Oscillations and waves. Oscillations are the repeated repetition of identical or close to identical processes. The process of propagation of vibrations in a medium is called wave. The line indicating the direction of propagation of the wave is called a ray, and the boundary that defines the oscillating particles from particles of the medium that have not yet begun to oscillate is called the wave front.

The time during which a complete cycle of oscillations occurs is called period T and is measured in seconds. The value ƒ = 1 / T, showing how many times per second the oscillation is repeated, is called frequency and is measured in s -1.

The quantity ω, showing the number of complete revolutions of a point around a circle in 2T s, is called the circular frequency ω = 2 π / T = 2 π ƒ and is measured in radians per second (rad/s).

The wave phase is a parameter that shows how much of the period has passed since the start of the last cycle of oscillations.

Wavelength λ is the minimum distance between two points oscillating in the same phase. Wavelength is related to frequency ƒ and speed with the relation: λ = c / ƒ. A plane wave propagating along the horizontal X axis is described by the formula:

u = U cos (ω t - kh) ,

where k = 2 π /λ. - wave number; U is the amplitude of oscillations.

From the formula it is clear that the value of u changes periodically in time and space.

The displacement of particles from the equilibrium position u and the acoustic pressure p are used as quantities that change during oscillations.

In ultrasonic (US) flaw detection, vibrations with a frequency of 0.5...15 MHz (longitudinal wavelength in steel 0.4...12 mm) and a displacement amplitude of 10 -11 ...10 -4 mm (arising in steel at a frequency of 2 MHz acoustic stresses are 10... 10 8 Pa).

The intensity of wave I is equal to I = р 2 /(2ρс),

where ρ is the density of the medium in which the wave propagates.

The intensity of the waves used for control is very low (~10 -5 W/m 2). During flaw detection, it is not the intensity, but the amplitude of waves A that is recorded. Usually, the attenuation of the amplitude A" relative to the amplitude of the oscillations A o (probing pulse) excited in the product is measured, i.e. the ratio A "/A o. For this, logarithmic units of decibels (dB) are used, i.e. A" / A o = 20 Ig A" / A o.

Types of waves. Depending on the direction of particle oscillations relative to the beam, several types of waves are distinguished.

A longitudinal wave is a wave in which the oscillatory motion of individual particles occurs in the same direction in which the wave propagates (Fig. 1).

A longitudinal wave is characterized by the fact that the medium alternates between areas of compression and rarefaction, or high and low pressure, or high and low density. Therefore, they are also called pressure, density or compression waves. Longitudinal can spread in solids, liquids, and gases.

Rice. 1. Vibration of medium particles v in a longitudinal wave.

Shear (transverse) called a wave in which individual particles oscillate in a direction perpendicular to the direction of propagation of the wave. In this case, the distance between the individual vibration planes remains unchanged (Fig. 2).

Rice. 2. Oscillation of medium particles v in a transverse wave.

Longitudinal and transverse waves, collectively called “body waves,” can exist in an unlimited medium. These are most widely used for ultrasonic flaw detection.

Sound wave propagation speed c is the speed of propagation of a certain state in a material medium (for example, compression or rarefaction for a longitudinal wave). The speed of sound is different for different types of waves, and for transverse and longitudinal waves it is a characteristic of the medium that does not depend on the parameters of the ultrasonic wave.

The speed of propagation of a longitudinal wave in an unbounded solid body is determined by the expression

where E is Young's modulus, defined as the ratio between the magnitude of the tensile force applied to a certain rod and the resulting deformation; v - Poisson's ratio, which is the ratio of the change in the width of the rod to the change in its length, if the rod is stretched along its length; ρ is the density of the material.

The velocity of a shear wave in an unbounded solid is expressed as follows:

Since in metals v ≈ 0.3, there is a relationship between the longitudinal and transverse waves

c t ≈ 0.55 s l.

Surface waves(Rayleigh waves) are elastic waves that propagate along the free (or lightly loaded) boundary of a solid body and quickly decay with depth. A surface wave is a combination of longitudinal and transverse waves. Particles in a surface wave perform oscillatory motion along an elliptical trajectory (Fig. 3). The major axis of the ellipse is perpendicular to the boundary.

Since the longitudinal component included in the surface wave decays with depth faster than the transverse component, the elongation of the ellipse changes with depth.

The surface wave has a speed of s = (0.87 + 1.12v) / (1+v)

For metals with s ≈ 0.93c t ≈ 0.51 c l.

Depending on the geometric shape of the front, the following types of waves are distinguished:

• spherical - a sound wave at a short distance from a point source of sound;
• cylindrical - a sound wave at a short distance from the sound source, which is a long cylinder of small diameter;
• flat - it can be emitted by an endlessly oscillating plane.

The pressure in a spherical or plane sound wave is determined by the relation:

where v is the magnitude of the oscillatory velocity.

The quantity ρс = z is called acoustic resistance or acoustic impedance.

Rice. 3. Oscillation of medium particles v in a surface wave.

If the acoustic impedance is large, then the medium is called hard, but if the impedance is small, it is called soft (air, water).

Normal (waves in plates), are called elastic waves propagating in a solid plate (layer) with free or lightly loaded boundaries.

Normal waves come in two polarizations: vertical and horizontal. Of the two types of waves, Lamb waves are the most widely used in practice - normal waves with vertical polarization. They arise due to resonance during the interaction of the incident wave with repeatedly reflected waves inside the plate.

To understand the physical essence of waves in plates, let us consider the issue of the formation of normal waves in a liquid layer (Fig. 4).

Rice. 4. On the issue of the emergence of normal will in the liquid layer.

Let a plane wave of thickness h fall from the outside at an angle β. Line AD shows the front of the incident wave. As a result of refraction at the boundary, a wave with a CB front appears in the layer, propagating at an angle α and undergoing multiple reflections in the layer.

At a certain angle of incidence β, the wave reflected from the lower surface is in phase with the direct wave coming from the upper surface. This is the condition for the occurrence of normal waves. The angle a at which this phenomenon occurs can be found from the formula

h cos α = n λ 2 / 2

Here n is an integer; λ 2 - wavelength in the layer.

For a solid layer, the essence of the phenomenon (resonance of body waves during oblique incidence) is preserved. However, the conditions for the formation of normal waves are very complicated due to the presence of longitudinal and transverse waves in the plate. Different types of waves that exist at different values ​​of n are called normal wave modes. Ultrasonic waves with odd values n are called symmetrical, since the movement of particles in them is symmetrical relative to the axis of the plate. Waves with even values ​​of n are called antisymmetric(Fig. 5).

Rice. 5. Oscillation of medium particles v in a normal wave.

Head waves. In real conditions of ultrasonic testing with an inclined transducer, the ultrasonic wave front of the radiating piezoelectric element has a non-planar shape. From the emitter, the axis of which is oriented at the first critical angle to the interface, longitudinal waves with angles slightly smaller and slightly larger than the first critical angle also fall on the boundary. In this case, a number of types of ultrasonic waves are excited in the steel.

A non-uniform longitudinal surface wave propagates along the surface (Fig. 6). This wave, consisting of surface and volumetric components, is also called leaky or creeping. Particles in this wave move along trajectories in the form of ellipses, close to circles. The phase velocity of the leaking wave c in slightly exceeds the speed of the longitudinal wave (for steel with b = 1.04 c l).

These waves exist at a depth approximately equal to the wavelength and decay rapidly as they propagate: the wave amplitude decays 2.7 times faster at a distance of 1.75λ. along the surface. The weakening is due to the fact that at each point of the interface, transverse waves are generated at an angle α t2 equal to the third critical angle, called lateral waves. This angle is determined from the relation

sin α t2 = (c t2 - c l2)

for steel α t2 = 33.5°.

Rice. 6. Acoustic field of the head wave transducer: PEP - piezoelectric transducer.

In addition to the leaking wave, the head wave is also excited, which is widely used in the practice of ultrasonic testing. The head wave is a longitudinal-subsurface wave excited when an ultrasonic beam is incident on the interface at an angle close to the first critical one. The speed of this wave is equal to the speed of the longitudinal wave. The head wave reaches its amplitude value under the surface along the beam with an input angle of 78°.

Rice. 7. Amplitude of head wave reflection depending on the depth of flat-bottomed holes.

The head wave, like the leaky wave, generates lateral transverse ultrasonic waves at the third critical angle to the interface. Simultaneously with the excitation of the longitudinal surface wave, a reverse longitudinal surface wave is formed - the propagation of an elastic disturbance in the direction opposite to the direct radiation. Its amplitude is ~100 times less than the amplitude of the direct wave.

The head wave is insensitive to surface irregularities and responds only to defects lying under the surface. The attenuation of the amplitude of a longitudinal-subsurface wave along a beam of any direction occurs as in an ordinary volumetric longitudinal wave, i.e. proportional to l/r, where r is the distance along the beam.

In Fig. Figure 7 shows the change in the amplitude of the echo signal from flat-bottomed holes located at different depths. Sensitivity to defects near the surface is close to zero. The maximum amplitude at a distance of 20 mm is achieved for flat-bottomed holes located at a depth of 6 mm.

Other related pages

Ultrasound is the name given to longitudinal mechanical waves with vibration frequencies above 20 kHz. Like sound waves, an ultrasonic wave is an alternation of condensations and rarefactions of the medium. In each medium, the speed of propagation 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).

Ultrasound sources are special electromechanical emitters. One type of emitter works based on the phenomenon of magnetostriction, when the dimensions of some 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, voltage is applied to the 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 emitters operates on the basis of the piezoelectric effect, when the dimensions of some bodies—ferroelectric materials—change in an alternating electric field. For this type of emitters, higher frequency oscillations can be obtained - up to 500 MHz. From an alternating current source, voltage is also applied to the side faces of a rod made of ferroelectric (quartz, tourmaline), 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.

Ultrasound receivers operate on the principle of the inverse phenomena of magnetostriction and the piezoelectric effect: an ultrasonic wave causes fluctuations in the linear dimensions of bodies, when the bodies are in the field of an ultrasonic wave, the size fluctuations are accompanied by the appearance of either alternating magnetic or alternating electric fields in the material. These fields arising in the corresponding sensor are recorded by some kind of indicator, for example an oscilloscope. The more intense the ultrasound, the greater the amplitude of the mechanical vibrations of the sample - sensor and the greater the amplitude of the resulting alternating magnetic or electric fields.

Features of ultrasound.

As mentioned above, in each medium the speed of propagation of both sound and ultrasound is the same. The most important feature of ultrasound is the narrowness of the ultrasonic beam, which allows it to influence any objects locally. In inhomogeneous media with small inhomogeneities, when the sizes of inclusions are approximately equal but larger than the wavelength (L ≈ λ), the phenomenon of diffraction occurs. If the size of the inclusions is much larger than the wavelength (L >> λ), the propagation of ultrasound occurs straight. In this case, it is possible to obtain ultrasonic shadows from such inclusions, which is used in various types of diagnostics - both technical and medical. An important theoretical point when using ultrasound is the passage of ultrasound from one medium to another. Such a characteristic of waves as frequency does not change. On the contrary, the speed and wavelength can change. 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– Figure 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 = ρ 1 * V 1, Z 2 = ρ 2 * V 2. The penetration coefficient is greatest - 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 general, coefficient β is calculated using the formula:

For the transition of ultrasound from air to human skin β = 0.08%, for the transition from glycerin to skin β = 99.7%.

Absorption of ultrasound in various environments.

In homogeneous media, ultrasound is absorbed, like any type of radiation, according to the law of the exponential function:

The value L' - called the half-absorption layer - is the distance at which the wave intensity is halved. The half absorption layer depends on the ultrasound frequency 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 ultrasound absorption occur:

Substance	Water	Blood	Cartilage	Bone
L'	300 cm	2 – 8 cm	0.24 cm	0.05 cm

The effect of ultrasound on body tissue.

There are three types of ultrasound action:

Mechanical,

Thermal,

Chemical.

The degree of impact of one or another type is determined by intensity. In this regard, in medicine there is a distinction three levels of ultrasound intensities:

Level 1 - up to 1.5 W / cm 2,

Level 2 - from 1.5 to 3 W / cm 2,

Level 3 - from 3 to 10 W/cm2.

All three types of ultrasound effects on tissue are associated with the phenomenon of cavitation - these are short-term (half periods of vibration of medium particles) occurrence of microscopic cavities in places where the medium is rarefied. These cavities are filled with liquid vapor, and in the high-pressure phase (the other half of the oscillation period of the medium particles), the resulting cavities collapse. At high wave intensities, the collapse of cavities with liquid vapors contained in them can lead to destructive mechanical action. Naturally, the collapse of microcavities is accompanied by a thermal effect. The chemical action of ultrasound is also associated with the process of collapse of microcavities, since in this case the particles of the medium reach high speeds of translational motion, which can cause the phenomenon of ionization, breaking of chemical bonds, and the formation of radicals. The resulting radicals can interact with proteins, proteins, and nucleic acids and cause undesirable chemical effects.

6. Features of blood flow through large vessels, medium and small vessels, capillaries;
blood flow when a vessel narrows, sound effects.

The speed of blood flow in different vessels is different. Approximate values ​​of this speed are presented in table. 2.1.

Table 2.1. Blood speed and pressure in various vessels

At first glance, it seems that the given values ​​contradict the continuity equation - in thin capillaries the blood flow speed is less than in arteries. However, this discrepancy is apparent. The fact is that in the table. Figure 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).

Subsequently, when the capillaries merge into venules, into veins, up to the vena cava, the total lumen of the vessels again decreases and the speed of blood flow increases again. However, for a number of reasons, the speed of blood flow when the vena cava enters the heart does not increase to the initial value, but to approximately ½ of it (Fig. 2.7).

Aorta arteries arterioles capillaries venules veins vena cava

Rice. 2.7. Distribution of blood flow velocities in different sections

of cardio-vascular system

In the capillaries and veins the blood flow is constant, in other parts of the cardiovascular system there are pulse waves.

A 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 the arteries extending from it. If the walls of these vessels were rigid, then the pressure arising in the blood at the exit from the heart would be transmitted to the periphery at the speed of sound. However, the elasticity of the walls of blood vessels leads to the fact that during systole, the blood pushed out by the heart stretches the aorta, arteries and arterioles. Large vessels receive more blood during systole than flows to the periphery. The normal systolic pressure (S) of a person is approximately 16 kPa. During relaxation of the heart (diastole), the distended blood vessels collapse and the potential energy imparted to them by the heart through the blood is converted into kinetic energy of the blood flow, while maintaining a diastolic pressure (PP) of approximately 11 kPa.

R, Pa R, Pa

1 - in the aorta 2 - in arterioles

Rice. 2.8. Pressure fluctuations in blood 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 P 0, max is equal to the difference between systolic (P C) and diastolic (PD) pressures: P 0, max = P C - P D. The attenuation of the amplitude of the pulse wave as it propagates 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 blood particles = 0.3 - 0.5 m/s. During the expulsion of blood from the ventricles (systole time) t s = 0.3 s, the pulse wave manages to spread over a distance

L p = ·t » 2m,

that is, to cover all large vessels - the aorta and arteries. This means that the front of the pulse wave will reach the extremities before the pressure in the aorta begins to decline.

Experimental determination of pulse wave speed is the basis for diagnosing the condition of blood vessels. With age, the elasticity of blood vessels increases 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 pulse wave front

3 - pulse wave in the artery 4 - decrease in high 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 elastic modulus (Young’s modulus); ρ is the density of the vessel’s substance; h is the thickness of the vessel wall; 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, the modulus of elasticity of blood vessels increases with age, therefore, the speed of the pulse wave also increases.

Along with the pulse wave, sound waves can also propagate in the “vessel-blood” system, 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 movement processes 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).

Blood flow in 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 produced 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 air flow in the nasal cavity is normally laminar. However, with inflammation or any other deviations from the norm, it can become turbulent, which will lead to additional work of the respiratory muscles.

The transition from a laminar flow form to a turbulent one occurs not only during flow in a pipe (channel), it is characteristic of almost all flows of a viscous fluid. In particular, the flow of liquid around the profile of a ship or submarine, the body of a fish, or the wing of an airplane or bird is also characterized by a laminar-turbulent transition; in this case, the characteristic size of the streamlined body and a constant depending on the shape of the body must be substituted into the formula.

Related information.

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 greater 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 right angle, 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 instruments 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 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 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.”

13. Acoustics(from the Greek ἀκούω (akuo) - hear) - the science of sound, studying the physical nature of sound and problems associated with its occurrence, distribution, perception and impact. Acoustics is one of the areas of physics (mechanics) that studies elastic vibrations and waves from the lowest (conventionally from 0 Hz) to high frequencies.

Acoustics is an interdisciplinary science that uses a wide range of disciplines to solve its problems: mathematics, physics, psychology, architecture, electronics, biology, medicine, hygiene, music theory and others.

Sometimes (in everyday life) under acoustics also understand an acoustic system - an electrical device designed to convert a variable frequency current into sound vibrations using electro-acoustic conversion. The term acoustics is also applicable to denote vibrational properties associated with the quality of sound propagation in any system or any room, for example, “good acoustics of a concert hall.”

The term "acoustics" (French) acoustique) was introduced in 1701 by J. Sauveur.

Tone in linguistics, the use of pitch to distinguish meaning within words/morphemes. Tone should be distinguished from intonation, that is, changes in pitch over a relatively large speech segment (statement or sentence). Various tone units that have a semantic-distinctive function can be called tonemes (by analogy with a phoneme).

Tone, like intonation, phonation and stress, refers to suprasegmental, or prosodic, features. The carriers of tone are most often vowels, but there are languages ​​where consonants, most often sonants, can also play this role.

A tonal, or tonal, language is a language in which each syllable is pronounced with a specific tone. A variety of tone languages ​​are also languages ​​with musical stress, in which one or more syllables in a word are emphasized, and different types of emphasis are contrasted with tone features.

Tone contrasts can be combined with phonation ones (such are many languages ​​of Southeast Asia).

Noise- random oscillations of various physical natures, characterized by the complexity of their temporal and spectral structure. Originally the word noise referred exclusively to sound vibrations, but in modern science it was extended to other types of vibrations (radio, electricity).

Noise- a set of aperiodic sounds of varying intensity and frequency. From a physiological point of view, noise is any unfavorable perceived sound.

Acoustic, sonic boom- this is the sound associated with the shock waves created by the supersonic flight of an aircraft. A sonic boom creates a huge amount of sound energy, similar to an explosion. The sound of a whip is a clear example of an acoustic boom. This is the moment when the plane breaks the sound barrier, then, breaking through its own sound wave, it creates a powerful, instantaneous sound that spreads to the sides. But on the plane itself it is not audible, since the sound “lags behind” it. The sound resembles the shot of a super-powerful cannon, shaking the entire sky, and therefore supersonic aircraft are recommended to switch to supersonic distance from cities so as not to disturb or frighten citizens

Physical parameters of sound

Oscillatory speed measured in m/s or cm/s. In terms of energy, real oscillatory systems are characterized by a change in energy due to partial expenditure on work against friction forces and radiation into the surrounding space. In an elastic medium, vibrations gradually die out. For characteristics damped oscillations Damping coefficient (S), logarithmic decrement (D) and quality factor (Q) are used.

Attenuation coefficient reflects the rate at which the amplitude decreases over time. If we denote the time during which the amplitude decreases by e = 2.718 times, then:

The decrease in amplitude per cycle is characterized by a logarithmic decrement. The logarithmic decrement is equal to the ratio of the oscillation period to the damping time:

If an oscillatory system with losses is acted upon by a periodic force, then forced oscillations , the nature of which to one degree or another repeats changes in external forces. The frequency of forced oscillations does not depend on the parameters of the oscillatory system. On the contrary, the amplitude depends on the mass, mechanical resistance and flexibility of the system. This phenomenon, when the amplitude of the oscillatory velocity reaches its maximum value, is called mechanical resonance. In this case, the frequency of forced oscillations coincides with the frequency of natural undamped oscillations of the mechanical system.

At impact frequencies significantly lower than the resonant one, the external harmonic force is balanced almost exclusively by the elastic force. At excitation frequencies close to resonance, friction forces play the main role. Provided that the frequency of the external influence is significantly greater than the resonant one, the behavior of the oscillatory system depends on the force of inertia or mass.

The ability of a medium to conduct acoustic energy, including ultrasonic energy, is characterized by acoustic resistance. Acoustic impedance environment is expressed by the ratio of sound density to the volumetric velocity of ultrasonic waves. The specific acoustic resistance of a medium is determined by the ratio of the amplitude of sound pressure in the medium to the amplitude of the vibrational velocity of its particles. The greater the acoustic resistance, the higher the degree of compression and rarefaction of the medium for a given amplitude of vibration of the particles of the medium. Numerically, the specific acoustic resistance of the medium (Z) is found as the product of the density of the medium () and the speed (c) of propagation of ultrasonic waves in it.

Specific acoustic impedance is measured in pascal-second on meter(Pa s/m) or dyne s/cm³ (GHS); 1 Pa s/m = 10 −1 dyne s/cm³.

The value of the specific acoustic resistance of a medium is often expressed in g/s cm², with 1 g/s cm² = 1 dyne s/cm³. The acoustic impedance of a medium is determined by the absorption, refraction and reflection of ultrasonic waves.

Sound or acoustic pressure in a medium is the difference between the instantaneous value of pressure at a given point in the medium in the presence of sound vibrations and static pressure at the same point in their absence. In other words, sound pressure is a variable pressure in a medium caused by acoustic vibrations. The maximum value of variable acoustic pressure (pressure amplitude) can be calculated through the amplitude of particle vibration:

where P is the maximum acoustic pressure (pressure amplitude);

At a distance of half the wavelength (λ/2), the amplitude value of the pressure changes from positive to negative, that is, the pressure difference at two points spaced from each other by λ/2 along the wave propagation path is equal to 2P.

To express sound pressure in SI units, Pascal (Pa) is used, equal to a pressure of one newton per square meter (N/m²). Sound pressure in the SGS system is measured in dyn/cm²; 1 dyne/cm² = 10 −1 Pa = 10 −1 N/m². Along with the indicated units, non-system units of pressure are often used - atmosphere (atm) and technical atmosphere (at), with 1 atm = 0.98·10 6 dynes/cm² = 0.98·10 5 N/m². Sometimes a unit called a bar or microbar (acoustic bar) is used; 1 bar = 10 6 dynes/cm².

The pressure exerted on the particles of the medium during wave propagation is the result of the action of elastic and inertial forces. The latter are caused by accelerations, the magnitude of which also increases during the period from zero to maximum (amplitude value of acceleration). In addition, during the period the acceleration changes its sign.

The maximum values ​​of acceleration and pressure that arise in a medium when ultrasonic waves pass through it do not coincide in time for a given particle. At the moment when the acceleration difference reaches its maximum, the pressure difference becomes zero. The amplitude value of acceleration (a) is determined by the expression:

If traveling ultrasonic waves encounter an obstacle, it experiences not only variable pressure, but also constant pressure. The areas of condensation and rarefaction of the medium that arise during the passage of ultrasonic waves create additional changes in pressure in the medium in relation to the external pressure surrounding it. This additional external pressure is called radiation pressure (radiation pressure). This is the reason why, when ultrasonic waves pass through the boundary of a liquid with air, fountains of liquid are formed and individual droplets are separated from the surface. This mechanism has found application in the formation of aerosols of medicinal substances. Radiation pressure is often used to measure the power of ultrasonic vibrations in special meters - ultrasonic balances.

Intensitysound (absolute) - a value equal to the ratio flow of sound energy dP through a surface perpendicular to the direction of propagation sound, to the square dS this surface:

Unit - watt per square meter(W/m2).

For a plane wave, the sound intensity can be expressed in terms of amplitude sound pressure p 0 And oscillatory speed v:

,

Where Z S - environment.

Sound volume is a subjective characteristic that depends on the amplitude, and therefore on the energy of the sound wave. The greater the energy, the greater the pressure of the sound wave.

Intensity level is an objective characteristic of sound.

Intensity is the ratio of sound power incident on a surface to the area of ​​that surface. It is measured in W/m2 (watts per square meter).

The intensity level determines how many times the sound intensity is greater than the minimum intensity perceived by the human ear.

Since the minimum sensitivity perceived by a person, 10 -12 W/m2, differs from the maximum sensitivity, which causes pain - 1013 W/m2, by many orders of magnitude, the logarithm of the ratio of sound intensity to the minimum intensity is used.

Here k is the intensity level, I is the sound intensity, I 0 is the minimum sound intensity perceived by a person or threshold intensity.

The meaning of the logarithm in this formula is if intensity I changes by an order of magnitude, then the intensity level changes by unity.

The unit of intensity level is 1 B (Bell). 1 Bell - an intensity level that is 10 times higher than the threshold.

In practice, intensity level is measured in dB (decibells). Then the formula for calculating the intensity level is rewritten as follows:

Sound pressure- variable redundant pressure, arising in an elastic medium when passing through it sound wave. Unit - pascal(Pa).

The instantaneous value of sound pressure at a point in the medium changes both with time and when moving to other points of the medium, therefore the root mean square value of this quantity, associated with sound intensity:

Where - sound intensity, - sound pressure, - specific acoustic impedance environment, - time averaging.

When considering periodic oscillations, the amplitude of sound pressure is sometimes used; so, for a sine wave

where is the sound pressure amplitude.

Sound pressure level (English SPL, Sound Pressure Level) - measured by relative scale sound pressure value referred to reference pressure = 20 μPa corresponding to the threshold audibility sinusoidal sound wave frequency 1 kHz:

dB.

Sound volume- subjective perception strength sound(absolute value of auditory sensation). Volume mainly depends on sound pressure, amplitudes And frequencies sound vibrations. Also, the volume of a sound is influenced by its spectral composition, localization in space, timbre, duration of exposure to sound vibrations and other factors (see. , ).

The unit of the absolute loudness scale is background . Volume of 1 phon is the volume of a continuous pure sine tone with frequency 1 kHz, creating sound pressure 2 mPa.

Sound volume level- relative value. It is expressed in backgrounds and is numerically equal to the level sound pressure(V decibels- dB) produced by a sine wave of frequency 1 kHz the same volume as the sound being measured (equal loudness to the given sound).

Dependence of volume level on sound pressure and frequency

The figure on the right shows a family of equal loudness curves, also called isophones. They are standardized graphs (international standard ISO 226) dependences of the sound pressure level on frequency at a given volume level. Using this diagram, you can determine the volume level of a pure tone of any frequency, knowing the level of sound pressure it creates.

Sound surveillance equipment

For example, if a sine wave with a frequency of 100 Hz creates a sound pressure level of 60 dB, then by drawing straight lines corresponding to these values ​​on the diagram, we find at their intersection an isophone corresponding to a volume level of 50 von. This means that this sound has a volume level of 50 background.

Isophone “0 background”, indicated by a dotted line, characterizes hearing threshold sounds of different frequencies for normal hearing.

In practice, what is often of interest is not the volume level expressed in backgrounds, but the value indicating how much louder a given sound is than another. Another interesting question is how the volumes of two different tones add up. So, if there are two tones of different frequencies with a level of 70 background each, this does not mean that the total volume level will be equal to 140 background.

Dependence of volume on sound pressure level (and sound intensity) is purely nonlinear

curve, it has a logarithmic character. When the sound pressure level increases by 10 dB, the sound volume will increase by 2 times. This means that volume levels of 40, 50 and 60 von correspond to volumes of 1, 2 and 4 sones.

physical basis of sound research methods in the clinic

Sound, like light, is a source of information, and this is its main significance. The sounds of nature, the speech of people around us, the noise of operating machines tell us a lot. To imagine the meaning of sound for a person, it is enough to temporarily deprive yourself of the ability to perceive sound - close your ears. Naturally, sound can also be a source of information about the state of a person’s internal organs.

A common sound method for diagnosing diseases is auscultation (listening). For auscultation, a stethoscope or phonendoscope is used. A phonendoscope consists of a hollow capsule with a sound-transmitting membrane that is applied to the patient’s body, from which rubber tubes go to the doctor’s ear. A resonance of the air column occurs in the hollow capsule, as a result of which the sound intensifies and the au-cultation improves. When auscultating the lungs, breathing sounds and various wheezing characteristic of diseases are heard. By changes in heart sounds and the appearance of murmurs, one can judge the state of cardiac activity. Using auscultation, you can determine the presence of peristalsis of the stomach and intestines and listen to the fetal heartbeat.

To simultaneously listen to a patient by several researchers for educational purposes or during a consultation, a system is used that includes a microphone, an amplifier and a loudspeaker or several telephones.

To diagnose the state of cardiac activity, a method similar to auscultation and called phonocardiography (PCG) is used. This method consists of graphically recording heart sounds and murmurs and their diagnostic interpretation. A phonocardiogram is recorded using a phonocardiograph, consisting of a microphone, an amplifier, a system of frequency filters and a recording device.

Fundamentally different from the two sound methods outlined above is percussion. With this method, the sound of individual parts of the body is listened to when they are tapped. Schematically, the human body can be represented as a set of gas-filled (lungs), liquid (internal organs) and solid (bone) volumes. When hitting the surface of a body, vibrations occur, the frequencies of which have a wide range. From this range, some vibrations will fade out quite quickly, while others, coinciding with the natural vibrations of the voids, will intensify and, due to resonance, will be audible. An experienced doctor determines the condition and location (tonography) of internal organs by the tone of percussion sounds.

15. Infrasound(from lat. infra- below, under) - sound waves having a frequency lower than that perceived by the human ear. Since the human ear is usually capable of hearing sounds in the frequency range 16 - 20,000 Hz, 16 Hz is usually taken as the upper limit of the frequency range of infrasound. The lower limit of the infrasound range is conventionally defined as 0.001 Hz. Oscillations of tenths and even hundredths of hertz, that is, with periods of tens of seconds, may be of practical interest.

The nature of the occurrence of infrasonic vibrations is the same as that of audible sound, therefore infrasound is subject to the same laws, and the same mathematical apparatus is used to describe it as for ordinary audible sound (except for concepts related to sound level). Infrasound is weakly absorbed by the medium, so it can spread over considerable distances from the source. Due to the very long wavelength, diffraction is pronounced.

Infrasound generated in the sea is called one of the possible reasons for finding ships abandoned by the crew (see Bermuda Triangle, Ghost Ship).

Infrasound. The effect of infrasound on biological objects.

Infrasound- oscillatory processes with frequencies below 20 Hz. Infrasounds– are not perceived by human hearing.

Infrasound has an adverse effect on the functional state of a number of body systems: fatigue, headache, drowsiness, irritation, etc.

It is assumed that the primary mechanism of action of infrasound on the body is of a resonant nature.

Ultrasound, methods of its production. Physical characteristics and features of the propagation of ultrasonic waves. Interaction of ultrasound with matter. Cavitation. Applications of ultrasound: echolocation, dispersion, flaw detection, ultrasonic cutting.

Ultrasound –(US) are mechanical vibrations and waves whose frequencies are more than 20 kHz.

To obtain ultrasound, devices called Ultrasound – emitter. The most widespread electromechanical emitters, based on the phenomenon of inverse piezoelectric effect.

By its physical nature Ultrasound represents elastic waves and in this it is no different from sound. from 20,000 to a billion Hz. The fundamental physical feature of sound vibrations is the wave amplitude, or displacement amplitude.

Ultrasound in gases and, in particular, in air, it propagates with great attenuation. Liquids and solids (especially single crystals) are generally good conductors. Ultrasound, attenuation, in which is significantly less. For example, in water, the attenuation of Ultrasound, all other things being equal, is approximately 1000 times less than in air.

Cavitation– compression and rarefaction created by ultrasound lead to the formation of discontinuities in the continuity of the liquid.

Ultrasound application:

Echolocation - a method by which the position of an object is determined by the delay time of the return of the reflected wave.

Dispersing - Grinding of solids or liquids under the influence of ultrasonic vibrations.

Flaw detection - search defects in the product material using the ultrasonic method, that is, by emitting and receiving ultrasonic vibrations, and further analyzing their amplitude, arrival time, shape, etc. using special equipment - ultrasonic flaw detector.

Ultrasonic cutting- based on the transmission of ultrasonic mechanical vibrations to the cutting tool, which significantly reduces the cutting force, the cost of equipment and improves the quality of manufactured products (threading, drilling, turning, milling). Ultrasonic cutting is used in medicine for cutting biological tissues.

The effect of ultrasound on biological objects. The use of ultrasound for diagnosis and treatment. Ultrasound surgery. Advantages of ultrasonic methods.

Physical processes caused by the influence of ultrasound cause the following main effects in biological objects.

Microvibrations at the cellular and subcellular level;

Destruction of biomacromolecules;

Restructuring and damage to biological membranes, changes in membrane permeability;

Thermal action;

Destruction of cells and microorganisms.

Biomedical applications of ultrasound can be mainly divided into two areas: diagnostic and research methods and intervention methods.

Diagnostic method:

1) include location methods and the use mainly of pulsed radiation.

Z: encephalography– detection of tumors and cerebral edema, ultrasound cardiography– measurement of heart size in dynamics; in ophthalmology – ultrasonic location to determine the size of the ocular media. Using the Doppler effect, the pattern of movement of the heart valves is studied and the speed of blood flow is measured.

2) Treatment includes ultrasound physiotherapy. Typically, the patient is exposed to a frequency of 800 kHz.

The primary mechanism of ultrasound therapy is mechanical and thermal effects on tissue.

For the treatment of diseases such as asthma, tuberculosis, etc. I use aerosols of various medicinal substances obtained using ultrasound.

During operations, ultrasound is used as an “ultrasonic scalpel”, capable of cutting both soft and bone tissue. Currently, a new method has been developed for “welding” damaged or transplanted bone tissue using ultrasound (ultrasonic osteosynthesis).

The main advantage of ultrasound over other mutagens (X-rays, ultraviolet rays) is that it is extremely easy to work with.

The Doppler effect and its use in medicine.

Doppler effect call the change in the frequency of waves perceived by an observer (wave receiver) due to the relative movement of the wave source and the observer.

The effect was first describedChristian DopplerV1842 year.

The Doppler effect is used to determine the speed of blood flow, the speed of movement of the valves and walls of the heart (Doppler echocardiography) and other organs.

The manifestation of the Doppler effect is widely used in various medical devices, which, as a rule, use ultrasonic waves in the MHz frequency range.

For example, ultrasound waves reflected from red blood cells can be used to determine the speed of blood flow. Similarly, this method can be used to detect the movement of the fetal chest, as well as to remotely monitor heartbeats.

16. Ultrasound- elastic vibrations with a frequency beyond the audibility limit for humans. Usually the ultrasonic range is considered to be frequencies above 18,000 hertz.

Although the existence of ultrasound has been known for a long time, its practical use is quite young. Nowadays, ultrasound is widely used in various physical and technological methods. Thus, the speed of sound propagation in a medium is used to judge its physical characteristics. Velocity measurements at ultrasonic frequencies make it possible to determine, for example, the adiabatic characteristics of fast processes, the specific heat capacity of gases, and the elastic constants of solids with very small errors.

The frequency of ultrasonic vibrations used in industry and biology lies in the range of the order of several MHz. Such vibrations are usually created using piezoceramic transducers made of barium titanite. In cases where the power of ultrasonic vibrations is of primary importance, mechanical ultrasound sources are usually used. Initially, all ultrasonic waves were received mechanically (tuning forks, whistles, sirens).

In nature, ultrasound is found both as components of many natural noises (in the noise of wind, waterfall, rain, in the noise of pebbles rolled by the sea surf, in the sounds accompanying thunderstorm discharges, etc.), and among the sounds of the animal world. Some animals use ultrasonic waves to detect obstacles and navigate in space.

Ultrasound emitters can be divided into two large groups. The first includes emitters-generators; oscillations in them are excited due to the presence of obstacles in the path of a constant flow - a stream of gas or liquid. The second group of emitters are electroacoustic transducers; they convert already given fluctuations in electrical voltage or current into mechanical vibrations of a solid body, which emits acoustic waves into the environment.

Physical properties of ultrasound

The use of ultrasound in medical diagnostics is associated with the possibility of obtaining images of internal organs and structures. The basis of the method is the interaction of ultrasound with the tissues of the human body. The actual image acquisition can be divided into two parts. The first is the emission of short ultrasonic pulses directed into the tissues being examined and the second is the formation of an image based on the reflected signals. Understanding the operating principle of an ultrasound diagnostic unit, knowledge of the basic physics of ultrasound and its interaction with the tissues of the human body will help you avoid mechanical, thoughtless use of the device and, therefore, approach the diagnostic process more competently.

Sound is a mechanical longitudinal wave in which the vibrations of particles are in the same plane as the direction of energy propagation (Fig. 1).

Rice. 1. Visual and graphical representation of changes in pressure and density in an ultrasonic wave.

A wave carries energy, but not matter. Unlike electromagnetic waves (light, radio waves, etc.), sound requires a medium to propagate - it cannot propagate in a vacuum. Like all waves, sound can be described by a number of parameters. These are frequency, wavelength, speed of propagation in the medium, period, amplitude and intensity. Frequency, period, amplitude and intensity are determined by the sound source, the speed of propagation is determined by the medium, and the wavelength is determined by both the sound source and the medium. Frequency is the number of complete oscillations (cycles) over a period of time of 1 second (Fig. 2).

Rice. 2. Ultrasonic wave frequency 2 cycles in 1 s = 2 Hz

The units of frequency are hertz (Hz) and megahertz (MHz). One hertz is one vibration per second. One megahertz = 1,000,000 hertz. What makes the sound "ultra"? This is the frequency. The upper limit of audible sound, 20,000 Hz (20 kilohertz (kHz)), is the lower limit of the ultrasonic range. Ultrasonic bat locators operate in the range of 25÷500 kHz. Modern ultrasound devices use ultrasound with a frequency of 2 MHz and higher to obtain images. The period is the time required to obtain one complete cycle of oscillations (Fig. 3).

Rice. 3. Period of ultrasonic wave.

The units of period are second (s) and microsecond (µsec). One microsecond is one millionth of a second. Period (µsec) = 1/frequency (MHz). The wavelength is the length that one vibration occupies in space (Fig. 4).

Rice. 4. Wavelength.

Units of measurement are meter (m) and millimeter (mm). The speed of ultrasound is the speed at which the wave travels through a medium. The units of ultrasound propagation speed are meters per second (m/s) and millimeters per microsecond (mm/µsec). The speed of ultrasound propagation is determined by the density and elasticity of the medium. The speed of ultrasound propagation increases with increasing elasticity and decreasing density of the medium. Table 2.1 shows the speed of propagation of ultrasound in some tissues of the human body.

Table 2.1. Speed ​​of propagation of ultrasound in soft tissues
Textile	Ultrasound propagation speed in mm/µsec
Adipose tissue
Soft tissue (averaging)
Water (20°C)

The average speed of propagation of ultrasound in the tissues of the human body is 1540 m/s - most ultrasound diagnostic devices are programmed for this speed. The speed of propagation of ultrasound (C), frequency (f) and wavelength (λ) are related to each other by the following equation: C = f × λ. Since in our case the speed is considered constant (1540 m/s), the remaining two variables f and λ are related to each other by an inversely proportional relationship. The higher the frequency, the shorter the wavelength and the smaller the size of objects that we can see. Another important environmental parameter is acoustic impedance (Z). Acoustic resistance is the product of the density of the medium and the speed of propagation of ultrasound. Resistance (Z) = density (p) × speed of propagation (C).

To obtain an image in ultrasound diagnostics, it is not ultrasound that is emitted by a transducer continuously (constant wave), but ultrasound emitted in the form of short pulses (pulse). It is generated by applying short electrical pulses to the piezoelectric element. Additional parameters are used to characterize pulsed ultrasound. Pulse repetition rate is the number of pulses emitted per unit of time (second). Pulse repetition frequency is measured in hertz (Hz) and kilohertz (kHz). Pulse duration is the time duration of one pulse (Fig. 5).

Rice. 5. Ultrasonic pulse duration.

Measured in seconds (s) and microseconds (µsec). The occupancy factor is the fraction of time during which ultrasound is emitted (in the form of pulses). The spatial pulse extension (SPR) is the length of space in which one ultrasonic pulse is placed (Fig. 6).

Rice. 6. Spatial extent of the pulse.

For soft tissues, the spatial extent of the pulse (mm) is equal to the product of 1.54 (ultrasound propagation speed in mm/µsec) and the number of oscillations (cycles) in the pulse (n) divided by the frequency in MHz. Or PPI = 1.54 × n/f. Reducing the spatial extent of the pulse can be achieved (and this is very important for improving axial resolution) by reducing the number of oscillations in the pulse or increasing the frequency. The amplitude of the ultrasonic wave is the maximum deviation of the observed physical variable from the average value (Fig. 7).

Rice. 7. Ultrasonic wave amplitude

Ultrasound intensity is the ratio of wave power to the area over which the ultrasonic flow is distributed. It is measured in watts per square centimeter (W/sq.cm). With equal radiation power, the smaller the flux area, the higher the intensity. The intensity is also proportional to the square of the amplitude. So, if the amplitude doubles, then the intensity quadruples. The intensity is non-uniform both over the flow area and, in the case of pulsed ultrasound, over time.

When passing through any medium, there will be a decrease in the amplitude and intensity of the ultrasonic signal, which is called attenuation. Ultrasonic signal attenuation is caused by absorption, reflection and scattering. The unit of attenuation is decibel (dB). The attenuation coefficient is the attenuation of an ultrasonic signal per unit path length of this signal (dB/cm). The attenuation coefficient increases with increasing frequency. The average soft tissue attenuation coefficients and the decrease in echo signal intensity as a function of frequency are presented in Table 2.2.

Table 2.2. Average attenuation coefficients in soft tissues
Frequency, MHz	Average attenuation coefficient for soft tissues, dB/cm	Reducing intensity with depth
		1 cm (%)	10 cm (%)

Electrocardiography is a method of studying the heart muscle by recording the bioelectric potentials of the beating heart. Contraction of the heart is preceded by excitation of the myocardium, accompanied by the movement of ions through the membrane of the myocardial cell, as a result of which the potential difference between the outer and inner surfaces of the membrane changes. Measurements using microelectrodes show that the potential change is about 100 mV. Under normal conditions, the parts of the human heart are covered by excitation sequentially, therefore, a changing potential difference between already excited and not yet excited areas is recorded on the surface of the heart. Due to the electrical conductivity of body tissues, these electrical processes can also be detected by placing electrodes on the surface of the body, where the change in potential difference reaches 1-3 mV.

Experimental electrophysiological studies of the heart were carried out back in the 19th century, but the introduction of the method into medicine began after the research of Einthoven in 1903-1924, who used a low-inertia string galvanometer, developed the designation of the elements of the recorded curve, a standard registration system and the main evaluation criteria.

The high information content and relative technical simplicity of the method, its safety and the absence of any inconvenience for the patient have ensured the widespread use of ECG in medicine and physiology. The main components of a modern electrocardiograph are an amplifier, a galvanometer and a recording device. When recording a changing pattern of distribution of electrical potentials on moving paper, a curve is obtained - an electrocardiogram (ECG), with sharp and rounded teeth, repeated during each systole. The teeth are usually designated by the Latin letters P, Q, R, S, T and U.

The first of them is associated with the activity of the atria, the remaining teeth are associated with the activity of the ventricles of the heart. The shape of the teeth in different leads is different. ECG recording in different individuals is achieved using standard registration conditions: the method of applying electrodes to the skin of the limbs and chest (usually 12 leads are used), determined by the sensitivity of the device (1 mm = 0.1 mV) and the speed of paper movement (25 or 50 mm/sec.) . The subject is in a supine position, under resting conditions. When analyzing an ECG, the presence, size, shape and width of the waves and the intervals between them are assessed, and on this basis they judge the characteristics of electrical processes in the heart as a whole and, to some extent, the electrical activity of more limited areas of the heart muscle.

In medicine, the ECG is of greatest importance for recognizing heart rhythm disturbances, as well as for detecting myocardial infarction and some other diseases. However, ECG changes reflect only the nature of the disturbance of electrical processes and are not strictly specific to a particular disease. ECG changes can occur not only as a result of the disease, but also under the influence of normal daily activity, food intake, drug treatment and other reasons. Therefore, the diagnosis is made by the doctor not by ECG, but by a combination of clinical and laboratory signs of the disease. Diagnostic capabilities increase when comparing a series of sequentially taken ECGs with an interval of several days or weeks. The electrocardiograph is also used in cardiac monitors - devices for round-the-clock automatic monitoring of the condition of seriously ill patients - and for telemetric monitoring of the condition of a working person - in clinical, sports, and space medicine, which is ensured by special methods of applying electrodes and radio communication between the galvanometer and the recording device.

The bioelectrical activity of the heart can be recorded in another way. The potential difference is characterized by a magnitude and direction specific for a given moment, that is, it is a vector and can be conventionally represented by an arrow occupying a certain position in space. The characteristics of this vector change during the cardiac cycle so that its starting point remains stationary, and the final point describes a complex closed curve. When projected onto a plane, this curve looks like a series of loops and is called a vectorcardiogram (VCG). Approximately, it can be constructed graphically based on an ECG in different leads. It can also be obtained directly using a special device - a vectorcardiograph, the recording device of which is a cathode ray tube, and for removal two pairs of electrodes are used, placed on the patient in the appropriate plane.

By changing the position of the electrodes, it is possible to obtain VCG in different planes and obtain a more complete spatial understanding of the nature of electrical processes. In some cases, vectorcardiography complements electrophysiological studies as a diagnostic method. The study of electrophysiological foundations and clinical applications of electrophysiological studies and vectorcardiography, improvement of devices and recording methods is the subject of a special scientific branch of medicine - electrocardiology.

In veterinary medicine, electrocardiography is used in large and small animals to diagnose changes in the heart that occur as a result of certain non-contagious or infectious diseases. With the help of electrocardiography, heart rhythm disturbances, enlargement of heart chambers and other changes in the heart are determined in animals. Electrocardiography allows you to monitor the effect of drugs used or tested on the heart muscle of an animal.