Medical Physics. Ivan Tanev Ivanov. Thracian University. 2016

Medical Physics. Ivan Tanev Ivanov. Thracian University. 2016

CHAPTER 5. HIGH FREQUENCY ELECTROMAGNETIC FIELD

5.1. Spectrum of electromagnetic radiation. Generation and biological effects of electromagnetic radiation

The electromagnetic field is a form of matter, and may exist and propagate both in the dielectric media and vacuum. It contains two ingredients, alternating electric field and an alternating magnetic field, both giving rise to each other. The changes in magnetic field induce an alternating electric field (the principle of electromagnetic induction) and, in turn, the alternating electric field generates an alternating magnetic field (principle of Maxwell). When these changes have high frequency, the magnetic and electric fields exist as a single entity - electromagnetic field. The high frequency electromagnetic field propagates in space as recurrent oscillations (electromagnetic wave). The electromagnetic wave is a transverse wave, since the vectors of the electric and magnetic fields oscillate perpendicularly to one another and at the same time, perpendicularly to the direction of propagation - Fig. 5.1.1. The plane where the vector of electric field oscillates is called a plane of polarization of the electromagnetic wave.

Fig. 5.1.1. The electromagnetic wave consists of a wave of electric field and a magnetic field wave. Both waves vibrate in mutually perpendicular planes.

In vacuum the electromagnetic wave propagates at a speed of Со =1/(εoμo)1/2 = 299792.5 km/s, whereas in optically dense media this speed is lower and is called a phase velocity of light, C. The ratio Co/C = n is called absolute refractive index of the medium. The wavelength, , of this oscillation (Fig. 5.1.1) is the path the wave travels for the time of one period, T, ie,  = C.T = C/n, where n is the oscillation frequency of these waves. From the expression T = 1/n it is clear that a short-wave oscillations have a higher frequency and vice versa. For some media (glass, water) n, respectively C, is a function of the wavelength. This phenomenon is called dispersion of the light.

The high frequency electromagnetic field is a flow of electromagnetic energy. The amount of energy that is transmitted through a unit area for one second is called intensity of the wave (radiation). Hence, electromagnetic field is a physical factor that exerts strong and specific effects on living organisms. The strength and nature of the effects depend essentially on two parameters, independent of each other, the frequency of oscillation (wave length) and the amplitude of the oscillation (the intensity of the wave). It should be borne in mind that in living organisms the effects produced by the electromagnetic field are mainly due to the electric component of the wave, because the tissues and cells have highly pronounced dielectric and weekly demonstrated magnetic properties.

When incandescent bodies emit electromagnetic radiation, the energy of the radiation is usually unevenly distributed over the interval of wavelengths. The spectrum of radiation gives the distribution of the energy of radiation over different wavelengths and can be continuous, linear and band spectra. If a body (matter) radiates in all wavelengths in a certain range (e.g., in the entire visible region), then its spectrum is continuous. If the substance emits only at certain wave lengths, the spectrum will consists of separate lines i.e., it will be linear (intermittent). Such spectra have the atoms of all chemical elements. If the spectrum contains separate broad bands, each consisting of a tightly arranged adjacent lines (as in the continuous spectrum), it is a band spectra. Such spectra have substances made of molecules. If the molecules are small the bands are narrow and with sharp borders. Substances made up of large molecules have spectrum containing large bands with fuzzy edges.

Depending on the wavelength, , the spectrum of electromagnetic waves is divided into different ranges - radio waves, infrared rays, visible light, ultraviolet waves, X-rays and gamma-rays (Fig. 5.1.2). Of all the electromagnetic waves only those with  between 380 and 760 nm are visible to the human eye and are known as visible light. In this narrow frequency area, however, we differentiate 6 basic color areas and a large number of intermediate color hues. All other electromagnetic radiations are invisible to the eye and can be detected only by means of appropriate technical means (detectors). The presented figure indicates that such different by their properties rays as radio, infra-red (IR) light, visible light, ultraviolet (UV) rays, X-rays and g-rays have the same nature - these are oscillations (waves) of the electromagnetic field.

In general, the ability of electromagnetic radiation to penetrate the tissues of human body strongly diminishes with decreasing the wavelength. An important exception to this rule are the X-rays and gamma rays which penetrate much deeper than UV- rays and the rays of visible light.

The radio waves from the meter, decimeter and centimeter range are used in radar, radio and television transmitters and mobile phones. The dielectric media and human tissues slightly absorb these types of electromagnetic waves. Such waves are emitted by all conductive media including metal conductors when alternating electric current flows through them. They are usually generated in oscillating circuits composed of capacitor and inductor (coil).

Radio waves with very high intensity produce heat in human tissues. At low intensity, this radiation increases the temperature of tissues with no more than 0.1°C, nevertheless, it induce various effects, referred to as non-thermal effects. They include; increase in the permeability of plasma membranes, inhibition of the activity of enzymes and protein channels, influence on immunocompetent cells and on reproduction of bacteria, influence on the flow of Ca2+ ions in the brain tissue coupled to concequent change in the rhythm of brain potentials and functions of the cerebral cortex, effects on DNA. Very often these effects are frequency dependent. In human they cause fatigue, insomnia, forgetfulness and weakened memory. When the amplitude of these waves is less than a certain threshold the above effects can not occur.

Fig. 5.1.2. Scale (specter) of electromagnetic waves.

Mobile phones emit electromagnetic waves with frequencies from 450 to 1900 MHz and can be a potential risk to large groups of people. This risk is based on several mechanisms for non-thermal effects of electromagnetic radiation. One such mechanism is due to the resonance absorption of electromagnetic radiation by biological macromolecules and biomembranes which have natural frequencies of vibration in this range. For example, in biomembranes polar heads of phospholipids perform rotary motions with a frequency of about 109 Hz, the characteristic frequencies of the bound water are in the range 108-109 Hz, and these of the free water are of the order of 1010 Hz. The area of 1010 - 1011 Hz contains the characteristic oscillations of some functional groups like COO-, NH3+, playing an important role in the functioning of protein molecules. These are frequencies in the microwave range of electromagnetic radiation and therefore biomacromolecules and biomembranes can resonancely absorb the energy of this radiation.

Microwave radiation represents millimeter radio waves and has a large capability to penetrate into the tissues of the human body (10 - 20 cm). It is used in the treatment of deeply localized inflammatory and other disorders to heat the internal tissues with high water content. The strongest heating is induced in tissues containing a lot of water. Water strongly absorbs microwave radiation due to the high value of its dielectric permittivity.

Infrared rays are emitted from the hot bodies and, therefore, they are also referred to as heat radiation. The higher the body temperature, the greater is the intesity of heat radiation, and the rays have a shorter wavelength. The infrared rays very efficiently heat the bodies which absorb them. Their penetration in human tissues, however, is small (2-3 cm).

Together with their thermal radiation, highly heated bodies emit also visible and even ultraviolet light. In addition, visible and ultraviolet lights are emitted through luminescence whereas some kind of energy, other than heat, excites atoms and molecules of the body. The exited atoms and molecules release their energy by light emittion. Visible and UV radiation are used in medicine for phototherapy and photodynamic therapy. Their penetration in the tissues and skin, however, is very small, only about 1 mm. In photochemistry, visible and ultraviolet light is used to create a photographic image, to initiate fusion reactions and polymerization.

Reontgen, or X-rays, are generated by slowing down fast-moving charged particles. In medicine they are produced in the X-ray tubes and in linear accelerators of electrons. Unlike the long-wave UV radiation and visible light, X-rays have abnormal, extremely high ability to penetrate into bodies and in human tissues. This penetration capacity is used to obtain images of the internal parts of the bodies, internal organs of the human body in medicine; internal defects of various products in defectoscopy; at the border check of passengers' luggage.

Fig. 5.1.3. Frequency profile (spectrum) of the absorption of electromagnetic radiation by Earth's atmosphere.

Gamma rays are produced only during the radioactive decay of unstable nuclei of certain isotopes. These rays as well as short-wave X-rays have a highly damaging effect on the structure of the bodies as they ionize and excite atoms and molecules and produce free radicals. In chemotherapy these effects are used to kill the cancerous tissues. In industry these rays are applied in the non-destructive gamma ray defectoscopy.

Shortwave radiation (gamma rays, X-rays and ultraviolet rays) falling on a certain type of substances (luminophors) elicits luminescence, that is emission of visible light. Directing these types of radiation to a screen covered with a luminophor induces luminescence allowing them to become visible to human eye.

The sun is a powerful source of electromagnetic radiation, which delivers an energy flux with density of about 1100 W/m2 to the Earth's surface perpendicular to the beam of propagation. It emits all types of electromagnetic radiation, listed above, although mostly intense is its radiation in the yellow-green interval of the visible range, where the sensitivity of human eye is greatest. However, the Sun radiation is largely absorbed by the Earth atmosphere, depending on the wavelength (Fig. 5.1.3). The gamma-rays, X-rays and hard UV rays, which are harmful to the life on Earth, are absorbed by the ozone layer of the Earth's atmosphere. The Earth's atmosphere also absorbs most of the heat radiation allowing a temperature, acceptable for the living organisms on the Earth's surface. Only a part of the thermal radiation, radio waves, the rays of the visible range and soft UV rays can penetrate to the Earth's surface.

Electromagnetic rays having a single frequency are referred to as monochromatic (of the same color). If two monochromatic waves have the same phase angles, they are called coherent waves. This means that these waves change consistently over time. Coherent waves demonstrate the phenomenon called interference (superposition). When two coherent waves fall and overlay on a small surface, their electric field vectors add or subtract to each other, depending on their phase angle difference. Thereat, the amplitude of the resultant electric field is increased in some areas of the surface and decreased or eliminated in the adjacent areas. This interference explains the fact that electromagnetic waves (e.g. visible light), coming out of a small hole, spread further as a straight line beam. The propagation in the form of beem is typical of the short-wave radiation - infrared, visible and ultraviolet, all designated as optical radiation.

Yet another phenomenon, diffraction, is typical for the electromagnetic waves. When such a wave encounters a barrier with dimensions, L, much larger than its wavelength (L l), the wave is reflected and does not penetrate behind the barrier. When the size of the barrier is comparable to l, (i.e., L @l), the wave circumvents the barrier; this is called diffraction of the wave.

Interferention and diffraction are inherent of each wave process. The fact that electromagnetic radiations, including light, also demonstrate such phenomena proves that light and other electromagnetic radiations are waves. However, electromagnetic radiations, especially shortwave ones, elicit effects that are characteristic for the flow of particles. Such effect, for example, is the photoelectric effect – ejection of valence electrons when the atoms are irradiated with visible light. Based on experimental data, quantum mechanics formulates that each electromagnetic wave can be regarded as a stream of particles (corpuscles, microvolumes with the size of ), in which the energy of the electric field is concentrated. Any such corpuscle, called photon of electromagnetic radiation or electromagnetic quantum, has the energy E = h.n, according to the equation of the German physicist Max Planck. Here h is Planck's constant, h = 6.626196 x10-34 J s.

The monochromatic electromagnetic field carries an energy flow, which is equal to the number of photons crossing a surface of 1 m2, multiplied by the mean energy of the individual photons. The strongest corpuscular character is exhibited by the high frequencies radiations - ultraviolet, X-ray and, in particular, the gamma-rays. Their photons have enough energy to break chemical bonds in molecules, to produce photoelectric effect, to excite and ionize the atoms and molecules. Hence, these types of radiation are called ionizing radiations. The main factor determining the type of the effects elicited by the radiation is the energy of individual photons, i.e., the frequency of the electromagnetic wave. In turn the number of absorbed photons, i.e., the intensity of the wave determines the amplitude of the arised effect. Radiation whose photons have not enough energy (visible light, infrared light, microwaves and radiowaves) are referred to as non-ionizing.

Whether or not the electromagnetic radiation will be regarded as waves or a stream of particles depends on what object they interact with. If the dimensions of the object are much larger than , the electromagnetic radiation behaves like waves. If the dimensions of the objects are close to or smaller than , the radiations behave like a stream of particles. Interestingly, the corpuscular streams of elementary particles - electrons, protons, and many others, exhibit similar dual character. Based on experimental facts the quantum mechanics formulates that each elementary particle exhibits properties of a wave with  = h/(mv) (wave of Louis de Broglie). Here, m is the mass of the particle, and v is its speed. This conception is used in the electron microscope and in the newer proton microscope. In these instruments a flow of electrons and protons is used to obtain magnified image of micro bioobjects with space resolution as small as the  of the Louis de Broglie’wave of these particles.