Physics of ultrasound

Author: Irina Bragina, dermatocosmetologist, physiotherapist, consultant doctor at SportMedImport Group of Companies (Russia)

Ultrasound and its parameters

Ultrasound is elastic sound vibrations of high frequency. The human ear is capable of perceiving elastic waves propagating in a medium with a frequency of approximately 16–20 kHz. Vibrations that are beyond the limit of audibility, that is, with a higher frequency, are ultrasound. Typically, the ultrasonic range is considered to be a frequency range from 20,000 to several billion Hz.

The depth and strength of the impact of ultrasound on biological tissue depends on:

• frequencies;
• shapes of propagating ultrasonic waves;
• intensity.

Let's take a closer look at these parameters.

Frequency

This is the number of complete oscillations of the particles of the medium per unit of time (per second), measured in hertz (Hz). The lower the ultrasound frequency, the deeper its penetration.

In medical and cosmetology practice, ultrasound is used in a fixed frequency range: high-frequency ultrasound – 800–3,000 kHz, low-frequency – 22–44 kHz. Low-frequency ultrasound, unlike high-frequency ultrasound, penetrates deeper into tissues and has a pronounced bactericidal, decongestant, loosening and depolymerizing effect. Significantly increases vascular and epithelial permeability and has phoretic activity.

As the ultrasound frequency increases, the penetration depth decreases. At a frequency of 0.8 MHz, the penetration depth in adipose tissue is 6.8 cm, in muscle tissue - 3.6 cm, in adipose and muscle tissue together - 4.9 cm. And at a frequency of 2.4 MHz, the intensity of ultrasound passing through fat and muscle tissue is halved and is about 1.5 cm. Some modern devices for the treatment of cellulite emit an ultrasound wave with a frequency of 3 MHz. In this case, the penetration depth is slightly more than 1 cm.

High-frequency ultrasound with a frequency of up to 1,500 kHz penetrates to a depth of 4-5 cm and is used for body procedures. Ultrasound with a frequency of over 2,000 kHz penetrates to a depth of 1.5 cm and is used for facial procedures.

Low frequency ultrasound with frequencies of 25–44 kHz is used primarily for ultrasonic peeling. In this procedure, the depth of impact will be influenced not so much by the frequency as by the shape of the ultrasonic wave propagation in biological tissues, which can be longitudinal, transverse, or combined.

Ultrasonic Wave Propagation Shape

The form of ultrasonic wave propagation in biological tissues can be transverse, longitudinal, or combined.

In transverse (or shear) waves, elastic vibrations occur perpendicular to the line of propagation. Transverse waves arise only at the interface between two media (for example, on the surface of water, at the skin-air interface). The mechanical function of a transverse wave differs from a similar function of a longitudinal wave solely by its surface effect. The depth of penetration of a transverse wave into biological tissue does not exceed 0.2 mm.

Low-frequency cosmetology devices excite a surface transverse ultrasonic wave using emitters in the form of a “shovel,” oscillating at a frequency of 22–44 kHz. With the help of such devices, superficial and deep peeling is performed - cleansing the skin and pores of dead cells, sebum, bacteria, cosmetic residues, toxic substances and other skin contaminants.

Ultrasound, spraying liquid applied to the treated surface, causes the formation of microscopic gas bubbles, promotes depolarization and catalysis of the skin surface, penetration of water into the deep layers, and moisturizes.

In a longitudinal wave, compression and rarefaction of the medium occurs along the direction of propagation of the wave, deep into the tissue. High-frequency cosmetology devices generate longitudinal waves (up to 3,000 kHz). Ultrasound emitters are usually flat membranes that perform mechanical vibrations along the direction of wave propagation.

Ultrasonic waves can be reflected from the boundaries of various media. Reflection and refraction of ultrasound depends on the acoustic resistance of the media. We observe this effect, for example, at the interface of biological tissues and air, which significantly absorbs ultrasound. Therefore, ultrasonic exposure must be carried out through an aqueous or oily contact medium (gel, cream, water).

Also, the reflection of ultrasonic waves depends on the angle of their incidence on the affected area. The greater the deflection angle, the greater the reflection coefficient. Therefore, during the procedure, the emitter must be pressed tightly against the skin.

During ultrasound therapy, a stable (fixed position of the emitter) or labile (movement of the emitter) procedure technique can be used. The labile technique is used much more often. The emitter moves slowly, without pressure, in a circular or spiral motion. The recommended movement speed is 0.5–2 cm per second.

Ultrasound intensity

This is the energy passing in one second through an area equal to 1 cm2, located perpendicular to the direction of propagation of the wave, and is measured in watts per square centimeter (W/cm2).

The intensity of ultrasonic vibrations used in physiotherapy and cosmetology practice is conventionally divided into:

• small (0.05–0.4 W/cm2) – stimulating effect;
• medium (0.5–0.8 W/cm2) – corrective effect (anti-inflammatory, analgesic, etc.);
• large (0.9–1.2 W/cm2) – absorbable effect.

Ultrasonic wave generation modes

There are two modes of ultrasonic wave generation: continuous and pulsed (Table 1).

Table 1. Ultrasonic wave generation modes, effects, indications

A continuous ultrasound wave is generated at a constant intensity, which is most effective for the treatment of scar changes on the face and body (including post-acne), hematomas, dark circles under the eyes, stretch marks, hyperpigmentation, as well as in the chronic period of diseases.

A pulsed ultrasonic wave is generated at a time-varying intensity. It is used to obtain non-thermal effects, namely in the treatment of inflammatory, pustular diseases, rosacea, during procedures on sensitive skin, and in acute pain syndrome.

The intensity of the generated ultrasonic vibrations in continuous mode can be 0.05–2.0 W/cm2, in pulsed mode – 0.1–3 W/cm2.

Ultrasound effects in biological tissues

When ultrasound acts on biological objects, pressure differences from units to tens of atmospheres can arise in irradiated tissues. Such intense impacts lead to a variety of biological effects, the physical nature of which is determined by the combined action of mechanical, thermal and physicochemical phenomena accompanying the propagation of ultrasound in the environment (Table 2).

Table 2. Ultrasound effects in tissues

The action of the mechanical factor is associated with the acoustic pressure of ultrasonic waves. As a result of alternating zones of compression and rarefaction in the tissues, a vibration “micro-massage” occurs at the cellular level. The permeability of cell membranes increases, intracellular processes are activated: protein synthesis, enzymes, ATP. Collagen and elastin fibers formed under the influence of ultrasonic vibrations have increased, twice or more, elasticity and strength compared to “non-sounded” tissue.

The thermal effect of ultrasound is associated with the conversion of mechanical energy into thermal energy in tissues. In tissues containing molecules with large linear dimensions, due to significant absorption of ultrasonic vibration energy, the temperature rises to 1°C. The greatest amount of heat is generated not in the thickness of homogeneous tissues, but at the interfaces of tissues with different acoustic impedance - collagen-rich superficial layers of the skin, fascia, scars, ligaments, synovial membranes, articular menisci and periosteum, which increases elasticity and expands the range of physiological stresses (vibrothermolysis ). Local dilation of microvasculature leads to an increase in volumetric blood flow in tissues with reduced blood supply (2-3 times), increased metabolism, improved skin elasticity and reduced swelling.

The physicochemical factor is a consequence of the influence of mechanical and thermal effects on the body and has a stimulating effect on biochemical and biophysical processes. Ultrasound increases the permeability of biological membranes, as a result of which metabolic processes are accelerated due to diffusion, also promotes the synthesis of ATP, nucleic acids, proteins, lipids, polysaccharides and other cellular components, acts as a catalyst, changes the pH value of tissues to alkali, promotes the formation of biologically active substances . Due to the activation of membrane enzymes and depolymerization of hyaluronic acid, swelling decreases and resolves, and compression of nerve conductors in the affected area decreases. Subsequently, the mechanisms of nonspecific immunological resistance of the body are activated due to increased binding of biologically active substances (kinins, histamine) to blood proteins and their breakdown by enzymes.

The action of all three factors is closely related and complements each other. In the area of ultrasound, blood and lymph circulation improves, metabolic and regeneration processes accelerate, phagocytosis increases, and immunity is restored. Ultrasound has a fiber-breaking effect on fibrous tissue, which gives good results in the treatment of scars, contractures, and fibrous cellulite.

The nervous system is most sensitive to ultrasound. Complex tissue and endocrine changes in the body under the influence of ultrasound actively affect the functioning of the central nervous system. Redox processes in neurons are activated, ATP synthesis increases, oxygen absorption by nerve cells and glycogen utilization improves, which has a normalizing effect on the dynamics of basic nervous processes and the psychasthenic state of a person. The above effects neutralize the effect of botulinum toxin A, promoting the growth of an additional axon.

The biological effect of ultrasound is associated with its ability to cause analgesic, antispastic, vasodilating, absorbable, and anti-inflammatory effects in tissues. The ability of ultrasound to increase epithelial and vascular permeability determines the phoretic effect.

Ultrasound in aesthetic medicine

Ultraphonophoresis and its differences from electrophoresis

Ultraphonophoresis is a combined effect on the body of ultrasound and medicinal and cosmetic products applied to the skin, which retain their structure and pharmacotherapeutic activity when exposed to a mechanical wave. The introduction of necessary substances into the body during ultraphonophoresis occurs through the excretory ducts of the sweat and sebaceous glands, and transcellular and intercellular penetration routes are also carried out. Moreover, the penetration of the phoretic substance is deeper (maximum up to 6-7 cm) than with electrophoresis (galvanic current - 1 cm, pulsed - up to 3 cm). But, unlike electrophoresis, ultrasound does not manage to accumulate medicinal substances in the skin in sufficient concentration, and they act for a relatively short time. Despite this, as a result of the combined action of phonophoresis and various therapeutic effects of ultrasound (mechanical, thermal, chemical), the therapeutic effects are potentiated and are quite pronounced. When carrying out the procedure, the injected substance must be included in the contact medium, the basis of which can be glycerin, lanolin, petroleum jelly, dimethyl sulfoxide, etc.

Unfortunately, not every agent can be administered using ultrasound, since phoretic activity depends both on the structure of the injected substance and on the degree of dispersion, determined by the size of the molecules and the nature of the solvent. As the structure becomes more complex, phoretic mobility decreases significantly. It is maximum when using 5–10% aqueous solutions of the substance. In this case, the amount of substance introduced into the body does not exceed 1–3% of that applied to the surface of the skin and depends on :

• ultrasound frequency (the lower it is, the greater the amplitude of the induced vibrational displacements of the particles of the drug substance);
• intensity (phoretic activity increases when the intensity increases to 0.8 W/cm2, and with a further increase it begins to decrease);
• duration of exposure (the amount of the phoretic substance is directly proportional to the exposure time);
• ultrasonic wave generation mode (with continuous mode of exposure, the phoretic ability is greater than with pulsed mode);
• methods of carrying out the procedure (with a labile method, the amount of substance administered is higher than with a stable one).

Cavitation

Cavitation (from the Latin сavitas - emptiness) is the process of formation of cavities (cavitation bubbles, or caverns) in a liquid filled with gas, steam or a mixture of them. Cavitation can be hydrodynamic or acoustic.

Hydrodynamic cavitation occurs as a result of a decrease in pressure in the liquid, which can be caused by an increase in the speed of its movement.

Acoustic cavitation occurs when a high-intensity acoustic wave passes through a liquid, and it is this type of cavitation that is used in aesthetic medicine.

A cavitation bubble can move from an area of low pressure to an area of high pressure, while changing its size. It may go through several periods of increase and decrease. Moving with the flow to an area of higher pressure or during the half-cycle of compression, the cavitation bubble can collapse, emitting a shock wave and releasing a large amount of energy.

The main task of ultrasonic (acoustic) cavitation in cosmetology is the fight against excess fat deposits. At the same time, I would like to note that cavitation is not treated with ultrasound. Ultrasound is used here not as a method of influence, but as a physical factor that causes the effect of cavitation in adipose tissue.

By changing the parameters of the ultrasonic wave (frequency and intensity), it is possible to obtain different types of cavitation, which will have different effects on adipose tissue: either reduce the number of adipocytes or reduce their volume.

Stable non-explosive cavitation occurs at an intensity of 0.8 to 3 W/cm2, when stable pressure of varying strength causes compression of adipocytes, resulting in the disintegration of the fat droplet into a finely dispersed state and removal through pores in the cell membrane. In this case, the adipocyte retains its cellular structure, only the volume of the fat drop decreases.

Unstable explosive cavitation occurs with high intensity vibrations (60 W/cm2) directed at a small surface. In this case, cell fragmentation is observed: they “explode”.

In cosmetology, the technique of cavitation non-surgical liposuction is used. The combination of low-frequency ultrasound (38–41 KHz), low pressure (0.6 kPa) and a certain flow density in fat cells causes the effect of acoustic cavitation, which produces a maximum number of bubbles of the required size, which, increasing in size, emulsify fat and displace it from adipocytes. When the bubbles collapse inside the fat cell, a hydrodynamic push occurs, a kind of microexplosion with the release of a large amount of energy. These microexplosions damage the cell membranes of adipocytes. In this case, the membranes of the cells most filled with fat are damaged first, due to their greatest tension. Released triglycerides are removed from the intercellular space through natural metabolic processes. 90% of breakdown products are excreted through the lymphatic system and 10% are absorbed into the bloodstream, where, as a result of the reaction, triglycerides are converted into glucose molecules.

At the same time, other cells and tissues (muscle fibrils, epidermal cells, vascular endothelium, etc.) are not damaged by cavitation, since they are relatively strong and have a sufficient elasticity coefficient. Many scientific studies have been conducted that have proven the effectiveness and safety of cavitation.

HIFU (High Intensity Focused Ultrasound) - focused ultrasound

One of the most interesting new areas of medical ultrasound is associated with the possibility of remote destruction of biological tissue due to the absorption of a powerful focused acoustic wave in it.

Because ultrasound can be focused in the same way as light, it is achievable to achieve local heating of tissue within the human body to a temperature sufficient to cause blood clotting or soft tissue necrosis, without overheating the tissue on the way to focus. Ultrasound focusing can be achieved in several ways. The simplest of them is the use of a transducer, the emitting surface of which is shaped like a spherical concave shell made of piezoelectric material. The focus of such an emitter lies on its main axis and is located near the center of curvature of the shell. Although this method can produce a heated area with clearly defined boundaries, it is not easy to control the depth of the affected area. Using a flat emitter in conjunction with various acoustic lenses, it is possible to change the depth of the affected area. Since acoustic lenses are usually made of a material that has a speed of sound greater than in water, to create a converging beam it is necessary to make the lenses concave. Modern cosmetology machines use both focusing methods. With the help of focusing, a power of several tens of watts can be achieved in a small volume of adipose tissue.

Along the path of the ultrasonic wave to the focal zone, all the properties of linear ultrasound are preserved. Particularly important is the defibrosing effect of ultrasound on connective tissue structures in the hypodermis, depolymerization of triglycerides and increasing their fluidity. Due to the phenomenon of stable cavitation, lipolysis and the removal of fat droplet breakdown products beyond the adipocytes are significantly accelerated.

In the focusing zone, ultrasonic waves combine (thermal and mechanical effects increase significantly), which leads to thermal destruction of protein structures and mechanical damage to the cell membrane. This makes it possible to use powerful ultrasound, for example, to destroy tumors, fatty tissue, and stop internal bleeding non-invasively, that is, without conventional surgical intervention. In cosmetology, focused ultrasound is used to work in the area of local fat deposits and to treat cellulite.

These mechanisms, their contribution to tissue destruction, and the possibility of using them to monitor the HIFU procedure in real time are not fully understood and represent an interesting object of physical research, both theoretically and experimentally.

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So, we have analyzed the main characteristics of ultrasound and ways of using it in aesthetic medicine. And finally, we compiled Table 3, in which we finally systematized all the data discussed in the article.

Table 3. The most important parameters of ultrasound and their use in aesthetic medicine

Source: Les Nouvelles Esthetiques Ukraine, No. 3 (85), 2014, pp. 54-60