阅读说明：本技术 混合弹性成像方法、用于混合弹性成像的探头和装置 (Hybrid elastography method, probe and device for hybrid elastography ) 是由 洛朗·桑德兰 雨果·伯纳德·马丁·洛雷 于 2019-02-26 设计创作，主要内容包括：包括以下步骤的混合弹性成像方法(P)：?步骤(CW)，施加连续低频振动并使用与粘弹性介质接触的超声波换能器产生第一连串的超声波采集，所述第一连串的超声波采集包括超声波采集组，所述超声波采集组以第一重复率产生，每个超声波采集组包括至少一个探测，所述连续振动在所述粘弹性介质内产生弹性波；?施加低频脉冲并且使用所述超声波换能器产生(TI)第二连串的超声波采集，组成所述第二连串的所述超声波采集以第二重复率产生，所述低频脉冲产生在所述粘弹性介质内传播的瞬时剪切波；在施加所述低频脉冲之前停止通过第一振动器施加的所述连续振动。(Hybrid elastography method (P) comprising the following steps: -a step (CW) of applying a continuous low-frequency vibration and generating a first succession of ultrasound acquisitions using an ultrasound transducer in contact with a viscoelastic medium, said first succession of ultrasound acquisitions comprising ultrasound acquisition groups generated at a first repetition rate, each ultrasound acquisition group comprising at least one probe, said continuous vibration generating an elastic wave within said viscoelastic medium; -applying a low frequency pulse and generating (TI) a second series of ultrasound acquisitions using the ultrasound transducer, the ultrasound acquisitions constituting the second series being generated at a second repetition rate, the low frequency pulse generating a transient shear wave propagating within the viscoelastic medium; stopping the continuous vibration applied by the first vibrator before applying the low frequency pulse.)

1. A hybrid elastography method (P), the method comprising the steps of:

-a step (CW) of applying a continuous low-frequency vibration using a first vibrator contained in a probe in contact with a viscoelastic medium and generating a first succession of ultrasound acquisitions using an ultrasound transducer in contact with the viscoelastic medium, the first succession of ultrasound acquisitions comprising sets of ultrasound acquisitions generated at a first repetition rate, each set of ultrasound acquisitions comprising at least one acquisition, the continuous low-frequency vibration generating an elastic wave within the viscoelastic medium;

-a step (CW _ P) of determining, from said first succession of ultrasonic acquisitions, at least one characteristic of the elastic waves within the viscoelastic medium, said characteristic of the elastic waves within the viscoelastic medium being used to calculate a real-time positioning indicator (RT _ IP) of the probe with respect to the viscoelastic medium to be studied;

-a step (TI) of applying a low-frequency pulse using a second vibrator contained in the probe in contact with the viscoelastic medium and generating a second succession of ultrasound acquisitions using the ultrasound transducer, the ultrasound acquisitions constituting the second succession being generated at a second repetition rate; the low frequency pulse generates a transient shear wave that propagates within the viscoelastic medium.

2. The method according to the preceding claim, characterized in that the continuous low-frequency vibration applied by the first vibrator is stopped before the application of the low-frequency pulses by the second vibrator and the generation of the second series of ultrasound acquisitions.

3. Hybrid elastography method (P) according to one of the preceding claims, further comprising the steps of:

-a step (TI _ P) of determining at least one characteristic of the instantaneous shear waves from the second series of ultrasound acquisitions.

4. Hybrid elastography method (P) according to any one of the preceding claims, characterized in that the continuous low-frequency vibrations and the low-frequency pulses are applied using the same vibrator.

5. Hybrid elastography method (P) according to any one of the preceding claims, characterized in that it further comprises a step (RT _ IP) of displaying said real-time localization indicator in real time.

6. Hybrid elastography method (P) according to any one of the preceding claims, characterized in that the steps (TI) of applying the low-frequency pulse and generating the second succession of ultrasound acquisitions are triggered only in the case where the localization indicator satisfies a predetermined condition.

7. Hybrid elastography method (P) according to any one of the preceding claims, characterized in that the steps (TI) of applying said low-frequency pulses and of generating said second succession of ultrasound acquisitions are triggered automatically.

8. Hybrid elastography method (P) according to any one of the preceding claims, characterized in that the step of applying continuous low-frequency vibrations is triggered only in the case of a contact force between the vibrator and the viscoelastic medium higher than a predetermined lower threshold value.

9. Hybrid elastography method (P) according to any one of the preceding claims, characterized in that the step of applying low-frequency pulses is triggered only in the case where the contact force between the vibrator and the viscoelastic medium is comprised between a predetermined lower and an upper threshold value.

10. Hybrid elastography method (P) according to any one of the preceding claims, characterized in that said first succession of ultrasound acquisitions is formed by repeating a group comprising at least two ultrasound acquisitions, said group comprising said at least two ultrasound acquisitions having an intra-group repetition rate (HPRF) between 500Hz and 10kHz, and a first repetition rate (LPRF) between 10Hz and 10 kHz.

11. Hybrid elastography method (P) according to any one of the preceding claims, characterized in that said first repetition rate is lower than the frequency of said continuous oscillations (csff).

12. Hybrid elastography method (P) according to any one of the preceding claims, characterized in that the amplitude of the low-frequency pulses is determined on the basis of the characteristics of the elastic waves generated within the viscoelastic medium by the continuous low-frequency vibrations.

13. Hybrid elastography method (P) according to any of claims 2 to 12, characterized in that the stop of the continuous oscillation of the vibrator and the application of the low-frequency pulses are separated by a time interval greater than 10 ms.

14. A hybrid elastography Probe (PR), comprising:

-a first vibrator configured to apply a continuous low frequency vibration to a viscoelastic medium, the continuous low frequency vibration generating an elastic wave within the viscoelastic medium;

-a second vibrator configured to apply a low frequency pulse to the viscoelastic medium, the low frequency pulse generating a transient shear wave within the viscoelastic medium;

-an ultrasonic transducer configured to emit:

a first series of ultrasound acquisitions, the first series of ultrasound acquisitions comprising ultrasound acquisition groups generated at a first repetition rate, each ultrasound acquisition group comprising at least one acquisition;

a second series of ultrasound acquisitions, the ultrasound acquisitions constituting the second series being generated at a second repetition rate;

the probe is further configured to stop applying the continuous vibration prior to applying the low frequency pulse.

15. Hybrid elastography Probe (PR) according to the preceding claim, characterized in that at least one vibrator has the same axis of symmetry as the ultrasound transducer.

16. Hybrid elastography Probe (PR) according to claim 14 or 15, characterized in that at least one vibrator is ring-shaped and arranged to surround the ultrasound transducer.

17. Hybrid elastography Probe (PR) according to any of claims 14 to 16, characterized in that it further comprises means for calculating and displaying a positioning indicator (RT _ IP).

18. A hybrid elastography Device (DEV), comprising:

-the hybrid elastography Probe (PR) according to any of the preceding claims;

-a central Unit (UC) connected to said Probe (PR) and comprising at least computing means for processing the reflected ultrasonic signals, display means (SC), and control and/or input means (ENT).

Technical Field

The present invention relates to the field of elastography for determining the viscoelastic properties of a viscoelastic medium having an ultrasonic signal after ultrasonic irradiation. First, the invention relates to a hybrid elastography method comprising a harmonic elastography step and a transient elastography step. Secondly, the invention relates to a probe for implementing said hybrid elastography method. Furthermore, the invention relates to a hybrid elastography device. The hybrid elastography method according to the invention is particularly suitable for determining properties of a viscoelastic medium such as a human or animal liver.

Background

Transient elastography (also known as pulse elastography) is one of the most well-known and effective methods for determining the elasticity of a viscoelastic medium. For example, transient elastography is widely used to determine the elasticity of the liver in humans or animals.

In transient elastography, a pulsed shear wave is generated and the velocity of propagation of the pulsed shear wave within the viscoelastic medium of interest is measured. The propagation velocity of the shear wave in turn makes it possible to calculate the young's modulus of the medium and thus measure its elasticity.

There are several techniques for implementing transient elastography.

For example, the applicant has developed and commercialized a vibration controlled transient elastic imaging (VCTE) technology. Devices for carrying out this technique are known asWhich is capable of measuring the elasticity of a human liver in a fast, non-invasive and reproducible manner. In such transient elastography devices, shear waves are generated by a vibrator placed in contact with the medium to be characterized. The propagation of shear waves is then monitored using a series of ultrasound acquisitions with high repetition rate achieved by the ultrasound transducer. Each ultrasound acquisition corresponds to at least one ultrasound transmission.Each ultrasonic emission can be associated with the detection and recording of the progression of echoes resulting from the presence of reflecting particles in a defined depth range of the medium under investigation. The reflected ultrasound signals are processed by cross-correlation to trace back the motion of the tissue resulting from the propagation of the shear wave as a function of time and position in the medium. The study of these movements makes it possible to reflect the speed of propagation of shear waves in viscoelastic media and thus the elasticity of the tissue, as explained in the document "Transient elastomer: a new non-invasive method for assessing liver fibrosis", published by L.Sandrin et al in Ultrasound in Medicine and Biology 2003, 29 < 1705 > 1713.

The VCTE technique is particularly advantageous because it can temporally separate the propagation of a shear wave from the propagation of a compression wave generated simultaneously with the shear wave, the two types of waves having very different propagation velocities. Compression waves propagate at about 1500m/s, the propagation speed of shear waves being typically comprised between 1 and 10m/s, compression waves can be considered infinitely fast compared to shear waves. In practice, this separation is important because the measurement of the propagation velocity of the compression wave to the shear wave, which is simultaneous with the shear wave, introduces systematic errors.

One major limitation of VCTE technology is that it is difficult to verify the positioning of the probe before performing the elasticity measurement, and therefore it is difficult to trigger the mechanical pulse. In fact, incorrect positioning of the probe may result in imperfect propagation of the shear wave or even in the absence of the shear wave. For example, the propagation of the shear wave may be disturbed by the presence of recoil associated with the vicinity of the edge of the organ under investigation, or the shear wave may not propagate at all in the presence of a liquid interface between the probe and the medium under investigation. Indeed, shear waves are known not to cross liquid obstacles; especially in the presence of ascites in the abdomen. The measurement of the obtained elasticity will therefore be invalid.

Ultrasound may be used today to guide the positioning of vibrators for transient elastography. For example, an ultrasonic imaging or targeting tool such as described in patent application EP2739211a1 may be used. However, these solutions are not satisfactory because they do not directly predict the incorrect propagation of shear waves associated with, for example, incorrect positioning of the probe or with the presence of a liquid interface.

Among other transient elastography techniques, one can cite techniques based on the generation of shear waves by radiation force or "acoustic radiation force pulses" -ARFI. This technique is described, for example, in the document "Acoustic Radiation Force Impulse Imaging: Ex-vivo and in-vivo visualization of transient shear wave propagation" published by K.Nightingale et al in IEEE Biomedical Imaging, 2002.

Another technique for transient elastography is described in J.Bercoff et al, in the document "Supersonic shear Imaging: A new technique for soft tissue elastography", IEEE Transactions on ultrasound, Ferroelectrics, and Frequency Control, 2004. According to this technique, radiation force is achieved by focusing an ultrasonic beam at different points of a medium, and shear waves are generated by the radiation force, which makes it possible to obtain shear waves having a plane wavefront.

However, none of these transient elastography techniques provide a simple and complete solution to the problem of positioning the probe in order to obtain an effective elastography measurement in some way.

So-called harmonic elastography techniques also exist. These techniques are based on the application of continuous vibrations with a frequency comprised between 30Hz and 100 Hz. The elastic wave generated within the medium is a superposition of quasi-standing, shear and compressional waves. Among these existing harmonic elastography techniques, mention may be made of:

so-called "magnetic resonance elastography" or MRE techniques, in which magnetic resonance imaging is used to view quasi-standing waves generated in a medium; this technique is described in the document "magnetic resonance imaging by direct visualization of propagating acoustic strain waves" published by r.muthutillai et al in Science 269,1995. This technique is guided by MRI;

the so-called sonoelastography technique, which is described, for example, in the document "a pulsed doppler ultrasound system for making non-invasive measurements of the mechanical properties of soft tissue", published by t.kroustop in Journal of rehabilitation Research and Development,24,1987. This technique is guided by echo imaging;

the so-called "time harmonic elastography" technique, which is described, for example, In the document "In vivo time-harmonic multifrequency elastography of the human liver" by h.tzschatzsch et al, published on phys.med.biol.,59,2004. This technique is guided by echo imaging;

even though these techniques do not require the generation of pulsed shear waves propagating in the medium to be characterized, they still have some difficulties.

For example, in harmonic elastography, it is not possible to separate shear and compressional waves that are generated simultaneously in the medium to be characterized. The elastic quasi-standing wave thus generated within the medium to be characterized is a superposition of a shear quasi-standing wave and a compression wave. Since the velocity of the shear wave is much lower than the velocity of the compression wave, the really observed vibration velocity does not correspond to the velocity of the shear wave. The effect of compressional wave propagation must therefore be taken into account before the propagation velocity of the shear wave can be measured. To do this, complex data must be recorded and displacements in three spatial directions x, y, z calculated.

The only harmonic elastography technique that can currently perform this correction is the MRE technique. However, this technique requires a very complex and expensive magnetic resonance imaging apparatus and is therefore much more difficult to implement than the VCTE technique.

Furthermore, these techniques are guided by conventional methods of the echographic imaging or magnetic resonance imaging type. They require the operator to be quite specialized, which is not conducive to the widespread use of this technology.

In addition, harmonic elastography techniques may be used to guide the treatment method. It relates to the treatment of tumours localized by harmonic elastography techniques, for example by hyperthermia-type methods.

Technical problem

Harmonic or transient elastography techniques rely on the guidance of measurements made using traditional imaging techniques (echographic imaging or magnetic resonance imaging) which require a rather specialized operator and cannot ensure optimal positioning of the tissue to be characterized on the basis of the propagation of shear waves. The result is that it is not possible to predict the accuracy of the elastography measurement to be performed. Finally, these techniques are not suitable for implementation with small devices that are easy to use.

Disclosure of Invention

To at least partially address these issues, the present invention describes a new elastography technique, which will be referred to as hybrid elastography in the remainder of the present application.

To this end, the invention firstly relates to a hybrid elastography method comprising the following steps:

-applying a continuous low frequency vibration using a first vibrator comprised in a probe in contact with a viscoelastic medium, and generating a first series of ultrasound acquisitions using an ultrasound transducer in contact with a viscoelastic medium, the first series of ultrasound acquisitions comprising sets of ultrasound acquisitions, the sets of ultrasound acquisitions being generated at a first repetition rate, each set of ultrasound acquisitions comprising at least one acquisition, the continuous vibration generating an elastic wave within the viscoelastic medium;

-applying a low frequency pulse using a second vibrator comprised in a probe in contact with a viscoelastic medium, and generating a second series of ultrasound acquisitions using said ultrasound transducer, the ultrasound acquisitions constituting said second series being generated at a second repetition rate, said low frequency pulse generating a transient shear wave propagating within said viscoelastic medium.

According to one embodiment, the application of continuous vibrations by the first vibrator is stopped before the application of the low frequency pulses by the second vibrator and the generation of the second series of ultrasound acquisitions.

Employing hybrid elastography to indicate a method for implementing an elastography technique, the method comprising: at least one of the steps of applying continuous low frequency vibrations and applying low frequency pulses. In other words, the hybrid elastography method according to the invention comprises both the generation of continuous vibrations, which are characteristic of harmonic elastography techniques, and the generation of low-frequency pulses, which are characteristic of transient elastography techniques.

Thus, the continuous low frequency oscillations differ from the low frequency pulses in that the continuous low frequency oscillations are continuous and the duration of the low frequency pulses is short. Typically the duration of the low frequency pulse is comprised between 1/2 × tsff and 20/tsff, tsff being the center frequency of the low frequency pulse.

Continuous low frequency vibration is used to indicate continuous generation of the waveform pattern. This pattern may be, for example, an ideal sine wave; this is then referred to as monochromatic vibration. The vibration may also consist of a reproduction of an arbitrary pattern. According to one embodiment, the continuous oscillation is interrupted during the switching to the low-frequency pulse mode in order to stop the measuring process or when the measuring conditions are no longer satisfactory. The measurement conditions can be, for example, conditions relating to the contact force with the medium under investigation. The center frequency of the continuous low frequency vibration is typically comprised between 5Hz and 500 Hz.

An elastic wave is used to indicate the superposition of a compression wave and a shear wave.

Ultrasound acquisition is employed to indicate the emission of an ultrasound beam. The ultrasonic emission can be associated with the detection and recording of the progression of echoes resulting from the presence of reflective particles in a defined depth range of the medium under investigation.

Thus, a first series of ultrasound acquisitions is formed by a repetition of acquisition groups. A set of acquisitions includes at least one ultrasound acquisition. The acquisition groups are transmitted or generated at a first repetition rate. The first repetition rate is also referred to as the inter-group repetition rate. The first repetition rate is typically comprised between 5Hz and 500 Hz.

When each set of acquisitions is formed by at least two ultrasound acquisitions, the ultrasound acquisitions forming the same set are transmitted or generated with an intra-set repetition rate typically comprised between 500Hz and 100 kHz.

Advantageously, the use of a first low repetition rate during the application of continuous vibration enables the measurement of the movement of viscoelastic tissue while limiting the acoustic energy delivered to the same tissue from exceeding peak and average acoustic power limits.

The term "displacement" is considered in this document in a broader sense. Including any motion parameters (such as displacement, velocity, deformation rate, deformation velocity) and any mathematical transformations applied to these parameters.

The use of low frequency pulses to indicate the center frequency typically includes pulses between 5Hz and 500 Hz.

A second series of ultrasound acquisitions is employed to indicate a series of ultrasound acquisitions transmitted or generated at a repetition rate greater than 500Hz and preferably comprised between 500Hz and 100 kHz.

During the application of continuous vibration, an elastic wave is generated within the viscoelastic medium.

A first series of ultrasonic acquisitions is used to study the propagation of elastic waves within a viscoelastic medium. Which is capable of detecting echoes or ultrasonic signals reflected by the viscoelastic medium and calculating, from these reflected ultrasonic signals, the displacement of the viscoelastic medium caused by the propagation of elastic waves generated by continuous vibrations within the viscoelastic medium.

For example, the displacement of the viscoelastic medium can be calculated by applying a cross-correlation technique to the ultrasound acquisitions that make up the same set of ultrasound acquisitions in the first series.

The properties of the elastic waves within the medium can then be measured and the location indicator calculated in real time from the measured properties. The indicator is displayed in real time to guide the operator. Examples of such properties are the amplitude and phase of the elastic wave measured as a function of depth in the tissue to be characterized. The phase velocity of the elastic wave can also be calculated. However, the elastic value may be derived from the phase velocity of the elastic wave, but is different from the elastic value subsequently derived with a pulse wave given the superposition of a shear wave and a compression wave during the application of continuous vibration.

The "location indicator" and the "real-time location indicator" in the remainder of this document represent the same real-time location indicator.

Real-time is used to indicate an indicator whose display is regularly refreshed during the examination. Typically, the refresh rate is about 20Hz but may also be of the order of 1 Hz.

It is important to note that continuous vibration is used to verify the positioning of the probe for hybrid elastography. As an example, continuous vibration may be used to verify the presence of hepatic parenchyma (hepatic parahyma) facing the probe. It is important to note that continuous vibration is not used to replace measurements performed with pulses; but it completes the measurement. In other words, an indirect measurement of the viscoelastic properties of the medium is possible but not indispensable during the step of applying continuous vibrations. This latter measurement is physically different from elasticity in the sense of young's modulus, but may be correlated with this value.

Application of a low frequency pulse generates a transient shear wave that propagates within the viscoelastic medium to be characterized. The detection of the propagation of the shear wave enables measurement of the viscoelastic properties of the tissue to be characterized, such as the propagation velocity of the shear wave, the elasticity of the tissue, the shear modulus of the tissue or the young's modulus of the tissue. Thanks to the method according to the invention, the measurement of the viscoelastic properties of the medium can be disregarded if the positioning of the probe is not satisfactory. In other words, the measure of elasticity can be verified a priori by the positioning indicators obtained during the step of applying the continuous vibration.

Alternatively, the application of the low frequency pulses may be triggered only if the positioning of the probe has been previously verified during the harmonic elastography step.

A second series of ultrasonic acquisitions, transmitted or generated at a second repetition rate, is used to study the propagation of transient shear waves within the viscoelastic medium to be characterized. The ultrasonic signals reflected by the viscoelastic medium may be recorded and the displacement of the viscoelastic medium caused by the propagation of the shear wave calculated from these reflected ultrasonic signals. The measurement of the displacement generated in the viscoelastic medium by the propagation then makes it possible to trace back the propagation velocity of the shear wave and thus to use the formula E-3 ρ V _s2 elasticity of retrospective media, where E is elasticity or Young's modulusρ is the density, V_sIs the shear rate.

The hybrid elastography method according to the invention thus makes it possible to verify the positioning of the probe using harmonic elastography techniques and then to measure the viscoelastic properties of the medium to be characterized by using transient or pulsed elastography techniques. In particular, once the positioning of the probe is verified, a measurement of the viscoelastic properties is performed during the transient elastography step. This measurement provides a more accurate value of the viscoelastic properties of the medium than with harmonic elastography, because compression and shear waves do not overlap in pulse elastography, unlike what is observed in harmonic elastography.

In other words, the first step of harmonic elastography makes it possible to guide the positioning of the probe relative to the tissue to be characterized by providing the operator with an indicator predicting the success of the measurements made by pulsed elastography. Once the positioning of the probe is verified, acquisition can be triggered in transient elastography, the transient shear waves propagating correctly within the medium.

Advantageously, the hybrid elastography method according to the invention enables measurements of the viscoelastic properties of the tissue to be characterized to be carried out in a reliable and reproducible manner using instantaneous elastography techniques, while simultaneously positioning the probe in a simple and accurate manner thanks to harmonic elastography techniques.

The hybrid elastography method according to the invention may also have one or more of the following features considered individually or according to all technically possible combinations thereof:

the hybrid elastography method according to the invention further comprises the steps of:

determining from the first series of ultrasonic acquisitions at least one characteristic of an elastic wave within the viscoelastic medium;

determining from the second series of ultrasound acquisitions at least one property of the instantaneous shear wave and a property of the viscoelastic medium;

the same vibrator is used for applying successive low frequency vibrations and low frequency pulses;

the characteristics of the elastic waves within the viscoelastic medium are used to calculate an indicator of the real-time positioning of the probe with respect to the viscoelastic medium to be studied;

the method according to the invention further comprises displaying the real-time positioning indicator in real time; the refresh rate of the display is, for example, greater than or equal to 5 Hz;

-the step of calculating and the step of displaying the positioning indicator and the displaying thereof are performed together.

-the step of applying a low frequency pulse and generating a second series of ultrasound acquisitions is triggered only if the real-time localization indicator satisfies a predetermined condition;

-the step of applying a low frequency pulse and generating a second series of ultrasound acquisitions is automatically triggered based on the value of the real-time location indicator;

-the step of applying low frequency pulses and generating a second series of ultrasound acquisitions is automatically triggered;

-the step of applying continuous low frequency vibrations is triggered only in case the contact force between the vibrator and the viscoelastic medium is higher than a predetermined lower critical value;

-the step of applying continuous low-frequency vibrations is triggered only if the contact force between the vibrator and the viscoelastic medium is comprised between a predetermined lower and an upper threshold value;

-the step of applying low frequency pulses is triggered only in case the contact force between the vibrator and the viscoelastic medium is higher than a predetermined lower critical value;

the upper and lower critical values of the contact force for applying the continuous oscillation are generally equal to 1N and 10N, respectively;

the upper and lower contact force threshold values for applying the low-frequency pulses are generally equal to 4N and 10N, respectively;

-the frequency cSWF of the continuous low-frequency vibration applied by the vibrator is comprised between 5Hz and 500 Hz;

-the amplitude of the continuous low-frequency vibration applied by the vibrator is comprised between 10 μm and 5 mm;

-the first succession of ultrasound acquisitions is formed by a repetition of groups comprising at least two ultrasound acquisitions with an intra-group repetition rate comprised between 500Hz and 10kHz and a first repetition rate comprised between 10Hz and 10 kHz;

-the first repetition rate is lower than the continuous vibration frequency;

-the central frequency tsff of the low-frequency pulses is comprised between 10Hz and 1000 Hz;

-the pulse duration is comprised between 1/(2 x tsff) and 20/tsff, tsff being the central frequency of the low frequency pulse;

-the repetition rate of the ultrasound beam of the second succession is comprised between 500Hz and 100 kHz;

-the amplitude of the low frequency pulses is comprised between 100 μm and 10 mm;

-the stop of the continuous oscillation of the vibrator and the application of the low-frequency pulses are separated by a time interval which can be greater than or equal to 10ms and is preferably comprised between 1ms and 50 ms;

the amplitude of the low frequency pulses is determined on the basis of the characteristics of the elastic wave generated by the continuous vibration.

The invention also relates to a probe for implementing the hybrid elastography method according to the invention. The probe according to the present invention comprises:

-a first vibrator configured to apply continuous low frequency vibrations to the viscoelastic medium, the continuous low frequency vibrations generating elastic waves within the viscoelastic medium;

-a second vibrator configured to apply a low frequency pulse to the viscoelastic medium, the low frequency pulse generating a transient shear wave within the viscoelastic medium;

-an ultrasonic transducer configured to transmit:

a first series of ultrasound acquisitions, the first series of ultrasound acquisitions including ultrasound acquisition groups, the ultrasound acquisition groups being generated at a first repetition rate, each ultrasound acquisition group including at least one acquisition;

a second series of ultrasound acquisitions, the ultrasound acquisitions making up the second series being generated at a second repetition rate;

the probe is further configured to stop applying the continuous vibration prior to applying the low frequency pulse.

The probe according to the invention enables the method according to the invention to be carried out.

According to one embodiment, the probe according to the invention comprises a single vibrator used to apply continuous vibrations to the viscoelastic medium during the harmonic elastography step and to apply low frequency pulses during the pulsed elastography step.

The probe is configured such that the application of the low frequency pulses and the stopping of the continuous vibration are separated by a time interval comprised between 1ms and 50 ms. Preferably, the time interval is greater than or equal to 10 ms.

An ultrasonic transducer is used to transmit a first series and a second series of ultrasonic acquisitions within a viscoelastic medium. The same ultrasonic transducer detects the ultrasonic signal reflected at each acquisition. The reflected ultrasonic signals are then processed to detect displacement of the viscoelastic medium caused by the continuous low frequency vibrations and low frequency pulses.

The probe for hybrid elastography according to the present invention may also have one or more of the following features considered individually or according to all technically possible combinations thereof:

the vibrator is an electric motor or an audio reel or an electric actuator;

-the ultrasonic transducer is mounted on the shaft of the vibrator;

the hybrid elastography probe according to the invention further comprises means for triggering the application of low frequency pulses;

the ultrasonic transducer is circular, with a diameter comprised between 2mm and 15 mm;

-the ultrasonic transducer has an operating frequency comprised between 1MHz and 15 MHz;

-the ultrasonic transducer is a convex abdomen probe (convex abdominal probe);

-the first vibrator and the second vibrator are axisymmetric;

at least one of the vibrators is axisymmetric;

-at least one vibrator has the same axis of symmetry as the ultrasonic transducer;

-at least one vibrator having an annular shape and being arranged to surround the ultrasonic transducer;

the probe further comprises means for calculating and displaying a real-time positioning indicator.

The invention also relates to a hybrid elastography device implementing the hybrid elastography method according to the invention.

Such a mixing device according to the invention comprises:

-a hybrid elastography probe according to the invention;

a central unit connected to the probe and comprising at least computing means for processing the reflected ultrasonic signals, display means, and control and/or input means.

According to one embodiment, the display device is configured to display the real-time location indicator in real-time.

Drawings

Other features and advantages of the invention will become apparent from the following description of the invention, given for the purpose of illustration and not of limitation, with reference to the accompanying drawings, in which:

figure 1 shows the steps of a hybrid elastography method according to the invention;

fig. 2 schematically shows the vibrations applied by the vibrator and the ultrasound acquisition during the implementation of the method according to the invention shown in fig. 1;

FIG. 3 schematically shows a particular embodiment of the elastography method shown in FIG. 1;

figure 4 shows the results obtained by implementing the part of the method according to the invention relating to the positioning of the vibrator;

FIG. 5 shows the result of an implementation of the method shown in FIG. 1;

figure 6 presents a hybrid elastography probe according to the invention;

figure 7a presents a particular embodiment of a hybrid elastography probe according to the present invention;

figure 7b presents a hybrid elastography device according to the invention.

Detailed Description

Fig. 1 shows the steps of a hybrid elastography method P according to the invention.

The first step CW of method P comprises applying continuous low frequency vibrations using a first vibrator comprised in a probe in contact with the viscoelastic medium.

The frequency of the continuous vibration is between 5Hz and 500 Hz.

The first step CW of method P further comprises generating a first series of ultrasound acquisitions by an ultrasound transducer. The first series of ultrasound acquisitions comprises an ultrasound acquisition group. Groups of ultrasound acquisitions are transmitted at a first repetition rate LPRF between 5Hz and 500Hz, each group including at least one ultrasound acquisition.

Ultrasound acquisition involves the emission of an ultrasound beam and the subsequent detection and recording of reflected ultrasound signals or echoes.

Continuous vibration is applied to a viscoelastic medium to generate elastic waves within the viscoelastic medium. The elastic wave is formed by the superposition of a shear wave and a compressional wave. The study of the properties of this elastic wave makes it possible to obtain information about the correct positioning of the probe with respect to the viscoelastic medium.

The viscoelastic medium to be characterized at least partially diffuses the ultrasound beam. It is thus possible to detect the ultrasound signals reflected during the emission of the first succession of ultrasound acquisitions.

The detection of the reflected ultrasonic signal may be performed using the same ultrasonic transducer used for transmission.

During a step CW _ P of determining at least one characteristic of an elastic wave within the viscoelastic medium, reflected ultrasonic signals detected during a step CW of generating a first succession of ultrasonic acquisitions are processed.

During this step, the reflected ultrasonic signals are subjected to a mutual correlation according to techniques known in the field of elastography and more generally ultrasound, so as to measure the displacement of the viscoelastic medium caused by the elastic waves generated by applying continuous vibrations.

From the measured displacements within the viscoelastic medium, characteristics of the elastic wave, such as amplitude and phase, may be calculated as a function of position within the viscoelastic medium. The position of a point within the viscoelastic medium is measured as the distance between the ultrasonic transducer and the point, calculated along the propagation direction of the ultrasonic wave emitted by the transducer. For this reason, the location of a point within the viscoelastic medium is often referred to as the depth.

Other parameters of the elastic wave within the viscoelastic medium, such as the phase velocity of the elastic wave, may also be determined.

The change in amplitude and phase of the elastic wave as a function of depth within the tissue may be calculated. By making adjustments between the theoretical model and the measured characteristics, adjustment quality parameters can be extracted. From this adjustment quality parameter and/or other characteristics of the elastic waves, a real-time positioning indicator RT _ IP of the vibrator relative to the tissue to be characterized can be calculated.

Due to the low first repetition rate used for the first series of ultrasound acquisitions, the real-time location indicator RT _ IP may be calculated in real-time.

According to one embodiment, the real-time positioning indicator RT _ IP is displayed accompanying the calculation of the real-time positioning indicator RT _ IP. In other words, the real-time location indicator is calculated and displayed in real-time. In other words, the step of calculating the real-time positioning indicator and the step of displaying the real-time positioning indicator are performed jointly.

For example, one of the theoretical models used provides a linear variation of the phase lag at the center frequency of the elastic wave with depth in the medium to be characterized. In this case, the adjustment is a linear adjustment and the adjustment quality parameter converts the linearity of the phase as a function of the depth in the medium. One possible indicator is to determine the coefficient R²Which gives the prediction quality of a linear regression of the phase lag curve as a function of depth in the depth range studied.

According to one embodiment, the step CW _ P of determining at least one characteristic of an elastic wave in the tissue is performed simultaneously with the step CW of applying continuous vibrations and detecting the first reflected ultrasonic signal.

Thanks to the method P according to the invention, it is thus possible to measure in real time the characteristics of the elastic waves in the tissue and to obtain in real time the real-time positioning indicator RT _ IP of the probe.

Advantageously, the low first repetition rate LPRF makes it possible to reduce the size of the data recorded during the step CW of generating the first succession of ultrasound acquisitions and to process these data in real time to obtain the location indicator RT _ IP.

If the value of the positioning indicator is not satisfactory, these two steps CW and CW _ P are repeated as indicated by the dashed arrow in fig. 1.

If the value of the positioning indicator is satisfactory, the probe is correctly positioned with respect to the viscoelastic medium and the elasticity measurements performed during the transient elastography step will be valid. In this case, the method P according to the invention leads to step TI.

According to one embodiment, the method P according to the invention comprises a step of displaying the positioning indicator RT _ IP in real time. The technique of positioning the indicator RT _ IP and its display are performed jointly.

According to one embodiment, the refresh rate of the display of the position indicator is greater than or equal to 5 Hz.

Step TI shown in fig. 1 comprises applying a low frequency pulse using a second vibrator.

As in any transient elastography technique, application of a low frequency pulse to a viscoelastic medium generates a transient or pulsed shear wave that propagates within the medium. By measuring the propagation velocity of the transient shear wave within the medium to be characterized, the elasticity of the medium can be reflected.

It is important to note that the continuous low frequency vibration is stopped during the application of the low frequency pulse and the following steps. Stopping the continuous vibration during the real-time transient elastography step is very important to enable a temporal separation of the compressional and shear waves, which enables a reliable measurement of the elasticity of the medium to be obtained.

According to one embodiment, there is a time interval between the stop of the continuous oscillation and the application of the low-frequency pulse, said time interval being comprised between 1ms and 50ms and preferably greater than or equal to 10 ms. This time interval enables the compression wave generated by the continuous vibration to be dissipated and can improve the accuracy and reliability of the measurement of viscoelastic properties such as the velocity of the instantaneous shear wave.

While applying the low frequency pulse, step TI comprises generating a second series of ultrasound acquisitions with an ultrasound transducer transmitting at a second repetition rate VHPRF.

The repetition rate VHPRF of the second series of ultrasound acquisitions is between 500Hz and 100 kHz.

From the reflected ultrasonic signal detected during step TI, at least one characteristic of the viscoelastic medium can be calculated during step TI _ P of method P according to the invention. This can be achieved by applying cross-correlation techniques well known in elastography. In particular, the propagation velocity of the shear wave and thus the elasticity of the viscoelastic medium can be calculated, as explained, for example, in the document "Transient elastomer mapping: a new non-invasive method for assessing liver fibrosis" by L.Sandrin et al.

For example, during step TI _ P of determining the characteristics of the viscoelastic medium, the propagation speed of the pulsed shear wave generated by the low-frequency pulse is determined. The elasticity, shear modulus or young's modulus of the viscoelastic medium can be traced back from the propagation velocity of the shear wave.

According to one embodiment, the steps of applying the low frequency pulse and generating the second series of ultrasound acquisitions are triggered only if the location indicator satisfies a predetermined condition.

Advantageously, this enables only valid elasticity measurements to be triggered, since the positioning indicator ensures the presence of a transient shear wave and its correct propagation.

The triggering of the step of applying the low frequency pulse and the following steps may be automatic or manual and initiated, for example, by an operator based on the value of the positioning indicator RT _ IP.

If the application of the low frequency pulse is initiated by the operator, the positioning indicator calculated in real time during step CW _ P is displayed in real time.

According to one embodiment, a simpler signal of the "position OK" or "position NOT OK" type may be displayed to communicate with the operator.

According to one embodiment, the refresh rate of the display of the position indicator is greater than 5 Hz.

This allows the operator to trigger the elasticity measurement from the moment when the correct propagation of the shear wave is observed, thus ensuring the validity of the measurement.

According to one embodiment, the continuous vibration is triggered only in case the contact force between the vibrator and the viscoelastic tissue is above a predetermined critical value (typically 1N).

According to one embodiment, the continuous vibration is triggered only in case the contact force between the vibrator and the viscoelastic tissue is below a predetermined critical value (typically 10N).

Advantageously, the lower threshold ensures sufficient coupling between the probe and the viscoelastic medium, and the upper threshold avoids continuous vibrational deformations and damages to the medium under study caused by excessive contact forces.

According to one embodiment, the low frequency pulse is triggered only if the contact force between the vibrator and the viscoelastic tissue is comprised between a predetermined lower threshold value and a predetermined upper threshold value. These two thresholds are typically 4N and 8N, respectively.

Advantageously, the lower threshold value ensures sufficient coupling between the probe and the viscoelastic medium, and the upper threshold value avoids deformation of the low-frequency pulses and damage to the medium under study caused by excessive contact forces.

Due to the continuous vibratory motion of the vibrator, the determination of the contact force between the vibrator and the medium is more complicated than in the case of standard transient elastography methods. In the presence of continuous low frequency vibrations, the contact force between the vibrator and the viscoelastic medium is given by the following equation:

in this formula, x is the displacement of the vibrator, k is the spring constant of a spring placed in the probe, A is the amplitude of the continuous vibration, f_lowIs the continuous vibration frequency.

Force F may be measured using a force sensor placed on the hybrid elastography probe. By then applying a low-pass filter to the signal measured by the force sensor, it is possible to eliminate the low-frequency part and to derive the average contact force:

F_Average＝k(x)

according to one embodiment of method P of the present invention, the low frequency pulses are only at F_AverageIs higher than a predetermined critical value.

Advantageously, the use of a minimum contact force value makes it possible to ensure good transmission of the low-frequency pulses to the viscoelastic medium and correct propagation of the transient shear waves generated within the medium.

According to one embodiment of the method P according to the invention, the stop of the continuous oscillation of the vibrator and the application of the low-frequency pulses are separated by a time interval comprised between 1ms and 50 ms. Preferably, the time interval is greater than or equal to 10 ms.

Advantageously, the use of a time interval separating the stop of the continuous vibration and the application of the low-frequency pulses enables the vibration generated by the continuous vibration to be attenuated. It is therefore possible to apply low frequency pulses and observe the propagation of pulsed shear waves in the absence of elastic waves. The co-existence of elastic waves including compression waves and instantaneous shear waves may introduce errors into the measurement of the propagation velocity of the instantaneous shear waves.

Fig. 2 schematically shows:

-continuous low frequency vibrations cSW applied by the first vibrator during step CW shown in fig. 1;

low frequency pulses tSW applied by the second vibrator during step TI shown in fig. 1;

a first succession of ultrasound acquisitions PA formed in the manner of probe groups G, generated by the ultrasound transducers during step CW shown in fig. 1;

a second series of ultrasound acquisitions DA generated by the ultrasound transducers during step TI shown in fig. 1.

During the step CW of applying continuous vibration, the vibrator oscillates at an amplitude between 10 μm and 5mm at a frequency between 5Hz and 500 Hz.

Advantageously, thanks to the low amplitude and low frequency of the continuous vibrations, the operator can easily keep the probe in contact with the viscoelastic medium.

According to one embodiment, the continuous low frequency vibration csff and the low frequency pulse tsff may be applied using the same vibrator.

While applying the continuous low-frequency vibrations, the ultrasound transducer emits a first succession of ultrasound acquisitions PA formed in the manner of an ultrasound acquisition group G. In the example shown in fig. 2, each group G comprises two ultrasound acquisitions.

The ultrasound acquisition group G is transmitted at a first repetition rate LPRF between 10Hz and 500 Hz. Ultrasound acquisitions belonging to the same group G are transmitted at an intra-group repetition rate HPRF between 500Hz and 10 kHz.

The ultrasonic transducer also detects the ultrasonic signal reflected during the generation of the ultrasonic acquisition PA as explained with reference to step CW shown in fig. 1. From the first series of ultrasound acquisitions PA, the displacements in the viscoelastic medium generated by the propagation of the elastic waves generated by the continuous vibrations applied by the vibrators can be calculated by means of a cross-correlation Corr step between the ultrasound signals belonging to the same group G.

Advantageously, by applying the cross-correlation technique to the ultrasound acquisitions belonging to the same group G, and thus closer in time, it is possible to detect small displacements of the order of 1 μm to 10 μm.

As explained with reference to step CW _ P shown in fig. 1, the displacement of the viscoelastic medium is then used to calculate characteristics of the elastic wave, such as the change in amplitude and phase of the elastic wave as a function of depth in the medium. By comparing the measured characteristics with theoretical models, the location indicator RT _ IP can be derived in real time.

For example, the localization indicator may be associated with a linearity of the phase of the elastic wave as a function of the depth in the medium to be characterized. The indicator then depends on the quality of the adjustment of the phase evolution as a function of the depth represented by the straight line.

For example, the localization indicator may be associated with a decrease in the amplitude of the elastic wave as a function of the depth in the medium to be characterized. The indicator then depends on being at 1/ZⁿWherein Z is depth and n is an integer coefficient comprised between 1 and 3.

For example, the real-time positioning indicator RT _ IP has a value between 0 and 1, wherein the value is close to 1 if the probe is correctly positioned with respect to the viscoelastic medium of interest.

If the value of the real-time positioning indicator RT _ IP is considered satisfactory, for example greater than a predetermined threshold value, the step TI of applying low-frequency pulses is triggered.

The center frequency of the low frequency pulses tSWF is between 10Hz and 1000 Hz. The duration of the low frequency pulse is between 1/(2 × tsvf) and 1/tsvf.

The amplitude of the low frequency pulses is between 100 μm and 10 mm.

According to one embodiment, the amplitude of the low frequency pulse may be modified based on the characteristics of the elastic wave measured at step CW _ P.

The amplitude of the displacement caused by the propagation of the elastic wave is measured in the region of interest. For example, consider the average amplitude HA measured in the region of interest_MAnd a reference average amplitude HA in the region of interest_R. Knowing that the displacement caused by the propagation of the pulsed shear wave may be more difficult to measure, it is possible to calculate the multiplication coefficient b to be applied to the set point of the low frequency pulse so that the amplitude of the resulting displacement is optimal. Amplitude AT of the setpoint of the low-frequency pulse is dependent on a reference amplitude AT as setpoint_RAnd the following equation for the function of coefficient b:

and

AT＝b×AT_R

the set point a (t) of the low frequency pulses is then defined for the period of pulse duration in the following manner:

where f is the center frequency of the low frequency pulse, also labeled tsff, and t is time.

According to one embodiment, several low-frequency pulses can be generated in succession, as described in patent application FR 1351405.

As described with reference to step TI of method P according to the invention shown in fig. 1, a second succession of ultrasound acquisitions DA is transmitted at a second repetition rate VHPRF, while applying the low-frequency pulses and propagating the transient shear waves.

The second repetition rate VHPRF is between 500Hz and 100 kHz. Each ultrasound beam has a center frequency between 1MHz and 15 MHz.

The ultrasonic transducer also detects a reflected ultrasonic signal resulting from the second series of ultrasonic acquisitions DA, as explained with reference to step TI shown in fig. 1. From the second series of ultrasound acquisitions, the displacement of the viscoelastic medium can be calculated by means of a cross-correlation Corr step. Said displacement of the viscoelastic medium is generated by the propagation of a transient shear wave generated by a low-frequency pulse applied by a vibrator. The displacement of the viscoelastic medium is then used to calculate the properties of the instantaneous shear wave, as explained with reference to step TI _ P shown in fig. 1. In particular, the propagation velocity V of the shear wave can be calculated_SAnd thus the elasticity E of the viscoelastic medium of interest. The young's modulus and/or shear modulus of the media may also be calculated.

As shown in fig. 2, after a measurement of the elasticity E of the viscoelastic medium has been obtained, the method may be repeated by restarting the step CW of applying continuous vibrations and subsequently performing the step of applying low frequency pulses (not shown in fig. 2).

Fig. 3 shows a particular implementation of steps CW and CW _ P of method P according to the invention, called strobe (strobo) mode.

The continuous sinusoidal line schematically represents the continuous vibration cSW applied by the first vibrator. The continuous vibration cSW has a center frequency cSWF of, for example, 50Hz corresponding to a period of 20 ms.

The continuous vertical lines indicate the ultrasound acquisition groups G forming the first series of ultrasound acquisitions PA. Group G is transmitted at a first repetition rate LPRF. According to the strobe acquisition mode, the first repetition rate LPRF is less than the center frequency of the continuous oscillation csff.

The intra-group repetition rate is between 500Hz and 100kHz, which makes it possible to measure small displacements of the order of 1 to 10 μm.

The white circles and arrows along the continuous vibration cSW correspond to the sampling performed by each ultrasound acquisition group G.

Thanks to the fact that the repetition rate LPRF of group G is lower than the center frequency of the continuous vibration cSW, the continuous vibration cSW can be sampled in a complete manner at the ends of several oscillation periods, as indicated by the white circles.

Advantageously, the strobe pattern allows the continuous vibration cSW to be sampled in a complete manner while using the first low repetition rate LPRF. The use of a low repetition rate enables the reflected signals to be processed in real time and thus the positioning indicator RT _ IP to be obtained in real time.

According to one embodiment, the first repetition rate LPRF is greater than the center frequency of the continuous oscillation csff. This enables, for example, two points to be acquired per vibration cycle. Thus using as many oscillation periods to obtain better sampling or less oscillation periods to obtain equal sampling.

Fig. 4 schematically shows the results obtained by implementing the part of the method P according to the invention relating to the positioning of the vibrator.

The diagram CW _ DISP shows the displacement (or any other motion parameter such as velocity, deformation rate) of the viscoelastic medium in the region of interest ROI as a function of depth Z and time T in the medium. The displacement is represented using a false color scale, and the light color represents the displacement along the positive direction of the axis D. The displacement is caused by continuous low frequency vibration applied by a vibrator and is measured by an ultrasonic transducer UT placed in contact (Z ═ 0) with the surface of the medium.

From the displacement CW _ DISP measured in the region of interest ROI within the viscoelastic medium, information RT _ INFO about the elastic wave generated by the continuous vibrations propagating within the medium can be extracted in real time. Examples of these characteristics are the amplitude a and the phase Ph of the elastic wave as a function of depth within the medium.

By comparing the measured values of a and Ph with predetermined critical values, a positioning indicator of the vibrator with respect to the viscoelastic medium can be determined. If the value of the location indicator is above a predetermined threshold, the elasticity measurement of the medium by transient elastography is considered valid.

Alternatively, the measured quantities A and Ph may be obtained as described inTuning quality parameters between theoretical models of amplitude and phase of an elastic wave propagating within a medium. In this case, the positioning indicator is obtained from the adjustment quality parameter AJ. For example, adjusting the quality parameter is determining the coefficient R²Which gives the prediction quality of the linear regression of the phase lag curve as a function of depth in the depth range studied.

According to one embodiment, said adjustment quality parameter AJ is comprised between 0 and 1.

Once calculated, the positioning indicator may be displayed in the form of numbers or letters or by using a color scale. Alternatively, the location indicator may be a simple visual indication of the "location OK" type indicating that the operator can trigger the instantaneous elastography step.

Fig. 5 shows the results obtained by implementing the method P according to the invention.

As already described with reference to fig. 4, the graph CW DISP represents the displacement measured in the presence of an elastic wave in the medium.

The graph RT-INFO represents the amplitude a and phase Ph of the standing wave measured in real time as explained with reference to fig. 4. From the map RT-INFO, the positioning indicators can be calculated and displayed in real time.

The graph TI _ DISP represents the displacement measured after application of the low frequency pulse as a function of the depth D in the medium and the time T. In other words, the graph TI _ DISP represents a pulse elastic graph. The displacement is expressed using a pseudo-color scale and corresponds to the propagation of the instantaneous shear wave within the viscoelastic medium.

From the displacement TI _ DISP, the propagation velocity of the instantaneous shear wave can be calculated and reflects the elasticity of the medium.

As explained with reference to fig. 1, 2 and 3, the graphs CW _ DISP, RT _ INFO and the positioning indicators of the vibrators are calculated and displayed together during the implementation of the method P according to the invention.

Advantageously, thanks to the structure of the first series of ultrasound acquisitions, the positioning indicator RT _ IP and the graph RT _ INFO can be calculated and displayed in real time.

In contrast, the calculation of the propagation velocity Vs of the shear wave of the graph TI _ DISP is displayed only in the case where the position of the vibrator facing the viscoelastic medium is verified and step TI is triggered.

Fig. 5 can also be considered as a representation of the results obtained during the implementation of the method P according to the invention and can be displayed on a screen and referred to by the operator during inspection or measurement.

Figure 6 schematically shows a hybrid elastography probe PR.

The probe PR includes:

a first vibrator VIB1 configured to apply continuous low frequency vibrations to the viscoelastic medium, the continuous low frequency vibrations generating elastic waves within the viscoelastic medium;

a second vibrator VIB2 configured to apply a low frequency pulse to the viscoelastic medium, said low frequency pulse generating an instantaneous shear wave within the viscoelastic medium;

-an ultrasonic transducer TUS configured to transmit:

a first series of ultrasound acquisitions, the first series of ultrasound acquisitions including ultrasound acquisition groups, the ultrasound acquisition groups being generated at a first repetition rate, each ultrasound acquisition group including at least one acquisition;

a second series of ultrasound acquisitions, the ultrasound acquisitions making up the second series being generated at a second repetition rate;

the probe is further configured to stop applying the continuous vibration prior to applying the low frequency pulse.

According to the embodiment shown in fig. 6, an ultrasonic transducer TUS is mounted on the shaft of a vibrator VIB2 which applies low frequency pulses.

According to one embodiment, the ultrasonic transducer TUS may be fixed to the body of the probe using a joint PT.

The first vibrator VIB1 oscillates the probe PR. During this oscillation, the ultrasonic transducer TUS is pushed against the viscoelastic medium, applying a continuous low frequency vibration and generating an elastic wave within the medium.

According to one embodiment, the first vibrator VIB1 for applying continuous low frequency vibrations comprises a vibration ring placed around the ultrasonic transducer TUS or around the probe tip PT.

The second vibrator VIB2 may apply a low frequency pulse to the viscoelastic medium according to several embodiments.

According to a first embodiment, the probe joint PT is movable and can be actuated by a second vibrator VIB 2. The ultrasonic transducer TUS is then pushed against the viscoelastic medium to apply vibration in the direction of arrow 2 of fig. 6.

According to a second embodiment, the probe PR is an inertial probe without moving parts. In this case, the movement of the second vibrator VIB2 within the probe PR causes movement of the probe and again applies continuous or pulsed vibrations by pushing the transducer TUS against the viscoelastic medium.

The axis of motion a of the vibrator is the axis of symmetry of the ultrasonic transducer TUS. For example, the ultrasonic transducer TUS may have a circular cross-section with an axis a passing through the center of the ultrasonic transducer TUS.

According to one embodiment, the probe PR comprises control means TOG for triggering the application of low-frequency pulses, for example during step TI of the method according to the invention.

Fig. 7a schematically shows an embodiment of a probe PR for hybrid elastography according to the invention.

The probe PR includes:

a vibrator VIB for applying continuous or pulsed vibrations to the viscoelastic medium of interest;

an ultrasonic transducer TUS for emitting an ultrasonic beam and detecting the reflected ultrasonic signal.

Thus, the probe PR according to FIG. 7a includes a single vibrator intended to apply both continuous low frequency vibrations and low frequency pulses.

According to one embodiment, the diameter of the ultrasonic transducer is comprised between 2mm and 15 mm.

According to one embodiment, the center frequency of the ultrasonic transducer is comprised between 1MHz and 15 MHz.

According to one embodiment, the ultrasonic transducer TUS is a convex abdomen probe.

According to one embodiment of the probe PR, the at least one vibrator is axisymmetric. In other words, at least one vibrator has an axis of symmetry.

According to one embodiment, the axis of symmetry of the axisymmetric vibrator corresponds to the axis of symmetry of the ultrasonic transducer TUS.

According to one embodiment, the at least one vibrator of the probe has a ring shape and is arranged to surround the ultrasonic transducer TUS.

According to one embodiment, the probe further comprises a calculation and display device for calculating and displaying the real-time positioning indicator RT _ IP.

For example, the computing device includes at least one microprocessor and a memory.

For example, the display device includes a screen and/or a positioning indicator.

According to one embodiment, the probe includes a positioning indicator that is triggered when the probe is properly positioned. The indicator may be a visual indicator, such as a change in color of a diode. Alternatively, the indicator may be a sound or tactile indicator, such as a change in the type or amplitude of vibration.

Fig. 7b shows a hybrid elastography device DEV according to the invention. The device DEV according to the invention comprises:

-a probe PR according to the invention;

a central unit connected to the probe PR.

The center unit may include:

-computing means for processing the reflected ultrasound signals;

-a screen SC for displaying the results obtained at the different steps of the method P according to the invention;

-control or input means ENT for controlling the device by an operator.

The central unit UC may be connected to the probe PR by a wire line or by wireless communication means.

According to one embodiment, the screen SC is adapted to display the results shown in fig. 5. The screen SC can also display in real time the positioning indicator RT _ IP calculated during step CW _ P of the method P according to the invention.

According to one embodiment, the central unit comprises means configured to automatically trigger the application of the low-frequency pulses based on the value of the positioning indicator RT _ IP calculated and displayed in real time.