Elastography device and method

Abstract

To provide an elastography device and method.SOLUTION: An elastography device includes a probe with a single ultrasonic transducer (6); or a plurality of ultrasonic transducers, and a low frequency vibrator (5) arranged to induce displacement of the single ultrasonic transducer or the plurality of ultrasonic transducers towards a tissue (8). The device is configured to emit a sequence of ultrasonic pulses and to acquire echo signals received in response thereto in order to track how elastic waves, induced by the displacement, travel in the tissue. The device is configured to generate, for one or a plurality of the ultrasonic pulses emitted, a temporal offset upon emission (δtTX), and / or a temporal offset upon reception (δtRX), so that a difference thereof varies as a function of 2.d / vUS (d is the displacement of the single transducer or plurality of ultrasonic transducers, and vUS is the speed of ultrasonic wave in the tissue).SELECTED DRAWING: Figure 7

Description

[Technical Field]

[0001] The disclosed technologies relate to elastography devices and methods. For more details: - By moving the tip that is in contact with the tissue, elastic waves are generated that move within the tissue to be characterized. - To characterize the stiffness of the tissue, an ultrasound pulse is transmitted, a corresponding echo is received, and how this elastic wave moves within the tissue is tracked. Regarding devices configured in this way. [Background technology]

[0002] For example, liver stiffness measured by vibration-controlled transient elastography has been shown to be a very useful tool for medical professionals to detect or characterize liver disease or injury, and more generally, to monitor the condition of a subject's liver. Figure 1 schematically represents a prior art elastography device 1a configured to measure liver stiffness by vibration-controlled transient elastography. This device: - A casing 3a that can be held by hand, - A tip portion 4a that can be moved relative to the casing 3a by the low-frequency vibrator 5a, - An ultrasonic transducer 6a (which may have a sealing film covering the ultrasonic transducer) attached to the end of the tip portion 4a, Includes probe 2a equipped with

[0003] During measurement, the probe casing 3a is held by hand so that the ultrasonic transducer 6a is slightly pressed against the subject's body 8. Then, a transient displacement of the tip 4a is triggered to move the ultrasonic transducer 6a toward and away from the subject's body, thereby generating low-frequency elastic waves (particularly low-frequency shear waves) within the tissue. The corresponding displacement d(t) of the ultrasonic transducer 6a is schematically represented over time t in Figure 2, where d(t) corresponds more precisely to the position of the transducer 6a at time t along axis z directed toward the subject's body (see Figure 1) relative to the transducer 6a's initial position (the position immediately before this transient vibration is triggered). As shown in Figure 2, a sequence S of ultrasonic pulses USP is emitted from the ultrasonic transducer 6a, starting from when the low-frequency vibration is triggered. These ultrasonic pulses allow tracking how the elastic waves induced in the tissue facing the probe move within that tissue. For this purpose, two echo signals corresponding to two consecutive ultrasound shots in this sequence are correlated together to determine the distance the tissue traveled between these two pulses for different depths within the tissue.

[0004] At each time point, a strain map (sometimes called an elastogram, displacement map, or shear wave propagation map) in the tissue is determined in this way, depending on the depth z in the tissue. Figure 3 shows the strain map in the subject's liver, resulting from the application of low-frequency vibrations as shown in Figure 2, as shown in relation to time t and depth z. This spatiotemporal representation of elastic waves propagating within the tissue being examined is called an elastogram. Elastograms can visualize very clearly how such elastic waves propagate through the tissue. The propagation speed of the shear wave in this tissue can be determined from the slope of the line indicating the position of the wavefront with respect to time and depth, as shown in this figure. The stiffness of the tissue can then be estimated from this propagation speed.

[0005] When analyzing and processing the echo signals recorded during such a measurement process, it is desirable to compensate for the displacement d(t) of the ultrasonic transducer 6a. In fact, in such a device, the ultrasonic transducer 6a is fixed to the moving tip 4a itself. Therefore, as the tip 4a moves, the distance between the transducer 6a and such tissue or a portion of such tissue changes. Without compensation, the apparent depth at which such or such distortion is observed will therefore be slightly different from the depth at which this distortion actually occurs.

[0006] Furthermore, for a couple of two consecutive shots emitted (by correlating two corresponding echo signals) to determine tissue strain at a given time, the displacement of the ultrasonic transducer between these two pulses superimposed on the actual tissue displacement caused by the elastic waves propagating through it results in an apparent overall displacement of the tissue. This offset is the same at each depth, but can be easily suppressed by calculating the z-derivative of the tissue displacement determined at the time considered, as described in Section III.A (p. 440) of the paper "Shear elasticity probe for soft tissues with 1-D transient elastography" L. Sandrin et al, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, vol. 49, no. 4, pp. 436-446, April 2002. Nevertheless, as described in that paper, it is highly desirable to compensate for the transducer displacement before correlating the two recorded echo signals. In fact, without compensation for transducer movement, the displacement measured by correlation becomes significantly larger, and therefore noisy, requiring more time to determine.

[0007] To compensate for the transducer displacement d(t) before performing correlation of the echo signals, the following technique is typically employed: As described above, a sequence S of ultrasonic pulses is emitted, and the echo signals received in response are recorded. These echo signals are then post-processed by a central electronic unit 7a, which has a computer structure and is operably connected to the probe 2a. This post-processing is described, for example, in the paper mentioned above: a) Estimating the transducer displacement d(t) from the echo signal itself, b) For each echo signal, in the frequency domain, the Fourier transform of this echo signal is exp(j2πfΔt) (where f is the frequency and Δt is 2.d / v). US (v US By multiplying by (equivalent to the velocity of ultrasound in the tissue), the transducer displacement d(t) estimated in (step a) is compensated, c) Correlating echo signals to determine tissue displacement according to depth z for each time t, d) Optionally, the z-derivative ∂* / ∂ of the spatiotemporal displacement map obtained in step c). Z Calculating and Includes.

[0008] In step a), the transducer displacement d(t) can be estimated by identifying strong back reflections associated with elements assumed to have a fixed position within the tissue in each echo signal. Alternatively, the transducer displacement d(t) can be estimated by assuming that tissue displacement is negligible at very deep tissues, and that the displacement observed at deep tissues actually corresponds to the transducer displacement between two pulses.

[0009] This procedure is generally satisfactory. Nevertheless, the inventors have found that for some of the recorded echo signals, step a) may fail, resulting in an incorrect value for the transducer displacement d(t) for the echo signal in question. Thus, these echo signals are incorrectly readjusted, resulting in a few noisy, destructive sequences that impair the final elastogram. Furthermore, this procedure requires significant computational resources in terms of storage, data transfer, and calculation, and therefore limits the pulse repetition rate of the ultrasonic pulses. This limitation is further restricted in the case of vibration-controlled harmonic elastography, where vibrations (e.g., sinusoidal vibrations) are repeatedly and continuously applied to tissue, and the resulting tissue deformation is monitored in real time. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] European Patent Application Publication No. 3315074 [Patent Document 2] European Patent Application Publication No. 3769691 [Non-patent literature]

[0011] [Non-Patent Document 1] “Shear elasticity probe for soft tissues with 1-D transient elastography,” L. Sandrin et al, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, vol. 49, no. 4, pp. 436-446, April 2002. [Non-Patent Document 2] H. Loree et al., September 2020 "Vibration-Guided Transient Elastography: A Novel Fibroscan(R) Examination with Improved Guidance for Liver Stiffness Measurement", Ultrasound In Medicine and Biology, Volume 46, pp. 2193-2206 [Overview of the Initiative] [Means for solving the problem]

[0012] To at least partially solve the above problem, an elastography device is provided, and this elastography device is - A probe that is held in the body of the subject, A single ultrasonic transducer, or multiple ultrasonic transducers, of a probe positioned to emit ultrasonic pulses in the tissue to be characterized, all ultrasonic transducers are stationary relative to each other. A low-frequency vibrator, wherein the low-frequency vibrator is arranged to induce the displacement of a single ultrasonic transducer or a plurality of ultrasonic transducers toward the tissue, A probe, including - An electronic unit configured to control a single or multiple ultrasonic transducer to emit a sequence of ultrasonic pulses, and configured to acquire echo signals received by the single or multiple ultrasonic transducers in response to the emitted ultrasonic pulses, in order to track how elastic waves induced in the tissue by the displacement of the single or multiple ultrasonic transducers propagate within the tissue. Includes, - The electronic unit analyzes one or more of the emitted ultrasonic pulses. The temporal offset at the time of emission, which causes the emission of ultrasonic pulses to be shifted. and / or, the time offset at reception in which the echo signal acquired in response to the emitted ultrasonic pulse is shifted, Further configured to generate, The temporal offset during emission and / or reception is adjusted according to the displacement of a single transducer or multiple ultrasonic transducers.

[0013] In particular, the temporal offset during radiation and / or the temporal offset during reception have a difference of 2.d / v US It may be adjusted to change according to, where d is, for example, the displacement of a single transducer or multiple ultrasonic transducers during radiation, and v US This is the speed of ultrasound within the tissue.

[0014] The temporal offset during emission and / or reception may therefore be generated to compensate for a temporal shift in the echo signal relative to other acquired echo signals caused by the displacement of the ultrasonic transducer or a plurality of ultrasonic transducers during the sequence of ultrasonic pulses.

[0015] The emission of these ultrasonic pulses is temporally shifted by a temporal offset at the time of emission, by delaying these emissions in a controllable manner, for example, using a controllable delay connected upstream of the probe's ultrasonic pulser. In other words, this temporal shift is realized in the time domain at the time of emission.

[0016] Each echo signal is formed by signals received over time by the transducer after the emission of the pulse. More precisely, it begins after this emission and lasts for a predetermined duration (for example, if you want to explore tissue at a depth of 7.5 cm, ultrasound v USThis is a signal received within a predetermined time window (see, for example, Figure 4) having a duration of 100 μs, where the speed must be 1.5 mm / μs. In practice, the signals received within this time window that form the echo signal are recorded, i.e., written to the probe's memory. The time offset at reception is obtained by shifting the start of this time window (which may also be the acquisition and recording time window, as described above). In any case, this time shift is realized here in the time domain.

[0017] In the elastography device described above, temporal shifts between different echo signals caused by the movement of one or more ultrasonic transducers during measurement are first compensated for in real time, at the time of emission and / or reception, i.e., before recording the echo signals or transferring them to the remote computing unit. This compensation scheme significantly reduces the computational burden compared to the post-processing method described in the background section. As a result, the disclosed compensation scheme improves the operation of the computing unit of the elastography device.

[0018] The displacement d(t) of one or more transducers can be measured directly by a displacement sensor attached to the probe. Alternatively, it can be inferred from a command signal controlling the vibrator, and the displacement induced by the vibrator is controlled by the displacement sensor and a control loop (implemented by the control loop electronic circuit) to match this command signal. In any case, in this device, the displacement d(t), or at least a signal representing it, is readily available and does not need to be determined by post-processing the echo signal itself. In this case as well, the computational burden is significantly reduced compared to the post-processing method introduced in the background section above. Moreover, problems associated with the possible failure of estimating the displacement d from the echo signal itself (i.e., the possible failure of step a above) are avoided in the disclosed device.

[0019] In the disclosed device, echo signals can be recalibrated in time directly at the ultrasonic sequencer and ultrasonic receiver levels (eliminating the need for the complex post-processing steps described above). Since the recalibrated echo signals are available immediately downstream of the ultrasonic receiver, correlation calculations between consecutive echo signals can be achieved immediately thereafter by dedicated electronic circuitry (such as a properly programmed FPGA). This significantly reduces the amount of data transferred and stored directly from the source, making it highly suitable for real-time applications. For example, a typical echo signal contains 5000 points of 2 bytes each (for a typical echo duration of 100 μs and a sampling rate of 50 MHz), while the corresponding strain lines obtained by correlation would each contain approximately 100 points of 4 bytes. Thus, the reduction in data volume performed directly at the source is typically more than 1 / 25th. Real-time applications where such data volume reduction is effective include technologies such as those described in the paper "Vibration-Guided Transient Elastography: A Novel Fibroscan(R) Examination with Improved Guidance for Liver Stiffness Measurement" by H. Loree et al., published in September 2020 in Ultrasound In Medicine and Biology, Volume 46, pp. 2193-2206, particularly vibration-guided harmonic elastography and vibration-inductive transient elastography technologies.

[0020] Regarding temporal offset adjustment, when an ultrasonic transducer (or multiple ultrasonic transducers) is moved toward tissue with its displacement d relative to a reference position (d is positive when the transducer is brought closer to the tissue), the flight time of the emitted ultrasonic pulse is 2.d / v for round trip toward the part of tissue to be probed. USNote that it becomes shorter by . Therefore, as shown in FIG. 4, in order to obtain an echo signal that remains aligned with other echo signals despite the displacement of the transducer (or transducers), the delay between the emission of the ultrasonic pulse and the acquisition of the corresponding echo signal is 2.d / v US should desirably be decreased according to . That is exactly, in the disclosed device, the difference is 2.d / v US by adjusting the time offset at emission and / or the time offset at reception such that it varies according to .

[0021] In particular, the electronic unit of the device is configured to adjust the time offset at emission and / or the time offset at reception such that the difference between the time offset at reception and the time offset at emission is Δt0 = -2.d / v US (Δt0 is a constant delay between the emission of the ultrasonic pulse and the acquisition of the echo signal received in response thereto). "Equal" means equal within a given accuracy (since absolute accuracy cannot actually be achieved), for example, equal within 20% accuracy (which can remove most of the time shift due to displacement), or more preferably equal within 10% accuracy, or equal within 5% accuracy.

[0022] For at least some of the ultrasonic pulses emitted, the electronic unit is configured such that the time offset at emission is δt TX,o +C.d / v US (δt TX,o is a constant delay at emission), and the time offset at reception is δt[[ID=***]] RX,o -(2 - C).d / v US (δt RX,o is a constant delay at reception and C is a constant coefficient between 0 and 2) such that it is equal to and may be further configured to adjust the time offset at reception.

[0023] In particular, C may be equal to 1, in which case the time offset at the time of emission is δt TX,o +d / v US It is equal to, on the other hand, the time offset at reception is δt RX,o -d / v US This becomes equal to: In other words, the electronic unit applies an overall time shift correction (i.e., -2.d / v) between the time offset at the time of emission and the time offset at the time of reception. US ) may be configured to distribute evenly. For this reason, the correction is applied only during radiation (the temporal offset during radiation is 2.d / v US (Change it as follows), or apply correction only when receiving (set the time offset at reception to -2.d / v US This requires specialized electronic equipment and / or programming, rather than simply changing the tissue over time. However, in return, it allows for optimal and accurate sampling of the tissue's deformation over time.

[0024] In fact, to probe the tissue displacement δ at depth z and time t, the emission of ultrasonic pulses is ideally d / v US The delay is only necessary so that, despite the transducer displacement d, the emitted pulse reaches the probed position at the initially planned time.

[0025] Upon comparison, the received echo signal was -2.d / v US If only the time is shifted, while the radiation is not, the z-shift of the echo signal due to the transducer displacement is sufficiently suppressed, but the resulting tissue deformation is not at time t, but at time td / v. USThis is likely the case. In other words, the tissue displacement δ obtained by the correlation of these readjusted echo signals is the actual tissue displacement at depth z without displacement offset (i.e., with appropriate z correction), but with a slight time error. In practice, this not-so-accurate time sampling of tissue deformation can slightly distort the wavefront in the elastogram. Therefore, it is desirable to time-shift both radiation and reception according to d as described above. In this regard, it should be noted that the prior art post-processing compensation techniques shown above do not enable such accurate time sampling of tissue deformation.

[0026] As already mentioned, compensation techniques implemented in instant devices require far fewer calculations than the post-processing techniques of the prior art. In particular, they can be performed in real time by ultrasonic sequencers implemented on FPGAs.

[0027] However, in exchange, this technology requires specific equipment. In fact, the ultrasonic pulser or transducer, and / or the electronic module that controls it, must be able to time-shift the radiation and / or reception in real time in response to a variable input signal (e.g., a displacement signal). And, to the best of our knowledge, ultrasonic pulsers or transducers and associated control electronics currently on the market generally do not allow such control of the radiation / reception delay based on an external signal: they are designed to transmit pulses of a predetermined (actually, pre-recorded) fixed sequence (a sequence that can sometimes be very complex, but is still predetermined), rather than a sequence that is adjusted in real time in response to a variable input signal. Therefore, to implement the technology disclosed herein, it is necessary to develop such a specific electronic module, which has required considerable development work.

[0028] The temporal offset during emission and / or reception is adjusted based on the displacement d of a single or multiple ultrasonic transducers at the time of emission of the ultrasonic pulse (to adequately compensate for the time shift caused by this displacement). However, it can be noted that a small time margin is acceptable (the displacement d considered may not be an accurate displacement at the time of emission of this ultrasonic pulse) only for the non-zero response time of the displacement sensor and the response time of the electronics controlling the temporal offset during emission / reception (a small delay usually called "pipeline time"). For example, for a pulse emitted at time t, the displacement considered to shift this pulse (or the corresponding echo signal) may be a displacement at the same time t with 0.2 ms accuracy, or a displacement with better temporal accuracy than 1 / 30 of the period (or typical fluctuation time) of the displacement induced by a low-frequency mechanical vibrator.

[0029] The electronic unit of the device may include electronics for driving / interfacing the probe's actuators, transducers, and sensors, such as amplifiers, pulsers, switches, or converters. This electronic unit may also include control electronics for generating appropriate control signals and processing acquired signals. These control electronics may include one or more electronic logic circuits, such as a microprocessor, a digital signal processor (DSP), a system-on-a-chip, multiple system-on-a-chips, or a combination thereof. Such electronic logic circuits may be, for example, an FPGA (Field-Programmable Gate Array).

[0030] The elastography devices presented above may include one or more of the following complementary and non-limiting features, which are considered individually or in all technically possible combinations: - The device further includes a displacement sensor arranged to output a measurement signal representing the displacement of the single or multiple ultrasonic transducers, and the electronic unit is further configured to generate a time offset at the time of emission and / or a time offset at the time of reception based on the measurement signal; - The displacement sensor is an inertial sensor (such as an accelerometer) arranged so that the output measurement signal represents the displacement of the single or multiple ultrasonic transducers relative to an inertial reference frame; - The probe includes a handheld probe casing, the single or multiple ultrasonic transducers are coupled to the probe casing in a motionless manner relative to the probe casing, and the vibrator is positioned to move a mass within the probe casing to guide the displacement of the single or multiple ultrasonic transducers toward the body of the subject; - The probe includes a probe casing, the single or multiple ultrasonic transducers are movable relative to the probe casing, and a displacement sensor is positioned such that the measurement signal it provides represents the displacement of the single or multiple ultrasonic transducers relative to the probe casing.

[0031] As explained above, when adjusting the temporal offset during emission based on displacement d, it should be noted that this displacement changes over time with each ultrasonic pulse, and therefore the duration between two consecutively emitted pulses (i.e., pulse repetition period) also changes in accordance with the temporal change in d (see, for example, Figure 9). This indicates that the above compensation technique can also be implemented by adjusting the pulse repetition period based on the temporal change in d.

[0032] Thus, improvised technologies also relate to elastography devices, including: - A probe that is held in the body of the subject, A single ultrasonic transducer; or multiple ultrasonic transducers, all of which are positioned to emit ultrasonic pulses in the tissue to be characterized, are not moving relative to each other. A low-frequency vibrator, wherein the low-frequency vibrator is arranged to induce the displacement of a single ultrasonic transducer or a plurality of ultrasonic transducers toward the tissue, A probe, including - An electronic unit configured to control a single or multiple ultrasonic transducer to emit a sequence of ultrasonic pulses, and configured to acquire echo signals received by the single or multiple ultrasonic transducers in response to the emitted ultrasonic pulses, in order to track how elastic waves induced in the tissue by the displacement of the single or multiple ultrasonic transducers propagate within the tissue. Includes, - The electronic unit is further configured such that, for at least some of the emitted ultrasonic pulses, the pulse repetition period that separates an ultrasonic pulse from the next emitted ultrasonic pulse varies depending on the displacement of a single ultrasonic transducer or multiple ultrasonic transducers, and the pulse repetition period is: When a single or multiple ultrasonic transducers move away from the tissue, the reference pulse repetition period T o Compared to that, it is shorter, When a single or multiple ultrasonic transducers move toward the tissue, the reference pulse repetition period T o It will be longer compared to [the previous one].

[0033] In particular, the pulse repetition period is v / v US It may be adjusted based on the displacement d of a single ultrasonic transducer or multiple ultrasonic transducers, i.e., the displacement d

number

[0034] Adjusting the pulse repetition period in this way is d / v US It should be noted that this produces the same, or substantially the same, result as adjusting the time offset at the time of emission in accordance with d. In fact, adjusting the pulse repetition period in this way is equivalent to adjusting the time derivative of this offset at the time of emission in accordance with the time derivative of d, rather than adjusting the offset itself based on d.

[0035] In particular, the electronic unit has a pulse repetition period (within a given precision, e.g., 20% or 10%)

number

[0036] Furthermore, the electronic unit has a pulse repetition period of T o ×(1+2.v / v US The pulse repetition period may be configured to be equal to ), which corresponds to the case where the time shift to be introduced is fully introduced at the time of emission (there is no time shift at the time of reception).

[0037] More generally, an electronic unit has a pulse repetition period of T o ×(1+Cv / v US The pulse repetition period may be configured to be adjusted to be equal to ), where C is a constant coefficient between 0 and 2.

[0038] Improvised techniques also relate to elastography methods performed by devices including probes equipped with the following: - A single ultrasonic transducer; or multiple ultrasonic transducers, all of which are positioned to emit ultrasonic pulses in the tissue to be characterized, are not moving relative to each other, and - A low-frequency vibrator, arranged to guide the displacement of the ultrasonic transducer or a plurality of ultrasonic transducers toward the tissue. The method includes the following steps: - A step of controlling a low-frequency vibrator so as to guide the displacement of the ultrasonic transducer or a plurality of ultrasonic transducers toward the tissue, - A step of controlling an ultrasonic transducer or multiple ultrasonic transducers to emit a sequence of ultrasonic pulses, and obtaining echo signals received by the ultrasonic transducer or multiple ultrasonic transducers in response to the emitted ultrasonic pulses in order to track how elastic waves induced in the tissue by the displacement of the ultrasonic transducer or multiple ultrasonic transducers propagate within the tissue. - The method involves, for one or more of the emitted ultrasonic pulses, To compensate for the temporal shift of the echo signal relative to other acquired echo signals caused by the displacement of an ultrasonic transducer or multiple ultrasonic transducers occurring during a sequence of ultrasonic pulses, A step of generating a time offset in the emission time in which the emission of ultrasonic pulses is shifted, and / or, further comprising the step of generating a time offset at reception in which the echo signal acquired in response to the emitted ultrasonic pulse is shifted, The temporal offset during transmission and / or reception is such that the difference is 2.d / v US It is adjusted to change according to, where d is the displacement of the transducer or multiple transducers at the time of radiation, and v US This is the speed of ultrasound in tissue.

[0039] The features of the different embodiments of the device described above can also be applied to this elastography method.

[0040] Other features and advantages of the disclosed technology will become apparent from the description given below, with reference to the figures, for illustrative purposes only. [Brief explanation of the drawing]

[0041] [Figure 1] Figure 1 is a schematic representation of an elastography device of prior art. [Figure 2] Figure 2 shows the displacement of the ultrasonic transducer of the device in Figure 1 during a transient elastography measurement, and the sequence of ultrasonic pulses transmitted during the measurement to track how the tissue moves in response to this displacement. [Figure 3] Figure 3 is a schematic representation of the elastogram obtained using the device shown in Figure 1. [Figure 4] Figure 4 schematically shows the radiation and reception times, which are shifted in time according to the transducer displacement. [Figure 5] Figure 5 is a schematic chronogram showing the displacement of an ultrasonic transducer and the time of emission of ultrasonic pulses during a typical transient elastography measurement. [Figure 6] Figure 6 is a schematic representation of an elastography device according to the first embodiment. [Figure 7] Figure 7 is a block diagram that schematically represents the electronic unit of the device shown in Figure 6. [Figure 8] Figure 8 schematically shows the temporal offsets during radiation and reception generated by the electronic unit in Figure 7, based on the transducer displacement. [Figure 9] Figure 9 schematically shows the sequence of ultrasonic pulses emitted over time by the device in Figure 6 during a typical elastography measurement. [Figure 10]Figure 10 is a schematic and partial representation of an elastography device according to a second embodiment. [Figure 11] Figure 11 is a schematic and partial representation of an elastography device according to a third embodiment. [Modes for carrying out the invention]

[0042] As described above, the improvised technology relates to a vibration-controlled elastography device—that is, an elastography device configured to track how the elastic waves move through the medium (in other words, how the medium moves due to the vibrations exerted upon it) by moving an element that contacts the surface of the medium, such as a probe tip, to generate elastic waves that propagate through the medium to be explored, and by transmitting ultrasonic pulses through the medium and recording the echo signals received in response.

[0043] Ultrasonic pulses and corresponding echo signals are transmitted and received by one or more ultrasonic transducers, which are fixed near or even in contact with the moving element (e.g., the tip of the probe), the medium being probed. During the process of elastic measurement, these transducers, like the moving element, are displaced, which causes a temporal lag in the acquired echo signals relative to each other. The elastography device presented herein is configured to compensate for the displacement of the ultrasonic transducers or transducers in order to temporally readjust different received echo signals.

[0044] For this purpose, the temporal offset during emission (or, equivalently, the pulse repetition period between consecutive pulses) and / or the temporal offset during reception are adjusted in real time according to the displacement of the ultrasonic transducer or transducer (see, for example, Figure 7).

[0045] Three embodiments of such devices, identified by reference numerals 1;1' and 1'' respectively, are shown in Figures 6, 10, and 11, respectively.

[0046] In these three embodiments, device 1;1'';1'' includes a single ultrasonic transducer 6. However, in other embodiments, the elastography device may include multiple ultrasonic transducers. In any case, in the device according to the art disclosed herein, all ultrasonic transducers positioned to emit ultrasonic pulses in the tissue to be characterized do not move relative to one another. They move together, and their motion is characterized by a displacement d which is the same for all transducers.

[0047] The term "tissue" is understood to mean a part of the subject's body (either human or animal). This term does not necessarily specify an entire organ or a single organ. The tissue 8 to which mechanical vibrations are applied and whose deformation is tracked by ultrasonic pulses is a part of the subject's body located near the device's probe, along the probe's axis z. Hereafter, the abbreviation U / S means "ultrasonic".

[0048] In the first embodiment, device 1 (Figures 6 and 7) has a temporal offset δt at the time of radiation, depending on the displacement d of the U / S transducer 6. TX and the time offset δt at the time of reception RX By adjusting this, it is configured to compensate for the aforementioned time shift.

[0049] Device 1' according to the second embodiment is similar to that of the first embodiment, but it directly adjusts the pulse repetition period T between consecutive U / S pulses, and optionally adjusts the time offset δt at reception. RX By adjusting this, it is configured to compensate for the time shift in the problem.

[0050] In devices 1 and 1', the U / S transducer 6 is movable relative to the casing 3 of the device's probe 2. The probe includes a low-frequency vibrator 5 positioned to move the U / S transducer 6 relative to the casing 3 (a casing held by the operator) to apply low-frequency vibrations to the tissue 8.

[0051] In contrast, in device 1'' according to the third embodiment, the U / S transducer 6 is coupled to the probe casing 3 in a state where it does not move relative to the probe casing. The probe 2'' includes a vibrator 5'' positioned to move a mass 12 within the probe casing 3 and vibrate the entire probe.

[0052] However, these three embodiments are similar to one another, and identical or corresponding elements of device 1;1'';1'' are identified by the same reference number as a whole.

[0053] next, Device 1 according to the first embodiment This will be explained in more detail with reference to Figures 6 to 9.

[0054] The elastography device 1 includes the probe 2, probe casing 3, vibrator 5, and U / S transducer 6 described above. The U / S transducer 6 is fixed to the end of the probe tip 4 and is actuated by a low-frequency vibrator (see Figure 6).

[0055] In this device, the vibrator 5 is rotationally symmetric about its vibrator axis, which coincides with the probe axis z. When the vibrator 5 vibrates, a longitudinal displacement is induced, primarily parallel to its axis.

[0056] Here, the vibrator 5 is positioned to move a shaft, with its tip forming the tip 4 of the probe. This shaft is centered on axis z, and the vibrator 5 is positioned to move this shaft along axis z. The vibrator 5 is a low-frequency vibrator in that it moves its tip at a center, average frequency less than 500 Hz, or even less than 100 Hz (in contrast to ultrasonic shots or echo signals, whose center frequency is typically higher than 1 MHz, e.g., 1 to 5 MHz). The vibrator is a low-frequency electromechanical actuator comprising one or more coils and magnets, similar to, for example, the actuator of a loudspeaker. As an alternative, an electric motor such as a brushless DC motor or an electronically rectified motor may be included. Such alternatives are well suited, for example, to vibration-induced transient elastography.

[0057] The ultrasonic transducer 6 is rotationally symmetric about its transducer axis and emits an ultrasonic beam centered on this axis. The transducer axis coincides with the axis of the vibrator and therefore with the probe axis z. The ultrasonic transducer 6 has, for example, a circular section through which the axis of the vibrator passes through the center of this section. This section is small, typically less than 1 square centimeter (it may have a diameter less than 1 centimeter, or a diameter less than 8 millimeters or 5 millimeters). The transducer 6 may be covered by a sealing membrane that comes into contact with the subject's body when the probe 2 is held in place to perform the measurement.

[0058] In practice, the displacement of the ultrasonic transducer 6 induced by the vibrator 5 has a peak-to-peak amplitude between 0.1 mm and 10 mm (for example, between 0.5 and 10 mm for its own transient elastography measurement, and potentially smaller for harmonic vibrations employed, for example, to induce an operator). In the examples in Figures 2 and 5, this amplitude is 2 mm.

[0059] Probe 2 receives a measurement signal S representing the displacement of the ultrasonic transducer 6. d Includes a displacement sensor 11 arranged to output a measurement signal S. d This represents the displacement of the ultrasonic transducer 6 relative to the probe casing 3. Part of the displacement sensor 11 is fixed to the shaft, and the other part of the sensor is fitted into the probe so as not to move relative to the casing 3. The displacement sensor 11 may be a Hall effect sensor, an inductive displacement sensor, an optical sensor including a ruler with alternating opaque / transparent zones, or any other suitable sensor.

[0060] The probe 2 is operably connected to a central unit 7, which has the structure of a computer (and may be a laptop, smartphone, or a dedicated electronic device arranged to control, interface with, and process the signals acquired by the probe). The central unit includes at least memory and a processor. The central unit also includes a user interface, such as a touchscreen. The probe may be connected to the central unit 7 by a connecting cable 9 or by a wireless link.

[0061] Device 1 also includes an electronic unit 10. A block diagram of this electronic unit is shown in Figure 7.

[0062] Some elements of this electronic unit 10 (for example, the signal conditioning module 32) can be housed in the probe 2, and other elements of this unit 10 may be part of the central unit 7. Alternatively, the entire electronic unit 10 can be housed in the probe 2, or conversely, it can be fully integrated into the central unit 7.

[0063] As shown in Figure 7, the electronic unit 10 includes a control module 20, an ultrasonic front end 40, and a motion controller 30 for controlling the vibrator 5. Both the ultrasonic front end 40 and the motion controller 30 are connected to the control module 20 (i.e., they can receive commands or control signals from the control module 20 and transmit data or measurement signals). The electronic unit also receives the measurement signal S output by the displacement sensor 11. d It also includes a signal adjustment module 32 for adjusting and digitizing the signal. This signal adjustment module 32 is, in this case, part of the motion controller 30.

[0064] The motion controller 30 also includes an amplifier 31 for driving the vibrator 5. From an electrical standpoint, the amplifier 31 is configured to convert the control signal into a form suitable for driving the vibrator. Therefore, the amplifier 31 may be, for example, a current amplifier or a power amplifier (for example, the LM3886 power amplifier from Texas Instruments).

[0065] The control module 20 is a device or system that includes a microprocessor coupled to non-volatile memory containing machine-executable instructions, and / or a programmable microcircuit such as an FPGA (Field-Programmable Gate Array), or other programmable circuit, which is an electrical circuit that processes data. The control module 20 may also include one or more RAM memories or registers. The control module 20 can take the form of, for example, an FPGA carrier board.

[0066] The control module 20 is configured (e.g., programmed via instructions stored in memory) to control the motion controller 30 to displace the shaft 40 (and thus the U / S transducer 6) when an elastography measurement is triggered. This measurement may be triggered manually by an operator (e.g., by a push button or the user interface described above) or automatically. The displacement of the shaft is controlled according to a predetermined command signal. Here, this displacement is controlled by a control loop including an amplifier 31, a displacement sensor 11, a signal conditioning module 32, and a vibration control module 24 such as a PID compensator (in another embodiment, the vibrator may be controlled in an open loop, i.e., without sensor feedback). In the embodiment considered herein, the shaft displacement induced by the vibrator is a transient displacement corresponding to one period of a sine wave with a duration between 5 ms and 50 ms, for example.

[0067] The ultrasonic front end 40 includes an ultrasonic (U / S) pulser 41, a U / S receiving module 42, and a switch 43 for alternately transmitting and receiving ultrasonic signals. The U / S pulser 41 receives a transmit control signal S output by the control module 20. TX Based on this, the system includes an electrical circuit configured to generate an electro-ultrasonic signal suitable for driving the U / S transducer 6. This electrical circuit may include an amplifier and a digital-to-analog converter (DAC), for example, an 8 to 16-bit DAC with a rate of 10 to 1000 megasamples / second. The U / S receiving module 42 includes an electrical circuit configured to acquire the electro-ultrasonic signal (echo signal) previously received by the U / S transducer 6 (transmitted to the U / S receiving module 42 via switch 43). The electrical circuit of the ultrasonic receiving module 42 may include a voltage amplifier, one or more filters, and an analog-to-digital converter (ADC), for example, an 8 to 16-bit ADC with a rate of 10 to 100 megasamples / second.

[0068] The control module 20 is configured to control the U / S front end 40 so that when an elastography measurement is triggered, the U / S transducer 6 emits a sequence of ultrasonic pulses USP, such as sequence S shown in Figure 2 (programmed, for example, via instructions stored in memory). The control module 20 is also configured to acquire echo signals received by the U / S transducer 6 in response to the emitted pulses in order to track how the elastic waves induced in the tissue 8 by the displacement of the U / S transducer 6 move within the tissue.

[0069] The center frequency of each ultrasonic pulse (USP) is, for example, between 0.5 and 10 megahertz. The ultrasonic pulses of the sequence described above may be transmitted one at a time, with two consecutive pulses separated by a pulse repetition period T, which is typically between 50 microseconds and 2 milliseconds (corresponding to a pulse repetition rate between 0.5 kilohertz and 20 kilohertz). The ultrasonic pulses of the sequence described above may also be transmitted in groups, for example, by groups of two pulses (to calculate the correlation between two corresponding echo signals). The two pulses in each group may be separated by a duration between 50 and 200 microseconds, while the pulse group itself may be separated by a longer duration, for example, longer than 0.5 milliseconds. It will be understood that different transmission sequences may also be considered in various embodiments.

[0070] In the case of vibration-controlled transient elastography (as shown in Figures 2, 3, and 5), the total duration of this U / s pulse sequence can be between 50 ms and 200 ms. This duration can be selected depending on the propagation velocity of the slower elastic waves and the depth of the site being observed. For example, at a depth of 80 mm and a propagation velocity of 1 m / s (typical for shear waves in a subject's liver), the sequence may have a duration of 80 ms.

[0071] The sequence of U / S pulses transmitted by device 1 is based on a fixed, predetermined reference sequence, and each pulse in the sequence is adjusted in real time according to the displacement of the U / S transducer 6, with a temporal offset δt at the time of emission. TX It is generated by shifting the time only by a certain amount.

[0072] For this purpose, when an elastography measurement is triggered, the control module 20 provides a reference transmission control signal S (based on a predetermined transmission sequence stored in the control module's memory, for example). TX,o A signal may be generated, which is then delayed in a controlled manner by a controllable delay 23 and sent to the U / S front end 40 as a transmit control signal S TX Generates.

[0073] Each acquired echo signal is formed by the signal received by the U / S transducer 6 over time t after the emission of one of the U / S pulses emitted during the sequence. More precisely, it starts after this emission and lasts for a predetermined duration t. echo This is a signal received within a predetermined time window (see Figure 4) having Δt. When the U / S transducer 6 is not moving, the delay between the U / S pulse emission and the start of this time window is Δt o For example, if organization 8 (more generally, the medium to be characterized) has a minimum depth z min From, maximum depth z max If probed up to a certain point, there is a constant delay Δt o , 2.z min / v US This can be set to , while the window duration t echo is 2.(z max -z min ) / v US It is set to z. min =20mm and z min =100mm (Region of Interest ROI extending from z=20mm to 80mm), Δt o and t echo These may be set to 27 μs and 107 μs, respectively.

[0074] The echo signals received by the U / S transducer 6 in response to the transmitted U / S pulse are temporally shifted according to the displacement of the U / S transducer. They are the temporal offset δt at reception, which changes according to the displacement of the U / S transducer. RX It is shifted by δt. More precisely, for each echo signal, the start of the above time window is shifted relative to the reception time initially planned for this pulse (in other words, the reference time of reception), assuming no transducer displacement. RX Only that will be shifted.

[0075] δt time shift at reception RX This can also be obtained by a controllable sequencer 22, which selects an appropriate set of values ​​in the digitized signals output by the amplifier and ADC 42 using a shift register or another type of digital buffer. Alternatively, it can be obtained by controlling the triggering time for initiating ADC operation. In this regard, it should be noted that various solutions different from those presented above are possible for implementing such controllable delays during radiation and / or reception. In particular, control of the temporal offset during radiation and / or reception may be achieved not by the control module 20, but by the U / S front-end itself (depending on the received adjustment signal).

[0076] In any case, the temporal offset δt at the time of emission TX and the time offset δt at the time of reception RX Both are offsets with respect to a (same) stable temporal reference, such as a clock signal, which is independent of the transducer's displacement.

[0077] The electronic unit 10 compensates for the temporal shift of the received echo signal with respect to the other, caused by the displacement of the ultrasonic transducer 6 that occurs during the sequence of ultrasonic pulse emission, by adjusting the time offset δt during emission. TX and the time offset δt at the time of reception RXIt is configured to be adjustable.

[0078] In fact, as shown in Figure 5, as transducer 6 moves during measurement, the distance (and therefore time of flight) between the U / S transducer and the element located at a given depth z in the tissue changes depending on the moment being considered. Figure 5 schematically represents, in chronogram form, the displacement of U / S transducer 6 and the time of emission of U / S pulses or groups (e.g., pairs) of pulses during a typical transient elastography measurement. In this example, the pulse repetition frequency (of the group of pulse repetition frequencies) is 500 Hz, and the transient oscillation applied to the transducer is a one-period sine wave with a duration of 20 ms and a peak-to-peak amplitude of 2 mm.

[0079] Here, as shown in Figures 7 and 8, the electronic unit 10 is, more specifically: - Time offset δt at the time of emission TX δt TX,o +d / v us The temporal offset δt at the time of emission is equal to TX Adjust (δt TX,o (where is a constant delay during radiation), and - The time offset at the time of reception is δt RX,o -d / v us Adjust the time offset at reception so that it becomes equal to (δt RX,o (This is a certain delay during reception.) It is configured in this way.

[0080] Here, the difference between these two time offsets is Δt = δt RX -δt TX This is equal to the delay Δt between the emission of one U / S pulse and the start of the echo signal recorded in response (these time offsets are two time offsets with respect to the same time reference or clock). And this time difference Δt is equal to Δt o =δt RX,o -δt TX,o So, △t o -2.d / vus is equal to

[0081] a constant delay δt during emission TX,o is δt TX can be set according to the maximum displacement expected for the transducer, such that δt remains positive. For example, if a maximum peak-to-peak displacement amplitude of 2 mm (1 mm towards the tissue and 1 mm backwards) is expected, δt TX,o may be selected to be higher than 0.7 μs (the speed of ultrasound in tissue should be equal to 1.5 mm / μs). Also, for the constant delay δt during emission RX,o for △t o = 2.z min / v us it may be set such that

[0082] As shown in FIG. 7, the correction module 21 determines a variable delay d / v from the digitized signal output by the signal adjustment module 32 (the digitized signal representing the signal output by the displacement sensor 11). In this embodiment, the displacement d of the transducer 6 is its displacement relative to the probe casing 3. The correction module 21 includes an electronic circuit for determining the variable delay d / v us This displacement corresponds to the displacement of the transducer 6 relative to the reference position of the transducer. This reference position is, for example, the position of the transducer (position relative to the casing) when the probe is pressed against the subject's body immediately before triggering elastography measurement. Also, in the case of harmonic elastography, it may be the average position of the transducer. The value of d (an algebraic value, positive or negative) increases as the ultrasonic transducer moves towards the tissue (towards the subject's body). d corresponds to the variation in position along the axis z directed towards the tissue. us

[0083] As described in the section entitled "Means for Solving the Problem", when the emission and reception are thus temporally shifted in accordance with the displacement d of the U / S transducer 6 during emission, it becomes possible to compensate for the temporal inconsistency between the echo signals caused by the displacement of the transducer during the acquisition of this series of echo signals.

[0084] Thanks to this delay compensation, the recorded different echo signals are temporarily readjusted with respect to each other. This means that, for each echo signal, the portion of the echo signal at a given time t after the start of the echo signal corresponds, for different echo signals, to the same depth z in the medium (i.e.: corresponds to the signal backscattered by the portion of tissue located at the same depth z in the medium).

[0085] Also, the electronic unit 10 may be configured to: - correlate the recorded echo signals (which have been temporally readjusted prior to this correlation) and determine the tissue displacement as a function of the depth z for each time t (i.e.: step c) described above in the "Background Art" section), and - calculate the z derivative ∂* / ∂ of the spatio-temporal displacement map thus obtained (to remove the small uncompensated residual z shift that may remain due to the incomplete compensation of the displacement of the transducer), Z as may be configured.

[0086] In the case of FIG. 7, the temporal offsets δt TX and δt RX during emission and reception are adjusted to be equal to δt TX,o + d / v us and δt RX,o - d / v us respectively.

[0087] Note that, in an alternative, these temporal offsets may be adjusted differently as long as their difference Δt = δt RX - δt TX varies in accordance with 2.d / v us ​​

[0088] For example, compensation is fully achieved at the time of radiation, δt TX Then, δt' TX,o +2.d / v us It is adjusted to be equal to δt RX It is possible to keep it constant over time (no adjustment required upon reception).

[0089] Conversely, compensation is fully achieved upon receipt, δt RX δt' RX,o -2.d / v us It is adjusted to be equal to δt TX It is possible to keep it constant over time (no adjustment during radiation).

[0090] More generally, the electronic unit 10 is: - Time offset δt at the time of emission TX δt TX,o +Cd / v us Adjust the time offset of the emission so that it is equal to, and - The time offset at the time of reception is δt RX,o -(2-C).d / v us Adjust the time offset at reception so that it becomes equal to (C is a constant coefficient between 0 and 2). It can be configured in this way.

[0091] Referring to Figures 7 and 8, as shown above, C=1 corresponds to the case shown. In this case, the overall time shift correction applied is -2.d / v. us The time offset is evenly distributed between the time offset at the time of emission and the time offset at the time of reception. As described in the "Means for Solving the Problem" section, this particular correction is optimal in terms of the time sampling accuracy of tissue deformation.

[0092] Figure 9 schematically represents the sequence S (chronogram b) of U / S pulses emitted by device 1 shown above, for an example of displacement d(t) of U / S transducer 6. It also shows the sequence S that would have been emitted without displacement.o This represents (chronogram a). In this example, the reference sequence S has no delay. o In this system, the U / S pulses are repeated periodically and regularly at a constant pulse repetition frequency PRF0. The pulse repetition period, i.e., the duration between any of these pulses and the immediately following pulse, is T o It is stated that it is constant. In contrast, in the sequence emitted by device 1, as can be seen in Figure 9, the pulse repetition period T is not constant in time.

[0093] This variation in the elapsed time between the two pulses is a result of the emission delay applied to the two pulses under consideration, and since the value of the displacement d(t) is different when the first pulse is emitted and when the second pulse is emitted, these two pulses have different delays.

[0094] Therefore, as shown in Figure 9, when the U / S transducer 6 does not move (i.e., when d(t) is zero, i.e., constant), the pulse repetition frequency PRF is constant and equal to PRF0. However, when the U / S transducer 6 moves toward the tissue (when d increases, which is between time t1 and time t2 in Figure 9), the pulse repetition frequency PRF becomes smaller than PRF0, and the pulse repetition period T becomes T o It becomes larger than . Conversely, when the U / S transducer 6 moves backward (when d decreases, which is between time 0 to time t1 and between time t2 to time t3 in Figure 9), the pulse repetition frequency PRF becomes higher than PRF0, and the pulse repetition period T becomes T o It will become smaller than that.

[0095] In a second embodiment of the elastography device 1', a sequence S without delay on reference is formed based on the displacement d. o Instead of time-shifting different pulses, what is directly adjusted according to the displacement d is the pulse repetition period T of the generated sequence. More specifically, the pulse repetition period T is the time derivative of d, i.e., the U / S transducer.

number

[0096] In the second embodiment, the electronic unit 10' of device 1', schematically represented as a block diagram in Figure 10, thus determines T=T based on the displacement d during radiation. o ×(1+v / v us It is configured to generate a sequence of pulses to be emitted with a pulse repetition period T that is adjusted in real time to achieve the following: o This is the repetition period of the underlying reference. This is equal to the duration between two consecutive pulses when the U / S transducer 6 is not moving. o This can be constant over time, as in the case of Figure 9.

[0097] As explained in the "Means for Solving the Problem" section, adjusting the pulse repetition period T in this way results in a temporal offset δt at the time of emission (as in the first embodiment). TXo δt TX,o +d / v us The temporal offset δt at the time of emission is equal to TX Adjusting this yields the same, or substantially the same, result. In fact, adjusting the pulse repetition period T in this way does not adjust the offset itself based on d, but rather the offset δt at the time of emission depends on the time derivative of d. TX This is equivalent to, or substantially equivalent to, adjusting the time derivative of .

[0098] A different element of device 1' (the device partially shown in Figure 10) according to the second embodiment is that the electronic unit 10' is configured differently with respect to U / S pulse radiation control (rather than adjusting each temporal offset at radiation depending on d,

number

[0099] More specifically, in device 1' according to the second embodiment, the probe 2 and central unit 7 may be identical to those of device 1 in the first embodiment, except that the control module 20' is configured differently. The controllable delay 23 in the first embodiment is replaced by a controllable U / S pulse sequence generator 23' that generates a signal to control the U / S pulser 41 based on the displacement d of the U / S transducer 6. This signal has a pulse repetition period T within it. o ×(1+v / v us It is generated so that it is equal to ).

[0100] To implement such controllable pulse repetition period generation, other electronic implementations different from those shown above can be considered. In particular, the different functions of the electronic units presented above are distributed differently among the elements and modules of the device.

[0101] Furthermore, the pulse repetition period T is T o ×(1+v / v us Instead of being configured to adjust the pulse repetition period T to be equal to ), the electronic unit 10' is: - T is T o ×(1+Cv / v us Adjust T so that it equals (where C is a constant coefficient between 0 and 2), and - Temporal offset δt at the time of reception RX δt RX,o -(2-C).d / v us The time offset δt at reception is set to be equal to RX Adjust It can be configured in this way.

[0102] As described above for the first embodiment, when C=1, the overall time shift correction to be applied (compensating for the displacement of the U / S transducer) is evenly distributed between radiation and reception, which is optimal in terms of the time sampling accuracy of tissue deformation.

[0103] Furthermore, in other cases as well, it is possible to compensate for most of the impact of U / S transducer movement.

[0104] For example, compensation is fully achieved during emission, and the pulse repetition period T is T o ×(1+2.v / v us It is adjusted to be equal to ) and δδt RX This remains constant over time (no adjustment upon reception), which corresponds to C=2.

[0105] In the example shown above, U / S pulses are transmitted one at a time, and when the transducer is not moving, two consecutive pulses are T o It is separated by [this]. Nevertheless, the pulse repetition period adjustment technique shown above can also be applied to other types of U / S pulse sequences. For example, a reference sequence S without delay. o It consists of pairs (more generally, groups of pulses) of pulses that repeat periodically, with a period T between two consecutive pairs of pulses. o There is a duration t between the two pulses of each pair. intra This is possible. In such a case, the pulse repetition period is adjusted depending on d as described above, and the quantity t intra T o Similarly, the correction coefficient (1 + Cv / v us ) is multiplied. More generally, a periodically repeating U / S-based sequence S o All temporal patterns are given by the coefficient (1 + Cv / v us ) is temporally expanded (or contracted depending on the sign of v).

[0106] Figure 11 is a schematic representation of the elastography device 1'' according to the third embodiment. As described above, in this embodiment, the U / S transducer 6 is coupled to the probe casing 3 so as not to move relative to the probe casing 3. The U / S transducer 6 is fixed to the end of the tip 4'' and is mounted on the casing 3 in a non-moving manner relative to the casing 3. The probe 2'' includes a mass 12 positioned to move relative to the casing along the axis z of the casing. The tip 4'' and the U / S transducer 6 are centered on this axis. The vibrator 5'', the mass 12, and the U / S transducer 6 are rotationally symmetric about axis z. The vibrator 5'' is positioned to move the mass 12 relative to the casing (in other words, to move the casing 3 relative to the inertial mass 12), and the rebound effect moves the entire probe toward and from the tissue. This inertial probe 2'' is similar to the one described in patent EP3315074 by Sandrin and Audiere.

[0107] In this embodiment, the displacement sensor 11'' is an inertial sensor mounted on the probe and does not move relative to the probe, and therefore does not move relative to the U / S transducer 6. Accordingly, the measurement signal output from the displacement sensor 11'' represents the displacement of a single ultrasonic transducer 6 relative to an inertial reference frame (this reference frame is related to the room or location where the measurement is performed). The displacement sensor 11'' is an accelerometer, for example, a MEMS accelerometer.

[0108] In this third embodiment, the electronic unit is identical or at least similar to the electronic unit 10 of the first embodiment (see Figure 7), except that it includes a dual-time integrator for converting acceleration signals into position signals.

[0109] Note that in the third embodiment, displacement d is a kind of absolute displacement, while in the first and second embodiments, it is a kind of relative displacement (i.e., the displacement of the transducer relative to the probe casing).

[0110] Therefore, in the third embodiment, the displacement d considered to adjust for the temporal offset during radiation and / or reception corresponds precisely or nearly precisely to the actual displacement of the U / S transducer relative to the subject's body (since the subject is at rest during such examinations). In this case, the time shift compensation is therefore optimal in principle (assuming the measurement is not impaired by bias or noise).

[0111] In the first and second embodiments, it was found that adequate time-shift compensation could be obtained even if the displacement considered was only the displacement relative to the probe. This may seem quite surprising at first glance. Indeed, in the first and second embodiments, when the vibrator pushes its tip toward the subject, a slight recoil of the probe is usually observed, even if the probe is held firmly. Thus, the displacement of the transducer 6 relative to the probe casing, which is the amount considered to compensate for the time shift, does not exactly match the displacement of the transducer relative to the subject's body (which should ideally be considered).

[0112] The explanation for the fact that both techniques (measurement of absolute displacement by inertial sensors, or measurement of displacement relative to the casing) yield suitable results is that the electronic unit is configured in both cases to calculate the z derivative ∂* / ∂z of the ultimately obtained spatiotemporal displacement map (obtained by correlating the echo signals readjusted using the techniques described above). Thus, even if the compensation of the transducer displacement is not perfectly accurate (due to the reaction of the probe described above, or due to noise and / or bias in the displacement inferred from the signal provided by the inertial sensor), any possible residual, uncompensated small z-shifts are removed by the z derivative. In other words, the main purpose of time-shift compensation is to remove most of the time shift due to displacement (which is achieved in both the first and third embodiments) in order to avoid having to calculate the correlation of echo signals with large constant offsets between them (which is time-consuming and increases noise that impairs the results).

[0113] From the above, it will be understood that although specific embodiments of the present invention have been described herein for illustrative purposes, various modifications can be made to the devices presented above in addition to those already described.

[0114] For example, a probe may contain not just one, but multiple U / S transducers. In this case, all ultrasound transducers of the probe, positioned to emit ultrasound pulses in the tissue being characterized, are kept stationary relative to each other, as already mentioned. These transducers may be distributed symmetrically with respect to each other with respect to the probe axis z, so as to maintain the rotational symmetry of the probe. Alternatively, instead of being exactly symmetrical with respect to each other, they may be regularly distributed around this axis.

[0115] Furthermore, in the first and second embodiments (transducers movable relative to the casing), the displacement sensor may be, for example, an inertial sensor fixed on the shaft 40. Alternatively, the device may include both an inertial sensor that does not move relative to the casing and a displacement sensor such as the sensor 11 described above (both sensors are employed to determine the displacement of the transducer relative to the subject's body).

[0116] In another embodiment, the transducer displacement d(t) is measured by the measurement signal S d Instead of being inferred from that, it can be inferred from the command signals that control the vibrator.

[0117] In an alternative embodiment, the transducer displacement d(t) considered to compensate for the time shift caused by the displacement may be obtained by reading pre-recorded displacement data stored in the device's memory. This displacement data may be obtained by acquiring a signal representing the transducer displacement during a typical elastography measurement sequence. This displacement data may be acquired during a preliminary testing phase in which the device is tested and characterized. Using such pre-recorded data (instead of measuring the transducer displacement in situ each time an elastography measurement is triggered again) enables reliable time shift compensation, especially when the vibrator is controlled by a control loop (in fact, in such cases, the obtained displacement will be the same or at least similar for each elastography measurement performed thanks to the control loop, and therefore the same pre-recorded displacement signal may be employed).

[0118] The time-shift compensation technique described above has been explained in detail for vibration-controlled transient elastography, but it can also be similarly applied to vibration-controlled harmonic elastography, such as the one described in the patent application published as EP3769691.

[0119] It will be understood that the various embodiments described above can be combined in any technically permissible combination. [Explanation of symbols]

[0120] 1, 1', 1'' Elastography device 2, 2'' probe 3. Probe casing 5, 5'' Low-frequency vibrator 6. Ultrasonic transducer 8 organization 10, 10' Electronic Unit 11, 11'' Displacement sensor 12 squares δt TX Time offset during emission δt RX Temporal offset at reception USP Ultrasonic Pulse

Claims

1. - A probe (2; 2'') held against the body of a subject, a single ultrasonic transducer (6), or a plurality of ultrasonic transducers, all ultrasonic transducers of the probe arranged to emit ultrasonic pulses in a tissue (8) to be characterized, being immovable relative to each other, an ultrasonic transducer, a low-frequency oscillator (5; 5''), arranged to induce displacement of the single ultrasonic transducer (6) or the plurality of ultrasonic transducers towards the tissue (8), a low-frequency oscillator comprising a probe (2; 2''), - configured to control a single ultrasonic transducer (6) or a plurality of ultrasonic transducers to emit a sequence (S) of ultrasonic pulses (USP), and to acquire echo signals received by the single ultrasonic transducer (6) or the plurality of ultrasonic transducers in response to the emitted ultrasonic pulses (USP) in order to track how elastic waves induced in the tissue (8) by the displacement of the single ultrasonic transducer (6) or the plurality of ultrasonic transducers propagate within the tissue, an electronic unit (10) comprising, - the electronic unit (10) being further configured to generate, for one or more of the emitted ultrasonic pulses, a temporal offset (δt ＴＸ ) at emission at which the emission of the ultrasonic pulse is shifted, and / or a temporal offset (δt ＲＸ ) at reception at which the echo signal acquired in response to the emitted ultrasonic pulse is shifted and the temporal offset at emission (δt ) and / or the temporal offset at reception (δt ＴＸ ) being adjusted in response to the displacement of the single transducer (6) or the plurality of ultrasonic transducers, an elastography device (1; 1''). ＲＸ

2. The time offset (δt ＴＸ ) during emission and / or the time offset (δt ＲＸ ) during reception is adjusted so that the difference thereof changes in accordance with 2.d / v ＵＳ , where d is the displacement of a single transducer (6) or a plurality of ultrasonic transducers during emission, and v ＵＳ is the speed of ultrasonic waves in the tissue (8), the elastography device (1; 1´´) according to claim 1.

3. The electronic unit is configured such that the difference is equal to Δt ｏ = -2.d / v ＵＳ (Δt ｏ is a constant delay between the emission of an ultrasonic pulse (USP) and the acquisition of an echo signal received in response thereto), the elastography device (1; 1´´) according to claim 2.

4. The electronic unit (10) is such that, for one or more of the radiated ultrasonic pulses (USP), the time offset during emission is equal to δt ＴＸ，ｏ + C.d / v ＵＳ (δt ＴＸ，ｏ is a constant delay during emission), the time offset during emission, and the time offset during reception is equal to δt ＲＸ，ｏ - (2 - C).d / v ＵＳ (δt ＲＸ，ｏ is a constant delay during reception and C is a constant coefficient between 0 and 2), the time offset during reception, is adjusted, the elastography device (1; 1´´) according to claim 3.

5. C = 1, the elastography device (1; 1´´) according to claim 4.

6. Further comprising a displacement sensor (11; 11'') arranged to output a measurement signal representing the displacement of the single ultrasonic transducer (6) or the plurality of ultrasonic transducers, and the electronic unit (10) is based on the measurement signal to generate a time offset (δt ＴＸ ) and / or a time offset (δt ＲＸ ) during reception, and is further configured to generate an elastography device (1; 1'') according to any one of claims 1 to 5.

7. The displacement sensor (11'') is an inertial sensor arranged such that the output measurement signal represents the displacement of the single ultrasonic transducer (6) or the plurality of ultrasonic transducers with respect to an inertial reference frame, and the elastography device (1'') according to claim 6.

8. The probe (2'') includes a probe casing (3) that can be held by hand, and the single ultrasonic transducer (6) or the plurality of ultrasonic transducers are coupled to the probe casing (3) in a non-movable state with respect to the probe casing, and the vibrator (5'') is arranged to move a mass (12) within the probe casing in order to induce the displacement of the single ultrasonic transducer or the plurality of ultrasonic transducers towards the subject's body, and the elastography device (1'') according to claim 7.

9. The probe (2) includes a probe casing (3), the single ultrasonic transducer (6) or the plurality of ultrasonic transducers are movable with respect to the probe casing (3), and the displacement sensor (11) is arranged such that the measurement signal it provides represents the displacement of the single ultrasonic transducer (6) or the plurality of ultrasonic transducers with respect to the probe casing (3), and the elastography device (1) according to claim 1.

10. - A probe held against the subject's body, A single ultrasonic transducer (6), or a plurality of ultrasonic transducers, all of the ultrasonic transducers of a probe arranged to emit ultrasonic pulses in the tissue (8) to be characterized, being immovable relative to each other, an ultrasonic transducer, and A low-frequency oscillator (5) arranged to induce displacement of the single ultrasonic transducer (6) or the plurality of ultrasonic transducers towards the tissue (8), a low-frequency oscillator (5) Including a probe, - Configured to control a single ultrasonic transducer (6) or a plurality of ultrasonic transducers to emit a sequence of ultrasonic pulses (USP), and to obtain echo signals received by the single ultrasonic transducer (6) or the plurality of ultrasonic transducers in response to the emitted ultrasonic pulses in order to track how the elastic waves induced in the tissue (8) by the displacement of the single ultrasonic transducer (6) or the plurality of ultrasonic transducers propagate in the tissue, an electronic unit (10´) Including, - The electronic unit (10´) is further configured such that, for at least some of the ultrasonic pulses to be emitted, the pulse repetition period (T) that separates the ultrasonic pulses (USP) from the next ultrasonic pulse (USP) to be emitted varies according to the displacement (d) of the single ultrasonic transducer (6) or the plurality of ultrasonic transducers, and the pulse repetition period (T) is When the single ultrasonic transducer or the plurality of ultrasonic transducers move away from the tissue, shorter than the reference pulse repetition period T ｏ Compared to, When the single ultrasonic transducer or the plurality of ultrasonic transducers move towards the tissue, longer than the reference pulse repetition period T ｏ An elastography device (1´).

11. The electronic unit (10´) has a pulse repetition period of T ｏ × (1 + C.v / v ＵＳ configured to adjust a pulse repetition period (T) according to a displacement (d) of a single ultrasonic transducer or a plurality of ultrasonic transducers such that it is equal to), where v is a displacement velocity of a single ultrasonic transducer or a plurality of ultrasonic transducers, and v ＵＳ is a velocity of ultrasonic waves in the tissue, and C is a constant coefficient between 0 and 2, the elastography device (1´) according to claim 10. **Claim 12** The elastography device (1´) according to claim 11, wherein C = 1. **Claim 13** - A single ultrasonic transducer (6), or a plurality of ultrasonic transducers, all of the ultrasonic transducers of a probe arranged to emit ultrasonic pulses in a tissue (6) to be characterized, being immovable relative to each other, an ultrasonic transducer, and - A low-frequency oscillator (5; 5´´) arranged to direct the displacement of the ultrasonic transducer (6) or the plurality of ultrasonic transducers towards the tissue (8) An elastography method implemented by a device (1; 1´´) including a probe (2; 2´´) including - Controlling a low-frequency oscillator (5; 5´´) so as to direct the displacement of the ultrasonic transducer (6) or the plurality of ultrasonic transducers towards the tissue; and - Controlling the ultrasonic transducer (6) or the plurality of ultrasonic transducers to emit a sequence (S) of ultrasonic pulses (USP), and acquiring echo signals received by the ultrasonic transducer (6) or the plurality of ultrasonic transducers in response to the emitted ultrasonic pulses (USP) to track how elastic waves induced in the tissue (8) by the displacement of the ultrasonic transducer (6) or the plurality of ultrasonic transducers propagate within the tissue including For one or more of the emitted ultrasonic pulses A time offset (δt ＴＸ ) during emission that shifts the emission of the ultrasonic pulse and / or a time offset (δt ＲＸ ) during reception that shifts the echo signal acquired in response to the emitted ultrasonic pulse are further included The time offset (δt ＴＸ ) during emission and / or the time offset (δt ＲＸ ) during reception are adjusted according to the displacement of the transducer or a plurality of transducers, an elastography method.

14. The time offset (δt ＴＸ ) during emission and / or the time offset (δt ＲＸ ) during reception are adjusted such that their difference is 2·d / v ＵＳ where d is the displacement of the transducer or a plurality of transducers during emission, and v ＵＳ is the speed of ultrasound in the tissue, the elastography method according to claim 13.

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