RU2794039C2 - Hybrid elastography method, probe and device for hybrid elastography - Google Patents

Abstract

FIELD: medicine.

SUBSTANCE: hybrid elastography method includes the following steps: applying continuous low frequency vibration and generating, using an ultrasonic transducer in contact with a viscoelastic medium, a first series of ultrasonic data acquisition signals; applying a low frequency pulse and generating, using an ultrasonic transducer, a second series of ultrasonic data acquisition signals; moreover, the continuous vibration applied by the first vibrator stops before the application of the low frequency pulse. The hybrid elastography probe includes: a first vibrator configured to apply continuous low frequency vibration to the viscoelastic medium, wherein the continuous low frequency vibration generates an elastic wave within the viscoelastic medium; a second vibrator configured to apply to the viscoelastic medium a low-frequency pulse that generates a non-stationary shear wave inside the viscoelastic medium; an ultrasonic transducer configured to emit: a first series of ultrasonic data acquisition signals; a second series of ultrasonic data acquisition signals; wherein said probe is further configured to terminate the application of continuous vibration prior to application of the low frequency pulse. The hybrid elastography device includes: a hybrid elastography probe; a central unit connected to the probe and including a calculation means for processing reflected ultrasonic signals, a display means, and a control and/or input means.

EFFECT: using this group of inventions will provide optimal determination of the location of the tissue.

18 cl, 7 dwg

Description

FIELD OF TECHNOLOGY TO WHICH THE INVENTION RELATES

The invention relates to the field of elastography for determining the viscoelastic properties of a viscoelastic medium that generates an ultrasonic signal after irradiation with ultrasound. First, the invention relates to a hybrid elastography method including a harmonic elastography step and a transient elastography step. Secondly, the invention relates to a probe for implementing a hybrid elastography method. Thirdly, the invention relates to a device for hybrid elastography. The hybrid elastography method of the present invention is particularly suitable for determining the properties of a viscoelastic medium such as the human or animal liver.

BACKGROUND OF THE INVENTION

Transient elastography (also called pulsed elastography) is one of the best known and most 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 its propagation velocity is measured within the viscoelastic medium of interest. Then the speed of propagation of the shear wave makes it possible to calculate the modulus of elasticity of the medium and, thereby, measure its elasticity.

There are several methods for implementing transient elastography.

For example, the Applicant has developed and marketed the Vibration Controlled Transient Elastography (VCTE) technique. The device that implements this method, called Fibroscan®, is able to quickly, non-invasively and reproducibly measure the elasticity of the human liver. In such a transient elastography device, a shear wave is generated by a vibrator placed in contact with the medium to be characterized. The shear wave propagation is then monitored using a series of ultrasonic data acquisition signals implemented by an ultrasonic transducer with a high pulse repetition rate. Each ultrasonic data acquisition signal corresponds to at least one emission of ultrasonic waves. Each emission of ultrasonic waves can be associated with the reception and recording in real time of echo signals generated by reflective particles present in the medium under study for a given depth range. Reflected ultrasonic signals are correlated to solve the inverse problem of determining tissue displacements caused by shear wave propagation as a function of time and position in the medium. The study of these displacements allows solving the inverse problem of determining the shear wave propagation velocity within the viscoelastic medium and, consequently, the elasticity of tissues, as explained in the document “Transient elastography: a new non-invasive method for the assessment of hepatic fibrosis”, L. Sandrin et al., Ultrasound in Medicine and Biology, Vol. 29, pages 1705-1713, 2003.

The VCTE method is useful, in particular, because it allows time separation of the shear wave propagation and the propagation of compression waves formed simultaneously with the shear wave, two types of waves with very different propagation velocities. The compression wave propagates at a speed of approximately 1500 m/s, which can be considered infinite compared to the shear wave propagation speed, which is typically 1-10 m/s. Indeed, such a separation is important because the presence of compression waves simultaneously with the shear wave introduces a systematic error in the measurement of the shear wave propagation velocity.

One of the main limitations of the VCTE method is the difficulty in verifying that the probe is positioned correctly before performing the elasticity measurement and therefore triggering the mechanical impulse. Indeed, incorrect probe positioning can result in poor shear wave propagation or even no shear wave. For example, the propagation of a shear wave may be disturbed by the presence of reflections associated with the proximity of the boundaries of the organ under study, or may not occur at all in the presence of a liquid boundary between the probe and the medium under study. It is indeed known that shear waves do not cross liquid barriers; in particular, this occurs in the presence of ascites in the abdominal cavity. Therefore, the result of the elasticity measurement will be unreliable.

Ultrasonic waves can now be used to control the positioning of a vibrator for transient elastography. For example, ultrasound imaging or a targeting tool such as described in patent application EP2739211 A1 can be used. However, these solutions are not sufficient, since they do not allow direct prediction of incorrect shear wave propagation associated, for example, with incorrect positioning of the probe or the presence of a liquid boundary.

Among other methods of transient elastography, we can mention methods based on the formation of a shear wave by means of radiation pressure or the “Acoustic Radiation Force Impulse (ARFI)” method (radiation pressure of an ultrasonic pulse). This method is described, for example, in the document “Acoustic Radiation Force Impulse Imaging: Ex-vivo and in-vivo demonstration of transient shear wave propagation”, K. Nightingale et al., IEEE Biomedical Imaging, 2002.

Another method of transient elastography is described in the document “Supersonic Shear Imaging: A new technique for soft tissue elasticity mapping”, J. Bercoff et al., IEEE Transactions on Utrasonics, Ferroelectrics, and Frequency Control, 2004. According to this method, shear waves are formed by the force of radiation pressure by focusing the ultrasonic beam at different points in the medium, which makes it possible to obtain shear waves having a flat wave front.

However, none of the above transient elastography methods provides a simple and complete solution to the problem of probe positioning in order to obtain a measure of elasticity that is definitely reliable.

There are also methods of so-called harmonic elastography. These methods are based on the application of continuous vibration having a frequency in the range of 30 Hz to 100 Hz. Elastic waves generated inside the medium are quasi-stationary waves, superpositions of shear waves and compression waves.

Of the existing methods of harmonic elastography, the following can be mentioned:

- The method of the so-called "magnetic resonance elastography" or MRE, which uses magnetic resonance imaging to observe quasi-stationary waves generated in the medium; this method is described in the document “Magnetic resonance elastography by direct visualization of propagating acoustic strain waves”, R. Muthupillai et al., Science 269, 1995. This method is performed under the guidance of magnetic resonance imaging (MRI);

- The method of so-called sonoelastography, described, for example, in the document “A pulsed doppler ultrasonic system for making noninvasive measurements of the mechanical properties of soft tissues”, T. Krouskop, Journal of Rehabilitation Research and Development, 24, 1987. This method is performed under ultrasonography control;

- The method of the so-called "time-harmonic elastography", described, for example, in the document "In vivo time-harmonic multifrequency elastography of the human liver", H. Tzschatzsch et al., Phys. Med. Biol., 59, 2004. This method is performed under the control of ultrasonography.

Even if the above methods do not require the creation of a pulsed shear wave that propagates in the medium to be characterized, they are associated with some difficulties.

For example, harmonic elastography does not provide the ability to separate shear waves and compression waves that are generated simultaneously in the medium to be characterized. Therefore, the quasi-stationary elastic wave generated inside the medium to be characterized is a superposition of the quasi-stationary shear and compression waves. Since the speed of shear waves is much less than the speed of compression waves, the actual observed vibration speed does not correspond to the speed of the shear wave. Therefore, the effect of compression wave propagation must be taken into account in order to be able to measure the propagation velocity of shear waves. To do this, it is necessary to record complex data and calculate offsets in three spatial directions, x, y, z.

The only method of harmonic elastography that currently allows such a correction is the MRE method. However, this method requires a very complex and expensive MRI device and is therefore much more difficult to implement than the VCTE method.

Moreover, these methods are performed under the control of traditional methods such as ultrasonography or magnetic resonance imaging. Therefore, they require extensive experience from the operator, which is not conducive to widespread adoption of the technology.

In addition, the method of harmonic elastography can be applied to control methods of treatment. This approach includes, for example, the treatment of tumors, the location of which is determined by the method of harmonic elastography, using methods of hyperthermia therapy.

TECHNICAL PROBLEM

Harmonic or transient elastography methods rely on measurement control by using a conventional imaging method (ultrasoundography or magnetic resonance imaging), which requires extensive experience from the operator and does not provide optimal localization of the tissue to be characterized in relation to shear wave propagation. As a result, it is not possible to predict the reliability of the elastographic measurement that is intended to be performed. Finally, these methods are not suitable for implementation with devices that are small and easy to use.

DISCLOSURE OF THE INVENTION

In order to at least partially solve these problems, the present invention provides a new method of elastography, which will be referred to as hybrid elastography in the remainder of this document.

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

applying, using a first vibrator contained in a probe in contact with a viscoelastic medium, continuous low-frequency vibration and generating, using an ultrasonic transducer in contact with a viscoelastic medium, a first series of ultrasonic data acquisition signals, wherein said first series of ultrasonic acquisition signals the data includes groups of ultrasonic data acquisition signals, wherein the groups of ultrasonic data acquisition signals are generated at a first pulse repetition rate, each group of ultrasonic data acquisition signals includes at least one data acquisition signal, and continuous vibration generates an elastic wave inside the viscoelastic medium;

- applying, using a second vibrator contained in a probe in contact with a viscoelastic medium, a low-frequency pulse and generating, using an ultrasonic transducer, a second series of ultrasonic data acquisition signals, wherein the ultrasonic data acquisition signals constituting the second series are generated at a second pulse repetition rate , and the low-frequency pulse forms a non-stationary shear wave propagating inside the viscoelastic medium.

In accordance with one embodiment, the continuous vibration applied by the first vibrator is terminated prior to the application of a low frequency pulse by the second vibrator and generation of a second series of ultrasonic data acquisition signals.

By hybrid elastography is meant a method for implementing the elastography method, including at least one step of applying continuous low-frequency vibration and a step of applying a low-frequency pulse. In other words, the hybrid elastography method according to the invention includes both the generation of continuous vibration, which is characteristic of the harmonic elastography method, and the generation of a low frequency pulse, which is characteristic of the transient elastography method.

Thus, the continuous low frequency vibration, which is continuous, and the low frequency pulse, whose duration is short, are different. Typically, the duration of the low frequency pulse is in the range from 1/2×tSWF to 20/tSWF, where tSWF is the center frequency of the low frequency pulse.

By continuous low-frequency vibration is meant the continuous reproduction of the wave pattern. This pattern can be, for example, an ideal sinusoid; and, in that case, it is called monochromatic vibration. Vibration can also be generated by reproducing an arbitrary pattern. According to one embodiment, the continuous vibration is interrupted during a switch to the low frequency pulse mode to terminate the measurement process, or when the measurement conditions are no longer satisfactory. The measurement conditions may be, for example, a condition imposed on the force of contact with the test medium. The center frequency of continuous low frequency vibration is usually in the range of 5-500 Hz.

An elastic wave is understood as a superposition of a compression wave and shear waves.

An ultrasonic data acquisition signal refers to the emission of an ultrasonic pulse. The mentioned emission of ultrasonic waves can be associated with the real-time reception and recording of echo signals generated by reflective particles present in the medium under study for a given depth range.

Thus, the first series of ultrasonic acquisition signals is formed by repeating groups of acquisition signals. The data acquisition signal group includes at least one ultrasonic data acquisition signal. Groups of data acquisition signals are emitted or formed at a first pulse repetition rate. The first pulse repetition rate is called the intergroup pulse repetition rate. The first pulse repetition frequency is usually in the range of 5-500 Hz.

When each group of acquisition signals is generated by at least two ultrasonic acquisition signals, the ultrasonic acquisition signals forming the same group are emitted or generated at an intra-group pulse repetition rate, typically in the range of 500 Hz to 100 kHz.

Advantageously, the use of a first low pulse repetition rate during continuous vibration application makes it possible to measure movements of viscoelastic tissue while limiting the acoustic energy directed into the same tissue so as not to exceed peak and average acoustic power limits.

The term "bias" is used in this document in a broad sense. It includes any movement parameter such as displacement, speed, strain, strain factor, strain rate, and any mathematical transformation applicable to those parameters.

A low-frequency pulse is understood as a pulse with a center frequency, usually in the range of 5-500 Hz.

By the second series of ultrasonic data acquisition signals is meant a series of ultrasonic data acquisition signals emitted or generated at a pulse repetition rate greater than 500 Hz and preferably in the range of 500 Hz to 100 kHz.

During the application of continuous vibration, an elastic wave is formed inside the viscoelastic medium.

The first series of ultrasonic data acquisition signals is used to investigate the propagation of an elastic wave within a viscoelastic medium. In this way, echoes or ultrasonic signals reflected by the viscoelastic medium can be detected, and from these reflected ultrasonic signals, displacements of the viscoelastic medium caused by the propagation of an elastic wave generated by continuous vibration inside the viscoelastic medium can be calculated.

For example, it is possible to calculate the displacements of the viscoelastic medium by correlation processing the ultrasonic acquisition signals constituting the same acquisition signal group of the first series of ultrasonic acquisition signals.

The properties of the elastic wave inside the medium can then be measured and a real-time positioning indicator calculated from the measured properties. This indicator is displayed in real time to serve as a guide to the operator. Examples of such properties are the amplitude and phase of an elastic wave measured as a function of depth into the tissue to be characterized. In addition, the phase velocity of the elastic wave can be calculated. Although the elasticity value can be derived from the phase velocity of the elastic wave, this value is different from the elasticity value derived using the impulse wave when there is a superposition of shear waves and compression waves in the process of applying continuous vibration.

In the remainder of this document, "positioning indicator" and "real-time positioning indicator" refer to the same real-time positioning indicator.

Real time means that the display of the indicator is updated periodically during the examination. In general, the refresh rate is around 20 Hz, but can be as low as 1 Hz as well.

It is important to note that continuous vibration is used to control the positioning of the probe used for hybrid elastography. For example, continuous vibration can be used to check for the presence of liver parenchyma facing the probe. It is important to note that continuous vibration is not used in lieu of a pulsed measurement; it completes this dimension. In other words, at the stage of application of continuous vibration, it is possible, but not necessarily, to indirectly measure the viscoelastic properties of the medium. This last measurement is not physically identical to elasticity in terms of modulus of elasticity, but can be correlated with this value.

The application of a low-frequency pulse generates a non-stationary shear wave propagating inside the viscoelastic medium that needs to be characterized. Shear wave propagation monitoring measures the viscoelastic properties of the tissue to be characterized, such as shear wave propagation velocity, tissue elasticity, tissue shear modulus, or tissue elastic modulus. When using the method in accordance with the invention, the measurement of the viscoelastic properties of the medium can be ignored if the positioning of the probe is unsatisfactory. In other words, the validity of the elasticity measurement can be checked a priori by the positioning indicator obtained in the continuous vibration application step.

Alternatively, the application of a low-frequency pulse can only be switched on if the correct positioning of the probe has previously been confirmed in the harmonic elastography step.

A second series of ultrasonic data acquisition signals, emitted or generated at a second pulse repetition rate, is used to investigate the propagation of a non-stationary shear wave within the viscoelastic medium to be characterized. It is possible to record ultrasonic signals reflected by a viscoelastic medium and calculate from these reflected ultrasonic signals the displacements of the viscoelastic medium caused by the propagation of a shear wave. Measuring the displacements caused in a viscoelastic medium by said propagation allows one to further solve the inverse problem of determining the speed of propagation of a shear wave and, therefore, determining the elasticity of the medium using the formula E=3ρV _s ² , where E is the elasticity or modulus of elasticity, ρ means density, and V _s is the shear rate.

Thus, the hybrid elastography method according to the invention makes it possible to check the correct positioning of the probe using the harmonic elastography method and then measure the viscoelastic properties of the medium to be characterized using the transient or pulsed elastography method. In particular, after the correct positioning of the probe has been verified, the viscoelastic properties are measured in the transient elastography step. This measurement provides a more accurate indication of the viscoelastic properties of the medium than harmonic elastography, since there is no superposition of compression and shear waves in pulsed elastography, unlike what is observed in harmonic elastography.

In other words, the first step of harmonic elastography allows you to control the positioning of the probe relative to the tissue to be characterized by providing the operator with an indicator predicting the success of the pulsed elastography measurement. After correct probe positioning has been verified, transient elastography data acquisition can be initiated, with the non-stationary shear wave propagating correctly within the medium.

Advantageously, the hybrid elastography method according to the invention makes it possible to measure the viscoelastic properties of the tissue to be characterized in a reliable and reproducible manner using the transient elastography method, while simultaneously simple and accurate probe positioning using the harmonic elastography method.

The hybrid elastography method according to the invention may also have one or more of the following characteristics, considered individually or in accordance with all their technically possible combinations:

- the hybrid elastography method according to the invention further comprises the following steps:

determining, based on the first series of ultrasonic data acquisition signals, at least one elastic wave property within the viscoelastic medium;

determining, from the second series of ultrasonic data acquisition signals, at least one property of the transient shear wave and a property of the viscoelastic medium;

- the same vibrator is used to apply continuous low frequency vibration and low frequency pulse;

- the property of the elastic wave inside the viscoelastic medium is used to calculate the real-time positioning indicator for the probe relative to the viscoelastic medium to be investigated;

the method according to the invention further includes the step of displaying a real-time positioning indicator in real time; while the refresh rate of the display has a value, for example, not lower than 5 Hz;

the step of calculating the positioning indicator and the step of displaying it are performed simultaneously;

- the stage of applying a low-frequency pulse and generating a second series of ultrasonic data acquisition signals is initiated only if the real-time positioning indicator satisfies the specified condition;

- the step of applying a low-frequency pulse and generating a second series of ultrasonic data acquisition signals is initiated automatically based on the value of the real-time positioning indicator;

- the stage of application of a low-frequency pulse and the formation of the second series of ultrasonic data acquisition signals is initiated automatically;

- the stage of application of continuous low-frequency vibration is initiated only if the contact force between the vibrator and the viscoelastic medium exceeds the specified lower threshold value;

- the stage of application of continuous low-frequency vibration is initiated only if the contact force between the vibrator and the viscoelastic medium is in the range between the specified lower threshold value and the specified upper threshold value;

- the stage of applying a low-frequency pulse is initiated only if the contact force between the vibrator and the viscoelastic medium exceeds the specified lower threshold value;

- the lower and upper threshold values of the contact force for the application of continuous vibration are usually 1 N and 10 N, respectively;

- the lower and upper threshold values of the contact force for the application of a low-frequency pulse are usually 4 N and 10 N, respectively;

- the frequency of continuous low-frequency vibration, cSWF, applied by the vibrator is in the range of 5-500 Hz;

- the amplitude of the continuous low-frequency vibration applied by the vibrator is in the range from 10 µm to 5 mm;

- the first series of ultrasonic data acquisition signals is formed by repeating groups, including at least two ultrasonic data acquisition signals having an intra-group pulse repetition rate in the range from 500 Hz to 10 kHz, and a first pulse repetition frequency in the range from 10 Hz to 10 kHz ;

- the first pulse repetition frequency has a value below the frequency of continuous vibration;

- the center frequency tSWF of the low-frequency pulse is in the range from 10 Hz to 1000 Hz;

- the pulse duration is in the range from 1/(2×tSWF) to 20/tSWF, where tSWF is the center frequency of the low frequency pulse;

- the pulse repetition frequency of the second series of ultrasonic pulses is in the range from 500 Hz to 100 kHz;

- the amplitude of the low-frequency pulse is in the range from 100 µm to 10 mm;

- the termination of the continuous vibration of the vibrator and the application of the low-frequency pulse are separated by a time interval, and the time interval can be at least 10 ms and preferably is in the range from 1 ms to 50 ms;

- the amplitude of the low-frequency pulse is determined based on the properties of the elastic wave obtained with continuous vibration.

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

- The first vibrator configured to apply continuous low frequency vibration to the viscoelastic medium, wherein the continuous low frequency vibration generates an elastic wave within the viscoelastic medium;

- The second vibrator, configured to apply to the viscoelastic medium a low-frequency pulse that generates a non-stationary shear wave inside the viscoelastic medium;

- An ultrasonic transducer capable of emitting:

of the first series of ultrasonic data acquisition signals, wherein said first series of ultrasonic data acquisition signals includes groups of ultrasonic data acquisition signals, wherein the groups of ultrasonic data acquisition signals are formed at a first pulse repetition rate, and each group of ultrasonic data acquisition signals includes at least at least one data acquisition signal;

a second series of ultrasonic data acquisition signals, wherein the ultrasonic data acquisition signals constituting the second series are generated at a second pulse repetition rate;

wherein said probe is further configured to terminate the application of continuous vibration prior to application of the low frequency pulse.

The probe according to the invention makes it possible to carry out the method according to the invention.

In accordance with one embodiment, the probe according to the invention includes a single vibrator that is used to both apply continuous vibration to a viscoelastic medium in the harmonic elastography step and to apply a low frequency pulse in the pulsed elastography step.

The probe is designed so that the application of a low-frequency pulse and the cessation of continuous vibration are separated by a time interval in the range from 1 ms to 50 ms. The time interval is preferably at least 10 ms.

The ultrasonic transducer is used to transmit the first and second series of ultrasonic data acquisition signals into the interior of the viscoelastic medium. The same ultrasonic transducer detects reflected ultrasonic signals for each ultrasonic acquisition signal. The reflected ultrasonic signals are then processed to determine displacements of the viscoelastic medium caused by continuous low frequency vibration and low frequency pulse.

The probe for hybrid elastography according to the invention may also have one or more of the following characteristics, considered individually or in accordance with all their technically possible combinations:

- the vibrator is an electric motor or a voice coil, or an electrodynamic drive;

- ultrasonic transducer is installed on the axis of the vibrator;

the hybrid elastography probe according to the invention further includes means for enabling the application of a low frequency pulse;

- the ultrasonic transducer has the shape of a circle with a diameter ranging from 2 mm to 15 mm;

- ultrasonic transducer has an operating frequency in the range from 1 MHz to 15 MHz;

- the ultrasonic transducer is a convex abdominal probe;

- the first and second vibrators are axisymmetric;

- at least one of the two vibrators is axisymmetric;

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

- at least one vibrator has an annular shape and is located around the ultrasonic transducer;

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

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

Such a hybrid device according to the invention includes:

- probe for hybrid elastography in accordance with the invention;

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

According to one embodiment, the display means is used to display a real-time positioning indicator in real time.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention will become apparent from the description which is given below by way of explanation and by no means limitation, with reference to the accompanying drawings, in which:

Fig. 1 is a representation of the steps of the hybrid elastography method according to the invention;

Fig. 2 is a schematic representation of the vibrations applied by the vibrator and the ultrasonic data acquisition signals during the method according to the invention shown in FIG. 1;

Fig. 3 is a schematic representation of a particular embodiment of the elastography method shown in FIG. 1;

Fig. 4 is a representation of the results obtained by carrying out part of the method according to the invention with respect to the positioning of the vibrator;

Fig. 5 is a representation of the results of the method shown in FIG. 1;

Fig. 6 is a representation of a hybrid elastography probe according to the invention;

Fig. 7a is a representation of a specific embodiment of a hybrid elastography probe according to the invention;

Fig. 7b is a representation of a hybrid elastography device according to the invention.

IMPLEMENTATION OF THE INVENTION

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

The first step CW of method P includes applying continuous low frequency vibration using a first vibrator contained in a probe in contact with a viscoelastic medium.

The frequency of continuous vibration is in the range of 5-500 Hz.

The first step of the CW method P includes generating, by the ultrasonic transducer, a first series of ultrasonic data acquisition signals. The first series of ultrasonic data acquisition signals includes groups of ultrasonic data acquisition signals. Groups of ultrasonic acquisition signals are emitted at a first LPRF (low pulse repetition frequency) in the range of 5 Hz to 500 Hz, with each group including at least one ultrasonic acquisition signal.

The ultrasonic data acquisition signal includes the emission of an ultrasonic pulse, followed by detection and recording of reflected ultrasonic signals or echoes.

The application of continuous vibration to a viscoelastic medium generates an elastic wave inside the same medium. An elastic wave is formed by a superposition of shear waves and compression waves. The study of the properties of this elastic wave makes it possible to obtain information about the correct positioning of the probe relative to the viscoelastic medium.

The viscoelastic medium to be characterized dissipates, at least partially, the ultrasonic pulses. Therefore, it is possible to detect ultrasonic signals reflected during the emission of the first series of ultrasonic acquisition signals.

Detection of reflected ultrasonic signals can be performed using the same ultrasonic transducer that is used for radiation.

The reflected ultrasonic signals detected in the step CW of generating the first series of ultrasonic data acquisition signals are processed in the step CW_P of determining at least one property of the elastic wave within the viscoelastic medium.

At this stage, the reflected ultrasonic signals are correlated with each other in order to measure the displacements of the viscoelastic medium caused by the elastic wave generated by the application of continuous vibration, in accordance with a method known in the field of elastography and, more generally, ultrasound research.

From the measured displacements within the viscoelastic medium, it is possible to calculate the properties of the elastic wave, such as amplitude and phase, depending on the position within the viscoelastic medium. The position of a point within the viscoelastic medium is measured as the distance between the ultrasonic transducer, and said point is calculated along the propagation direction of the ultrasonic waves emitted by the transducer. For this reason, the position of a point within a viscoelastic medium is generally referred to as depth.

In addition, other parameters of the elastic wave inside the viscoelastic medium can be determined, such as the phase velocity of the elastic wave.

Changes in the amplitude and phase of the elastic wave can be calculated as a function of the depth within the tissue. By performing a theoretical model correction on the measured properties, a correction quality parameter can be calculated. Based on this correction quality parameter and/or other properties of the elastic wave, a real-time positioning indicator RT_IP of the vibrator relative to the tissue to be characterized can be calculated.

Due to the use of a low first pulse repetition rate LPRF for the first series of ultrasonic acquisition signals, a real-time real-time positioning indicator RT_IP can be calculated.

According to one embodiment, the real-time positioning indicator RT_IP is displayed simultaneously with its calculation. In other words, the real-time positioning 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 executed simultaneously.

For example, one of the theoretical models used is to linearly change the phase delay at the center frequency of the elastic wave as a function of depth in the medium to be characterized. In this case, the correction is a linear correction, and the correction quality parameter conveys phase linearity with depth in the medium. A possible indicator is the coefficient R ² determination, which determines the quality of the prediction of the linear regression of the phase delay curve depending on the depth in the range of the investigated depths.

According to one embodiment, the step CW_P of determining at least one property of the elastic wave inside the tissue is performed simultaneously with the step of applying continuous vibration CW and detecting the first reflected ultrasonic signals.

By means of the method P according to the invention, it is possible to measure in real time the properties of the elastic wave inside the tissue and obtain in real time a real time positioning indicator RT_IP for the probe.

As an advantage, the low first pulse repetition rate LPRF makes it possible to reduce the amount of data recorded in the CW step of generating the first series of ultrasonic acquisition signals and process this data in real time to obtain a positioning indicator RT_IP.

If the value of the positioning indicator is unsatisfactory, then the two steps CW and CW_P are repeated as shown by the dashed arrow in FIG. 1.

If the positioning indicator value is satisfactory, then the probe is positioned correctly in relation to the viscoelastic medium and the elasticity measurement made in the transient elastography step will be reliable. In this case, the method P in accordance with the invention involves the transition to step TI.

According to one embodiment, the method P according to the invention includes the step of displaying a real-time positioning indicator RT_IP. The calculation of the positioning indicator RT_IP and its display are performed simultaneously.

In accordance with one embodiment, the refresh rate of the visual display of the positioning indicator is at least 5 Hz.

The TI step shown in FIG. 1 includes applying a low frequency pulse using a second vibrator.

Just as in any transient elastography method, the application of a low-frequency pulse to a viscoelastic medium generates a non-stationary or pulsed shear wave propagating within the medium. By measuring the propagation velocity of a non-stationary shear wave inside the medium to be characterized, it is possible to solve the inverse problem of determining the elasticity of the medium.

It is important to note that during the application of the low frequency pulse and subsequent steps, the continuous low frequency vibration stops. Stopping continuous vibration during the transient elastography step is very important to allow separation of compression and shear waves in time, which makes it possible to obtain a reliable measurement of the elasticity of the medium.

According to one embodiment, between the termination of the continuous vibration and the application of the low frequency pulse, there is a time interval ranging from 1 ms to 50 ms, and preferably not less than 10 ms. This time interval allows the attenuation of compression waves generated by continuous vibration and increases the accuracy and reliability of measuring such a viscoelastic property as the speed of a non-stationary shear wave.

Simultaneously with the application of the low frequency pulse, the TI step includes generating, using an ultrasonic transducer, a second series of ultrasonic data acquisition signals emitted at a second pulse repetition frequency VHPRF (very high frequency).

The pulse repetition frequency VHPRF of the second series of ultrasonic data acquisition signals is in the range from 500 Hz to 100 kHz.

From the reflected ultrasonic signals detected in step TI, at least one property of the viscoelastic medium can be calculated in step Ti_P of the method P according to the invention. This can be done by applying the correlation methods widely known in elastography. In particular, as explained, for example, in the document "Transient elastography: a new non-invasive method for assessment of hepatic fibrosis", L. Sandrin et al., it becomes possible to calculate the speed of propagation of a shear wave and, therefore, the elasticity of a viscoelastic medium.

For example, in the step TI_P of determining the properties of the viscoelastic medium, the propagation velocity of the pulsed shear wave generated by the low-frequency pulse is determined. Using the speed of propagation of a shear wave, one can solve the inverse problem of determining the elasticity, shear modulus, or modulus of elasticity of a viscoelastic medium.

In accordance with one embodiment, the steps of applying a low frequency pulse and generating a second series of ultrasonic data acquisition signals are initiated only if the positioning indicator satisfies a predetermined condition.

As an advantage, this makes it possible to initiate only reliable elasticity measurements, since the existence of a non-stationary shear wave and its correct propagation is ensured by the position indicator.

The initiation of the low frequency pulse application steps and subsequent steps may be automatic or manual and is activated, for example, by the operator based on the value of the positioning indicator RT_IP.

If the application of a low frequency pulse is initiated by the operator, then the position indicator calculated in real time in the CW_P step is displayed in real time.

According to one embodiment, a simpler "positioning acceptable" or "unacceptable positioning" signal may be displayed to the operator.

In accordance with one embodiment, the refresh rate of the visual display of the positioning indicator is greater than 5 Hz.

This allows the operator to initiate an elasticity measurement from the point at which the correct shear wave propagation is observed to ensure measurement validity.

According to one embodiment, continuous vibration is only enabled if the contact force between the vibrator and the viscoelastic tissue is above a predetermined threshold, which is typically 1 N.

According to one embodiment, continuous vibration is only enabled if the contact force between the vibrator and the viscoelastic tissue is below a predetermined threshold, which is typically 10 N.

Advantageously, the lower threshold ensures satisfactory coupling between the probe and the viscoelastic medium, and the upper threshold eliminates continuous vibration distortion caused by excessive contact force and damage to the medium under test.

According to one embodiment, the low frequency pulse is only turned on if the contact force between the vibrator and the viscoelastic tissue is between a predetermined lower threshold and a predetermined upper threshold. These two threshold values are usually 4 N and 8 N, respectively.

Advantageously, the lower threshold ensures satisfactory coupling between the probe and the viscoelastic medium, and the upper threshold eliminates continuous vibration distortion caused by excessive contact force and damage to the medium under test.

Due to the continuous oscillatory movement of the vibrator, determining the contact force between the vibrator and the medium is more difficult than in the case of the standard method of transient elastography. In the presence of continuous low-frequency vibration, the contact force between the vibrator and the viscoelastic medium is determined by the following formula:

F =k(x+ A ×cos(2 πf _low ∙t))

In this formula, x means the displacement of the vibrator, k is the constant of elasticity of the spring placed in the probe, A means the amplitude of continuous vibration, and f _low is the frequency of continuous vibration.

The force F can be measured using a force sensor placed on the hybrid elastography probe. Consistently, by applying a low-pass filter to process the signal measured in this way, it is possible to eliminate the low-frequency component and calculate the average contact force:

FAverage =k(x) _.

In accordance with one embodiment of the method P in accordance with the invention, the low frequency pulse is turned on only if the F _Average value exceeds a predetermined threshold value.

As an advantage, the application of a minimum value of the contact force allows a satisfactory application of the low frequency pulse to the viscoelastic medium and the correct propagation of the non-stationary shear wave formed within the medium.

According to one embodiment of the method P according to the invention, the cessation of continuous vibration of the vibrator and the application of the low frequency pulse are separated by a time interval in the range of 1 ms to 50 ms. The time interval preferably has a value of at least 10 ms.

As an advantage, the use of a time interval separating the cessation of the continuous vibration and the application of the low frequency pulse allows the damping of the vibration generated by the continuous vibration. Therefore, it is possible to apply a low-frequency pulse and observe the propagation of a pulsed shear wave in the absence of an elastic wave. The simultaneous presence of an elastic wave, including a compression wave and a non-stationary shear wave, can introduce an error into the measurement of the propagation velocity of the non-stationary shear wave.

Fig. 2 schematically depicts:

- Continuous low frequency vibration cSW applied by the first vibrator in the CW step shown in FIG. 1;

- The low frequency pulse tSW applied by the second vibrator in the TI step shown in FIG. 1;

- The first series of ultrasonic data acquisition signals PA generated by groups G of ultrasonic data acquisition signals and generated by the ultrasonic transducer in the step CW shown in FIG. 1;

- The second series of ultrasonic data acquisition signals DA generated by the ultrasonic transducer in the TI step shown in FIG. 1.

During the CW stage of continuous vibration application, the vibrator oscillates at a frequency in the range of 5-500 Hz, with an amplitude in the range of 10 µm to 5 mm.

As an advantage, due to the low amplitude and low frequency of continuous vibration, the operator can easily keep the probe in contact with the viscoelastic medium.

According to one embodiment, the same vibrator can be used to apply a continuous low frequency vibration cSWF and a low frequency pulse tSWF.

Simultaneously with the application of continuous low frequency vibration, the ultrasonic transducer emits a first series of ultrasonic data acquisition signals PA formed by groups G of ultrasonic data acquisition signals. In the example shown in FIG. 2, each group G includes two ultrasonic acquisition signals.

Groups G of ultrasonic data acquisition signals are emitted at a first pulse repetition rate LPRF in the range of 10 Hz to 500 Hz. Acquisition ultrasonic signals belonging to the same group G are emitted at an intra-group HPRF ranging from 500 Hz to 10 kHz.

The ultrasonic transducer also detects the ultrasonic signals reflected in the process of generating the ultrasonic data acquisition signals PA, as explained with reference to the CW step shown in FIG. 1. From the first series of ultrasonic data acquisition PA, it is possible to calculate, in the correlation step Corr between ultrasonic signals belonging to the same group G, the displacements caused in a viscoelastic medium by the propagation of an elastic wave generated by a continuous vibration applied by a vibrator.

Advantageously, by applying the correlation method to ultrasonic data acquisition signals belonging to the same group G and therefore contiguous in time, small offsets of the order of 1 µm to 10 µm can be detected.

As explained with reference to the CW_P step shown in FIG. 1, the displacements of the viscoelastic medium are then used to calculate the properties of the elastic wave, such as the change in their amplitude and their phase as a function of depth in the medium. By comparing the measured properties with the theoretical model, the positioning indicator RT_IP can be calculated in real time.

For example, the positioning indicator may be related to the linearity of the phase of the elastic wave as a function of depth in the medium to be characterized. Then the indicator depends on the quality of the correction of the phase change depending on the depth by a straight line segment.

For example, a position indicator can be associated with a decrease in elastic wave amplitude with depth in the medium to be characterized. The indicator then depends on the quality of the fit to 1/Z ⁿ , where Z is the depth and n is an integer exponent ranging from 1 to 3.

For example, the value of the real-time positioning indicator RT_IP ranges from 0 to 1, with values close to 1 if the probe is positioned correctly relative to the viscoelastic medium of interest.

If the value of the real-time positioning indicator RT_IP is considered satisfactory, for example, above a predetermined threshold value, then the low frequency pulse application step TI is initiated.

The center frequency of the low frequency pulse tSFW is in the range from 10 Hz to 1000 Hz. The duration of the low frequency pulse is in the range from 1/(2×tSFW) to 1/tSFW.

The amplitude of the low-frequency pulse is in the range from 100 µm to 10 mm.

In accordance with one embodiment, the amplitude of the low frequency pulse may be modified based on the properties of the elastic wave measured in the CW_P step.

The amplitude of the displacements caused by the propagation of an elastic wave is measured in the region of interest. For example, consider that HA _M is the average amplitude measured in the area of interest, and HA _R is the reference average amplitude in the area of interest. Considering that the displacements caused by the propagation of an impulsive shear wave can be measured with great difficulty, it is possible to calculate the coefficient b to apply to the set value of the low frequency pulse so that the amplitude of the displacements induced is optimal. The amplitude AT of the set value of the low frequency pulse is calculated as a function of AT _R , the reference amplitude of the set value, and the coefficient b is calculated according to the equations:

And

AT = b × AT _R .

Then the set value a(t) of the low-frequency pulse is determined as follows for the pulse duration of a certain period:

.

.

Where f is the center frequency of the low frequency pulse, also denoted tSWF, and t represents time.

In accordance with one embodiment, several low frequency pulses can be generated in series, as described in patent application FR 1351405.

As described with reference to step TI of the method P according to the invention shown in FIG. 1, simultaneously with the application of the low frequency pulse and the propagation of the non-stationary shear wave, a second series of ultrasonic data acquisition signals DA at a second pulse repetition frequency VHPRF is emitted.

The second pulse repetition frequency VHPRF is in the range of 500 Hz to 100 kHz. The center frequency of each ultrasonic pulse is in the range from 1 MHz to 15 MHz.

The ultrasonic transducer also detects reflected ultrasonic signals originating from the second series of ultrasonic data acquisition signals DA, as explained with reference to the TI step shown in FIG. 1. From the second set of ultrasonic acquisition signals, the Corr correlation step can calculate the displacements of the viscoelastic medium. Said displacements of the viscoelastic medium are caused by the propagation of a non-stationary shear wave generated by a low-frequency pulse applied by a vibrator. As explained with reference to the TI_P step shown in FIG. 1, then the displacements of the viscoelastic medium are used to calculate the properties of the non-stationary shear wave. In particular, the shear wave propagation velocity VS and hence the elasticity E of the viscoelastic medium of interest can be calculated. In addition, the modulus of elasticity and/or the shear modulus of the medium can be calculated.

As shown in FIG. 2, having obtained the measurement of the elasticity E of the viscoelastic medium, it is possible to repeat the method by restarting from the step of applying continuous vibration CW, followed by the step of applying a low frequency pulse (not shown in FIG. 2).

Fig. 3 represents a specific embodiment of the steps CW and CW_P of the method P according to the invention, referred to as the strobe mode.

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

The continuous vertical lines represent groups G of ultrasonic acquisition signals forming the first series of ultrasonic acquisition signals PA. The G groups are emitted at the first pulse repetition rate LPRF. According to the stroboscopic acquisition mode, the first pulse repetition rate LPRF has a value lower than the continuous vibration center frequency cSWF.

The intragroup pulse repetition rate is in the range from 500 Hz to 100 kHz, which makes it possible to measure small displacements with a value of the order of 1 μm to 10 μm.

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

Because the pulse repetition rate LPRF in the G groups is below the continuous vibration center frequency cSW, continuous vibration measurements cSW can be taken exhaustively at the end of several oscillation periods, as indicated by the white circles.

As an advantage, the stroboscopic mode makes it possible to take measurements of continuous vibration cSW in an exhaustive manner, using the first low pulse repetition rate LPRF. By using a low pulse repetition rate, it is possible to process the reflected signals in real time and thus obtain a real-time positioning indicator RT_IP.

In accordance with one embodiment, the first pulse repetition frequency LPRF exceeds the continuous vibration center frequency cSWF. This makes it possible, for example, to collect data on two points over a period of vibration. Thus, more frequent sampling with the same number of vibration periods or an equal number of measurements with fewer vibration periods is obtained.

Fig. 4 schematically represents the results obtained by carrying out part of the method P according to the invention in relation to the positioning of the vibrator.

The CW_DISP plot shows the displacement (or any other displacement parameter, e.g. velocity, strain, strain rate) of a viscoelastic medium in the region of interest, ROI, as a function of depth Z in the medium and time T. Displacements are represented using a pseudo-color scale with lighter colors, representing the displacement along the positive direction of the D axis. The displacements are caused by continuous low frequency vibration applied by the vibrator and are measured by the ultrasonic transducer UT placed in contact with the surface of the medium, at the position Z=0.

From the measured displacement CW_DISP in a region of interest, ROI, within a viscoelastic medium, real-time information RT_INFO related to an elastic wave propagating within the medium and generated by continuous vibration can be extracted. Examples of such properties are the amplitude A and the phase Ph of an elastic wave depending on the depth inside the medium.

By comparing the measured values of A and Ph with predetermined threshold values, an indication of the positioning of the vibrator relative to the viscoelastic medium can be determined. If the value of the positioning indicator exceeds the specified threshold value, then the measurement of the elasticity of the medium by the method of transient elastography is considered reliable.

Alternatively, it is possible to obtain the correction quality parameter AJ of a theoretical model describing the amplitude and phase of an elastic wave propagating inside the medium from the measured values of A and Ph. In this case, the positioning indicator is obtained from the correction quality parameter AJ. For example, the correction quality parameter is the R² coefficient of determination, indicating the quality of the prediction of the linear regression of the phase delay curve versus depth in the depth range of interest.

According to one embodiment, the correction quality parameter AJ is in the range of 0 to 1.

After calculation, the position indicator can be displayed in the form of a number or letter, or using a color bar. Alternatively, the positioning indicator may be a simple visual indicator such as "acceptable positioning" indicating that the operator can initiate the transient elastography step.

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

The CW_DISP plot represents displacements measured in the presence of an elastic wave in a medium, as described above with reference to FIG. 4.

The RT_INFO plot represents the amplitude A and the phase Ph of a stationary wave measured in real time, as explained with reference to FIG. 4. Based on the RT_INFO chart, the positioning indicator can be calculated and displayed in real time.

The TI_DISP plot represents the measured displacements following the application of a low frequency pulse as a function of the depth D in the medium and the time T. In other words, the TI_DISP plot represents the so-called impulse elastogram. Displacements are represented using a pseudo-color scale and correspond to the propagation of a non-stationary shear wave inside a viscoelastic medium.

From the displacements TI_DISP, one can calculate the propagation velocity Vs of a non-stationary shear wave and solve the inverse problem of determining the elasticity of the medium.

As explained with reference to FIG. 1, 2 and 3, during the execution of the method P according to the invention, the CW_DISP, RT_INFO graphs and the vibrator position indicator are calculated and displayed simultaneously.

As an advantage, due to the structure of the first series of ultrasonic acquisition signals, the position indicator RT_IP as well as the RT_INFO graph can be calculated and displayed by computed in real time.

Otherwise, only the TI_DISP plot and the result of the calculation of the shear wave velocity Vs are displayed if the position of the vibrator facing the viscoelastic medium is checked and if the TI step is initiated.

Fig. 5 can also be considered as a graphical representation of the results obtained during the implementation of the method P in accordance with the invention, and displayed on the screen and taken into account by the operator during the examination or measurement.

Fig. 6 is a schematic representation of a PR hybrid elastography probe.

Probe PR includes:

- The first vibrator VIB1 configured to apply continuous low frequency vibration to the viscoelastic medium, wherein the continuous low frequency vibration generates an elastic wave within the viscoelastic medium;

- The second vibrator VIB2, configured to apply to the viscoelastic medium a low-frequency pulse that generates a non-stationary shear wave inside the viscoelastic medium;

- Ultrasonic transducer TUS, configured to emit:

a first series of ultrasonic data acquisition signals, wherein said first series of ultrasonic data acquisition signals includes groups of ultrasonic data acquisition signals, wherein the groups of ultrasonic data acquisition signals are formed at a first pulse repetition rate, and each group of ultrasonic data acquisition signals includes at least one data acquisition signal;

a second series of ultrasonic data acquisition signals, wherein the ultrasonic data acquisition signals constituting the second series are generated at a second pulse repetition rate;

wherein said probe is further configured to terminate the application of continuous vibration prior to application of the low frequency pulse.

According to the embodiment shown in FIG. 6, an ultrasonic transducer TUS is mounted on the axis of a vibrator VIB2 applying a low frequency pulse.

In accordance with one embodiment, the TUS ultrasonic transducer may be attached to the probe body using a PT probe tip.

The first vibrator VIB1 causes the PR probe to oscillate. During these oscillations, the ultrasonic transducer TUS is pressed against the viscoelastic medium, applying continuous low frequency vibration and creating an elastic wave within the medium.

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

The second vibrator VIB2 may apply a low frequency pulse to the viscoelastic medium, in accordance with several embodiments.

According to the first embodiment, the tip of the PT probe is movable and can be driven by a second vibrator VIB2. In such a case, the ultrasonic transducer TUS is pressed against the viscoelastic medium to apply vibration in the direction of arrow 2 in FIG. 6.

According to the second embodiment, the PR probe is an inertial probe with no moving parts. In this case, movement of the second vibrator VIB2 inside the PR probe causes the probe to move, and continuous or pulsed vibration is again applied by pushing the TUS transducer against the viscoelastic medium.

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

According to one embodiment, the PR probe includes a TOG control for enabling the application of a low frequency pulse, for example, during the TI step of the method according to the invention.

Fig. 7a is a schematic representation of an embodiment of a PR probe for hybrid elastography in accordance with the invention.

Probe PR includes:

- Vibrator VIB for applying continuous or pulsed vibration to a viscoelastic medium of interest;

- Ultrasonic transducer TUS for emitting ultrasonic pulses and detecting reflected ultrasonic signals.

Thus, the PR probe according to FIG. 7a includes a single vibrator designed to apply both continuous low frequency vibration and low frequency pulse.

In accordance with one embodiment, the diameter of the ultrasonic transducer is in the range of 2-15 mm.

In accordance with one embodiment, the center frequency of the ultrasonic transducer is in the range of 1 MHz to 15 MHz.

According to one embodiment, the TUS ultrasound transducer is a convex abdominal probe.

According to one embodiment of the PR probe, at least one of the vibrators 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.

In accordance with one embodiment, at least one of the probe vibrators has an annular shape and is located around the ultrasonic transducer TUS.

According to one embodiment, the probe further includes a calculator and display means for calculating and displaying a real-time position indicator RT_IP.

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

For example, the display means includes a screen and/or a position indicator.

According to one embodiment, the probe includes a positioning indicator that turns on when the probe is correctly positioned. This indicator can be a visual indicator, such as a change in the color of the diodes. Alternatively, the indicator may be an audible or tactile indicator, such as a change in vibration type or amplitude.

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

- Probe PR in accordance with the invention;

- UC central unit connected to PR probe.

The central block may include:

- Calculation tool for processing reflected ultrasonic signals;

- Screen SC for displaying the results obtained at different stages of the method P in accordance with the invention;

- ENT control or input facility for operator control of the device.

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

According to one embodiment, the screen SC is suitable for displaying the results shown in FIG. 5. The screen SC may also display in real time the positioning indicator RT_IP calculated in step CW_P of the method P according to the invention.

According to one embodiment, the central unit includes means configured to automatically turn on the application of the low frequency pulse based on the value of the positioning indicator RT_IP calculated and displayed in real time.

Claims (29)

1. Hybrid elastography method (P) comprising the following steps: applying, using a first vibrator contained in a probe in contact with a viscoelastic medium, continuous low-frequency vibration and generating (CW) using an ultrasonic transducer in contact with a viscoelastic medium, a first series of ultrasonic data acquisition signals, wherein said first series of ultrasonic the acquisition signal includes groups of ultrasonic acquisition signals, wherein the groups of ultrasonic acquisition signals are formed at a first pulse repetition rate, each group of ultrasonic acquisition signals includes at least one acquisition signal, and continuous low-frequency vibration forms an elastic wave inside a viscoelastic medium; determining (CW_P) from the first series of ultrasonic data acquisition signals at least one property of an elastic wave within the viscoelastic medium, wherein the property of the elastic wave within the viscoelastic medium serves to calculate a real-time positioning indicator (RT_IP) of the probe relative to the viscoelastic medium to be investigated; applying, using a second vibrator contained in a probe in contact with a viscoelastic medium, a low-frequency pulse and generating (TI) using an ultrasonic transducer a second series of ultrasonic data acquisition signals, wherein the ultrasonic data acquisition signals constituting the second series are generated at a second repetition rate pulses, and a low-frequency pulse forms a non-stationary shear wave propagating inside a viscoelastic medium. 2. The method according to the previous claim, characterized in that the continuous low frequency vibration applied by the first vibrator is stopped before the application of the low frequency pulse by the second vibrator and the formation of the second series of ultrasonic data acquisition signals. 3. The hybrid elastography method (P) according to one of the preceding claims, further comprising the following step: determining (TI_P) from the second series of ultrasonic data acquisition signals at least one property of a non-stationary shear wave. 4. Hybrid elastography method (P) according to one of the preceding claims, characterized in that the same vibrator is used to apply continuous low frequency vibration and low frequency pulse. 5. The hybrid elastography method (P) according to the previous claim, characterized in that it includes a real-time display step of a real-time positioning indicator (RT_IP). 6. The hybrid elastography method (P) according to the previous claim, characterized in that the step (TI) of applying a low frequency pulse and generating a second series of ultrasonic data acquisition signals is initiated only if the positioning indicator satisfies a predetermined condition. 7. The hybrid elastography method (P) according to the previous claim, characterized in that the step (TI) of applying a low frequency pulse and generating a second series of ultrasonic data acquisition signals is initiated automatically. 8. Hybrid elastography method (P) according to one of the preceding claims, characterized in that the step of applying continuous low frequency vibration is initiated only if the contact force between the vibrator and the viscoelastic medium exceeds a predetermined lower threshold value. 9. Hybrid elastography method (P) according to one of the preceding claims, characterized in that the step of applying a low frequency pulse is initiated only if the contact force between the vibrator and the viscoelastic medium is in the range between a predetermined lower threshold value and a predetermined upper threshold value. 10. The method (P) of hybrid elastography according to one of the preceding claims, characterized in that the first series of ultrasonic data acquisition signals is formed by repetition of groups, including at least two ultrasonic data acquisition signals having an intra-group frequency (HPRF) of pulse repetition in the range from 500 Hz to 10 kHz, and the first pulse repetition frequency (LPRF) is in the range from 10 Hz to 10 kHz. 11. The hybrid elastography method (P) according to one of the preceding claims, characterized in that the first pulse repetition frequency has a value below the continuous vibration frequency (cSWF). 12. The hybrid elastography method (P) according to the previous claim, characterized in that the amplitude of the low frequency pulse is determined based on the properties of the elastic wave generated inside the viscoelastic medium by continuous low frequency vibration. 13. Method (P) hybrid elastography according to one of paragraphs. 2-12, characterized in that the cessation of continuous vibration of the vibrator and the application of a low-frequency pulse are separated by a time interval of more than 10 ms. 14. Probe (PR) for hybrid elastography, including: a first vibrator configured to apply continuous low frequency vibration to the viscoelastic medium, wherein the continuous low frequency vibration generates an elastic wave within the viscoelastic medium; a second vibrator configured to apply to the viscoelastic medium a low-frequency pulse that generates a non-stationary shear wave inside the viscoelastic medium; ultrasonic transducer configured to emit: of the first series of ultrasonic data acquisition signals, and said first series of ultrasonic data acquisition signals includes groups of ultrasonic data acquisition signals, and groups of ultrasonic data acquisition signals are formed with a first pulse repetition rate, and each group of ultrasonic data acquisition signals includes at least one data acquisition signal; a second series of ultrasonic data acquisition signals, wherein the ultrasonic data acquisition signals constituting the second series are generated at a second pulse repetition rate; wherein said probe is further configured to terminate the application of continuous vibration prior to application of the low frequency pulse. 15. Probe (PR) for hybrid elastography according to the previous paragraph, characterized in that at least one vibrator has the same axis of symmetry as the ultrasonic transducer. 16. Probe (PR) for hybrid elastography according to claim 14 or 15, characterized in that at least one vibrator has an annular shape and is located around the ultrasonic transducer. 17. Probe (PR) for hybrid elastography according to one of paragraphs. 14-16, characterized in that the probe further includes means for calculating and displaying a positioning indicator (RT_IP). 18. Device (DEV) for hybrid elastography, including: probe for hybrid elastography (PR) according to claim 14; a central unit (UC) connected to the probe (PR) and including at least calculation means for processing reflected ultrasonic signals, display means (SC), and control and/or input means (ENT).

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