EP2084702A2

EP2084702A2 - Method for generating mechanical waves by creating an interfacial acoustic radiation force

Info

Publication number: EP2084702A2
Application number: EP07866491A
Authority: EP
Inventors: Mathieu Pernot; David Savery; Jérémy BERCOFF; Claude Cohen-Bacrie
Original assignee: SuperSonic Imagine SA
Current assignee: SuperSonic Imagine SA
Priority date: 2006-10-25
Filing date: 2007-10-25
Publication date: 2009-08-05
Anticipated expiration: 2027-10-25
Also published as: FR2907692B1; US8037766B2; CA2667527A1; FR2907692A1; CN101589426A; CA2667527C; WO2008050072A3; US20080276709A1; EP2084702B1; WO2008050072A2; CN101589426B

Abstract

The present invention relates to a method for generating mechanical waves in a viscous-elastic medium (11), that comprises the step of generating an acoustic radiation force (15) in the viscous-elastic medium (11) by the application of acoustic waves focused on an interface (13) separating two areas (11, 14) having different acoustic properties.

Description

Title of the invention

"Process for generating mechanical waves by interfacial acoustic radiation force generation"

Background of the invention

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

More particularly, the invention is concerned with the generation of mechanical waves within a viscoelastic medium, such mechanical waves being capable of being imaged in order to determine the properties of the viscoelastic medium.

The present invention thus relates more precisely to the field of elastography.

This medical imaging technique makes it possible to map the mechanical properties of a viscoelastic medium and to quantify the rheology of the viscoelastic medium. According to this technique, a mechanical stimulus is generated and causes tissue displacement. We then measure the spatiotemporal response of the tissue to this mechanical excitation. The measurement of the spatiotemporal response is advantageously performed by means of an imaging modality, for example by ultrasound or by magnetic resonance, etc.

Once the movement resulting from the known mechanical excitation, it is possible to determine the mechanical properties of the medium.

In transient elastography, mechanical excitation consists of a short mechanical pulse or a small number of pulses created either on the surface of the body or inside the tissue itself.

The quality of the transient elastography images crucially depends on the amplitude of the displacements that can be generated by the excitatory mechanical stimulation. It is noted that in transient elastography by external stress, the amplitude of displacement is limited only by the maximum surface vibration that can be induced in contact with the medium without damaging it. The movements in the tissue thus generated easily have amplitudes of the order of 100 microns.

Thus, in general, the displacements resulting from the mechanical excitation must be sufficiently large to be measurable with a minimum of errors, while being limited to avoid any harmful effect in the medium, especially when it comes to a biological tissue.

The power generated is therefore satisfactory, but it is known that the use of external solicitation creates technical problems, such as the size of the device required for this solicitation, the synchronization of the mechanical excitation with the imaging, the localization of the device. mechanical excitation, optimization of the amplitude of the wave in the areas of interest at depth, etc.

There is also transient elastography where the mechanical stress of the observed medium is created by an acoustic radiation force. This radiation force is obtained by focusing an ultrasonic beam inside the medium. The focusing of the beam can here take place in a single zone of the medium or successively in a plurality of zones of the medium.

The focal point, on which the ultrasonic beam converges, is then moved at a speed greater than the propagation speed of the elastic waves to generate an elastic displacement wave of maximum amplitude of the order of 10 to 100 μm.

This displacement wave then propagates in the medium. The measurement of the propagation properties of the wave, observed by ultrasound, MRI or other imaging modality, makes it possible to determine characteristic mechanical quantities of the tissues. investigated. It is possible to determine, among other things, a shear modulus or a viscosity, etc.

The displacement generated by the acoustic radiation force is related to the energy deposited in the tissue, and the amplitude of the generated mechanical wave is limited by the maximum acoustic power that can be sent into the medium observed without altering thermally or mechanically tissue.

The ultrasonic solution offers simplicity of manipulation, reproducibility of the way in which the stress is generated, an assurance as to the synchronization of the excitation with the imagery and an assurance as to the location of the excitation, but suffers from a lack of power.

Object and summary of the invention

The main purpose of the present invention is therefore to overcome such drawbacks by proposing a method for generating mechanical waves within a viscoelastic medium comprising a step of generating an acoustic radiation force within the viscoelastic medium by applying acoustic waves focused on an interface delimiting two zones having distinct acoustic properties.

With such a method of generating mechanical waves within a viscoelastic medium, the amplitudes of the induced displacements are higher than with a simple ultrasonic stressing by focussing within a tissue.

According to the invention, acoustic waves are focused at depth and in the direction of a surface interface.

The interface on which the acoustic waves are focused, can be a gel / skin or water / skin separation surface or water / membrane / skin, etc. The membrane can be a membrane deformable or not. The interface may also be located between a solid medium and a liquid medium within the imaged tissue, or between two media of different acoustic properties within the tissue. This is, for example, the case with a biological medium comprising a cyst. With the method according to the invention, the amplitude of the displacements generated is of the order of 100 μm.

According to a preferred embodiment of the invention, the step of generating an acoustic radiation force is coupled with an imaging step of the medium, the coupling being such as to image the propagation of the mechanical waves generated in the middle.

The imaging of wave propagation can be performed in one, two or three dimensions. In such a preferred embodiment, an elastography measurement of the medium is performed. This is the preferred application of the invention, focusing at the interface according to the invention allowing a remarkable improvement in the quality of the imaging thus performed.

According to an advantageous characteristic, the acoustic waves are ultrasonic waves.

The ultrasonic frequencies are, in fact, particularly suited to the generation of a radiation force allowing, in particular, the creation of shear waves within a medium. Such shear waves are commonly used in elastography. Such shear waves belong to the mechanical waves as generated according to the method of the invention and they are the ones which are generally imaged according to the elastographic methods.

According to one particular characteristic, the interface on which the acoustic waves are focused is an interface present between two zones of distinct acoustic properties present within the viscoelastic medium. With such a characteristic, the visibility and the characterization of the interfacial zones within a medium are considerably improved. Indeed, the observation of the propagation of shear waves created at the interfaces naturally present in the human body makes it possible to further characterize these interfaces and the media they separate.

This characteristic will be particularly interesting in the case of the presence of a liquid cyst, blood vessels or structures harder than soft tissues, such as bone and cartilage.

According to another particular characteristic of the invention, the interface on which the acoustic waves are focused is an artificial membrane placed in contact with the surface of the viscoelastic medium and surrounding a so-called coupling medium placed between a device intended to apply the acoustic waves. and the surface of the viscoelastic medium, the coupling medium and the viscoelastic medium defining two zones of distinct acoustic properties.

This characteristic is particularly interesting in applications where the presence of an artificial medium is necessary. This is the case, in particular, in focused ultrasound therapy processes where a thin membrane surrounding a coupling medium is generally used to effect contact with the biological tissue.

According to the invention, it is then possible to use such an interface to generate shear waves. Successively with excitation, one advantageously uses an elastographic mode where an imaging of the medium and the propagation of shear waves is carried out. In this way, the viscoelastic properties of the tissue are then evaluated and monitored during a therapeutic treatment.

Such monitoring is particularly relevant because it is well known that the elasticity of biological tissues changes when they are denatured after cellular thermal necrosis. According to an advantageous characteristic, the artificial membrane has a composition chosen to minimize the acoustic impedance contrast while increasing the amplitude of the mechanical waves.

According to another advantageous characteristic, the artificial membrane has a thickness chosen to minimize the acoustic impedance contrast while increasing the amplitude of the mechanical waves.

These last two features make it easy to adapt an artificial membrane according to the intended application, by modifying its composition, its shape and / or its thickness.

It turns out that the method of generating mechanical waves according to the invention is of great interest for imaging the elasticity of the surface areas of biological media.

Indeed, as the shear waves are generated at the interface, this allows to obtain waves of very large amplitude at the surface of the tissue. This characteristic is not possible to achieve with the radiation pressure technique in volume since the generated waves generally reach the surface of the medium very attenuated.

The use of an artificial membrane, for example the membrane of a water bag, makes it possible to generate a mechanical pulse at a predetermined location on the surface of the medium. The technique according to the invention is therefore very interesting for elastographic imaging of the skin, for example at the level of a melanoma or superficial lesions such as for example certain lesions of the breast.

However, it can be interesting to be able to generate deep shear waves in a medium. Thus, according to a particularly advantageous characteristic of the invention, the artificial membrane has a non-uniform and spatially determined composition so as to increase the amplitude of the mechanical waves in a region of interest of the viscoelastic medium.

Alternatively or in addition to the preceding characteristic, the artificial membrane may have a non-uniform thickness and determined spatially so as to increase the amplitude of the mechanical waves in a region of interest of the viscoelastic medium.

With these characteristics of the membrane, it is possible to use the directivity of the shear waves to concentrate the mechanical waves in a zone of interest. The amplitude of the mechanical waves in this area is therefore increased.

It is also possible for the application of acoustic waves focused on an interface delimiting two zones having distinct acoustic properties to be successively performed at a plurality of points of the interface, this plurality of points and the succession of the focal points being determined in a manner to increase the amplitude of the mechanical waves in a region of interest of the viscoelastic medium.

With this dynamic focus feature, one can then, somehow, draw a pattern on the interface. Depending on the shape of this pattern, the amplitude of the mechanical waves in a certain area of interest is increased by an interference phenomenon. In the dynamic succession of ultrasonic beam focusing, the relative delay of each ultrasound beam focused at a given point is chosen wisely so that the interference is positive at the level of the area of interest. The mechanical shear waves are then as focused in the area of interest. In an advantageous application of the invention, the method is coupled with an ultrasonic treatment method to monitor the effect of the treatment.

Advantageously, the ultrasonic treatment method is capable of being controlled according to the results of the imaging step of the medium.

The invention also relates to an artificial membrane intended to be partially placed in contact with the surface of a viscoelastic medium and intended to surround a so-called coupling medium placed between an acoustic wave generating device and a viscoelastic medium to serve as a interface during the implementation of a method according to the invention.

Brief description of the drawings

Other characteristics and advantages of the present invention will emerge more clearly on reading the following description, given in an illustrative and nonlimiting manner, with reference to the appended drawings in which:

FIG. 1 schematically illustrates a generation of mechanical waves according to the method of the invention,

FIG. 2 schematically illustrates the directivity of shear waves in a biological medium,

FIG. 3 represents a first embodiment of an artificial membrane according to the invention,

FIGS. 4a and 4b represent in section and in partial view from above a second embodiment of an artificial membrane according to the invention,

Figure 5 shows a particular embodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION Figure 1 schematically illustrates the generation of mechanical waves in a medium 11 using a method according to the invention. In this figure, the method is applied using a transducer 12 applying focussed acoustic waves at an interface 13. In FIG. 1, the focusing of the waves is schematized in the plane in a conventional manner by two lines. dotted substantially hyperbolic symmetrical with respect to the center line of the transducer 12 and approaching each other at the depth of focus. According to the method of the invention, this depth of focus is precisely chosen as corresponding to the depth of the interface.

The focused waves are advantageously ultrasonic waves. In the example of FIG. 1, the interface 13 is made using an artificial membrane surrounding an artificial medium 14.

Transfers of quantity of movements between media 14 and

11 allow the creation of an acoustic radiation force 15 which, pressing on the interface 13 of the medium 11, will push it and generate a mechanical wave within the medium 11.

According to the invention, the medium is thus mechanically stimulated by using an acoustic radiation force generated at the interface 13 of two media 11 and 14 having different acoustic properties.

The acoustic radiation force is a characteristic phenomenon of any acoustic propagation. Applied to a particle volume V, located in the propagation medium 11, it is created following a non-zero balance between the incoming and outgoing flux of momentum carried by the acoustic wave. This average non-zero balance over many ultrasonic cycles results in a force F described by:

F = / - | (pw n + n) βj, where p denotes the density of the medium, p the pressure, v the particle velocity, n the unitary vector normal to an element dS of the surface of the volume V, and the square brackets denote the time averaging.

Thus, in order to compare the amplitudes of the acoustic radiation force generated by focusing within a medium and the radiation force obtained with focusing on an interface, it is necessary to look at the forces of volumic radiation generated by absorption of acoustic energy and the surface radiation forces generated at the interface of media having properties of different speed and density.

Considering the propagation of an acoustic wave of intensity I and velocity c in a direction denoted Oz in a dissipative medium with an ultrasonic absorption coefficient denoted α, it is common to express the radiation force by its density density. f according to the formula:

f = 2alejc.

Furthermore, the propagation of an ultrasound wave in a first medium 14 to an interface 13 with a medium 11 is considered.

Due to a particular effect of the interface 13, a surface radiation force 15 is generated locally on the interface 13, which causes the displacement of the medium 11 located nearby.

This thrust of the interface makes it possible to generate, as previously seen, mechanical waves of high amplitude that propagate in the biological medium 11.

Created by a plane ultrasonic wave incident perpendicular to the interface 13, the radiation force 15 per unit area at the interface 13, denoted π, can be written (according to Shutilov VA, Fundamental Physics of Ultrasound, p 133, CRC , 1988): π = 1 + Λ - (1 - .R) -

C ₁ ,

where R is the reflection coefficient (in terms of energy) of the interface 13, Ci ₄ and Cn are the ultrasonic celerities in the media 14 and 11, and I is the energy of the incident ultrasonic beam.

Considering a particle volume V of height H in the medium 11, a particle volume of which one of the borders coincides with the interface 13 on a section A, it is possible to compare the relative contributions of the two types of forces generated when a plane wave intensity I propagates in the particle volume V of the medium 14.

The volume V is then subjected to a volume force F _VO ι due to the acoustic absorption in the medium 11, and subjected to a surface force F _on f on the section A due to the contrast between the two media

14 and 11. The surface strength F _on f is written F _sω , the volume radiation force created by absorption can be written as a first approximation F fah _ml = 2a = _u l IC ₁₄ AH (I - R)

In reality, these orders of magnitude of force are applied to a half-focal zone centered axially on the interface 13 having a section A equal to the thickness of the focused acoustic beam and having a height H equal to half a depth of field.

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

_FIF _ _α . ⁱ + aq-WO + r. ⁾ Considering that the contrasts R and - - 1 = γ _c are small, c,

then the ratio of the two forces is expressed F _s . IF _ml ≈ 2R - Yc

2a _u H

The values of this ratio depends mainly on the choice of the material from which is made the interface 13. The term 2R-γ _c is indeed based on the choice of interface material. As for the term

2αuH, taking the depth of field of a transducer with aperture p - = 1 and 5 MHz center frequency, and considering

the typical attenuation in the breast (ldB / MHz / cm), we find 2αuH≈0,12. It can thus be seen that it is sufficient to choose the interface material so that 2R-γ _c is of the order of 0.25, so that the surface strength is of amplitude twice as large as the force of volume.

For this purpose, in order to increase the contrast of celerity, it is possible for example to use an elastic membrane. Such a membrane may be, for example, made from latex, polyurethane, silicone, etc. It is found that the latex is particularly well suited for the manufacture of a membrane useful in the implementation of the invention.

Advantageously, the transducer 12 is capable of performing an ultra-fast imaging step of the medium 11. Depending on the transducer, the image may be two-dimensional or three-dimensional. It can also be reduced to one dimension (a line of sight) if a single element of stationary transducer is used. This ultrasound ultrafast imaging step is coupled with the step of applying the focused ultrasonic waves at the membrane 13. The occurrences of these steps are then synchronized as a function of the propagation velocity of the mechanical waves created by application of ultrasonic waves. In order to obtain an image of good quality, it is therefore necessary to be careful to limit the reflection coefficient at the interface 13, so as not to harm the ultrasound imaging because of the loss of energy transmitted. This leads to choosing a medium surrounded by the membrane having an impedance close to that of the medium to be imaged, which makes it possible to minimize reflection at the interface. Examples of suitable materials are given below.

As the invention is especially aimed at elastography, it is necessary to focus in particular on the generation, by the method according to the invention, of shear waves at the interface 13.

In order to specify the characteristics of the field of displacement corresponding to the mechanical waves resulting from a surface excitation, it is necessary to study the theory of elastic wave propagation induced by a solicitation on the surface of a semi-infinite solid. .

Such a semi-infinite solid is an isotropic elastic propagation medium 11. Four types of waves can then propagate: three waves of volume and a surface wave. Volume waves consist of a head wave, a compression wave, and a shear wave.

With regard to shear waves, the calculation of the function of Green (according to Gakenheimer and Miklowitz, Transient excitation of a half space by a point load on the surface I, J.Appl.Mech., 1969) shows that the shear waves generated in the volume have directivity lobes. This comes from the dipolar behavior of the point shear source.

FIG. 2 schematically illustrates the directivity of the shear waves generated by a source zone 26, on which ultrasonic waves are focused, located on an interface 23, placed on the surface of a medium 21. The ultrasonic radiation force 25 generates shear waves according to directivity lobes 27 and 27 ', whose maxima are located 35 ° from the normal to the interface 23 and illustrating these mechanical shear waves.

Indeed, in a large medium, the main lobe is at 35 ° relative to the normal at the interface 23 when considering a medium whose mechanical characteristics are typical of biological tissues.

It is thus found that, in order to maximize the amplitude of the shear wave in a defined spatial area of particular interest, it is relevant to place the point shear source at 35 ° with respect to this zone.

It is also known that the compression wave propagates at very high speed and it is observed for example that c _L ≈300c _τ where c _τ is the speed of the shear wave and c _L that of the compression wave. Since the mechanical pulse must be short in order to be imaged, the compression wave will tend to escape from the imaged region very quickly.

It suffices to wait a few tens of microseconds, for example about 30 μs for an area located at a depth of 4 cm, so that the field of displacement is no more than the manifestation of the other celestial waves approximately equal to the speed of the shear waves.

The head wave ensures the continuity of the stresses and has zero amplitude at the interface. It propagates on the surface in the form of a compression wave, yielding part of its energy in volume in the form of a shear wave in a given direction. This specific angle is given by the formula:

0 = αsin (^), where c _τ is the speed of the shear wave and c _L is the velocity of the compression wave.

However, the speed values of the shear and compression waves are respectively of the order of 5 m / s and 1 500 m / s. Consequently, the specific angle is almost zero and this head wave does not enter the medium. It will therefore not be observable since we will image deep, even weak, in the middle.

The surface wave, or Rayleigh R wave, is actually capable of being detected in volume because it has a normal evanescent component along the Z axis. This component extends over a depth of approximately one wavelength, about 1 cm in biological media.

The propagation velocity of this surface wave is given with good precision by Viktorov's formula:

c _R 0.718 - (c _r / cJ ² _ 0.718 - (5/1500) ² _ _, c _τ ^~ 0.75 - (c _r / cJ ^{2 ~} 0.75 - (5 / 150O) ²

where C _R is the speed is the speed of the surface wave.

The surface wave therefore has a speed almost identical to that of shear waves.

It can be seen, therefore, that it is not really possible to temporally separate the R wave and the shear wave. On the other hand, here too, since we image in depth, even weak, this wave does not come superimposed on the shear waves. Moreover, even in the case of a superposition to the shear wave, its presence will only slightly alter the measurement of the speed c _τ since c _R ≈Cτ.

FIG. 3 shows a first embodiment of an artificial membrane according to the invention. This embodiment is particularly adapted to be combined with a focused ultrasound therapy method. Indeed, such a method of therapy requires the presence of a coupling medium between ultrasonic transducers and a biological medium. Such a coupling medium is generally a water bag consisting of a membrane filled with water and which can be advantageously used to implement the invention.

Now it is known that in the presence of such a water bag, it is almost impossible to generate a shear wave by direct mechanical contact, precisely because of the coupling medium.

This is detrimental when one wants to image the biological medium by elastography to follow the evolution of the elastic properties related to the progression of the treatment. In addition, even if one managed to generate a volumetric radiation force within the biological medium, the volume radiation pressure that can be generated in the medium would be greatly diminished because of the loss of ultrasonic energy. at the interface between the water bag and the middle.

The embodiment of the invention presented in FIG. 3 precisely makes it possible to overcome this disadvantage by making it possible to generate mechanical shear waves in a biological medium 31, despite the presence of the water bag.

The assembly shown in FIG. 3 uses an imaging probe 38 carrying ultrasound transducers 32. This imaging probe 38 is applied to a water bag, defining a coupling medium 34 surrounded by a membrane 34 '. The water bag is placed on the surface of a biological medium 31, for example a breast, thus defining an interface 33.

The method according to the invention uses the interface effect at the level of the membrane 34 'to create mechanical waves, more precisely shear waves in the medium 31. By then imaging these shear waves, it is possible to map the elasticity of the medium 31 observed at any time.

When the method according to the invention is used during a focused ultrasound treatment, it thus becomes possible to easily follow the variation of the elasticity of the treated area by using one and the same imaging probe 38. Such a probe of Imaging 38 is then programmed not only to carry out the treatment but also to punctually trigger a measurement of elasticity by performing a step of generating mechanical waves and, successively, in a synchronized manner, an imaging step of the medium 31.

In addition, the invention makes it possible to adjust the parameters of the interface as a function of the observation that one wishes to make of the medium 31.

Indeed, unlike the volumetric radiation force which depends mainly on the acoustic parameters of the medium 31 and the intensity of the ultrasonic beam, the radiation force 35 generated on the interface 33 between the two media 34 and 31 depends on other parameters that are likely to be adjusted by the experimenter.

The interfacial radiation force depends, in fact, the ratio of acoustic impedances, the ratio of sound velocities in the two media or, again, the thickness of the membrane.

In particular, it is possible to use a well-chosen membrane material to adjust these parameters in order to amplify the radiation pressure at the interface 33.

It is also wise that the acoustic impedances of the two media 31 and 34 are similar, but that the two media 31 and 34 have very different sound velocities. This makes it possible to obtain a higher radiation pressure while avoiding reflections at the interface 33 which are harmful to ultrasound imaging. For such a purpose, it will be advantageous to use an elastic membrane filled either with silicone or with chloroform, or else mono chlorobenzene, or nitromethane or potassium.

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

FIG. 4 illustrates a second embodiment of an artificial membrane according to the invention. In this embodiment, the membrane 44 'forming the interface 43, is such that it is possible to confine and amplify the amplitude and the directivity of the mechanical waves in a zone of interest 66 located in a medium 41 .

In fact, when several sources of surface-vibrating shear are suitably arranged, a region is defined in which the amplitude of the mechanical wave, more particularly of its axial component, is increased.

In the example of Figure 4, is used a non-constant thickness and composition membrane. Spatialization of the surface sources can, in fact, be carried out using a membrane whose thickness and / or the composition is non-homogeneous at the interface 43 with the medium 41.

FIGS. 4a and 4b thus describe a particular embodiment for a membrane 44 'surrounding a coupling medium 44, able to focus the mechanical waves on an area of interest 66.

Figure 4a is a section A-A and Figure 4b is a partial top view as seen in section B-B.

The zone of interest 66 is located at a depth Z and the characteristics of the membrane 44 'are determined as a function of this depth Z in terms of thickness or composition. In the example of FIG. 4, the thickness of the membrane 44 'is increased on a crown zone 49 shown in Figure 4b, so that the area of interest 66 and the ring 49 form a cone of about 35 °.

When an acoustic wave is emitted towards the membrane 44 ', axial displacements occur more significantly, by acoustic radiation force 45, at the level of the ring 49 since the membrane thickness or the membrane composition have been locally optimized at this point. end.

By symmetry around the axis AX of revolution of the ring 49, the axial displacements add up and, by propagation, are of maximum amplitude in the zone of interest 66, placed in each of the main emission lobes of the membrane sources.

It can be seen that there are therefore different possibilities of constitutions of the membrane aiming to reach zones of interest 66 of distinct depths Z.

It is also noted that the heterogeneities of the membrane 44 'can be made according to variable geometries, not only in crown, but also in rectangle, etc. Instead of a continuous relief surface, spikes can also be arranged in a ring.

Finally, FIG. 5 shows a particular embodiment of the invention in which a biological interface 53 present within a biological medium 51 is used according to the method of the invention. According to the invention, transducers 52 are used to apply focused ultrasound waves at interface 53, i.e. at the depth of the interface and towards it.

By an interface effect, the ultrasonic waves generate a surface radiation force 55 that induces mechanical shear waves within a biological medium 54 included in the biological medium 51. The transducers 52 are then used to image the propagation of these shear waves and deduce from this observation the mechanical properties of the medium 54.

It may be noted that, when the method according to the invention is used, as represented in FIG. 5, to characterize a biological medium 54 present in the biological medium 51, it is also possible to deduce from it the mechanical properties of the medium 51. Indeed, not only the second interface 53 'present in the direction Oz also generates shear waves within the medium 51 but also the size of the biological medium 54 is generally such that the shear waves generated at the interface 53 also propagates in the medium 51. By imaging the whole of the medium, one can then deduce properties on each of the media 51 and 54 and their interface 53, 53 '.

Note finally that various implementations can be made according to the principles of the invention as defined in the following claims.

Claims

A method of generating mechanical waves within a viscoelastic medium [11] comprising a step of generating an acoustic radiation force [15] within the viscoelastic medium [11] coupled with an imaging step of the medium [21], the coupling being such as to image the propagation of the mechanical waves [27] generated in the medium [21], the generation step being performed by applying acoustic waves focused on an interface [13] delimiting two zones [11, 14] having distinct acoustic properties.

2. Method according to claim 1, characterized in that the acoustic waves are ultrasonic waves.

3. Method according to one of the preceding claims, characterized in that the interface [53] on which the acoustic waves are focused is an interface between two zones [51,54] of distinct acoustic properties present within the viscoelastic medium [51].

4. Method according to one of claims 1 and 2, characterized in that the interface [33] on which the acoustic waves are focused is an artificial membrane [34 '] placed in contact with the surface of the viscoelastic medium [31] and surrounding a so-called coupling medium [34] placed between a device [38,32] for applying the acoustic waves and the surface of the viscoelastic medium [31], the coupling medium [34] and the viscoelastic medium [31] defining two zones of distinct acoustic properties.

5. Method according to claim 4, characterized in that the artificial membrane [34 '] has a composition chosen to minimize the acoustic impedance contrast while increasing the amplitude of the mechanical waves.

6. Method according to claim 4, characterized in that the artificial membrane [34 '] has a thickness chosen to minimize the acoustic impedance contrast while increasing the amplitude of mechanical waves.

7. Method according to one of claims 4 to 6, characterized in that the artificial membrane [34 '] has a non-uniform composition and determined spatially so as to increase the amplitude of mechanical waves [27] in a region of interest of the viscoelastic medium [31].

8. Method according to one of claims 4 to 6, characterized in that the artificial membrane [44 '] has a thickness [49] non-uniform and determined spatially so as to increase the amplitude of the mechanical waves [27] in a region of interest [66] of the viscoelastic medium [41].

9. Method according to one of the preceding claims, characterized in that the application of acoustic waves focused on the interface [33] is performed successively in a plurality of points of the interface [33], this plurality of points. and the sequence of focusing being determined so as to increase the amplitude of the mechanical waves [27] in a region of interest of the viscoelastic medium [31].

10. Method according to one of the preceding claims, characterized in that said method is coupled with an ultrasonic treatment method to monitor the effect of the treatment.

11. The method of claim 11, characterized in that the ultrasonic treatment process is capable of being controlled according to the results of the imaging step of the medium.

12. Artificial membrane [34 '] having a composition and / or a thickness chosen to minimize the acoustic impedance contrast while increasing the amplitude of the mechanical waves to be partially placed in contact with the surface of an environment viscoelastic [31] and intended to surround a so-called coupling medium [34] placed between an acoustic wave generating device [32,38] and a viscoelastic medium [31] to serve as an interface [33] when setting implement a method according to one of the preceding claims.