KR101411099B1 - Method for generating mechanical waves by creating an interfacial acoustic radiation force - Google Patents

Abstract

The present invention relates to: A method for producing a viscoelastic medium (11) comprising applying acoustic waves concentrated on an interface (13) which confines two zones (11, 14) with specific acoustic characteristics to generate an acoustic radiation force (15) To a method for generating a mechanical wave in the waveguide.

Mechanical wave generation method

Description

METHOD FOR GENERATING MECHANICAL WAVES BY CREATING AN INTERFACIAL ACOUSTIC RADIATION FORCE FIELD OF THE INVENTION [0001]

The present invention relates to the general field of medical imaging.

More particularly, the present invention relates to a method of generating a mechanical wave in a viscoelastic medium, wherein such mechanical wave is easily imaged to determine the characteristics of the viscoelastic medium.

Thus, the present invention more specifically relates to elastic ultrasound imaging.

These medical imaging techniques measure the mechanical properties of viscoelastic media and quantify the rheology of viscoelastic media. According to this technique, mechanical stimulation occurs and causes tissue displacement. The spatio-temporal response of the tissue to this mechanical excitation is measured. This spatio-temporal reaction is preferably measured using an imaging method, such as echography or magnetic resonance.

Once the motion resulting from the mechanical excitation is known, the mechanical properties of the medium can be determined.

In transient elastic ultrasound imaging, mechanical excitation consists of short mechanical pulses or fewer pulses occurring on the surface of the body or inside the tissue itself.

The quality of a transient elastic ultrasound image depends critically on the amplitude of the movement that can be caused by mechanical stimulation.

In transient elastic ultrasound images due to external stress, the travel amplitude is limited only by the maximum surface vibrations that can be induced in contact with the medium without damaging the medium. The resulting movement in the tissue has a size of about 100 탆.

In this way, in general, movement due to mechanical excitation should be sufficient to be able to be measured with minimal error, but at the same time be restricted to avoid deleterious effects in the medium, especially in the case of biological tissues.

It is thus known that the use of external stress causes technical problems such as the space required for the apparatus required for this stress, the synchronization of imaging and mechanical excitation, the localization of mechanical excitation, and the optimization of the wave amplitude of the region at the depth of interest Despite this, the power generated is satisfactory.

There is also a temporary elastic ultrasound image in which the mechanical stress of the observed medium is generated by the acoustic radiation force. This radiation force is obtained by concentrating the ultrasonic beam in the medium. Concentration of the beam can occur in a single zone of the medium or continuously in multiple zones of the medium.

The focal point of the ultrasonic beam moves at a velocity higher than the propagation velocity of the acoustic wave to generate an elastic traveling wave having a maximum amplitude of about 10 to 100 탆.

This traveling wave propagates in the medium. By observing by ultrasonography, MRI or other imaging methods, measuring the propagation characteristics of the waves can determine the mechanical parameters characteristic of the irradiated tissue. In particular, shear modules or even viscous properties can be determined.

Movement caused by acoustic radiation forces is coupled to energy accumulated in the tissue, and thus the amplitude of the generated mechanical waves is limited by the maximum acoustic power that can be sent to the medium being observed without thermally or mechanically changing the tissue.

Ultrasonic solutions are simple to handle, reproducible in the way stresses occur, ensure imaging and synchronization, and ensure localization here, but lack power.

The main object of the present invention is to propose a method of generating a mechanical wave in a viscoelastic medium to eliminate such disadvantages, wherein the method comprises applying acoustic waves concentrated on the interface defining two zones having individual acoustic properties, And generating a force of attraction.

Through this method of generating mechanical waves in a viscoelastic medium, the amplitude of the movement that occurs is greater than the simple ultrasonic stress due to concentration in the tissue.

According to the present invention, the sound waves are concentrated at a predetermined depth in the direction of the surface interface.

The interface where the sound waves are concentrated can be gel / skin or water / skin or water / membrane / skin separation surface. The membrane may or may not be a deformable membrane. The interface may be located between the solid medium and the liquid medium in the tissue being imaged, or may be located between two media having different acoustic properties in the tissue. For example, this is the case of biological media containing cysts. In the method according to the present invention, the amplitude of the generated movement is on the order of 100 mu m.

According to a preferred embodiment of the present invention, the step of generating the acoustic radiation force is connected with the imaging step of the medium, and the connection is such that the propagation of the mechanical waves generated in the medium is imaged.

Imaging wave propagation can be one-dimensional, two-dimensional or three-dimensional. In a preferred embodiment, an elastic ultrasound imaging method of the medium is used. This is a preferred application method of the present invention, and focusing on the interface according to the present invention can greatly improve the quality of imaging performed.

According to a preferred characteristic, the sound wave is ultrasonic.

The ultrasonic frequency is particularly suitable for generating a radiation force to create a shearing wave in the medium. These shear waves are commonly used in elastic ultrasound imaging. These shear waves belong to the mechanical waves as generated by the method of the present invention and are generally imaged according to the elastic ultrasound imaging method.

According to a particular feature, the interface at which sound waves are concentrated is an interface present between the two zones having individual acoustic characteristics within the viscoelastic medium.

Because of this feature, the visibility and characterization of the interfacial zone in the media is significantly improved. In fact, observing the propagation of shear waves created at the height of the interface naturally present in the human body helps to better characterize these interfaces and the media from which they separate.

Thus, this feature is particularly interesting when there is a stiffer structure than soft tissue, such as liquid cysts, blood vessels or bones or cartilage.

According to another particular feature of the invention, the interface at which sound waves are concentrated is an artificial membrane which is in contact with the surface of the viscoelastic medium and which surrounds the medium known as the connecting medium, which is located between the device applying the sound wave and the surface of the viscoelastic medium And between the connecting medium and the viscoelastic medium defines two zones of individual acoustic characteristics.

This feature is interesting for applications, especially because artificial media are needed. In particular, this is the case where a microscopic membrane surrounding the connective medium is generally used to contact biological tissue in a treatment with ultrasound concentration.

According to the present invention, such an interface can be used to generate a shear wave. Following excitation, an elastic ultrasound mode is preferably used, imaging of the medium and imaging of the shear wave propagation is achieved. In this way, the viscoelastic properties of the tissue can be assessed and monitored during treatment.

This monitoring is particularly suitable because it is well known that the elasticity of the biological tissue changes when the biological tissue is denatured after necrosis of the cell.

According to a preferred feature, the artificial membrane has a composition selected to minimize the acoustic impedance contrast while increasing the amplitude of the mechanical wave.

According to another preferred feature, the artificial membrane has a thickness selected to minimize the acoustic impedance contrast while increasing the amplitude of the mechanical wave.

These two features adjust the composition, shape and / or thickness of the artificial membrane to make it easier to fit the artificial membrane according to the particular application.

The method of generating mechanical waves in accordance with the present invention is primarily concerned with imaging the elasticity of the surface area of the biological medium.

In fact, since shear waves are generated at the interface, this results in significant amplitude waves at the tissue surface height. This property can not be realized with volumetric radial pressure technology because the generated waves generally attenuate and reach the surface of the medium.

If an artificial membrane is used, for example, the membrane of the water pocket generates a mechanical pulse at a predetermined point on the surface of the medium. The technique according to the present invention is therefore primarily concerned with the imaging of elastic ultrasound images of the skin, for example at the level of superficial disorders such as melanoma or certain chest disorders.

However, it may also be related to generating a shear wave at a predetermined depth in the medium.

Thus, according to a particularly preferred feature of the present invention, the artificial membrane has a non-uniform composition spatially determined to increase the amplitude of mechanical waves in the region of interest of the viscoelastic medium.

Alternatively or additionally to this feature, the artificial membrane has a non-uniform thickness spatially determined to increase the amplitude of mechanical waves in the region of interest of the viscoelastic medium.

With this feature of the membrane, the directionality of the shear waves can be used to focus the mechanical impulses to the area of interest. Thus, the amplitude of the mechanical wave in this zone is very large.

Applying the sound waves focused at the interface defining the two zones with individual acoustic characteristics can be made continuously at a plurality of points of the interface and the plurality of points and successive concentrations are caused by the mechanical waves in the region of interest of the viscoelastic medium Is determined to increase the amplitude.

With this dynamic focusing property, a pattern can appear on the interface. Depending on the form of this pattern, the amplitude of the mechanical wave of a particular area of interest is increased by the interference effect. In the dynamic continuation of the concentration of the ultrasound beam, the relative delay of each ultrasound beam focused at a given point is carefully chosen such that the interference is positive at the height of the area of interest. The mechanical shear waves are concentrated in the area of interest.

In a preferred application of the present invention, the method can be associated with an ultrasonic treatment method to monitor the therapeutic effect.

Preferably, the ultrasonic treatment method is suitable to be controlled according to the result of the imaging step of the medium.

The invention also relates to a device for generating a sound wave and a medium known as a connecting medium located between the viscoelastic medium, positioned in partial contact with the surface of the viscoelastic medium for performing the interface role during the performance of the method according to the invention The present invention relates to an artificial membrane enclosing the same.

Other features and advantages of the present invention will become more apparent from the following description taken in an illustrative and non-limiting manner, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic representation of generating mechanical waves in accordance with the method of the present invention.

2 is a diagram schematically showing the directionality of a shear wave in a biological medium.

3 is a view showing a first embodiment of the artificial membrane according to the present invention.

4A and 4B are a cross-sectional view and a partial plan view of a second embodiment of the artificial membrane according to the present invention.

Fig. 5 shows a specific embodiment of the present invention.

Figure 1 schematically shows the generation of mechanical waves in the medium 11 using the method according to the invention. In this figure, the method is carried out by a transducer 12 using a focused sound wave at the height of the interface 13. Figure 1 typically shows the focusing of the waves in a plane with two dashed lines that are substantially hyperbolic and symmetrical about the centerline of the transducer 12 and approaching each other at an intense depth. According to the method of the present invention, this concentration depth is accurately selected corresponding to the depth of the interface.

The concentrated waves are preferably ultrasonic waves. In the embodiment of Figure 1, the interface 13 is made using an artificial membrane that surrounds the artificial medium 14.

The transfer of the momentum between the media 14 and 11 produces an acoustic radiation force 15 which at the interface 13 of the medium 11 pushes this interface and generates a mechanical impulse .

According to the invention, the medium is therefore mechanically stimulated using the acoustic radiation force 15 generated at the interface 13 of the two media 11 and 14 with different acoustic properties.

Acoustic radiation power is a phenomenon characteristic of all acoustic propagation. When applied to the basic volume V located in the propagating medium 11, there is a non-zero balance between the inlet flow amount and the outlet flow amount of the motion transmitted by the sound waves, and an acoustic radiation force is generated. The force (F) due to this imbalance obtained by averaging multiple ultrasonic cycles can be obtained from the following equation.

Where ρ is the density of the media, p is the pressure, v is the base velocity, n is the unit vector perpendicular to the element (dS) of the surface of the volume (V), and the angle brackets indicate the average time.

Therefore, in order to compare the amplitude of the acoustic radiation force generated by concentration in the medium with the amplitude of the radiation force concentrated on the interface, the volume radiation force generated by absorbing the acoustic energy, and the interface There is a reason to be interested in the surface radiation force generated from the surface.

Considering the propagation of a sound wave having intensity (I) and speed (c) in a specific direction (Oz) in a dissipation medium having an ultrasonic absorption coefficient (?), The radiation force is expressed by its volume density (f) .

In addition, it is possible to consider the propagation of ultrasonic waves in the first medium 14 up to the interface 13 with the medium 11.

Due to the special effect of the interface 13, a surface radial force 15 is generated locally at the interface 13, which causes movement of the medium 11 located nearby.

As described above, by pushing the interface like this, a mechanical vibration of the main amplitude propagating in the biological medium 11 occurs.

The radiation force 15 per unit surface (denoted by pi) at the interface 13 when generated by a plane-incidence ultrasonic wave perpendicular to the interface 13 (Shutilov VA, Fundamental Physics of Ultrasound, page 133, CRC 1988 Can be written as:

Where R is the reflection coefficient of the interface 13 (in terms of the energy terms), c ₁₄ and c ₁₁ are the ultrasound velocities in the media 14 and medium 11, and I is the energy of the incident ultrasound beam.

Considering the specific volume V of the height H in the medium 11 (one of the interfaces of this particular volume coincides with the interface 13 of section A) It is possible to compare the relative contribution of the two types of forces that occur when the volume 14 is spread over a particular volume V of the medium 14.

으로 쓸 수 있고, 흡수에 의해 발생되는 체적 방사력은 1 차 근사화를 통해

로 쓸 수 있다.The volume V is subjected to the volume force F _vol due to acoustic absorption in the medium 11 and also receives the surface force F _surf on the section A due to the contrast between the two media 14 and 11 do. The surface force (F _surf )

, And the volumetric radiation force generated by the absorption is firstly approximated

Can be written as.

In practice, such a magnitude of force is generated by a partial focus region 12 having a section A such as the thickness of the acoustic beam to be focused and axially centered on the interface 13 having the same height H as the partial depth of the field (focal demi zone).

The ratio of the two forces acting on the focal volume zone can be written as:

값이 낮기 때문에, 두 힘의 비는 다음과 같이 표현된다.The contrast (R) and

Since the value is low, the ratio of the two forces is expressed as:

이다. 따라서, 2R-γ_c 가 0.25 정도가 되고, 표면력이 체적력의 진폭의 두 배가 되도록 계면 재료를 선택하면 충분하다는 것이 명백하다.The values of this ratio mainly depend on the choice of material constituting the interface 13. The term 2R-γ _c actually depends on the choice of the interface material. For the 2α ₁₁ H term, considering the field depth of the transducer with the number of openings F / D = 1 and the center frequency of 5 MHz, and taking into account the typical attenuation at the chest (1 dB / MHz / cm)

to be. Therefore, it is clear that it is sufficient to select the interface material so that 2R -? _C becomes about 0.25 and the surface force becomes twice the amplitude of the volume force.

For this purpose, an elastic membrane can be used, for example, in order to increase the speed contrast. Such a membrane can be made of, for example, latex, polyurethane, silicone or the like. It is clear that latex is particularly suitable for making membranes useful in the present invention.

Advantageously, the transducer 12 is suitable for carrying out a very fast imaging step of the medium 11. Depending on the transducer, the image can be two-dimensional or three-dimensional. Also, if a simple translator element that does not move is employed, the image may be one dimensional (line of sight). The very fast ultrasound imaging step is associated with the step of applying focused ultrasound at the height of the membrane 13. The occurrence of this phase is synchronized as a function of the propagation velocity of the mechanical wave produced by applying ultrasonic waves.

From the viewpoint of obtaining a high quality image, the reflection coefficient is limited at the height of the interface 13 so as not to damage the ultrasonic image due to the energy loss transmitted. This means that the medium surrounded by the film having an impedance close to the medium to be imaged is selected to effectively minimize reflection at the interface. Examples of suitable materials are shown below.

Since the present invention focuses on elastic ultrasound images, it is necessary to be particularly interested in generating a shear wave at the interface 13 using the method according to the present invention.

To explain the characteristics of the shift field corresponding to the mechanical waves generated from the surface stimuli, it is necessary to be interested in the propagation theory of the elastic waves caused by the stress applied to the surface of semi-infinite solids.

This semi-infinite solid is an isotropic elastic propagating medium 11. Four kinds of waves can be propagated, three are volumetric waves and one is surface waves. The volumetric wave consists of a head wave, a compression wave and a shearing wave.

For the shear wave, the Green function calculation (Gakenheimer and Miklowitz, Transient excitation of a half space by a point load moving on the surface), I, J. Appl .Mech., 1969) show that the shear wave generated in the volume represents a directional lobe. This results from the bipolar behavior of the local shearing source.

2 schematically shows the directionality of the shear wave generated in the front-end circular section 26 located at the interface 23 located on the surface of the medium 21, and the ultrasonic waves are concentrated in this section.

The ultrasonic radiation force 25 generates a shear wave in accordance with the directivity lobes 27 and 27 ', the maximum of which lies at 35 degrees from the normal at the interface 23 and represents this mechanical shear wave.

In fact, in a medium of large size, in the case of a medium having typical mechanical properties for biological tissue, the main lobe is located at 35 degrees from the normal of the interface 23.

It is therefore apparent that it is appropriate to place the local shear source at 35 DEG relative to this zone in order to maximize the amplitude of the shear wave in a given spatial zone of special interest.

인 것으로 관찰되는 것 또한 잘 알려져 있다. 기계적 펄스는 이미지화되기 위해 짧아야 하며, 따라서 압축파는 이미지화된 영역에서 매우 빠르게 빠져나가는 경향을 갖게 될 것이다.If the compression wave propagates at a very high speed, for example c _T is the speed of the shear wave and c _L is the speed of the compression wave

Is also well known. The mechanical pulses must be short enough to be imaged, and thus the compression wave will tend to go out very quickly in the imaged area.

Thus, it is sufficient to reach several tens of microseconds, such as approximately 30 [mu] s for a zone located, for example, at a depth of 4 cm, so that the traveling field only represents another velocity wave that is approximately the same as the velocity of the shear wave.

The first wave causes the stress to continue, and the amplitude at the interface becomes zero. The first wave propagates on the surface in the form of a compression wave in the form of a shear wave in a given direction of part of its energy. This particular angle is given by

Where c _T is the velocity of the shear wave and c _L is the velocity of the compression wave.

However, the velocity values of the shear wave and the compression wave are about 5 m / s and 1500 m / s, respectively. As a result, this particular angle is nearly zero, and this leading wave does not penetrate the medium. Therefore, since the image is formed even at a slight depth in the medium, the head wave can not be observed.

The surface wave or Rayleigh wave (R) is actually easily observed because it has a slight normal direction component along the Z axis. This component can be as deep as a wavelength in the biological medium, or as deep as 1 cm.

The propagation speed of this surface wave is given very accurately by the expression of Viktorov.

Where c _R is the surface wave velocity.

Surface waves are therefore approximately equal in speed to shear waves.

이므로 그의 존재는 속도 (c_T) 를 단지 약간만 바꾸게 된다.As a result, it is apparent that it is practically impossible to temporally separate the surface wave R and the shear wave temporarily. However, surface imaging also does not overlap with shear waves because imaging is also done at some depth. Further, even when overlapping with the shearing wave,

, So his presence changes the speed (c _T ) only slightly.

Fig. 3 shows a first embodiment of the artificial membrane according to the present invention.

This embodiment is particularly used with concentrated ultrasound treatment methods. In fact, this method of treatment requires the presence of a link medium between the ultrasonic transducer and the biological medium. Such a connecting medium is generally a water bag consisting of a membrane filled with water which can be preferably used to practice the present invention.

It is clear that in the presence of such water pockets, it is almost impossible to produce a shear wave through direct mechanical contact, precisely because of the connecting medium.

This is harmful when imaging the biological medium by elastic ultrasound imaging to monitor the evolution of the elastic properties associated with the progress of the treatment. In addition, even if a volumetric radiation force can be generated in the biological medium, the volumetric radiation pressure that can be generated in the medium is considerably reduced due to the loss of ultrasonic energy at the interface between the water pocket and the medium.

The embodiment of the present invention shown in Figure 3 generates mechanical shear waves in the biological medium 31 to precisely eliminate these disadvantages despite the presence of water pockets.

The assembly shown in FIG. 3 uses an imaging probe 38 having an ultrasonic transducer 32. This image probe 38 is used in a water pouch, which defines a connecting medium 34 surrounded by a membrane 34 '. The water bladder is located on the surface of the biological medium 31 forming the interface 33, for example the surface of the chest.

The method according to the present invention makes use of the interfacial effect at the height of the film 34 'to produce a mechanical wave, more precisely a shear wave, in the medium 31.

The elasticity of the observed medium 31 can be mapped at any moment by imaging such shearing waves.

When the method according to the present invention is used in concentrated ultrasound therapy, it is possible to easily follow the change in the elasticity of the treatment area by using one and the same imaging probe 38. This imaging probe 38 is programmed to locally trigger the measurement of elasticity by performing a step of generating a mechanical wave as well as a treatment, followed by a synchronized imaging step of the medium 31.

In addition, the present invention can control the parameters of the interface by observing what is happening in the medium 31.

In fact, the radiation force 35 generated at the interface 33 between the two media 34 and 31, in contrast to the acoustic parameters of the medium 31 and the intensity of the ultrasonic beam, Depending on other parameters that are likely to be encountered. The interfacial radiation force actually depends on the ratio of the acoustic impedance (the ratio of the speed of sound in the two media) or even the thickness of the film.

In particular, a well-selected membrane material may be used to adjust these parameters to amplify the radiation pressure at the interface 33.

Although the acoustic impedances of the two media 31 and 34 are similar, it is also desirable that the two media 31 and 34 have very different sonic speeds. This results in a higher radial pressure and at the same time avoids reflections at the interface 33, which is detrimental to ultrasonic imaging.

With this in mind, an elastic membrane filled with silicone or chloroform or even mono-chlorobenzene or nitromethane or even potassium is preferably used.

These materials in fact have acoustic impedances close to those of biological media, but sound speeds are very different.

Fig. 4 shows a second embodiment of the artificial membrane according to the present invention. In this embodiment, the membrane 44 'forming the interface 43 is adapted to limit and magnify the amplitude and direction of the mechanical waves in the region of interest 66 located in the medium 41.

In fact, if several shear circles oscillating at the surface are properly positioned, this defines the amplitude of the mechanical wave and, more particularly, the area in which the axial component increases.

In the embodiment of FIG. 4, a film having an uneven thickness and composition is used. It is possible to actually spatialize the surface source using a film whose thickness and / or composition is not constant at the height of the interface 43 with the medium 41.

4A and 4B illustrate a particular embodiment of a membrane 44 ' suitable for concentrating the mechanical waves on the region of interest 66 surrounding the connecting medium 44. As shown in Fig.

4A is a cross-sectional view along the line A-A, and FIG. 4B is a partial plan view along the cross-section B-B.

The region of interest 66 is located at depth Z and the properties of the film 44 'are determined by this depth Z in terms of thickness and composition. 4, the thickness of the membrane 44 'increases in the crown zone 49 shown in Fig. 4b such that the region of interest 66 and crown 49 are cone- .

When a sound wave is transmitted to the membrane 44 ', a substantial axial movement occurs at the height of the crown 49 via the acoustic radiation force 45 because the film thickness or film composition is locally optimized for this reason.

Since the crown 49 is symmetrical about the axis of rotation Ax, the axial movement is added and propagated to the maximum amplitude at the region of interest 66 located in each of the main release lobes of the membrane source.

Other possibilities for constructing the membrane to obtain the area of interest 66 at a particular depth Z are also evident.

Other types of membranes 44 ' may be used depending on various configurations, such as a crown shape as well as a rectangle. Instead of a continuous relief surface, a spike may also be placed in the crown.

Finally, FIG. 5 shows a specific embodiment of the invention in which the biological interface 53 in the biological medium 51 is used in accordance with the method of the present invention. According to the present invention, the transducer 52 is used to apply ultrasonic waves concentrated in the height of the interface 53, that is, in the direction of the interface with the depth of the interface.

By the interfacial effect, the ultrasonic waves produce a surface radiation force 55 that causes mechanical shear waves in the biological medium 54 contained in the biological medium 51. The transducer 52 is used to image the propagation of this shear wave and deduce the mechanical properties of the medium 54 through this observation.

When the method according to the present invention is used to characterize the biological medium 54 present in the biological medium 51 as shown in Fig. 5, the mechanical properties of the medium 51 can be deduced therefrom. Indeed, the second interface 53 'existing in the Oz direction generates a shear wave in the medium 51, as well as the size of the biological medium 54, ). In imaging the entire medium, properties for media 51 and 54 and their interfaces 53 and 53 'can be deduced.

Finally, as defined in the following claims, there are various implementations in accordance with the concepts of the present invention.

Claims (12)

(13, 23, 33, 43, 53) defining the viscoelastic media (11, 21, 31, 41, 51) and the viscoelastic media and other media Generating acoustic radiation forces (15, 25, 35, 45, 55) in the viscoelastic medium (11, 21, 31, 41, 51) by applying concentrated acoustic waves; And A method of generating and imaging mechanical waves in a viscoelastic medium (11, 21, 31, 41, 51) comprising imaging said viscoelastic medium, Wherein the generating and imaging steps are performed together to cause the propagation of the mechanical waves generated within the viscoelastic medium to be imaged, wherein the mechanical waves are generated and imaged in a viscoelastic medium. The method according to claim 1, Characterized in that the sound waves are ultrasonic waves. 3. The method according to claim 1 or 2, Characterized in that the interface at which the sound waves are concentrated is a biological interface in the viscoelastic medium and the other medium is a biological medium surrounded by the interface and wherein the viscoelastic medium and the biological medium have separate acoustic properties. ≪ / RTI > 3. The method according to claim 1 or 2, Wherein the interface at which the sound waves are concentrated is an artificial membrane (34 ', 44') located in contact with the surface of the viscoelastic medium and the other medium comprises a device (32, 38; 42, 48) Wherein the connecting medium and the viscoelastic medium have separate acoustical properties. ≪ RTI ID = 0.0 > 11. < / RTI > 5. The method of claim 4, Characterized in that the artificial membranes (34 ', 44') have a composition selected to minimize the acoustic impedance contrast while increasing the amplitude of the mechanical waves. 5. The method of claim 4, Characterized in that the artificial membranes (34 ', 44') have a thickness selected to minimize the acoustic impedance contrast while increasing the amplitude of the mechanical waves. 5. The method of claim 4, Characterized in that the artificial membranes (34 ', 44') have a non-uniform composition spatially determined to increase the amplitude of the mechanical waves (27) in the region of interest of the viscoelastic medium. How to. 5. The method of claim 4, Characterized in that the artificial membrane has a non-uniform thickness (49) spatially determined to increase the amplitude of the mechanical waves in the region of interest (66) of the viscoelastic medium. 3. The method according to claim 1 or 2, Applying the sound waves focused on the interface is made continuously at a plurality of points of the interface, Wherein the plurality of points and successive concentrations are determined to increase the amplitude of the mechanical waves in the area of interest of the viscoelastic medium. 3. The method according to claim 1 or 2, A method of generating and imaging mechanical waves in a viscoelastic medium, the method further comprising applying the ultrasound therapy to monitor the effectiveness of the ultrasound therapy. 11. The method of claim 10, Characterized in that the result of the imaging step is used to control the ultrasonic treatment method. An artificial membrane (34 ', 44') serving as an interface during performing the method according to claim 1 or 2, The artificial membrane having at least one of a selected composition and thickness selected to minimize the acoustic impedance contrast while increasing the amplitude of the mechanical wave and being located in partial contact with the surface of the viscoelastic medium, 38. An artificial membrane enclosing a connection medium located between an apparatus (32, 38; 42, 48) for generating sound waves and the viscoelastic medium.

Images