METHOD FOR ANALYZING A MEDIUM THAT CAN REDUCE THE EFFECT OF ARTIFICIAL OBJECTS CAUSED BY STATIC DEFORMATION OF THE MEDIUM

Abstract

The present invention relates to a method for analyzing a region of interest in a viscoelastic medium, the method comprising a step (200) of acquiring signals representative of the motion of at least one shear propagation component and a static deformation component, and a processing step (300) of determining at least one property of the medium, the processing step comprising the following substeps: # determining an image of the region of interest; # estimating a motion image by comparing the images of the region of interest; # filtering the motion image to attenuate the static deformation component; # determining the properties of the medium from the filtered motion image.

Description

[Technical Field]

[0001] The present invention relates to the general technical field of imaging objects or diffuse media such as human or animal tissue.

[0002] More particularly, the present invention relates to an apparatus and method for measuring the viscoelastic properties of biological tissue of interest.

[0003] In particular, it is applied to the measurement of viscoelastic parameters of the human or animal liver, which measurements correlate with the amount of fibrosis present in the liver. [Background technology]

[0004] 1. General theory It is known to use shear wave elastography to measure the viscoelastic properties of tissue.

[0005] This technique involves observing the movement of shear waves passing through a region of interest (ROI) in a medium and determining parameters of the shear waves within the ROI (such as the propagation velocity of the shear waves), which are directly related to the properties of the analyzed medium (e.g., the shear modulus of the analyzed medium).

[0006] For this purpose, shear waves are generated in the ROI, for example by applying mechanical or acoustic stress.

[0007] The generated shear waves induce time-varying motion as they travel from their point of generation to multiple locations in the analysis medium.

[0008] These movements can be detected by a variety of imaging techniques, including magnetic resonance imaging (or "magnetic resonance elastography"), optical imaging (or "optical integrity elastography"), and ultrasound imaging (or "ultrasound elastography").

[0009] Ultrasound imaging uses an imaging probe, which consists of a casing and a transducer array mechanically fixed to the casing, to emit high frequency ultrasound waves and receive acoustic echoes that can track the movement of shear waves.

[0010] In particular, by tracking the motion due to shear waves as a function of time at multiple locations, it is possible to estimate the shear velocity and, consequently, one (or more) properties of the analyzed medium.

[0011] Characterization of viscoelastic properties of a medium, such as shear stiffness, using shear rate estimation has important medical applications because these properties are closely related to the state of the medium with respect to pathology. Typically, at least a portion of a tissue may become stiffer than the surrounding tissue, which indicates the onset or presence of a disease or condition such as cancer, tumor, or fibrosis.

[0012] However, conventional shear rate imaging techniques suffer from artifacts due to the presence of static deformations caused by probe movement or patient organ movement, which reduces the quality of the signal acquired by the probe and makes it difficult to accurately measure the viscoelastic properties of the medium.

[0013] The object of the present invention is to propose a method for analyzing a region of interest (ROI) of a viscoelastic medium, which makes it possible to reduce the effects associated with artifacts (due to the presence of static deformations) when determining one (or more) properties of the viscoelastic medium.

[0014] Before describing the present invention, the basic principles associated with shear wave elastography will be detailed to better understand the concepts associated with the static deformation problem described above.

[0015] 2. Theory of shear wave elastography 2.1 Wave propagation in elastic media Review the basic principles of wave propagation in elastic media.

[0016] For this purpose, we consider a homogeneous, isotropic, elastic solid medium. The wave equation in the absence of forces can be expressed as:

number

[0017] The wave equation assumes two propagating components in a medium: Compression propagation component (P wave) This is the shear propagation component (S wave).

[0018] These propagating components behave differently in the monochromatic approximation (plane wave). The compressional propagating component has zero divergence, is vertically polarized, and has a velocity c p Propagate with:

number

number

[0019] Therefore, these two propagating components can propagate at different velocities depending on the relative values ​​of the two Lamé coefficients. In biological tissue, which is nearly incompressible (λ >> μ), the speeds of the propagating components of the compressional and shear waves are c p and C S have very different values, typically around 1500 m / s for compressional waves and around 1-5 m / s for shear waves.

[0020] Furthermore, since biological tissue is mostly incompressible, the compressional propagation component has negligible amplitude compared to the shear propagation component, and the motion observed in the context of elastography techniques can be considered to be related only to shear effects.

[0021] The compressional propagation component is therefore understood to be that used in conventional ultrasound imaging (which allows to achieve acceptable spatial resolution at frequencies of the order of MHz) used to image motion within the framework of the present invention, while the shear propagation component is based on the estimation of the viscoelastic parameters of the tissue in elastography methods.

[0022] 2.2 Harmonic, transient and passive elastography Referring to Figure 1, the principle of harmonic elastography is based on the use of a mechanical source to generate mechanical waves in a medium containing low-frequency, shear-propagating components CIS, CIS' (and compression-propagating components CMP, CMP', which have negligible amplitude in biological tissues), the propagation of which is monitored by ultrafast ultrasound imaging.

[0023] In particular, Figure 1 shows how a shear propagation component CIS and a compression propagation component CMP occur during compression of a medium by a probe S.

[0024] By monitoring the propagation of the shear propagation component over time, the propagation velocity and Young's modulus of the medium can be determined.

[0025] The basic idea is based on a simple rheological model: indeed, if we assume that the medium is nearly incompressible (a valid approximation for biological tissues), the shear modulus (and therefore the shear wave velocity) is proportional to the Young's modulus of the medium as follows:

number

[0026] Therefore, by measuring the velocity of the shear propagation component, the Young's modulus of the medium can be estimated.

[0027] Ultrasound tissue Doppler, combined with ultrafast ultrasound to avoid aliasing problems, can monitor the shear propagation component and measure its velocity to determine Young's modulus.

[0028] Various techniques for generating shear propagating components have been discussed in the prior art.

[0029] The first approach uses a harmonic source external to the transducer array to generate the shear propagation component. The main problem with this approach is the use of an external vibration source, which significantly complicates the measurement implementation.

[0030] The second technique, called transient elastography, popularized by the "Vibration Controlled Transient Elastography (VCTE™)" method implemented in Fibroscan® (Echosens, Paris, France), solves this problem by using the vibration of a piezoelectric element to generate a shear-propagating component and then tracking the propagation of that shear-propagating component. The advantage of this device is that, due to symmetry effects, the longitudinally polarized shear-propagating component, i.e., the near-field term, is separated from the other components and captured by Doppler processing along the line recorded by the piezoelectric element. Therefore, the velocity of the shear-propagating component is measured by a simple Doppler processing that is scanned along the line recorded by the piezoelectric element. However, this second elastography technique developed in Fibroscan does not enable two-dimensional elastography and is not suitable for capturing stiffness changes within a medium.

[0031] A third technique, called ARFI or supersonic shear wave, uses a high-intensity focused ultrasound beam that generates a mechanical force called "radiative" mechanical force through nonlinear effects, depending on the implementation, to generate shear waves within the tissue itself. The same ultrasound transducer is used to generate the shear waves and measure their motion with ultrafast acquisition. This technique has the advantage of not generating static deformation of the medium itself, but it cannot prevent static deformations (e.g., those caused by heartbeats) from interfering with the generated waves in passive elastography, as described below.

[0032] The fourth approach, called passive natural wave elastography, utilizes shear waves naturally induced by the living body, such as those exerted by the heart on the liver during the cardiac cycle. The original approach is based on the diffusive motion field assumption: shear propagation components propagate in all directions. Under this assumption, the time derivative of the spatial cross-correlation of the motion field at a point relates to the causal and anticausal Green's functions at that point. The spatiotemporal properties of these Green's functions can be used to estimate the Young's modulus of the medium. In this context, the presence of static deformation introduces a bias in the cross-correlation measurement and, therefore, in the stiffness measurement. Such a solution is necessary for passive liver elastography applications, where the heart induces strong static deformations. However, no solution has been developed to reduce the effects of static deformation for this third approach.

[0033] 2.3 Static Deformation Problems Tissue Doppler allows the motion of a medium at each point to be measured as a function of time. For passive and transient elastography, this motion is the sum of three components: - two propagating components, compression and shear, as described in 2.1; - static deformation components as described below.

[0034] The static deformation component is caused by stresses on the medium: - probe, - an external shear wave generator, such as an external vibrator; - Or it is caused by an organ.

[0035] In the present invention, the terms "static deformation component" or "quasi-static deformation component" refer to the deformation of a medium induced by the application of stress, for example, from the surface of the medium, which manifests non-propagating shear phenomena. This static deformation component is different at each instant of time and therefore adds to the propagating component in the measured motion. This is therefore problematic for all methods based on measuring the propagating component, such as passive elastography and transient elastography techniques.

[0036] In biological tissue, this component is strong near the stress source and approaches zero as the distance from the stress source increases.

[0037] Such static deformation components may be due to probe motion and / or organ motion, such as beat-to-beat motion of the heart and lungs.

[0038] Below we describe a static deformation problem involving a probe: - casing, - an array of transducers housed within a casing and configured as follows: o emitting excitation ultrasound waves towards the medium to be analyzed (organ, biological tissue, etc.); o receiving acoustic echoes (i.e., ultrasound waves reflected at different interfaces of the medium being analyzed); - An exciter housed in a casing and consisting of: o Generates vibrations o communicated to the probe, By mechanically generating and observing shear waves, the properties of the medium being analyzed can be measured.

[0039] FIG. 2a shows a stationary probe S placed on a medium M to be analyzed.

[0040] When the vibrator is activated and generates and transmits vibrations to the probe S (generating shear propagation components), these vibrations continuously generate support stresses (Fig. 2b) and release stresses (Fig. 2c) on the medium of the probe S, resulting in local motion and static deformation.

[0041] As shown in Figures 2b and 2c, in the reference frame of the probe S, the movement of points located on the surface of the medium (such as point A) relative to the probe is zero (due to continuity), but the amplitude of the movement of points located below the surface of the medium (such as points B and C) relative to the probe increases with depth and tends towards an asymptote corresponding to the amplitude of the probe movement for the deepest points (such as point D).

[0042] To better understand this phenomenon, it is possible to use a physical model of the medium, as shown in Figure 3. This model consists of a stack of springs with stiffness coefficients CR1, CR2, and CR3 that increase as a function of depth, depending on the boundary conditions of the problem.

[0043] In incompressible media, the stiffness coefficients of these springs are related to the shear stress and depend on the Young's modulus of the medium and the geometry of the problem.

[0044] From the physical model shown in Figure 3, we can investigate the movement of points B and C located at different depths within the medium, from point A on the medium surface to ΔL, when a force F is applied by the probe.

[0045] From the force balance, we can deduce that the compressive forces ΔL2 and ΔL3 at points b and c are given by:

number

[0046] Thus, the movement measured at each of points A, B, and C in the medium due to the application of a force near point A varies with depth.

[0047] To illustrate, Figure 4a shows the profile of the static deformation component (i.e., the motion of the medium due to the probe motion and the motion relative to the probe) as a function of depth. Such a profile generally has a logarithmic shape that tends to an asymptote.

[0048] As shown in Figure 4b, this static deformation component DS (i.e., the movement of the medium due to the movement of the probe) is superimposed on the propagation component CP (i.e., the movement of the medium due to the shear propagation component in particular, in Figure 4c; the compression propagation component is negligible), making it difficult to observe only the shear propagation component and therefore difficult to measure the properties of the medium being analyzed.

[0049] Indeed, the artifact in Figure 4a (due to static deformation components caused by probe motion, patient organ motion, etc.) is particularly troublesome, as it: - The temporal frequency of the motion induced by this static deformation component DS is similar to that of the motion induced by the shear propagation component, making it difficult to suppress by pure temporal filtering; - The amplitude of the movement caused by this static deformation component DS is: o is significantly larger (1–2 orders of magnitude) than the motion induced by the underlying shear propagation component, and o This is because the amplitude of the motion induced by the shear propagation component decreases with depth (due to the effects of dispersion and absorption).

[0050] Description of the Prior Art A variety of methods have been used to avoid this artifact.

[0051] a. The first method is to use broadband excitation, as implemented in the Fibroscan® system, or equivalently, to hold an external source of vibration to generate shear waves.

[0052] More specifically, the Fibroscan® system proposes damping the vibrations of an external source to generate shear waves over short time periods, as described in WO 2018 / 078002. This damping (or "damping") facilitates the separation of the medium motion due to the static deformation component (which returns to zero after an oscillation cycle) from the medium motion due to the propagation of the shear propagation component.

[0053] Thus, the shear propagation component and the static deformation component are decoupled in time. Shear is observed when the static deformation component is very small in the medium.

[0054] A major problem with this method is that it is energy inefficient, as a lot of energy is wasted in generating broadband vibrations (i.e., in exciting and damping the external source of the shear-propagating component).

[0055] b. The second method, named after D.C. Mellema et al. and described in the paper "Probe Oscillation Shear Elastography (PROSE): A High Frame-Rate Method for Two-Dimensional Ultrasound Shear Wave Elastography," uses stroboscopy to systematically sample the same embodiment of the static deformation component.

[0056] Thus, the static deformation component no longer biases the measured shear propagation component.

[0057] The problem with this method is that the time sampling frequency (less than twice the vibration frequency of the external vibration source) is too low to effectively analyze the propagation of shear propagation components at low frequencies, e.g., 50 Hz.

[0058] Furthermore, this method has proven to be quite ineffective in vivo due to the inability to completely eliminate the static deformation component, as described in the paper "Probe Oscillation Shear Wave Elastography: Initial In Vivo Results in Liver" by D.C. Mellema et al.

[0059] c. Finally, particularly in magnetic resonance elastography, many methods based on filtering techniques have been investigated to reduce parasitic motion (due to compressional propagation components, but especially shear propagation components in unwanted directions) from motion due to the shear propagation components of interest. These methods include, among others, one-dimensional or two-dimensional filtering. An example of such a method is described in the paper "Spatio-temporal directional filtering for improved inversion of MR elastography images" by Manduca et al.

[0060] The drawback of these methods is that they require the use of two- or three-dimensional Fourier transforms, which are computationally expensive and can cause performance issues at film boundaries.

[0061] Furthermore, some of these methods are based on nonlinear directional filtering (thresholding in the frequency domain), which can introduce significant artifacts, e.g., the propagation of phantom waves where there is no propagation.

[0062] Inspired by these methods, filtering techniques using bandpass finite impulse response (FIR) filters were tested, but were unsuccessful due to their lack of robustness in separating the static deformation component from the shear propagation component. These unsuccessful attempts were demonstrated in a paper by D.C. Mellema et al. entitled "Probe Oscillation Shear Wave Elastography: Initial In Vivo Results in Liver," which showed those skilled in the art that the use of finite impulse response filters is not suitable for filtering the static deformation component.

[0063] Filtering techniques using infinite impulse response (IIR) filters have also been investigated, but these filters introduce phase distortion and transient effects, resulting in significant artifacts in the propagation velocity measurements of shear-propagating components.

[0064] The object of the invention is to propose a method for analyzing a region of interest of a medium, making it possible to overcome at least one of the aforementioned drawbacks. Summary of the Invention

[0065] To this end, the invention provides a method for analyzing a region of interest in a viscoelastic medium, the method comprising: acquiring signals representative of at least one shear propagation component and static deformation component of motion, said acquiring comprising the steps of: o continuously emitting acquisition signals in said viscoelastic medium over time; o detecting and recording received signals received from said viscoelastic medium, each received signal being associated with a respective acquired signal; A method comprising: The method further comprises a processing step (300) for determining at least one characteristic of the medium, the processing step comprising the following substeps: determining images of the region of interest of the medium from the received signals, each image being determined from a respective received signal and containing information representative of the region of interest at a different time; o estimating motion images by comparing images of said region of interest, each motion image containing information representative of a shear propagation component and a static deformation component; o filtering the motion images to attenuate static deformation components; o determining said at least one characteristic of said medium from said filtered motion image; characterized in that it comprises

[0066] In the present invention, the term "mechanical vibration" means a vibration that includes: - shear-propagating components generated naturally (i.e. passive elastography, where the shear-propagating components are generated by an organ such as the heart during the cardiac cycle) or artificially (i.e. harmonic or transient elastography, where the shear-propagating components are generated by the force of ultrasound radiation or by an external source, such as an inertial vibration exciter of the type described in WO 2022 / 084502); - and compressional propagation components, which are also generated naturally or artificially and whose amplitude in biological tissues is smaller than that of the shear propagation components. Because the amplitude of this compressional propagation component is much smaller than that of the shear propagation component, it will be ignored in the following explanation.

[0067] In the present invention, "shear wave" or "shear propagating component" refers to a transversely polarized "low frequency" wave in the far field, i.e., a wave that induces motion in the medium in a direction perpendicular to the direction of wave propagation, and whose frequency is typically comprised between 10 Hz and 1500 Hz.

[0068] In the present invention, the term "compression wave" or "compression propagation component" refers to a longitudinally polarized "low frequency" wave in the far field, i.e., a wave that causes motion of the medium in a direction parallel to the direction of wave propagation, and whose frequency is typically comprised between 10 Hz and 1500 Hz.

[0069] In the present invention, the term "static deformation component" refers to the deformation induced by stresses applied to the medium. In biological media, this static deformation component is related to shear phenomena.

[0070] In the present invention, the term "ultrasound" refers to longitudinally polarized "high frequency" compressional waves, i.e., waves that induce motion in a medium parallel to the direction of wave propagation, and whose frequency is typically comprised between 15 kHz and 20 MHz. In the context of the present invention, ultrasound is used as a means of obtaining images of the motion that can be used to monitor the propagation of shear propagation components.

[0071] For purposes of the present invention, the term "group velocity" of a wave refers to the speed at which the overall shape of the envelope of the wave's amplitude, known as the wave envelope, propagates through space.

[0072] Preferred, but non-limiting, aspects of the analytical method according to the present invention are as follows: - The filtering step consists of applying a finite impulse response filter, the characteristics of which are o The mean frequency of the shear propagation components must be between 10 Hz and 1500 Hz; and o Maximum group velocity of the shear-propagating component in the region of interest It is defined as a function of - A finite impulse response filter consists of a passband and a stopband, o The passband has at least a lower boundary K -6 wherein the lower boundary K is defined by -6 is equal to the ratio of the mean frequency of the shear component to the maximum group velocity of the shear component, the passband attenuation is less than 6 dB, and o The stopband has a lower boundary K -6 The blocking boundary K is proportional to -30 The stopband attenuation is at least 30 dB, and the lower bound K -6 and the blocking boundary K -30 defines the stiffness coefficient r of the finite impulse response filter, said stiffness coefficient being between 1 and 4.5; The minimum and maximum speeds may be determined based on predefined minimum and maximum hardness in the region of interest; The mean frequency of the shear component can be estimated from a signal representative of the time spectrum of the motion image; - the method may further comprise an excitation stage comprising the step of generating a shear propagation component by applying to the viscoelastic medium an excitation having the form of a low frequency impulse with a central frequency f comprised between 10 Hz and 1500 Hz; The finite impulse response filter is from 0 to 50 m ―1 It can be of high-pass type in the useful frequency range between - the finite impulse response filter can be obtained by combining an all-pass filter and a low-pass filter in the useful frequency range; The low-pass filter can be a rectangular window low-pass filter or a filter whose impulse response has a maximum value less than or equal to twice the mean value.

[0073] Other advantages and characteristics of the analysis method according to the invention will become more apparent from the accompanying drawings and from the following description of some variant embodiments given as non-limiting examples. [Brief explanation of the drawings]

[0074] [Figure 1] 1 is a schematic diagram showing the generation of compressional and shear waves when a medium is compressed. [Figure 2a] 1 is a schematic diagram of a stationary probe positioned above a medium to be analyzed. [Figure 2b] 1 is a schematic diagram showing the generation of static deformation by pressing a probe into a medium to be analyzed. [Figure 2c] 1A-1C are schematic diagrams showing the generation of static deformation by releasing a probe on a medium to be analyzed. [Figure 3] Schematic diagram of the spring model. [Figure 4a] Schematic illustration of the movement profile induced by the static deformation component as a function of the propagation direction of the shear propagation component σ. [Figure 4b] The motion profile induced by the superposition of the static deformation component and the shear propagation component is shown schematically as a function of the propagation direction of the shear propagation component. [Figure 4c] 4b is a schematic illustration of the movement profile induced by the shear propagation component as a function of the propagation direction of the shear propagation component after the static deformation component indicated by DS in FIG. 4b has been damped using the method described according to the present invention. [Figure 5] 1 is a schematic diagram of an analytical device according to the present invention; [Figure 6]1 is a schematic diagram of a variant embodiment of the analysis method according to the invention; [Figure 7] FIG. 4 is a schematic diagram of the impulse response of an exemplary filter that may be used in the filtering substep of the analysis method. DETAILED DESCRIPTION OF THE INVENTION

[0075] Various embodiments of the analysis method according to the invention will now be explained in more detail with reference to the figures, in which like elements are designated by the same numerical references in the different figures.

[0076] Below we explain how to process and analyze data acquired using an ultrasound elastography probe: - generating at least one low frequency elastic wave (called a "shear wave") in the medium; - Simultaneously with the generation of low frequency waves: o Emits high frequency ultrasound, o Receive acoustic echoes due to reflection of ultrasound waves in the medium; Observe the propagation of low-frequency elastic waves in the medium.

[0077] Those skilled in the art will appreciate that the analysis method can be implemented using passive elastography techniques (where shear waves are naturally generated in the body), where the probe may consist of an array of transducers for emitting high-frequency depth-dependent ultrasound (15 kHz to 20 MHz) and receiving acoustic echoes to observe the propagation of one or more shear waves naturally generated by the body.

[0078] It will also be clear to those skilled in the art that the analysis method according to the present invention can be carried out using imaging techniques other than ultrasound imaging (or "ultrasound elastography"), in particular magnetic resonance imaging (or "magnetic resonance elastography") or optical imaging ("optical coherence elastography").

[0079] 1. General theory FIG. 5 shows an example of an apparatus capable of carrying out the method for analyzing a medium described below.

[0080] This device: - Signal acquisition probe S - a control and processing unit Uc, o Probe S operation, and o for processing the signals acquired by the probe S; The device is configured to include:

[0081] 1.1 Impulse Elastography Probe The probe S is configured as follows: - an array of transducers for emitting ultrasound and receiving acoustic echoes; - An exciter that generates shear waves.

[0082] The exciter may be an inertial vibration exciter of the type described in WO 2022 / 084502, which is used to generate vibrations, the fixed part of which is mechanically fixed to the transducer array in order to transmit the vibrations to the probe in order to mechanically generate the shear waves necessary to measure the viscoelastic properties of the tissue being analyzed.

[0083] By using an inertial shaker, a probe of limited weight and size can be obtained.

[0084] Inertial vibration exciters can be used to generate shear waves of sufficient power to measure the viscoelastic properties of tissue.

[0085] Although it is possible to place the excitation device external to the probe and move the entire probe, this approach is not optimal due to size considerations.

[0086] Such probes are known in the art and will not be described in detail below.

[0087] 1.2 Control and Processing Unit The control and processing unit Uc is connected to the probe S by wired or wireless communication means.

[0088] Additionally, an inertial vibration exciter can be operated to generate one or more shear waves in the medium to be analyzed, which may be an organ of the patient, such as the liver.

[0089] More precisely, the control and processing unit Uc: - Controlling an inertial excitation device to generate vibrations that generate shear waves in the medium under analysis, - controlling the irradiation of the medium to be analyzed with ultrasound, - controlling the transducer to receive echoes reflected by the medium to be analyzed and convert them into received signals; - Process the received signal This makes it possible.

[0090] The control and processing unit Uc may be configured in one or more separate physical entities that may be separate from the probe S.

[0091] The control and processing unit Uc is configured, for example, as follows: one (or more) controllers 11, such as a smartphone, a personal assistant (or "PDA", an abbreviation for "Personal Digital Assistant"), or any type of mobile terminal known to those skilled in the art; and one (or more) computing devices 12, such as one or more computer(s), one or more microcomputer(s), one or more workstation(s), and / or other devices known to those skilled in the art including one or more processor(s), one or more microcontroller(s), one or more programmable logic controller(s), one or more application specific integrated circuit(s), and / or other programmable circuitry; - one (or more) storage devices 13, including one (or more) memories such as ROM / RAM memory, USB key, memory of a central server, etc.

[0092] In addition to storing data relating to the analysis of the media, the storage device 13 may also store programming code instructions for carrying out the steps of the analysis method described below.

[0093] 1.3 Operating principle With reference to FIGS. 5 and 6, the operation principle of the imaging device is as follows.

[0094] The control device 11 causes the vibration device to generate vibrations in the viscoelastic medium during the vibration step (step 100), thereby generating mechanical waves in the medium that include shear propagation components and compression propagation components that are negligible in biological tissue.

[0095] The motion of the viscoelastic medium induced by the propagation of mechanical waves in the medium is measured by ultrasound during an observation step (step 200) accompanying the excitation step (100).

[0096] For this purpose, the control device 11 controls the emission of ultrasonic waves (for example, at frequencies comprised between 15 kHz and 20 MHz) by the transducer of the probe S. These ultrasonic waves They penetrate the medium, where they are reflected by scattering particles contained in said medium (for example, collagen particles contained in the medium if it is human tissue), allowing their movement to be tracked. The ultrasound waves thus emitted can be "planar" (i.e., waves whose wavefronts are linear in the analysis plane (D, Z)), spiral, divergent or focused.

[0097] In particular, the observation step (200) consists of the following substeps: - the control device 11 controls the emission by the array of transducers of a succession of ultrasonic shots into the medium (at a rate comprised between 10 and 10,000 shots per second); The control device 11 controls the detection by the array of transducers and the recording (in real time) in a storage device 13 of the received acoustic signals consisting of echoes generated by the ultrasound waves due to their interaction with the diffusing particles of the medium.

[0098] The acoustic signals recorded in the storage device 13 are processed by a computer (typically with a time delay) in a processing step 300 .

[0099] More specifically, processing step 300 comprises the substeps of determining, by a conventional beamforming process, images of a region of interest (i.e., an area isolated by ultrasound) of a medium contained in the field of view, each image representing the region of interest after a respective shot. Since the shots are performed sequentially over time, each image represents the region of interest at a different time.

[0100] These successive images of the region of interest are compared by correlation, in particular cross-correlation: - In pairs - either a reference image (which can be a pre-determined displacement image or an image of the area of ​​interest obtained in a preliminary observation step) By comparison, an image of the media movement is determined.

[0101] These motion images represent the propagation of mechanical waves in the medium.

[0102] Such motion images also contain static deformation components, which can severely interfere with monitoring the propagation of mechanical waves used to determine one (or more) properties of the medium.

[0103] For this reason, the inventors have developed a substep of filtering the received acoustic signal, in particular the motion image, in order to remove the static deformation component of the measured motion and leave only the propagating component, i.e. the mechanical waves, in the motion image.

[0104] This filtering substep will now be described in more detail.

[0105] 2. Filtering Substep The filtering substep consists of applying a finite impulse response filter, the characteristics of which are - the mean frequency of the shear-propagating component, which is typically between 10 Hz and 1500 Hz, and - Maximum velocity of shear propagation components in the target region (especially maximum group velocity) is defined as:

[0106] The mean frequency of the shear propagation component depends on the solution selected to generate the shear propagation component. However, in all cases, this mean frequency is known or can be measured. For example, if a vibration exciter is used, the vibration exciter is configured to generate the shear propagation component at a desired mean frequency (e.g., a mean frequency of 50 Hz). In this case, the mean frequency of the shear propagation component is known in advance. If the solution selected to generate the shear wave is natural (i.e., shear wave generation by the patient's organs), the mean frequency of the shear propagation component can be calculated from the motion images by any technique known to those skilled in the art.

[0107] Similarly, the maximum group velocity of the shear-propagating component in a region of interest will vary depending on the type of region of interest being observed. However, for each type of region of interest, this maximum group velocity is known in the literature or can be measured. For example, if the region of interest consists of soft tissue (liver, spleen, prostate, breast, muscle, etc.), the maximum group velocity of the shear-propagating component is known to be on the order of 6 m / s.

[0108] Thus, the mean frequency and maximum group velocity of the shear propagation component are known, and the finite impulse response filter shown below can be determined.

[0109] 2.1 Filter Characteristics The frequency response (variation of the filter gain as a function of the frequency of the signal applied as input) of one example of a filter that can be used in the filtering substep is described with reference to Figure 7. This frequency response is a curve that represents the attenuation (dB) as a function of frequency (Hz).

[0110] In this embodiment, the finite impulse response filter is a "high pass" filter.

[0111] As shown in Figure 7, this frequency response (or Bode gain plot) is Stop band BA, Passband BP The transition band BT between the stopband BA and the passband BP It is composed of:

[0112] 2.1.1 Stopband The stop band BA corresponds to the frequency range over which a signal applied to the filter input appears strongly attenuated at the filter output. In this frequency range, the amplitude of a signal applied to the filter input is attenuated (approaching zero) at the filter output.

[0113] For a high-pass filter, the stop band BA can be defined by a stop boundary. The stop boundary (stop wavenumber) is the ratio of frequency to group velocity (which represents the propagation speed of the shear-propagating component) expressed in cycles per meter. This stop boundary wavenumber indicates the frequency at which the filter no longer produces an "attenuation" effect.

[0114] In the present invention, this "attenuation effect" is considered to occur when the amplitude of the signal applied to the filter input is divided by 32 (or more) at the filter output (-30 dB). -30 The term is used to characterize the upper limit of the stopband.

[0115] Therefore, the stop boundary K of the stop band BA -30 (or the stopping wavenumber K -30 ) represents the frequency at which the amplitude of the signal applied to the filter input is divided by 32 at the filter output (-30 dB).

[0116] In this stop band BA, the amplitude of the signal applied to the filter input is therefore attenuated by an attenuation factor FA2 of more than 30 dB.

[0117] 2.1.2 Passband The passband BP corresponds to the frequency range over which a signal applied to the filter input is little or not attenuated at the filter output. In this frequency range, the amplitude of a signal applied to the filter input is substantially unchanged (or only slightly attenuated) at the filter output.

[0118] For a high-pass filter, the passband BP can be defined by a lower boundary. This boundary (or wavenumber) corresponds to the ratio of frequency to group velocity (which represents the propagation speed of the shear-propagating component) in periods per meter. This boundary indicates the frequency at which the filter no longer has a "pass" effect.

[0119] In the present invention, this "pass effect" is considered to occur when the amplitude of the signal applied to the filter input is divided by 2 (or less) at the filter output (-6 dB). Therefore, in the following description, we use the term K to characterize the lower side of the passband. -6 The term is used.

[0120] Therefore, the lower boundary K of the passband BP -6 represents the frequency at which the amplitude of the signal applied to the input of the filter is divided by 2 at the output of the filter (-6 dB).

[0121] In this passband BP, the amplitude of the signal applied to the filter input is therefore attenuated with an attenuation factor FA1 of less than or equal to 6 dB.

[0122] Lower boundary K -6 teeth, - the mean frequency of the shear propagation component and - maximum group velocity of the shear propagation component and Equivalent to the ratio:

[0123] As mentioned above, the mean frequency of the shear-propagating component is known, as is the maximum group velocity of the shear-propagating component (on the order of 6 m / s for soft tissue).

[0124] Therefore, the lower boundary K of the passband BP -6 can be defined as a function of the type of tissue being analyzed and the type of excitation considered to generate the shear propagation component.

[0125] 2.1.3 Transition Band The transition band BT corresponds to the frequency range in which the input signal is neither strongly attenuated (stop band BA) nor substantially changed (pass band BP).

[0126] This transition band BT characterizes the filter's ability to retain or cut off frequencies with some abruptness, i.e., it describes the "responsiveness" of the filter.

[0127] This transition band is characterized by a stiffness factor (or "stiffness constant") that corresponds to the ratio between: - Lower boundary K of the passband BP -6 , and - the stop boundary K of the stop band BA -30 .

[0128] Therefore, the stiffness coefficient is defined as the ratio:

number

[0129] This stiffness factor indicates the slope of the amplitude response of the filter in the transition band between the region where the filter is strongly attenuated and the region where it is no longer attenuated.

[0130] The inventors have determined that a stiffness factor of 4.5 or less is preferred so as to be able to significantly attenuate the static deformation component of the received acoustic signal without unacceptably affecting the low shear frequencies that correspond to high shear wave velocities and therefore high stiffness.

[0131] The filter efficiency depends on two criteria of the transition band: - cutoff frequency, - Stiffness coefficient.

[0132] The cutoff frequency of a filter corresponds to the limit frequency at which the filter operates effectively. More precisely, the cutoff frequency can be expressed as the frequency at which the filter begins to have a "pass" effect. Therefore, the lower boundary K of the passband BP is -6 represents the cutoff frequency of the filter.

[0133] 2.2 Filter Variations Other types of filters may also be used to attenuate static distortion components in the signal received by the probe.

[0134] For example, a bandpass filter can be used instead of the highpass filter shown in FIG.

[0135] In the case of a bandpass filter, the passband BP is further limited by an upper limit K, which is equal to the ratio of the mean frequency of the shear-propagating component to the minimum group velocity of the shear-propagating component. max is defined by

[0136] If the values ​​of the mean frequency and minimum group velocity are known (either measurable or known from the literature depending on the tissue being imaged), then this upper limit K max is easily determined.

[0137] In any case, the selected filter must be within a useful frequency range, e.g., 0-50 m. -1 It has the behavior of a high-pass filter configured between

[0138] Advantageously, this finite impulse response filter can be obtained by combining "all-pass" and "low-pass" type filters in said useful frequency range, which makes it easy to obtain an effective filter with the following effects: - damping of static deformation components, and - Preservation of shear propagation components.

[0139] 2.3 Theory of the Invention To significantly reduce the influence of the static deformation component, we propose a filtering method along the direction of the static deformation component. This method is based on the idea that the static deformation component and the shear propagation component have very different spatial characteristics: - The short wavelength of the shear propagation component (at most a few cm at 50 Hz) and the group velocity in soft tissues such as the spleen and liver vary between 0.5 m / s and 6 m / s. - Long wavelength of static deformation component (tens of μm at 50 Hz).

[0140] 2.3.1 High-pass FIR filter In one embodiment, the filtering technique is based on a high-pass finite impulse response (FIR) filter (not necessarily a linear phase response filter), whose frequency characteristics are the mean frequency f of the investigated shear wave, the minimum group velocity C of interest, min and maximum group velocity C max It is defined as a function of

[0141] The average frequency of the shear waves can be estimated by the following formula:

number

[0142] Typical values ​​for the mean frequency of shear waves are between 10 Hz and 500 Hz. The value of the mean frequency is assumed to be known in most cases. For example, when shear waves are naturally generated (passive elastography), the value of the mean frequency can be measured. When shear waves are artificially generated (transient elastography), the value of the mean frequency is inferred from the properties of the mechanical source used.

[0143] section:

number

number

[0144] Typical values ​​of shear wave group velocity are between 0.5 m / s and 5.5 m / s in soft tissues such as the liver.

[0145] In particular, two values ​​of the wave number are of interest: K -6 This corresponds to the value of the wavenumber (cycles per meter) at which the filter attenuation is equal to 6 dB. K -30 It corresponds to the value of the wavenumber (cycles per meter) at which the filter attenuation is equal to 30 dB.

[0146] As an indication, we recall that the "wave number" (or "repetition") is a quantity proportional to the inverse of the wavelength. The "wave number" is the spatial analogue of the temporal frequency, and characterizes the propagation of a wave in space just as the temporal frequency does in time.

[0147] The above attenuation is calculated based on the passband filter gain values ​​as follows:

number

[0148] This amount can be, for example, the maximum value of the absolute value of the frequency characteristic in the pass band or the average value of the absolute value of the frequency characteristic in the pass band.

[0149] Combining these two values, the stiffness coefficient of the high-pass filter response is defined as:

number

[0150] This stiffness factor characterizes the average slope of the filter's frequency response between these two values:

number

[0151] At a higher level, it can be seen as quantifying the effectiveness of a high-pass filter.

[0152] This filter is very generic and can be created by one skilled in the art using finite impulse response filter synthesis tools such as the "firwin" and "firwin2" functions in the "scipy signal processing" library of the Python programming language.

[0153] Advantageously, the stiffness factor of the filter is selected to provide sufficient filter efficiency, for example, the stiffness factor value can be selected to be less than 4.5.

[0154] This type of high-pass filter can be created by those skilled in the art by constructing a phase-distortion FIR filter. Below is a code example for a shear wave, assuming a group frequency of 50 Hz:

[0155] A 2.5 cm long finite impulse response filter is defined.

number

[0156] In this case, the filter characteristics are as follows:K -6 =24.6m -1 , K. -30 =5.71m -1 , and r = 4.31.

[0157] The reader will understand that it is difficult to achieve a stiffness constraint of less than 4.5 using the "firwin2" function because the "firwin2" function imposes a phase linearity constraint (or equivalently, filter antisymmetry) to prevent phase distortion.

[0158] The following example illustrates this difficulty in cases such as the one above:

number

[0159] In this case, the stiffness coefficient is 16.7.

[0160] It is possible to vary the length of the filter and other parameters to lower the stiffness factor. Thus, in the example below, using "firwin" and a filter of size 3.2 cm, a stiffness lower than 4.5 can be achieved.

number

[0161] In this case, the filter characteristics are as follows:K -6 =25.1m -1 , K. -30 =5.6m -1 , and r = 4.48.

[0162] In this way, a filter can be created according to the stiffness specification, but the 3.2 cm length required to create the filter is lost for visualization of shear wave velocity (reducing the depth of the region of interest).

[0163] Of course, you can use other Matlab or python functions (other than fiwin2 functions).

[0164] 2.3.2 FIR Bandpass Filter In one embodiment of the present invention, the filtering technique is based on a one-dimensional bandpass filter applied along the transformation.

[0165] Similar to the high-pass filter defined in 2.4.1, we focus on the following quantities: -K -6 corresponds to the value of the wavenumber (cycles per meter) at which the attenuation of the filter is equal to 6 dB in the low transition band, - stiffness coefficient of the filter.

[0166] It should be noted that the corresponding stiffness coefficients are defined in this case for the lower transition band of the bandpass filter, which are subject to the same constraint as the highpass filter (i.e., stiffness coefficients of 4.5 or less). No constraints are imposed on the stiffness coefficients of the upper transition band.

[0167] The length of the impulse response should not exceed a few centimeters, with a typical size being 1 to 3 cm, in order to limit boundary effects while maintaining a sufficiently large area within the target medium.

[0168] Those reading this specification will appreciate that many modifications may be made to the invention as described above without materially departing from the novel teachings and advantages set forth herein.

[0169] Accordingly, all such modifications are intended to be included within the scope of the appended claims.

Claims

1. 1. A method for analyzing a region of interest in a viscoelastic medium, the method comprising acquiring (200) signals representative of the motion of at least one shear propagation component and a static deformation component, the acquiring (200) comprising the following steps: o continuously emitting acquired signals in said viscoelastic medium over time; o detecting and recording received signals received from said viscoelastic medium, each received signal being associated with a respective acquired signal; A method comprising: The method further comprises a processing step (300) for determining at least one characteristic of the medium, the processing step comprising the following sub-steps: determining images of the region of interest of the medium from the received signals, each image being determined from a respective received signal and containing information representative of the region of interest at a different time; o estimating motion images by comparing images of said region of interest, each motion image containing information representative of a shear propagation component and a static deformation component; o filtering the motion images to attenuate static deformation components; o determining said at least one characteristic of said medium from said filtered motion image; A method comprising:

2. The filtering step comprises applying a finite impulse response filter having the following characteristics: o the mean frequency of said shear propagating component is between 10 Hz and 1500 Hz; and o The maximum group velocity of the shear propagation component in the region of interest The analytical method of claim 1, wherein the function is defined as:

3. the finite impulse response filter comprises a passband (BP) and a stopband (BA), o said passband (BP) has at least a lower boundary K －６ and the lower boundary K －６ is equal to the ratio between the mean frequency of the shear component and the maximum group velocity of the shear component, and the attenuation in the passband is 6 dB or less; o The stop band (BA) has at least a lower boundary K －６ The blocking boundary K is proportional to －３０ and the attenuation in the stopband is greater than or equal to 30 dB, with a lower bound K －６ and blocking boundary K －３０ defines a stiffness coefficient r of the finite impulse response filter, said stiffness coefficient being between 1 and 4.5; The analytical method according to claim 2:

4. 4. The method of claim 2, wherein the minimum and maximum velocities are determined based on predefined minimum and maximum hardnesses in the region of interest.

5. 5. The method according to claim 2, wherein the mean frequency of the shear component is estimated from a signal representative of the time spectrum of a motion image.

6. 5. The method according to claim 2, further comprising an excitation stage (100) comprising generating the shear propagation component by applying to the viscoelastic medium an excitation having the form of a low frequency impulse with a center frequency f comprised between 10 Hz and 1500 Hz.

7. The finite impulse response filter is configured to filter a range of 0 to 50 m ―１ 7. The method according to claim 2, wherein the useful frequency range is high-pass type.

8. 8. The method of claim 7, wherein the finite impulse response filter is obtained by combining an all-pass filter and a low-pass filter in the useful frequency range.

9. 9. The method of claim 8, wherein the low-pass filter is a rectangular window low-pass filter or a filter whose impulse response has a maximum value that is less than or equal to twice its average value.

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