IL294313A - Elastography device and method - Google Patents

Description

ELASTOGRAPHY DEVICE AND METHOD FIELD The disclosed technology concerns an elastography device and method. It concerns more particularly such a device arranged: - to generate an elastic wave that travels in a tissue to be characterized, by moving a tip that is in contact with said tissue, - to transmit ultrasound pulses and to receive corresponding echoes, to track how this elastic wave travels in this tissue, in order to characterize the tissue stiffness.

BACKGROUND Liver stiffness, measured for instance by Vibration-Controlled Transient Elastography, has been shown to be a very useful tool to help health care professionals to detect or to characterize liver disease or damages, and more generally to monitor the condition of the liver of a subject. Figure 1 schematically represents an elastography device 1a of the prior art, that is adapted to measure liver stiffness by Vibration-Controlled Transient Elastography. This device comprises a probe 2a with: - a casing 3a, to be handheld; - a tip 4a, that can be moved relative to the casing 3a by a low frequency vibrator 5a; and - an ultrasound transducer 6a, mounted at an end of the tip 4a (possibly with a sealing membrane covering the ultrasound transducer).

During measurement, the probe casing 3a is handheld in such a way that the ultrasound transducer 6a is slightly pressed against the body 8 of the subject under examination. A transient displacement of the tip 4a is then triggered, causing the ultrasound transducer 6a to move towards the body of the subject, and back, thereby generating low frequency elastic waves (and, in particular, a low frequency shear wave) in the tissue. The corresponding displacement d(t) of the ultrasound transducer 6a is schematically represented over time t in figure 2. d(t) corresponds more precisely to the position of the transducer 6a along an axis z directed towards the subject’s body (see figure 1), at time t, relative to an initial position of the transducer 6a (which is its position just before triggering this transient vibration). As represented in figure 2, a sequence S of ultrasound pulses USP, starting when the low frequency vibration is triggered, is emitted by the ultrasound transducer 6a. These ultrasound pulses enable to track how elastic waves, induced in the tissue facing the probe, travel in this tissue. To this end, two echo signals, corresponding to two successive ultrasound shots of this sequence, are correlated together to determine, for different depths in the tissue, the distance over which the tissue has moved between these two pulses.

At each time, a strain map (sometimes called elastogram or displacements or shear wave propagation map) in the tissue is thus determined as a function of the depth z in this tissue. Figure 3 represents, as a function of time t and depth z, the strain map in the liver of a subject that results from imparting a low frequency vibration such as the one represented in figure 2. This spatio-temporal representation of the elastic waves propagating in the tissue under examination is called an elastogram. It enables one to visualize very clearly how such waves propagates in the tissue. The speed of propagation of shear waves in this tissue can be determined from the slope of the straight line represented in this figure, which shows the position of the wavefront as a function of time and depth. The tissue stiffness is then deduced from this speed of propagation.

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 ultrasound transducer 6a. Indeed, in such a device, the ultrasound transducer 6a is fixed on the moving tip 4a itself. So, when the tip 4a moves, the distance between the transducer 6a and such or such part of the tissue varies. With no correction, the apparent depth, at which such or such strain would be observed would thus be slightly different from the depth at which this strain actually occurs.

Besides, for a couple of two successive shots emitted to determine the strain in the tissue at a given time (by correlating the two corresponding echo signals), the displacement of the ultrasound transducer between these two pulses results in an apparent overall displacement of the whole tissue, superimposed to the actual tissue displacement caused by the elastic waves travelling in it. This offset, which is the same for each depth, can be readily suppressed by computing the z-derivative of the tissue displacement determined at the time considered, as explained in section III.A (p. 440) of the following article: "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. Still, as explained in this article, it is highly desirable to compensate for the transducer’s displacement before correlating the two echo signals recorded. Indeed, with no compensation of the transducer’s movement, the displacement measured by correlation is significantly larger, and thus noise and more time consuming to determine.

To compensate for the transducer’s displacement d(t) prior to carrying out echo signals correlation, the following technique is usually employed. The sequence of ultrasound pulses S is emitted, and the echo signals received in response are recorded, as explained above. These echo signals are then post-processed by a central electronic unit 7a, that has the structure of a computer and is operatively connected to the probe 2a. This post-processing, described for instance in the article mentioned above, comprises: a) estimating the transducer’s displacement d(t) from the echo signals themselves; b) for each echo signal, compensating for the displacement d(t) of the transducer (that has been estimated in step a)), in the frequency domain, by multiplying the Fourier transform of this echo signal by

Claims (2)

1.CLAIMS 1. An elastography device (1; 1’’) comprising: - a probe (2; 2’’), to be held against the body of a subject, the probe comprising: o a single ultrasound transducer (6); or a plurality of ultrasound transducers, all ultrasound transducers of the probe that are arranged to emit ultrasound pulses in a tissue (8) to be characterized being motionless with respect to each other, and o a low frequency vibrator (5; 5’’), arranged to induce a displacement of said single ultrasound transducer (6) or plurality of ultrasound transducers towards said tissue (8), and - an electronic unit (10), configured to control the single ultrasound transducer (6) or plurality of ultrasound transducers to emit a sequence (S) of ultrasound pulses (USP), and configured to acquire echo signals received by the single ultrasound transducer (6) or plurality of ultrasound transducers in response to the ultrasound pulses (USP) emitted, in order to track how elastic waves, induced in the tissue (8) by the displacement of the single ultrasound transducer (6) or the plurality of ultrasound transducers, travel in said tissue, - the electronic unit (10) being further configured to generate, for one or more of the ultrasound pulses emitted: o a temporal offset upon emission ( tTX), by which the emission of an ultrasound pulse is shifted, o and/or a temporal offset upon reception ( tRX), by which an echo signal, acquired in response to said emitted ultrasound pulse, is shifted, the temporal offset upon emission ( tTX) and/or the temporal offset upon reception ( tRX) being adjusted as a function of the displacement of the single transducer (6) or plurality of ultrasound transducers. 2. The elastography device (1; 1’’) of claim 1, wherein the temporal offset upon emission ( tTX) and/or the temporal offset upon reception ( tRX) is adjusted so that a difference 30 thereof varies as a function of 2.d/vus, where d is the displacement of the single transducer (6) or plurality of ultrasound transducers at the time of emission, and where vus is the speed of ultrasound in said tissue (8). 3. The elastography device (1; 1’’) of claim 2, wherein the electronic unit is configured so that said difference is equal to to - 2.d/vus, to being a constant delay between the emission of the ultrasound pulse (USP), and the acquisition of the echo signal received in response. 4. The elastography device (1; 1’’) of claim 3, wherein the electronic unit (10) is configured to adjust, for said one or more of the ultrasound pulses (USP) emitted: o the temporal offset upon emission , so that it is equal to tTX,o + C.d/vus, tTX,o being a constant delay upon emission, o and the temporal offset upon reception, so that it is equal to tRX,o - (2-C).d/vus, tRX,o, being a constant delay upon reception, C being a constant coefficient between 0 and 2. 5. The elastography device (1; 1’’) of claim 4, wherein C=1. 6. The elastography device (1; 1’’) of any of claim 1 to 5, further comprising a displacement sensor (11; 11’’) arranged to output a measurement signal representative of the displacement of said single ultrasound transducer (6) or plurality of ultrasound transducers, and wherein the electronic unit (10) is further configured to generate the temporal offset upon emission ( tTX) and/or the temporal offset upon reception ( tRX) based on said measurement signal. 7. The elastography device (1’’) of claim 6, wherein said displacement sensor (11’’) is an inertial sensor, arranged so that the measurement signal it outputs is representative of the displacement of said single ultrasound transducer (6) or plurality of ultrasound transducers relative to an inertial frame of reference. 8. The elastography device (1’’) of claim 7, wherein the probe (2’’) comprises a probe (3) casing, to be handheld, and wherein said single ultrasound transducer (6) or plurality of ultrasound transducers is bound to the probe casing (3) with no motion with respect to the probe casing, the vibrator (5’’) being arranged to move a mass (12) 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. 9. The elastography device (1) of any of claim 1 to 7, wherein the probe (2) comprises a probe casing (3), said single ultrasound transducer (6) or plurality of ultrasound transducers being movable with respect to the probe casing (3), and wherein the displacement sensor (11) is arranged so that the measurement signal it delivers is representative of the displacement of said single ultrasound transducer (6) or plurality of ultrasound transducers, relative to the probe casing (3). 10. An elastography device (1’) comprising: - a probe, to be held against the body of a subject, the probe comprising: o a single ultrasound transducer (6); or a plurality of ultrasound transducers, all ultrasound transducers of the probe that are arranged to emit ultrasound pulses in a tissue (8) to be characterized being motionless with respect to each other, and o a low frequency vibrator (5), arranged to induce a displacement of said single ultrasound transducer (6) or plurality of ultrasound transducers towards said tissue (8), and - an electronic unit (10’), configured to control the single ultrasound transducer (6) or plurality of ultrasound transducers to emit a sequence of ultrasound pulses (USP), and configured to acquire echo signals received by the single ultrasound transducer (6) or plurality of ultrasound transducers in response to the ultrasound pulses emitted, in order to track how elastic waves, induced in the tissue (8) by the displacement of the single ultrasound transducer (6) or the plurality of ultrasound transducers, travel in said tissue, - the electronic unit (10’) being further configured so that, for at least some of the ultrasound pulses emitted, a pulse repetition period (T), that separates an ultrasound pulse (USP) from a next ultrasound pulse (USP) emitted, varies depending on the displacement (d) of the single ultrasound transducer (6) or plurality of ultrasound transducers, the pulse repetition period (T): o being shortened compared to a base pulse repetition period To when the single ultrasound transducer or plurality of ultrasound transducers moves away from said tissue, o and being lengthened compared to the base pulse repetition period To when the single ultrasound transducer or plurality of ultrasound transducers moves towards said tissue. 11. The elastography device (1’) of claim 10, wherein the electronic unit (10’) is configured to adjust the pulse repetition period (T), depending on the displacement (d) of the single ultrasound transducer or plurality of ultrasound transducers, so that the pulse repetition period is equal to To×(1+ C. v/vUS), where v is the speed of displacement of the single ultrasound transducer or plurality of ultrasound transducers, where vus is the speed of ultrasound in said tissue, and where C is a constant coefficient between 0 and 2. 12. The elastography device (1’) of claim 11, where C=1. 13. An elastography method, implemented by a device (1; 1’’) that comprises a probe (2; 2’’) including: - a single ultrasound transducer (6); or a plurality of ultrasound transducers, all ultrasound transducers of the probe that are arranged to emit ultrasound pulses in a tissue (8) to be characterized being motionless with respect to each other, and - a low frequency vibrator (5; 5’’), arranged to induce a displacement of said ultrasound transducer (6) or plurality of ultrasound transducers towards said tissue (8), the method comprising: - controlling the low frequency vibrator (5; 5’’) to induce a displacement of said ultrasound transducer (6) or plurality of ultrasound transducers towards said tissue, - controlling said ultrasound transducer (6) or plurality of ultrasound transducers to emit a sequence (S) of ultrasound pulses (USP), and acquiring echo signals received by the ultrasound transducer (6) or plurality of ultrasound transducers in response to the ultrasound pulses (USP) emitted, to track how elastic waves, induced in the tissue (8) by the displacement of the ultrasound transducer (6) or plurality of ultrasound transducers, travel in the tissue, 30 - the method further comprising, for one or more of the ultrasound pulses emitted: o generating a temporal offset upon emission ( tTX), by which the emission of an ultrasound pulse is shifted, o and/or generating a temporal offset upon reception ( tRX), by which an echo signal acquired in response to the emitted ultrasound pulse is shifted, the temporal offset upon emission ( tTX) and/or the temporal offset upon reception ( tRX) being adjusted as a function of the displacement of the transducer or plurality of transducers. 14. The elastography method of claim 13, wherein the temporal offset upon emission ( tTX) and/or the temporal offset upon reception ( tRX) is adjusted so that a difference thereof varies as a function of

2.d/vus, where d is the displacement of the transducer or plurality of transducers at the time of emission, and where vus is the speed of ultrasound in said tissue