Ultrasonic speed. Ultrasound velocity measurement and ultrasonic equipment
1. Ultrasound emitters and receivers.
2. Absorption of ultrasound in a substance. Acoustic flows and cavitation.
3. Ultrasound reflection. Sound vision.
4. Biophysical effect of ultrasound.
5. Use of ultrasound in medicine: therapy, surgery, diagnostics.
6. Infrasound and its sources.
7. Impact of infrasound on humans. Use of infrasound in medicine.
8. Basic concepts and formulas. Tables.
9. Tasks.
Ultrasound - elastic vibrations and waves with frequencies from approximately 20x10 3 Hz (20 kHz) to 10 9 Hz (1 GHz). The ultrasound frequency range from 1 to 1000 GHz is commonly called hypersound. Ultrasonic frequencies are divided into three ranges:
ULF - low frequency ultrasound (20-100 kHz);
USCh - mid-frequency ultrasound (0.1-10 MHz);
UHF - high frequency ultrasound (10-1000 MHz).
Each range has its own characteristics of medical use.
5.1. Ultrasound emitters and receivers
Electromechanical emitters And ultrasound receivers use the phenomenon of the piezoelectric effect, the essence of which is illustrated in Fig. 5.1.
Crystalline dielectrics such as quartz, Rochelle salt, etc. have pronounced piezoelectric properties.
Ultrasound emitters
Electromechanical Ultrasound emitter uses the phenomenon of the inverse piezoelectric effect and consists of the following elements (Fig. 5.2):
Rice. 5.1. A - direct piezoelectric effect: compression and stretching of the piezoelectric plate leads to the emergence of a potential difference of the corresponding sign;
b - reverse piezoelectric effect: depending on the sign of the potential difference applied to the piezoelectric plate, it is compressed or stretched
Rice. 5.2. Ultrasonic emitter
1 - plates made of a substance with piezoelectric properties;
2 - electrodes deposited on its surface in the form of conductive layers;
3 - a generator that supplies alternating voltage of the required frequency to the electrodes.
When alternating voltage is applied to the electrodes (2) from the generator (3), the plate (1) experiences periodic stretching and compression. Forced oscillations occur, the frequency of which is equal to the frequency of voltage changes. These vibrations are transmitted to particles of the environment, creating a mechanical wave with the corresponding frequency. The amplitude of oscillations of the particles of the medium near the emitter is equal to the amplitude of oscillations of the plate.
The features of ultrasound include the possibility of obtaining waves of high intensity even with relatively small vibration amplitudes, since at a given amplitude the density
Rice. 5.3. Focusing an ultrasonic beam in water with a plano-concave plexiglass lens (ultrasound frequency 8 MHz)
energy flow is proportional squared frequency(see formula 2.6). The maximum intensity of ultrasound radiation is determined by the properties of the material of the emitters, as well as the characteristics of the conditions of their use. The intensity range for US generation in the USF region is extremely wide: from 10 -14 W/cm 2 to 0.1 W/cm 2 .
For many purposes, significantly higher intensities are required than those that can be obtained from the surface of the emitter. In these cases, you can use focusing. Figure 5.3 shows the focusing of ultrasound using a plexiglass lens. For getting very large ultrasound intensities use more complex focusing methods. Thus, at the focus of a paraboloid, the inner walls of which are made of a mosaic of quartz plates or piezoceramics of barium titanite, at a frequency of 0.5 MHz it is possible to obtain ultrasound intensities of up to 10 5 W/cm 2 in water.
Ultrasound receivers
Electromechanical Ultrasound receivers(Fig. 5.4) use the phenomenon of the direct piezoelectric effect. In this case, under the influence of an ultrasonic wave, vibrations of the crystal plate (1) occur,
Rice. 5.4. Ultrasound receiver
as a result of which an alternating voltage appears on the electrodes (2), which is recorded by the recording system (3).
In most medical devices, an ultrasonic wave generator is also used as a receiver.
5.2. Absorption of ultrasound in a substance. Acoustic flows and cavitation
In its physical essence, ultrasound does not differ from sound and is a mechanical wave. As it spreads, alternating areas of condensation and rarefaction of particles of the medium are formed. The speed of propagation of ultrasound and sound in media is the same (in air ~ 340 m/s, in water and soft tissues ~ 1500 m/s). However, the high intensity and short length of ultrasonic waves give rise to a number of specific features.
When ultrasound propagates in a substance, an irreversible transition of the energy of the sound wave occurs into other types of energy, mainly into heat. This phenomenon is called absorption of sound. The decrease in the amplitude of particle vibrations and the intensity of ultrasound due to absorption is exponential:
where A, A 0 are the amplitudes of vibrations of particles of the medium at the surface of the substance and at a depth h; I, I 0 - corresponding intensities of the ultrasonic wave; α - absorption coefficient, depending on the frequency of the ultrasonic wave, temperature and properties of the medium.
Absorption coefficient - the reciprocal of the distance at which the amplitude of the sound wave decreases by a factor of “e”.
The higher the absorption coefficient, the more strongly the medium absorbs ultrasound.
The absorption coefficient (α) increases with increasing ultrasound frequency. Therefore, the attenuation of ultrasound in a medium is many times higher than the attenuation of audible sound.
Along with absorption coefficient, Ultrasound absorption is also used as a characteristic half-absorption depth(H), which is related to it by an inverse relationship (H = 0.347/α).
Half-absorption depth(H) is the depth at which the intensity of the ultrasound wave is halved.
The values of the absorption coefficient and half-absorption depth in various tissues are presented in table. 5.1.
In gases and, in particular, in air, ultrasound propagates with high attenuation. Liquids and solids (especially single crystals) are, as a rule, good conductors of ultrasound, and the attenuation in them is much less. For example, in water, the attenuation of ultrasound, other things being equal, is approximately 1000 times less than in air. Therefore, the areas of use of ultrasonic frequency and ultrasonic frequency refer almost exclusively to liquids and solids, and in air and gases only ultrasonic frequency is used.
Heat release and chemical reactions
The absorption of ultrasound by a substance is accompanied by the transition of mechanical energy into the internal energy of the substance, which leads to its heating. The most intense heating occurs in areas adjacent to the interfaces, when the reflection coefficient is close to unity (100%). This is due to the fact that as a result of reflection, the intensity of the wave near the boundary increases and, accordingly, the amount of absorbed energy increases. This can be verified experimentally. You need to attach the ultrasound emitter to your wet hand. Soon, a sensation (similar to pain from a burn) appears on the opposite side of the palm, caused by ultrasound reflected from the skin-air interface.
Tissues with a complex structure (lungs) are more sensitive to ultrasound heating than homogeneous tissues (liver). Relatively much heat is generated at the interface between soft tissue and bone.
Local heating of tissues by a fraction of a degree promotes the vital activity of biological objects and increases the intensity of metabolic processes. However, prolonged exposure may cause overheating.
In some cases, focused ultrasound is used to locally influence individual structures of the body. This effect makes it possible to achieve controlled hyperthermia, i.e. heating to 41-44 °C without overheating adjacent tissues.
The increase in temperature and large pressure drops that accompany the passage of ultrasound can lead to the formation of ions and radicals that can interact with molecules. In this case, chemical reactions can occur that are not feasible under normal conditions. The chemical effect of ultrasound is manifested, in particular, in the splitting of a water molecule into H + and OH - radicals, followed by the formation of hydrogen peroxide H 2 O 2.
Acoustic flows and cavitation
Ultrasonic waves of high intensity are accompanied by a number of specific effects. Thus, the propagation of ultrasonic waves in gases and liquids is accompanied by the movement of the medium, which is called acoustic flow (Fig. 5.5, A). At frequencies in the ultrasonic frequency range in an ultrasonic field with an intensity of several W/cm2, liquid gushing may occur (Fig. 5.5, b) and spraying it to form a very fine mist. This feature of ultrasound propagation is used in ultrasonic inhalers.
Among the important phenomena that arise when intense ultrasound propagates in liquids is acoustic cavitation - growth of bubbles from existing ones in an ultrasonic field
Rice. 5.5. a) acoustic flow that occurs when ultrasound propagates at a frequency of 5 MHz in benzene; b) a fountain of liquid formed when an ultrasonic beam falls from inside the liquid onto its surface (ultrasound frequency 1.5 MHz, intensity 15 W/cm2)
submicroscopic nuclei of gas or vapor in liquids up to a fraction of a mm in size, which begin to pulsate at an ultrasonic frequency and collapse in the positive pressure phase. When gas bubbles collapse, large local pressures of the order of thousand atmospheres spherical shock waves. Such an intense mechanical effect on particles contained in a liquid can lead to a variety of effects, including destructive ones, even without the influence of the thermal effect of ultrasound. Mechanical effects are especially significant when exposed to focused ultrasound.
Another consequence of the collapse of cavitation bubbles is the strong heating of their contents (up to a temperature of about 10,000 °C), accompanied by ionization and dissociation of molecules.
The phenomenon of cavitation is accompanied by erosion of the working surfaces of the emitters, damage to cells, etc. However, this phenomenon also leads to a number of beneficial effects. For example, in the area of cavitation, increased mixing of the substance occurs, which is used to prepare emulsions.
5.3. Ultrasound reflection. Sound vision
Like all types of waves, ultrasound is characterized by the phenomena of reflection and refraction. However, these phenomena are noticeable only when the size of the inhomogeneities is comparable to the wavelength. The length of the ultrasonic wave is significantly less than the length of the sound wave (λ = v/v).
Thus, the lengths of sound and ultrasonic waves in soft tissues at frequencies of 1 kHz and 1 MHz are respectively equal: λ = 1500/1000 = 1.5 m;
1500/1,000,000 = 1.5x10 -3 m = 1.5 mm. In accordance with the above, a body 10 cm in size practically does not reflect sound with a wavelength of λ = 1.5 m, but is a reflector for an ultrasonic wave with λ = 1.5 mm. The reflection efficiency is determined not only by geometric relationships, but also by the reflection coefficient r, which depends on the ratio wave resistance of the media x
(see formulas 3.8, 3.9): For values of x close to 0, the reflection is almost complete. This is an obstacle to the transfer of ultrasound from air to soft tissues (x = 3x10 -4, r = 99.88%). If an ultrasound emitter is applied directly to a person’s skin, the ultrasound will not penetrate inside, but will be reflected from a thin layer of air between the emitter and the skin. In this case, small values X = 99.88%). If an ultrasound emitter is applied directly to a person’s skin, the ultrasound will not penetrate inside, but will be reflected from a thin layer of air between the emitter and the skin. In this case, small values play a negative role. To eliminate the air layer, the surface of the skin is covered with a layer of appropriate lubricant (water jelly), which acts as a transition medium that reduces reflection. On the contrary, to detect inhomogeneities in the medium, small values
are a positive factor.
The intensity of the received reflected signal depends not only on the value of the reflection coefficient, but also on the degree of absorption of ultrasound by the medium in which it propagates. Absorption of an ultrasonic wave leads to the fact that the echo signal reflected from a structure located in depth is much weaker than that formed when reflected from a similar structure located near the surface.
Based on the reflection of ultrasonic waves from inhomogeneities sound vision, used in medical ultrasound examinations (ultrasound). In this case, ultrasound reflected from inhomogeneities (individual organs, tumors) is converted into electrical vibrations, and the latter into light, which allows you to see certain objects on the screen in an environment opaque to light. Figure 5.6 shows an image
Rice. 5.6. Image of a 17-week-old human fetus obtained using 5 MHz ultrasound
human fetus aged 17 weeks, obtained using ultrasound.
An ultrasonic microscope has been created at frequencies in the ultrasonic range - a device similar to a conventional microscope, the advantage of which over an optical microscope is that for biological research, preliminary staining of the object is not required. Figure 5.7 shows photographs of red blood cells obtained with optical and ultrasound microscopes.
Rice. 5.7. Photographs of red blood cells obtained by optical (a) and ultrasound (b) microscopes
As the frequency of ultrasonic waves increases, the resolution increases (smaller inhomogeneities can be detected), but their penetrating ability decreases, i.e. the depth at which structures of interest can be examined decreases. Therefore, the ultrasound frequency is chosen so as to combine sufficient resolution with the required depth of investigation. Thus, for ultrasound examination of the thyroid gland, located directly under the skin, waves of a frequency of 7.5 MHz are used, and for examination of the abdominal organs, a frequency of 3.5-5.5 MHz is used. In addition, the thickness of the fat layer is also taken into account: for thin children, a frequency of 5.5 MHz is used, and for overweight children and adults, a frequency of 3.5 MHz is used.
5.4. Biophysical effect of ultrasound
When ultrasound acts on biological objects in irradiated organs and tissues at distances equal to half the wavelength, pressure differences from units to tens of atmospheres can arise. Such intense impacts lead to a variety of biological effects, the physical nature of which is determined by the combined action of mechanical, thermal and physicochemical phenomena accompanying the propagation of ultrasound in the environment.
General effects of ultrasound on tissues and the body as a whole
The biological effect of ultrasound, i.e. changes caused in the life activity and structures of biological objects when exposed to ultrasound are determined mainly by its intensity and duration of irradiation and can have both positive and negative effects on the life activity of organisms. Thus, mechanical vibrations of particles that occur at relatively low ultrasound intensities (up to 1.5 W/cm 2) produce a kind of micromassage of tissues, promoting better metabolism and a better supply of tissues with blood and lymph. Local heating of tissues by fractions and units of degrees, as a rule, promotes the vital activity of biological objects, increasing the intensity of metabolic processes. Ultrasonic waves small And average intensities cause positive biological effects in living tissues, stimulating the occurrence of normal physiological processes.
The successful use of ultrasound at these intensities is used in neurology for the rehabilitation of diseases such as chronic radiculitis, polyarthritis, neuritis, and neuralgia. Ultrasound is used in the treatment of diseases of the spine and joints (destruction of salt deposits in joints and cavities); in the treatment of various complications after damage to joints, ligaments, tendons, etc.
High-intensity ultrasound (3-10 W/cm2) has a harmful effect on individual organs and the human body as a whole. High ultrasound intensity can cause
in biological environments of acoustic cavitation, accompanied by mechanical destruction of cells and tissues. Long-term intense exposure to ultrasound can lead to overheating of biological structures and their destruction (denaturation of proteins, etc.). Exposure to intense ultrasound can also have long-term consequences. For example, with prolonged exposure to ultrasound with a frequency of 20-30 kHz, which occurs in some industrial conditions, a person develops nervous system disorders, fatigue increases, the temperature rises significantly, and hearing impairment occurs.
Very intense ultrasound is fatal to humans. Thus, in Spain, 80 volunteers were exposed to ultrasonic turbulent engines. The results of this barbaric experiment were disastrous: 28 people died, the rest were completely or partially paralyzed.
The thermal effect produced by high-intensity ultrasound can be very significant: with ultrasound irradiation at a power of 4 W/cm2 for 20 s, the temperature of body tissues at a depth of 2-5 cm increases by 5-6 °C.
In order to prevent occupational diseases among people working on ultrasonic installations, when contact with sources of ultrasonic vibrations is possible, it is necessary to use 2 pairs of gloves to protect hands: outer rubber and inner cotton.
The effect of ultrasound at the cellular level
The biological effect of ultrasound may also be based on secondary physicochemical effects. Thus, during the formation of acoustic flows, mixing of intracellular structures can occur. 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 biological membranes, as a result of which metabolic processes are accelerated due to diffusion. A change in the flow of various substances through the cytoplasmic membrane leads to a change in the composition of the intracellular environment and the cell microenvironment. This affects the rate of biochemical reactions involving enzymes that are sensitive to the content of certain or
other ions. In some cases, a change in the composition of the environment inside a cell can lead to an acceleration of enzymatic reactions, which is observed when cells are exposed to low-intensity ultrasound.
Many intracellular enzymes are activated by potassium ions. Therefore, with increasing ultrasound intensity, the effect of suppressing enzymatic reactions in the cell becomes more likely, since as a result of depolarization of cell membranes, the concentration of potassium ions in the intracellular environment decreases.
The effect of ultrasound on cells can be accompanied by the following phenomena:
Violation of the microenvironment of cell membranes in the form of changes in the concentration gradients of various substances near the membranes, changes in the viscosity of the environment inside and outside the cell;
Changes in the permeability of cell membranes in the form of acceleration of normal and facilitated diffusion, changes in the efficiency of active transport, disruption of membrane structure;
Violation of the composition of the intracellular environment in the form of changes in the concentration of various substances in the cell, changes in viscosity;
Changes in the rates of enzymatic reactions in the cell due to changes in the optimal concentrations of substances necessary for the functioning of enzymes.
A change in the permeability of cell membranes is a universal response to ultrasound exposure, regardless of which of the ultrasound factors acting on the cell dominates in a particular case.
At a sufficiently high intensity of ultrasound, membrane destruction occurs. However, different cells have different resistance: some cells are destroyed at an intensity of 0.1 W/cm 2, others at 25 W/cm 2.
In a certain intensity range, the observed biological effects of ultrasound are reversible. The upper limit of this interval of 0.1 W/cm 2 at a frequency of 0.8-2 MHz is accepted as the threshold. Exceeding this limit leads to pronounced destructive changes in cells.
Destruction of microorganisms
Ultrasound irradiation with an intensity exceeding the cavitation threshold is used to destroy bacteria and viruses present in the liquid.
5.5. Use of ultrasound in medicine: therapy, surgery, diagnostics
Deformations under the influence of ultrasound are used when grinding or dispersing media.
The phenomenon of cavitation is used to obtain emulsions of immiscible liquids and to clean metals from scale and fatty films.
Ultrasound therapy
The therapeutic effect of ultrasound is determined by mechanical, thermal, and chemical factors. Their combined action improves membrane permeability, dilates blood vessels, improves metabolism, which helps restore the body’s equilibrium state. A dosed ultrasound beam can be used to perform a gentle massage of the heart, lungs and other organs and tissues.
In otolaryngology, ultrasound affects the eardrum and nasal mucosa. In this way, rehabilitation of chronic runny nose and diseases of the maxillary cavities is carried out.
PHONOPHORESIS - introduction of medicinal substances into tissues through the pores of the skin using ultrasound. This method is similar to electrophoresis, however, unlike an electric field, an ultrasonic field moves not only ions, but also uncharged particles. Under the influence of ultrasound, the permeability of cell membranes increases, which facilitates the penetration of drugs into the cell, whereas with electrophoresis, drugs are concentrated mainly between the cells.
AUTOHEMOTHERAPY - intramuscular injection of a person's own blood taken from a vein. This procedure turns out to be more effective if the blood taken is irradiated with ultrasound before infusion.
Ultrasound irradiation increases the sensitivity of cells to the effects of chemicals. This allows you to create less harmful
vaccines, since in their manufacture chemical reagents of lower concentration can be used.
Preliminary exposure to ultrasound enhances the effect of γ- and microwave irradiation on tumors.
In the pharmaceutical industry, ultrasound is used to produce emulsions and aerosols of certain medicinal substances.
In physiotherapy, ultrasound is used for local impact, carried out using an appropriate emitter, applied contactally through an ointment base to a specific area of the body.
Ultrasound surgery
Ultrasound surgery is divided into two types, one of which is associated with the effect of sound vibrations on tissue, the second with the application of ultrasonic vibrations to a surgical instrument.
Destruction of tumors. Several emitters mounted on the patient's body emit ultrasound beams that focus on the tumor. The intensity of each beam is not sufficient to damage healthy tissue, but in the place where the beams converge, the intensity increases and the tumor is destroyed by cavitation and heat.
In urology, using the mechanical action of ultrasound, they crush stones in the urinary tract and thereby save patients from operations.
Welding soft tissues. If you put two cut blood vessels together and press them together, a weld will form after irradiation.
Welding bones(ultrasonic osteosynthesis). The fracture area is filled with crushed bone tissue mixed with a liquid polymer (cyacrine), which quickly polymerizes under the influence of ultrasound. After irradiation, a strong weld is formed, which gradually dissolves and is replaced by bone tissue.
Application of ultrasonic vibrations to surgical instruments(scalpels, files, needles) significantly reduces cutting forces, reduces pain, and has hemostatic and sterilizing effects. The vibration amplitude of the cutting tool at a frequency of 20-50 kHz is 10-50 microns. Ultrasonic scalpels make it possible to perform operations in the respiratory organs without opening the chest,
operations in the esophagus and blood vessels. By inserting a long and thin ultrasonic scalpel into a vein, cholesterol thickenings in the vessel can be destroyed.
Sterilization. The destructive effect of ultrasound on microorganisms is used to sterilize surgical instruments.
In some cases, ultrasound is used in combination with other physical influences, for example cryogenic, for surgical treatment of hemangiomas and scars.
Ultrasound diagnostics
Ultrasound diagnostics is a set of methods for studying a healthy and sick human body, based on the use of ultrasound. The physical basis of ultrasound diagnostics is the dependence of the parameters of sound propagation in biological tissues (sound speed, attenuation coefficient, wave impedance) on the type of tissue and its condition. Ultrasound methods make it possible to visualize the internal structures of the body, as well as to study the movement of biological objects inside the body. The main feature of ultrasound diagnostics is the ability to obtain information about soft tissues that vary slightly in density or elasticity. The ultrasound examination method is highly sensitive, can be used to detect formations that are not detected by x-ray, does not require the use of contrast agents, is painless and has no contraindications.
For diagnostic purposes, ultrasound frequency from 0.8 to 15 MHz is used. Low frequencies are used when studying deeply located objects or when studying through bone tissue, high frequencies - for visualizing objects located close to the surface of the body, for diagnostics in ophthalmology, when studying superficially located vessels.
The most widely used in ultrasound diagnostics are echolocation methods based on the reflection or scattering of pulsed ultrasound signals. Depending on the method of obtaining and the nature of presentation of information, devices for ultrasound diagnostics are divided into 3 groups: one-dimensional devices with type A indication; one-dimensional instruments with type M indication; two-dimensional devices with type B indication.
During ultrasound diagnostics using a type A device, a radiator emitting short (lasting about 10 -6 s) ultrasound pulses is applied to the area of the body being examined through a contact substance. In the pauses between pulses, the device receives pulses reflected from various inhomogeneities in the tissues. After amplification, these pulses are observed on the screen of the cathode ray tube in the form of beam deviations from the horizontal line. The complete pattern of reflected pulses is called one-dimensional echogram type A. Figure 5.8 shows an echogram obtained during echoscopy of the eye.
Rice. 5.8. Echoscopy of the eye using the A-method:
1 - echo from the anterior surface of the cornea; 2, 3 - echoes from the anterior and posterior surfaces of the lens; 4 - echo from the retina and structures of the posterior pole of the eyeball
Echograms of tissues of various types differ from each other in the number of pulses and their amplitude. Analysis of a type A echogram in many cases allows one to obtain additional information about the condition, depth and extent of the pathological area.
One-dimensional devices with type A indication are used in neurology, neurosurgery, oncology, obstetrics, ophthalmology and other fields of medicine.
In devices with type M indication, reflected pulses, after amplification, are fed to the modulating electrode of the cathode ray tube and are presented in the form of dashes, the brightness of which is related to the amplitude of the pulse, and the width is related to its duration. The development of these lines in time gives a picture of individual reflecting structures. This type of indication is widely used in cardiography. An ultrasound cardiogram can be recorded using a cathode ray tube with memory or on a paper tape recorder. This method records the movements of the heart elements, which makes it possible to determine mitral valve stenosis, congenital heart defects, etc.
When using type A and M recording methods, the transducer is in a fixed position on the patient's body.
In the case of type B indication, the transducer moves (scans) along the surface of the body, and a two-dimensional echogram is recorded on the screen of the cathode ray tube, reproducing the cross section of the examined area of the body.
A variation of method B is multiscanning, in which the mechanical movement of the sensor is replaced by sequential electrical switching of a number of elements located on the same line. Multiscanning allows you to observe the sections under study in almost real time. Another variation of method B is sector scanning, in which there is no movement of the echo probe, but the angle of insertion of the ultrasound beam changes.
Ultrasound devices with type B indication are used in oncology, obstetrics and gynecology, urology, otolaryngology, ophthalmology, etc. Modifications of type B devices with multiscanning and sector scanning are used in cardiology.
All echolocation methods of ultrasound diagnostics make it possible, in one way or another, to register the boundaries of areas with different wave impedances inside the body.
A new method of ultrasound diagnostics - reconstructive (or computational) tomography - gives the spatial distribution of sound propagation parameters: attenuation coefficient (attenuation modification of the method) or sound speed (refractive modification). In this method, the section of the object under study is sounded repeatedly in different directions. Information about the coordinates of the sound and the response signals is processed on a computer, as a result of which a reconstructed tomogram is displayed on the display.
Recently, the method has begun to be introduced elastometry for the study of liver tissue both normally and at various stages of microsis. The essence of the method is this. The sensor is installed perpendicular to the body surface. Using a vibrator built into the sensor, a low-frequency sound mechanical wave is created (ν = 50 Hz, A = 1 mm), the speed of propagation of which through the underlying liver tissue is assessed using ultrasound with a frequency of ν = 3.5 MHz (in essence, echolocation is carried out ). Using
modulus E (elasticity) of the fabric. A series of measurements (at least 10) are taken for the patient in the intercostal spaces in the projection of the position of the liver. All data is analyzed automatically; the device provides a quantitative assessment of elasticity (density), which is presented both numerically and in color.
To obtain information about the moving structures of the body, methods and instruments are used, the operation of which is based on the Doppler effect. Such devices usually contain two piezoelements: an ultrasonic emitter operating in continuous mode and a receiver of reflected signals. By measuring the Doppler frequency shift of an ultrasonic wave reflected from a moving object (for example, from the wall of a vessel), the speed of movement of the reflecting object is determined (see formula 2.9). The most advanced devices of this type use a pulse-Doppler (coherent) location method, which makes it possible to isolate a signal from a certain point in space.
Devices using the Doppler effect are used to diagnose diseases of the cardiovascular system (determination
movements of parts of the heart and the walls of blood vessels), in obstetrics (study of the fetal heartbeat), for studying blood flow, etc.
The organs are examined through the esophagus, with which they border.
Comparison of ultrasonic and x-ray “candling”
In some cases, ultrasonic scanning has an advantage over x-ray. This is due to the fact that X-rays provide a clear image of “hard” tissue against a background of “soft” tissue. For example, bones are clearly visible against the background of soft tissue. To obtain an X-ray image of soft tissues against the background of other soft tissues (for example, a blood vessel against the background of muscles), the vessel must be filled with a substance that absorbs X-ray radiation well (contrast agent). Ultrasonic transillumination, thanks to the already mentioned features, provides an image in this case without the use of contrast agents.
X-ray examination differentiates the density difference up to 10%, and ultrasound – up to 1%.
5.6. Infrasound and its sources
Infrasound- elastic vibrations and waves with frequencies lying below the range of frequencies audible to humans. Typically, 16-20 Hz is taken as the upper limit of the infrasound range. This definition is conditional, since with sufficient intensity, auditory perception also occurs at frequencies of a few Hz, although the tonal nature of the sensation disappears and only individual cycles of vibrations become distinguishable. The lower frequency limit of infrasound is uncertain; its current area of study extends down to about 0.001 Hz.
Infrasonic waves propagate in air and water, as well as in the earth's crust (seismic waves). The main feature of infrasound, due to its low frequency, is low absorption. When propagating in the deep sea and in the atmosphere at ground level, infrasonic waves with a frequency of 10-20 Hz attenuate at a distance of 1000 km by no more than a few decibels. It is known that sounds
Volcanic eruptions and atomic explosions can circle the globe many times. Due to the long wavelength, infrasound scattering is also low. In natural environments, noticeable scattering is created only by very large objects - hills, mountains, tall buildings.
Natural sources of infrasound are meteorological, seismic and volcanic phenomena. Infrasound is generated by atmospheric and oceanic turbulent pressure fluctuations, wind, sea waves (including tidal waves), waterfalls, earthquakes, and landslides.
Sources of infrasound associated with human activity are explosions, gun shots, shock waves from supersonic aircraft, impacts of piledrivers, the operation of jet engines, etc. Infrasound is contained in the noise of engines and technological equipment. Vibrations of buildings created by industrial and domestic pathogens, as a rule, contain infrasonic components. Transport noise makes a significant contribution to infrasonic pollution of the environment. For example, passenger cars at a speed of 100 km/h create infrasound with an intensity level of up to 100 dB. In the engine compartment of large ships, infrasonic vibrations created by operating engines have been recorded with a frequency of 7-13 Hz and an intensity level of 115 dB. On the upper floors of high-rise buildings, especially in strong winds, the infrasound intensity level reaches
Infrasound is almost impossible to isolate - at low frequencies, all sound-absorbing materials almost completely lose their effectiveness.
5.7. Impact of infrasound on humans. Use of infrasound in medicine
Infrasound, as a rule, has a negative effect on humans: it causes a depressed mood, fatigue, headache, and irritation. A person exposed to low-intensity infrasound experiences symptoms of seasickness, nausea, and dizziness. A headache appears, fatigue increases, and hearing weakens. At a frequency of 2-5 Hz
and an intensity level of 100-125 dB, the subjective reaction is reduced to a feeling of pressure in the ear, difficulty swallowing, forced modulation of the voice and difficulty speaking. Exposure to infrasound negatively affects vision: visual functions deteriorate, visual acuity decreases, the field of vision narrows, accommodative ability is weakened, and the stability of the eye’s fixation of the observed object is impaired.
Noise at a frequency of 2-15 Hz at an intensity level of 100 dB leads to an increase in the tracking error of the dial indicators. Convulsive twitching of the eyeball and dysfunction of the balance organs appear.
Pilots and cosmonauts exposed to infrasound during training were slower in solving even simple arithmetic problems.
There is an assumption that various anomalies in the condition of people in bad weather, explained by climatic conditions, are actually a consequence of the influence of infrasonic waves.
At moderate intensity (140-155 dB), fainting and temporary loss of vision may occur. At high intensities (about 180 dB), paralysis can occur with a fatal outcome.
It is believed that the negative impact of infrasound is due to the fact that the natural vibration frequencies of some organs and parts of the human body lie in the infrasound region. This causes unwanted resonance phenomena. Let us indicate some frequencies of natural oscillations for humans:
Human body in a lying position - (3-4) Hz;
Chest - (5-8) Hz;
Abdomen - (3-4) Hz;
Eyes - (12-27) Hz.
The effects of infrasound on the heart are especially harmful. With sufficient power, forced oscillations of the heart muscle occur. At resonance (6-7 Hz), their amplitude increases, which can lead to hemorrhage.
Use of infrasound in medicine
In recent years, infrasound has become widely used in medical practice. Thus, in ophthalmology, infrasound waves
with frequencies up to 12 Hz are used in the treatment of myopia. In the treatment of eyelid diseases, infrasound is used for phonophoresis (Fig. 5.9), as well as for cleansing wound surfaces, improving hemodynamics and regeneration in the eyelids, massage (Fig. 5.10), etc.
Figure 5.9 shows the use of infrasound to treat lacrimal duct abnormalities in newborns.
At one stage of treatment, massage of the lacrimal sac is performed. In this case, the infrasound generator creates excess pressure in the lacrimal sac, which contributes to the rupture of embryonic tissue in the lacrimal canal.
Rice. 5.9. Scheme of infrasound phonophoresis
Rice. 5.10. Massage of the lacrimal sac
5.8. Basic concepts and formulas. Tables
Table 5.1. Absorption coefficient and half-absorption depth at a frequency of 1 MHz
Table 5.2. Reflection coefficient at the boundaries of different tissues
5.9. Tasks
1. The reflection of waves from small inhomogeneities becomes noticeable when their sizes exceed the wavelength. Estimate the minimum size d of a kidney stone that can be detected by ultrasound diagnostics at a frequency ν = 5 MHz. Ultrasound wave speed v= 1500 m/s.
Solution
Let's find the wavelength: λ = v/ν = 1500/(5*10 6) = 0.0003 m = 0.3 mm. d > λ.
Answer: d > 0.3 mm.
2. Some physiotherapeutic procedures use ultrasound with frequency ν = 800 kHz and intensity I = 1 W/cm2. Find the vibration amplitude of soft tissue molecules.
Solution
The intensity of mechanical waves is determined by formula (2.6)
The density of soft tissues is ρ « 1000 kg/m 3 .
circular frequency ω = 2πν ≈ 2x3.14x800x10 3 ≈ 5x10 6 s -1 ;
ultrasound speed in soft tissues ν ≈ 1500 m/s.
It is necessary to convert the intensity to SI: I = 1 W/cm 2 = 10 4 W/m 2 .
Substituting numerical values into the last formula, we find:
Such a small displacement of molecules during the passage of ultrasound indicates that its effect is manifested at the cellular level. Answer: A = 0.023 µm.
3. Steel parts are checked for quality using an ultrasonic flaw detector. At what depth h in the part was a crack detected and what is the thickness d of the part if, after emitting an ultrasonic signal, two reflected signals were received at 0.1 ms and 0.2 ms? The speed of propagation of an ultrasonic wave in steel is equal to v= 5200 m/s.
Solution
2h = tv →h = tv/2. Answer: h = 26 cm; d = 52 cm.
1. The speed of propagation of ultrasound depends on the temperature and pressure in the pipeline. The speed of ultrasound at various values of water temperature and atmospheric pressure is given in Table D.1.
Table E.1
Alexandrov A.A., Trakhtengerts M.S. Thermophysical properties of water at atmospheric pressure. M. Standards Publishing House, 1977, 100 p. (State Standard Reference Data Service. Ser. Monographs).
2. When using a flow meter to measure the flow and volume of water in water and heat supply systems, the ultrasonic speed is determined according to the data in table. D.2 by linear interpolation method for temperature and pressure in accordance with the formula:
where c(t,P) is the speed of ultrasound in the liquid flowing through the pipeline, m/s;
c(t1) – table value of ultrasound speed at a temperature lower than measured, m/s;
c(t2) – table value of ultrasound speed at a temperature higher than measured, m/s;
c(P1) – table value of ultrasound speed at a pressure less than measured, m/s;
c(P2) – table value of ultrasound speed at a pressure greater than measured, m/s;
t – water temperature in the pipeline, ºС;
P – water pressure in the pipeline, MPa;
t1, t2 – table temperature values, ºС;
P1, P2 – table pressure values, MPa;
NOTE.
1. The values of c(t1) and c(t2) are determined according to the data in table. D.1. The values of c(P1) and c(P2) are determined according to the data in Table. D 2. at a temperature closest to the water temperature in the pipeline.
2. Measurements of temperature and pressure of water in the pipeline must be carried out with an error of no more than ±0.5 ºС and ±0.5 MPa, respectively.
Table E.2
Continuation of Table E.2
Alexandrov A.A., Larkin D.K. Experimental determination of ultrasound speed in a wide range of temperatures and pressures. Journal "Thermal Power Engineering", No. 2, 1976, p. 75.
3. In the absence of tables of the dependence of ultrasonic speed on liquid temperature, the ultrasonic speed can be determined using the device shown in Fig. E.1. Immediately before measuring the ultrasonic speed, the body of the device (steel bracket) is immersed in the test liquid, and the thickness gauge is adjusted to measure the ultrasonic speed. Then an ultrasonic thickness gauge directly measures the ultrasonic speed.
To measure the speed of ultrasound in a liquid, it is also possible to use the US-12 IM device (ShchO 2.048.045 TO) or other types of thickness gauges.
Fig.D.1. A device for measuring the speed of ultrasound in a liquid.
The section of ultrasound physics is quite fully covered in a number of modern monographs on echography. We will focus only on some of the properties of ultrasound, without knowledge of which it is impossible to understand the process of obtaining ultrasound imaging.
Ultrasound speed and specific wave resistance of human tissue (according to V.N. Demidov)
An ultrasonic wave, having reached the boundary of two media, can be reflected or travel further. The ultrasound reflection coefficient depends on the difference in ultrasonic resistance at the interface: the larger this difference, the stronger the degree of reflection. The degree of reflection depends on the angle of incidence of the beam on the interface between the media: the more the angle approaches the straight line, the stronger the degree of reflection.
Thus, knowing this, it is possible to find the optimal ultrasonic frequency that gives maximum resolution with sufficient penetration.
Basic principles on which the operation of ultrasound diagnostic equipment is based, - This spreading And ultrasound reflection.
The operating principle of diagnostic ultrasound devices is reflection of ultrasonic vibrations from the interfaces of tissues with a certain amount of acoustic resistance. It is believed that the reflection of ultrasonic waves at the interface occurs when the difference in acoustic densities of the media is at least 1%. The magnitude of the reflection of sound waves depends on the difference in acoustic density at the interface, and the degree of reflection depends on the angle of incidence of the ultrasonic beam.
Receiving ultrasonic vibrations
The production of ultrasonic vibrations is based on the direct and inverse piezoelectric effect, the essence of which is that when electric charges are created on the surface of the crystal faces, the latter begins to compress and stretch. The advantage of piezoelectric transducers is the ability of the ultrasound source to simultaneously serve as its receiver.
Diagram of the structure of an ultrasonic sensor
The sensor contains a piezoelectric crystal, on the edges of which electrodes are fixed. Behind the crystal there is a layer of substance that absorbs ultrasound, which propagates in the direction opposite to the required one. This improves the quality of the resulting ultrasound beam. Typically, the ultrasonic beam generated by the transducer has maximum power in the center and decreases at the edges, resulting in different ultrasound resolutions at the center and at the periphery. At the center of the beam, it is always possible to obtain stable reflections from both more and less dense objects, while at the periphery of the beam, less dense objects can give a reflection, and more dense ones can be reflected as less dense.
Modern piezoelectric materials allow sensors to send and receive ultrasound over a wide range of frequencies. It is possible to control the shape of the spectrum of the acoustic signal, creating and maintaining a Gaussian signal shape, which is more resistant to frequency band distortion and center frequency shift.
In recent designs of ultrasonic devices, high resolution and image clarity are ensured by the use of a dynamic focus system and a wideband echo filter for focusing incoming and outgoing ultrasonic beams using a microcomputer. This ensures ideal profiling and improvement of the ultrasound beam and the lateral resolution characteristics of the image of deep structures obtained with sector scanning. Focus parameters are set according to frequency and sensor type. The Wideband Echo Filter provides optimal resolution by combining frequencies to match the absorption of echoes passing through soft tissue. The use of high-density multi-element sensors helps eliminate false echoes caused by side and rear diffraction.
Today in the world there is fierce competition among firms to create high-quality visual systems that meet the highest requirements.
In particular, Acuson Corporation has set a specific standard for image quality and clinical variety and developed the 128 XP TM Platform, a core module for continuous improvement that allows clinicians to expand the scope of clinical research depending on their needs.
The Platform uses 128 electronically independent channels that can be used simultaneously on both transmit and receive, providing exceptional spatial resolution, tissue contrast and image uniformity across the entire field of view.
Ultrasound diagnostic instruments are divided into three classes: one-dimensional, two-dimensional and three-dimensional.
In one-dimensional scanners, information about an object is represented in one dimension along the depth of the object, and the image is recorded as vertical peaks. The amplitude and shape of the peaks are used to judge the structural properties of the tissue and the depth of the areas where echo signals are reflected. This type of device is used in echo-encephalography to determine the displacement of the midline structures of the brain and volumetric (liquid and solid) formations, in ophthalmology - to determine the size of the eye, the presence of tumors and foreign bodies, in echopulsography - to study the pulsation of the carotid and vertebral arteries in the neck and their intracranial branches, etc. For these purposes, a frequency of 0.88-1.76 MHz is used.
2D scanners
2D scanners They are divided into manual scanning devices and those operating in real time.
Currently, only devices operating in real time are used to study surface structures and internal organs, in which information is continuously reflected on the screen, which makes it possible to dynamically monitor the state of the organ, especially when studying moving structures. The operating frequency of these devices is from 0.5 to 10.0 MHz.
In practice, sensors with frequencies from 2.5 to 8 MHz are more often used.
3D scanners
Their use requires certain conditions:
- the presence of a formation that has a round or well-contoured shape;
- the presence of structural formations located in fluid spaces (fetus in the uterus, eyeball, gallstones, foreign body, polyp in a fluid-filled stomach or intestine, appendix against the background of inflammatory fluid, as well as all abdominal organs against the background of ascitic fluid );
— inactive structural formations (eyeball, prostate, etc.).
Thus, taking into account these requirements, three-dimensional scanners can be successfully used for research in obstetrics, for volumetric pathology of the abdominal cavity for more accurate differentiation from other structures, in urology for studying the prostate in order to differentiate the structural penetration of the capsule, in ophthalmology, cardiology, neurology and angiology.
Due to the complexity of use, high cost of equipment, and the presence of many conditions and restrictions, they are currently rarely used. However 3D scanning — this is the echography of the future.
Doppler ultrasound
The principle of Doppler ultrasound is that the frequency of an ultrasonic signal when reflected from a moving object changes in proportion to its speed and depends on the frequency of the ultrasound and the angle between the direction of propagation of the ultrasound and the direction of the flow. This method is successfully used in cardiology.
The method is also of interest for internal medicine due to its ability to provide reliable information about the state of the blood vessels of internal organs without introducing contrast agents into the body.
It is more often used in a comprehensive examination of patients with suspected portal hypertension in its early stages, when determining the severity of portal circulation disorders, determining the level and cause of blockade in the portal vein system, as well as to study changes in portal blood flow in patients with cirrhosis of the liver when administering medications (beta blockers, ACE inhibitors, etc.).
All devices are equipped with two types of ultrasonic sensors: electromechanical and electronic. Both types of sensors, but more often electronic, have modifications for use in various fields of medicine when examining adults and children.
In the classic real-time version, 4 electronic scanning methods are used : sector, linear, convex and trapezoidal, each of which is characterized by specific features regarding the field of observation. The researcher can choose a scanning method depending on the task facing him and the location.
Sector scanning
Advantages:
- large field of view when exploring deep areas.
Application area:
— craniological studies of newborns through the large fontanel;
— cardiological studies;
- general abdominal examinations of the pelvic organs (especially in gynecology and prostate examination), organs of the retroperitoneal system.
Line scan
Advantages:
— large field of view when examining shallow areas of the body;
— high resolution when examining deep areas of the body thanks to the use of a multi-element sensor;
Application area:
— surface structures;
— cardiology;
— examination of the pelvic organs and perinephric region;
- in obstetrics.
Convex scanning
Advantages:
— small contact area with the surface of the patient’s body;
— large field of observation when exploring deep areas.
Application area:
- general abdominal examinations.
Trapezoidal scanning
Advantages:
— large field of observation when examining close to the surface of the body and deep-lying organs;
— easy identification of tomographic sections.
Application area:
— general abdominal examinations;
- obstetric and gynecological.
In addition to the generally accepted classical scanning methods, the designs of the latest devices use technologies that allow them to be qualitatively supplemented.
Vector scan format
Advantages:
— with limited access and scanning from the intercostal space, it provides acoustic characteristics with a minimum sensor aperture. The vector imaging format provides a wider view in the near and far field.
The scope is the same as for sector scanning.
Scanning in Zoom Zone Select mode
This is a special scanning of a zone of interest selected by the operator to enhance the acoustic information content of the image in two-dimensional and color Doppler mode. The selected area of interest is displayed using full acoustic and raster lines. Improved image quality results in optimized line and pixel density, increased resolution, higher frame rates and larger images.
With a normal area, the same acoustic information remains, and with the usual RES zoom zone selection format, an image enlargement with increased resolution and greater diagnostic information is achieved.
Multi-Hertz Visualization
Broadband piezoelectric materials provide modern sensors with the ability to operate over a wide range of frequencies; provide the ability to select a specific frequency from a wide range of frequencies available in sensors while maintaining image uniformity. This technology allows you to change the frequency of the sensor with just the press of a button, without wasting time on replacing the sensor. This means that one sensor is equivalent to two or three specific characteristics, which increases the value and clinical versatility of sensors (Acuson, Siemens).
The necessary ultrasonic information in the latest device instructions can be frozen in different modes: B-mode, 2B-mode, 3D, B+B mode, 4B-mode, M-mode and recorded using a printer on special paper, on a computer cassette or video tape with computer information processing.
Ultrasound visualization of organs and systems of the human body is constantly being improved, new horizons and opportunities are constantly opening up, however, the correct interpretation of the information received will always depend on the level of clinical training of the researcher.
In this regard, I often recall a conversation with a representative of the Aloca company, who came to us to commission the first real-time device, Aloca SSD 202 D (1982). To my admiration that Japan had developed the technology of an ultrasound device with computer image processing, he replied: “A computer is good, but if another computer (pointing to his head) does not work well, then that computer is worth nothing.”
The speed of propagation of ultrasound in concrete ranges from 2800 to 4800 m/s, depending on its structure and strength (Table 2.2.2).
Table 2.2.2
| Material | ρ, g/cm3 | v p p , m/s |
| Steel | 7.8 | |
| Duralumin | 2.7 | |
| Copper | 8.9 | |
| Plexiglas | 1.18 | |
| Glass | 3.2 | |
| Air | 1.29x10 -3 | |
| Water | 1.00 | |
| Transf oil | 0.895 | |
| Paraffin | 0.9 | |
| Rubber | 0.9 | |
| Granite | 2.7 | |
| Marble | 2.6 | |
| Concrete (more than 30 days) | 2.3-2.45 | 2800-4800 |
| Brick: | ||
| silicate | 1.6-2.5 | 1480-3000 |
| clay | 1.2-2.4 | 1320-2800 |
| Solution: | ||
| cement | 1.8-2.2 | 1930-3000 |
| lime | 1.5-2.1 | 1870-2300 |
Measuring such a speed in relatively small areas (on average 0.1-1 m) is a relatively complex technical problem that can only be solved with a high level of development of radio electronics. Of all the existing methods for measuring the speed of propagation of ultrasound, from the point of view of the possibility of their use for testing building materials, the following can be distinguished:
Acoustic interferometer method;
Resonance method;
Traveling wave method;
Pulse method.
To measure the speed of ultrasound in concrete, the pulse method is most widely used. It is based on repeatedly sending short ultrasonic pulses into concrete with a repetition rate of 30-60 Hz and measuring the propagation time of these pulses at a certain distance, called the sounding base, i.e.
Therefore, in order to determine the speed of ultrasound, it is necessary to measure the distance traveled by the pulse (sounding base) and the time during which the ultrasound propagates from the point of emission to reception. The sounding base can be measured with any device with an accuracy of 0.1mm. The propagation time of ultrasound in most modern devices is measured by filling electronic gates with high-frequency (up to 10 MHz) counting pulses, the beginning of which corresponds to the moment of emission of the pulse, and the end to the moment of its arrival at the receiver. A simplified functional diagram of such a device is shown in Fig. 2.2.49.
The scheme works as follows. The master oscillator 1 generates electrical pulses with a frequency of 30 to 50 Hz, depending on the design of the device, and starts a high-voltage generator 2, which generates short electrical pulses with an amplitude of 100 V. These pulses enter the emitter, in which, using the piezoelectric effect, they are converted into a pack ( from 5 to 15 pcs.) mechanical vibrations with a frequency of 60-100 kHz and are introduced through acoustic lubricant into the controlled product. At the same time, the electronic gates open, which are filled with counting pulses, and the scanning unit is triggered, and the electron beam begins to move across the screen of the cathode ray tube (CRT).
Rice. 2.2.49. Simplified functional diagram of an ultrasonic device:
1 - master oscillator; 2 - generator of high-voltage electrical pulses; 3 - ultrasonic pulse emitter; 4 - controlled product; 5 - receiver; 6 - amplifier; 7 - gate formation generator; 8 - counting pulse generator; 9 - scanner; 10 - indicator; 11 - processor; 12 - coefficient input block; 13 - digital value indicator t,V,R
The head wave of a pack of ultrasonic mechanical vibrations, having passed through a controlled product of length L, spending time t, enters the receiver 5, in which it is converted into a pack of electrical pulses.
The arriving packet of pulses is amplified in amplifier 6 and enters the vertical scanning unit for visual monitoring on the CRT screen, and the first pulse of this packet closes the gate, stopping the access of counting pulses. Thus, the electronic gates were open for counting pulses from the moment ultrasonic vibrations were emitted until they arrived at the receiver, i.e. time t. Next, the counter counts the number of counting pulses that filled the gate, and the result is displayed on indicator 13.
Some modern devices, such as Pulsar-1.1, have a processor and a coefficient input unit, with the help of which the analytical equation of the speed-strength relationship is solved, and the digital display displays time t, speed V and concrete strength R.
To measure the speed of propagation of ultrasound in concrete and other building materials, ultrasonic devices UKB-1M, UK-10P, UK-10PM, UK-10PMS, UK-12P, UV-90PTs, Beton-5 were mass-produced in the 80s, which well recommended.
In Fig. 2.2.50 shows a general view of the UK-10PMS device.
Rice. 2.2.50. Ultrasonic device UK-10PMS
Factors influencing the speed of propagation of ultrasound in concrete
All materials in nature can be divided into two large groups,” relatively homogeneous and with a large degree of heterogeneity or heterogeneity. Relatively homogeneous materials include materials such as glass, distilled water and other materials with a constant density under normal conditions and the absence of air inclusions. For them, the speed of propagation of ultrasound under normal conditions is almost constant. In heterogeneous materials, which include most building materials, including concrete, the internal structure, the interaction of microparticles and large constituent elements is not constant both in volume and in time. Their structure includes micro- and macropores, cracks, which can be dry or filled with water.
The relative position of large and small particles is also variable. All this leads to the fact that the density and speed of propagation of ultrasound in them is inconsistent and fluctuates within wide limits. In table 2.2.2 shows the values of density ρ and ultrasound propagation speed V for some materials.
Next, we will consider how changes in such parameters of concrete as strength, composition and type of coarse aggregate, amount of cement, humidity, temperature and the presence of reinforcement affect the speed of propagation of ultrasound in concrete. This knowledge is necessary for an objective assessment of the possibility of monitoring the strength of concrete using the ultrasonic method, as well as to eliminate a number of errors in monitoring associated with changes in these factors
Effect of concrete strength
Experimental studies show that as the strength of concrete increases, the speed of ultrasound increases.
This is explained by the fact that the value of speed, as well as the value of strength, depends on the conditions of intrastructural connections.
As can be seen from the graph (Fig. 2.2.51), the “speed-strength” relationship for concrete of various compositions is not constant, which means that this relationship, in addition to strength, is influenced by other factors.
Rice. 2.2.51. The relationship between ultrasound speed V and strength R c for concrete of various compositions
Unfortunately, some factors affect ultrasonic speed more than strength, which is one of the serious disadvantages of the ultrasonic method.
If we take concrete of a constant composition, and change the strength by adopting a different W/C, then the influence of other factors will be constant, and the speed of ultrasound will change only from the strength of the concrete. In this case, the speed-strength relationship will become more defined (Fig. 2.2.52).
Rice. 2.2.52. The speed-strength relationship for a constant concrete composition, obtained at the reinforced concrete plant No. 1 in Samara
Influence of the type and brand of cement
Comparing the results of testing concrete using ordinary Portland cement and other cements, we can conclude that the mineralogical composition has little effect on the speed-strength relationship. The main influence is exerted by the content of tricalcium silicate and the fineness of cement grinding. A more important factor influencing the speed-strength relationship is the consumption of cement per 1 m 3 of concrete, i.e. its dosage. As the amount of cement in concrete increases, the speed of ultrasound increases more slowly than the mechanical strength of concrete.
This is explained by the fact that ultrasound, when passing through concrete, propagates both through the coarse aggregate and through the mortar part connecting the aggregate granules, and its speed largely depends on the speed of propagation in the coarse aggregate. However, the strength of concrete mainly depends on the strength of the mortar component. The effect of the amount of cement on the strength of concrete and the speed of ultrasound is shown in Fig. 2.2.53.
Rice. 2.2.53. Effect of cement dosage on dependence
"speed-strength"
1- 400 kg/m3; 2 - 350 kg/m3; 3 - 300 kg/m 3 ; 4 - 250 kg/m3; 5 - 200 kg/m 3
Effect of water-cement ratio
As the W/C decreases, the density and strength of concrete increase, and the ultrasound speed increases accordingly. With increasing W/C, an inverse relationship is observed. Consequently, a change in W/C does not introduce significant deviations into the established speed-strength relationship. Therefore, when constructing calibration graphs to change the strength of concrete, it is recommended to use different W/C.
Influence of speciesAnd amount of coarse aggregate
The type and amount of coarse aggregate have a significant impact on the change in the speed-strength relationship. The speed of ultrasound in aggregate, especially in such as quartz, basalt, hard limestone, granite, is much higher than the speed of its propagation in concrete.
The type and amount of coarse aggregate also affect the strength of concrete. It is generally accepted that the stronger the aggregate, the higher the strength of the concrete. But sometimes you have to deal with a phenomenon where the use of less durable crushed stone, but with a rough surface, allows you to obtain concrete with a higher Re value than when using durable gravel, but with a smooth surface
With a slight change in the consumption of crushed stone, the strength of concrete changes slightly. At the same time, such a change in the amount of coarse aggregate has a great influence on the speed of ultrasound.
As the concrete becomes saturated with crushed stone, the ultrasonic speed increases. The type and amount of coarse aggregate influence the speed-strength relationship more than other factors (Fig. 2.2.54 – 2.2.56)
Rice. 2.2.54. The influence of the presence of coarse aggregate on the speed-strength relationship:
1 - cement stone; 2 - concrete with aggregate up to 30 mm in size
Rice. 2.2.55. Speed-strength relationship for concrete with different aggregate sizes: 1-1 mm; 2-3 mm; 3-7 mm; 4-30 mm
Rice. 2.2.56. Velocity-strength relationship for concrete with filler from:
1-sandstone; 2-limestone; 3-granite; 4-basalt
It is clear from the graphs that an increase in the amount of crushed stone per unit volume of concrete or an increase in the speed of ultrasound in it leads to an increase in the speed of ultrasound in concrete more intensely than strength.
Effect of humidity and temperature
The moisture content of concrete has an ambiguous effect on its strength and ultrasound speed. With increasing moisture content of concrete, the compressive strength decreases due to changes in intercrystalline bonds, but the speed of ultrasound increases as air pores and microcracks are filled with water, A speed in water is greater than in air.
The temperature of concrete in the range of 5-40 ° C has virtually no effect on strength and speed, but increasing the temperature of hardened concrete beyond the specified range leads to a decrease in its strength and speed due to an increase in internal microcracks.
At negative temperatures, the speed of ultrasound increases due to the transformation of unbound water into ice. Therefore, it is not recommended to determine the strength of concrete using the ultrasonic method at subzero temperatures.
Propagation of ultrasound in concrete
Concrete in its structure is a heterogeneous material, which includes a mortar part and coarse aggregate. The mortar part, in turn, is a hardened cement stone with the inclusion of quartz sand particles.
Depending on the purpose of concrete and its strength characteristics, the ratio between cement, sand, crushed stone and water varies. In addition to ensuring strength, the composition of concrete depends on the manufacturing technology of reinforced concrete products. For example, with cassette production technology, greater plasticity of the concrete mixture is required, which is achieved by increased consumption of cement and water. In this case, the mortar portion of the concrete increases.
In the case of bench technology, especially with immediate stripping, rigid mixtures with reduced cement consumption are used.
The relative volume of coarse aggregate in this case increases. Consequently, with the same strength characteristics of concrete, its composition can vary within wide limits. The structure formation of concrete is influenced by the manufacturing technology of products: the quality of mixing of the concrete mixture, its transportation, compaction, thermal and moisture treatment during hardening. It follows from this that the properties of hardened concrete are influenced by a large number of factors, and the influence is ambiguous and random in nature. This explains the high degree of heterogeneity of concrete both in composition and in its properties. The heterogeneity and different properties of concrete are also reflected in its acoustic characteristics.
At present, despite numerous attempts, a unified scheme and theory of ultrasound propagation through concrete has not yet been developed, which is explained by ) first of all, by the presence of the above numerous factors, which have different effects on the strength and acoustic properties of concrete. This situation is aggravated by the fact that a general theory of the propagation of ultrasonic vibrations through a material with a high degree of inhomogeneity has not yet been developed. This is the only reason why the speed of ultrasound in concrete is determined for a homogeneous material by the formula
where L is the path traveled by ultrasound, m (base);
t is the time spent traveling this path, μs.
Let us consider in more detail the scheme of propagation of pulsed ultrasound through concrete as through a heterogeneous material. But first, we will limit the area in which our reasoning will be valid by considering the composition of the concrete mixture most common in reinforced concrete factories and construction sites, consisting of cement, river sand, coarse aggregate and water. In this case, we will assume that the strength of coarse aggregate is higher than the strength of concrete. This is true when using limestone, marble, granite, dolomite and other rocks with a strength of about 40 MPa as coarse aggregates. Let us conventionally assume that hardened concrete consists of two components: a relatively homogeneous mortar part with density ρ and speed V and coarse aggregate with ρ and V.
Taking into account the noted assumptions and limitations, hardened concrete can be considered as a solid medium with acoustic impedance:
Let's consider a diagram of the propagation of the head ultrasonic wave from emitter 1 to receiver 2 through hardened concrete of thickness L (Fig. 2.2.57).
Rice. 2.2.57. Scheme of propagation of the head ultrasonic wave
in concrete:
1 - emitter; 2 - receiver; 3 - contact layer; 4 - wave propagation in granules; 5 - wave propagation in the solution part
The head ultrasonic wave from the emitter 1 first hits the contact layer 3 located between the radiating surface and the concrete. In order for an ultrasonic wave to pass through the contact layer, it must be filled with a conductive liquid or lubricant, which is most often used as technical petroleum jelly. Having passed through the contact layer (during time t 0), the ultrasonic wave is partially reflected in the opposite direction, and the rest will enter the concrete. The thinner the contact layer compared to the wavelength, the less of the wave will be reflected.
Having entered the thickness of the concrete, the head wave will begin to propagate in the mortar part of the concrete over an area corresponding to the diameter of the emitter. After traveling a certain distance Δ l 1, after time Δ t 1 head wave in a certain area will encounter one or more granules of coarse aggregate, will be partially reflected from them, and the majority will enter the granules and begin to propagate into them. Between the granules, the wave will continue to propagate through the solution part.
Taking into account the accepted condition that the ultrasound speed in the coarse aggregate material is greater than in the mortar part, the distance d is equal to the average value of the diameter of the crushed stone, the wave that propagated through the granules at a speed V 2 will pass first, and the wave passing through the mortar part will be delayed .
Having passed through the first granules of coarse aggregate, the wave approaches the interface with the mortar part, is partially reflected, and partially enters it. In this case, the granules through which the head wave passed can later be considered as elementary spherical sources of ultrasonic wave radiation into the mortar part of the concrete, to which Huygens’ principle can be applied.
Having passed through the solution the minimum distance between neighboring granules, the head wave will enter them and begin to propagate through them, turning them into the next elementary sources. Thus, after time t, having passed through the entire thickness of concrete L and the second contact layer 3, the head wave will enter receiver 2, where it will be converted into an electrical signal.
From the considered diagram it follows that the head wave from the emitter 1 to the receiver 2 propagates along a path passing through the granules of coarse aggregate and the mortar part connecting these granules, and this path is determined from the condition of the minimum elapsed time t.
Hence the time t is
where is the time spent passing the solution part connecting the granules;
Time taken to pass through the granules. The path L traveled by ultrasound is equal to
where: - the total path traveled by the head wave through the solution part;
The total path traveled by the head wave through the granules.
The total distance L that the head wave will travel may be greater than the geometric distance between the emitter and the receiver, since the wave travels along the path of maximum speed, and not along the minimum geometric distance.
The time spent by ultrasound passing through the contact layers must be subtracted from the total measured time.
The waves that follow the head wave also propagate along the path of maximum speed, but during their movement they will encounter reflected waves from the interface between the coarse aggregate granules and the mortar part. If the diameter of the granules turns out to be equal to the wavelength or half of it, then an acoustic resonance may occur inside the granule. The effect of interference and resonance can be observed by spectral analysis of a packet of ultrasonic waves passing through concrete with different aggregate sizes.
The scheme of propagation of the head wave of pulsed ultrasound discussed above is valid only for concrete with the properties indicated at the beginning of the section, i.e. the mechanical strength and speed of propagation of ultrasound in the material from which the coarse aggregate granules are obtained exceed the strength and speed in the mortar part of the concrete. Most concretes used in reinforced concrete factories and construction sites that use crushed stone from limestone, marble, and granite have these properties. For expanded clay concrete, foam concrete, and concrete with tuff filler, the ultrasonic propagation pattern may be different.
The validity of the considered scheme is confirmed by experiments. So, from Fig. 2.2.54 it can be seen that when a certain amount of crushed stone is added to the cement part, the ultrasonic speed increases with a slight increase (and sometimes decrease) in the strength of concrete.
In Fig. 2.2.56 it is noticeable that with an increase in the speed of ultrasound in the coarse aggregate material, its speed in concrete increases.
The increase in speed in concrete with larger aggregate (Fig. 2.2.55) is also explained by this scheme, since with increasing diameter the path of ultrasound through the aggregate material lengthens.
The proposed scheme for the propagation of ultrasound will allow us to objectively evaluate the capabilities of the ultrasonic method in flaw detection and monitoring the strength of concrete.