to form images of the body. 2. RF ablation: A technique that uses contact electrodes to deliver low-frequency voltages in a wide variety of medical therapies. For over half a century, an electrosurgical knife (electrosurgery) has been used by surgeons as a replacement for the scalpel to cut and cauterize tissues. 3. RF telemetry: RF telemetry transmitters encapsulated in a small pill have been used to monitor internal body temperature and other physiological parameters. 4. Hyperthermia: A type of cancer treatment involving exposure of the human body to high temperatures, i.e., the human tissue is exposed to intense electromagnetic radiation to damage and kill the cancer cells.

6.3 Physical Characterization of RF Fields For accurate measurement of RF radiation exposure, the physical aspects of the RF fields have to be understood and their impact on RF exposure has to be taken into account. Thus, this section discusses the most important physical parameters of the RF fields [8,15].

6.3.1 Electric and Magnetic Field Vectors RF radiation consists of waves of electric and magnetic energy moving together (radiating) through space. Therefore, the RF field is characterized by a pair of vector fields of electric field E and magnetic field H (or magnetic induction B). Each field vector has

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three components that can vary throughout space and over time in terms of their magnitude and direction. Therefore, it is necessary to determine six functions of time at each point in space to characterize the field completely.

6.3.2 Emitted and Received Power The RF signal emitter by an RF source is characterized by the power, which decreases as the distance from the source increases in accordance with the inverse square law, P = k/r2. Thus, it is important to differentiate between the power emitted by the source (radiated power) and the power absorbed by an object (received power).

6.3.3 Modulation One of the fundamental aspects of any RF transmission system is modulation, i.e., the way in which the information is superimposed on the radio carrier. For a steady radio signal or “radio carrier” to carry information, it must be changed or modulated so that the information can be conveyed from one place to another.

6.3.4 Polarization The orientation of an electric/magnetic field vector in the plane orthogonal to the direction of propagation is called polarization. If the electric/magnetic field vector is oriented in a given direction, the wave is linearly polarized. If the electric field vector rotates around the direction of propagation, maintaining a constant magnitude, the wave is circularly polarized. If the extremity of the electric field vector traces an ellipse, the wave is elliptically polarized. The rotation of the electric field vector occurs in one of two directions, clockwise or counterclockwise.

6.3.5 Radiation Pattern Electromagnetic waves are radiated into space by means of antennas. The radiation pattern of the antenna determines the spatial distribution of the radiated energy. A pattern taken in a plane containing the electric field vector is referred to as an E-plane pattern, and a pattern taken in a plane perpendicular to an E-plane is called an H-plane pattern. The directional pattern of an antenna describes how much energy is concentrated in one direction in preference to radiation in other directions.

6.3.6 Fading Obstacles such as buildings, trees, etc. may cause an RF signal to be reflected during its propagation, thereby resulting in the change of its amplitude. Fading of the RF field signal is an important aspect to be considered in the estimation of RF exposure.

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Physical Aspects of RF Dosimetry TABLE 6.1

RF Exposure Parameters

Densitometric Parameters

Dosimetric Parameters

Parameter Name (symbol)

Unit of Measurement

Received RF power (P) RF power density (S or PD) Electric field strength (E) Magnetic field strength (H) Specific absorption rate (SAR) Specific absorption (SA) Current density (Id)

W or dBm W/m2 V/m A/m W/kg J/kg A/m2

6.4 RF Exposure Parameters Besides the fundamental parameters (frequency, wavelength, and amplitude), RF radiation is also characterized by a set of physical parameters that determine the level of RF exposure. The exposure parameters can be divided into two groups: • Densitometric parameters, and • Dosimetric parameters. Densitometric parameters describe the RF exposure in free space, whereas the dosimetric parameters provide information on the absorbed RF radiation in tissue. The most important densitometric and dosimetric RF radiation parameters, and the corresponding measurement units, are listed in Table 6.1 [8]. By measuring the densitometric RF field parameters, it is possible to only estimate the intensity of RF radiation in free space, thereby enabling the determination of the possible risks from RF radiation exposure. However, for estimation of the dosimetric aspects of RF radiation, i.e., the influence of RF radiation on the human body in terms of thermal and nonthermal excitation, the energy deposited in tissue has to be evaluated. Specific absorption rate (SAR) is used for quantifying the thermic effects of RF radiation, which are dominant at frequencies above 100 kHz, and is determined by measuring either the electric field in the body or the induced change of temperature. The current density is essential for characterization of the stimulation (nonthermal) effects for frequencies below 100 kHz, and is determined by measuring the current induced in certain parts of the body such as hands and wrists.

6.5 RF Field Zones For practical assessment of RF radiation exposure, an understanding of the RF field variations and relations between the RF field parameters is essential. The intensity of the RF field decreases with the distance from the source in accordance with the inverse square law (∼ 1/r2) [16].

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Besides the intensity of the field, the RF radiation pattern changes depending on the distance between the source of the RF field and the receiver of the RF field. The RF field zones can thus be divided into two categories [17,18] (Figure 6.1): • Near-field zone • Far-field zone. In the near zone, i.e., close to the source, the RF field is complex and the relationship between the electric and magnetic field components is not constant. Therefore, it is necessary to measure electric and magnetic fields separately to determine the contribution of each component to the total field. The near zone can be further divided into two zones: reactive zone and radiative zone. The reactive zone is closest to the source (transmission antenna), and in this zone the electric and magnetic fields are independent of each other. On the other hand, the radiative zone is further away from the source. The electric and magnetic fields are well correlated in the radiative zone, but there are changes in the angular distribution of electric and magnetic fields with increasing distance. In the distant (far) zone, the magnetic and electric fields are orthogonal to one another, and therefore it is much easier to determine their interdependence. The relationship between the electric and magnetic fields is constant in the distant (far) zone, as a result of the plane wave character of the field, which means that it is sufficient to measure only one component of the field and based on this the other component can be calculated. Of course, in practical applications, it is important to know the boundaries between the RF field zones. In principle, there is no clearly defined border, but it depends on the characteristics of the field, i.e., the wavelength of the signal to be transmitted, as well as the dimensions of the transmitting antenna. According to the international conventions, the criteria for determining the boundaries between RF field zones were established, and these are listed in Table 6.2.

Far Farfield field

Near field Non-radiative (reactive)

FIGURE 6.1

RF fields in near and far zones.

Radiative Radiative

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Physical Aspects of RF Dosimetry TABLE 6.2

Criteria for Boundaries of RF Field Zones Reactive Near-Field

Distance between transmitting and reception antenna (r) Relation between E and H Parameter to be measured

Radiating Near-Field 2

Far-Field

r 2D2/λ

E/H ≠ Z0 E and H

E/H ≈ Z0 E or H

E/H = Z0 E or H

In Table 6.2, λ is the wavelength of the emitted electromagnetic signal, while Z0 is the characteristic impendace of air and its value is 377 Ω. D is the largest linear dimension of the antenna. The data given in Table 6.2, with the parameter D, are related to larger antennas such as transmitting GSM antennas. For smaller antennas, i.e., antennas whose dimensions are smaller than the wavelength of the emitted electromagnetic signal, the following relations are used, λ < r < 10λ, for near field

r > 10λ, for far field According to the criteria defined in Table 6.2, the relation between the electric and magnetic field components is constant in the far field and can be expressed as,

E = Z 0, H where Z0 denotes the characteristic impedance of space, its value is 377 Ω. With known E or H parameters for the far-field condition, the power density can easily be determined from the relation [9],

PD =

E2 = H 2 × 377. 377

Measurement instruments or other objects in the reactive near field of a source can alter the field strengths and phases of E and H. For example, the presence of measurement personnel or an instrument at an arbitrary location in the reactive near field of a source may change the E- and H-fields at any other nearby locations. Therefore, sensors that are used to measure fields in this region must be very small compared not only to the wavelength but also to the field gradients.

6.6 RF Exposure Measurement The instruments for the measurement of electromagnetic radiation are classified as instruments for measuring the field levels in free space, i.e., densitometric parameters, and instruments for measuring the radiation absorbed dose, i.e., dosimetric parameters.

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From the aspect of the operating frequency band, the instruments for measuring RF radiation parameters can be classified into two distinct groups: narrowband (frequencyselective) instruments and broadband instruments [8]. The frequency selective instruments measure both the field level and frequency of the received signal, and thus can determine the contributions of all frequencies to the total field level. On the other hand, the broadband instruments measure the total field levels within a designated frequency range, but cannot distinguish between different frequencies. Frequency-selective instruments, also known as spectrometers, are suitable for use when the frequency of the source is unknown, while the broadband instruments are useful when the frequency of the RF radiation source is known. Exposimeters are small instruments worn on the body that are used for measuring the RF exposure parameters in free space and are intended for use by professional personnel exposed to RF. Basically, every RF radiation measurement system is composed of three main components: an RF field sensor, an RF processing unit, and a data acquisition unit [11]. Most commercially available general-purpose RF field meters are designated for measuring the densitometric RF field parameters in free space, while special instruments are used for measuring the dosimetric parameters (Figure 6.2). The sensor interacts with the field propagating through space, extracts the useful information, and converts it into an electrical signal (current or voltage). The most common field sensors are conventional antennas, but other types of field sensors are also available, such as the Hall sensor for detecting magnetic field and Pockels sensor for detecting electric fields. The antennas are categorized as electric field antennas or magnetic fields antennas. Monopole and dipole antennas are typically used for sensing electric fields, while loop antennas are used for detecting magnetic fields. Antennas are usually directional, i.e., they detect only the field propagating through one axis, but the use of three orthogonal antennas can detect RF field in all three directions—such a configuration is known as an isotropic antenna.

Field sensor Data link cables 166.8 V/m

Display and data collection unit

FIGURE 6.2 General architecture of an RF radiation meter. (From ICNIRP. Exposure to highfrequency electromagnetic fields, biological effects and health consequences (100 kHz–300 GHz), International Commission on Non-Ionizing Radiation Protection. Available at http://www.emf. ethz.ch/archive/var/ICNIRP_effekte_RFReview.pdf., 2009.)


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The output of the RF field sensor requires further processing such as amplification or conversion into appropriate electric form. When an antenna is used as a field sensor, the conditioning circuit is basically an RF detector that converts the RF signal from the antenna into a DC voltage suitable for further processing, where the DC voltage is proportional to a certain parameter of the input signal (voltage or power) [19]. The two basic types of RF detectors are: thermal based (thermistor and thermocouple) and diode based. Besides, currently, there is a wide range of advanced RF detectors in the form of integrated circuits based on complex amplification chains. These detectors have far better performance in comparison to conventional diode and thermal detectors. The two common types of integrated RF detectors are logarithmic and rms RF detectors [19]. The data acquisition unit measures the DC voltage generated at the output of the RF processing unit, performs appropriate processing of the measured data to obtain the corresponding RF field parameters, and displays results on a suitable display. The core of the data acquisition unit is a processor-based element such as a microcontroller. Depending on the type of measurement instrument, the data acquisition unit can provide only the conversion of electrical parameters into RF field parameters, or it may be equipped with frequency selective logic to determine the frequency of the received signal. Besides, the data acquisition unit can store the measured values as well as act as an interface for communication with a personal computer. Practically, RF measurement instruments can be either compact or modular. A compact instrument includes all the elements embedded into a single unit. On the other hand, in the modular design, the sensor and the RF processing unit are considered as one unit, commonly known as the probe, and the processing unit is separate, while the connection between these two units is established via cable or through direct connection. The conventional instruments used for measuring the EMF parameters can be classified as: • • • • •

Electric and magnetic field meters Spectrum analyzers Power and power density meters Induced current meters Contact current meters

6.6.1 Electric and Magnetic Field Strength Meters Electric and magnetic field strength meters are narrowband devices. They consist of an antenna, cable(s) to carry the signal from the antenna, and a signal conditioning/ readout instrument. Field strength meters may use linear antennas, such as monopoles, dipoles, loops, biconical or conical log spiral antennas, horns, or parabolic reflectors. The appropriate field parameters can be determined from a measurement of voltage or power at the selected frequency and at the antenna terminal. The electric (or magnetic) field strength can be derived from information on the antenna gain or antenna factor and the loss in the connecting cable.

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6.6.2 Spectrum Analyzers Spectrum analyzers are essentially broadband tunable receivers whose reception bandwidth may be set over a wide range of frequencies. They are used to measure the power at the antenna terminal at the selected frequency(ies). If used in combination with a narrowband selective antenna, the overall device becomes conceptually similar to a field strength meter. However, spectrum analyzers can also be connected to relatively short antennas to produce a broad response over a given frequency range. In this case, the analyzer will display the spectrum of ambient signals and permit ascertaining the frequencies involved and their relative contribution to the overall power density.

6.6.3 Power and Power Density Meters Power and power density meters are generally isotropic and broadband devices. However, there are conceptual differences among these devices in the way the fields are detected and processed. The instruments described in the following sections have essentially the same basic components (i.e., a probe, a connecting cable, and a conditioning display unit). They are limited to those types that are currently available and can provide reasonable accuracy in both near-field and far-field situations. Measurements conducted with a power density meter may produce erroneous readings when the connecting cables are inadvertently aligned with the electric field. This is due to the fact that high-resistance leads carrying the signal act as a more efficient antenna.

6.6.4 Induced Current Meters Induced current meters display the current induced through the body to the ground when an individual is exposed to an electric field created by a high-power transmitter. These currents can provide an indication of energy absorbed by the body. Induced current meters are generally stand-on devices that measure the induced current flowing through the subject’s feet to the ground. The stand-on baseplate is made of two stainless steel plates and is in fact a capacitor/resistor network. The meter reads the current flowing through the resistor connected between the capacitor plates. The size of the baseplate is kept small to minimize any pickup of electric field from the sides of the baseplate. There are also induced current meters that can measure the induced current in arms and legs directly using clamp-on sensors. The typical frequency range of commercial meters is from 10 kHz to 100 MHz.

6.6.5 Contact Current Meters Contact current meters measure the amount of current through the body caused by contact with a “hot” metallic object located in the vicinity of a high-power transmitter. Contact current meters generally feature an insulated contact probe for contact with the “hot” object. Together with a stainless steel baseplate and internal circuitries, the measured current simulates the equivalent induced current by a barefoot individual gripping the “hot” metallic object. The typical frequency range of this type of meter is from 3 kHz to 30 MHz.


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6.7 Uncertainty in RF Measurement Despite the fact that a wide range of RF measurement instrumentation is available, measuring RF radiation exposure is not an easy task. The measured results are subject to a certain degree of uncertainty, which must be taken into account to obtain precise information on the RF exposure levels [20–23]. The accuracy of the measured field strength levels can be affected by the uncertainties of the actual measurement and the uncertainties of the instrument used to perform the measurement. Actual measurement uncertainties can be minimized by following proper measurement procedures, and instrumentation uncertainties can be reduced by correct calibration and careful selection of the instrument. In carrying out calibrations in standard laboratory environments, the effect of scattering objects and the conducting parts of the RF instrumentation being calibrated will disturb the incident field. In general, it would be expected that the uncertainty should not exceed 2 dB and in some circumstances may be less. Uncertainty for TEM cell calibrations may be as little as 5%, but 10% is more typical. For Gigahertz Transverse Electromagnetic cell (GTEM), where the field strength cannot simply be calculated from the power and cell geometry, it is likely that a transfer standard field sensor will provide the lowest uncertainty for calibration. In addition to the uncertainty in the calibration procedures, there are other measurement factors that will affect the overall uncertainty when using RF field instrumentation in particular situations. These will include temperature and drift effects, resolution of the display, issues related to the relative location of the RF source and the measurement probe, positioning of the sensor, nature of polarization, perturbation of measurement by people, and the degree of repeatability. All of these will contribute to the derivation of the expanded uncertainty budget, which may be much larger than the calibration uncertainty but may be reduced by adopting approaches to minimize the uncertainty on some of the abovementioned factors. The measurement given by the instrument is only an estimate of the measured (the subject to measurement), and thus it is complete only if associated with a statement of the uncertainty parameter that characterizes the dispersion of the values that could be reasonably attributed to the measured value. All the components contributing to uncertainty should then be identified with reference to both the measuring instruments used and the measurement procedures and conditions. The evaluation of uncertainty becomes crucial when comparing a result of measurement with a field limit value fixed by a standard. Besides the uncertainty associated with the use of a field meter, other contributions also have to be considered when evaluating uncertainty of a field measurement. These contributions depend both on the measurement procedures and conditions and on the characteristics of the field source. The following are some of the most common uncertainty contributions in RF measurements [20,21]: • Probe calibration, which should be carried out in an accredited laboratory. • Frequency interpolation, due to the fact that the probe calibration curve is determined for discrete frequencies of the reference EMF.

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• The measuring procedure followed to estimate the measured quantity, and differences arising due to different staff carrying out the same type of measurement. • The effects of environmental conditions (i.e., temperature, humidity) in the measurement. • The anisotropy of the RF antenna.

6.8 RF Dosimetry RF dosimetry establishes the relationship between an EMF distribution in free space and the induced fields inside biological tissues, generally the human body. The dosimetric properties of electromagnetic exposure are evaluated using two standard dosimetric quantities: • SAR • Current density

6.8.1 SAR Measurement SAR is used as a primary indicator of RF energy absorbed in the body when quantifying the biological effects and thus defining the basic exposure limits. It is defined as the absorbed power per unit mass and is expressed in the unit of watt per kilogram (W/kg). In other words, SAR can be expressed as the time derivative of the incremental energy (dW) absorbed by an incremental mass (dm) contained in a volume element (dV) of given mass density (ρ), SAR =

d  dW  d  dW  .  = dt  dm  dt  ρdV 

For practical assessment of SAR, it is possible to use two alternative equations. One is, SAR =

σ eff (E local )2 , ρ

where Elocal denotes the electric field strength (expressed in V/m) in the tissue, σeff is the effective conductivity of the tissue in S/m, and ρ is the density of tissue in kg/m 3. The other equation is, SAR = c p

∆T , ∆t

where cp is the heat capacity of the tissue expressed in J/kg K, and ΔT is the temperature change within the time interval Δt. Calculations of SAR from temperature rise can be done only if the temperature rise is linear with time. This method is appropriate for local SAR measurement when the exposure levels (irradiating fields) are intense enough so that heat transfer within and


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out of the body does not influence temperature rise. On the other hand, determining the SAR from the electric field is appropriate for local SAR measurement when the exposure levels (irradiating fields) are intense enough that heat transfer within and out of the body does not cause significant fluctuations of the body temperature. From these equations, it is clear that for SAR measurement it is necessary to measure the strength of an electric field or temperature changes in the organism being tested. There are special instruments for this purpose, but such a procedure is invasive and is not easy, especially when it comes to scientific research. Therefore, an alternative method is based on using a special artificial simulation model human tissue as well as appropriate mathematical models that allow SAR estimation analytically, i.e., without measuring (Figure 6.3). SAR can be classified as average and local. Average SAR represents the ratio of the total power absorbed throughout the body and the weight of the body. On the other hand, the local SAR refers to a specific part of the body, i.e., to a particular mass. Values of SAR depend on the following parameters: • Parameters of the field and the distance between the source and body. • Physical dimensions of the body. • The effects of the grounding and reflection for other bodies in the vicinity of the body exposed to radiation. The average SAR, i.e., SAR for the whole body, depends on the position of the body in relation to the electric and magnetic field components. The highest SAR value is achieved when the body is parallel to the propagation direction of the electric field in the far zone. Usually, SAR is measured only in research laboratories because they are relatively difficult and require specialized equipment and conditions. Three basic techniques are used for measuring SARs. One is to measure the E-field inside the body, using implantable E-field probes, and then to calculate the SAR; this requires knowing the conductivity of the material. This technique is suitable for measuring the SAR only at specific

FIGURE 6.3 Human head phantom (left), and measurement probe inserted in the phantom (right). (From Psenakova, Z., and Benova, M., Adv. Electr. Electron. Eng., 7, 1–2, 350, 2008.)

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points in an experimental animal. Even in models using tissue-equivalent synthetic material, measuring the internal E-field at more than a few points is often not practical. A second basic technique for measuring SAR is to measure the temperature change due to heat produced by the radiation, and then to calculate the SAR from that. Probes inserted into experimental animals or models can measure local temperatures, and then the SAR at a given point can be calculated from the temperature rise. Such a calculation is easy if the temperature rise is linear with time; that is, the irradiating fields are intense enough so that heat transfer within and out of the body has but negligible influence on the temperature rise. Generating fields intense enough is sometimes difficult. If the temperature rise is not linear with time, calculation of the SAR from temperature rise must include heat transfer, which is much more difficult. Another problem is that the temperature probe sometimes perturbs the internal E-field patterns, thus producing artifacts in the measurements. This problem has led to the development of temperature probes using optical fibers or high-resistance leads instead of ordinary wire leads. A third technique for measuring SAR is to calculate the absorbed power as the difference between incident power and scattered power in a radiation chamber. This is called the differential power method. Whole-body (average) SAR in small animals and small models can be calculated from the total heat absorbed, as measured with whole-body calorimeters. Whole-body SARs have also been determined in saline-filled models by shaking them after irradiation to distribute the heat and then measuring the average temperature rise of the saline.

6.8.2 Current Density Measurement Current density is the electrical current per unit area and is expressed in A/m2. This parameter determines stimulatory effects of RF radiation. Tests have shown that the largest current density is induced in the hands and feet. This is explained by the fact that the hands and feet have the smallest cross section. Accordingly, special instruments are developed to measure the current density in these parts of the body. The current measurement is undertaken in one of three ways: 1. By placing a human subject on a conducting, standard-size plate and measuring the current between the plate and the surface (of the earth) using a thermocouple, 2. By measuring the voltage drop on a resistance between the plate and the ground, or 3. By using a current transformer. These principles of current measurement are illustrated in Figure 6.4, and a practical current measurement procedure is illustrated in Figure 6.5. It should be noted that the measurement of the induced currents has two important advantages. First, there is the possibility of comparing the current induced by the RF field with the currents that occur naturally in the body, i.e., in the nerves. Second, comparing the current that is induced in the body with currents that occur in the surrounding conductive bodies can be determined by the marginal value of electric shocks that may occur in contact between the human body and any other conductive object.

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I Thermocouple

or

R

V

FIGURE 6.4

Principles of measuring induced current in the body.

FIGURE 6.5

Instrument for measuring induced current.

6.8.3 Experimental Dosimetry Experimental dosimetry uses instrumentation and measurements to directly measure the dosimetric quantities in exposed subjects or in artificial models called phantoms. In vivo measurements of induced current densities and SAR in humans exposed to EMFs are highly invasive and are thus almost impossible for ethical reasons. Measurements on animals pose fewer ethical problems, but their results cannot be easily extended to humans. Thus, researchers have to resort to: • Measurement of basic quantities in phantoms • In vivo measurement of some derived quantities such as current.

6.8.4 Analytical Dosimetry Analytical dosimetry is aimed at finding a solution to the set of Maxwell equations that describe the coupling of the EMF with the exposed body, taking source characteristics and environmental properties into account, with reference to some particularly simplified geometries, like the sphere, the cylinder, the spheroid, and the ellipsoid, in free space or over an infinite, perfectly conducting ground plane.

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6.9 Biological Effects of RF Radiation When an RF field interacts with a biological body, it is reflected, transmitted, refracted, or scattered by the biological body. The refracted and scattered fields may proceed in directions different from that of the incident RF field. These phenomena are described and governed by the well-known Maxwell’s equations of electromagnetic theory. The transmitted and refracted fields from the RF exposure induce electric and magnetic fields in the biological systems that interact with cells and tissues in a variety of ways depending on the frequency, waveform, and strength of the induced fields, and the energy deposited or absorbed in the biological systems. Thus, to achieve a biological impact, the electric, magnetic or electromagnetic field must exert its influence on the biological system (tissue) in such a way that the deposited energy produces a detectable change in the biological system. An important consideration in RF exposure is the coupling of RF fields and their distribution inside the body. This association is also valuable in human epidemiological investigations on the health effects of RF field usage. The coupling of RF electromagnetic energy into biological systems may be quantified by the induced electric and magnetic fields, power deposition, energy absorption, and their distribution and penetration into biological tissues. These quantities are all functions of the source and its frequency or wavelength, and their relationship to the physical configuration and dimension of the biological body. Furthermore, the coupling is more complicated in that the same exposure or incident field does not necessarily provide the same field inside biological systems of different species, size, or constitution. Additionally, the interaction of RF energy with biological systems depends on electric field polarization, especially for elongated bodies with a large height-towidth ratio. It is emphasized that the quantity of induced field is the primary driving force underlying the interaction of electromagnetic energy with biological systems. The induced field in biological tissue is a function of body geometry, tissue property, and exposure conditions. Moreover, determination of the induced field is important because: (1) it relates the field to specific responses of the body, (2) it facilitates understanding of biological phenomena, and (3) it applies to all mechanisms of interaction. Once the induced field is known, quantities such as current density (J) and specific energy absorption rate (SAR) are related to it by simple conversion formulas. It is important to assess the health and safety risk of RF energy to determine not only the fields induced in biological tissues but also the mechanisms underlying its biological interactions with cells, tissues, and the human body. RF radiation causes health effects that can be divided into two main groups [24–28]: 1. Thermal effects and 2. Stimulating (nonthermal) effects. Thermal effect is pronounced at frequencies >100 kHz (typically defined in the range from 100 kHz to 10 GHz). Basically, thermal (or heat) effect is the change in temperature of the body that is exposed to radiation. In other words, the tissue is heated. The thermal EMFs can be shown by measurement with thermovision camera, measurement with


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special phantoms of biological tissue, or measurement with water phantoms. The organs most sensitive to temperature are the eye lens, brain, and seminal vesicles. A very high percentage of the human body is made up of water, and water molecules are polar molecules liable to be influenced by impinging EMFs. Hence, those tissues having a significant water content are most liable to be influenced by the RF fields. The effect of RF on such body tissues is to cause polar molecules to attempt to follow the reversals of the cycles of RF energy. Due to the frequency and inability of the polar molecules to follow these alternations, the vibrations lag on them, resulting in a gain of energy from the field in the form of heat, which causes an increase in the temperature of the tissue concerned [2]. This stimulatory effect is pronounced at frequencies lower than 100 kHz and is reflected in the appearance of irritation of nerve and muscle cells, which occurs as a result of current flow that is induced in the tissue under the influence of radiation. Nonthermal effects include nonspecific symptoms that are caused by mobile radiation. It is believed that nonthermal effects can lead to headache, dizziness, and insomnia. These types of symptoms might result from a unilateral influence on the vestibular system in the middle ear, which arise from absorption of the telephone’s EMF [29]. The existence of thermal effects is scientifically proven and is the subject of numerous studies whose primary objective is to determine the exact consequences of thermal effects of RF radiation and measures for the protection of the same. The depth of penetration of electromagnetic waves into the tissue is not sufficient to explore the biological effects caused by EMF penetration, because the distribution of energy in the tissues is very complex. Different local distribution of electromagnetic energy in the tissue leads to thermal and non-thermal excitation which may be associated with a variety of effects that have not been yet fully explored. Therefore, the most common parameter for characterization of biological effects of electromagnetic radiation is the absorbed energy or SAR.

6.10 Exposure Standards To define guidelines for RF radiation safety, a number of reference exposure limits have been specified both for environmental and occupational conditions. The RF radiation limits are defined by national and international regulatory bodies, and the most important regulatory organizations are: 1. 2. 3. 4.

ICNIRP (International Commission on Non-Ionizing Radiation Protections) ANSI (American National Standards Institution) CENELEC (European Committee for Electrotechnical Standardization) FCC (U.S. Federal Communications Commission)

Electromagnetic radiation standards, such as the ICNIRP Guidelines, usually indicate the maximum allowable values (called exposure limit values) for the basic quantities. These values are set with reference to the thresholds of biological effects, applying proper safety margins. While exposure guidelines such as ICNIRP do not apply to exposures to patients for medical purposes, they do apply to occupational exposures to medical staff; and compliance with the guidelines and associated possible health risks need to be examined.

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6.11 Conclusion As a result of the rapid expansion of wireless communication technologies and services, exposure to RF radiation has increased tremendously, and this has in turn raised the concerns of both public and general specialists regarding the possible adverse health effects of RF radiation. On the other hand, it has been confirmed through extensive research that controlled RF radiation exposure can be very helpful in medical treatment. Thus, the investigation of the physical aspects and biological effects of RF radiation is very important for better understanding of both the undesired and the beneficial effects of RF radiation exposure. This chapter presents a review of the fundamental physical aspects of electromagnetic RF radiation dosimetry. The basic physical characteristics, measurement procedures, and dosimetric aspects of RF radiation have been analyzed, providing fundamental knowledge for conducting general RF exposure measurements.

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