WO2020016192A1

WO2020016192A1 - Device for generating mechanical shear waves inside a soft material, shear wave imaging apparatus for the same

Info

Publication number: WO2020016192A1
Application number: PCT/EP2019/069042
Authority: WO
Inventors: Zhishen SUN; Stefan Catheline
Original assignee: Institut National De La Sante Et De La Recherche Medicale (Inserm); Université Claude Bernard Lyon 1; Centre Léon-Bérard
Priority date: 2018-07-16
Filing date: 2019-07-15
Publication date: 2020-01-23

Abstract

The present invention relates to the field of acoustic imaging of soft materials, such as shear wave elastography. A method for generating shear waves in a soft material includes placing at least one patch including a metallic material along and in contact with an external boundary of a soft material, and applying a variable magnetic field on the patch using an electromagnet, in order to generate eddy currents circulating inside each metal strip and a Lorentz force imparting an oscillation movement of the patch.

Description

DEVICE FOR GENERATING MECHANICAL SHEAR WAVES INSIDE A SOFT MATERIAL, SHEAR WAVE IMAGING APPARATUS FOR THE SAME

TECHNICAL FIELD

The present invention relates to a method and a device for generating mechanical shear waves inside a soft material. The invention also relates to a shear wave imaging method and an apparatus for the same.

BACKGROUND

Shear wave elastography is a non-invasive acoustic imaging technique used to map the elasticity and stiffness of a soft material, such as a biological tissue, for example in order to detect local variations of elasticity properties caused by tumors or by diseased organs. Elastography is based on the emission of mechanical shear waves, such as ultrasound waves, into the soft material. The emitted waves are detected using a measurement probe such as a transducer after propagation through the soft material.

A drawback of elastography is that the required mechanical shear waves are often complicated to create inside the soft material. Usually, shear waves must be generated by an apparatus including one or several dedicated electro-mechanical transducers which must be placed in direct contact with the soft material. Also, in some cases, the generated shear waves are too weak for use in imaging applications, especially for clinical applications.

For example, US-6, 770, 033-B discloses an apparatus including a computer- controlled transducer such as a loudspeaker adapted to apply a low-frequency pulse on the surface of a soft material. This apparatus is bulky and cumbersome to use. A lot of care is required to place the transducer properly on the soft material to be imaged and to keep it properly placed at the same location during the whole imaging session.

SUMMARY

The object of the present invention is therefore to provide a simpler method for generating mechanical shear waves inside a soft material.

To that end, the invention relates to a method for generating mechanical shear waves inside a soft material, wherein the method comprises steps of:

a) placing a patch along and in contact with an external boundary of a soft material, said patch including a metallic material, b) applying a variable magnetic field on the patch using an electromagnet, in order to generate eddy currents circulating inside each metal strip, the applied magnetic field interacting with the generated eddy currents to generate a Lorentz force imparting an oscillation movement of the metal strip.

The application of the magnetic field on the patch causes each metal strip to oscillate, causing in turn the boundary of the soft material to vibrate. This creates mechanical shear waves inside the soft material.

Thanks to the invention, generating shear waves is simpler than using mechanical transducers that must be placed in contact with the soft material, since only the patch needs to be in contact with the soft material.

According to advantageous aspects, the invention comprises one or more of the following features, considered alone or according to all possible technical combinations:

-The variable magnetic field is a pulsed magnetic field, the electromagnet being powered by a pulse generator.

-The magnetic field pulses have a duration comprised between 0.05ms and 1 s.

-The amplitude of the magnetic field is higher than or equal to 100mT.

-The changing rate of the magnetic field is higher than or equal to 100T/s.

-The patch includes a sheet of said metallic material, said sheet preferably having a thickness lower than or equal to 5mm.

-During step a) several patches are placed on the same soft material next to each other and during step b) the magnetic field is applied simultaneously to said patches.

-The metallic material of the patch includes a ferromagnetic material.

-The patch has the shape of a strip.

-The patch has the shape of a flat ring.

-The area of the surface of the patch in contact with the external boundary is equal to or lower than 50cm².

-The angle between the main direction of the magnetic field and the main face of the patch is equal to 45°.

According to another aspect, the invention also relates to a shear wave imaging method, including steps of:

c) generating at least one shear wave inside a target soft material,

d) detecting a propagation pattern of the at least one generated shear wave in the target soft material, step c) being performed according to the invention.

According to another aspect, a diagnostic method, comprises steps of:

y) imaging an anatomical region of interest of a patient using a shear wave imaging method as described above, and z) identifying a clinical condition of said patient from the acquired images of said anatomical region of interest.

According to yet another aspect, the invention also relates to a device for generating mechanical shear waves inside a soft solid, said device comprising at least one patch including a metallic material and adapted to be placed along and in contact with an external boundary of a soft material and an electromagnet powered by a generator, wherein the electromagnet being configured to apply a variable magnetic field on the metal patches, when the patch is placed along and in contact the external boundary of the soft material, in order to generate eddy currents circulating inside the patch, the applied magnetic field interacting with the generated eddy currents to generate a Lorentz force imparting an oscillation movement of the patch.

According to still another aspect, the invention also relates to an apparatus for shear wave imaging, including a first device for generating at least one shear wave in a target soft material and a second device for detecting a propagation pattern of the at least one shear wave inside the soft material, the first device being according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood upon reading the following description, provided solely as an example, and made in reference to the appended drawings, in which:

-Fig. 1 is a simplified diagram of a shear wave imaging apparatus including a shear wave emission device according to the invention;

-Fig. 2 is a simplified representation of a portion of the apparatus of Fig. 1 ;

-Fig. 3 is a flow-chart illustrating a method for generating shear waves in a soft material according to the invention;

-Fig. 4 is a flow-chart illustrating a shear wave imaging method according to the invention;

-Fig. 5 is an image representing the value of the shear wave velocity across an imaging slice of an example soft material sample, obtained experimentally using the method of Fig. 4.

DETAILED DESCRIPTION OF SOME EMBODIMENTS

Figure 1 illustrates a soft material 2 comprising an external boundary 4. For example, the soft material 2 is a biological tissue.

The soft material 2 may correspond to a human body or animal body. In that case, the external boundary 4 corresponds to a skin associated to said body. The soft material 2 may be an internal or external organ being part of said body or being taken alone independently from said body.

In some other embodiments, the soft material 2 may be a vegetal body, or be something other than a biological tissue, such as a synthetic soft material or a food item.

The exemplary soft material 2 of Figure 1 is depicted having a cuboid shape with essentially flat faces for exemplary purposes only and in practice many other shapes are possible.

A wave generation device 6 is adapted to generate shear mechanical waves inside the soft material 2. In other words, the soft material 2 acts as a target material in which shear mechanical waves are generated.

In preferred embodiments, the mechanical shear waves are ultrasound waves. However, in some alternative embodiments, the shear waves may have a different frequency, for example a lower frequency corresponding to infrasound waves.

In the illustrated example, the wave generation device 6 is part of a shear wave imaging apparatus 8. The imaging apparatus 8 includes, in addition to the wave generation device 6, a detection device 10 adapted to detect the propagation of a shear wave inside the soft material 2. However, in many other embodiments, the wave generation device 6 is used independently from the apparatus 8. In that case, the detection device 10 may be omitted.

In some embodiments, the imaging apparatus 8 is meant to implement an elastography imaging method using shear waves, for example to detect an inhomogeneity of elasticity of the soft material 2, i.e. a local variation of elasticity and/or stiffness inside the soft material 2. In other embodiments, the imaging apparatus 8 may be used for non medical imaging methods, such as non-destructive industrial testing.

The wave generation device 6 comprises an electromagnet 20, or solenoid, and a generator 22 for powering the electromagnet 20.

The electromagnet 20 includes one or several coils adapted to generate a magnetic field, preferably a variable magnetic field, i.e. a magnetic field that varies with time. The generated magnetic field (or magnetic induction) bears the reference“B” in what follows. The generator 22 is connected to the coil(s) of the electromagnet 20 using one or several cables.

In some embodiments, the magnetic field amplitude is higher than or equal to 100 milliTesla (mT). For example, the changing rate of the magnetic field is comprised between 10³ and 10⁶ Tesla per second (T/s) and preferably comprised between 10⁴ T/s and 10⁵ T/s. According to preferred embodiments, the magnetic field B is a pulsed magnetic field. In that case, the generator 22 includes a pulse generator. Preferably, each of the magnetic field pulses has a duration comprised between 0.05 milliseconds (ms) and 1 second (s), or preferably comprised between 0.1 ms and 0.5s.

As a purely illustrative and non-limiting example, the generator 22 is the high- intensity current pulse generator sold by the company MAGSTIM, under the commercial reference “Magstim 200²” and the electromagnet 20 is the excitation coil sold by MAGSTIM under the commercial reference“D70²”. Other embodiments are possible.

The wave generating device 6 further comprises at least one patch 24 including a metallic material, for example including a sheet or layer of a metallic material.

For example, each patch includes one or several joined superimposed layers, at least one of which is a metal layer.

In some embodiments, the wave generation device 6 includes several patches 24. Preferably, the patches 24 are essentially similar or identical to each other, although in some case the patches 24 may have differing shapes and/or dimensions.

In the illustrated embodiment of Fig. 1 , three patches 24 are depicted for exemplary purposes, although it is understood that any number of patches 24 can be used in practice. In what follows, wherever only one patch 24 is described in detail, the corresponding description can nonetheless be transposed to embodiments where several patches 24 are used, unless specified otherwise.

Each patch 24 is adapted to be placed along and in contact with the external boundary 4 of the soft material 2.

For example, patches 24 are pasted onto the external boundary 4 using an adhesive substance or are simply laid onto the external boundary 4.

Each patch 24 has a main face which is in direct contact with the external boundary 4 of the soft material 2. For example, each metal sheet 24 has a planar shape. Each patch 24 has two opposed main faces, one of which is in contact with the external boundary 4, either directly or indirectly.

The area of the patch 24 which is in contact with the external boundary is equal to or lower than 50cm² or preferably lower than 10cm². Preferably, this area is higher than or equal to 5mm².

In the illustrated example, the patch 24 has the shape of a strip, such as an elongated and/or essentially rectangular strip. For example, the metal sheets of the patches 24 are metal strips or metal foils. According to some other embodiments, patches 24 may have the shape of a flat ring, such as a circular ring or a rectangular ring or any ring-shaped arrangement in which a flat strip is closed on itself so as to surround a central hole.

Preferably, the patches 24 are flexible. For example, the patches 24 are foils of metal.

As an illustrative example, the thickness of the metal sheet of the patches 24 is lower than or equal to 5mm, preferably lower than or equal to 1 mm.

In some cases, the patch 24 may include a support layer attached to the metal sheet, such as a non-metallic support layer. Preferably, the support layer is also flexible.

According to some embodiments, the metallic material of the patch 24 is made of a non-ferromagnetic metal, such as aluminum or copper or a non-ferromagnetic metal alloy.

However, in some embodiments, said metallic material may include of a ferromagnetic material, such as iron or cobalt or any suitable metal or composition or alloy.

Preferably, when several patches 24 are used, they are placed next to each other on a same face or a same region of the external boundary 4.

The electromagnet 20 is arranged so as to apply the magnetic field B onto the patches 24. In other words, if several patches 24 are placed on the same soft material 2 next to each other, then the magnetic field B is applied simultaneously onto all said metal sheets 24.

The direction of the magnetic field B relative to the patches 24 may depend on the coil properties of the electromagnet 20. For example, the main direction of the magnetic field B is perpendicular to the main face of the patches 24. In another example, the angle between the main direction of the magnetic field B and the main face of the metal sheet is equal to 45°.

When the magnetic field B is applied to the patch 24, eddy currents are generated in the patch 24 (i.e. in the metallic material of the patch 24) and circulate in the patch 24, here along a geometrical plane essentially parallel to the main plane of the patch 24.

The applied magnetic field B further interacts with said eddy currents to generate a Lorentz force, which imparts an oscillation movement of the patch 24, here along a direction essentially perpendicular to the main plane of the patch 24.

The detection device 10 also comprises a measurement probe 26 and an acquisition unit 28.

The measurement probe 26 is adapted to detect a propagation pattern of the shear wave(s) generated inside the soft material. For example, the measurement probe 26 includes a transducer, such as an ultrasonic transducer. Preferably, the measurement probe 26 is aligned with the patch 24 along a propagation direction of the generated shear waves.

The acquisition unit 28 is adapted to collect a measurement signal generated by the measurement probe 26 in response to the detection of the wave propagation pattern. For example, the acquisition unit 28 includes an analog-to-digital converter connected to the measurement probe 26.

In the illustrated example, the apparatus 8 further comprises an electronic control unit 30, such as a computer including a central processing unit and a memory storing executable instructions for automatically controlling the wave generation method and the imaging method described above.

The control unit 30 is programmed to process the signal collected by the acquisition unit 28, for example in order to generate a reconstructed digital image of the shear wave propagation velocity inside an imaging region (or imaging slice) of the soft material 2.

For example, the control unit 30 is programmed to implement an image reconstruction method, such as a velocity-reconstruction algorithm, for building one or several digital images from the collected measurement signals.

Advantageously, the control unit 30 is also adapted to control the generator 22 and/or to synchronize operation of the wave generation device 6 with the imaging device 10.

Figure 2 illustrates an exemplary setup of a wave generation system 6 in a cutout view along a geometrical plane extending along two fixed axes X and Z which are part of a reference system including axes X, Y and Z perpendicular to each other.

In the illustrated example, the measurement probe 26 is aligned with the patch 24 along the Z-axis. The measurement probe 26 and the patch 24 are placed on opposite faces of the soft material 2. The magnetic field B is applied along the Z axis. The main face of the patch 24 extends along a geometrical plane parallel to axes X and Y.

Reference“32” denotes an imaging slice of the soft material 2, corresponding to the portion of the soft material 2 which can be imaged using the measurement probe 26. In practice, the shape of the imaging slice 32 is at least in part defined by the arrangement of the sensing elements of the measurement probe 26. In this example, the measurement probe 26 includes a 1 -dimensional array of sensors and thus the imaging slice 32 is a 2- dimension slice of the soft material 2.

Reference “34” denotes lines of force (or field lines) of the magnetic field B generated by the solenoid 20. In practice, the direction of the magnetic field B depends on the properties of the solenoid 20. For example the direction and intensity of the magnetic field B is given by Ampere’s circuital law, where Ji is the current density inside the coil of the solenoid 20:

V X b = mo·/i

In this example, the axis of the solenoid 20 is aligned along the Z axis, i.e. perpendicular to the main face of the patch 24. The excitation electrical current circulates counter-clockwise inside the solenoid 20.

Eddy currents generated inside the patch 24 follow Faraday’s law of induction and dA dA

are defined as follows: B = V x A, E = -- d—t V0 and ] = sE =—s—

dt where J is the eddy ^J current density.

In this example, the eddy currents circulate in loops inside the X-Y plane of the metal layers of the patch 24. The Lorenz force resulting from the interaction of the applied field B with the eddy currents is aligned along the Z axis.

Therefore, the Lorenz force causes the patch 24 to oscillate along the Z axis. As a result, shear waves W propagate inside the soft material 2 from the emission region corresponding to the main face of the patch 24 which is in contact with the external boundary 4.

An exemplary method for generating shear waves W inside soft material 2 is now described in reference to the flow-chart of Figure 3.

First, during a step 100, at least one patch 24 is placed along and in contact with the external boundary 4 of the soft material.

Optionally, several patches 24 may be placed on the external boundary 4, preferably on a same face or emission region of the external boundary 4.

Then, the electromagnet 20 is placed relative to the patch 24 (or relative to the patches 24) as described above.

During a step 102, the variable magnetic field B is applied on the installed patch. As a result, eddy currents are created inside the metallic material of the patch 24. The interaction between the applied magnetic field B and the eddy currents gives rise to a Lorentz force, which imparts an oscillation movement of the patch 24.

As the patch 24 is attached to the external boundary 4, the oscillation of the patch 24 forces the external boundary 4 to oscillate. The oscillation movement of the external boundary 4 propagates inside the soft solid 2, causing the generation of shear waves W which then propagate inside and across the soft material 2.The solenoid 20 can be kept at a distance from the soft material 2. Thanks to the invention, generating shear waves is simpler than in known methods in which electro-mechanical transducers have to be placed in contact with the soft material. Instead, only the patch 24 is in contact with the soft material 2.

The shear waves W are generated without requiring any contact between the soft material 2 and the electromagnet 20 and without requiring any cable between the generator 22 and the soft solid 2.The wave generation device 6 is therefore less bulky and less cumbersome to use.

Furthermore, the amplitude of the generated shear waves is sufficiently strong for use in imaging methods such as elastography.

If a ferromagnetic metal is used inside the patch 24, then the applied magnetic field B also interacts with said ferromagnetic material to generate an additional magneto motive force which increases the oscillation movement amplitude of the patch 24. Therefore, shear waves W can be generated with still greater amplitude.

If several patches 24 are used, each patch 24 behaves as a shear wave source when the magnetic field B is applied. Therefore, the emission pattern of the shear waves can be personalized by placing patches 24 at different locations, e.g. to create interferences between shear waves emitted by different neighboring sources or to create a specific predefined emission pattern. This is simpler to implement than using multiple dedicated transducers.

An exemplary method for imaging the soft material 2 using the apparatus 8 is now described in reference to Figure 4.

First, during a step 1 10, one or several shear waves W are generated inside the soft material 2, for example using the wave generation device 6 according to the previously- described steps 100 and 102.

Then, during a step 1 12, the propagation pattern of the generated shear wave(s) is detected using the imaging device 10. For example, the measurement probe 26 measures said propagation pattern and, in response, generates a measurement signal that is collected by the acquisition unit 28.

The collected measurement signal may then be forwarded towards the control unit 30 and a digital image of the shear wave propagation velocity may be generated using an image reconstruction method implemented by the control unit 30.

According to a non-limiting illustrative example, one or more embodiments of the imaging method can be used to implement a diagnostic method, comprising steps of imaging an anatomical region of interest of a patient using the shear wave imaging method, and identifying a clinical condition of said patient from the acquired images of said anatomical region of interest. Turning now to Figure 5, experimental data obtained using the above method is illustrated. The image 50 represents the value of the propagation velocity of shear waves moving across an imaging slice 32 in a sample of soft material 2. The shear waves are generated using the device 6 and using the method of Fig. 3. The image 50 is obtained using the device 10 and the method of Figure 4.

In this example, the sample of soft material 2is made of a phantom gel, for example a mixture of agar gel, graphite powder and water. More precisely, the sample of soft material 2 includes a main portion 52 and a cylindrical inclusion 54 inserted inside the main portion.

The main portion 52 and the inclusion 54 have different elasticity properties. Preferably, the inclusion 54 is stiffer than the main portion 52. For example, the main portion 52and the inclusion 54are both made of phantom gel albeit with different compositions.

In the example, the imaging slice 32 has a width (measured along the X axis) equal to 40mm and a depth equal to 100mm. The depth is measured along the Z axis from the external boundary 4.

The propagation velocity is comprised between 0 and 10 meters per second (m/s) and is represented by a pixel color and/or intensity level, as defined by the scale 56.

The image 50 was acquired using the apparatus 8, in which the at least one metal sheet 24 is placed against the external boundary 4 (i.e. at a depth equal to 0mm) and the measurement probe 26 placed on the opposite external boundary (i.e. at a depth equal to 100mm).

In the image 50, the inclusion 54 can be distinguished from the background corresponding to the main portion 52, due to the difference of pixel intensity level due to the different wave propagation speed values inside the main portion 52 and inside the inclusion 54. In this example, the inclusion 54 appears lighter than the surrounding main portion 52.

The embodiments and alternatives described above may be combined with each other in order to generate new embodiments of the invention.

Claims

1.-A method for generating mechanical shear waves (W) inside a soft material (2), wherein the method comprises steps of:

a) placing (100) a patch (24) along and in contact with an external boundary (4) of a soft material (2), said patch (24) including a metallic material,

b) applying (102) a variable magnetic field (B) on the patch (24) using an electromagnet (20), in order to generate eddy currents circulating inside the patch (24), the applied magnetic field interacting with the generated eddy currents to generate a Lorentz force imparting an oscillation movement of the patch (24).

2. -The method of claim 1 , wherein the variable magnetic field(B) is a pulsed magnetic field, the electromagnet being powered by a pulse generator (22).

3. -The method of claim 2, wherein the magnetic field pulses have a duration comprised between 0.05ms and 1 s.

4.-The method according to any of the previous claims, wherein the amplitude of the magnetic field (B) is higher than or equal to 100mT and wherein the changing rate of the magnetic field (B) is higher than or equal to 100T/s.

5.-The method according to any of the previous claims, wherein the patch(24) includes a sheet of said metallic material, said sheet preferably having a thickness lower than or equal to 5mm.

6.-The method according to any of the previous claims, wherein during step a) several said patches (24) are placed on the same soft material (2) next to each other and wherein during step b) the magnetic field (B) is applied simultaneously to said patches (24).

7. -The method according to any of the previous claims, wherein the metallic material of the patch (24) includes a ferromagnetic material.

8.-The method according to any of the previous claims, wherein the patch (24) has the shape of a strip.

9. -The method according to any of the claims 1 to 7, wherein the patch (24) has the shape of a flat ring.

10. -The method according to any of the previous claims, wherein the area of the face of the patch (24)in contact with the external boundary (4) is equal to or lower than 50cm².

1 1 .-The method according to any of the previous claims, wherein the angle between the main direction of the magnetic field (B) and the main face of the patch(24) is equal to 45°.

12. -A shear wave imaging method, including steps of:

c) generating (1 10) at least one shear wave (W) inside a target soft material (2), d) detecting (1 12) a propagation pattern of the at least one generated shear wave(W) in the target soft material (2),

wherein step c) is performed according to any one of the previous claims.

13. -A diagnostic method, comprising steps of:

y) imaging an anatomical region of interest of a patient using the shear wave imaging method of claim 12, and

z) identifying a clinical condition of said patient from the acquired images of said anatomical region of interest.

14.-A device (6) for generating mechanical shear waves inside a soft solid, wherein said device comprises at least one patch (24) including a metallic material and adapted to be placed along and in contact with an external boundary (4) of a soft material(2) and an electromagnet (20) powered by a generator (22),

and wherein the electromagnet (20) is configured to apply a variable magnetic field on the metal strips, when the patch (24) is placed along and in contact the external boundary (4) of the soft material (2), in order to generate eddy currents circulating inside the patch, the applied magnetic field interacting with the generated eddy currents to generate a Lorentz force imparting an oscillation movement of the patch.

15. -An apparatus (8) for shear wave imaging, including a first device (6) for generating at least one shear wave (W) in a target soft material (2) and a second device (10) for detecting a propagation pattern of the at least one shear wave (W) inside the soft material (2), wherein the first device (6) is according to the device of claim 13.