Classifications

• • G—PHYSICS
  • G10—MUSICAL INSTRUMENTS; ACOUSTICS
  • G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
  • G10K11/00—Methods or devices for transmitting, conducting or directing sound in general; Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
  • G10K11/18—Methods or devices for transmitting, conducting or directing sound
  • G10K11/26—Sound-focusing or directing, e.g. scanning
  • G10K11/30—Sound-focusing or directing, e.g. scanning using refraction, e.g. acoustic lenses

Definitions

• the present invention relates to the general field of medical imaging.
• 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.
• a mechanical stimulus is generated and causes tissue displacement.
• the measurement of the spatiotemporal response is advantageously performed by means of an imaging modality, for example by ultrasound or by magnetic resonance, etc.
• mechanical excitation 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.
• 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.
• 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 ⁇ m.
• 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.
• 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.
• the amplitudes of the induced displacements are higher than with a simple ultrasonic stressing by focussing within a tissue.
• 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.
• the amplitude of the displacements generated is of the order of 100 ⁇ m.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• the artificial membrane has a composition chosen to minimize the acoustic impedance contrast while increasing the amplitude of the mechanical waves.
• the artificial membrane has a thickness chosen to minimize the acoustic impedance contrast while increasing the amplitude of the mechanical waves.
• an artificial membrane for example the membrane of a water bag
• 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.
• 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.
• 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.
• the method is coupled with an ultrasonic treatment method to monitor the effect of the treatment.
• 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.
• 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.
• FIG. 1 schematically illustrates the generation of mechanical waves in a medium 11 using a method according to the invention.
• the method is applied using a transducer 12 applying focussed acoustic waves at an interface 13.
• 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.
• the interface 13 is made using an artificial membrane surrounding an artificial medium 14.
• 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:
• 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.
• 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
• I is the energy of the incident ultrasonic beam.
• volume V is then subjected to a volume force F VO ⁇ 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
• an elastic membrane for this purpose, in order to increase the contrast of celerity, it is possible for example to use an elastic membrane.
• 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.
• the transducer 12 is capable of performing an ultra-fast imaging step of the medium 11.
• 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.
• 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.
• 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.
• 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.
• 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.
• the compression wave propagates at very high speed and it is observed for example that c L ⁇ 300c ⁇ where c ⁇ 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.
• 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:
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• FIG. 4 illustrates a second embodiment of an artificial membrane according to the invention.
• 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 .
• a region is defined in which the amplitude of the mechanical wave, more particularly of its axial component, is increased.
• 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.
• 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 °.
• 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.
• 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.
• 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.
• transducers 52 are used to apply focused ultrasound waves at interface 53, i.e. at the depth of the interface and towards it.
• 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.

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• Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• Acoustics & Sound (AREA)
• Multimedia (AREA)
• Ultra Sonic Daignosis Equipment (AREA)
• Surgical Instruments (AREA)
• General Electrical Machinery Utilizing Piezoelectricity, Electrostriction Or Magnetostriction (AREA)

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• 2006
  • 2006-10-25 FR FR0654502A patent/FR2907692B1/fr active Active
• 2007
  • 2007-10-25 CN CN2007800396492A patent/CN101589426B/zh active Active
  • 2007-10-25 EP EP07866491.9A patent/EP2084702B1/de active Active
  • 2007-10-25 CA CA2667527A patent/CA2667527C/en not_active Expired - Fee Related
  • 2007-10-25 US US12/092,406 patent/US8037766B2/en active Active
  • 2007-10-25 WO PCT/FR2007/052247 patent/WO2008050072A2/fr not_active Ceased

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