Velocity of propagation of ultrasound in various media. Physical principles of ultrasound imaging of human tissues and organs
Ultrasound is called longitudinal mechanical waves with oscillation frequencies above 20 kHz. Like sound waves, an ultrasonic wave is an alternation of condensation and rarefaction of the medium. In each medium, the propagation speed of both sound and ultrasound is the same. In view of this, the length of ultrasonic waves in air is less than 17 mM (V = λ * ν; Vair = 330 m/s).
The sources of ultrasound are special electromechanical emitters. One type of emitters work on the basis of the phenomenon of magnetostriction, when the dimensions of certain bodies (for example, a nickel rod) change in an alternating magnetic field. Such emitters make it possible to obtain oscillations with frequencies from 20 to 80 kHz. From an alternating current source with the indicated frequencies, a voltage is applied to a nickel rod, the longitudinal size of the rod changes with the frequency of the alternating current, and an ultrasonic wave is emitted from the side faces of the sample (Fig. 4).
The second type of radiators works on the basis of the piezoelectric effect, when the dimensions of certain bodies - materials made of ferroelectrics - change in an alternating electric field. For this type of radiators, higher frequency oscillations can be obtained - up to 500 MHz. From the alternating current source, voltage is also applied to the side faces of the rod made of ferroelectric (quartz, tourmaline), while the longitudinal size of the rod changes with the frequency of the alternating current, and an ultrasonic wave is emitted from the side faces of the sample (Fig. 5). In both the first and second cases, ultrasound is emitted due to vibrations of the side faces of the rod; in the latter case, these faces are metallized to supply current to the sample.
Ultrasonic receivers operate on the principle of inverse phenomena of magnetostriction and piezoelectric effect: an ultrasonic wave causes fluctuations in the linear dimensions of bodies, when the bodies are in the field of an ultrasonic wave, size fluctuations are accompanied by the appearance of either alternating magnetic or alternating electric fields in the material. These fields, which appear in the corresponding sensor, are recorded by some indicator, for example, an oscilloscope. The more intense the ultrasound, the greater the amplitude mechanical vibrations sample - the sensor and the greater the amplitude of the resulting alternating magnetic or electric fields.
Features of ultrasound.
As mentioned above, in each medium, the propagation speed of both sound and ultrasound is the same. The most important feature of ultrasound is the narrowness of the ultrasonic beam, which allows you to influence any objects. locally. In inhomogeneous media with small inhomogeneities, when the sizes of inclusions are approximately equal to but greater than the wavelength (L ≈ λ), the phenomenon of diffraction takes place. If the dimensions of the inclusions are much larger than the wavelength (L >> λ), then the propagation of ultrasound is rectilinear. In this case, it is possible to obtain ultrasonic shadows from such inclusions, which is used when various types diagnostics - both technical and medical. An important theoretical point in the use of ultrasound is the passage of ultrasound from one medium to another. Such a characteristic of the waves as frequency does not change in this case. On the contrary, the speed and wavelength can change in this case. So in water the speed of acoustic waves is 1400 m/s, when in air it is 330 m/s. The penetration of ultrasound into another medium is characterized by the penetration coefficient (β). It is defined as the ratio of the intensity of the wave entering the second medium to the intensity of the incident wave: β = I 2 / I 1– Fig. 6. This coefficient depends on the ratio of the acoustic impedances of the two media. Acoustic impedance is the product of the density of a medium and the speed of wave propagation in a given medium: Z 1 \u003d ρ 1 * V 1, Z 2 \u003d ρ 2 * V 2. The penetration coefficient is the largest - close to unity, if the acoustic impedances of the two media are approximately equal: ρ 1 * V 1 ,≈ ρ 2 * V 2. If the impedance of the second medium is much greater than the first, the penetration coefficient is negligible. In the general case, the coefficient β is calculated by the formula:
For the transition of ultrasound from the air into the human skin β = 0.08%, for the transition from glycerol to the skin β = 99.7%.
Absorption of ultrasound in various media.
In homogeneous media, ultrasound is absorbed, like any type of radiation - according to the law exponential function: 
The value of L ' - called the half-absorption layer - is the distance at which the wave intensity is halved. The half-absorption layer depends on the frequency of the ultrasound and the tissue itself - the object. With increasing frequency, the value of L 1/2 -decreases. For various tissues of the body, the following values of the degree of absorption of ultrasound take place:
| Substance | Water | Blood | Cartilage | Bone |
| L' | 300 cm | 2 - 8 cm | 0.24 cm | 0.05 cm |
The effect of ultrasound on body tissues.
There are three types of ultrasound action:
mechanical,
thermal,
Chemical.
The degree of impact of one or another type is determined by the intensity. In this regard, in medicine, there are three levels of ultrasound intensities:
1 level - up to 1.5 W / cm 2,
level 2 - from 1.5 to 3 W / cm 2,
Level 3 - from 3 to 10 W / cm 2.
All three types of impact of ultrasound on tissues are associated with the phenomenon of cavitation - these are short-term (half of the periods of oscillation of the particles of the medium) the appearance of microscopic cavities in places where the medium is rarefied. These cavities are filled with liquid vapor, and in the phase high blood pressure(the other half of the oscillation period of the particles of the medium), the formed cavities collapse. At high wave intensities, the collapse of cavities with liquid vapors in them can lead to a destructive mechanical effect. Naturally, the collapse of microcavities is accompanied by a thermal effect. The process of collapse of microcavities is also associated with the chemical action of ultrasound, since in this case the particles of the medium reach high speeds of translational motion, which can cause the phenomenon of ionization, rupture chemical bonds, the formation of radicals. The resulting radicals can interact with proteins, lampids, nucleic acids and cause undesirable effects of a chemical nature.
6. Features of blood flow through large vessels, medium and small vessels, capillaries;
blood flow during vasoconstriction, sound effects.
The rate of blood flow in different vessels is different. Approximate values of this speed are presented in table. 2.1.
Table 2.1. Velocity and pressure of blood in various vessels
At first glance, it seems that the given values contradict the continuity equation - in thin capillaries, the blood flow velocity is less than in arteries. However, this discrepancy is apparent. The point is that in Table 2.1 shows the diameter of one vessel, but as the vessels branch, the area of each of them decreases, and the total branching area increases. Thus, the total area of all capillaries (approximately 2000 cm 2) is hundreds of times greater than the area of the aorta - this explains such a low blood velocity in the capillaries (500 - 600 times less than in the aorta).
In the future, when the capillaries merge into venules, into veins, up to the vena cava, the total lumen of the vessels decreases again and the blood flow rate increases again. However, due to a number of reasons, the blood flow velocity when the vena cava enters the heart does not increase to the initial value, but approximately up to ½ of it (Fig. 2.7).
Aorta arteries arterioles capillaries venules veins vena cava
Rice. 2.7. Distribution of blood flow velocities in different departments
of cardio-vascular system
In capillaries and veins, the blood flow is constant; in other parts of the cardiovascular system, pulse waves.
The wave of increased pressure propagating through the aorta and arteries, caused by the ejection of blood from the left ventricle of the heart during systole, is called a pulse wave.
When the heart muscle contracts (systole), blood is ejected from the heart into the aorta and arteries extending from it. If the walls of these vessels were rigid, then the pressure arising in the blood at the outlet of the heart would be transmitted to the periphery at the speed of sound. However, the elasticity of the walls of the vessels leads to the fact that during systole, the blood pushed out by the heart stretches the aorta, arteries and arterioles. Large vessels perceive during the systole more blood than it flows to the periphery. The systolic pressure (P C) of a person is normally approximately 16 kPa. During the relaxation of the heart (diastole), the distended blood vessels subside and the potential energy communicated to them by the heart through the blood is converted into the kinetic energy of the blood flow, while maintaining a diastolic pressure (D) of approximately 11 kPa.
R, Pa R, Pa
1 - in the aorta 2 - in the arterioles
Rice. 2.8. Fluctuations in pressure in the vessels during the passage of pulse waves
The amplitude of the pulse wave P 0 (x) (pulse pressure) is the difference between the maximum and minimum pressure values at a given point of the vessel (x). At the beginning of the aorta, the amplitude of the wave Р 0, max is equal to the difference between systolic (Р С) and diastolic (Р D) pressures: Р 0, max = Р С - Р D. The attenuation of the pulse wave amplitude during its propagation along the vessels can be represented by the dependence:
where β is the attenuation coefficient, which increases with decreasing vessel radius.
The speed of propagation of the pulse wave, measured experimentally, is » 6 - 8 m / s, which is 20 - 30 times greater than the speed of movement of blood particles = 0.3 - 0.5 m / s. During the time of expulsion of blood from the ventricles (systole time) t s \u003d 0.3 s, the pulse wave has time to propagate to a distance
L p \u003d t s "2m,
that is, to cover all large vessels - the aorta and arteries. This means that the pulse wave front will reach the extremities before the pressure drop in the aorta begins.
Experimental determination of the pulse wave velocity is the basis for diagnosing the state of blood vessels. With age, the elasticity of blood vessels increases by 2-3 times, and, consequently, the speed of the pulse wave also increases.
As is clear from experiments and from general ideas about the work of the heart, the pulse wave is not sinusoidal.
(harmonic) (Fig. 2.9).
1 - artery after passing 2 - passes through the artery
pulse wave front of the pulse wave
3 - pulse wave in the artery 4 - decrease in high blood pressure
Rice. 2.9. Profile of an artery during the passage of a pulse wave.
The speed of the pulse wave in large vessels depends on their parameters as follows (Moens-Korteweg formula):
, where E is the modulus of elasticity (Young's modulus); ρ is the density of the substance of the vessel; h is the vessel wall thickness; d is the diameter of the vessel.
It is interesting to compare this formula with the expression for the speed of sound propagation in a thin rod:
, E - Young's modulus; ρ - density of the rod substance
In humans, with age, the modulus of elasticity of blood vessels increases, therefore, the speed of the pulse wave also increases.
Along with the pulse wave in the "vessel-blood" system, sound waves can also propagate, the speed of which is very high compared to the speed of movement of blood particles and the speed of the pulse wave. Thus, in the vessel-blood system, three main processes of movement can be distinguished:
1) movement of blood particles ( = 0.5 m/s);
2) pulse wave propagation (~ 10 m/s);
3) propagation of sound waves (~ 1500 m/s).
The flow of blood in the arteries is normally laminar, with slight turbulence occurring near the valves. In pathology, when the viscosity is less than normal, the Reynolds number may exceed the critical value and the movement will become turbulent. Turbulent flow is associated with additional energy consumption during fluid movement, which in the case of blood leads to additional work of the heart.
The noise generated by turbulent blood flow can be used to diagnose diseases. This noise is heard on the brachial artery when measuring blood pressure using the Korotkoff sound method.
The flow of air in the nasal cavity is normally laminar. However, with inflammation or any other abnormality, it can become turbulent, which will entail additional work of the respiratory muscles.
The transition from a laminar flow to a turbulent one occurs not only with a flow in a pipe (channel), but is characteristic of almost all flows of a viscous fluid. In particular, the fluid flow around the profile of a ship or submarine, the body of a fish or the wing of an aircraft or a bird is also characterized by a laminar-turbulent transition, while the characteristic size of the streamlined body and a constant depending on the shape of the body must be substituted into the formula.
Similar information.
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 | |
| plexiglass | 1.18 | |
| Glass | 3.2 | |
| Air | 1.29x10-3 | |
| Water | 1.00 | |
| Transfer 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, in terms of the possibility of their application for testing building materials, the following can be distinguished:
Acoustic interferometer method;
Resonance method;
Traveling wave method;
impulse method.
To measure the speed of ultrasound in concrete, the pulse method is most widely used. It is based on repeated sending of short ultrasonic pulses with a repetition rate of 30-60 Hz into concrete 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 (the sounding base), and the time it takes for the ultrasound to propagate from the place of emission to reception. The sound base can be measured with any device with an accuracy of 0.1 mm. 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 the pulse is emitted, and the end corresponds to the moment it arrives 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 pieces) of mechanical vibrations with a frequency of 60-100 kHz and are introduced through acoustic lubrication into the controlled product. At the same time, the electronic gate opens, which are filled with counting pulses, and the scanner is triggered, the movement of the electron beam along the screen of the cathode ray tube (CRT) begins.
Rice. 2.2.49. Simplified functional diagram of an ultrasonic device:
1 - master generator; 2 - generator of high-voltage electrical impulses; 3 - emitter of ultrasonic pulses; 4 - controlled product; 5 - receiver; 6 - amplifier; 7 - gate formation generator; 8 - generator of counting pulses; 9 - scanner; 10 - indicator; 11 - processor; 12 - coefficient input block; 13 - digital indicator of values t,V,R
The head wave of a pack of ultrasonic mechanical oscillations, having passed through the controlled product of length L, while spending time t, enters the receiver 5, in which it is converted into a pack of electrical impulses.
The incoming burst of pulses is amplified in amplifier 6 and enters the vertical scanner for visual control on the CRT screen, and the first pulse of this burst closes the gate, stopping the access of counting pulses. Thus, the electronic gates were open for counting pulses from the moment the ultrasonic vibrations were emitted to the moment 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 "velocity-strength" dependence is solved, and time t, speed V and concrete strength R are displayed on the digital display.
To measure the propagation velocity of ultrasound in concrete and other building materials in the 80s, ultrasonic devices UKB-1M, UK-10P, UK-10PM, UK-10PMS, UK-12P, UF-90PTs, Beton-5 were mass-produced, which themselves well recommended.
On fig. 2.2.50 shows a general view of the device UK-10PMS.
Rice. 2.2.50. Ultrasonic device UK-10PMS
Factors affecting 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 of the 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 mutual arrangement of large and small particles is also unstable. All this leads to the fact that the density and speed of propagation of ultrasound in them are not constant and fluctuate over a wide range. In table. 2.2.2 shows the values of the density ρ and the propagation velocity of ultrasound V for some materials.
Next, we will consider how changes in concrete parameters such 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 testing the strength of concrete by the ultrasonic method, as well as for eliminating a number of errors in the control associated with a change in these factors.
Influence of concrete strength
Experimental studies show that with an increase in the strength of concrete, 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 condition of intrastructural bonds.
As can be seen from the graph (Fig. 2.2.51), the "speed-strength" dependence for concrete of various compositions is not constant, from which it follows that other factors, in addition to strength, also influence this dependence.
Rice. 2.2.51. Relationship between ultrasonic velocity V and strength R c for concretes of various compositions
Unfortunately, some factors affect the speed of ultrasound more than strength, which is one of the serious disadvantages of the ultrasonic method.
If we take concrete of constant composition, and change the strength by adopting 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" dependence will become more definite (Fig. 2.2.52).
Rice. 2.2.52. Dependence "speed-strength" for a constant composition of concrete, obtained at the concrete goods plant No. 1 in Samara
Influence of the type and brand of cement
Comparing the results of testing concretes on ordinary Portland cement and on other cements, it can be concluded that the mineralogical composition has little effect on the "speed-strength" dependence. 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. his dosage. With an increase in the amount of cement in concrete, the speed of ultrasound increases more slowly than the mechanical strength of concrete.
This is explained by the fact that when passing through the concrete, ultrasound propagates both in the coarse aggregate and in the mortar part connecting the aggregate granules, and its speed to a greater extent depends on the propagation velocity in the coarse aggregate. However, the strength of concrete mainly depends on the strength of the mortar component. The influence 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 dependency
"speed-strength"
1 - 400 kg / m 3; 2 - 350 kg / m 3; 3 - 300 kg / m 3; 4 - 250 kg / m 3; 5 - 200 kg/m3
Influence of water-cement ratio
With a decrease in W / C, the density and strength of concrete increase, respectively, the speed of ultrasound increases. With an increase in W / C, an inverse relationship is observed. Consequently, the change in W / C does not introduce significant deviations in the established dependence "velocity-strength. Therefore, when constructing calibration curves for changing the strength of concrete, it is recommended to use different W / C.
View Influenceand amount of coarse aggregate
The type and amount of coarse filler have a significant impact on the change in the "speed-strength" dependence. The speed of ultrasound in the 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 such a phenomenon when the use of less durable crushed stone, but with a rough surface, allows you to get 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 filler has a great influence on the speed of ultrasound.
As the concrete is saturated with crushed stone, the value of the ultrasonic velocity increases. The type and amount of coarse aggregate affect the "speed - strength" bond 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 dependence "speed-strength":
1 - cement stone; 2 - concrete with aggregate size up to 30 mm
Rice. 2.2.55. Dependence "speed-strength" for concretes with different fineness of aggregates: 1-1 mm; 2-3 mm; 3-7 mm; 4-30 mm
Rice. 2.2.56. "Speed-strength" dependence for concrete with filler from:
1-sandstone; 2-limestone; 3-granite; 4-basalt
It can be seen 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 intensively than strength.
Influence of humidity and temperature
The moisture content of concrete has an ambiguous effect on its strength and ultrasonic velocity. With an increase in the moisture content of concrete, the compressive strength decreases due to a change in intercrystalline bonds, but the speed of ultrasound increases, since air pores and microcracks are filled with water, a faster in water than in air.
The temperature of concrete in the range of 5-40 ° C has practically no effect on strength and speed, but an increase in 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 by the ultrasonic method at a negative temperature.
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 particles of quartz sand.
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 technology of manufacturing reinforced concrete products. For example, with a cassette production technology, a greater plasticity of the concrete mixture is required, which is achieved by an increased consumption of cement and water. In this case, the mortar part of the concrete increases.
In the case of bench technology, especially for 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. From this it follows that the property of hardened concrete is influenced by a large number of factors, and the influence is ambiguous and is of a random 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 the propagation of ultrasound through concrete has not yet been developed, which is explained by ) First of all, the presence of the above numerous factors that affect the strength and acoustic properties of concrete in different ways. This situation is exacerbated by the fact that there is not yet developed general theory propagation of ultrasonic vibrations through the material with a high degree heterogeneity. This is the only reason why the speed of ultrasound in concrete is determined as for a homogeneous material by the formula
where L is the path traveled by ultrasound, m (base);
t is the time spent on the passage of this path, μs.
Let us consider in more detail the scheme of propagation of pulsed ultrasound through concrete as through an inhomogeneous material. But first, we will limit the area in which our reasoning will be valid by considering the composition of the concrete mix, which is most common in reinforced concrete plants 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 a coarse aggregate. Let us conditionally assume that hardened concrete consists of two components: a relatively homogeneous mortar part with density ρ and velocity V and coarse aggregate with ρ and V .
Given the above assumptions and limitations, hardened concrete can be considered as a solid medium with an acoustic impedance:
Let us consider the scheme of propagation of the head ultrasonic wave from the emitter 1 to the receiver 2 through the hardened concrete with the 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 of all enters the contact layer 3 located between the radiating surface and concrete. To pass through the contact layer of an ultrasonic wave, it must be filled with a conductive liquid or lubricant, which is most often used as technical vaseline. After passing through the contact layer (in 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 smaller part 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 passing a certain distance Δ l 1, after time Δ t 1 head wave on a certain area will meet one or more coarse aggregate granules, partially reflected from them, and most of them will enter the granules and begin to propagate in them. Between the granules, the wave will continue to propagate through the solution part.
Taking into account the accepted condition that the speed of ultrasound in the coarse filler material is greater than in the mortar part, the distance d, equal to the average value of the crushed stone diameter, the wave that propagated through the granules at a speed V 2 will be the first to pass, and the wave that has passed through the mortar part will be delayed .
After passing through the first coarse aggregate granules, the wave will approach the interface with the mortar part, be partially reflected, and partially enter it. In this case, the granules through which the head wave passed can be further considered as elementary spherical sources of ultrasonic wave radiation into the mortar part of concrete, to which the 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 next elementary sources. Thus, after time t, having passed the entire thickness of concrete L and the second contact layer 3, the head wave will enter the receiver 2, where it will be converted into an electrical signal.
It follows from the considered scheme that the head wave from the emitter 1 to the receiver 2 propagates along the path passing through the coarse aggregate granules and the mortar part connecting these granules, and this path is determined from the condition of the minimum time spent t.
Hence the time t is
where is the time spent on the passage of the mortar part connecting the granules;
Time taken to pass through the granules. The path L traveled by ultrasound is equal to
where: is the total path traveled by the head wave through the mortar part;
The total path traveled by the head wave through the granules.
The total distance L that the bow wave will travel may be greater than the geometric distance between the transmitter and receiver, since the wave propagates along the path of maximum velocity, and not along the minimum geometric distance.
The time taken by ultrasound to pass 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 coarse aggregate granules and the mortar part. If the diameter of the granules is equal to the wavelength or half of it, then acoustic resonance may occur inside the granule. The effect of interference and resonance can be observed in the spectral analysis of a pack of ultrasonic waves transmitted through concrete with different aggregate sizes.
The scheme of propagation of the head wave of pulsed ultrasound considered above is valid only for concretes 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 coarse aggregate granules are obtained exceed the strength and speed in the mortar part of concrete. Such properties are possessed by the majority of concretes used in reinforced concrete plants and construction sites, which use crushed stone from limestone, marble, granite. For expanded clay concrete, foam concrete, concrete with tuff filler, the ultrasound propagation scheme 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 speed of ultrasound increases with a slight increase (and sometimes decrease) in the strength of concrete.
On fig. 2.2.56 it is noticeable that with an increase in the speed of ultrasound in the material of coarse aggregate, its speed in concrete increases.
The increase in velocity in concrete with larger aggregates (Fig. 2.2.55) is also explained by this scheme, since with an increase in diameter, the path of ultrasound through the aggregate material lengthens.
The proposed scheme of ultrasound propagation will make it possible to objectively assess the capabilities of the ultrasonic method for flaw detection and concrete strength control.
The section of the physics of ultrasound 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.
Velocity of ultrasound and specific wave resistance of human tissues (according to V.N. Demidov)
An ultrasonic wave, having reached the boundary of two media, can be reflected or go further. The reflection coefficient of ultrasound depends on the difference in ultrasonic resistance at the interface between the media: 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 media interface: the more the angle approaches a straight line, the stronger the degree of reflection.
Thus, knowing this, it is possible to find the optimal ultrasonic frequency, which gives the maximum resolution with sufficient penetrating power.
The basic principles on which the operation of ultrasonic diagnostic equipment is based, - this is Spread and reflection of ultrasound.
The principle of operation of diagnostic ultrasound devices is to reflection of ultrasonic vibrations from the interfaces of tissues with a certain value of acoustic resistance. It is believed that the reflection of ultrasonic waves at the interface occurs when the difference between the 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 between the media, and the degree of reflection depends on the angle of incidence of the ultrasonic beam.
Obtaining ultrasonic vibrations
The production of ultrasonic vibrations is based on the direct and inverse piezoelectric effect, the essence of which lies in the fact that when electric charges are created on the surface of the crystal faces, the latter begins to shrink 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 the ultrasonic sensor
The sensor contains a piezocrystal, on the faces of which electrodes are fixed. Behind the crystal is a layer of substance that absorbs ultrasound, which propagates in the direction opposite to that required. This improves the quality of the resulting ultrasonic beam. Typically, the ultrasonic beam generated by the transducer has a maximum power in the center, and it decreases at the edges, as a result of which the resolution of ultrasound is different in the center and around the periphery. At the center of the beam, you can always get stable reflections from both more and less dense objects, while at the periphery of the beam, less dense objects can reflect, and denser objects can be reflected as less dense ones.
Modern piezoelectric materials allow transducers 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 waveform that is more resistant to distortion of the frequency band and offset of the center frequency.
In the latest designs of ultrasonic devices, high resolution and image clarity are provided by using a dynamic focus system and a broadband echo filter for focusing incoming and outgoing ultrasonic beams by means of a microcomputer. In this way, ideal profiling and enhancement of the ultrasound beam and the lateral resolution characteristics of images of deep structures obtained by sector scanning are ensured. Focus parameters are set according to frequency and type of sensor. The broadband echo filter provides optimum resolution by perfectly matching frequencies to absorb soft tissue echoes. The use of high-density multi-element sensors helps eliminate false echoes due to side and back diffraction.
Today in the world there is a fierce competition between companies 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 has developed the 128 XP™ Platform, a foundational module for continuous improvement that allows physicians to expand the scope of clinical research based on needs.
The Platform uses 128 electronically independent channels that can be used simultaneously for both transmission and reception, 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 presented 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 reflection areas of the echo signals. 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 on the neck and their intracranial branches, etc. For these purposes, a frequency of 0.88-1.76 MHz is used.
2D scanners
2D scanners are divided into manual scanning and real-time scanning devices.
Currently, for the study of surface structures and internal organs, only real-time instruments are used, 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 a frequency of 2.5 to 8 MHz are more often used.
3D scanners
For their use, certain conditions are required:
- the presence of a formation that has a rounded or well-contoured shape;
- the presence of structural formations located in the liquid spaces (fetus in the uterus, eyeball, stones in the gallbladder, foreign body, polyp in the stomach or intestines filled with liquid, appendix against the background of inflammatory fluid, as well as all abdominal organs against the background of ascitic fluid );
- sedentary structural formations (eyeball, prostate, etc.).
Thus, taking into account these requirements, three-dimensional scanners can be successfully used for research in obstetrics, with volume pathology of the abdominal cavity for more accurate differentiation from other structures, in urology for examining the prostate in order to differentiate the structural penetration of the capsule, in ophthalmology, cardiology, neurology and angiology.
Due to the complexity of use, the high cost of equipment, the presence of many conditions and restrictions, they are rarely used at present. However 3D scanning — this is echography of the future.
Doppler echography
The principle of Doppler sonography 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 ultrasound and the angle between the direction of propagation of ultrasound and the direction of flow. This method has been successfully applied in cardiology.
The method is also of interest for internal medicine in connection with its ability to provide reliable information about the state of the blood vessels of internal organs without the introduction of contrast agents into the body.
It is more often used in a comprehensive examination of patients with suspected portal hypertension in its early stages, in determining the severity of portal circulation disorders, determining the level and cause of blockade in the portal vein system, and also to study changes in portal blood flow in patients with liver cirrhosis when administering medications. (beta-blockers, ACE inhibitors, etc.).
All devices are equipped with ultrasonic sensors of two types: electromechanical and electronic. Both types of sensors, but more often electronic ones, have modifications for use in various areas medicine in the examination of adults and children.
AT classic version 4 methods of electronic scanning are applied in real time : sector, linear, convex and trapezoidal, each of which is characterized by specific features in relation to the field of observation. The researcher can choose the scanning method depending on the task before him and the location.
Sector Scan
Advantages:
- large field of view when examining deep areas.
Application area:
– craniological studies of newborns through a large fontanel;
– cardiological studies;
- general abdominal examinations of the pelvic organs (especially in gynecology and in the study of the prostate), organs of the retroperitoneal system.
Line scan
Advantages:
- a large field of view when examining shallow areas of the body;
- high resolution in the study of deep areas of the body due to the use of a multi-element sensor;
Application area:
— surface structures;
— cardiology;
– examination of the pelvic organs and perirenal region;
- in obstetrics.
Convex scanning
Advantages:
- a small area of contact with the surface of the patient's body;
- a large field of observation in the study of deep areas.
Application area:
- general abdominal examinations.
Trapezoidal Scan
Advantages:
- a large field of observation when examining close to the surface of the body and deeply located 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 gives a wider view in the near and far fields.
The scope is the same as for sector scanning.
Scanning in zoom area selection mode
This is a special scanning of the area 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 with full use of acoustic and raster lines. Improving image quality is expressed in optimal line and pixel density, higher resolution, higher frame rate and larger image.
With a normal section, the same acoustic information remains, while with the usual RES zoom zone selection format, image magnification with increased resolution and more diagnostic information is achieved.
Visualization Multi-Hertz
Broadband piezoelectric materials provide modern sensors with the ability to operate over a wide frequency range; provide the ability to select a specific frequency from a wide band of frequencies available in the sensors while maintaining image uniformity. This technology allows you to change the frequency of the sensor with just the push of a button, without wasting time to replace the sensor. And this means that one sensor is equivalent to two or three particular 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 registered using a printer on special paper, on a computer cassette or video tape with computer processing of information.
Ultrasound imaging 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 put into operation the first real-time device Aloca SSD 202 D (1982). To my admiration that Japan had developed computer-assisted ultrasonic technology, he replied: “A computer is good, but if another computer (pointing to the head) does not work well, then that computer is worthless.”
1. The speed of propagation of ultrasound depends on the temperature and pressure in the pipeline. Ultrasonic speed at different values water temperature and atmospheric pressure is given in Table D.1.
Table E.1
Alexandrov A.A., Trakhtengerts M.S. Thermophysical properties water at atmospheric pressure. M. Publishing house of standards, 1977, 100s. ( public service standard reference data. Ser. monographs).
2. When using a flow meter to measure the flow and volume of water in water and heat supply systems, the speed of ultrasound is determined from the data in Table. E.2 by the method of linear interpolation in temperature and pressure in accordance with the formula:
where c(t,P) is the speed of ultrasound in the fluid flowing through the pipeline, m/s;
c(t1) is the tabular value of the speed of ultrasound at a temperature lower than the measured one, m/s;
c(t2) is the tabular value of the speed of ultrasound at a temperature higher than the measured one, m/s;
c(P1) is the tabular value of the speed of ultrasound at a pressure less than the measured one, m/s;
c(P2) - table value of the speed of ultrasound at a pressure greater than the measured one, m/s;
t is the water temperature in the pipeline, ºС;
P is the water pressure in the pipeline, MPa;
t1, t2 - tabular values of temperatures, ºС;
P1, P2 - tabular values of pressure, MPa;
NOTE.
1. The values c(t1) and c(t2) are determined from the data in Table. D.1. The values c(P1) and c(P2) are determined from the data in Table. D 2. at a temperature closest to the temperature of the water in the pipeline.
2. Measurements of temperature and pressure of water in the pipeline should be carried out with an error of no more than ±0.5 ºС and ±0.5 MPa, respectively.
Table E.2
Continuation of table D.2
Aleksandrov A.A., Larkin D.K. Experimental determination of the speed of ultrasound in a wide range of temperatures and pressures. Journal "Heat power", №2, 1976, p.75.
3. In the absence of tables of the dependence of the speed of ultrasound on the temperature of the liquid, the speed of ultrasound can be determined using the device shown in Fig. E.1. Immediately before measuring the ultrasonic velocity, the body of the device (steel bracket) is immersed in the test liquid, and the thickness gauge is adjusted to measure the ultrasonic velocity. Then an ultrasonic thickness gauge directly measures the speed of ultrasound.
To measure the velocity of ultrasound in a liquid, it is also possible to use the US-12 IM device (SCHO 2.048.045 TO) or other types of thickness gauges.
Fig. E.1. A device for measuring the speed of ultrasound in a liquid.