ultrasound

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ultrasound

or

sonography,

in medicine, technique that uses sound waves to study and treat hard-to-reach body areas. In scanning with ultrasound, high-frequency sound waves are transmitted to the area of interest and the returning echoes recorded (for more detail, see ultrasonicsultrasonics,
study and application of the energy of sound waves vibrating at frequencies greater than 20,000 cycles per second, i.e., beyond the range of human hearing. The application of sound energy in the audible range is limited almost entirely to communications, since
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). First developed in World War II to locate submerged objects, the technique is now widely used in virtually every branch of medicinemedicine,
the science and art of treating and preventing disease. History of Medicine
Ancient Times

Prehistoric skulls found in Europe and South America indicate that Neolithic man was already able to trephine, or remove disks of bone from, the skull
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. In obstetrics it is used to study the age, sex, and level of development of the fetus and to determine the presence of birth defects or other potential problems. Its use to determine fetal sex has led to the widespread abortion of female fetuses in some countries, such as China and India, where male offspring are more highly valued. Ultrasound is used in cardiology to detect heart damage and in ophthalmology to detect retinal problems. It is also used to heat joints, relieving arthritic joint pain, and for such procedures as lithotripsy, in which shock waves break up kidney stones, eliminating the need for surgery. Ultrasound is noninvasive, involves no radiation, and avoids the possible hazards—such as bleeding, infection, or reactions to chemicals—of other diagnostic methods.

Ultrasound

 

elastic vibrations and waves with frequencies greater than 1.5-2 x 104 hertz (Hz), or 15-20 kilohertz (kHz). In a more restricted sense, the range of ultrasonic frequencies is regarded as extending up to 109 Hz, or 1 gigahertz. Frequencies of 109 to 1012–1013 Hz are often referred to as hypersonic (see HYPERSOUND). The range of ultrasonic frequencies may be divided into three subranges: low-frequency (1.5 x 10M05 Hz), medium-frequency (10M07 Hz), and high-frequency (107–109Hz). Each subrange exhibits distinctive characteristics of the generation, reception, propagation, and application of ultrasound.

Physical properties and characteristics of propagation. Ultrasonic waves are elastic waves. In this respect they do not differ from audible sound waves. The frequency boundary between ultrasonic waves and audible sound waves is therefore artificial; it is determined by the subjective properties of human hearing and corresponds to the average upper limit of audible sound.

The propagation of ultrasonic waves, however, exhibits a number of features specific to such waves owing to their higher frequencies and, consequently, shorter wavelengths. For example, the wavelengths of high-frequency ultrasonic waves range from 3.4 x 10–3 to 3.4 x 10–5 cm in air, from 1.5 x 10–2 to 1.5 x 10–4 cm in water, and from 5 x 10–2to5 x 10–4 cm in steel.

Ultrasonic waves propagating in gases—particularly in air— undergo considerable attenuation (seeABSORPTION OF SOUND). Liquids and solids, particularly single crystals, are generally good ultrasonic conductors; the attenuation in such substances is much weaker. For example, the attenuation of ultrasonic waves in water, other conditions being equal, is less than the attenuation in air by a factor of 1,000. Therefore, medium- and high-frequency ultrasonic waves are used almost exclusively in liquids and solids, and only low-frequency ultrasonic waves are used in air and other gases.

Because of the short wavelength of ultrasound, its propagation is affected by the molecular structure of the medium; consequently, by measuring the velocity of propagation c and the absorption coefficient α it is possible to assess the molecular properties of a substance. Molecular acoustics deals with these problems. A characteristic feature of the propagation of ultrasonic waves in gases and liquids is the existence of distinct regions of dispersion, which is accompanied by a sharp increase in absorption (seeDISPERSION OF SOUND). The absorption coefficient for ultrasonic waves in a number of fluids substantially exceeds that calculated by classical theory and does not display the increase proportional to the square of the frequency that is predicted by this theory. These effects find explanation in relaxation theory, which describes the propagation of ultrasonic waves in any medium and provides the theoretical basis of modern molecular acoustics. The basic experimental method is to measure the dependence of c and, especially, α on frequency and ambient conditions, such as temperature and pressure.

The compressions and rarefactions that accompany the propagation of an ultrasonic wave form a diffraction grating for light; the diffraction of light waves by the grating can be observed in optically transparent media. Because of the short wavelengths of ultrasonic waves, the propagation of such waves can in many cases be studied by the methods of geometrical acoustics. Physically, this fact leads to a ray picture of propagation, from which there follow such properties of ultrasound as the possibility of geometric reflection, refraction, and focusing.

Another important characteristic of ultrasound is that high intensities can be obtained even at comparatively low vibration amplitudes, since at a given amplitude the energy flux density is proportional to the square of the frequency. High-intensity ultrasonic waves are accompanied by a number of effects that can be described only by the laws of nonlinear acoustics. For example, the propagation of ultrasonic waves in gases and liquids is accompanied by the motion of the medium called sound streaming. The sound streaming velocity depends on the viscosity of the medium, the intensity of the ultrasonic waves, and the frequency of the waves; in general the velocity is low and amounts to a fraction of a percent of the velocity of the ultrasonic waves.

Among the important nonlinear effects that occur when intense ultrasonic waves propagate in liquids is acoustic cavitation. In this phenomenon submicroscopic nuclei of gas or vapor existing in the liquid grow in the ultrasonic field into bubbles the size of fractions of a millimeter; the gas bubbles begin pulsating with the ultrasonic frequency and collapse in the positive pressure phase. When the bubbles collapse, high local pressure of the order of thousands of atmospheres arise, and spherical shock waves are formed. Sound microstreaming arises near the pulsating bubbles. The phenomena in a cavitation field have both useful and harmful effects. Thus, cavitation may be used to produce emulsions and to clean contaminated components; an example of a harmful effect is the erosion of ultrasonic radiators. The ultrasonic frequencies at which cavitation is made use of in engineering lie in the low-frequency ultrasonic range. The intensity corresponding to the cavitation threshold depends on such factors as the type of liquid, the ultrasonic frequency, and the temperature. In water at a frequency of 20 kHz, this intensity is about 0.3 watts per square centimeter (W/cm2). An ultrasonic field with an intensity of a few W/cm2 or more and a frequency in the middle range can cause the appearance of a fountain in a liquid and the atomization of the liquid with the formation of an extremely finely dispersed mist.

Generation. The various devices used to generate ultrasonic vibrations may be divided into two main groups: mechanical devices, in which the ultrasound source is the mechanical energy of a gas or liquid flow, and electromechanical devices, in which ultrasonic energy is produced by the conversion of electrical energy.

Mechanical ultrasonic generators, such as air and liquid whistles and sirens, are distinguished by comparative simplicity of design and operation, do not require costly high-frequency electrical energy, and have an efficiency of 10-20 percent. The principal shortcomings of mechanical generators are their comparatively broad spectrum of emitted frequencies and their frequency and amplitude instability. They consequently cannot be used for monitoring and measuring purposes; they are employed chiefly in industrial ultrasonic processes and, to some extent, as signaling devices.

The most important method of radiating ultrasound is to convert in some way electrical oscillations into mechanical vibrations. Electrodynamic and electrostatic generators can be used in the low-frequency ultrasonic range. In addition, generators based on the property of magnetostriction in nickel, a number of special alloys, and ferrites have found broad application in this frequency range. Ultrasonic waves in the middle-frequency and high-frequency ultrasonic ranges are generated primarily by means of the piezoelectric effect. The main piezoelectric materials used in ultrasonic generators are quartz, lithium niobate, and potassium dihydrogenphosphate; in the low-frequency and middle-frequency ranges, various piezoelectric ceramics are generally used. Magnetostrictive generators consist of a rod- or ring-shaped core and a coil through which an alternating current flows. Piezoelectric generators consist of a plate or rod of a piezoelectric material with metal electrodes to which an alternating voltage is applied (Figure 1). Many piezoelectric generators used in the low-frequency ultrasonic range have a piezoelectric-ceramic plate mounted between metal blocks or plates. The vibration of magnetostrictive and piezoelectric elements at their natural resonance frequency is generally used to increase the amplitude of the vibrations and the power radiated into the medium.

Figure 1. The generation of longitudinal waves L in a solid by a plate vibrating in the thickness direction: (1) X-cut quartz plate of thickness λ/2, where λ is the wavelength in quartz; (2) metal electrodes; (3) liquid (transformer oil) ensuring acoustic contact; (4) generator of electrical oscillations; (5) solid. The plate may also be used as a receiver.

The limiting intensity of ultrasonic radiation is determined by the strength and nonlinear properties of the material of the radiator and by the specific characteristics of the use of the radiator. The range of intensity for ultrasonic generation in the middle-frequency region is extremely broad: intensities from 10–1415 W/cm2 to 0.1 W/cm2 are considered low. Intensities are often required that are much higher than those that can be obtained from the surface of a radiator. To achieve such intensities, the ultrasonic waves must be focused. Thus, at the focus of a paraboloid whose inner walls are made of a mosaic of quartz plates or the piezoelectric ceramic barium titanate, ultrasonic intensities higher than 105 W/cm2 can be obtained in water at a frequency of 0.5 megahertz. Rod-type ultrasonic concentrators, or intensifiers, that make it possible to obtain displacement amplitudes up to 10–4 cm are often used in the low-frequency ultrasonic region to increase the amplitude of vibrations of solids (seeACOUSTIC INTENSIFIER).

The selection of the method of ultrasonic generation depends on the ultrasonic frequency range, the character of the medium (gas, liquid, or solid), the type of elastic waves involved, and the required intensity of the radiation.

Reception and detection. Because of the reversibility of the piezoelectric effect, the effect is often made use of in the reception of ultrasonic waves. The study of an ultrasonic field can also be carried out by optical methods: since ultrasonic waves propagating in a medium cause a change in the medium’s optical index of refraction, the ultrasonic field can be visualized if the medium is transparent. The science of acoustooptics, which studies the interaction between acoustic waves and light, has undergone considerable development, particularly since the appearance of continuous-wave gas lasers. Research is being carried out on the diffraction of light by ultrasound and on various applications of such diffraction.

Applications. The applications of ultrasound are extremely diverse. It provides a powerful means of investigating various phenomena in many fields of physics. For example, ultrasonic methods are used in solid-state and semiconductor physics. A new branch of physics called acoustoelectronics has arisen; on the basis of its achievements various devices are being developed for processing signaling information in microelectronics. Ultrasound plays an important role in the study of matter. Along with the methods of molecular acoustics for liquids and gases, in the study of solids the measurement of the velocity c and absorption coefficient α is used to determine the elastic moduli and dissipative characteristics of substances. Considerable research has been carried out on the interaction of phonons, or quanta of elastic disturbances, with electrons, magnons, and other quasiparticles and elementary excitations in solids. Ultrasound is extensively used in engineering, and ultrasonic methods are finding increasing application in biology and medicine.

ENGINEERING. Various processes in many engineering problems are monitored on the basis of measurements of c and a; for example, the concentration of a mixture of gases or the composition of different liquids may be monitored. The reflection of ultrasonic waves at the interface of different media is made use of in ultrasonic devices for measuring the dimensions of products (for example, ultrasonic thickness gauges) and for determining the level of liquid in large tanks that are inaccessible to direct measurement. Ultrasound of comparatively low intensity (up to ~0.1 W/cm2) is widely used for nondestructive testing of products made of solid materials, such as rails, large castings, and high-quality rolled stock (see).

A particularly rapidly developing branch of flaw detection is that concerned with acoustic emission. When a mechanical stress is applied to a solid specimen (structure), bursts of ultrasonic waves are emitted (just as a tin rod produces a “crackling” sound when bent). The cause of this phenomenon is the movement of dislocations, which under certain conditions that have not yet been fully explained become sources (like the dislocations and submicroscopic cracks in the tin rod) of acoustic pulses with a spectrum containing ultrasonic frequencies. By making use of these ultrasonic bursts, the formation and development of a crack can be detected, and the location of cracks in critical parts of various structures can be determined.

Another important application of ultrasonic waves is ultrasonic imaging, that is, the use of ultrasound to see objects in a medium that is opaque to light (seeACOUSTO-OPTICAL IMAGING). In ultrasonic imaging, ultrasonic vibrations are converted into electrical oscillations, which are then converted into visible pictures. High-frequency ultrasonic waves are made use of by the ultrasonic microscope, which is similar to the conventional, optical microscope but has the advantage for biological research that preliminary staining of the specimen is not required. The development of holography has led to progress in ultrasonic holography.

Ultrasound plays an extremely important role in hydroacoustics, since elastic waves are the only waves that propagate well in seawater. The operation of, for example, sonic depth finders and sonar is based on the reflection of ultrasonic pulses from obstacles lying in their path.

Nonlinear effects of high-intensity ultrasound, chiefly in the low-frequency ultrasonic range, are made use of in various manufacturing processes; such effects include cavitation and sound streaming. For example, a number of processes of heat and mass transfer in metallurgy can be accelerated by means of high-power ultrasound. By exposing melts directly to ultrasonic vibrations, a finer texture and more uniform structure can be obtained in a metal. Ultrasonic cavitation is used extensively to remove contaminants from small parts in watch-making, instrument-making, and the electronics industry and from large parts made, for example, from electrical or rolled steel. By using ultrasound it is possible to solder aluminum parts. Ultrasonic bonding of fine conductors to sputtered metallic films or directly to semiconductors is employed in microelectronics and semiconductor technology. Ultrasonic welding is used to join, for example, plastic parts, polymer films, and synthetic fabrics; all these cases involve such processes as ultrasonic cleaning, local heating under the action of ultrasound, acceleration of diffusion processes, and alteration of the state of the polymer. Ultrasonic energy is used to machine brittle materials—such as glass or ceramics—and parts of complex configuration; the vibrations of the ultrasonic tool cause the particles of the abrasive suspension to cut the material being worked. V. A. KRASIL’NIKOV

BIOLOGY. When organs and tissues are irradiated with ultrasonic waves, pressure differences ranging from a few atmospheres to several dozen atmospheres may arise over a distance of a half wavelength. Ultrasonic influences of such intensity lead to various biological effects, whose physical nature is determined by the joint action of the mechanical, thermal, and physicochemical phenomena that accompany the propagation of ultrasonic waves in a medium. The biological effects of ultrasound—that is, the changes induced in the vital activity and structures of biological objects when exposed to ultrasound—are determined principally by the intensity of the ultrasound and by the duration of irradiation. The influence on the vital activity of organisms may be positive or negative. For example, the mechanical vibrations performed by particles at comparatively low ultrasonic intensities (up to 1-2 W/cm2) produce a “micromassage” of tissues that promotes better metabolism and better blood and lymph supply to the tissues. At higher intensities there may occur acoustic cavitation in biological media accompanied by the mechanical destruction of cells and tissues; the gas bubbles present in biological media serve as the cavitation nuclei.

When ultrasound is absorbed in biological objects, acoustic energy is converted into thermal energy. The local heating of tissues by a fraction of a degree or by a few degrees generally promotes the vital activity of organisms by increasing the rate of metabolic processes. Exposures of greater intensity and duration, however, may lead to the overheating and destruction of biological structures—for example, the denaturation of proteins.

Secondary physicochemical effects may also underlie the biological effects of ultrasound. For example, the occurrence of streaming may be accompanied by mixing of intracellular structures. Cavitation leads to the breaking of molecular bonds in biopolymers and other vital compounds and to the development of redox reactions. Ultrasound increases the permeability of membranes; as a result, metabolic processes are sped up owing to diffusion. Under real conditions all the effects mentioned occur in a biological object jointly. It therefore is difficult and sometimes impossible to investigate separately processes having different physical natures. L. R. GAVRILOV

MEDICINE. Ultrasound is used for diagnosis and treatment in various fields of clinical medicine. Since it penetrates without significant absorption the soft tissues of an organism and is reflected from acoustical inhomogeneities, ultrasound is used to study internal organs. In many cases the structure of tissues can be distinguished more finely by ultrasonic diagnostic methods than by X-ray methods. For example, ultrasonic techniques may often reveal in soft tissues tumors that are indistinguishable by other methods. Ultrasound is used in obstetrics for diagnostic investigation of the fetus and pregnant woman, in neurosurgery for identification of brain tumors (echoencephalography), and in cardiology for studying hemodynamics and ascertaining hypertrophy of the myocardium. The micromassaging of tissues, activation of metabolic processes, and local heating of tissues under the action of ultrasound are used in medicine for therapeutic purposes.

The surgical uses of ultrasound fall into two groups: (1) the destruction of tissues by acoustic vibrations and (2) the application of ultrasonic vibrations to surgical instruments. The first group employs focused ultrasonic waves with frequencies of the order of 106–107 Hz, and the second group employs vibrations with frequencies of 20-75 kHz and an amplitude of 10-50 micrometers. The use of ultrasonic instruments to separate soft and bone tissues makes possible a considerable reduction in cutting force, blood losses, and painful sensations. In traumatology and orthopedics ultrasound is used to unite broken bones: in such operations the space between the bone pieces is filled with bone chips mixed with a liquid plastic; union of the bone pieces occurs under the action of ultrasound.

Ultrasound has various applications in biological and medical laboratory practice. Examples are the dispersion of biological structures, the effecting of relatively small changes in cell structure, the sterilization of instruments and medicines, and the production of aerosols. In such fields as bacteriology and immunology the uses of ultrasound include the production of enzymes and antigens from bacteria and viruses and the study of the morphological features and antigenic activity of bacterial cells.

NATURE. A number of animals are capable of perceiving and radiating elastic waves with frequencies much higher than 20 kHz. For example, birds react with pain to ultrasonic frequencies above 25 kHz. This fact is made use of, for example, to frighten gulls away from bodies of water that are sources of drinking water. In flight small insects generate ultrasonic waves. The types of bats with poor vision or no vision at all orient themselves in flight and seek prey by means of an ultrasonic location system. They radiate ultrasonic pulses with their vocal apparatus with a repetition frequency of a few hertz and a carrier frequency of 50-60 kHz. Dolphins emit and perceive ultrasonic waves at frequencies of up to 170 kHz; their method of ultrasonic location appears to be even more highly developed than that of the bat.

Research. A large number of institutes and laboratories in the USSR and abroad are engaged in the study of ultrasound and its applications. Such laboratories are found at the Acoustics Institute of the Academy of Sciences of the USSR, the Institute of Radio Engineering and Electronics of the Academy of Sciences of the USSR, and the physics departments of Moscow State University, Leningrad State University, and other universities in the USSR. In the USA research is carried on in laboratories at such universities as the University of California, Stanford University, and Brown University; important work is also done at the Bell Telephone Laboratories. Other countries with institutes and university laboratories studying ultrasound include Great Britain, Japan, France, the Federal Republic of Germany, and Italy. Papers on ultrasound are published in Akusticheskii zhurnal AN SSSR (English translation published as Soviet Physics-Acoustics), the Journal of the American Acoustical Society, the European journals Ultrasonics and Acústica, and many other physics and engineering journals.

Historical survey. The first work on ultrasound was carried out in the 19th century. In 1830 the French scientist F. Savart attempted to establish the upper frequency limit of audibility for the human ear. Others investigating ultrasound included the British scientist F. Galton (1883), the German physicist W. Wien (1903), and the Russian physicist P. N. Lebedev and his students (1905).

An important contribution was made by the French physicist P. Langevin, who in 1916 became the first to make use of the piezoelectric properties of quartz for the generation and reception of ultrasonic waves in the detection of submarines and the measurement of sea depths. In 1925, G. W. Pierce of the USA built an instrument, the Pierce interferometer, for measuring with high accuracy the velocity and absorption of ultrasonic waves in gases and liquids. Several important contributions were made by the American physicist R. Wood. In addition to achieving unprecedented ultrasonic intensities in liquids, he observed the production of a fountain in a liquid under the action of an ultrasonic beam and investigated the effects of ultrasound on living organisms (1927). In 1928 the Soviet scientist S. Ia. Sokolov initiated ultrasonic flaw detection for metal products by proposing the use of ultrasound to detect cracks, cavities, and other defects in solids.

In 1932, R. Lucas and P. Biquard in France and P. Debye and F. W. Sears in Germany demonstrated the diffraction of light by ultrasonic waves; such diffraction subsequently came to play an important role in the study of the structure of liquids and solids and in a number of technical applications. In the early 1930’s H. O. Kneser in Germany discovered the anomalous absorption and dispersion of ultrasonic waves in polyatomic gases; this phenomenon later was also observed in a number of complex liquids, such as organic liquids. The correct theoretical explanation of these relaxation phenomena was given in general form in 1937 by Soviet scientists L. I. Mandel’shtam and M. A. Leontovich. Relaxation theory thus became the basis of molecular acoustics.

In the 1950’s and 1960’s considerable work was done on various industrial process applications of ultrasonics. An important contribution to the development of the physical principles underlying these applications was made by L. D. Rozenberg and coworkers in the USSR. As increasingly high ultrasonic intensities were attained, research was begun on the characteristics of the propagation of powerful ultrasonic waves in gases, liquids, and solids. Nonlinear acoustics has been developing rapidly; seminal work in this area has been done by such Soviet scientists as N. N. Andreev, V. A. Krasil’nikov, and R. V. Khokhlov and by various American and British scientists. Substantial advances were made in acoustoelectronics in the 1970’s. Much work in the field was stimulated by the demonstration of ultrasonic amplification and generation in piezoelectric semiconductors by Hutson, McFee, and White of the USA in 1961.

REFERENCES

Bergmann, L. Ul’trazvuk. Moscow, 1956. (Translated from German.)
Krasil’nikov, V. A. Zvukovye i ul’trazvukovye volny v vozdukhe, vode i tverdykh telakh, 3rd ed. Moscow, 1960.
Fizicheskaio akustika, vols. 1-7. Edited by W. Mason. Moscow, 1966-74. (Translated from English.)
Fizika i tekhnika moshchnogo ul’trazvuka, vols. 1-3. Edited by L. D. Rozenberg. 1967-69.
Mikhailov, I. G., V. A. Solov’ev, and Iu. P. Syrnikov. Osnovy mole-kuliarnomkustiki. Moscow, 1964.
Viktorov, I. A. Fizicheskie osnovy primeneniia ul’trazvukovykh voln Releía i Lemba v tekhnike. Moscow, 1966.
Metody nerazrushaiushchikh ispytanii. Edited by R. Sharpe. Moscow, 1972. (Translated from English.)
Ul’trazvukovoe rezanie. Moscow, 1962.
U’trazvukovaia tekhnologiia. Edited by B. A. Agranat. Moscow, 1974.
El’piner, I. E. Biofizika ul’trazvuka. Moscow, 1973.
Beier, W., and E. Dôrner. Ul’trazvuk v biologii i meditsine. Leningrad, 1958. (Translated from German.)
Interaction of Ultrasound and Biological Tissues: Proceedings of aWorkshop.... Edited by J. M. Reid and M. R. Sitkov. Washington, D.C., 1972.

V. A. KRASIL’NIKOV

ultrasound

[′əl·trə‚sau̇nd]
(acoustics)
Sound with a frequency above about 20,000 Hz, the upper limit of human hearing.

ultrasound

Acoustic oscillations having a frequency above the high-frequency limit of audible sound, i.e., above 20,000 Hz.

ultrasound

ultrasonic waves at frequencies above the audible range (above about 20 kHz), used in cleaning metallic parts, echo sounding, medical diagnosis and therapy, etc.
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