EP2084702B1 - Method for generating mechanical waves by creating an interfacial acoustic radiation force - Google Patents

Description

Invention background

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 the displacement of the tissues. We then measure the spatiotemporal response of the tissue to this mechanical excitation. The measurement of the spatiotemporal response is advantageously carried out thanks to an imaging modality, for example by ultrasound or magnetic resonance, etc.

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

In transient elastography, mechanical excitation consists of a short mechanical impulse or a small number of impulses created either on the surface of the body, or even inside the tissue.

The quality of transient elastography images crucially depends on the amplitude of the displacements that can be generated by excitatory mechanical stimulation.

It can be seen that in transient elastography by external stress, the amplitude of displacement is limited only by the maximum surface vibration which can be induced on contact with the medium without damaging it. The displacements in the tissue thus generated easily exhibit amplitudes of the order of 100 μm.

Thus, in general, the displacements resulting from the mechanical excitation must be large enough to be measurable with a minimum of errors, while remaining limited to avoid any harmful effect in the environment, in particular when it is a question of biological tissue.

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

There is also a transient elastography where the mechanical stress of the observed medium is created by a force of acoustic radiation. 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 speed of propagation of the elastic waves to generate an elastic wave of displacement of maximum amplitude of the order of 10 to 100 μm.

This displacement wave then propagates in the medium. The measurement of wave propagation properties, observed by ultrasound, MRI or another imaging method, makes it possible to determine mechanical quantities characteristic of 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 linked to the energy deposited in the tissue, and the amplitude of the mechanical wave generated is therefore limited by the maximum acoustic power that can be sent into the environment observed without altering thermally or mechanically the fabric.

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

US5477736 (A ) describes an ultrasonic transducer which generates waves with a focusing inside a medium to be analyzed.

US5903516 (A ) discloses a generator of an acoustic radiation force using two secant waves.

DE4229631 (A1 ) relates to a lens comprising a variable focus.

Subject and summary of the invention

The present invention relates to a method for imaging a viscoelastic medium according to claim 1 and an imaging probe according to claim 10.

The present disclosure proposes to overcome such drawbacks by proposing a method for generating mechanical waves within a viscoelastic medium according to claim 1 comprising a step of generating an acoustic radiation force within the viscoelastic medium by application of 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 stress by focusing within a tissue.

According to the disclosure, acoustic waves are focused to the depth and towards a surface interface.

The interface on which the acoustic waves are focused, can be a gel / skin or water / skin separation surface or even water / membrane / skin, etc. The membrane can be a membrane deformable or not. The interface can also be located between a solid medium and a liquid medium inside the imaged tissue, or between two media with different acoustic properties inside 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, the step of generating an acoustic radiation force is coupled with a step of imaging the medium, the coupling being such that the propagation of the mechanical waves generated in the medium is image.

Wave propagation imagery can be performed in one, two or three dimensions. In such a preferred embodiment, an elastography measurement of the medium is carried out. 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 carried out.

According to a characteristic of the invention, the acoustic waves are ultrasonic waves.

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

According to a 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 an environment are considerably improved. Indeed, the observation of the propagation of the shear waves created at the level of the interfaces naturally present in the human body, makes it possible to characterize these interfaces all the better.

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

According to another particular characteristic, 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 advantageous in applications where the presence of an artificial medium is necessary. This is the case, in particular, in focused ultrasound therapy methods where a thin membrane surrounding a coupling medium is generally used to make contact with the biological tissue.

According to the disclosure, it is then possible to take advantage of such an interface to generate shear waves. Following the excitation, an elastographic mode is advantageously used where an imaging of the medium and of the propagation of the 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 thermal cell necrosis.

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

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

These last two characteristics make it possible to easily 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 makes it possible to obtain waves of very large amplitude at the surface of the tissue. This characteristic is not possible to achieve with the volume radiation pressure technique since the waves generated generally reach the surface of the medium very attenuated.

The use of an artificial membrane, for example the membrane of a water pocket, makes it possible to generate a mechanical impulse at a predetermined location on the surface of the medium. The technique according to the invention is therefore very advantageous 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 may be advantageous to be able to generate shear waves deep in an environment.

Thus, according to a particularly advantageous characteristic of the invention, the artificial membrane has a non-uniform composition and determined spatially 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 spatially determined 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 an area of interest. The amplitude of the mechanical waves in this zone is therefore all the more increased.

It is also possible that the application of acoustic waves focused on an interface delimiting two zones having distinct acoustic properties is carried out successively at a plurality of points of the interface, this plurality of points and the succession of the focal points being determined so increasing the amplitude of the mechanical waves in a region of interest of the viscoelastic medium.

With this dynamic focusing characteristic, we can then, in a way, 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 the focusing of the ultrasonic beams, the relative delay of each ultrasonic beam focused at a given point is judiciously chosen so that the interference is positive at the level of the area of interest. The mechanical shear waves are then focused in the area of interest.

In the disclosure, 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 as a function of the results of the stage of imaging the medium.

The disclosure also relates to an imaging probe carrying the transducer according to the invention and 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 a generation device. acoustic waves and a viscoelastic medium to serve as an interface during the implementation of a method according to the invention.

Brief description of the drawings

• La figure 1 illustre schématiquement une génération d'ondes mécaniques selon le procédé de l'invention,
• La figure 2 illustre schématiquement la directivité des ondes de cisaillement dans un milieu biologique,
• La figure 3 représente un premier mode de réalisation d'une membrane artificielle selon l'invention,
• Les figures 4a et 4b représente en coupe et en vue partielle de dessus un second mode de réalisation d'une membrane artificielle selon l'invention,
• La figure 5 représente un mode de réalisation particulier de l'invention.

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

• The figure 1 schematically illustrates a generation of mechanical waves according to the method of the invention,
• The figure 2 schematically illustrates the directivity of shear waves in a biological medium,
• The figure 3 represents a first embodiment of an artificial membrane according to the invention,
• The Figures 4a and 4b shows in section and in partial view from above a second embodiment of an artificial membrane according to the invention,
• The figure 5 represents a particular embodiment of the invention.

Detailed description of the embodiments of the invention

The 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 acoustic waves focused at an interface 13. On the figure 1 , the focusing of the waves is shown diagrammatically in the plane in a conventional manner by two dotted lines which are substantially hyperbolic symmetrical with respect to the center line of the transducer 12 and which approach each other at the focusing depth. According to the method of the invention, this focusing depth is precisely chosen as corresponding to the depth of the interface.

Focused waves are ultrasonic waves. In the example of the figure 1 , the interface 13 is produced using an artificial membrane surrounding an artificial medium 14.

The transfers of momentum between the 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 therefore mechanically stimulated by using an acoustic radiation force 15 generated at the interface 13 of two media 11 and 14 having different acoustic properties.

où p désigne la densité du milieu, p la pression, v la vitesse particulaire, n le vecteur unitaire normal à un élément dS de la surface du volume V, et les crochets désignent la prise de moyenne temporelle.The force of acoustic radiation is a phenomenon characteristic of all 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 flows of momentum carried by the acoustic wave. This non-zero balance averaged over numerous ultrasonic cycles results in a force F described by: $$\mathbf{F}=〈-{\displaystyle \underset{V}{\int }\left(\rho \mathbf{vv}\cdot \mathbf{not}+p\mathbf{not}\right)\mathit{dS}}〉,$$

where p denotes the density of the medium, p the pressure, v the particle velocity, n the unit vector normal to an element dS of the surface of volume V, and the square brackets denote the taking of time average.

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

By considering the propagation of an acoustic wave of intensity I and speed c in a direction noted Oz in a dissipative medium with an ultrasonic absorption coefficient noted a, it is common to express the force of radiation by its density f according to the formula: $$\mathbf{f}=2\mathit{\alpha I}{\mathbf{e}}_z/\mathrm{vs}.$$

Furthermore, we consider the propagation of an ultrasonic wave in a first medium 14 to an interface 13 with a medium 11.

Thanks 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 pushing of the interface makes it possible to generate, as seen previously, mechanical waves of high amplitude which propagate in the biological medium 11.

où R est le coefficient de réflexion (en terme d'énergie) de l'interface 13, c₁₄ et c₁₁ sont les célérités ultrasonores dans les milieux 14 et 11, et I est l'énergie du faisceau ultrasonore incident.Created by an incident plane ultrasonic wave 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 ): $$\pi =\frac{I}{{\mathrm{vs}}_{14}}\left(1+R-\left(1-R\right)\frac{{\mathrm{vs}}_{14}}{{\mathrm{vs}}_{11}}\right),$$

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

By considering a particle volume V of height H in the medium 11, particle volume of which one of the boundaries 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 of intensity I propagates in the particle volume V of the medium 14.

la force de radiation volumique créée par absorption peut s'écrire en première approximation F_vol = fAH = 2α ₁₁ I/c ₁₄ AH(1 - R),The volume V is then subjected to a volume force F _vol due to the acoustic absorption in the medium 11, and subjected to a surface force F _surf on the section A due to the contrast between the two media 14 and 11. The surface force F _{surf is} written $F_{\mathit{surf}}=\mathit{\pi A}=\frac{\mathit{IA}}{{\mathrm{vs}}_{14}}\left(1+R-\left(1-R\right)\frac{{\mathrm{vs}}_{14}}{{\mathrm{vs}}_{11}}\right),$

the volume radiation force created by absorption can be written as a first approximation F _vol = fAH = 2α ₁₁ I / c ₁₄ AH (1 - R ),

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

The ratio of the two forces acting on the focal volume zone can then be written: $$F_{\mathit{surf}}/F_{\mathit{flight}}=\left(1+{\gamma }_{\mathrm{vs}}\right)\frac{1+R-\left(1-R\right)/\left(1+{\gamma }_{\mathrm{vs}}\right)}{2{\alpha }_{11}H\left(1-R\right)}.$$

sont faibles, alors le ratio des deux forces s'exprime $F_{\mathit{surf}}/F_{\mathit{vol}}\approx \frac{2R-{\gamma }_c}{2{\alpha }_{11}H}.$

Considering that the contrasts R and $\frac{{\mathrm{vs}}_{11}}{{\mathrm{vs}}_{14}}-1={\gamma }_{\mathrm{vs}}$

are weak, then the ratio of the two forces is expressed $F_{\mathit{surf}}/F_{\mathit{flight}}\approx \frac{2R-{\gamma }_{\mathrm{vs}}}{2{\alpha }_{11}H}.$

et de fréquence centrale 5 MHz, et en considérant l'atténuation typique dans le sein (1dB/MHz/cm), on trouve 2α₁₁H≈0,12 On voit donc qu'il suffit de choisir le matériau d'interface de manière à ce que 2R-γ_c soit de l'ordre de 0,25, de manière à ce que la force surfacique soit d'amplitude deux fois plus grande que la force de volume.The values taken by this ratio depend mainly on the choice of material in which the interface 13 is made. The term 2R-γ _c is indeed a function of this choice of interface material. As for the term 2α ₁₁ H, taking the depth of field of a transducer with an opening number $\frac{F}{D}=1$

and with a central frequency of 5 MHz, and considering the typical attenuation in the breast (1dB / MHz / cm), we find 2α ₁₁ H120.12 We therefore see that it suffices to choose the interface material so that 2R-γ _c is of the order of 0.25, so that the surface force is of amplitude twice greater than the volume force.

For this purpose, in order to increase the speed contrast, one can for example use an elastic membrane. Such a membrane could, for example, be made from latex, polyurethane, silicone, etc. It can be seen that the latex is particularly well suited for the manufacture of a membrane useful in the implementation of the invention.

The transducer 12 is capable of performing a step of ultra fast imaging of the medium 11. Depending on the transducer, the image can be two-dimensional or three-dimensional. It can also be reduced to one dimension (a line of sight) if a simple stationary transducer element is used. This ultra-fast ultrasonic imaging step is coupled with the step of applying ultrasonic waves focused at the membrane 13. The occurrences of these steps are then synchronized as a function of the speed of propagation of the mechanical waves created by application of ultrasonic waves.

In order to obtain a good quality image, it is therefore necessary to take care to limit the reflection coefficient at the level of the interface 13, so as not to harm the ultrasound imaging due to 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 the reflection at the interface. Examples of suitable materials are given below.

As the invention specifically targets elastography, it is necessary to pay particular attention to the generation, by the method according to the invention, of shear waves at the interface 13.

In order to specify the characteristics of the displacement field corresponding to the mechanical waves resulting from a surface excitation, it is necessary to be interested in the theory of the propagation of the elastic waves induced by a stress on the surface of a semi infinite solid .

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

Concerning shear waves, the calculation of the Green's function (according to Gakenheimer and Miklowitz, Transient excitation of a half space by a point load traveling 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 source of shearing.

The figure 2 schematically illustrates the directivity of the shear waves generated by a source zone 26, on which ultrasonic waves are focused, situated 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 ′, the maxima of which are located at 35 ° from the normal at the interface 23 and which illustrate these mechanical shear waves.

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

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

We also know that the compression wave propagates at very high speed and we observe for example that c _L ≈300c _T where c _T is the speed of the shear wave and c _L that of the compression wave. Insofar as the mechanical pulse must be short in order to be able to be imaged, the compression wave will therefore tend to escape from the imaged region very quickly.

It therefore 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 waves of celerities approximately equal to the celerity of the shear waves.

où c_T est la vitesse de l'onde de cisaillement et c_L, celle de l'onde de compression.The head wave ensures the continuity of the constraints and has a zero amplitude at the interface. It propagates on the surface in the form of a compression wave, by yielding part of its energy in volume in the form of a shear wave in a determined direction. This specific angle is given by the formula: $$\theta =\mathrm{at}\sin \left(\frac{{\mathrm{vs}}_T}{{\mathrm{vs}}_L}\right),$$

where c _T is the speed of the shear wave and c _L , that of the compression wave.

However, the values of speed of 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 as soon as we imagine deep, even shallow, in the environment.

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

où c_R est la vitesse est la vitesse de l'onde de surface.The speed of propagation of this surface wave is given with good precision by Viktorov's formula: $$\frac{{\mathrm{vs}}_R}{{\mathrm{vs}}_T}=\frac{0.718-{\left({\mathrm{vs}}_T/{\mathrm{vs}}_L\right)}^2}{0.75-{\left({\mathrm{vs}}_T/{\mathrm{vs}}_L\right)}^2}=\frac{0.718-{\left(5/1500\right)}^2}{0.75-{\left(5/1500\right)}^2}=0.95,$$

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 therefore be seen that it is not really possible to separate the R wave and the shear wave in time. On the other hand, here too, as soon as one images in depth, even a weak one, this wave does not come to be superimposed on the shear waves. In addition, even in the case of a superposition on the shear wave, its presence will only slightly alter the measurement of the speed c _T since c _R ≈c _T.

The figure 3 presents a first embodiment of an artificial membrane according to the invention.

This embodiment is particularly suitable for being combined with a method of focused ultrasound therapy. Indeed, such a therapy method 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 we know that in the presence of such a water pocket, 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 linked to the progression of the treatment. Furthermore, even if it were possible to generate a volume radiation force within the biological medium, the volume radiation pressure which it is possible to generate in the medium, would be greatly reduced due to the loss of ultrasonic energy. at the interface between the water bag and the medium.

The embodiment of the invention presented on the figure 3 precisely overcomes this drawback by allowing mechanical shear waves to be generated in a biological medium 31, despite the presence of the water bag.

The assembly presented on the figure 3 uses an imaging probe 38 carrying ultrasonic 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 therefore becomes possible to easily follow the variation in the elasticity of the area treated using a single imaging probe 38. Such a scanning probe imaging 38 is then programmed not only to carry out the treatment but also to, punctually, trigger a measure of elasticity by carrying out a step of generating mechanical waves and, successively, in a synchronized manner, a step of imaging 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 volume radiation force which mainly depends 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 can be adjusted by the experimenter. The interfacial radiation force depends, in fact, on the ratio of the acoustic impedances, on the ratio of the speeds of sound in the two media or, again, on 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 judicious that the acoustic impedances of the two media 31 and 34 are close, but that the two media 31 and 34 have very different speeds of sound. This makes it possible to obtain a higher radiation pressure while avoiding reflections at the interface 33 which are detrimental to ultrasound imaging.

For this purpose, use will advantageously be made of an elastic membrane filled with either silicone or 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 speeds of sound.

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

In fact, when several sources of shearing vibrating at the surface are adequately arranged, a region is defined where the amplitude of the mechanical wave, more particularly of its axial component, is increased.

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

The Figures 4a and 4b thus describe a particular embodiment for a membrane 44 'surrounding a coupling medium 44, capable of focusing the mechanical waves on an area of interest 66.

The figure 4a is a AA cup and the figure 4b is a partial top view as seen according to section BB.

The area 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 the figure 4 , the thickness of the membrane 44 'is increased over a crown zone 49 represented on the figure 4b , so that the area of interest 66 and the crown 49 form a cone of approximately 35 °.

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

By symmetry about the axis AX of revolution of the crown 49, the axial displacements add up and, by propagation, are of a 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 constituting the membrane aimed at reaching zones of interest 66 of distinct depths Z.

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

Finally, the figure 5 presents a particular embodiment of the invention where 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 ultrasonic waves focused at the interface 53, that is to say at the depth of the interface and in the direction of the latter.

By interface effect, the ultrasonic waves generate a surface radiation force 55 which 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 can be noted that, when the method according to the invention is used, as shown in figure 5 , to characterize a biological medium 54 present in the biological medium 51, one can also deduce from it 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 imagining the whole of the medium, we can then deduce properties on each of the mediums 51 and 54 and on their interface 53, 53 '.

Finally, it is noted that various implementations can be carried out according to the principles of the invention as defined in the following claims.

Claims (10)

1. A method for imaging a viscoelastic medium (11) comprising a step for generating an acoustic radiation force (15) within the viscoelastic medium (11) which generates the propagation of the mechanical waves in the medium, and an ultra-rapid imaging step of the medium (21) coupled with the generation step, the step for generating being carried out by application of ultrasound acoustic waves focussed on an interface (13, 33, 53) delimiting two zones (11, 14) having distinct acoustic properties,
   characterized in that the step for generating an acoustic radiation force creates mechanical shearing and compression waves,
   the imaging step of the medium (21) is an ultrasound imaging step, and
   the occurrences of these step for generating the acoustic radiation force (15) and imaging step of the medium (21) being synchronised as a function of the propagation speeds of the mechanical waves generated in the medium (21) such that the propagation of the mechanical waves (27) generated in the medium (21) is imaged.
2. The method according to Claim 1, characterized in that the interface (53) the acoustic waves on which are focussed is present within the viscoelastic medium (11).
3. The method according to any of Claims 1 or 2, characterized in that the interface (33) on which the acoustic waves are focussed is an artificial membrane (34') placed in contact with the surface of the viscoelastic medium (31) and enclosing a medium known as 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 the two zones of distinct acoustic properties.
4. The method according to Claim 3, characterized in that the artificial membrane (34') has a composition selected to minimise the acoustic impedance contrast while increasing the amplitude of the mechanical waves.
5. The method according to Claim 3, characterized in that the artificial membrane (34') has thickness selected to minimise the acoustic impedance contrast while increasing the amplitude of the mechanical waves.
6. The method according to any of Claims 3 to 5, characterized in that the artificial membrane (34') has a non-uniform composition determined spatially so as to increase the amplitude of the mechanical waves (27) in a region of interest of the viscoelastic medium (31).
7. The method according to any of Claims 3 to 5, characterized in that the artificial membrane (34') has a non-uniform thickness (49) determined spatially so as to increase the amplitude of the mechanical waves (27) in a region of interest (66) of the viscoelastic medium (31).
8. The method according to any of the preceding Claims, characterized in that the application of acoustic waves focussed on the interface (13, 33, 53) is completed successively at a plurality of points of the interface (13, 33, 53), this plurality of points and the succession of the focussings being determined so as to increase the amplitude of the mechanical waves (27) in a region of interest of the viscoelastic medium (31).
9. The method according to any of the preceding Claims, characterized in that the ultrasound treatment method is suitable for being controlled as a function of the results of the imaging step of the medium.
10. A system comprising an imaging probe (38) bearing transducers (32) for generating mechanical waves within a viscoelastic medium (31) which is programmed to apply the method according to any of claims 3 to 7 wherein the occurrences of the step for generating an acoustic radiation force (15) and ultra-rapid ultrasound imaging step of the medium (21) are synchronised as a function of the propagation speeds of the mechanical waves generated in the medium (31) and comprising an artificial membrane (34') intended to be placed partially in contact with the surface of a viscoelastic medium (31) and intended to enclose a medium known as coupling medium (34) placed between the probe (38) for generation of acoustic waves and the viscoelastic medium (31) to serve as interface (33).

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