Hybrid elastography method, probe and device for hybrid elastography

Abstract

A hybrid elastic imaging method (P) comprising the following steps: - step (CW), applying continuous low-frequency vibration and generating a first series of ultrasonic acquisitions using an ultrasonic transducer in contact with a viscoelastic medium, the first series of ultrasonic acquisitions comprising ultrasonic acquisition groups, the ultrasonic acquisition groups being generated at a first repetition rate, each ultrasonic acquisition group comprising at least one probe, the continuous vibration generating elastic waves in the viscoelastic medium; - applying low-frequency pulses and generating (TI) a second series of ultrasonic acquisitions using the ultrasonic transducer, the ultrasonic acquisitions constituting the second series being generated at a second repetition rate, the low-frequency pulses generating transient shear waves propagating in the viscoelastic medium; stopping the continuous vibrations applied by the first vibrator before applying the low-frequency pulses.

Description

Technical Field

[0001] The present invention relates to the field of elastic imaging for determining the viscoelastic properties of a viscoelastic medium having an ultrasonic signal after ultrasonic irradiation. Firstly, the present invention relates to a hybrid elastic imaging method comprising a harmonic elastic imaging step and a transient elastic imaging step. Secondly, the present invention relates to a probe for implementing the hybrid elastic imaging method. Furthermore, the present invention relates to a hybrid elastic imaging device. The hybrid elastic imaging method according to the present invention is particularly suitable for determining the properties of a viscoelastic medium such as a human or animal liver. Background Art

[0002] Transient elastography (also called pulsed elastography) is one of the best known and effective methods for determining the elasticity of viscoelastic media. For example, transient elastography is widely used to determine the elasticity of the liver in humans or animals.

[0003] In transient elastography, a pulsed shear wave is generated and its propagation velocity is measured within the viscoelastic medium of interest. The propagation velocity of the shear wave in turn makes it possible to calculate the Young's modulus of the medium and thereby measure its elasticity.

[0004] There are several techniques for performing transient elastography.

[0005] For example, the applicant has developed and commercialized a vibration controlled transient elastography (VCTE) technique. The device implementing this technique is called It is capable of measuring the elasticity of the human liver in a fast, non-invasive and reproducible manner. In this transient elastic imaging device, shear waves are generated by a vibrator placed in contact with the medium to be characterized. The propagation of the shear waves is then monitored using a series of ultrasonic acquisitions with a high repetition rate achieved by an ultrasonic transducer. Each ultrasonic acquisition corresponds to at least one ultrasonic emission. Each ultrasonic emission can be associated with the detection and recording of the progress of an echo generated by reflecting particles present in a limited depth range of the medium under study. The reflected ultrasonic signal is processed by cross-correlation to trace back the movement of the tissue caused by 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 propagation speed of shear waves in viscoelastic media and thus reflect the elasticity of tissues, as explained in the document "Transient Elastography: a new non-invasive method for assessment of hepatic fibrosis" published by L. Sandrin et al. in Ultrasound in Medicine and Biology, Issue 29, 2003, Pages 1705-1713.

[0006] The VCTE technique is particularly advantageous because it allows temporal separation of the propagation of shear waves and the propagation of compression waves generated simultaneously with the shear waves, these two types of waves having very different propagation speeds. Compression waves propagate at about 1500 m / s, the propagation speed of shear waves being typically comprised between 1 and 10 m / s, and compression waves can be considered infinitely fast compared to shear waves. In practice, this separation is important because the presence of compression waves simultaneously with shear waves introduces systematic errors into the measurement of the propagation speed of the shear waves.

[0007] A major limitation of the VCTE technique is the difficulty in verifying the positioning of the probe before performing elasticity measurements, and therefore in triggering the mechanical pulse. Indeed, an incorrect positioning of the probe can lead to an imperfect propagation of the shear waves or even to their absence. For example, the propagation of the shear waves may be disturbed by the presence of rebounds associated with the vicinity of the edges of the organ under investigation, or the shear waves may not propagate at all in the presence of a liquid interface between the probe and the medium under investigation. Indeed, it is known that shear waves do not cross liquid obstacles; in particular in the presence of ascites in the abdomen. The measurements of elasticity obtained will therefore be invalid.

[0008] Ultrasound can now be used to guide the positioning of the vibrator for transient elastography. For example, ultrasound imaging or aiming tools such as those described in patent application EP2739211A1 can be used. However, these solutions are not satisfactory because they cannot directly predict incorrect propagation of shear waves associated with, for example, incorrect positioning of the probe or with the presence of a liquid interface.

[0009] Among other transient elastic imaging techniques, one can cite techniques based on the generation of shear waves by radiation force or "acoustic radiation force impulse" - ARFI. This technique is described, for example, in the document "Acoustic Radiation Force Impulse Imaging: Ex-vivo and in-vivo demonstration of transient shear wave propagation" by K. Nightingale et al., published in IEEE Biomedical Imaging, 2002.

[0010] Another transient elastic imaging technique is described in the document “Supersonic Shear Imaging: A new technique for soft tissue elasticity mapping” by J. Bercoff et al., published in IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 2004. According to this technique, shear waves are generated by radiation force achieved by focusing an ultrasonic beam at different points of a medium, which makes it possible to obtain shear waves with a plane wavefront.

[0011] However, none of these transient elastography techniques provide a simple and complete solution to the problem of positioning the probe in such a way as to obtain valid elasticity measurements.

[0012] There are also so-called harmonic elastography techniques. These techniques are based on the use of continuous vibrations with a frequency comprised between 30 Hz and 100 Hz. The elastic waves generated in the medium are a superposition of quasi-standing waves, shear waves and compression waves. Among these existing harmonic elastography techniques, one may cite:

[0013] - the so-called "magnetic resonance elastography" or MRE technique, in which magnetic resonance imaging is used to visualize quasi-standing waves generated in a medium; this technique is described in the document "Magnetic resonance elastography by direct visualization of propagating acoustic strain waves" by R. Muthupillai et al., Science 269, 1995. This technique is guided by MRI;

[0014] - the so-called sonar elastography technique, which is described, for example, in the document "A pulsed dopplerultrasonic system for making noninvasive measurements of the mechanical properties of soft tissues" by T. Krouskop, Journal of Rehabilitation Research and Development, 24, 1987. This technique is guided by echographic imaging;

[0015] - 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., Phys. Med. Biol., 59, 2004. This technique is guided by echographic imaging;

[0016] Even though these techniques do not require the generation of pulsed shear waves propagating in the medium to be characterized, they still present some difficulties.

[0017] For example, in harmonic elastography, it is not possible to separate the shear waves and compression waves that are generated simultaneously in the medium to be characterized. The elastic quasi-standing waves generated in the medium to be characterized are therefore a superposition of shear quasi-standing waves and compression waves. Since the speed of the shear wave is much lower than that of the compression wave, the actual observed vibration velocity does not correspond to the speed of the shear wave. Therefore, the influence of the propagation of the compression wave must be taken into account before the propagation velocity of the shear wave can be measured. To do this, complex data must be recorded and the displacements in the three spatial directions x, y, z calculated.

[0018] The only harmonic elastography technique currently capable of performing such a 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.

[0019] Furthermore, these techniques are guided by conventional methods of the echographic or magnetic resonance imaging type and therefore require the operator to be quite specialized, which is not conducive to the widespread dissemination of this technique.

[0020] Furthermore, harmonic elastography can be used to guide therapeutic approaches, which involve, for example, treating a tumor localized by harmonic elastography by means of a hyperthermia-type approach.

[0021] Technical issues

[0022] Harmonic or transient elastography techniques rely on the guidance of measurements performed using conventional imaging techniques (echo-image imaging or magnetic resonance imaging) which require a fairly specialized operator and are based on the propagation of shear waves and do not ensure optimal positioning of the tissue to be characterized. As a result, it is impossible to predict the accuracy of the elastography measurement to be performed. Finally, these techniques are not suitable for implementation with small, easy-to-use devices. Summary of the invention

[0023] To at least partially address these problems, the present invention describes a new elastography technique, which will be referred to as hybrid elastography in the remainder of this application.

[0024] To this end, the present invention firstly relates to a hybrid elastography method comprising the following steps:

[0025] - applying continuous low-frequency vibrations using a first vibrator included in the probe in contact with the viscoelastic medium, and generating a first series of ultrasonic acquisitions using an ultrasonic transducer in contact with the viscoelastic medium, the first series of ultrasonic acquisitions comprising groups of ultrasonic acquisitions, the groups of ultrasonic acquisitions being generated at a first repetition rate, each group of ultrasonic acquisitions comprising at least one acquisition, the continuous vibrations generating elastic waves in the viscoelastic medium;

[0026] - applying low frequency pulses using a second vibrator included in the probe in contact with the viscoelastic medium, and using the ultrasonic transducer to generate a second series of ultrasonic acquisitions, the ultrasonic acquisitions constituting the second series being generated at a second repetition rate, the low frequency pulses generating transient shear waves propagating within the viscoelastic medium.

[0027] According to one embodiment, before applying the low frequency pulses through the second vibrator and generating the second series of ultrasonic acquisitions, applying continuous vibration through the first vibrator is stopped.

[0028] Hybrid elastic imaging is used to indicate a method for implementing an elastic imaging technique, the method comprising: at least one step of applying a continuous low-frequency vibration and a step of applying a low-frequency pulse. In other words, the hybrid elastic imaging method according to the present invention comprises both generating a continuous vibration and generating a low-frequency pulse, wherein generating a continuous vibration is a feature of the harmonic elastic imaging technique, and generating a low-frequency pulse is a feature of the transient elastic imaging technique.

[0029] Therefore, the difference between continuous low-frequency vibration and low-frequency pulse is that continuous low-frequency vibration is continuous while the duration of low-frequency pulse is shorter. Usually the duration of low-frequency pulse is included between 1 / 2*tSWF and 20 / tSWF, tSWF is the center frequency of the low-frequency pulse.

[0030] Continuous low-frequency vibrations are used to indicate the continuous generation of a waveform pattern. This pattern can be, for example, an ideal sine wave; this is then called single-frequency vibration. The vibrations can also consist of the recurrence of an arbitrary pattern. According to one embodiment, the continuous vibrations are interrupted during the switch to the low-frequency pulse mode in order to stop the measurement process or when the measurement conditions are no longer satisfactory. The measurement conditions can be, for example, conditions regarding the contact force with the medium under investigation. The center frequency of the continuous low-frequency vibrations is typically comprised between 5 Hz and 500 Hz.

[0031] Elastic waves are used to indicate the superposition of compression waves and shear waves.

[0032] Ultrasonic acquisition is employed to indicate the emission of an ultrasonic beam which can be associated with the detection and recording of the travel of echoes generated by reflective particles present in a defined depth range of the medium under investigation.

[0033] Thus, the first series of ultrasonic acquisitions is formed by repetitions of acquisition groups. A group of acquisitions includes at least one ultrasonic acquisition. The acquisition group is emitted or generated at a first repetition rate. The first repetition rate is also called the inter-group repetition rate. The first repetition rate is typically included between 5 Hz and 500 Hz.

[0034] When each group of acquisitions is formed of at least two ultrasound acquisitions, the ultrasound acquisitions forming the same group are emitted or generated at an intra-group repetition rate typically comprised between 500 Hz and 100 kHz.

[0035] Advantageously, using a first low repetition rate during application of continuous vibrations enables measurement of viscoelastic tissue motion while limiting the acoustic energy delivered to the same tissue to not exceed peak and average acoustic power limits.

[0036] The term "displacement" is considered in this document in a broader sense. It includes any motion parameter (such as displacement, velocity, deformation, deformation rate, deformation speed) and any mathematical transformation applied to these parameters.

[0037] Using low frequency pulses to indicate the center frequency typically includes pulses between 5 Hz and 500 Hz.

[0038] The second train of ultrasonic acquisitions is taken to refer to a series of ultrasonic acquisitions transmitted or generated at a repetition rate greater than 500 Hz and preferably comprised between 500 Hz and 100 kHz.

[0039] During the application of continuous vibration, elastic waves are generated in the viscoelastic medium.

[0040] The first series of ultrasonic acquisitions is used to study the propagation of elastic waves in the viscoelastic medium. It is able to detect echoes or ultrasonic signals reflected by the viscoelastic medium and calculate the displacement of the viscoelastic medium caused by the propagation of elastic waves generated by continuous vibration in the viscoelastic medium from these reflected ultrasonic signals.

[0041] For example, the displacement of the viscoelastic medium can be calculated by applying a cross-correlation technique to the ultrasound acquisitions making up the same set of acquisitions in the first series of ultrasound acquisitions.

[0042] The properties of the elastic waves in the medium can then be measured and a positioning 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 waves measured as a function of depth in the tissue to be characterized. The phase velocity of the elastic waves can also be calculated. However, the elasticity values ​​that can be derived from the phase velocity of the elastic waves are different from the elasticity values ​​that are subsequently derived using pulse waves given the superposition of shear waves and compression waves during the application of continuous vibrations.

[0043] "Location indicator" and "real-time location indicator" in the remainder of this document refer to the same real-time location indicator.

[0044] Real time is used to refer to an indicator whose display is regularly refreshed during the examination. Typically, the refresh rate is about 20 Hz but can also be of the order of 1 Hz.

[0045] 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 can be used to verify the presence of hepatic parenchyma facing the probe. It is important to note that continuous vibration is not used to replace the measurement performed with pulses; rather, it completes the measurement. In other words, during the step of applying continuous vibration, an indirect measurement of the viscoelastic properties of the medium is possible but not indispensable. This latter measurement is physically different from elasticity in the sense of Young's modulus, but may be cross-correlated with this value.

[0046] Applying low-frequency pulses generates transient shear waves that propagate within the viscoelastic medium to be characterized. Detection of the propagation of shear waves enables the measurement of viscoelastic properties of the tissue to be characterized, such as the propagation velocity of the shear waves, 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 measurement of the elasticity can be verified a priori by the positioning indicator obtained during the step of applying continuous vibrations.

[0047] 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.

[0048] A second series of ultrasonic acquisitions, emitted or generated at a second repetition rate, is used to study the propagation of transient shear waves in the viscoelastic medium to be characterized. The ultrasonic signals reflected by the viscoelastic medium can be recorded and the displacement of the viscoelastic medium caused by the propagation of the shear waves can be calculated from these reflected ultrasonic signals. The measurement of the displacement caused by the propagation in the viscoelastic medium then makes it possible to trace back the propagation speed of the shear waves and thus use the formula E=3ρV s 2 The elasticity of the back-tracking medium, where E is the elastic or Young's modulus, ρ is the density, V s is the shear rate.

[0049] The hybrid elastography method according to the invention thus makes it possible to verify the positioning of the probe using the harmonic elastography technique and then to measure the viscoelastic properties of the medium to be characterized by using the transient or pulse elastography technique. In particular, once the positioning of the probe has been verified, the measurement of the viscoelastic properties is performed during the transient elastography step. This measurement provides more accurate values ​​of the viscoelastic properties of the medium than with harmonic elastography, since, unlike what is observed in harmonic elastography, in pulse elastography the compression and shear waves are not superimposed.

[0050] 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 indicators predicting the success of the measurements made by pulsed elastography. Once the positioning of the probe is verified, the acquisition can be triggered in transient elastography, the transient shear wave correctly propagating within the medium.

[0051] Advantageously, the hybrid elastography method according to the invention enables the measurement of the viscoelastic properties of the tissue to be characterized to be performed in a reliable and reproducible manner using the transient elastography technique while positioning the probe in a simple and accurate manner thanks to the harmonic elastography technique.

[0052] The hybrid elastography method according to the invention may also have one or more of the following features, considered individually or in all technically possible combinations thereof:

[0053] The hybrid elastic imaging method according to the present invention further comprises the following steps:

[0054] o determining at least one characteristic of elastic waves within the viscoelastic medium from the first series of ultrasonic acquisitions;

[0055] o determining at least one characteristic of the transient shear wave and a characteristic of the viscoelastic medium from the second series of ultrasonic acquisitions;

[0056] - The same vibrator is used to apply both continuous low-frequency vibration and low-frequency pulses;

[0057] - the properties of elastic waves in the viscoelastic medium are used to calculate real-time positioning indicators of the probe relative to the viscoelastic medium to be investigated;

[0058] - The method according to the present 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;

[0059] - The step of calculating and the step of displaying the positioning indicator and its display are performed together.

[0060] - the step of applying the low frequency pulse and generating a second series of ultrasonic acquisitions is triggered only if the real-time positioning indicator satisfies a predetermined condition;

[0061] - the step of applying the low frequency pulses and generating a second series of ultrasonic acquisitions is automatically triggered based on the value of the real-time positioning indicator;

[0062] - the step of applying low frequency pulses and generating a second series of ultrasonic acquisitions is automatically triggered;

[0063] - the step of applying continuous low-frequency vibration is triggered only if the contact force between the vibrator and the viscoelastic medium is above a predetermined lower threshold value;

[0064] - the step of applying a continuous low-frequency vibration is triggered only if the contact force between the vibrator and the viscoelastic medium is comprised between a predetermined lower threshold value and a predetermined upper threshold value;

[0065] - the step of applying the low-frequency pulses is triggered only if the contact force between the vibrator and the viscoelastic medium is above a predetermined lower threshold value;

[0066] - The upper and lower critical contact forces for applying continuous vibration are usually equal to 1N and 10N respectively;

[0067] - The upper and lower contact force thresholds for applying low-frequency pulses are usually equal to 4N and 10N respectively;

[0068] - the frequency cSWF of the continuous low-frequency vibration applied by the vibrator is comprised between 5 Hz and 500 Hz;

[0069] - the amplitude of the continuous low-frequency vibration applied by the vibrator is comprised between 10 μm and 5 mm;

[0070] the first series of ultrasound acquisitions being formed by repetitions of a group of at least two ultrasound acquisitions, the group of at least two ultrasound acquisitions having an intra-group repetition rate comprised between 500 Hz and 10 kHz and a first repetition rate comprised between 10 Hz and 10 kHz;

[0071] - the first repetition rate is lower than the continuous vibration frequency;

[0072] - the center frequency tSWF of the low-frequency pulses is comprised between 10 Hz and 1000 Hz;

[0073] - the pulse duration is comprised between 1 / (2*tSWF) and 20 / tSWF, tSWF being the center frequency of the low-frequency pulse;

[0074] - the repetition rate of the second series of ultrasonic beams is comprised between 500 Hz and 100 kHz;

[0075] - the amplitude of the low-frequency pulses is comprised between 100 μm and 10 mm;

[0076] - the cessation of the continuous vibration 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 10 ms and is preferably comprised between 1 ms and 50 ms;

[0077] - The amplitude of the low-frequency pulse is determined based on the characteristics of the elastic waves generated by the continuous vibration.

[0078] The present invention also relates to a probe for implementing the hybrid elastic imaging method according to the present invention. The probe according to the present invention comprises:

[0079] - a first vibrator configured to apply continuous low-frequency vibration to the viscoelastic medium, the continuous low-frequency vibration generating elastic waves in the viscoelastic medium;

[0080] - a second vibrator configured to apply low-frequency pulses to the viscoelastic medium, the low-frequency pulses generating transient shear waves in the viscoelastic medium;

[0081] - an ultrasonic transducer configured to transmit:

[0082] o a first series of ultrasonic acquisitions, the first series of ultrasonic acquisitions comprising ultrasonic acquisition groups, the ultrasonic acquisition groups being generated at a first repetition rate, each ultrasonic acquisition group comprising at least one acquisition;

[0083] o a second series of ultrasonic acquisitions, the ultrasonic acquisitions constituting the second series being generated at a second repetition rate;

[0084] The probe is further configured to stop applying the continuous vibration before applying the low frequency pulse.

[0085] The probe head according to the invention makes it possible to implement the method according to the invention.

[0086] 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 pulse elastography step.

[0087] The probe is configured so that the application of the low frequency pulse and the cessation of the continuous vibration are separated by a time interval comprised between 1 ms and 50 ms. Preferably, the time interval is greater than or equal to 10 ms.

[0088] An ultrasonic transducer is used to send a first series and a second series of ultrasonic acquisitions in the viscoelastic medium. The same ultrasonic transducer detects the ultrasonic signal reflected at each acquisition. The reflected ultrasonic signal is then processed to detect the displacement of the viscoelastic medium caused by the continuous low-frequency vibration and the low-frequency pulse.

[0089] The probe for hybrid elastography according to the invention may also have one or more of the following features, considered individually or in all technically possible combinations thereof:

[0090] - the vibrator is an electric motor or an audio reel or an electric actuator;

[0091] -The ultrasonic transducer is mounted on the shaft of the vibrator;

[0092] - the hybrid elastography probe according to the present invention further comprises means for triggering the application of low frequency pulses;

[0093] - the ultrasonic transducer is circular, with a diameter comprised between 2 mm and 15 mm;

[0094] - the ultrasonic transducer has an operating frequency comprised between 1 MHz and 15 MHz;

[0095] - The ultrasound transducer is a convex abdominal probe;

[0096] - the first vibrator and the second vibrator are axisymmetric;

[0097] - at least one of the vibrators is axisymmetric;

[0098] - at least one vibrator has the same axis of symmetry as the ultrasonic transducer;

[0099] - at least one vibrator has a ring shape and is arranged around the ultrasonic transducer;

[0100] - The probe further comprises means for calculating and displaying a real time positioning indicator.

[0101] The present invention also relates to a hybrid elastography device for implementing the hybrid elastography method according to the present invention.

[0102] The mixing device according to the present invention comprises:

[0103] - a hybrid elastography probe according to the invention;

[0104] - a central unit connected to the probe and comprising at least computing means for processing the reflected ultrasound signals, display means, and control and / or input means.

[0105] According to one embodiment, the display device is used to display the real-time positioning indicator in real time. BRIEF DESCRIPTION OF THE DRAWINGS

[0106] Other features and advantages of the invention will become apparent from the following description of the invention given for illustrative purposes and not intended to be limiting, with reference to the accompanying drawings, in which:

[0107] - Figure 1 The steps of the hybrid elastic imaging method according to the present invention are shown;

[0108] - Figure 2 Schematically showing the implementation Figure 1 Vibrations applied by a vibrator and ultrasonic wave acquisition during the method according to the invention are shown;

[0109] - Figure 3 Schematically shows Figure 1 A specific embodiment of the elastic imaging method shown;

[0110] - Figure 4 shows the results obtained by implementing the part of the method according to the invention relating to the positioning of the vibrator;

[0111] - Figure 5 Show Figure 1 The results of the implementation of the method shown;

[0112] - Figure 6 Presenting a hybrid elastography probe according to the present invention;

[0113] - Figure 7a Presenting a particular embodiment of a hybrid elastography probe according to the present invention;

[0114] - Figure 7b A hybrid elastography device according to the present invention is presented. DETAILED DESCRIPTION

[0115] Figure 1 The steps of the hybrid elastography method P according to the invention are shown.

[0116] The first step CW of method P consists in applying a continuous low-frequency vibration using a first vibrator included in the probe in contact with the viscoelastic medium.

[0117] The frequency of continuous vibration is between 5 Hz and 500 Hz.

[0118] The first step CW of the method P also comprises generating a first series of ultrasonic acquisitions by means of an ultrasonic transducer. The first series of ultrasonic acquisitions comprises groups of ultrasonic acquisitions. The groups of ultrasonic acquisitions are emitted at a first repetition rate LPRF between 5 Hz and 500 Hz, each group comprising at least one ultrasonic acquisition.

[0119] Ultrasonic acquisition involves the emission of an ultrasonic beam and the subsequent detection and recording of the reflected ultrasonic signal or echo.

[0120] Continuous vibration is applied to the viscoelastic medium to generate elastic waves in the viscoelastic medium. The elastic waves are formed by the superposition of shear waves and compression waves. The study of the characteristics of this elastic wave makes it possible to obtain information about the correct positioning of the probe relative to the viscoelastic medium.

[0121] The viscoelastic medium to be characterized at least partially diffuses the ultrasound beam. It is therefore possible to detect ultrasound signals reflected during the transmission of the first series of ultrasound acquisitions.

[0122] The detection of the reflected ultrasonic signal may be performed using the same ultrasonic transducer as used for transmission.

[0123] During the step CW_P of determining at least one characteristic of an elastic wave in the viscoelastic medium, the reflected ultrasonic signals detected during the step CW of generating the first series of ultrasonic acquisitions are processed.

[0124] During this step, the reflected ultrasonic signals are cross-correlated with one another, according to techniques known in the field of elastography and more generally in ultrasound, so as to measure the displacement of the viscoelastic medium caused by the elastic waves generated by the application of the continuous vibrations.

[0125] From the measured displacements within the viscoelastic medium, properties of the elastic wave, such as amplitude and phase, can 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 position of a point within the viscoelastic medium is often referred to as depth.

[0126] Other parameters of elastic waves in viscoelastic media may also be determined, such as the phase velocity of the elastic waves.

[0127] The change in amplitude and phase of the elastic wave as a function of depth in the tissue can be calculated. By making an adjustment between the theoretical model and the measured characteristics, an adjustment quality parameter can be extracted. From this adjustment quality parameter and / or other characteristics of the elastic wave, a real-time positioning indicator RT_IP of the vibrator relative to the tissue to be characterized can be calculated.

[0128] Thanks to the low first repetition rate used for the first series of ultrasound acquisitions, the real-time positioning indicator RT_IP can be calculated in real-time.

[0129] According to one embodiment, the real-time positioning indicator RT_IP is displayed along with the calculation of the real-time positioning indicator RT_IP. 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 performed together.

[0130] 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 the 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. A possible indicator is the coefficient of determination R 2 , which gives the prediction quality of the linear regression of the phase lag curve as a function of depth in the depth range studied.

[0131] 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 a continuous vibration and detecting a first reflected ultrasonic signal.

[0132] Thanks to the method P according to the present invention, the characteristics of elastic waves in the tissue can be measured in real time and the real-time positioning indicator RT_IP of the probe can be obtained in real time.

[0133] 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 positioning indicator RT_IP.

[0134] If the value of the positioning indicator is not satisfactory, then Figure 1 As shown by the dashed arrows, repeat these two steps CW and CW_P.

[0135] If the value of the positioning indicator is satisfactory, the probe is correctly positioned relative 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.

[0136] According to one embodiment, the method P according to the invention comprises a step of displaying in real time the positioning indicator RT_IP. The technique of the positioning indicator RT_IP and its display are performed jointly.

[0137] According to one embodiment, the refresh rate of the display of the positioning indicator is greater than or equal to 5 Hz.

[0138] Figure 1 Step TI shown comprises applying low frequency pulses using a second vibrator.

[0139] As in any transient elastography technique, applying a low-frequency pulse to a viscoelastic medium generates a transient or pulsed shear wave that propagates in the medium. By measuring the propagation speed of the transient shear wave in the medium to be characterized, the elasticity of the medium can be reflected.

[0140] Importantly, it should be noted that the continuous low frequency vibration is stopped during the application of the low frequency pulses and the following steps. Stopping the continuous vibration during the real-time transient elastography step is very important to enable a temporal separation of compression and shear waves, which enables reliable measurements of the elasticity of the medium to be obtained.

[0141] According to one embodiment, there is a time interval between stopping the continuous vibration and applying the low-frequency pulse, said time interval being comprised between 1 ms and 50 ms and preferably being greater than or equal to 10 ms. This time interval allows the compression waves generated by the continuous vibration to dissipate and can improve the accuracy and reliability of the measurement of viscoelastic properties (such as the velocity of the transient shear wave).

[0142] While applying the low frequency pulses, step TI includes using the ultrasonic transducer to generate a second series of ultrasonic acquisitions transmitted at a second repetition rate VHPRF.

[0143] The repetition rate VHPRF of the second series of ultrasound acquisitions is between 500 Hz and 100 kHz.

[0144] From the reflected ultrasound signal detected during step TI, at least one property of the viscoelastic medium can be calculated during step Ti_P of the method P according to the invention. This can be achieved by applying the cross-correlation technique known in elastic imaging. In particular, as explained, for example, in the document "Transient Elastography: a new non-invasive method for assessment of hepatic fibrosis" by L. Sandrin et al., the propagation velocity of the shear wave and thus the elasticity of the viscoelastic medium can be calculated.

[0145] For example, during the step TI_P of determining the properties of the viscoelastic medium, the propagation speed of the pulsed shear wave generated by the low-frequency pulse is determined. From the propagation speed of the shear wave the elasticity, shear modulus or Young's modulus of the viscoelastic medium can be retrieved.

[0146] According to one embodiment, the steps of applying the low frequency pulses and generating the second series of ultrasound acquisitions are triggered only if the positioning indicator meets a predetermined condition.

[0147] Advantageously, this enables triggering of only valid elastic measurements, since the positioning indicator ensures the presence of the transient shear wave and its correct propagation.

[0148] The triggering of the step of applying low-frequency pulses 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.

[0149] If the application of the low frequency pulses is initiated by the operator, the positioning indicator calculated in real time during step CW_P is displayed in real time.

[0150] According to one embodiment, a simpler signal of the "position OK" or "position NOT OK" type may be displayed to communicate to the operator.

[0151] According to one embodiment, the refresh rate of the display of the location indicator is greater than 5 Hz.

[0152] This allows the operator to trigger the elasticity measurement from the moment the correct propagation of the shear wave is observed, thus ensuring the validity of the measurement.

[0153] According to one embodiment, the continuous vibration is triggered only if the contact force between the vibrator and the viscoelastic tissue is above a predetermined threshold value (typically 1 N).

[0154] According to one embodiment, the continuous vibration is triggered only when the contact force between the vibrator and the viscoelastic tissue is below a predetermined critical value (typically 10N).

[0155] Advantageously, the lower critical value ensures sufficient coupling between the probe and the viscoelastic medium, and the upper critical value avoids excessive contact forces that would cause continuous vibration deformation and damage to the medium under investigation.

[0156] According to one embodiment, the low-frequency pulses are triggered only when 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 threshold values ​​are typically 4N and 8N, respectively.

[0157] Advantageously, the lower threshold ensures sufficient coupling between the probe and the viscoelastic medium, and the upper threshold avoids excessive contact forces that would cause low-frequency pulses to deform and damage the medium under investigation.

[0158] Due to the continuous vibration motion of the vibrator, the determination of the contact force between the vibrator and the medium is more complicated than in the case of the standard transient elastography method. In the presence of continuous low-frequency vibrations, the contact force between the vibrator and the viscoelastic medium is given by the following formula:

[0159]

[0160] In this formula, x is the displacement of the vibrator, k is the elastic constant of the spring placed in the probe, A is the amplitude of the continuous vibration, and f low is the continuous vibration frequency.

[0161] The force F can be measured using a force sensor placed on the hybrid elastography probe. Then by applying a low-pass filter to the signal measured by the force sensor, the low-frequency component can be removed and the average contact force can be derived:

[0162] F Average = k(x)

[0163] According to one embodiment of the method P according to the invention, the low-frequency pulses are only Average The trigger is when the value of is higher than the predetermined threshold.

[0164] Advantageously, the use of a minimum contact force value makes it possible to ensure a good transfer of the low-frequency pulses into the viscoelastic medium and a correct propagation of the transient shear waves generated in the medium.

[0165] According to one embodiment of the method P of the invention, the cessation of the continuous vibration of the vibrator and the application of the low-frequency pulses are separated by a time interval comprised between 1 ms and 50 ms. Preferably, the time interval is greater than or equal to 10 ms.

[0166] Advantageously, the use of a time interval separating the cessation of the continuous vibration and the application of the low frequency pulse enables the vibrations generated by the continuous vibration to be attenuated. It is thus possible to apply the low frequency pulse and observe the propagation of the pulsed shear wave in the absence of the elastic wave. The co-presence of elastic waves including compression waves and transient shear waves may introduce errors into the measurement of the propagation velocity of the transient shear wave.

[0167] Figure 2 Schematically showing:

[0168] -exist Figure 1 The continuous low-frequency vibration cSW applied by the first vibrator during the step CW shown;

[0169] -exist Figure 1 The low frequency pulse tSW applied by the second vibrator during step TI shown;

[0170] -exist Figure 1 During the step CW shown, a first series of ultrasonic acquisitions PA is generated by the ultrasonic transducer in the form of a detection group G;

[0171] -exist Figure 1 The second series of ultrasonic waves DA generated by the ultrasonic transducer during step TI is shown.

[0172] During the step of applying continuous vibration CW, the vibrator oscillates at a frequency between 5 Hz and 500 Hz with an amplitude between 10 μm and 5 mm.

[0173] Advantageously, thanks to the low amplitude and low frequency of the continuous vibration, the operator can easily keep the probe in contact with the viscoelastic medium.

[0174] According to one embodiment, the continuous low frequency vibration cSWF and the low frequency pulses tSWF may be applied using the same vibrator.

[0175] While applying continuous low-frequency vibration, the ultrasonic transducer emits a first series of ultrasonic acquisition PA formed in the form of ultrasonic acquisition groups G. Figure 2 In the example shown, each group G includes two ultrasound acquisitions.

[0176] The ultrasound acquisitions of the group G are transmitted at a first repetition rate LPRF between 10 Hz and 500 Hz. The ultrasound acquisitions belonging to the same group G are transmitted at an intra-group repetition rate HPRF between 500 Hz and 10 kHz.

[0177] The ultrasonic transducer also detects the ultrasonic signal reflected during the generation of the ultrasonic acquisition PA, as shown in FIG. Figure 1From the first series of ultrasonic acquisitions PA, the displacements generated in the viscoelastic medium by the propagation of elastic waves generated by the continuous vibrations applied by the vibrator can be calculated by means of the cross-correlation Corr step between the ultrasonic signals belonging to the same group G.

[0178] Advantageously, by applying a cross-correlation technique to ultrasound acquisitions belonging to the same group G, and therefore close in time, it is possible to detect small displacements of the order of 1 μm to 10 μm.

[0179] As reference Figure 1 As explained in the illustrated step CW_P, the displacement of the viscoelastic medium is then used to calculate the properties of the elastic waves, such as the change in the elastic wave amplitude and the elastic wave 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 derived in real time.

[0180] For example, the positioning indicator can be associated with the 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.

[0181] For example, a localization indicator may be associated with a decrease in the amplitude of an elastic wave as a function of depth in the medium to be characterized. The indicator then depends on the position of the wave at 1 / Z n The quality of the fit in , where Z is the depth and n is an integer coefficient between 1 and 3 included.

[0182] For example, the value of the real-time positioning indicator RT_IP is between 0 and 1, wherein the value is close to 1 if the probe is correctly positioned relative to the viscoelastic medium of interest.

[0183] If the value of the real-time positioning indicator RT_IP is considered satisfactory, for example greater than a predetermined critical value, a step TI of applying a low-frequency pulse is triggered.

[0184] The center frequency of the low-frequency pulse tSWF is between 10 Hz and 1000 Hz, and the duration of the low-frequency pulse is between 1 / (2*tSWF) and 1 / tSWF.

[0185] The amplitude of the low frequency pulses is between 100 μm and 10 mm.

[0186] 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.

[0187] The amplitude of the displacement caused by the propagation of elastic waves is measured in the region of interest. For example, consider the average amplitude HA measured in the region of interest M and the reference mean amplitude HA in the region of interestR 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 factor b to be applied to the set point of the low-frequency pulse so that the amplitude of the resulting displacement is optimal. The amplitude AT of the set point of the low-frequency pulse is based on the reference amplitude AT as the set point R The following equation is used to calculate the function of the coefficient b:

[0188]

[0189] and

[0190] AT=b×AT R

[0191] The set point a(t) of the low frequency pulse is then defined for the time period of the pulse in the following way:

[0192]

[0193] Where f is the center frequency of the low frequency pulse, also denoted as tSWF, and t is time.

[0194] According to one embodiment, as described in patent application FR 1351405, several low-frequency pulses are generated in succession.

[0195] As reference Figure 1 According to step TI of method P of the present invention shown in , while applying low-frequency pulses and propagating transient shear waves, a second series of ultrasonic acquisitions DA are emitted at a second repetition rate VHPRF.

[0196] The second repetition rate VHPRF is between 500 Hz and 100 kHz. The center frequency of each ultrasound beam is between 1 MHz and 15 MHz.

[0197] The ultrasonic transducer also detects the reflected ultrasonic signal obtained from the second series of ultrasonic acquisition DA, as shown in FIG. Figure 1 The steps TI shown are explained. From the second series of ultrasonic acquisitions, the displacement of the viscoelastic medium can be calculated by the cross-correlation Corr step. The displacement of the viscoelastic medium is caused by the propagation of transient shear waves generated by the low-frequency pulses applied by the vibrator. As shown in reference Figure 1 As explained in the steps TI_P shown, the displacement of the viscoelastic medium is then used to calculate the properties of the transient shear wave. In particular, the propagation velocity V of the shear wave can be calculated. S And thus calculate the elasticity E of the viscoelastic medium of interest. It is also possible to calculate the Young's modulus and / or the shear modulus of the medium.

[0198] like Figure 2As shown, after the measurement of the elasticity E of the viscoelastic medium has been obtained, it is possible to restart the step of applying continuous vibration CW and then the step of applying low-frequency pulses ( Figure 2 The method is repeated (not shown).

[0199] Figure 3 A particular implementation of steps CW and CW_P of method P according to the invention is shown, referred to as stroboscopic mode.

[0200] 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 50 Hz corresponding to a period of 20 ms, for example.

[0201] The continuous vertical lines represent the ultrasonic acquisition groups G forming the first series of ultrasonic acquisitions PA. The groups G are emitted at a first repetition rate LPRF. According to the stroboscopic acquisition mode, the first repetition rate LPRF is less than the center frequency of the continuous vibration cSWF.

[0202] The intra-group repetition rate is between 500 Hz and 100 kHz, which enables small displacements of the order of 1 μm to 10 μm to be measured.

[0203] The white circles and arrows along the continuous vibration cSW correspond to the sampling performed by each ultrasonic acquisition group G.

[0204] Thanks to the fact that the repetition rate LPRF of group G is lower than the center frequency of the continuous oscillation cSW, the continuous oscillation cSW can be sampled in a complete manner at the end of several oscillation periods, as indicated by the white circles.

[0205] Advantageously, the strobe mode makes it possible to sample the continuous vibration cSW in a complete manner while using the first low repetition rate LPRF. The use of a low repetition rate enables real-time processing of the reflected signal and thus obtaining the positioning indicator RT_IP in real-time.

[0206] According to one embodiment, the first repetition rate LPRF is greater than the center frequency of the continuously oscillating cSWF. This enables, for example, to acquire two points per oscillation period. Thus, better sampling is obtained using as many oscillation periods or equal sampling is obtained using fewer oscillation periods.

[0207] Figure 4 The results obtained by implementing the part of the method P according to the invention relating to the positioning of the vibrator are schematically shown.

[0208] Graph CW_DISP shows the displacement (or any other motion parameter, such as velocity, deformation, deformation rate) of a viscoelastic medium in a region of interest ROI as a function of depth Z in the medium and time T. A false color scale is used to represent the displacement, with light colors representing displacements along the positive direction of axis D. The displacements are caused by continuous low-frequency vibrations applied by a vibrator and are measured by an ultrasonic transducer UT placed in contact with the surface of the medium (Z=0).

[0209] From the displacements CW_DISP measured in a region of interest ROI in the viscoelastic medium, information RT_INFO about the elastic waves generated by the continuous vibrations propagating in the medium can be extracted in real time. Examples of these characteristics are the amplitude A and the phase Ph of the elastic waves as a function of the depth in the medium.

[0210] By comparing the measured values ​​of A and Ph with predetermined threshold values, a position indicator of the vibrator relative to the viscoelastic medium can be determined. If the value of the position indicator is above the predetermined threshold value, the elasticity measurement of the medium by transient elastography is considered valid.

[0211] Alternatively, it is possible to obtain an adjustment quality parameter between the measured quantities A and Ph and a theoretical model describing the amplitude and phase of an elastic wave propagating in a medium. In this case, the positioning indicator is obtained from the adjustment quality parameter AJ. For example, the adjustment quality parameter is the coefficient of determination R 2 , which gives the prediction quality of the linear regression of the phase lag curve as a function of depth in the depth range studied.

[0212] According to one embodiment, said adjustment quality parameter AJ is comprised between 0 and 1.

[0213] Once calculated, the positioning indicator may be displayed in the form of numbers or letters or by using a color scale. Alternatively, the positioning indicator may be a simple visual indication of the "position OK" type indicating to the operator that the transient elastography procedure can be triggered.

[0214] Figure 5 The results obtained by implementing the method P according to the invention are shown.

[0215] If referenced Figure 4 As described, graph CW_DISP represents the displacement measured in the presence of elastic waves in the medium.

[0216] Figure RT-INFO shows as reference Figure 4 The amplitude A and phase Ph of the standing wave are measured in real time as explained. From the RT-INFO diagram, the positioning indicator can be calculated and displayed in real time.

[0217] Diagram TI_DISP represents the displacement measured after application of a low frequency pulse as a function of depth D in the medium and time T. In other words, diagram TI_DISP represents a pulse elastic diagram. The displacement is represented using a false color scale and corresponds to the propagation of an instantaneous shear wave in a viscoelastic medium.

[0218] From the displacement TI_DISP, the propagation velocity of the instantaneous shear wave can be calculated and reflects the elasticity of the medium.

[0219] As reference Figure 1 , 2 As explained in Figures 3 and 3, during the implementation of the method P according to the invention, the graphs CW_DISP, RT_INFO and the positioning indicator of the vibrator are calculated and displayed together.

[0220] Advantageously, thanks to the structure of the first series of ultrasonic acquisitions, the positioning indicator RT_IP and the graphic RT_INFO can be calculated and displayed in real time.

[0221] Conversely, the calculation of the propagation velocity Vs of the shear wave of diagram TI_DISP is shown only if the position of the vibrator facing the viscoelastic medium is verified and step TI is triggered.

[0222] Figure 5 It can also be considered as a graphical 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 an operator during an inspection or measurement.

[0223] Figure 6 A hybrid elastography probe PR is schematically represented.

[0224] Probe PR includes:

[0225] - a first vibrator VIB1 configured to apply continuous low-frequency vibrations to the viscoelastic medium, wherein the continuous low-frequency vibrations generate elastic waves in the viscoelastic medium;

[0226] a second vibrator VIB2 configured to apply low-frequency pulses to the viscoelastic medium, said low-frequency pulses generating transient shear waves in the viscoelastic medium;

[0227] - an ultrasonic transducer TUS, configured to transmit:

[0228] o a first series of ultrasonic acquisitions, the first series of ultrasonic acquisitions comprising ultrasonic acquisition groups, the ultrasonic acquisition groups being generated at a first repetition rate, each ultrasonic acquisition group comprising at least one acquisition;

[0229] o a second series of ultrasonic acquisitions, the ultrasonic acquisitions constituting the second series being generated at a second repetition rate;

[0230] The probe is further configured to stop applying the continuous vibration before applying the low frequency pulse.

[0231] according to Figure 6 In the embodiment shown, the ultrasonic transducer TUS is mounted on the shaft of the vibrator VIB2 which applies low frequency pulses.

[0232] According to one embodiment, the ultrasonic transducer TUS may be fixed to the body of the probe using a connector PT.

[0233] The first vibrator VIB1 oscillates the probe PR. During this oscillation, the ultrasonic transducer TUS is pushed against the viscoelastic medium, applying continuous low-frequency vibrations and generating elastic waves in the medium.

[0234] According to one embodiment, the first vibrator VIB1 for applying continuous low-frequency vibrations comprises a vibrating ring placed around the ultrasonic transducer TUS or around the probe tip PT.

[0235] The second vibrator VIB2 may apply low frequency pulses to the viscoelastic medium according to several embodiments.

[0236] According to a first embodiment, the probe tip PT is movable and can be actuated by the second vibrator VIB2. The ultrasonic transducer TUS is then pushed against the viscoelastic medium to move along the Figure 6 Vibration is applied in the direction of arrow 2.

[0237] 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 inside the probe PR causes the movement of the probe and again applies a continuous or pulsed vibration by pushing the transducer TUS against the viscoelastic medium.

[0238] The movement axis A of the vibrator is the symmetry axis of the ultrasonic transducer TUS. For example, the ultrasonic transducer TUS may have a circular cross section, with the axis A passing through the center of the ultrasonic transducer TUS.

[0239] According to one embodiment, the probe head 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.

[0240] Figure 7a An embodiment of a probe head PR for hybrid elastography according to the invention is schematically represented.

[0241] Probe PR includes:

[0242] - a vibrator VIB for applying continuous or pulsed vibrations to the viscoelastic medium of interest;

[0243] - Ultrasonic transducer TUS for emitting an ultrasonic beam and detecting the reflected ultrasonic signal.

[0244] Therefore, according to Figure 7a The probe PR comprises a single vibrator intended to apply both continuous low frequency vibrations and low frequency pulses.

[0245] According to one embodiment, the diameter of the ultrasound transducer is comprised between 2 mm and 15 mm.

[0246] According to one embodiment, the center frequency of the ultrasound transducer is comprised between 1 MHz and 15 MHz.

[0247] According to one embodiment, the ultrasound transducer TUS is a convex belly probe.

[0248] According to one embodiment of the probe PR, at least one vibrator is axisymmetric. In other words, at least one vibrator has an axis of symmetry.

[0249] According to one embodiment, the axis of symmetry of the axisymmetric vibrator corresponds to the axis of symmetry of the ultrasonic transducer TUS.

[0250] According to one embodiment, the at least one vibrator of the probe has a ring shape and is arranged around the ultrasound transducer TUS.

[0251] According to one embodiment, the probe further comprises a calculation and display device for calculating and displaying a real-time positioning indicator RT_IP.

[0252] For example, a computing device includes at least one microprocessor and a memory.

[0253] For example, the display device includes a screen and / or a positioning indicator.

[0254] According to one embodiment, the probe includes a positioning indicator that is triggered when the probe is correctly positioned. The indicator may be a visual indicator, such as a change in the color of a diode. Alternatively, the indicator may be an audible or tactile indicator, such as a change in the type or amplitude of a vibration.

[0255] Figure 7b A hybrid elastic imaging device DEV according to the present invention is shown. The device DEV according to the present invention comprises:

[0256] - a probe PR according to the invention;

[0257] - A central unit connected to the probe head PR.

[0258] The central unit may include:

[0259] - computing means for processing the reflected ultrasonic signals;

[0260] - a screen SC for displaying the results obtained at the different steps of the method P according to the invention;

[0261] - Control or input means ENT for controlling the device by an operator.

[0262] The central unit UC may be connected to the probe PR by wired lines or by wireless communication means.

[0263] According to one embodiment, the screen SC is suitable for displaying Figure 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.

[0264] According to one embodiment, the central unit comprises means configured to automatically trigger the application of low-frequency pulses based on the value of the positioning indicator RT_IP calculated and displayed in real time.

Claims

1. A hybrid elastic imaging method (P), the method comprising the following steps: - step (CW), applying continuous low-frequency vibrations using a first vibrator included in a probe in contact with a viscoelastic medium, and generating a first series of ultrasonic acquisitions using an ultrasonic transducer in contact with the viscoelastic medium, the first series of ultrasonic acquisitions comprising groups of ultrasonic acquisitions, the groups of ultrasonic acquisitions being generated at a first repetition rate, each group of ultrasonic acquisitions comprising at least one acquisition, the continuous low-frequency vibrations generating elastic waves in the viscoelastic medium; - a step (CW_P) of determining, from the first series of ultrasound acquisitions, at least one characteristic of the elastic waves in the viscoelastic medium, said characteristic of the elastic waves in the viscoelastic medium being used to calculate a real-time positioning indicator (RT_IP) of the probe relative to the viscoelastic medium to be studied, said real-time positioning indicator indicating whether the probe is correctly positioned relative to the viscoelastic medium; -When the value of the real-time positioning indicator is greater than a predetermined critical value, a step (TI) is performed to calculate the elasticity of the viscoelastic medium by applying a low-frequency pulse using a second vibrator included in the probe in contact with the viscoelastic medium and generating a second series of ultrasonic acquisitions using the ultrasonic transducer, wherein the second series of ultrasonic acquisitions is generated at a second repetition rate; the low-frequency pulses generate transient shear waves that propagate in the viscoelastic medium.

2. The method according to claim 1, characterized in that Before applying the low frequency pulses through the second vibrator and generating the second series of ultrasonic wave acquisitions, the continuous low frequency vibration applied by the first vibrator is stopped.

3. The hybrid elastic imaging method (P) according to claim 1 further comprises the following steps: - A step (TI_P) of determining at least one characteristic of said transient shear wave from said second series of ultrasound acquisitions.

4. The hybrid elastic imaging method (P) according to claim 1, characterized in that: The continuous low frequency vibration and the low frequency pulse are applied using the same vibrator.

5. The hybrid elastic imaging method (P) according to claim 1, characterized in that: The method further comprises a step (RT_IP) of displaying the real-time positioning indicator in real time.

6. The hybrid elastic imaging method (P) according to claim 1, characterized in that: The step (TI) of applying the low frequency pulses and generating the second series of ultrasonic acquisitions is triggered only if the positioning indicator meets a predetermined condition.

7. The hybrid elastic imaging method (P) according to claim 1, characterized in that: The step (TI) of applying said low frequency pulses and generating said second series of ultrasound acquisitions is automatically triggered.

8. The hybrid elastic imaging method (P) according to claim 1, characterized in that: The step of applying continuous low-frequency vibration is triggered only if the contact force between the first vibrator and the viscoelastic medium is above a predetermined lower threshold value.

9. The hybrid elastic imaging method (P) according to claim 1, characterized in that: The step of applying low-frequency pulses is triggered only if the contact force between the second vibrator and the viscoelastic medium is comprised between a predetermined lower threshold value and a predetermined upper threshold value.

10. The hybrid elastography method (P) according to claim 1, characterized in that: The first series of ultrasound acquisitions is formed by repeating a group of at least two ultrasound acquisitions, the group of at least two ultrasound acquisitions having an intra-group repetition rate (HPRF) between 500 Hz and 10 kHz, and a first repetition rate (LPRF) between 10 Hz and 10 kHz.

11. The hybrid elastography method (P) according to claim 1, characterized in that: The first repetition rate is lower than a frequency of the continuous low frequency oscillation (cSWF).

12. The hybrid elastography method (P) according to claim 1, characterized in that: The amplitude of the low frequency pulse is determined based on the characteristics of the elastic wave generated in the viscoelastic medium by the continuous low frequency vibration.

13. The hybrid elastography method (P) according to any one of claims 2 to 12, characterized in that The stopping of the continuous low-frequency vibration of the first vibrator and the applying of the low-frequency pulses of the second vibrator are separated by a time interval greater than 10 ms.

14. A hybrid elastic imaging probe (PR), comprising: - a first vibrator, the first vibrator being configured to apply a continuous low-frequency vibration to the viscoelastic medium, the continuous low-frequency vibration generating elastic waves in the viscoelastic medium; - a second vibrator, the second vibrator being configured to apply a low-frequency pulse to the viscoelastic medium, the low-frequency pulse generating a transient shear wave in the viscoelastic medium; - an ultrasonic transducer configured to emit: a first series of ultrasonic acquisitions, the first series of ultrasonic acquisitions comprising ultrasonic acquisition groups, the ultrasonic acquisition groups being generated at a first repetition rate, each ultrasonic acquisition group comprising at least one acquisition; a second series of ultrasonic acquisitions, the ultrasonic acquisitions constituting the second series being generated at a second repetition rate; wherein the probe is further configured to stop applying the continuous low-frequency vibration before applying the low-frequency pulse, wherein the continuous low-frequency vibration and the first series of ultrasonic acquisitions are used to calculate a real-time positioning indicator (RT_IP) of the probe relative to the viscoelastic medium to be studied, the real-time positioning indicator indicating whether the probe is correctly positioned relative to the viscoelastic medium, and When the value of the real-time positioning indicator is greater than a predetermined critical value, the second vibrator is prompted to apply the low-frequency pulse to the viscoelastic medium and the ultrasonic transducer is prompted to emit the second series of ultrasonic acquisitions, wherein the low-frequency pulse and the second series of ultrasonic acquisitions are used to calculate the elasticity of the viscoelastic medium.

15. The hybrid elastic imaging probe (PR) according to claim 14, characterized in that At least one vibrator has the same axis of symmetry as the ultrasonic transducer.

16. The hybrid elastography probe (PR) according to claim 14 or 15, characterized in that: At least one vibrator is annular and is arranged to surround the ultrasonic transducer.

17. The hybrid elastography probe (PR) according to claim 14 or 15, characterized in that: It further includes means for calculating and displaying a real-time location indicator (RT_IP).

18. A hybrid elastic imaging device (DEV), comprising: - The hybrid elastography probe (PR) according to any one of claims 14 to 17; - a central unit (UC) connected to the probe (PR) and comprising at least computing means for processing the reflected ultrasound signals, display means (SC), and control and / or input means (ENT).

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