ELASTOGRAPHY DEVICE AND METHOD

Abstract

An elastography device includes a probe with a single ultrasound transducer; or a plurality of ultrasound transducers, and a low frequency vibrator arranged to induce a displacement of said single ultrasound transducer or plurality of ultrasound transducers towards a tissue. The device is configured to emit a sequence of ultrasound pulses and to acquire echo signals received in response to track how elastic waves, induced by the displacement, travel in the tissue. The device is configured to generate, for one or more of the ultrasound pulses emitted a temporal offset upon emission, and/or a temporal offset upon reception, so that a difference thereof varies as a function of 2.d/.sub.Vus, where d is the displacement of the single transducer or plurality of ultrasound transducers, and where .sub.Vus is the speed of ultrasound in said tissue.

Claims

1. An elastography device comprising: a probe, to be held against the body of a subject, the probe comprising: a single ultrasound transducer; or a plurality of ultrasound transducers, all ultrasound transducers of the probe that are arranged to emit ultrasound pulses in a tissue to be characterized being motionless with respect to each other, and a low frequency vibrator, arranged to induce a displacement of said single ultrasound transducer or plurality of ultrasound transducers towards said tissue, and an electronic unit, configured to control the single ultrasound transducer or plurality of ultrasound transducers to emit a sequence of ultrasound pulses, and configured to acquire echo signals received by the single ultrasound transducer or plurality of ultrasound transducers in response to the ultrasound pulses emitted, in order to track how elastic waves, induced in the tissue by the displacement of the single ultrasound transducer or the plurality of ultrasound transducers, travel in said tissue, the electronic unit being further configured to generate, for one or more of the ultrasound pulses emitted: a temporal offset upon emission, by which the emission of an ultrasound pulse is shifted, and/or a temporal offset upon reception, by which an echo signal, acquired in response to said emitted ultrasound pulse is shifted, so as to compensate for a temporal shift of said echo signal with respect to other echo signals acquired, caused by the displacement of the ultrasound transducer or plurality of ultrasound transducers occurring during said sequence of ultrasound pulses, the temporal offset upon emission and/or the temporal offset upon reception being adjusted so that a difference thereof varies as a function of 2.d/.sub.Vus, where d is the displacement of the single transducer or plurality of ultrasound transducers at the time of emission, and where .sub.Vus is the speed of ultrasound in said tissue.

2. The elastography device of claim 1, wherein the electronic unit is configured so that said difference is equal to Δt.sub.o - 2.d/.sub.Vus, Δt.sub.o being a constant delay between the emission of the ultrasound pulse, and the acquisition of the echo signal received in response.

3. The elastography device of claim 2, wherein the electronic unit is configured to adjust, for said one or more of the ultrasound pulses emitted: the temporal offset upon emission, so that it is equal to δt.sub.TX,o + C.d/.sub.Vus, δt.sub.Tx,o being a constant delay upon emission, and the temporal offset upon reception, so that it is equal to δt.sub.Rx,o - (2-C).d/.sub.Vus, δt.sub.Rx,o, being a constant delay upon reception, C being a constant coefficient between 0 and 2.

4. The elastography device of claim 3, wherein C=1.

5. The elastography device of claim 1, further comprising a displacement sensor arranged to output a measurement signal representative of the displacement of said single ultrasound transducer or plurality of ultrasound transducers, and wherein the electronic unit is further configured to generate the temporal offset upon emission and/or the temporal offset upon reception based on said measurement signal.

6. The elastography device of claim 5, wherein said displacement sensor is an inertial sensor, arranged so that the measurement signal it outputs is representative of the displacement of said single ultrasound transducer or plurality of ultrasound transducers relative to an inertial frame of reference.

7. The elastography device of claim 6, wherein the probe comprises a probe casing, to be handheld, and wherein said single ultrasound transducer or plurality of ultrasound transducers is bound to the probe casing with no motion with respect to the probe casing, the vibrator being arranged to move a mass inside the probe casing in order to induce said displacement of the single ultrasound transducer or plurality of ultrasound transducers, towards the body of the subject.

8. The elastography device of claim 1, wherein the probe comprises a probe casing, said single ultrasound transducer or plurality of ultrasound transducers being movable with respect to the probe casing, and wherein the displacement sensor is arranged so that the measurement signal it delivers is representative of the displacement of said single ultrasound transducer or plurality of ultrasound transducers, relative to the probe casing.

9. An elastography device comprising: a probe, to be held against the body of a subject, the probe comprising: a single ultrasound transducer; or a plurality of ultrasound transducers, all ultrasound transducers of the probe that are arranged to emit ultrasound pulses in a tissue to be characterized being motionless with respect to each other, and a low frequency vibrator, arranged to induce a displacement of said single ultrasound transducer or plurality of ultrasound transducers towards said tissue, and an electronic unit, configured to control the single ultrasound transducer or plurality of ultrasound transducers to emit a sequence of ultrasound pulses, and configured to acquire echo signals received by the single ultrasound transducer or plurality of ultrasound transducers in response to the ultrasound pulses emitted, in order to track how elastic waves, induced in the tissue by the displacement of the single ultrasound transducer or the plurality of ultrasound transducers, travel in said tissue, the electronic unit being further configured so that, for at least some of the ultrasound pulses emitted, a pulse repetition period, that separates an ultrasound pulse from a next ultrasound pulse emitted, varies depending on the displacement of the single ultrasound transducer or plurality of ultrasound transducers, the pulse repetition period: being shortened compared to a base pulse repetition period T.sub.o when the single ultrasound transducer or plurality of ultrasound transducers moves away from said tissue, and being lengthened compared to the base pulse repetition period T.sub.o when the single ultrasound transducer or plurality of ultrasound transducers moves towards said tissue.

10. The elastography device of claim 9, wherein the electronic unit is configured to adjust the pulse repetition period, depending on the displacement of the single ultrasound transducer or plurality of ultrasound transducers, so that the pulse repetition period is equal to T.sub.o×(1+ C. v/vus), where v is the speed of displacement of the single ultrasound transducer or plurality of ultrasound transducers, where .sub.Vus is the speed of ultrasound in said tissue, and where C is a constant coefficient between 0 and 2.

11. The elastography device of claim 10, where C=1.

12. An elastography method, implemented by a device that comprises a probe including: a single ultrasound transducer; or a plurality of ultrasound transducers, all ultrasound transducers of the probe that are arranged to emit ultrasound pulses in a tissue to be characterized being motionless with respect to each other, and a low frequency vibrator, arranged to induce a displacement of said ultrasound transducer or plurality of ultrasound transducers towards said tissue, the method comprising: controlling the low frequency vibrator to induce a displacement of said ultrasound transducer or plurality of ultrasound transducers towards said tissue, controlling said ultrasound transducer or plurality of ultrasound transducers to emit a sequence of ultrasound pulses, and acquiring echo signals received by the ultrasound transducer or plurality of ultrasound transducers in response to the ultrasound pulses emitted, to track how elastic waves, induced in the tissue by the displacement of the ultrasound transducer or plurality of ultrasound transducers, travel in the tissue, the method further comprising, for one or more of the ultrasound pulses emitted: generating a temporal offset upon emission, by which the emission of an ultrasound pulse is shifted, and/or generating a temporal offset upon reception, by which an echo signal acquired in response to the emitted ultrasound pulse is shifted, so as to compensate for a temporal shift of said echo signal with respect to other echo signals received, caused by the displacement of the ultrasound transducer or plurality of ultrasound transducers occurring during said sequence of ultrasound pulses, the temporal offset upon emission and/or the temporal offset upon reception being adjusted so that a difference thereof varies as a function of 2.d/.sub.Vus, where d is the displacement of the transducer or plurality of transducers at the time of emission, and where .sub.Vus is the speed of ultrasound in said tissue.

13. An elastography device comprising: a probe, to be held against the body of a subject, the probe comprising: a single ultrasound transducer; or a plurality of ultrasound transducers, all ultrasound transducers of the probe that are arranged to emit ultrasound pulses in a tissue to be characterized being motionless with respect to each other, and a low frequency vibrator, arranged to induce a displacement of said single ultrasound transducer or plurality of ultrasound transducers towards said tissue, and an electronic unit, configured to control the single ultrasound transducer or plurality of ultrasound transducers to emit a sequence of ultrasound pulses, and configured to acquire echo signals received by the single ultrasound transducer or plurality of ultrasound transducers in response to the ultrasound pulses emitted, in order to track how elastic waves, induced in the tissue by the displacement of the single ultrasound transducer or the plurality of ultrasound transducers, travel in said tissue, the electronic unit being further configured to generate, for one or more of the ultrasound pulses emitted: a temporal offset upon emission, by which the emission of an ultrasound pulse is shifted, and/or a temporal offset upon reception, by which an echo signal, acquired in response to said emitted ultrasound pulse is shifted, the temporal offset upon emission and/or the temporal offset upon reception being adjusted as a function the displacement of the single transducer or plurality of ultrasound transducers.

14. The elastography device of claim 13, wherein the temporal offset upon emission and/or the temporal offset upon reception is adjusted so that a difference thereof varies as a function of 2.d/.sub.Vus, where d is the displacement of the single transducer or plurality of ultrasound transducers at the time of emission, and where .sub.Vus is the speed of ultrasound in said tissue.

15. The elastography device of claim 14, wherein the electronic unit is configured so that said difference is equal to Δt.sub.o - 2.d/.sub.Vus, Δt.sub.o being a constant delay between the emission of the ultrasound pulse, and the acquisition of the echo signal received in response.

16. The elastography device of claim 15, wherein the electronic unit is configured to adjust, for said one or more of the ultrasound pulses emitted: the temporal offset upon emission, so that it is equal to δt.sub.TX,o + C.d/.sub.Vus, δt.sub.TX,o being a constant delay upon emission, and the temporal offset upon reception, so that it is equal to δt.sub.RX,o - (2-C).d/.sub.Vus, δt.sub.Rx,o, being a constant delay upon reception, C being a constant coefficient between 0 and 2.

17. The elastography device of claim 16, wherein C=1.

18. The elastography device of claim 13, further comprising a displacement sensor arranged to output a measurement signal representative of the displacement of said single ultrasound transducer or plurality of ultrasound transducers, and wherein the electronic unit is further configured to generate the temporal offset upon emission and/or the temporal offset upon reception based on said measurement signal.

19. The elastography device of claim 18, wherein said displacement sensor is an inertial sensor, arranged so that the measurement signal it outputs is representative of the displacement of said single ultrasound transducer or plurality of ultrasound transducers relative to an inertial frame of reference.

20. The elastography device of claim 19, wherein the probe comprises a probe casing, to be handheld, and wherein said single ultrasound transducer or plurality of ultrasound transducers is bound to the probe casing with no motion with respect to the probe casing, the vibrator being arranged to move a mass inside the probe casing in order to induce said displacement of the single ultrasound transducer or plurality of ultrasound transducers, towards the body of the subject.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0077] Other characteristics and benefits of the disclosed technology will become clear from the description which is given below, by way of example and nonrestrictively, in reference to the figures, in which:

[0078] FIG. 1 schematically represents an elastography device of the prior art.

[0079] FIG. 2 represents a displacement of the ultrasound transducer of the device of FIG. 1, during a transient elastography measurement, and an ultrasound pulses sequence transmitted during this measurement to track how a tissue is moved in response to this displacement.

[0080] FIG. 3 represents schematically an elastogram obtained by means of the device of FIG. 1.

[0081] FIG. 4 represents schematically emission and reception times, temporally shifted as a function of a transducer’s displacement.

[0082] FIG. 5 represents schematically, as a chronogram, the displacement of an ultrasound transducer and times of emission of the ultrasound pulses, during a typical transient elastography measurement.

[0083] FIG. 6 schematically represents an elastography device according to a first embodiment.

[0084] FIG. 7 schematically represents an electronic unit of the device of FIG. 6, as a block diagram.

[0085] FIG. 8 schematically represents temporal offsets upon emission and upon reception generated by the electronic unit of FIG. 7, based on a transducer’s displacement.

[0086] FIG. 9 schematically represents a sequence of ultrasound pulses emitted over time by the device of FIG. 6, during a typical elastography measurement.

[0087] FIG. 10 schematically and partially represents an elastography device according to a second embodiment.

[0088] FIG. 11 schematically and partially represents an elastography device according to a third embodiment.

DETAILED DESCRIPTION

[0089] As mentioned above, the instant technology concerns a vibration-controlled elastography device, that is an elastography device configured: - to generate elastic waves, propagating in a medium to be explored, by moving an element which is in contact with the surface of the medium, such as a probe tip, and - to track how the elastic wave travels in the medium (or, in other words, how the medium is moved by the vibration exerted upon it), by transmitting ultrasound pulses in the medium, and recording echo signals received in response.

[0090] The ultrasound pulses and corresponding echo signals are transmitted and received by means of one or more ultrasound transducers, that are fixed on the moving element in question (for instance at an end of the probe’s tip), close to or even in contact with the medium to be probed. In the course of an elasticity measurement, these transducers are thus displaced, just as the moving element, which causes a temporal de-alignment of the echo signal acquired, with respect to each others. The elastography device presented here is configured to compensate for the displacement of the ultrasound transducer or transducers, so as to re-align temporally the different echo signals received.

[0091] To this end, temporal offsets upon emission (or, equivalently, a pulse repetition period between successive pulses), and/or temporal offsets upon reception are adjusted in real time, depending on the displacement of the ultrasound transducer or transducers (see FIG. 7, for instance).

[0092] Three embodiments of such a device, respectively identified by the reference numbers 1; 1′ and 1″, are represented in FIGS. 6, 10 and 11 respectively.

[0093] In these three embodiments, the device 1; 1′; 1″ comprises a single ultrasound transducer 6. Still, in other embodiments, the elastography device could comprise a plurality of ultrasound transducers. But anyhow, in the device according to the technology disclosed here, all ultrasound transducers that are arranged to emit ultrasound pulses in the tissue to be characterized are motionless with respect to each other. So, they move together, and their motion is characterized by a displacement d which is the same for all the transducers.

[0094] The expression “tissue” is understood to mean a part of the body of a subject (either a human or an animal). This expression does not necessarily designate a whole organ or a single organ. The tissue 8, to which mechanical vibrations are delivered and the deformation of which is tracked by the ultrasound pulses, is a part of the subject’s body located in the vicinity of the device’s probe, along an axis z of the probe. In the following, the abbreviation U/S stands for “ultrasound”.

[0095] The device 1 according to the first embodiment (FIGS. 6 and 7) is configured to compensate for the time-shifts mentioned above by adjusting a temporal offset upon emission δt.sub.TX, and a temporal offset upon reception δt.sub.RX, depending on the displacement d of the U/S transducer 6.

[0096] The device 1′ according to the second embodiment is similar to the one of the first embodiment, but it is configured to compensate for the time-shifts in question by adjusting directly a pulse repetition period T between successive U/S pulses, and, optionally, by adjusting also the temporal offsets upon reception δt.sub.RX.

[0097] In the devices 1 and 1′, the U/S transducer 6 is movable with respect to a casing 3 of the probe 2 of the device. And the probe comprises a low frequency vibrator 5, arranged to move the U/S transducer 6 relative to the casing 3 (casing which is hand-held by an operator), to exert a low frequency vibration on the tissue 8.

[0098] By contrast, in the device 1″ according to the third embodiment, the U/S transducer 6 is bound to the probe casing 3 with no motion with respect to the probe casing. The probe 2″ comprises a vibrator 5″ arranged to move a mass 12 inside the probe casing 3 to make the whole probe vibrating.

[0099] These three embodiments are however similar to each other, and the identical or corresponding elements of the device 1; 1′; 1″ will generally be identified by the same references.

[0100] The device 1 according to the first embodiment is now described in more details, with reference to FIGS. 6 to 9.

[0101] This elastography device 1 comprises the probe 2, the probe’s casing 3, the vibrator 5 and the U/S transducer 6 mentioned above. The U/S transducer 6 is fixed at an end of a tip 4 of the probe, which is actuated by the low frequency vibrator (see FIG. 6).

[0102] In this device, the vibrator 5 is rotationally symmetrical around a vibrator axis, which coincide with the probe axis z. When the vibrator 5 vibrates, it induces displacements that are mainly longitudinal, parallel to its axis.

[0103] Here, the vibrator 5 is arranged to move a shaft, the end of which forming the tip 4 of the probe. This shaft is centered onto the axis z, and the vibrator 5 is arranged to move this shaft along the axis z. The vibrator 5 is a low frequency vibrator in that it moves the tip with a central, average frequency smaller than 500 hertz, or even smaller than 100 hertz (in contrast with ultrasound shots or echo signals, whose central frequency is typically higher than 1 megahertz, for instance between 1 and 5 megahertz). The vibrator is a low-frequency electro-mechanical actuator, for instance with one or several coils and magnets, similar to a loud-speaker actuator. In an alternative, it may also comprise an electric motor such as a brushless DC motor or an electronically commutated motor. Such an alternative is well adapted to, e.g., vibration-guided transient elastography.

[0104] The ultrasound transducer 6 is rotationally symmetrical around a transducer axis and emits ultrasound beams centered on this axis. The transducer axis coincides with the vibrator’s axis, and so with the probe axis z. The ultrasound transducer 6 has for instance a circular section, the vibrator’s axis passing through the center of this section. This section is small, typically smaller than 1 square centimeters (it may have a diameter smaller than 1 centimeter, or smaller than 8 or even 5 millimeters). The transducer 6 may be covered by a sealing membrane, this sealing membrane being in contact with the subject’s body when the probe 2 is held in position, to make a measurement.

[0105] In practice, the displacement of the ultrasound transducer 6, induced by the vibrator 5, has a peak-to-peak amplitude between 0.1 mm and 10 mm (for instance between 0.5 and 10 mm for the transient elastography measurement in itself, and possibly smaller for a harmonic vibration, employed for instance to guide the operator). In the examples of FIGS. 2 and 5, this amplitude is 2 mm.

[0106] The probe 2 comprises a displacement sensor 11, arranged to output a measurement signal S.sub.d representative of the displacement of the ultrasound transducer 6. In this embodiment, the measurement signal S.sub.d is representative of the displacement of the ultrasound transducer 6 relative to the probe casing 3. A part of the displacement sensor 11 is fixed on the shaft mentioned above while another part of the sensor is fitted in the probe, with no motion with respect to the casing 3. The displacement sensor 11 may be a Hall-effect sensor, an induction displacement sensor, an optical sensor comprising a ruler with opaque/transparent alternating zones, or any other suitable sensor.

[0107] The probe 2 is operatively connected to a central unit 7, which has the structure of a computer (and that could be a laptop, a smartphone, or a dedicated electronic device arranged to control and to interface the probe, and to process the signals acquired). The central unit comprises at least a memory and a processor. Here, it comprises also a user interface, such as a touch screen. The probe may be connected to the central unit 7 by means of a connection cable 9, or by means of a wireless link.

[0108] The device 1 comprises also an electronic unit 10. A block diagram of this electronic unit is represented in FIG. 7.

[0109] Some of the elements of this electronic unit 10 (such as the signal conditioning module 32, for instance) can be housed in the probe 2 while other elements of this unit 10 may be part of the central unit 7. Alternatively, the entire electronic unit 10 could be housed in the probe 2, or, on the contrary, it could be entirely integrated into the central unit 7.

[0110] As represented in FIG. 7, the electronic unit 10 comprises a control module 20, an ultrasound front end 40, and a motion controller 30 to control the vibrator 5. The ultrasound front end 40 and the motion controller 30 are both connected to the control module 20 (that is to say that they can receive instructions or control signals from the control module 20 or send data or measurement signals to it). The electronic unit comprises also a signal conditioning module 32, to condition and digitalize the measurement signal Sd outputted by the displacement sensor 11. This signal conditioning module 32 is part of the motion controller 30, here.

[0111] The motion controller 30 comprises also an amplifier 31, to drive the vibrator 5. The amplifier 31 is configured to convert a control signal into a form suitable to drive the vibrator, from an electrical point of view. The amplifier 31 may thus be a current amplifier or a power amplifier (such as the LM3886 power amplifier by texas instrument, for instance), for instance.

[0112] The control module 20 is a device or system comprising electric circuitry for processing data, such as a microprocessor coupled to a non-volatile memory comprising machine executable instructions and/or a programmable microcircuit like an FPGA (field programmable gate array) or another programmable circuit. The control module 20 may also comprise one or several RAM memories or registers. The control module 20 can be in the form of an FPGA carrier board, for instance.

[0113] The control module 20 is configured (for instance, programmed via instruction stored in a memory) to control the motion controller 30 in order to displace the shaft 40 (and so, to displace the U/S transducer 6) when an elastography measurement is triggered. This measurement may be triggered manually, by an operator (by means of a push button or a by means of the user interface mentioned above, for instance) or automatically. The shaft displacement is controlled according to a predetermined command signal. Here, this displacement is controlled by means of a control-loop comprising the amplifier 31, the displacement sensor 11, the signal conditioning module 32, and a vibration control module 24, such as a PID corrector (still, in alternative embodiments, the vibrator may be controlled by means of an open loop - that is with no sensor feedback). In the embodiment considered here, the displacement of the shaft induced by the vibrator is a transient displacement, corresponding for instance to one period of a sinusoid having a duration between 5 ms and 50 ms.

[0114] The ultrasound front end 40 comprises an ultrasound (U/S) pulser 41, an U/S receiver module 42 and a switch 43 for alternatively transmitting and receiving ultrasonic signals. The U/S pulser 41 comprises an electric circuit configured to generate an electric ultrasonic signal appropriate to drive the U/S transducer 6, based on a transmission control signal S.sub.TX outputted by the control module 20. This electric circuit may comprise an amplifier and a digital to analog converter (DAC), for instance an 8 to 16 bits DAC with a 10 to 1000 Mega-sample per second rate. The U/S receiver module 42 comprises an electric circuit configured to acquire an electric ultrasonic signal (an echo signal), previously received by the U/S transducer 6 (and transmitted to the U/S receiver module 42 via the switch 43). The electric circuit of the ultrasonic receiver module 42 may comprise a voltage amplifier, one or more filters and an analog to digital converter (ADC), for instance an 8 to 16 bits ADC with a 10 to 100 Mega-sample per second rate.

[0115] The control module 20 is configured (for instance, programmed via instruction stored in a memory) to control the U/S front end 40 so that the U/S transducer 6 emits a sequence of ultrasound pulses USP, such as the sequence S represented in FIG. 2, when the elastography measurement is triggered. The control module 20 is configured also to acquire echo signals received by the U/S transducer 6 in response to the pulses em itted, in order to track how elastic waves, induced in the tissue 8 by the displacement of the U/S transducer 6, travels in the tissue.

[0116] The central frequency of each ultrasound pulse USP is comprised for instance between 0.5 and 10 megahertz. The ultrasound pulses of the sequence mentioned above may be transmitted one a time, two successive pulses being separated by a pulse repetition period T, this pulse repetition period being typically between 50 microseconds and 2 milliseconds (which corresponds to a pulse repetition rate between 0.5 kilohertz and 20 kilohertz). The ultrasound pulses of the sequence mentioned above may also be transmitted by groups, for instance by groups of two pulses (to compute correlations between the two corresponding echo signals). The two pulses of each group may be separated by duration between 50 and 200 microseconds, while the groups of pulses themselves are separated by a longer duration, for instance higher than 0.5 ms. It will be appreciated that different transmission sequences can also be considered in various embodiments.

[0117] In the case of Vibration-Controlled Transient Elastography (like in FIGS. 2, 3 and 5), the total duration of this sequence of U/s pulses may be between 50 ms and 200 ms. This duration may be selected depending on the speed of propagation of the elastic wave which is the slower and depending on the depth of the region to be observed. For instance, for an 80 mm depth and a speed of propagation of the 1 m/s (typical for shear waves in the liver of a subject), the sequence may have a duration of 80 ms.

[0118] The sequence of U/S pulses transmitted by the device 1 is generated based on a fixed, predetermined reference sequence, by shifting temporally each pulse of the sequence by a temporal offset upon emission δt.sub.TX which is adjusted in real time depending on the displacement of the U/S transducer 6.

[0119] To this end, the control module 20 may generate a reference transmission control signal S.sub.TX,O (based on a predetermined transmission sequence stored in a memory of the control module, for instance), when an elastography measurement is triggered, this signal being then delayed in a controlled manner, by means of a controllable delay 23, to produce the transmission control signal S.sub.TX sent to the U/S front end 40.

[0120] Each echo signal acquired is formed by a signal, received over time t by the U/S transducer 6 after the em ission of one of the U/S pulses em itted during said sequence. It is more precisely the signal received within a given temporal window (see FIG. 4) starting after this emission and having a given duration t.sub.echo. When the U/S transducer 6 does not move, the delay between the U/S pulse emission, and the beginning of this temporal window is Δt.sub.o. For instance, if the tissue 8 (more generally, the medium to be characterized) is probed, from a minimum depth z.sub.min, to a maximum depth z.sub.max, the constant delay Δt.sub.o may be set to 2.z.sub.min/v.sub.US, while the window duration t.sub.echo is set to 2.(z.sub.max -z.sub.min)/v.sub.US. For instance, for z.sub.min = 20 mm and z.sub.min = 100 mm (a Region of interest ROI extending from z=20 mm to 80 mm), Δt.sub.o and t.sub.echo may be set to 27 .Math.s and 107 .Math.s respectively.

[0121] The echo signals, received by the U/S transduced 6 in response the U/S pulses transmitted, are shifted temporally, depending on the U/S transducer’s displacement. They are shifted by a temporal offset upon reception δt.sub.RX which varies as a function of the U/S transducer’s displacement. More precisely, for each echo signal, the beginning of the temporal window mentioned above is shifted by δt.sub.RX with respect to a time of reception, initially planned for this pulse supposing there is no displacement of the transducer (in other words, a refence time of reception).

[0122] The temporal shift upon reception δt.sub.RX may be obtained by means of a controllable sequencer 22, selecting the appropriate series of values in a digitalized signal outputted by the amplifier and ADC 42, using a shift register or another kind of digital buffer. It may also be obtained by controlling a triggering time for the beginning of an ADC operation. In this regard, it may be noted that various solutions, possibly different from the ones presented above, can be considered to implement such a controllable delay upon emission and/or upon reception. In particular, the control of the temporal offset upon emission and/or upon reception, could be achieved by the U/S front end itself (depending on an adjustment signal received), instead of being achieved by the control module 20.

[0123] Anyhow, both the temporal offset upon emission δt.sub.TX and the temporal offset upon reception δt.sub.RX are offsets with respect to a (same) stable temporal reference, such as a clock signal, which is independent of the transducer’s displacement.

[0124] The electronic unit 10 is configured to adjust the temporal offset upon emission δt.sub.TX and the temporal offset upon reception δt.sub.RX so as to compensate for temporal shifts of the echo signals received, with respect to the other, caused by the displacement of the ultrasound transducer 6 occurring during the sequence of ultrasound pulses emissions.

[0125] Indeed, as represented in FIG. 5, as the transducer 6 moves during the measurement, the distance (and so, the time of flight) between the U/S transducer and an element located at a given depth z within the tissue varies depending on the moment considered. FIG. 5 represents schematically, in the form of a chronogram, the displacement of the U/S transducer 6 and the times of emission of the 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 pulses repetition frequency) is 500 Hz, while the transient vibration imparted to the transducer is a one-period of sinusoid having a duration of 20 ms and a peak-to-peak amplitude of 2 mm.

[0126] Here, as represented in FIGS. 7 and 8, the electronic unit 10 is configured more particularly: [0127] to adjust the temporal offset upon emission δt.sub.TX so that it is equal to δt.sub.TX,o + d/v.sub.us, δt.sub.TX,o being a constant delay upon em ission, and [0128] to adjust the temporal offset upon reception, so that it is equal to δt.sub.RX,o [0129] d/v.sub.us, δt.sub.RX,o, being a constant delay upon reception.

[0130] Here, the difference Δt = δt.sub.RX - δt.sub.TX between these two temporal offsets is equal to the delay Δt between the em ission of one of the U/S pulses em itted, and the start of the echo signal recorded in response (these temporal offsets being two temporal offsets relative to a same temporal refence, or clock). This time difference Δt is then equal to Δt.sub.o - 2.d/v.sub.US with Δt.sub.o = δt.sub.Rx,0 - δt.sub.TX,0.

[0131] The constant delay upon emission δt.sub.TX,o may be set, depending on the maximum displacement expected for the transducer, so that the δt.sub.TX remains positive. If a maximum peak-to-peak displacement amplitude of 2 mm (1 mm towards the tissue, and 1 mm backward) is expected, for instance, δt.sub.TX,o may be chosen higher than 0.7 .Math.s (should the speed of ultrasound in the tissue be equal to 1.5 mm/.Math.s). Regarding the constant delay upon emission δt.sub.RX,o, it may be set so that Δt.sub.o = 2.z.sub.min/v.sub.US.

[0132] As represented in FIG. 7, a correction module 21 determines the variable delay d/v.sub.US, from the digitalized signal outputted by the signal conditioning module 32 (digitalized signal which is representative of the signal outputted by the displacement sensor 11). In this embodiment, the displacement d of the transducer 6 is its displacement relative to the probe’s casing 3. The correction module 21 includes electronic circuitry to determine the variable delay d/v.sub.US. This displacement corresponds to the displacement of the transducer 6 relative to a refence position of the transducer. This reference position is for instance the position of the transducer (position relative to the casing) when the probe is held against the subject’s body, just before triggering the elastography measurement. It may also be an average position of the transducer, in the case of harmonic elastography. The value of d (which is an algebraic value, that can be either positive or negative) increases when the ultrasound transducer moves towards the tissue (towards the body of the subject) ; d corresponds to a variation of position along the axis z, directed towards the tissue.

[0133] As explained in the section entitled “summary”, shifting temporally the emission and reception in this way, depending on the displacement d of the U/S transducer 6 at the time of emission, enables one to compensate for a temporal misalignment between echo signals caused by the displacement of the transducer during the acquisition of this series of echo signals.

[0134] Thanks to this delay compensation, the different echo signals recorded are temporarily re-aligned with each other. It means that, in each echo signal, the portion of the echo signal at a time given t after the beginning of the echo signal corresponds to the same depth z in the medium (i.e.: corresponds to the signal backscattered by the portion of the tissue located at the same depth z in the medium), for the different echo signals.

[0135] The electronic unit 10 may also be configured: [0136] to correlate the echo signals recorded (these echo signals having been re-aligned tem porally prior to this correlation), to determ ine the tissue displacement as a function of depth z, for each time t (that is: step c) mentioned above in the “background” section), and [0137] to compute the z-derivative ∂ ∗/∂z of the spatio-temporal displacement map thus obtained (to remove possible residual, un-compensated small z-shifts that may remain due to a non-completely perfect compensation of the transducer’s displacement).

[0138] In the case of FIG. 7, the temporal offsets upon emission and reception δt.sub.TX and δt.sub.RX are adjusted so as to be equal to δt.sub.TX,o + d/v.sub.us and δt.sub.RX,o - d/v.sub.us respectively.

[0139] Still, in an alternative, these tem poral offsets could be adjusted differently, as long as their difference Δt = δt.sub.RX - δt.sub.TX varies as a function of 2.d/v.sub.us.

[0140] For instance, the compensation could be achieved entirely upon emission, δt.sub.TX being then adjusted so as to be equal to δt′.sub.TX,o + 2.d/v.sub.us, while δt.sub.RX would remain constant over time (no adjustment upon reception).

[0141] Conversely, the compensation could be achieved entirely upon reception, δt.sub.RX being then adjusted so as to be equal to δt′.sub.RX,o - 2.d/v.sub.us, while δt.sub.TX would remain constant over time (no adjustment upon emission).

[0142] More generally, the electronic unit 10 could be configured: [0143] to adjust the temporal offset upon emission δt.sub.TX so that it is equal to δt.sub.TX,o + C. d/v.sub.us, and [0144] to adjust the temporal offset upon reception, so that it is equal to δt.sub.RX,o (2-C).d/v.sub.us, C being a constant coefficient between 0 and 2.

[0145] The case presented above, with reference to FIGS. 7 and 8, corresponds to C=1. In this case, the overall time-shift correction to be applied (that is - 2.d/v.sub.us) is distributed evenly between the temporal offset upon emission and the temporal offset upon reception. As explained in the section “summary”, this specific correction is optimal, in terms of tim e-sam pling accuracy of the tissue deform ations.

[0146] FIG. 9 represents schematically the sequence S of U/S pulses emitted by the device 1 presented above (chronogram b)), for an example of displacement d(t) of the U/S transducer 6. It also represents the sequence S.sub.o that would have been emitted without displacement (chronogram a)). In this example, in the un-delayed, reference sequence So, the U/S pulses are repeated periodically, regularly, with a constant Pulse Repetition Frequency PRF0. The pulse repetition period, that is, the duration between any of these pulses and the pulse immediately after it, is then noted To and is constant. In contrast, in the sequence emitted by device 1, the pulse repetition period T is not constant over time, as can be seen in FIG. 9.

[0147] This variation of the time lapse between two pulses is a consequence of the delays upon emission applied to the two pulses considered, delays that are different for these two pulses as the value of the displacement d(t) is different when the first pulse is em itted, and when the second one is emitted.

[0148] So, as illustrated in FIG. 9, when the U/S transducer 6 does not move (i.e.: when d(t) is zero, or constant), then, the pulse repetition frequency PRF is constant, equal to PRF0. But when the U/S transducer 6 moves towards the tissue (when d increases, which is the case between time t1 and time t2 in FIG. 9), then the pulse repetition frequency PRF is smaller than PRF0, and the pulse repetition period T is higher than To. Conversely, when the U/S transducer 6 moves backwards (when d decreases, which is the case between 0 and time t1, and between time t2 and time t3, in FIG. 9), then the pulse repetition frequency PRF is higher than PRF0, and the pulse repetition period T is smaller than To.

[0149] In the second embodiment of the elastography device, 1′, instead of time-shifting the different pulses of a reference, un-delayed sequence So based on the displacement d, it is directly the pulse-repetition period T of the sequence generated which is adjusted, depending on the displacement d. The pulse-repetition period T is adjusted more specifically depending on the time-derivative of d, that is, depending on the speed of displacement of the U/S transducer v = ḋ = d(d)/dt.

[0150] In the second embodiment, the electronic unit 10′ of the device 1′, represented schematically on FIG. 10 as a block-diagram, is thus configured to generate the sequence of pulses to be em itted with a pulse repetition period T which is adjusted in real time, based on the displacement d at the time of emission, so that T=T.sub.o×(1+v/v.sub.US). As mentioned above, T.sub.o is a base, reference repetition period. It is equal to the duration between two successive pulses when the U/S transducer 6 does not move. To may be constant over time, like in the case of FIG. 9.

[0151] As explained in the section “summary”, adjusting the pulse repetition period T in this way produces the same result, or substantially the same result as adjusting the temporal offset upon emission the temporal offset upon emission δt.sub.TX so that it is equal to δt.sub.TX,o + d/v.sub.us (like in the case of the first embodiment). Indeed, adjusting the pulse repetition period T in this way is equivalent, or substantially equivalent to adjusting the time-derivative of the offset upon emission δt.sub.TX depending on the time-derivative of d, instead of adjusting the offset itself based on d.

[0152] The different elements of the device 1′ according to the second embodiment (device which is represented partially in FIG. 10) are identical or at least similar to those of the device 1 of the first embodiment, except that the electronic unit 10′ is configured differently regarding the U/S pulses emission control (as it is configured to adjust the pulse repetition period upon emission depending on ḋ, instead of adjusting each temporal offset upon emission depending on d).

[0153] More specifically, in the device 1′ according to the second embodiment, the probe 2 and the central unit 7 may be identical those of the device 1 of the first embodiment, except that the control module 20′ is arranged differently. The controllable delay 23 of the first embodiment is replaced by a controllable U/S pulses sequence generator 23′ which generates the signal that controls the U/S pulser 41, based on the displacement d of the U/S transducer 6. This signal is generated so that the pulse repetition period T within this signal is equal to T.sub.o×(1+v/v.sub.US).

[0154] Other electronic implementations, possibly different from the one presented above, can be considered to implement such a controllable pulse repetition period generation. In particular, the different functions of the electronic unit presented above being distributed differently among the elements and module of the device.

[0155] Besides, instead of being configured to adjust the pulse repetition period T so that it is equal to T.sub.o×(1+v/v.sub.US), the electronic unit 10′ could be configured: [0156] to adjust T so that it is equal to T.sub.o×(1+C.v/v.sub.US), C being a constant coefficient between 0 and 2, and [0157] to adjust the temporal offset upon reception δt.sub.RX so that it is equal to δt.sub.RX,o -(2-C).d/v.sub.us.

[0158] As explained above about the first embodiment, the case C=1 is a case in which the overall time-shift correction to be applied (to compensate for the U/S transducer displacement) is distributed evenly between emission and reception, which is optimal, in terms of time-sampling accuracy of the tissue deformations.

[0159] Still, other cases also enable to compensate for most of the effects of the movement of the U/S transducer.

[0160] For instance, the compensation could be achieved entirely upon em ission, the pulse repetition period T being adjusted so that it is equal to T.sub.o×(1+2.v/v.sub.US) while δδt.sub.RX would remain constant over time (no adjustment upon reception), which corresponds to C=2.

[0161] In the example presented above, the U/S pulses are transmitted one at a time, two successive pulses being separated by T.sub.o when there is no transducer’s motion. Still, the pulse repetition period adjustment technique presented above can also be applied to other kinds of U/S pulses sequences. For instance, the un-delayed, reference sequence S.sub.o could be composed of pairs of pulses (more generally, groups of pulses), repeated periodically, with a period T.sub.o between two successive pairs of pulses and a duration t.sub.intra between the two pulses of each pair. In such a case, the pulse repetition period is adjusted depending on d, as described above, and the quantity t.sub.intra is also multiplied by the correcting factor (1+C.v/vus), just as To. More generally, all the temporal patterns of the U/S base sequence So, which are repeated periodically, are temporally expanded (or reduced, depending on the sign of v) by the factor (1+C.v/vus).

[0162] FIG. 11 represents schematically the elastography device 1″ according to the third embodiment. As mentioned above, in this embodiment, the U/S transducer 6 is bound to the probe casing 3 with no motion with respect to the probe casing. The U/S transducer 6 is fixed at an end oh the tip 4″, which is fitted to the casing 3 with no motion with respect to the casing. The probe 2″ comprises a mass 12 arranged so that it can move with respect to the casing, along the axis z of the casing. The tip 4″ and the U/S transducer 6 are centered onto this axis. The vibrator 5″, the mass 12 and the U/S transducer 6 are rotationally symmetrical around the axis z. The vibrator 5″ is arranged to move the mass 12 with respect to the casing (or, in other words, to move the casing 3 with respect to the inertia mass 12), to make the whole probe moving towards the tissue and back, by virtue of a recoil effect. This inertial probe 2″ is similar to the one described in the patent n°EP3315074 by Sandrin and Audiere.

[0163] In this embodiment, the displacement sensor 11″ is an inertial sensor, fitted to the probe, with no motion with respect to the probe, and so, with no motion with respect to the U/S transducer 6. The measurement signal outputted by the displacement sensor 11″ is thus representative of the displacement of the single ultrasound transducer 6 relative to an inertial frame of reference (this frame of reference being the one associated to the room or the place where the measurement is carried on). The displacement sensor 11″ is an accelerometer, for instance a MEMS accelerometer.

[0164] In this third embodiment, the electronic unit is identical, or at least similar to the electronic unit 10 of the first embodiment (see FIG. 7), except that it comprises a double temporal integrator, to convert the acceleration signal into a position signal.

[0165] One may note that, in the third embodiment, the displacement d is a kind of absolute displacement, while in the first and second embodiment, it a kind of relative displacement (namely, the transducer’s displacement with respect to the probe’s casing).

[0166] So, in the third embodiment, the displacement d taken into account to adjust the temporal offsets upon emission and/or reception corresponds exactly, or almost exactly to the actual displacement of the U/S transducer relative to the body of the subject (as the subject is at rest, during such an examination). In this case, the time-shifts compensation is thus optimal, in principle (if one assumes that the measurement is not impaired by biases or noises).

[0167] In the first and second embodiments, even if the displacement taken into account is only the displacement relative to the probe, it turns out that an adequate time-shift compensation is obtained, in fact. It may seem quite surprising, at first glance. Indeed, in the first and second embodiments, then the vibrator pushes the tip towards the subject, a slight recoil of the probe is usually observed, even if the probe is firmly held. And so, the displacement of the transducer 6 relative to the probe’s casing, which is the quantity taken into account to compensate for the times-shifts, does not match exactly the displacement of the transducer with respect to the subject’s body (which should be the one taken into account, ideally).

[0168] An explanation for the fact that both techniques (either measuring the absolute displacement by means of an inertial sensor, or measuring the displacement relative to the casing) lead to adequate results is that that electronic unit is configured, in both cases, to compute the z-derivative ∂ ∗/∂z of the spatio-temporal displacement map finally obtained (obtained by correlating the echo signals re-aligned using the technique mentioned above). So, even if the compensation of the displacement of the transducer is not entirely exact (either because of the probe recoil mentioned above, or because of the noise and/or biases of the displacement deduced form the signal provided by the inertial sensor), possible residual, un-compensated small z-shifts are removed by the z-derivative. In other words, the main goal of the time-shift compensation is in fact to remove most of the displacement-induced time shift (which is achieved both in the first and third embodiment), to avoid having to compute correlations for echo signals with big, constant offsets between echo signals (which would be time-consuming, and which would increase the noise impairing the result).

[0169] From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made to the device presented above, in addition to those already mentioned.

[0170] For instance, the probe could comprise several U/S transducers, instead of just one. In this case, all ultrasound transducers of the probe that are arranged to emit ultrasound pulses in the tissue to be characterized are motionless with respect 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 that the rotational symmetry of the probe is preserved. They could also be distributed regularly around this axis, instead of being exactly symmetrical with respect to each others.

[0171] Besides, in the case of the first and second embodiment (transducer movable with respect to the casing), the displacement sensor could also be an inertial sensor, fixed onto the shaft 40, for example. Alternatively, the device could comprise both an inertial sensor, motionless with respect to the casing, and a displacement sensor like the sensor 11 described above (both sensors being em ployed to determ ine the transducer’s displacement relative to the subject’s body).

[0172] In another embodiment, the displacement d(t) of the transducer could be deduced from the command signal controlling the vibrator, instead of being deduced form the measurement signal S.sub.d.

[0173] In an alternative embodiment, the displacement d(t) of the transducer, taken into account to compensate for the displacement-induced time-shift, could be obtained by reading pre-recorded displacement data stored in a memory of the device. This displacement data may be obtained by acquiring a signal representative of the transducer displacement during a typical elastography measurement sequence. This displacement data could be acquired during a preliminary test phase, during which the device is tested and characterized. Using such pre-recorded data (instead of measuring the transducer displacement on-the-fly, each time an elastography measurement is triggered again) enables a reliable time-shift compensation, in particular when the vibrator is controlled by means of a control loop (indeed, in such a case, the displacement obtained is the same or at least similar for each elastography measurement carried on – thanks to the control loop – and so, the same, pre-recorded displacement signal can be employed).

[0174] The time-shift compensation technique presented above has been described in detail in the case of Vibration-Controlled Transient Elastography, but it may be applied as well to Vibration-Controlled Harmonic Elastography, such as described in the patent application published as EP3769691, for instance.

[0175] It will be appreciated that the various embodiments described previously are combinable according to any technically permissible combinations.