WO2017098298A1

WO2017098298A1 - An imaging method and device using shear waves

Info

Publication number: WO2017098298A1
Application number: PCT/IB2015/002499
Authority: WO
Inventors: Laurent MARSAC
Original assignee: Super Sonic Imagine
Priority date: 2015-12-07
Filing date: 2015-12-07
Publication date: 2017-06-15

Abstract

An imaging method and device using shear waves An imaging method using shear waves for producing an image of an observation region inside a medium, comprising a firing step in which a plurality of firing sequences (FS1, FS2, FS3) are performed, each firing sequence (FS2, FS3) comprising observations (I₁₁ to I₃₄) that are time shifted (t_s) compared to those of the first sequence (FS1), and a processing step during which a movement parameter of the medium is determined at a multitude of points inside the observation region on the bases of signals received during the observation steps.

Description

An imaging method and device using shear waves

FIELD OF THE INVENTION

The present invention concerns an imaging method and device using shear waves and ultrasound echography for producing images representing physical characteristics inside a medium.

BACKGROUND OF THE INVENTION

The present invention concerns more precisely an imaging method using shear waves for producing an image of an observation region inside a medium. The method is implemented by a processing unit connected to an array of transducers in relation with said medium.

It is known from patents US 6,770,033 and US 7,252,004 to use a shear wave and ultrasound echography for producing an image of a medium. In the first patent, a shear wave is generated at the external surface of the medium by an acoustic transducer or vibrator. In the second patent, the array of ultrasound transducers used for echography imaging is also used to generate a focused ultrasound waves for generating the elastic shear wave directly inside the medium. Thanks to this method deeper imaging zones or shadow zones (masked by obstacles) can be imaged .

However, these methods still need to be improved.

OBJECTS AND SUMMARY OF THE INVENTION

One object of the present invention is to provide an imaging method having improved imaging capabilities.

To this effect, the imaging method according to the invention comprises the following steps:

a) a sequence number of at least two or more firing sequences, each firing sequence comprising:

al) an excitation step during which a shear wave is generated inside the medium, and a2) an observation step during which the propagation of said shear wave is observed at a plurality of points in the observation region, at a plurality of time instants relative to the shear wave generation of the firing sequence, said time instants being spaced of a period, and

wherein the time instants of each observation in the sequence being time shifted compared to a first firing sequence of a time shift different than the period, and

b) a processing step during which a movement parameter of the medium is determined at the multitude of points inside the observation region on the bases of signals received by the array during the observation step of the firing sequences.

Thanks to these features, the number of images representing movement parameters (displacements) of the medium inside the observation region can be increased. Therefore, a film illustrating the propagation of the shear wave inside the medium is obtained with a higher frame rate (more images per seconds) .

Then, the method is more accurate to estimate a propagation parameter (shear wave speed or shear modulus or Young' s modulus or shear elasticity) at some points in the observation region.

In various embodiments of the method, one and/or other of the following features may optionally be incorporated .

According to an aspect of the invention, the time shift is lower than the period.

According to an aspect of the method, the time shift is equal to:

ts = (k-1) . (T/N) ,

where

T is the period,

N is the sequence number of firing sequences, and k is an index of the firing sequence, k being an integer greater or equal to one, and being lower than N.

According to an aspect of the method, the sequence number depends on a distance between the array and the observation region.

According to an aspect of the method, the sequence number depends on a speed value of the shear wave.

According to an aspect of the method, the sequence number is determined on the bases of signals received during the observation step after the first firing sequence.

According to an aspect of the method, the firing sequences at step a) , comprises a first group of firing sequences and a second group of firing sequences, and wherein

- the first group of firing sequences comprises a first number of firing sequences, each firing sequence of the first group comprising an excitation step and an observation step during which the propagation of the shear wave is observed at time instants spaced of a first period, the time instants of each observation in the first group of firing sequences being time shifted compared to a first firing sequence in the first group of a first time shift different than the first period, and

- the second group of firing sequences comprises a second number of firing sequences, each firing sequence of the second group comprising an excitation step and an observation step during which the propagation of the shear wave is observed at time instants spaced of a second period, the time instants of each observation in the second group of firing sequences being time shifted compared to a first firing sequence in the second group of a second time shift different than the second period,

and wherein

- the first number and the first period of first group of firing sequences permits to observe a first portion of the observation region, said first portion being at a first depth range relative to the array,

- the second number and the second period of second group of firing sequences permits to observe a second portion of the observation region, said second portion being at a second depth range relative to the array.

According to an aspect of the method, the second depth range is at a higher depth compared to the first depth range .

According to an aspect of the method, the second depth range overlaps the first depth range.

According to an aspect of the method, the second number is greater than the first number.

According to an aspect of the method, each observation during the observation step a2) comprises the following sub-steps:

- emitting an unfocussed ultrasound wave into the medium by the array, and

- receiving and recording signals from the transducers, said signals corresponding to echoes generated by the medium in response to the emitted unfocussed ultrasound wave.

According to an aspect of the method, the unfocussed ultrasound waves used for each observation are plane waves, said plane waves having predetermined angles relative to a direction of the array.

According to an aspect of the method, the predetermined angles are not all identical during one observation step a2) of a firing sequence.

According to an aspect of the method, the processing step b) comprises the following sub-steps:

bl) for each firing sequence, a set of propagation images is determined on the bases of signals received during the observation steps,

b2) a movement parameter of the medium is determined at a plurality of points inside the observation region by comparing a propagation image to a predetermined image of said observation region.

According to an aspect of the method, the predetermined image is produced by at least one observation of the observation region during which no shear wave generated at excitation step al) is penetrating said observation region, and preferably the predetermined image is determined by a combination of a plurality of images produced by observations of the observation region during which no shear wave generated at excitation step al) is penetrating the observation region.

According to an aspect of the method, before sub- step b2), an image sequence is built by merging and interlacing the propagation images determined during sub- step bl) so as the images in the image sequence are ordered according to time evolution considering each firing sequence starting at a common initial time value.

According to an aspect of the method, the movement parameter is a displacement.

According to an aspect of the method, the processing step b) is followed by a mapping step c) during which a propagation parameter is calculated at at least some points inside the observation region on the basis of variation in the movement parameter over time, so as to determine a map of said propagation parameter in the observation region.

According to an aspect of the method, the propagation parameter calculated during the mapping step c) is selected from shear wave speed, shear modulus, Young's modulus, shear wave attenuation, shear elasticity, shear viscosity, and mechanical relaxation time.

Another object of the invention is to provide an imaging device for implementing a method as above, said imaging device using shear waves for producing an image of an observation region inside a medium, said device comprising a processing unit connected to an array of transducers in relation with said medium, and the processing unit is controlling :

al) an excitation step during which a shear wave is generated inside the medium, and

a2) an observation step during which the propagation of said shear wave is observed at a plurality of points in the observation region, at a plurality of time instants relative to the shear wave generation of the firing sequence, said time instants being spaced of a period, and

wherein the time instants of each observation in the firing sequence being time shifted compared to a first firing sequence of a time shift lower than the period, and b) a processing step during which a movement parameter of the medium is determined at the multitude of points inside the observation region on the bases of signals received by the array during the observation steps of the firing sequences.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will be apparent from the following detailed description of at least one of its embodiments given by way of non-limiting example, with reference to the accompanying drawings. In the drawings :

- Figure 1 is a schematic drawing showing a shear- wave imaging device according to one embodiment of the invention;

- Figure 2 is a block diagram showing details of the device of figure 1 ;

- Figure 3 is a diagram showing an example of imaging method according to the invention that is implemented in the imaging device of figure 1 ;

- Figures 4a to 4c are examples illustrating three firing sequences used in the method of figure 3;

- Figure 5a is a diagram showing in a compact form the successive observations (images) done during the three firings sequences of figures 4a to 4c;

- Figure 5b is a diagram showing the technical effect of the method, merging and re-ordering of the observations presented on figure 5a; and

- Figure 6 is a schematic drawing showing a shear- wave imaging device according to a variant of the implemented method of the invention.

MORE DE TAILLED DESCRIPTION

The imaging device 1 shown in Figure 1 is for studying and tracking the movements and propagation of elastic shear waves inside a viscoelastic medium 20 that diffuses ultrasound waves in compression, and said viscoelastic medium 20 may be constituted, for example:

by an inert body, in particular for quality control in industrial applications; or

- a living body, for example a portion of the body of a patient, in medical applications.

By way of example, the imaging device 1 comprises: a transducer array 2 (probe) , for instance a linear array typically including a few tens of transducers (for instance 100 to 300) juxtaposed along an axis X (horizontal or array direction X) as already known in usual probes (the array 2 is then adapted to perform a bidimensional (2D) imaging of the region 1, but the array 2 could also be a bidimensional array adapted to perform a 3D imaging of the medium 20);

an electronic bay 3 controlling the transducer array and acquiring signals therefrom;

a microcomputer 4 (possibly comprising an input interface 4b such as a keyboard, etc., and an output interface 4a such as a screen, etc.) for controlling the electronic bay 3 and viewing images obtained from the electronic bay (in a variant, a single electronic device could fulfil all the functionalities of the electronic bay 3 and of the microcomputer 4) .

The array 2 of transducers may comprise a number L of ultrasound transducers ΤΊ, T₂, T_±, T_L, where L is an integer greater than 1. The transducer array 2 may be linear or may be a convex array including a plurality of transducer aligned along a curved line.

The transducer array 2 is in contact or relation with the outside surface 20a of the medium 2 to generate and send ultrasound compression waves inside the medium 20 in the direction of an axis Z (designated in present document as a vertical or axial direction) that is perpendicular to the axis X.

These ultrasound compression waves interact with diffusing particles 22 (non-uniformity in the medium 20) contained in the medium 2, which particles are reflective for ultrasound compression waves. In echographic images, such particles form reflective points known as "speckle") .

The array 2 of transducers generates ultrasound compression wave pulses, which pulses have, for example, a frequency lying in the range 0.5 MHz to 100 MHz, and preferably in the range 0.5 MHz to 15 MHz, e.g. being about 4 MHz .

As shown on Figure 2, the electronic bay 3 may include for instance:

- L analog/digital converters 5 (A/Di-A/D_L) individually connected to the L transducers (Tl-TL) of the transducer array 2 ;

- L buffer memories 6 (Bi-B_n) respectively connected to the n analog/digital converters 5,

- a central processing unit 8 (CPU) communicating with the buffer memories 6 and the microcomputer 4,

- a memory 9 (MEM) connected to the central processing unit 8.

Optionally, the electronic bay 3 also comprise an additional processing unit 10 that can be used for executing some repetitive processing tasks so as to help the central processing unit 8. The additional processing unit 10 may be any kind of microprocessor, e.g. a digital signal processor 10 (DSP) or a graphic processing unit (GPU) connected to the central processing unit 8.

The electronic bay 3 can control the transducers Ti-T_L thus to emit selectively:

- Either an ultrasound compression wave that is "plane" (i.e. a wave whose wave front is rectilinear in the X-Z plane) , or any other type of unfocused wave illuminating the entire observation region in the medium 20,

- Or else an ultrasound compression wave that is focused on one or more points inside the medium 20.

The apparatus herein disclosed is a device for ultrasound imaging, the transducers are ultrasound transducers, and the implemented method is for producing ultrasound images of region 1. However, the apparatus may be any imaging device using other waves than ultrasound waves (waves having a wavelength different than an ultrasound wavelength) , the transducers and the electronic bay components being then adapted to said waves. The above imaging device 1 implements an imaging method 100 as illustrated on figure 3 for producing an image of an observation region 21 inside the medium 20. The imaging method 100 according to the invention comprises the following steps:

a) a sequence step 101 comprising a number of at least two or more firing sequences, each firing sequence comprising :

al) an excitation step 102 during which a shear wave is generated inside the medium 20, and

a2) an observation step 103 during which the propagation of said shear wave is observed simultaneously at a plurality of points in the observation region 21, and at a plurality of time instants relative to the shear wave generated at step al) belonging to the last firing sequence, and

b) a processing step 104 during which a movement parameter of the medium is determined at the multitude of points inside the observation region 21 on the bases of signals received by the array 2 during the observation steps of the firing sequences.

The time instants of observation step a2) are temporally spaced of a period T, said period T being preferably a constant period.

Moreover, the time instants of each observation in the firing sequence are time shifted compared to the first firing sequence of a time shift ts different than the period T.

Advantageously, the time shift ts is lower than the period T. In that case, the firing sequences after the first firing sequence FS1 have a length of time that is not significantly increased: These lengths of time are lower than a length of time being the first length of time of the first firing sequence FS1 plus one period T.

Eventually, time shift ts is higher than the period T. In that case, the first firing sequence FS1 comprises two or more observations of the observation region 21 at a various time instants, before a first observation of the observation region during a firing sequence following the first firing sequence.

More generally, the time instants of all the observations for all the firing sequences, said time instants being measured relative to the corresponding shear wave generation (step al)) time instant, are comprised between zero and a time limit. The time limit is the duration relative to the shear wave generation for which the shear wave exits the observation region 21 because of its velocity in the medium. Figures 4a to 4c are illustrating the above method by showing three firing sequences. In these figures, the axis R is a reference axis relative to a time instant of generation of each shear wave during the excitation step al) (102) . All the time instants ti, t₂, t₃ of generation of each shear wave SW1, SW2, SW3 are aligned on this axis although they are next one to another one (successive in time) .

During the first firing sequence FS1, represented on figure 4a:

- a shear wave SW1 (excitation step al)) is generated inside the medium at time ti,

- the propagation of the above shear wave SW1 is observed, for example at four time instants, i.e. at time instants tn, ti₂, ti₃ and ti₄, (observation step a2)), each time instants being separated of a period T. For each observation, a plurality of points in the observation region are measured at a very fast speed and substantially simultaneously by recording the received echoes signals in response to emission of an unfocused ultrasound wave ( insonification) so as the processing step b) is capable to calculate (produce) an echography image of said observation region 21. Then, an image In is a produced image with recorded signal in response to an insonification at time instant tn. An image 112 is a produced image in response to an insonification at time instant ti₂. An image 113 is a produced image in response to an insonification at time instant ti3, and an image I14 is a produced image in response to an insonification at time instant ti₄.

During the second firing sequence FS2, represented on figure 4b:

- a new shear wave SW2 (excitation step al)) is generated inside the medium at time t₂,

- the propagation of the above new shear wave SW2 is then observed similarly at four time instants t₂i, t₂₂, t_{23 f} 3-HCl t24 ·

The images Ι₂ι, I₂₂, ^23, and I₂ will be the produced images in response to each insonification at time instants t₂i, t₂₂, t₂₃, and t₂₄, respectively.

These four time instants (t₂i, t₂₂, t₂₃, and t₂₄) belonging to the second firing sequence FS2 are time shifted compared to the time instants (tn, ti₂, ti₃, and ti₄) belonging to the first firing sequence FS1 by a time shift ts, said time shift ts being lower than the period T of said observations in the firing sequence.

During the third firing sequence FS3, represented on figure 4c:

- again, a new shear wave SW3 (excitation step al)) is generated inside the medium at time t₃,

- the propagation of the above new shear wave SW3 is then observed similarly at four time instants t₃i, t₃₂, t₃₃ , and 134.

The images I₃i, I₃₂, I₃₃, and I₃₄ will be the produced images in response to each insonification at time instants t₃i, t₃₂, t₃₃, and t₃₄, respectively.

The four time instants (t₃i, t₃₂, t₃₃, and t₃₄) belonging to the third firing sequence F3 are also time shifted compared to the time instants (tn, ti₂, ti₃, and ti₄) belonging to the first firing sequence Fl by a new time shift ts, said new time shift ts being lower than the period T of said observations in the firing sequence.

In fact, all the time shifts ts can be a regular spaced time shift, and they may be determined as follow:

ts (k) = (k-1) . (T/N) ,

where

T is the period,

N is the number of firing sequences, and

k is an index of the firing sequence, k being an integer greater or equal to one, and being lower than N.

Each firing sequence is ending at a time instant, that is in present case, ti₅, t₂₅, and t₃₅. A next firing sequence can only start after this instant. For example, the instant t2 is after instant ti5, and so on...

Eventually, the firing sequences FS1, FS2, FS2 are joined without any waiting time between them, and t2 = ti5,

Then, the time value for each image in the image sequence is calculated by:

t(j,k) =(j-l) .T + (k-1) . (T/N) + k.Dt

where

j is an index of the time instant in a firing sequence, and

Dt id an initial shift period that is the difference of time between the first time instant of observation in a firing sequence and the time instant of the generation of the shear wave at the beginning of said firing sequence, e.g. Dt = t - ti.

The above formula assumes that all the difference of time (for all the firing sequences) are identical: Dt = Dt = t - ti = t₂i - t₂ = t₃i - t₃.

This initial shift period Dt is necessary to ensure that the shear wave SWk reaches the observation region 21.

The shear wave is indeed a slow wave (around 10 m/s) .

Additionally, it is preferred that the shear wave SWk is still at least partially inside the observation region 21 at the last time instant of the firing sequence FSk: i.e. in case of figure 4a, at the time instant ti₄, the first shear wave SW1 is preferably still inside the observation region 21.

The shear waves SWk (or at least part of them) should ideally cross the observation region 21 between the first time instant t_¾i (first observation in the firing sequence) and the last time instant t_¾4 (last observation in the firing sequence) .

The period T between each insonification (each observation) in a firing sequence corresponds to an observation rate Rt for observing the observation region 21. Obviously, we have the following relation: Rt = 1/T.

For example, each observation, which is repeated several times during one observation step a2), can be implemented as it is known in ultrafast echography techniques and comprises the following sub-steps:

- emitting an unfocussed ultrasound wave into the medium 20 by the array 2 (e.g. a succession of unfocused compression wave shocks at a rate of at least 500 shots per second) , and

- receiving and recording signals from the transducers, said signals corresponding to echoes generated by reflecting particles 22 inside the medium 20 in response to the emitted unfocussed compression wave.

The timing of the shear wave emitted during excitation step al), and the timing of the unfocused ultrasound waves are (as explained above) adapted so that at least some of said unfocused compression waves reach the observation region 21 (for observation) during the propagation of the shear wave through said observation region (for excitation), for at least some of the unfocused ultrasound wave emissions. Therefore, the shear wave and the echography observation unfocussed waves are simultaneously inside the observation region 21.

The above observation rate is limited is a high rate, e.g. in the range of 500 to 10,000 shots per seconds.

However, this observation rate is limited by the go-and-return travel time (time of flight) for the compression wave through the medium 2, from the outside surface 20a to the observation region 21. The echoes generated by the compression wave have to be received by the array 2 before a new unfocussed compression wave is sent. The period T is therefore higher than a lower limit depending on the depth of the observation region 21 under the outside surface 20a.

Fortunately, the unfocused ultrasound compression wave propagates through the medium 20 at a propagation speed that is much higher than the propagation speed of the shear waves: e.g., in the human body, a compression wave speed is of about 1500 meters per second (m/s) and a shear wave speed is of few meters per seconds (1-20 m/s) .

But, if the shear wave speed is too high and/or the size of the observation region is too small compared to said shear wave speed and compared to the period T (period that increases with depth) , the method of the invention proposes to generate several firing sequences that are time shifted for taking several intermediate images. Gathering all the images enable then to construct a more precise succession of images, i.e. to construct a film comprising more images.

The method however uses a plurality of shear waves SWk, and it assumes that all the fired shear waves SWk are substantially identical or can be normalized to a reference level.

Additionally, the method assumes that there are few uncontrolled movements in the medium, i.e. few movements such as heart motion, so as the built film is a film having a smooth continuity in the images.

In case of periodic uncontrolled movements (e.g. heart beating or breath) , the method may optionally use an input information so as to trigger the sequences and to keep continuity in the sequence of images.

The number N of firing sequences may be predetermined according to various considerations:

- the number N depends on the depth of the observation region 21, i.e. a distance between the array 2 of transducers and the observation region 21. It may be increased for high depth;

- the number N depends on the shear wave characteristics, e.g the frequency or frequency bandwidth, which may correspond to a scale of elastography analysis defined by the user; - the number N depends on a speed value of the shear wave inside the medium. It may be increased for high speed of shear waves;

- the number N is determined just after the first firing sequence FS1 on the bases of signals received during its observation step a2) . The signals could enable to determine a value of the shear wave speed c_s . And, this may enable to define a minimum value of number N of firing sequences so as to obtain a predefined image quality.

Moreover, according to a variant of the method, a first depth of the observation region 21 may be insonified with a first number Ni of firing sequences, and a second depth of the observation region 21 may be insonified with a second number N₂ of firing sequences. The second depth is greater than the first depth, and the second number is greater than the first number.

More precisely, this variant of the method is illustrated on figure 6 and comprises the following features :

The firing sequences 101 at step a) are split into a first group of firing sequences and a second group of firing sequences, each group being for example similar to the firing sequences already explained:

The first group of firing sequences comprises a first number Ni of firing sequences, each firing sequence of the first group comprising:

- an excitation step during which a shear wave is generated inside the medium, and

- an observation step during which the propagation of the above shear wave is observed at a plurality of points in a first portion 21i belonging to the observation region 21, at a plurality of time instants relative to the above shear wave generation, said time instants being spaced of a first period Ti, the time instants of each observation in the first group of firing sequences being time shifted compared to a first firing sequence in the first group of a first time shift tsi different than the first period ΊΊ. This first time shift is preferably lower than the first period ΊΊ to avoid waste of time in the method .

The second group of firing sequences comprises a second number N₂ of firing sequences, each firing sequence of the second group comprising:

- an observation step during which the propagation of the above shear wave is observed at a plurality of points in a second portion 21₂ belonging to the observation region 21, at a plurality of time instants spaced of a second period T₂, the time instants of each observation in the second group of firing sequences being time shifted compared to a first firing sequence in the second group of a second time shift ts₂ different than the second period (Ti) . This first time shift is preferably lower than the first period Ti to avoid waste of time in the method.

Advantageously, the first number Ni and the first period Ti of first group of firing sequences are adapted or tuned so as to permit to observe the first portion 21i of the observation region 21. The first portion is at a first depth range DRi relative to the array, i.e. for example at depths comprised between Z and z ₁₂ (zi₂>Zn) .

Similarly, the second number N₂ and the second period T₂ of second group of firing sequences are adapted or tuned so as to permit to observe the second portion 21₂ of the observation region 21. The second portion is at a second depth range DR₂ relative to the array, i.e. for example at depths comprised between z_2i and z₂₂ (z₂₂>z_2i) .

The second depth range DR₂ is possibly at a higher depth compared to the first depth range DRi, i.e. z_2i>Zi₂. In that case two separated portions of regions in the observation region are imaged.

Otherwise, the second depth range overlaps the first depth range DRi, i.e. z₂i<Zi₂. In that case two overlapping portions of regions in the observation region are imaged (as represented on figure 6) .

However, if the second depth range is adapted for a higher depth than the first depth range, i.e. Z₂₂>z_i2, the second number N₂ is preferably greater than the first number Ni, because the compression waves need more time to travel from the transducers to the second portion 21₂ of observation region compared to the travel of time from the transducers to the first portion 21i of observation region. And therefore, the number of observation in each observation step is more and more limited as the depth is increased (for a constant depth range) .

Additionally, the first and second portions 21i, 21₂ may also have different width in X direction as represented on figure 6. Advantageously, the second portion 21₂ (at a deeper distance from the transducers array 2) is wider than the first portion 21 .

The sum of the first number Ni and the second number N₂ is preferably equal to the sequence number N. The method is then generalised to two groups of firing sequences. However, similar generalisation can be done for more than two groups of firing sequences.

In these variants comprising several groups of firing sequences, each one adapted for various depths ranges inside the medium, the skilled man would easily deduce and experiments time diagrams of firings and images to be ordered as described for the figures 4a-4c and 5a, 5b.

In all the embodiments of the method, a plurality of excitation steps al) are performed. Each excitation step al) during which a shear wave is generated inside the medium can be implemented:

- either by emitting a low frequency pulse inside the medium 20 by an acoustic transducer or vibrator arranged against the outside surface 20a of the medium (skin of the patient) , said low frequency pulse having an amplitude of the order of 1mm and a central frequency between 20 and 5000 Hz, and typically of the order of 50 Hz, as used in the device of patent US 6,770,033;

- or by emitting one or a plurality of ultrasound waves that are focused inside the medium 20, said focused ultrasound waves being monochromatic waves, or a sum of two monochromatic signals producing an amplitude modulated wave, and/or comprising focused ultrasound waves that are focused on a plurality of points inside the medium so that to generate a desired front wave shape (a shear wave that is a plane wave or a shear wave that is a focused wave or a shear wave that is divergent) , as used in the device of patent US 7,252,004.

In these embodiments, the processing step b) determines a movement parameter of the medium 20 at the multitude of points inside the observation region 21 on the bases of signals received by the array 2 during the observation steps. The following description explains how such processing can be implemented.

After each shot of index j of an unfocused ultrasound compression wave, the transducers ΤΊ, T_L picks up signals s_t/t) . The signal s_t/t) as sampled and digitized in this way is then stored, likewise in real time, in a memory.

In deferred time, the central unit CPU causes these signals to be processed by beam-forming steps (path- forming) by summing delayed signals to generate beamformed signals S/x, z) each corresponding to the image of the observation field after insonification of index j . For example, it is possible to determine a beamformed signal S/x, z) by the following formula:

• .<n- y ,ίί.ν._-)..v. {T(x, z) + d_i(x,∑) / V) where:

- Sjj is the raw signal perceived by the transducer of index i after ultrasound compression wave emission;

t(x, z) is the time taken by the ultrasound compression wave to reach the point of the observation region 21 having coordinates (x,z), with t = 0 at the beginning of the emission;

- dj(x, z) is the distance between the point of the observation region 21 having coordinates (x, z) and transducer of index i, or an approximation to said distance ;

V is the mean propagation speed of ultrasound compression waves in the viscoelastic medium 20 under observation; and

- Aj(x, z) is a weighting coefficient taking account of apodization relationships (in practice, we may have A,(x, z) = 1) .

The above formula applies mutatis mutandis when the observation region is three-dimensional (with a two- dimensional array of transducers), with space coordinates (x, z) being replaced by (x, y, z) .

After the optional path-forming step, the central unit CPU stores in a memory the image signals Sx, z), each corresponding to insonification or observation of index j.

These images are then processed in deferred time in a correlation sub-step by a correlation method and advantageously by a cross-correlation either in pairs, or preferably with a reference predetermined image, which may be :

- either a displacement image determined previously as explained above and used as a reference predetermined image for subsequent displacement images (or for firing sequence) ;

- or else determined during a preliminary initial observation step, like the above-mentioned successive displacement images, and said initial image of the viscoelastic medium is determined by combining the successive preliminary images, and in particular by averaging the pixel values of said preliminary images.

During this cross-correlation sub-step process, a cross-correlation function <S_j(x, z), S_J+i(x, z)> is maximized in order to determine the displacement to which each particle 22 giving rise to an ultrasound echo has been subjected. Examples of such cross-correlation calculations are given in the state of the art, in particular by O'Donnell et al . in "Internal displacement and strain imaging using speckle tracking", IEEE transactions on ultrasound, ferroelectrics , and frequency control, Vol. 41, No. 3, May 1994, pp. 314-325, and by Ophir et al . in "Elastography: a quantitative method for imaging the elasticity of biological tissues", Ultrasound Imag., Vol. 13, pp. 111-134, 1991.

This produces a set of displacement vectors generated by the shear waves in each position of the medium 20 under the effect of the shear wave (these displacement vectors may optionally be reduced to a single component in the example described herein) .

This set of displacement vectors is stored in the memory and can be displayed, for example, in particular by means of the screen 4a, in the form of a slow motion picture in which the values of the displacements are illustrated for example by a gray level or a color level. The motion picture of shear wave propagation can also be superposed on a conventional echographic image.

As an alternative to displacement calculus, it is possible to calculate the deformations of each point inside the medium 2, i.e. vectors whose components are the derivatives of the displacement vectors respectively relative to the space variables. These deformation vectors can be also displayed in the form of a motion picture.

From the displacement or deformation fields, the method can then comprise a map-making step c) during which, on the basis of the way in which the movement parameter (displacement or deformation) varies over time in the observation region 21, it calculates at least one propagation parameter of the shear wave, either at certain points in the observation region.

The propagation parameter of the shear wave that is calculated during the map-making step is selected, for example, from amongst: the propagation speed c_s of shear waves, the shear modulus μ, or Young's modulus E = 3μ, the attenuation a of the shear waves, the shear elasticity μ, the shear viscosity μ, or the mechanical relaxation time T_S of the tissues.

For example, it is possible at various points in the observation region 21 to calculate:

the value of the propagation speed c_s of the shear wave, (hardness of the tissues);

- the value of the mechanical relaxation time T_S of the tissues, (local viscosity of the medium) .

To do this, the following propagation equation (1) is used, with the displacement ^{li "} ) generated by the shear waves at each position U of the medium satisfying this equation:

where

p is the density of the tissues,

Ts is the mechanical relaxation time of the tissues, c_s is the shear wave propagation speed.

The shear wave propagation speed is directly related to Young's modulus E of the tissue by the following relationship: c_s = sqrt(E/(3p))

In the Fourier domain, the above wave equation (1) can be written as follows:

co².plJ{r, ω) = c_s ².(1 + jcoT_s)AUF,ω)

Where

O(r,oj) is the Fourier transform of the displacement field it(r,t) measured at each point by the above described method steps, and AU(r,co) is the Fourier transform of the spatial Laplacian of the displacement field u(r,t) .

Given that ^ωτ, ^<< 1 , we have :

U{r,(o)

c_s " = or.p.

AU(r,a>) (3)

where

φ(χ) is the phase of the complex variable x. The functions U(i o)) and AU(r.aj) are known for each point of the echographic image, so it is possible to estimate or determine by the above equations (3) and (4), the propagation parameter (s) at each point, and thereby to draw a map of those parameters. It is also possible to greatly improve the quality of the mapping by averaging over a frequency band of frequencies carried by the shear wave. The method of calculation would be the same when using not displacements but deformations in the observed medium 20.

Claims

1. An imaging method using shear waves for producing an image of an observation region (21) inside a medium (20), wherein the method is implemented by a processing unit connected to an array (2) of transducers in relation with said medium, and the method comprises the following steps:

a) a sequence number (N) of at least two or more firing sequences (101), each firing sequence comprising:

al) an excitation step (102) during which a shear wave is generated inside the medium, and

a2) an observation step (103) during which the propagation of said shear wave is observed at a plurality of points in the observation region, at a plurality of time instants relative to the shear wave generation of the firing sequence, said time instants being spaced of a period (T) , and

wherein the time instants of each observation in the firing sequence being time shifted compared to a first firing sequence of a time shift (ts) different than the period (T) , and

b) a processing step (104) during which a movement parameter of the medium is determined at the multitude of points inside the observation region (21) on the bases of signals received by the array during the observation steps of the firing sequences.

2. The method according to claim 1, wherein the time shift (ts) is lower than the period.

3. The method according to claim 1, wherein the time shift (ts) is equal to:

ts = (k-1) . (T/N) ,

where

T is the period, N is the sequence number of firing sequences, and k is an index of the firing sequence, k being an integer greater or equal to one, and being lower than N.

4. The method according to any one of claims 1 to 3, wherein the sequence number (N) depends on a distance between the array and the observation region.

5. The method according to any one of claims 1 to 3, wherein the sequence number (N) depends on a speed value of the shear wave.

6 . The method according to any one of claims 1 to 3, wherein, the sequence number (N) is determined on the bases of signals received during the observation step after the first firing sequence.

7. The method according to claim 1, wherein the firing sequences (101) at step a), comprises a first group of firing sequences and a second group of firing sequences, and wherein

- the first group of firing sequences comprises a first number (Ni) of firing sequences, each firing sequence of the first group comprising an excitation step and an observation step during which the propagation of the shear wave is observed at time instants spaced of a first period (Ti) , the time instants of each observation in the first group of firing sequences being time shifted compared to a first firing sequence in the first group of a first time shift (tsi) different than the first period (Ti) , and

- the second group of firing sequences comprises a second number (N₂) of firing sequences, each firing sequence of the second group comprising an excitation step and an observation step during which the propagation of the shear wave is observed at time instants spaced of a second period (T₂) , the time instants of each observation in the second group of firing sequences being time shifted compared to a first firing sequence in the second group of a second time shift (tS₂) different than the second period (ΊΊ) ,

and wherein

- the first number ( Ni ) and the first period (ΤΊ) of first group of firing sequences permits to observe a first portion of the observation region (21), said first portion being at a first depth range relative to the array,

- the second number (N₂) and the second period (T₂) of second group of firing sequences permits to observe a second portion of the observation region (21), said second portion being at a second depth range relative to the array .

8. The method according to claim 7, wherein the second depth range is at a higher depth compared to the first depth range .

9. The method according to claim 7, wherein the second depth range overlaps the first depth range.

10. The method according to claim 8 or claim 9, wherein the second number (N₂) is greater than the first number ( Ni ) .

11. The method according to claim 1, wherein each observation during the observation step a2) comprises the following sub-steps:

- emitting an unfocussed ultrasound wave into the medium by the array, and

12. The method according to claim 11, wherein the unfocussed ultrasound waves used for each observation are plane waves, said plane waves having predetermined angles relative to a direction of the array.

13. The method according to claim 12, wherein the predetermined angles are not all identical during one observation step a2) of a firing sequence.

14. The method according to claim 1, wherein the processing step b) comprises the following sub-steps:

15. The method according to claim 14, wherein the predetermined image is produced by at least one observation of the observation region during which no shear wave generated at excitation step al) is penetrating said observation region, and preferably the predetermined image is determined by a combination of a plurality of images produced by observations of the observation region during which no shear wave generated at excitation step al) is penetrating the observation region.

16. The method according to claim 14, wherein before sub-step b2), an image sequence is built by merging and interlacing the propagation images determined during sub- step bl) so as the images in the image sequence are ordered according to time evolution considering each firing sequence starting at a common initial time value.

17. The method according to claim 1, wherein the movement parameter is a displacement.

18. The method according to claim 1, wherein the processing step b) is followed by a mapping step c) during which a propagation parameter is calculated at at least some points inside the observation region on the basis of variation in the movement parameter over time, so as to determine a map of said propagation parameter in the observation region.

19. The method according to claim 18, wherein the propagation parameter calculated during the mapping step c) is selected from shear wave speed, shear modulus, Young's modulus, shear wave attenuation, shear elasticity, shear viscosity, and mechanical relaxation time.

20. An imaging device for implementing a method according to any one of the preceding claims, said imaging device using shear waves for producing an image of an observation region (21) inside a medium (20), said device comprising a processing unit (8) connected to an array (2) of transducers in relation with said medium, and the processing unit is controlling :

wherein the time instants of each observation in the firing sequence being time shifted compared to a first firing sequence of a time shift different than the period, and

b) a processing step during which a movement parameter of the medium is determined at the multitude of points inside the observation region on the bases of signals received by the array during the observation steps of the firing sequences.