Patent Application
20240011947
The present invention relates to the general technical field of imaging a target object, or a diffuse medium such as a human or animal biological tissue.
More specifically, the present invention relates to a device and a method for measuring viscoelastic properties of a biological tissue of interest.
It applies in particular, but not exclusively, to the measurement of viscoelasticity parameters of the liver of a human or an animal, this measurement being correlated with the amount of fibrosis present in the liver.
In order to measure the tissue viscoelastic properties, it is known to use shear wave elastography.
This technique consists in measuring the speed of propagation of a shear wave in a tissue, this speed being directly related to the viscoelastic properties of the analyzed tissue.
The shear wave can be generated:
To estimate the viscoelastic properties of a tissue, an ultrasonic pulse elastography medical apparatus, called Fibroscan®, has already been proposed.
With reference to
The transducer 11 is attached to the end of the electrodynamic actuator 12. The electrodynamic actuator 12 allows to vibrate the transducer 11 to generate a shear wave. The principle of operation of the medical pulse elastography apparatus 1 is as follows.
The electrodynamic actuator 12 is activated to induce the movement of the transducer 11 and generate a low-frequency shear wave in the tissue to be analyzed. During the propagation of the low-frequency shear wave, the transducer 11 emits and receives high-frequency ultrasonic waves in order to allow the study of the propagation of the low-frequency shear wave.
The mode of generation of the shear wave proposed above is relatively effective in the transmission of mechanical energy to the tissue, since the ultrasonic transducer 11, in contact with the tissue, is directly moved by the electrodynamic actuator 12 held by the user.
However, a disadvantage of this technology is that it is unsuitable for two-dimensional (2D) mapping of a tissue. Indeed, the 2D mapping of a tissue requires the use of an ultrasonic transducer array. However, it is difficult (if not impossible) to attach such a transducer array to the end of an electrodynamic actuator due to:
It has also been proposed (in the document entitled “Shear Modulus Imaging with 2-D Transient Elastography” by Sandrin published in 2002) to separate the mechanical excitation from the ultrasonic probe. In particular,
In particular, the mechanical excitation means of the system include two bars 24 the movements of which are controlled by two magnetic electro-vibrators 22. The ultrasonic wave generation means in turn include a set of transduction elements 23 (composed of 128 elements) allowing to image the tissue to study the propagation of the elastic shear wave generated by the mechanical excitation means. The set of transducer elements 23 is disposed between the two bars 24.
This system allows the production of a 2D mapping during the estimation of the viscoelastic properties of a tissue. However, its design has many disadvantages. In particular, the positioning of the bars 24 on either side of the set of transduction elements 23:
More recently, document EP 3 315 074 has proposed a probe for transient elastography comprising:
The position sensor is arranged to measure the movement of the probe. The vibrator is able to vibrate in a frequency range comprised between 1 Hz and 5 kHz. It is made up of a fixed part and a mobile part whose mass is greater than or equal to 25% of the total mass of the probe. The mobile part is able to move in translation along a guide rod. This vibrator is arranged to induce a movement of the housing along the axis of symmetry A of the ultrasonic transducer. The feedback circuit uses the movement of the probe to control the movement of the vibrator(s) inside the housing and the shape of a low-frequency pulse applied by the probe.
A disadvantage of the probe described in EP 3 315 074 relates to its large size. Another disadvantage of the probe according to EP 3 315 074 relates to the high electrical energy consumption of the vibrator, which makes this probe unsuitable for a battery power supply. Finally, the damping of the vibrations generated by the vibrator(s) can vary depending on the orientation of the probe.
To overcome the disadvantages of the aforementioned apparatus 1 and system 2, it has been proposed to generate the shear wave using acoustic means rather than mechanical means.
Different solutions based on ultrasonic radiation pressure (which consists of a volumetric force generated in the medium during the propagation of a compression wave by momentum transfer with the medium) have thus been developed over the past twenty years.
These solutions are based on the following physical principle. Focusing a high-intensity ultrasonic beam gives rise to non-linear effects resulting in a force acting on the medium at the focus point. If the ultrasound energy is delivered over a short instant (ms fraction), it results in a transient point stress which generates a shear wave with partial spherical symmetry around the focus point.
In these different solutions:
This transducer array is then able to:
However, a disadvantage of this technique relates to the fact that the amplitude of the generated shear wave is low, and propagates spherically quickly losing the amplitude necessary for tissue movement by limiting the way in which it penetrates deep into the tissue, because it is a second-order effect.
To overcome this disadvantage, it has been proposed to emit ultrasound being focused successively at different depths to create pushes by radiation pressure. The constructive shear wave interference thus produced forms a supersonic “Mach cone” (in which the speed of the source is greater than that of the generated wave) and a conical shear wave is created. The transducer array then switches to an ultrafast imaging mode (where high-frequency ultrasonic waves are generated) to follow the shear wave as it propagates through the medium.
Although this technique allows to obtain a conical shear wave by using the ultrasound imaging transducer itself, it has the disadvantage of being costly in terms of energy.
In summary:
An object of the present invention is to propose an ultrasound imaging system allowing to overcome at least one of the aforementioned disadvantages.
More specifically, an object of the present invention is to provide a 2D pulse elastography system allowing, by minimizing the energy required to produce a shear wave, to measure the viscoelastic properties of a tissue.
To this end, the invention proposes a probe for measuring the viscoelastic properties of a medium, for example a human or animal biological tissue, such as a liver, said measurement consisting of:
Thus, the mass (of the mobile part of the exciter) and the stiffness coefficient (of the return spring(s)) are selected within ranges corresponding to the family of exciters that can be used at their respective resonance frequency to emit an elastic wave in a frequency range of interest.
This mass and this stiffness coefficient are further determined so that the resonance frequency of the exciter is equal to the frequency of the elastic wave. More specifically, if it is desired that the probe emits an elastic wave of frequency “F”, then the mass “m” of the mobile part of the exciter and the stiffness coefficient “k” of the spring(s) are selected so that
In practice, to size the probe correctly, knowing the desired frequency “F” for the elastic wave, the stiffness coefficient “k” (respectively the mass “m”) is fixed in the range comprised between 300 kg·s2 and 50 000 kg·s2 (respectively between 5 and 25% of the total weight of the probe), and the mass “m” (respectively the stiffness coefficient “k”) is calculated so as to satisfy the equation
In the context of the present invention, the term “free movement of the mobile part of the exciter” is understood to mean the fact that the mobile part moves without undergoing any stress exerted by a force external to the exciter, such as a force exerted by the user while gripping the probe. In particular, the only stresses undergone by the mobile part during its movement are gravity and the force(s) exerted by the exciter, namely a mechanical return force (exerted by a return spring) and an electromagnetic force in the case of an electromagnet exciter.
As will emerge from the following description, the fixed part mechanically integral with the transducer array can be attached:
In all cases, the mechanical securing of the fixed part to the transducer array induces the absence of relative movement of these two elements relative to each other.
Preferred but non-limiting aspects of the probe according to the invention are the following:
Other advantages and features of the probe according to the invention will emerge better from the description which will follow of several variant embodiments, given by way of non-limiting examples, from the appended drawings in which:
Different embodiments of the probe according to the invention will now be described in more detail with reference to the figures. In these various figures, the equivalent elements are designated by the same reference numeral.
With reference to
The probe 3 comprises:
The exciter 34 allows to generate vibrations. As will be described in more detail below, the fixed part of the exciter 34 is mechanically integral with the transducer array 33 to transmit the vibrations to the probe 3 in order to mechanically produce the shear wave necessary for the measurement of the viscoelastic properties of the tissue to be analyzed.
The use of an inertial vibration exciter 34 allows to obtain a probe in which the mass of the mobile part 342 generating the vibrations represents only 5 to 25% of the total mass of the probe, unlike the probe according to EP 3 315 074 as well as the apparatus and system illustrated in
Moreover, the reader will appreciate that, as illustrated in
The use of an inertial vibration exciter 34 further allows to generate a shear wave of sufficient power to measure the tissue viscoelastic properties while consuming less energy than that necessary for solutions based on ultrasonic radiation pressure.
The housing 31 of the probe 3 accommodates the probe acquisition means as well as the shear wave generation means. More specifically, the optional electronic card 32, the transducer array 33 and the inertial vibration exciter 34 are housed in the housing 31.
An advantage of such a probe is that all the mobile parts of the probe (in particular the mobile part of the inertial vibration exciter 34) are internal to the housing 31 and therefore protected. This facilitates the disinfection of the probe 3. Seen from the outside, the probe is non-deformable, which solves many problems of usability and facilitates its gripping by the user.
The probe also comprises (wired or wireless) communication means 35 for sending the acquired data (optionally preprocessed and beamformed) to a calculation unit (remote computer and/or tablet and/or smartphone, etc.) for the reconstruction of elementary images of a target object and/or the estimation and/or the display of the elasticity of the target object.
The transducer array 33 comprises a set of “n” ultrasonic transducers (“n” being an integer greater than or equal to one) disposed linearly, or in a curve, or in concentric circles, or in a matrix.
The transducer array 33 allows to emit ultrasonic excitation waves towards a medium to be explored (organ, biological tissue, etc.), and to receive acoustic echoes (that is to say ultrasonic waves reflected by the various interfaces of the medium to be explored). Each transducer consists for example of a plate of piezoelectric material of rectangular shape coated on its front and rear faces with electrodes. Such transducers are known to the person skilled in the art and will not be described in more detail below.
In the variant embodiment illustrated in
The optional electronic card 32 is connected to the transducer array 33. It allows to control the transducers of the array, and to process the data acquired by the transducers of the array. More specifically, the acquisition electronic card 32 allows:
The electronic card can also comprise a controller to drive the inertial vibration exciter 34, as will be described in more detail below.
The inertial vibration exciter 34 allows to vibrate the probe 3 to induce the generation of a shear wave.
More specifically, the movement of the mobile part 342—or “inertial mass”—of the exciter 34 in the direction opposite to that of the desired movement moves the transducer array 33 in the other direction, and produces the desired shear wave.
Advantageously in the context of the present invention, it is the fixed part 341 of the exciter 34 which is (“directly” or “indirectly”) mechanically integral with the transducer array 33, unlike the solutions of the prior art. Indeed:
This particular arrangement (use of an inertial vibration exciter 34 whose fixed part 341 is connected to the transducer array 33 to make it vibrate) allows to generate a shear wave of acceptable strength with:
In other words, the proposed solution has the advantages of the two technologies of the prior art without their respective disadvantages, namely:
In the embodiment illustrated in
These mechanically integral elements (that is to say housing 31, electronic card 32, transducer array 33 and fixed part 341 of the exciter 34) constitute a single mass, of the order of 200 to 300 grams.
The mobile part 342 of the exciter 34 (forming an inertial mass) has a mass comprised between 25 and 50 grams (and more generally from 5 to 25% of the weight of the probe). The fact that the mass of the mobile part 342 is comprised between 25 and 50 grams (and more generally from 5 to 25% of the total weight of the probe) allows to have an inertial mass:
The inertial vibration exciter is adapted to be used at its resonance frequency. More specifically:
In particular:
The fact that provision is made to operate the exciter at its resonance frequency allows to limit the amount of energy necessary for the generation of the shear waves. By limiting the energy consumption of the exciter, it is then possible to power supply the probe by battery.
With reference to
The plunger rod is slidably mounted inside a through conduit formed in the magnetic core 3422. The operating principle of electromagnet and plunger rod exciters is as follows.
When an electric current is applied in the winding 3421, the ferromagnetic material of the head 3411 is suddenly attracted downwards, which induces a “plunge” of the rod inside the through conduit of the magnetic core 3422. The head 3411 moves down, and the return spring 343 is compressed. As soon as the winding 3421 is no longer supplied with electric current, the return spring 343 returns the sliding rod to the “high” position.
In the present case, the base 3412 of the plunger rod being (directly or indirectly) attached to the housing 31 of the probe 3, it is the magnetic core 3422 which moves towards the head 3411 by compressing the spring 343 when applying an electric current to the winding 3421. After interruption of this electric current, the magnetic core 3422 returns to the “low” position due to the force applied by the return spring 343.
It is then possible to obtain a reciprocating movement of the magnetic core 3422 depending on whether the electric current is applied or interrupted in the winding 3421. This reciprocating movement of the magnetic core 3422 induces the vibration of the whole probe 3, which allows to produce a shear wave.
Preferably, the electrical excitation should be close to a Dirac (a few milliseconds in duration).
A disadvantage of this type of exciter 34 is that it is difficult to finely control the movement of the magnetic core 3422:
It is therefore the stiffness of the spring 343 which will set the frequency of the movements of the magnetic core 3422: there is no means of finely controlling the excitation, nor of damping it. Moreover, the magnetic force developed in such a system is very dependent on the penetration of the rod into the magnetic core 3422, becoming maximum when the rod contacts the magnetic core 3422.
Another disadvantage of this type of exciter relates to guiding the translational movement of the plunger rod. In particular, the friction associated with the movement of the plunger rod inside the conduit-forming a guiding slide-induce frictional damping. Such frictional damping of the mobile part has the disadvantage of being uncontrolled, variable over time and highly dependent on the orientation of the probe.
This is why the inventors have proposed other variant embodiments (described below) in which the inertial vibration exciter has no guiding slide cooperating by friction with a rod to ensure the translational movement of the mobile part. This allows to have an inertial vibration exciter which has the advantage of having an intrinsic oscillatory behavior that is invariable in time and in space.
In these different variants described below, the guiding of the (translational or rotational) movement of the mobile part is ensured by one (or more) return spring(s) 343 extending between the fixed part 341 and the mobile part 342.
A second variant of exciter allowing more control is illustrated with reference to
More specifically:
This exciter can be driven by a controller (not shown), for example integrated into the electronic card 32. This controller allows to emit an electrical excitation signal (from a few milliseconds to a few tens of milliseconds) to power supply the activation coil 3416 of the exciter 34 whose operating principle is as follows.
The activation coil 3416 mechanically attached to the housing 31 evolves in the air gap of the armature 3423 continuously biased by the annular permanent magnet 34231.
When the activation coil 3416 is power supplied with electric current (that is to say when the controller emits the electric excitation signal), it applies to the armature 3423 a vertical electromagnetic force proportional to the electric current, and whose direction (towards the base 34151 or opposite the base 34151), depends on the direction of the electric current.
This force induces the translational movement of the mobile part 342 along the central conduit (of the tubular support 34152) of the fixed part 341. More specifically, depending on the direction of the electric current, this force will:
After interruption of the electric current, the mobile part 342 returns to its initial position, by oscillating due to the return spring 343 which successively compresses and stretches until it returns to its rest position. It is then possible to obtain a back and forth movement of the mobile part 342 depending on whether the electric current is applied or interrupted in the activation coil 3416. This back and forth movement of the mobile part 342 induces the vibration of the whole probe 3 by reaction, which allows to produce a shear wave when the probe 3 is in contact with a tissue.
Such devices exist commercially and are called audio exciters. As an indication,
With reference to
Such an audio exciter is adapted to be attached on a support—such as a resonant plate—and to cause it to vibrate by inertia. The dimensions of this example of a commercial audio exciter are compatible with the width and thickness of the electronic cards used in existing probes. The force generated under an electric current of 1 A is 2.4 Newtons. The mass of the mobile part of such an exciter is 12.8 grams and oscillates at 100 Hz (for a total weight of the probe of 235 grams).
Thus, to cause such an exciter to oscillate at a resonance frequency of 50 Hz, it is necessary to increase the mass of the mobile part to 51.2 grams, for example by attaching an additional inertial mass of 38.4 grams to the permanent magnet 3424.
This additional inertial mass 3425 can be attached on the upper face of the permanent magnet 3424 as illustrated in
As a variant and as illustrated in
In the embodiment illustrated in
As indicated previously, each time an excitation signal is applied by the controller, the mobile part 342 of the exciter 34 oscillates relative to the fixed part 341 until it returns to its initial rest position. Advantageously, the vibration of the mobile part 342—in particular the oscillation of the mobile part caused by the return spring 343 until it returns to its initial rest position—can be attenuated (“damping”) by the controller to allow optimal control of the mechanical excitation. This attenuation can be adaptive. In particular, the attenuation can adapt to variations in the damping provided by the contact between the probe and the patient's body. For this purpose, it is necessary to know the real movements of the probe. This knowledge of the real movements of the probe can be obtained by various direct or indirect means. For example, these movements can be determined without using a specific position sensor:
Preferably, the device according to the invention uses information representative of the relative movement between the probe and the tissue (from the processing of the acoustic echoes received and/or from a measurement of the electric current flowing in the exciter coil) to calculate a signal allowing to attenuate the oscillation of the mobile part. This allows more effective attenuation of the oscillations of the mobile part than with information representative of the absolute movement of the probe.
Of course, the vibration of the mobile part 342 may not be attenuated. In this case, each time an excitation signal is applied by the controller, the mobile part 342 of the exciter 34 oscillates freely. This free oscillation causes the probe to move until the mobile part has returned to its initial rest position.
The movement of the probe (related to the free oscillation of the exciter) can be separated from the expression of the progress of the shear wave by digital filtering of the acquired data. Indeed, in the context of the present invention, the application of a post-processing allows to separate the movement of the probe from the expression of the movement of the shear wave. The fact of not attenuating the oscillation of the mobile part allows to reduce the electrical energy consumed by the probe. Indeed, in the case of a probe in which the oscillations of the mobile part of the exciter are attenuated, it is necessary:
Due to the absence of a guiding slide cooperating by friction with a guide rod to ensure the translational movement of the mobile part, the embodiments of the exciter illustrated in
Such exciters are excited at the resonance frequency, which allows to limit the energy consumed by the exciter to generate the shear wave, unlike in particular the vibrator described in EP 3 315 074.
The different variant embodiments of the exciter 34 illustrated in
In this figure:
The reader will appreciate that it is difficult to define an effective mass associated with the real mass “M” since such an effective mass depends in particular on:
In the rest of the calculations, it will be considered that the effective mass associated with the mass “M” is equal to twice the real mass of “M” (that is to say an effective mass comprised between 400 and 600 grams).
At an instant to =0, the return spring is in its rest position. An impulsive force is applied to the probe which tends to separate the two masses “M” and “m” for a duration of a few milliseconds, and with a total energy E.
As the same force acts on the two masses with an opposite sign, it is possible to write:
M*d(V)/d(t)=−m*d(v)/d(t)
This equation remains true after the initial pulse when the return spring reacts to the separation of the two masses since it always exerts forces of the same amplitude and of opposite sign on the two masses “M” and “m”. It is therefore possible to write:
d(M*V+m*v)/dt=0,
where: M*V+m*v=constant (which is nothing other than the law of conservation of momentum considered within the framework of the theory of jet propulsion).
We therefore obtain: v=M/m*V, and by integration:
x=M/m*X (1)
The relationship between the terms of equation (1) indicates that the speeds and movements are in the inverse ratio of the masses. This relation further allows to verify a first coherence of the system: to obtain a movement of 0.1 mm of the mass “M”, it is necessary to move the inertial mass “m” by 1 mm (which is ten times lighter than the mass “M”); such millimeter movement is realistic, especially using the audio exciter shown in
At the level of the energy expended, it is possible to write:
E=½*M*V2+½*m*v2.
Note Eu the useful energy; with: Eu=½*M*V2
Then:
E=E
u*(1+m/M*(v/V)2)=Eu(1+M/m)
The energy efficiency is therefore:
ξ=m/(m+M) (2)
If the masses “M” and “m” are in a ratio of 10, then the energy efficiency is 9%, which is lower than that of Fibroscan® (close to 100%), but much higher than that of “push” ultrasound (probably around 1/1000). We are therefore still in the coherence of the system.
The absolute value of this energy will now be studied.
After the initial pulse the two masses “M” and “m” oscillate at a frequency corresponding to a pulse Ω.
The conventional relationship between the spring stiffness, k, and Ω can easily be demonstrated:
Ω=√/(k*(1/m+1/M)) (3)
Moreover: vMax=Ω*xMax
Where:
The energy supplied to the probe therefore satisfies the following equation:
E=½*M*V2+½*m*v2=½*m/M*(m+M)*Ω2*XMax2
In the particular case where:
This energy is very low compared to the amount of energy allocated to the operation of the probe (typically less than five Watts). Even if it has to be assumed that the effective mass is three times greater than that of the probe, and that consequently xMax must also be three times greater than 1 mm, then the energy required will only be worth ten times more, that is to say barely more than 10 mJ, which is still very reasonable.
In conclusion, the orders of magnitude in terms of:
The use of an inertial vibration exciter allows to obtain a probe adapted to generate a shear wave having:
An example of a probe 3 is illustrated with reference to
Each exciter 34a, 34b includes a fixed part 341a, 341b and a mobile part 342a, 342b connected to each other by a return spring. The fixed part 341a, 341b of each exciter 34a, 34b is fixed to the edge of the electronic card 32 opposite the edge connected to the transducer array 33.
The inertial vibration exciters 34a, 34b can be driven by a controller, for example integrated into the electronic card 32. This controller allows to apply an electrical excitation signal (of a few milliseconds) to induce the generation of vibrations by the exciters 34a, 34b.
The advantage of this system is that:
Of course, the reader will have understood that the exciters 34a, 34b can be controlled in configurations other than in phase or in phase opposition, for example to generate movements with an arbitrary and predetermined time variation. Furthermore, the reader will appreciate that the probe can comprise more than two inertial vibration exciters arranged at variable positions in the housing.
Finally, the reader will appreciate that the exciters can be arranged according to other variant embodiments. For example, in the embodiment illustrated in
Two coupled oscillators are thus obtained. This allows to explore a wide range of oscillation frequencies in order to be able to evaluate the viscosity of the target object.
With reference to
In this embodiment, the inertial vibration exciter 34 is:
In this third embodiment, the positioning and orientation of the inertial vibration exciter 34 allow to generate movements of the mobile part 342 in the plane of the transducers of the array 33.
With reference to
More specifically in this embodiment, the probe comprises:
As illustrated in
The second electronic card 32b is, in turn, mechanically integral with the transducer array 33.
The principle of operation is as follows. When an activation signal (of a few milliseconds) is applied to the exciter 34, the permanent magnet 3424 and the first electronic card 32a move in translation in a direction opposite to the transducer array 33. The return spring 343 stretches. After interrupting the electrical activation signal, the return spring 343 applies a force to the magnet 3424 and the first electronic card 32a to bring them back towards the transducer array 33. The permanent magnet 3424 and the electronic card 32a move in translation towards the transducer array 33 and exceed their rest position so that the return spring 343 is compressed. The return spring then exerts on the permanent magnet 3424 (and the first electronic card 32a) a force tending to separate it from the transducer array. This damped oscillation continues until the permanent magnet and the electronic card return to their rest position.
To continue this vibration, it is possible to periodically apply the activation signal. A shear wave train is thus generated in the tissue when the probe is applied to the patient's skin.
Of course, other types of inertial vibration exciters can be used to allow the generation of the shear wave by the probe. For example, the probe including an inertial vibration exciter 34 with a motor driving an eccentric mass.
Regardless of the exciter 34 used or the arrangement retained for the probe 3, the principle of operation of the probe is as follows.
The actuator vibration exciter 34 is activated to induce the movement of the probe 3 in response to the movement of its mobile part 342 and generate a low-frequency elastic wave (the shear wave) in the tissue to be analyzed. Specifically, the controller emits an excitation signal to the exciter 34.
This signal induces the movement of the mobile part(s) relative to the fixed part(s) (simultaneously or successively, for example in phase opposition). When this signal (of a few milliseconds) is interrupted, the mobile part returns to its original position by oscillating—or not if the controller emits an attenuation signal to dampen the oscillation of the mobile part. The vibratory movement produced by the exciter is transmitted to the probe via the fixed part mechanically integral with the transducer array.
During the propagation of the low-frequency shear wave in the tissue in contact, the transducer array 33 emits and receives high-frequency ultrasonic waves in order to allow the study of the propagation of the low-frequency elastic wave.
The mode of generation of the shear wave proposed above is effective in the transmission of mechanical energy to the tissue, since the transducer array 33, in contact with the tissue, is directly moved by the inertial vibration exciter. It also allows to obtain a probe whose bulk is minimized.
Different solutions can be considered for integrating the inertial vibration exciter described above into the probe. For example, said exciter may be incorporated into a secondary receptacle 312, as illustrated in
Alternatively, the inertial vibration exciter can be integrated into the housing 31 of the probe 3 (
The reader will have understood that many modifications can be made to the invention described above without materially departing from the new teachings and advantages described here.
Accordingly, all such modifications are intended to be incorporated within the scope of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| FR2010857 | Oct 2020 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2021/079332 | 10/22/2021 | WO |
