Method and system for ultrasonic characterization of a medium

Abstract

To provide a method for evaluating ultrasonic characterization of a medium.SOLUTION: A method for ultrasonic characterization of a medium, comprises the steps of: generating a series of incident ultrasonic waves; generating an experimental reflection matrix Rui(t) defined between an emission basis (i) as input and a reception basis (u) as output; and determining a focused reflection matrix RFoc (rin, rout, δt) of the medium between an input virtual transducer (TVin) calculated based on a focusing as input to an experimental reflection matrix and an output virtual transducer (TVout) calculated based on a focusing as output from the experimental reflection matrix, the responses of the output virtual transducer (TVout) being obtained at a time instant that is shifted by an additional delay δt relative to a time instant of the responses of the input virtual transducer (TVin).SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present disclosure relates to a method and system for ultrasonic characterization of a medium, with particular application in medical imaging and non-destructive testing, and more generally in all fields where ultrasound imaging can be used. [Background technology]

[0002] The field of acoustic imaging involves the characterization of completely or partially unknown environments by active exploration with ultrasound, which is the principle behind ultrasound instruments used in medical imaging.

[0003] The resolution of an acoustic imaging system can be defined as the ability to distinguish fine details of an object. In principle, acoustic imaging systems are diffraction limited, so the theoretical resolution is given by λ / 2, where λ is the wavelength of sound in the medium, or by the finite aperture angle of the detector. In practice, however, the resolution is often reduced by variations in the speed of sound when the propagation medium is non-uniform.

[0004] In fact, for most of the time during acoustic imaging, the medium is homogeneous and has a constant speed of sound, c 0 It is assumed that the acoustic environment is uniform. However, the assumption of a uniform environment does not always hold true. For example, in the case of liver ultrasound, the probe is placed between the ribs of the patient. The acoustic waves propagate through layers of fat and muscle before reaching the target organ. Each soft tissue has different mechanical properties. Thus, the speed of sound is far from uniform and varies, for example, from 1450 m / s in fat tissue to 1600 m / s in the liver. The variation in the speed of sound causes different phase shifts in the waves depending on the area they are propagating. This leads to aberrations of the acoustic wavefront and thus to distortions of the resulting ultrasound image, which in turn leads to a deterioration of the resolution and contrast. These aberrations do not allow a reliable image reconstruction and may therefore impair, for example, the diagnostic results.

[0005] As shown in Figures 1A-1C, conventional ultrasound techniques use an array 10 of piezoelectric transducers 11 that can independently emit and / or receive ultrasound. Each transducer is located by a vector u. By pointing such an array at the medium of interest, it is possible to insonify and image the medium in a variety of ways.

[0006] The first method for generating an ultrasound image of the medium under investigation is to calculate the vector u in The first step is to emit an ultrasonic pulse from one of the transducers of the array, whose location is specified by (left diagram of FIG. 1A). This results in a diverging cylindrical (or spherical) incident wave in the case of a 1D or 2D array of transducers. This wave is reflected by scatterers 21 in the medium 20, and the backscattered field is recorded by each transducer 11 as a function of time (right diagram of FIG. 1A). By repeating this process using each transducer in turn as a source, a set of impulse responses R(u out ,u in ,t) is measured (where vector u out represents the detector position). These responses are expressed based on the transducer as a reflection matrix R uu (t). The advantage of such a measurement is that this matrix contains all the information about the analyzed medium and allows a series of matrix operations to be performed, for example for the purpose of forming an image of the medium. On the other hand, such an acquisition assumes that the medium remains fixed during the measurement period, which can be very difficult for in vivo use. Also, the energy emitted by one piezoelectric element is low, which can lead to a poor signal-to-noise ratio.

[0007] Another known method for generating an image of the analyzed medium is to focus the emission using a beamforming method. As shown in the left diagram of FIG. 1B, this method is based on a uniform velocity model and applies a set of appropriate delays to the transducer 11 to correct the propagation time of the waves and to obtain a position rin The assumed velocity of the waves employed is c 0 Due to the physical constraints of diffraction, the emitted ultrasound waves are concentrated in an area defined by the aperture of the ultrasound probe. A focusing step is also performed during reception to construct an ultrasound image. The set of echoes captured by the elements 11 of the array 10 is then processed to simulate the effect of the lens during reception, as shown in the right diagram of FIG. 1B. The signals received by the transducer are shifted in time to return them to the same phase. The delay is identical to that applied during emission. During the emission phase, all signals are focused on a position point r in At reception, the interference occurs at the same point r out =r in The signal from the ballistic time t=(||u out -r in ||+||u in -r in ||) / c 0 The signals at the input and output are electronically interfered as a sum of the signals at the input and output. This sum gives the final focusing result at the input. The method shown in FIG. 1B is known as confocal double focusing at transmit and receive and allows direct imaging of the reflectivity of a medium with lateral resolution limited by diffraction, good axial resolution limited only by the initial pulse duration, and good contrast. However, this method is time consuming as it requires physically focusing the emission to each point in the medium for each row of images, or at least to a given depth.

[0008] Another imaging technique that has been recently developed is to generate an image of a medium by insonifying the medium with a series of plane waves. Figure 1C shows the principle of this so-called plane wave ultrasound technique, which is described, for example, in Non-Patent Document 1. A delay is applied to each signal upon emission (left diagram of Figure 1C) and the signal is rotated at an angle θ in At reception (right side of Fig. 1C), the incident angle θ in The field R(u) backscattered by the medium for a series of different incident plane waves out ,θ in, t) for all position sensors u out These responses are measured by the reflection matrix R, which is defined between the spatial Fourier basis (or plane wave basis) as the input and the transducer basis as the output. uθ Once this matrix is ​​recorded, we shift the signals in time before coherently adding them together to produce a matrix for each location point r in The data is digitally focused during transmission and reception for 100 MHz to 100 MHz, thus advantageously reducing the number of data acquisitions required to form an ultrasound image compared to standard ultrasound (focused emission) for the same level of ultrasound image contrast and resolution.

[0009] FIG. 2 shows the effect of environmental aberrations on conventional ultrasound imaging (FIGS. 1A to 1C). These aberrations occur when the speed of sound in a medium, c(r), is proportional to the constant sound speed c 0 This appears when the delays initially determined based on this assumption and applied to each transducer of the array during transmission and reception are no longer optimal for evaluating the image of the medium. In FIG. 2, the aberration layer 22 induces a distortion of the incident wavefront. At stage 25 during emission or excitation, the delay law used is not capable of concentrating the acoustic energy in an area bounded by the diffraction limit (commonly called the focal spot). At stage 26 during reception, the delay law used is not capable of accurately selecting the ultrasonic signals resulting from the focal point of the medium, but mixes the signals resulting from equally aberrated focal points. This results in double aberrations in the image construction process that significantly degrade the resolution. A new delay law can then be recalculated to compensate for the effects of the aberration layer, for example adding an additional delay law to the delays commonly used in beamforming.

[0010] However, these aberration corrections do not completely correct the aberrations and the degradation of resolution. A better estimation of the focusing quality in the medium is needed.

[0011] In Non-Patent Document 2, the statistical properties of the fields reflected by a random medium under simple scattering conditions are studied. In particular, it is found that for a focused incident wave, the spatial covariance of the reflected field is proportional to the Fourier transform of the transmission aperture function from the far field. In other words, this theorem explains that the study of the statistical properties of the reflected field in the far field makes it possible to determine the quality of focusing of an incident wave in a medium.

[0012] However, because this technique requires statistical averaging of the corrections of the reflected field over a large number of random points, i.e., over a large number of focusing points of the incident wave, it only provides a global average estimate of the resolution of the ultrasound image, but does not allow for an accurate local assessment of the quality of focusing at each point in the image. Furthermore, this technique is only valid under simple scattering conditions.

[0013] Therefore, it is necessary to propose a method to overcome the above-mentioned drawbacks. [Prior art documents] [Non-patent literature]

[0014] [Non-Patent Document 1] G. Montaldo et al., “Coherent plane-wave compounding for very high frame rate ultrasonography and transient elastography”, IEEE Trans. Ultrason., Ferroelect. Freq. Control 56 489-506, 2009 [Non-Patent Document 2] Raoul Mallart and Mathias Fink, “The van Cittert-Zernike theorem in pulse echo measurements”, J. Acoust. Soc. Am. 90 (5), November 1991 [Non-Patent Document 3] Alfonso Rodriguez-Molares et al., “Specular Beamforming”, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Volume: 64, Issue: 9, Sept. 2017 Summary of the Invention [Means for solving the problem]

[0015] According to a first aspect, the present disclosure relates to a method for ultrasonic characterization of a medium to determine an integrated speed of sound value, the method comprising the following steps: - An array (10) of transducers (11) is used to direct a series of multiple incident ultrasound waves (US in ) (the series of incident ultrasonic waves being of emission (transmission, sending) basis (i)); - the experimental reflection matrix R defined between the emission basis (i) as input and the reception basis (u) as output ui generating (t); - spatial position r in Input virtual transducer (TV in ) and spatial position r out Output of virtual transducer (TV out ) is the focused reflection matrix RFoc(r in ,r out , δt) (output virtual transducer (TV out ) response of the input virtual transducer (TV in ) at a time shifted by an additional delay δt relative to the time of the response of - Input virtual transducer (TV) over additional delay range in) (the wavefront image is calculated based on the sound velocity c in the medium) 0 and the wavefront image is determined based on: - Focused reflection matrix RFoc(r in ,r out , δt) and - δt(Δr out ) = -sign(Δz out )·|Δr out | / c 0 Ballistic propagation relations of the type, During the ceremony, δt is the additional delay, |Δr out | is the input virtual transducer (TV in ) and output virtual transducer (TV out ) and Δr out =r out -r in and Δz out is the spatial position vector Δr out is the component along the depth axis Z); - the depth position Δz of the center of the focused spot in the wavefront image (0) (r in ) determining the - Composite sound speed c based on the following formula (1) (r in ) steps to calculate

number

[0016] With such a configuration, the method advantageously enables non-invasive probing of the medium locally at any point and direction at any time difference relative to the ballistic propagation time of ultrasound in the medium.

[0017] The method makes it possible to determine a focusing image of the waves in the ballistic reference frame, this image is called the "wavefront image", and makes it possible to follow the propagation of the waves towards a focusing point and to estimate the quality of the focusing, for which a (synthetic) sound speed value can then be derived.

[0018] The local determination of the composite sound velocity constitutes a new contrast in ultrasound imaging that complements the reflectivity image ("classical" ultrasound image). The determination of the sound velocity at the focal point allows to improve the quality of the ultrasound image by calculation without the need for new emission and / or repeated acquisitions, thus presenting economic advantages, for example in the case of human examinations, with significant savings in terms of staff, patients, equipment and time. It also allows the characterization of the propagation medium in near real time, which is a particularly important advantage for in vivo measurements.

[0019] Various embodiments of the method according to the present disclosure may optionally employ one or more of the following configurations.

[0020] According to one example, the center of the focused spot is determined by finding the point of maximum in the wavefront image.

[0021] According to one example, the wavefront image is determined only on the depth axis Z.

[0022] According to one example: - determining the wavefront image and the depth position Δz of the focused spot (0) (r in Between the steps of determining the coherence region, a step of linearly combining a set of wavefront images corresponding to a given coherence region to improve the wavefront image is performed, where each wavefront image in the set is located at a different spatial location r in Input selected in Virtual Transducer TV in andr out = Δr out +r in At a spatial position r such that outOutput of Virtual Transducer TV out and Δr out is predetermined and identical for all wavefront images in the set, and the reference input virtual transducer (TV in,ref The input virtual transducers selected to obtain an improved wavefront image for the reference input virtual transducer TV are close to each other. in,ref is a characteristic representative of the input virtual transducer of the set of wavefront images used for the selected coherence region ZC; - Depth position Δz (0) (r in In the step of determining the wavefront image, the improved wavefront image is used instead of the wavefront image, and the depth position of the center of the focused spot is determined by the reference input virtual transducer TV in,ref The depth position of the center of this focal spot is relative to the spatial position of the reference input virtual transducer TV in,ref The resultant sound speed c at the spatial position (1) (r in,ref ) can be estimated.

[0023] According to one example, a singular value decomposition (SVD) of a set of wavefront images is computed to find the singular vector (W 1 ), the linear combination is determined, and its singular vectors (W 1 ) is input to the reference input virtual transducer (TV in,ref ) resulting in an improved wavefront image corresponding to

[0024] According to one example, an input virtual transducer (TV) selected to substantially cover the entire region of interest in the medium for a linear combination of a set of wavefront images. in ) is used to calculate a composite sound velocity, thereby determining the optimum sound velocity in the medium.

[0025] According to one example: - Focused reflection matrix RFoc(r in ,r out, δt), determining a wavefront image, and determining a depth position Δz of the center of the focused spot in the wavefront image. (n) The step of determining the sound speed c used in the first step may be used instead of the previously used sound speed or 0 Instead of the synthetic sound speed c determined in the previous iteration, (n) Iterated using; In the step of calculating the resultant sound speed, the following recurrence formula is used:

number

[0026] According to one example: - Reverse the roles of the input virtual transducer and the output virtual transducer to obtain the synthetic sound velocity value c (1) (r out ) is determined; - synthetic sound speed c for the input virtual transducer (1) (r in ) and the composite sound velocity c for the output virtual transducer (1) (r out ) to obtain an improved synthetic sound speed.

[0027] According to one example, the method comprises: in Input virtual transducer (TV in ) or spatial position r in,ref Reference input virtual transducer (TV in,ref ) for a plurality of points in the medium, each of which corresponds to a respective one of the plurality of points, thereby determining a synthetic sound speed image.

[0028] According to one example, a synthetic sound speed image is determined and values ​​at each point of the sound speed image are calculated from the values ​​of the synthetic sound speed image.

[0029] According to one example, in the step of calculating the focusing reflection matrix: Input Virtual Transducer (TV in ) is calculated for the spatial position r in The emitting base and the input virtual transducer (TV in ) using the outward flight time of the wave between ui Corresponding to the focusing process at the input based on (t); Output Virtual Transducer (TV out ) is calculated for the spatial position r out The output virtual transducer (TV out ) and the return wave flight time between the transducer at the receiving base u, ui Corresponding to the focusing process at output based on (t); The additional delay δt is the time lag added to the outbound and inbound flight times during these focusing processes.

[0030] According to one example, the focusing reflection matrix is ​​calculated according to the following formula:

number

[0031] According to a second aspect, the present disclosure relates to a system for ultrasonic characterization of a medium, configured to carry out the method for ultrasonic characterization described above to determine a composite sound velocity value at every point in the medium. The system for ultrasonic characterization according to the second aspect comprises: an array of transducers for generating a series of incident ultrasonic waves into a region of the medium and for recording ultrasonic waves backscattered by the region as a function of time; and - a computation unit coupled to the array of transducers for carrying out the method according to the first aspect.

[0032] Other characteristics and advantages of the above technique will appear on reading the following detailed description, given by way of example and without limitation with reference to the drawings in which: [Brief description of the drawings]

[0033] [Figure 1A]A known transmission / reception mechanism for ultrasound imaging and quantification (as described above) is shown. [Figure 1B] A known transmission / reception mechanism for ultrasound imaging and quantification (as described above) is shown. [Figure 1C] A known transmission / reception mechanism for ultrasound imaging and quantification (as described above) is shown. [Diagram 2] 1 illustrates the effects of aberrations in ultrasound imaging according to the prior art (as discussed above). [Diagram 3] 1 illustrates an example of a system for ultrasound characterization for performing a method for ultrasound characterization according to the present disclosure. [Figure 4] 1 shows definitions used in the method for ultrasonic characterization according to the present disclosure. [Diagram 5] 1 shows an ultrasound image in which a location associated with an echogenic element is selected and propagation images near that location are obtained for multiple additive delays. [Figure 6] 1 shows an ultrasound image in which a location associated with a set of sub-resolution scatterers of comparable reflectivity is selected, and propagation images near that location are obtained for multiple additive delays. [Figure 7] Shown is an ultrasound image in which a set of locations associated with sub-resolution scatterers of comparable reflectivity are selected, and a coherent wave propagation image resulting from combining the propagation images associated with each selected location is obtained for multiple additive delays. [Figure 8] Figure 8A shows the time variation curves of the intensity of the center points of the propagation image and the coherent wave propagation image relative to a position corresponding to a sub-resolution scatterer of comparable reflectivity, and Figure 8B shows the frequency spectrum of the curves of Figure 8A. [Figure 9] Figure 9A shows an amplitude image of the wavefront associated with the same location as in Figure 5. Figure 9B shows the real part of the same wavefront image as Figure 9A. [Figure 10] 5. The amplitudes of several wavefront images obtained for three assumed sound velocities relative to the same positions chosen in FIG. 5 and the intensity curves on the ordinate axis Δz of these wavefront images are shown. [Figure 11] An ultrasound image and the corresponding image of the composite sound speed are shown. [Figure 12] Shown are an ultrasound image (Image A) obtained without aberration correction, an ultrasound image (Image B) obtained with conventional lateral aberration correction, and Image C obtained with axial aberration correction using the wavefront image measurement results according to the present disclosure. [Figure 13] Shown is an ultrasound image (A) containing multiple regions of muscle fibers with different orientations, and wavefront images (B, C, D, E) corresponding to different regions of the medium. [Figure 14] 1 shows a curve for calculating the angle of the preferred direction of inclination of muscle fibers in the medium. [Figure 15] 1 shows a matrix of determined preferred directions superimposed on an ultrasound image of a medium with muscle fibers. [Figure 16] Shown is an ultrasound image (A), a close-up of a region around a particular point on the ultrasound image (B), the real part of the local time signal for that particular point (C) and the estimated amplitude (D), and a spectral analysis of the local time signal of the medium. [Figure 17] Shown is an ultrasound image (A) and an estimate of the average spectrum as a function of depth Z for the corresponding ultrasound image in the figure. [Figure 18] 1 shows an ultrasound image (A) and a spectral correlation image determined for the corresponding ultrasound image in the figure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0034] In the various embodiments described with reference to the drawings, similar or identical elements have the same reference numerals unless otherwise specified.

[0035] In the following detailed description, only certain embodiments are described in detail for clarity of the disclosure; however, these examples do not limit the general scope of the principles emerging from the disclosure.

[0036] The various embodiments and aspects of the present disclosure may be combined or simplified in various ways. In particular, steps of the various methods may be repeated, reversed and / or performed in parallel, unless otherwise specified.

[0037] The present disclosure relates to a method and system for ultrasonic characterization of a medium, with particular application to medical imaging of living or non-living tissue. The medium may be, for example, a heterogeneous medium that one wishes to characterize in order to identify and / or characterize heterogeneities. Optionally, the method and system may also be applied to non-destructive testing of products such as metal parts. Thus, the characterization technique is non-invasive and medium-contained.

[0038] 3 shows an example of a system 40 for ultrasonic characterization for carrying out the method of ultrasonic characterization of a medium 20, such as a heterogeneous medium, according to the present disclosure. The system 40 comprises at least one array 10 of a plurality of transducers 11, for example a linear, two-dimensional or matrix array, the transducers being for example piezoelectric ultrasonic transducers in the conventional form of rigid rods in direct or indirect contact with the medium 20. The array of transducers is for example part of a probing device 41 (usually called a probe), the array of transducers being connected to a computing unit 42, itself connected to or associated with a display device 43, which transmits electrical signals to and records electrical signals from each transducer 11. The ultrasonic transducers then convert the electrical signals into ultrasonic waves and vice versa. A "connection" or "coupling" between the exploration device 41, the computing unit 42 and the display device 43 is understood to mean any kind of wired connection, electrical, optical or other kind of wireless connection, using protocols such as WiFi, Bluetooth, etc. Such connections or couplings may be unidirectional or bidirectional.

[0039] The calculation unit 42 is configured to perform calculation or processing steps, in particular to perform steps of the method according to the present disclosure. By convention, a spatial reference system for the medium 20 is defined with a first axis X and a second axis Z perpendicular to the first axis. For simplicity, the first axis X corresponds to the lateral direction in which the transducers 11 are aligned for a linear array, and the second axis Z corresponds to the depth of the medium 20 relative to the array 10 of transducers 11. This definition is adaptable in context and can be extended to, for example, a three-axis spatial reference system for a two-dimensional array 10.

[0040] In Figure 3, as in the rest of this specification, reference is made to one transducer array for transmission and reception, although in the more general case multiple transducer arrays can be used simultaneously. Similarly, an array can consist of one to N transducers of the same or different types. The transducers can be transceivers, or some can be only transmitters and others only receivers.

[0041] A transducer array may, for example, function as both a transmitter and a receiver, or may consist of several transducer subarrays, some of which are dedicated to emitting ultrasound waves and others to receiving them. The term "transducer array" is understood to mean at least one transducer, a row of aligned or non-aligned transducers, or a matrix of transducers.

[0042] When referring in this disclosure to the performance of computational or processing steps, particularly steps of the present methods, it is understood that each computational or processing step can be performed by software, hardware, firmware, microcode, or any suitable combination of such techniques or related techniques. When using software, each computational or processing step can be performed by, for example, interpretable or executable computer program instructions or code. Such instructions can be stored or transmitted to a storage medium readable by a computer (or computing unit) and / or executable by a computer (or computing unit) to perform the computational or processing step.

[0043] Analysis of points in a medium by a focused reflection matrix The present disclosure discloses a method and a system for ultrasonic characterization of a medium. In the realistic case, the medium is assumed to be inhomogeneous. The method and the system are based on the definition shown in FIG.

[0044] Define the following points in the medium: - spatial position r of the spatial reference system of the medium in The first point P1; - spatial position r of the spatial reference system of the medium out The second point P2.

[0045] These spatial positions r in andr out is shown in bold in the original text and is denoted as a position vector, where the vector is taken in the spatial reference frame (X,Z) of the medium. Other representations and definitions of the position of a point are possible and available to those skilled in the ultrasound art.

[0046] These two points P1 and P2 are selected to be at a short distance from each other at ultrasonic frequencies, ie a few millimeters from each other, for example less than twenty millimeters (20 mm).

[0047] As shown in FIG. 4 , the method for ultrasound characterization performed by the computing unit 42 of the system 40 comprises the following steps: - A series of multiple incident ultrasonic waves (US) are generated in a region of the medium using an array 10 of multiple transducers 11. in (wherein the series of multiple incident ultrasonic waves is of emission basis i); - the experimental reflection matrix R defined between the emission basis i as input and the reception basis u as output ui generating (t); - spatial position r in Input of virtual transducer TV in and spatial position r out Output of Virtual Transducer TV out The focused reflection matrix RFoc(r in ,r out , δt) (output virtual transducer TV out is the input virtual transducer TV in (The time is shifted by an additional delay δt relative to the time of the response of

[0048] Focused reflection matrix RFoc(r in ,r out , δt) response corresponds to the acoustic pressure field computed at any point in the medium.

[0049] The input emission (transmission, wave transmission) basis i is the basis of the waves generated by each transducer 11 of the array 10, respectively, as described above with respect to Figures 1A to 1C, or is the basis of a plane wave at an angle θ to the axis X.

[0050] The reception basis u is for example the basis of the transducer 11. Optionally, other reception basis can be used during reception.

[0051] Thus, the step of generating an ultrasonic wave is between the transmission basis i and the reception basis u. This step of generating an ultrasonic wave is therefore defined for any type of ultrasonic wave, focused or unfocused, for example, a plane wave.

[0052] In the matrix generation step, the empirical reflection matrix R ui (t) is defined between the emission basis i as input and the receiving basis u as output. This matrix is in Regarding the spatial coordinate u out It includes a set of time responses of the medium measured at a time t by each transducer 11 of the transducers 11 of the medium. It is to be understood that elements named with the subscript "in" refer to emission, transmission, or transmission (i.e., input) and elements named with the subscript "out" refer to reception, or reception (i.e., output). This empirical matrix can be recorded and / or stored, for example, in the memory of a computing unit or in any other medium, removable or non-removable, permanent or temporary storage.

[0053] More precisely, the focusing reflection matrix RFoc(r in ,r out , δt), the following applies: - Emission basis (i) and input virtual transducer TV in Using the time of flight of the wave between the two points, in The experimental reflection matrix R that generates the so-called input focal spot near the first point P1 of ui (t) is the input focusing process based on the input virtual transducer TV in corresponds to); - Output virtual transducer (TV out ) and the transducer at the receiving base (u), the return flight time is used to calculate the spatial position r out The experimental reflection matrix R that produces a so-called output focal spot near the second point P2 of ui (t) is the focusing process at the output based on the output virtual transducer TV out corresponds to); -Additional delay δt, which is the time lag added between the outbound flight time and the return flight time during the focusing process.

[0054] These convergence processes at the input and output become the actual input / output convergence processes and are referred to as convergence processes in the remainder of this specification.

[0055] That is, in this ultrasonic characteristic evaluation method, the input virtual transducer TV in is the spatial position r in the medium in It corresponds to an ultrasound "virtual source" located on the TV and outputs a virtual transducer out is the spatial position r out This virtual source and the virtual sensor correspond to an ultrasonic “virtual sensor” located at . The difference between their spatial positions Δr = r out -r in and separated in time by an additional delay δt, which is an arbitrarily adjustable delay independent of the spatial distance |Δr|. Thus, the method makes it possible to probe the medium in the vicinity of points P1 and / or P2 in space and / or time, obtaining new information in these two dimensions (space and time) regarding the wave propagation.

[0056] For example, the input virtual transducer TV due to the focusing process at the input and output in and output virtual transducer TV out The focusing reflection matrix RFoc(r in ,r out , δt) is an improved beamforming method and can be expressed as the following simplified Equation 1:

number

[0057] The time τ is the time between the transducer of emission base i and the spatial position r in Input virtual transducer TV (first point P1) in The time of flight τ of the ultrasonic wave in and spatial position r out Output of (second point P2) Virtual transducer TV out The return flight time τ of the ultrasonic wave between the transducer and the receiving base u out and an additional delay δt, which is expressed by Equation 2 below: τ(r in ,r out ,u out ,i in , δt)=τ in (r in ,i in )+τ out (r out ,u out )+δt (Equation 2)

[0058] Flight time τ in and τ out is calculated from the sound speed model. The simplest assumption is a constant sound speed c 0 It assumes a uniform (homogenous) medium, in which case the time of flight is obtained directly based on the distance between the probe transducer and the virtual transducer.

[0059] Number of elements of the emission basis N in is, for example, one or more (1 or more), and preferably two or more (2 or more). out is, for example, two or more (2 or more).

[0060] Therefore, this improved beamforming formula is based on the empirical reflection matrix R ui , i.e., the first sum is over the emission basis i, which represents the focusing at emission, and the second sum is over the reception basis u, which relates to the focusing at reception, and the calculation is in , r out The result of this improved beamforming formula is therefore a function of the two spatial coordinates (r in ,r out ), but is also a function of an additional delay δt between the input and the output, which can be adjusted arbitrarily.

[0061] Such beamforming formulations may also include input and output weighting terms (often referred to as receive and / or transmit apodization), and thus may be added to the remaining beamforming formulations by one skilled in the art.

[0062] Recorded experimental reflection matrix R ui (t) is a "real" matrix, i.e., it is composed of real coefficients in the time domain, and the electrical signals recorded by each transducer are real. Alternatively, this matrix can be a "complex" matrix, i.e., it is composed of complex numbers, as is the case, for example, for demodulation of in-phase and quadrature (IQ) beamforming.

[0063] Then, the focusing reflection matrix RFoc(r in ,r out , δt) is obtained. In the case of a linear probe, this focusing reflection matrix is in andr out t and five (5) dimensions for the additional delay δt, which is significantly different from and information-rich in the prior art focused reflection matrices.

[0064] In this analysis, due to the additional delay δt, the input virtual transducer TV in and output virtual transducer TV out is not determined at the same time, and the input virtual transducer TV in The first point P1 and the output virtual transducer TV out This additional delay δt can be positive or negative, making it possible to explore the focusing of the ultrasonic wave at the second point P2 before and after a reference point in the path of the ultrasonic wave in the medium.

[0065] This reference time is ballistic time t b This ballistic time is the time required for the transducer of emission basis i and the input virtual transducer TV in Between and output virtual transducer TV out is the round trip time of the ultrasound between the receiving base u and the receiving

[0066] This ballistic time t b is defined by Equation 3 below: t b =(||u out -r out ||+||u in -r in ||) / c 0 (Formula 3) In the formula, c 0 is the assumed speed of sound in the medium (the propagation speed of ultrasonic waves).

[0067] With such an arrangement, the method allows to probe the medium very locally at a second point P2 relative to a first point P1 with an additional delay δt between the signals coming from these two points. This local information is obtained by calculating the focusing reflection matrix RFoc(r in ,r out , δt) and can be used to characterize each point of the medium after the fact (without new releases and / or acquisitions).

[0068] Therefore, it is possible to derive an estimate of the reflectivity of the medium from the time response after beamforming, for spatial positions r that are equal at the input and output. in =r out This is made possible by considering the absolute value of the confocal signal, characterized by zero additional delay δt = 0 (i.e., ballistic time without additional delay). This estimate of the reflectivity of the medium is the value of a pixel in the ultrasound image of the medium. Thus, to construct the ultrasound image, we need a set of spatial locations r = r, which corresponds to a set of pixel locations in the ultrasound image. in =r out can be scanned or selected.

[0069] And r = r in =r out , δt=0, the focusing reflection matrix RFoc(r in ,r out , δt) based on the ultrasound image I (0) (r) can be constructed as follows: I (0) (r) = RFoc(r in ,r out =r in ,δt=0) (Equation 4)

[0070] Image of propagation near a point in a medium The method for ultrasound characterization performed by the computation unit 42 of the system 40 is to calculate the focused reflection matrix RFoc(r in ,r out , δt), which can be supplemented by constructing one or more “propagation images” based on a single input virtual transducer TV in (First point P1) and multiple output virtual transducer TV out (second point P2) for one or more values ​​of the additional delay δt, and their output virtual transducers TV out is the spatial position r in Input of virtual transducer TV in Nearby spatial position r out Located in.

[0071] In the case of a single propagation image, this propagation image is expressed as a focused reflection matrix RFoc(r in ,r out , δt).

[0072] This propagation image represents how ultrasound propagates between virtual transducers, for example in the vicinity of an input virtual transducer, at a time point (taken relative to ballistic time) equal to the additional delay.

[0073] System 40 can then optionally display one or more of the propagated images on display device 43.

[0074] The calculation unit 42 can also calculate a series of propagation images for multiple temporally consecutive additive delays, e.g. in A propagation film (image) of the ultrasound near the (first point P1) is constructed, which may optionally be displayed on a display device 43 or other media.

[0075] The time-sequential additional delays applied to construct this propagation film are applied within an additional time interval in this example.

[0076] For example, the additional delay range is the input virtual transducer TV at spatial position rin in from spatial position r out All outputs of the virtual transducer TV out This additional delay range can be, for example, [-δt min , + δt max ] and δt min =z out max -z in / c 0 , δt max =z out min -z in / c 0and z in and z out are spatial positions r in Input of virtual transducer TV in and spatial position r out Output of Virtual Transducer TV out is the depth in the positive direction of the second axis Z.

[0077] For example, the additional delay range is max and the value can be symmetric around zero (δt=0), and this additional delay range is [-δt max , + δt max For example, the output transducer used for the propagating image TV out Regarding δt max =max(|Δr|) / c 0 It can be defined as follows.

[0078] The image denoted by A in FIG. 5 is an ultrasound image of a sample medium or phantom having a given number of inhomogeneities. A rectangular analysis area ZA is considered in this medium. out (Multiple Output Virtual Transducer TV ou t) is constructed at the second point P2 at spatial position r in (In this case, the analysis area ZA out (located in the medium of the TV) in The analysis region is computationally scanned to construct one or more propagation images of the first P1 neighborhood. The analysis region can be positioned at any position, independent of the position of the input virtual transducer. However, of particular interest is the case where the analysis region surrounds the input virtual transducer.

[0079] In this reference image A, the input virtual transducer TV in The first point (P1) is located on or near a reflective element (echogenic target) of the medium.

[0080] The image shown in FIG. 5B is the analysis area ZA of the image A of FIG. 5 for five (5) additional delay values ​​δt. outThese additional delays are -3.86 μs, -1.93 μs, 0 μs, 1.93 μs, and 3.86 μs in this illustrative example. The composition of each propagation image is as follows: - The first image with index 1 (e.g. B 1 ) is the analysis area ZA out corresponds to the amplitude of the values ​​of the focusing reflection matrix for a set of points in - The second image with index 2 (e.g. B 2 ) is the analysis area ZA out , which corresponds to the real part of the value of the focusing reflection matrix for the same set of points in

[0081] In these images, the amplitude and real part levels are represented in grayscale. 1 and B 2 The points or pixels of these propagation images are denoted by Δr = r out -r in This is the input virtual transducer TV in Position of r in spatial position r out Output of Virtual Transducer TV out In the drawing showing this example, the coordinates of the image are represented by the abscissa Δx and the ordinate Δz.

[0082] These propagation images illustrate the explanation given above for the focusing reflection matrix calculated with the additive delay δt and make it possible to visualize the propagation of a coherent wave. In particular, for negative additive delays that tend towards zero, this coherent wave propagates through the input virtual transducer TV. in and ideally converges to a focal spot defined by the diffraction limit for zero additive delay (δt = 0). The coherent wave diverges for positive increasing additive delays.

[0083] This coherent wave is at spatial position r inIt results from a digital time-reversal process of echoes (measured by the probe transducer) coming from a virtual source located at the input virtual transducer of spatial position r in A set of spatial positions r near the input virtual transducer out By beamforming with various additional times δt at receive, the confocal position (i.e., r in =r out ) shows the focusing when receiving outside.

[0084] An input virtual transducer TV whose propagation image is located on or near a reflective element of the medium (echoic target) in Since the coherent wave is obtained for the first point P1, it is easily identifiable in the propagation image and exhibits a good signal-to-noise ratio compared to nearby signals.

[0085] The image indicated by A in FIG. 6 shows the same ultrasound image as that in FIG. 5, but with a different rectangular analysis area ZA' out (in this example, the same dimensions) are considered, and the spatial position r in Input of virtual transducer TV' in The propagation image is scanned by calculation to construct a propagation image in the vicinity of another first point P1'.

[0086] This input virtual transducer TV' in Another first point P1' is now associated with a resolution cell that contains a set of randomly located sub-resolution scatterers with comparable reflectivities. At the wavelength scale, such media is referred to as "ultrasonic speckle" and is characterized by random reflectivities due to destructive and constructive interactions between the sub-resolution scatterers, which are responsible for the granular effect in B-mode ultrasound images.

[0087] The images shown in FIG. 6 as B-F are the analysis areas ZA' of image A in FIG. 6 for the same five additional delay values ​​as images B-F in FIG. 5. out This is a propagation image of the

[0088] This other analytical area ZA' out The amplitudes and real parts of the values ​​of the focusing reflection matrix for a set of second points of are expressed in a similar manner.

[0089] The propagation image for the scatterer is calculated by the input virtual transducer TV' in Also shown is a coherent ultrasound wave converging to a first point P1' at the focal plane and then diverging. However, this coherent wave is difficult to distinguish, since the echoes generated by scatterers located upstream or downstream of the focal plane have reflectivities comparable to those of the virtual source being analyzed.

[0090] Also, unlike the previous definition of a propagation image, the multiple input virtual transducer TV in (First point P1) and one output virtual transducer TV out It is also possible to construct one or more propagation images between the two points. Then, the propagation images are expressed as the focusing reflection matrix RFoc(r in ,r out , δt), and these propagation images are constructed from one output virtual transducer TV out (Second point), Multiple input virtual transducer TV in (first point P1), one or more values ​​of the additional delay δt are determined, and a plurality of input virtual transducers TV in is the spatial position r out Output of Virtual Transducer TV out Nearby spatial position r in Located in.

[0091] Then, the definitions of the propagation images for the input and output transducers are directly reversed. Due to the reciprocity of wave propagation, the images generated are very similar, and the various calculations and determinations made from these propagation images can be performed in the same way, as described below. For simplicity, the detailed description of this application will only describe the first direction between an input transducer and multiple output virtual transducers. However, it should be understood that in each definition appearing in this specification, elements with the suffix "in" and elements with the suffix "out" are interchangeable, and the terms "input" and "output" are interchangeable.

[0092] It is also possible to use two propagation images (one in a first direction and one in a second direction) and combine or average the two propagation images to obtain an average propagation image that is more representative and contrasting of the wave propagation in the medium, and it is also possible to combine the results resulting from or determined from the two images to obtain results that are often more accurate.

[0093] The focusing reflection matrix RFoc(r in ,r out , δt) is the input virtual transducer TV in The spatial position r in and output virtual transducer TV out The spatial position r out These spatial positions are absolute positions in the spatial reference system. However, it is also possible to use a single absolute spatial position and a spatial position relative to that absolute spatial position. For example, the absolute spatial position r in and the relative spatial position Δr of the output virtual transducer out Take Δr out =r out -r in Conversely, the absolute spatial position r of the output virtual transducer can be expressed as out and the relative spatial position Δr of the input virtual transducer in Take Δr in =rin -r out Each calculation and / or each determination herein can be performed using the above definitions or other similar and / or equivalent definitions.

[0094] Extraction of coherent waves The method for ultrasound characterization performed by the computation unit 42 of the system 40 can be complemented by applying a combination step of performing a linear combination of a set of propagation films, each of which is located at a different spatial position r in Input selected in Virtual Transducer TV in andr out = Δr out +r in At a spatial position r such that out Output of Virtual Transducer TV out The photograph was taken between Δr out is predetermined and identical for all the propagation films in the set, and the selected input virtual transducers are close to each other.

[0095] That is, the input virtual transducer TV selected at multiple nearby spatial positions in A set of spatial locations is selected, which set forms a region of interest for correlation (more simply called spatial correlation region ZC) and allows to correlate the propagation films of these input virtual transducers. This spatial correlation region can be, for example, a rectangular region around a reference point, but can also be the entire image or any other region of symmetric or asymmetric shape. Neighboring spatial locations are, for example, spatial locations close to each other.

[0096] By combining a set of multiple propagation films, an improved "coherent wave propagation film" is obtained, for example with respect to coherence and contrast. The image of this new propagation film (called the coherent wave propagation film) is imaged at the same additional delay δt and the same relative position Δr out is obtained.

[0097] Then, this new coherent wave propagation film is placed at a spatial position r representing an input virtual transducer (input virtual transducer in the spatial correlation domain) selected from the set of propagation films. in,ref Reference Input Virtual Transducer TV in,ref can be associated with.

[0098] According to the first example, the reference input virtual transducer TV in,ref is an input virtual transducer whose spatial location corresponds to the average of the spatial locations of the selected input virtual transducers. Thus, in this example, the spatial location of the reference input virtual transducer can be expressed as Equation 5:

number

number

number

[0099] Another example is the reference input virtual transducer TV in,ref is an input virtual transducer whose spatial location corresponds to a weighted average of the spatial locations of a number of selected input virtual transducers, the weights being based, for example, on the reflectivity values ​​of each point of the selected input virtual transducer. Thus, in this example, the spatial location of the reference input virtual transducer can be expressed by the following Equation 6:

number

[0100] For example, this linear combination can be determined or implemented by singular value decomposition (SVD) to obtain a singular vector V 1 The singular value decomposition of the set of propagation films is computed to obtain the singular vectors V 1 For the same additional delay δt, the reference input virtual transducer TV in,ref It is a coherent wave propagation film related to

[0101] The set of transmission films are processed by singular value decomposition to combine the films, which represent the measured or experimental results of the acoustic disturbances in the region near the input virtual transducer, making it possible to advantageously improve the contrast of the transmission films and thus their ease of use.

[0102] To perform this singular value decomposition calculation (especially since current singular value decomposition tools work with two-dimensional matrices), a concatenated focused reflection matrix RFoc' can be constructed, whose rows are arranged at spatial positions r in Input of virtual transducer TV in , and the columns of this concatenated focused reflection matrix RFoc' are the indices of each selected input virtual transducer TV in The connected propagation film {Δr out ,t} (a set of images), where these propagation films are acquired at the same temporal succession with an additive delay δt. Thus, this concatenated focusing reflection matrix is in is the focusing reflection matrix RFoc that is refocused to the focusing point of

[0103] For example, the concatenated focusing reflection matrix RFoc' can be written as: RFoc' = [RFoc(r in ,{Δr out ,δt})]=[RFoc(r in ,{r in +Δr out ,δt})]

[0104] Then, this singular value decomposition SVD step is performed on the selected input virtual transducer TV in The singular vector V that maximizes the correlation between the sources of 1 The singular vector V 1 is associated with the maximum absolute singular value from the singular value decomposition. And the singular vector V 1 is the reference input virtual transducer TV for the same additional delay δt. in,ref This is a coherent wave propagation film related to the above.

[0105] Thus, the use of singular value decomposition SVD makes it possible to combine multiple wave-propagating films while avoiding random reflectivities introduced by speckle-type conditions. Coherent waves are common to each propagating film and therefore emerge during the combining process, and are the components of each input virtual transducer TV. in The contributions of external scatterers are cancelled out by destructive interference, which is equivalent to applying filtering to the propagation film to extract the coherent waves.

[0106] The image denoted by A in FIG. 7 shows the same ultrasound image as in FIG. 5 and FIG. 6. In the example of this figure, multiple selected input virtual transducers TV in The analysis region ZA associated with one input virtual transducer selected from the set out The selected virtual transducer is represented by a rectangular grid point in this image A. The grid points are located within a selected input virtual transducer TV, called the coherence region ZC, for coherent combining of the propagation films. in , i.e., the neighboring selected input virtual transducers.

[0107] The images shown in Fig. 7B-F are the analysis area ZA of image A in Fig. 7 for several additional delay values ​​δt. out is the coherent wave propagation image of the first singular vector V 1In this example, the same representation of the amplitude of the first image and the real part of the second image is used as in the previous figures.

[0108] The images in Fig. 7 show that it is possible to extract a coherent part of the ultrasound even from a set of first points (input virtual transducer TVin) located in the speckle. Indeed, in these images, a single coherent wave moves from bottom to top, and is detected by the input virtual transducer TV in However, the propagation images not processed by the singular value decomposition (SVD) process in this experiment resemble the images shown in Figure 6 B–F.

[0109] Singular value decomposition allows to extract coherent waves from a propagating image / film in a very reliable way. For example, in FIG. 8A, the first curve A1 is the input virtual transducer TV that belongs to the coherence region ZC. in corresponds to the amplitude of the signal at the confocal point as a function of the additional delay δt (here between -3 μs and +3 μs) of one of the transmission films obtained for Δx = Δz = |Δr| = 0(r in =r out ) in the propagation image. This curve A1 is very cluttered in the illustrated example, since the confocal position |r| corresponds to a "speckle" type area. The coherent wave is therefore completely or partially hidden by echoes from scatterers located upstream or downstream of this confocal area. Curve A2 in this figure shows the coherent wave propagation film (first singular vector V 1 ) which corresponds to the amplitude of the signal A2. This curve A2 shows a single peak centered at zero additional delay δt, demonstrating the good focusing of the waves even in this specific case containing poorly reflective elements.

[0110] Figure 8B shows the frequency spectrum of the signal of Figure 8A, where curve S1 corresponds to the frequency spectrum of the signal of curve A1 and curve S2 corresponds to the frequency spectrum of the signal of curve A2. A loss of time resolution of the coherent waves is nevertheless observed (visible in Figure 7), resulting in a reduction in the spectral width of the investigated signal. If necessary, this phenomenon can be corrected by applying a spectral equalization step.

[0111] Coherent wave propagation images are similar to the propagation images associated with echogenic scatterers, but with reduced spectral width.

[0112] Curves A2, S2 show the effectiveness of the combining / singular value decomposition steps to extract or filter a single-peak (single dominant wave) coherent wave propagating film.

[0113] Coherent waves in a ballistic reference frame Focused reflection matrix RFoc(r in ,r out , δt) is a model of the speed of ultrasound in a medium (e.g., a constant sound speed c 0 In fact, the flight time of the wave on the outward journey is τ in and the return flight time τ out is conventionally calculated using this assumption of constant sound speed and geometric formulas for calculating the distance between the transducer 11 and each point in the medium.

[0114] Therefore, the propagation images, propagation films, and coherent wave propagation films calculated above are calculated based on this constant sound speed c 0 In these images and films, the coherent waves result from a digital time-reversal process based on an assumed model of the speed of sound. The waves therefore move at the assumed speed of sound, c 0 At time δt=0, the input virtual transducer TV in (center of the drawing x) and Δz = 0. Therefore, the flight time of the coherent wave obeys the ballistic propagation relation in Equation 7: δt(Δr out ) = -sign(Δz out )·|Δr out | / c 0 (Formula 7) During the ceremony, c 0 is the speed of sound in the medium, |Δr out | is the input virtual transducer TV in and output virtual transducer TV out Vector Δr between out =r out -r in is the absolute value of δt is the additional delay, Δz out is the spatial position vector Δr out is the component along the second axis Z.

[0115] In other words, in these propagation images, the sound speed c 0 A theoretical wave propagating at in Input of virtual transducer TV in ) forms a circular arc with its center at . Therefore, the ballistic propagation relation is given by the sound speed c 0 The relative position Δr out to an additional delay δt. The negative sign emphasizes that this is a digital time-reversal process.

[0116] Then, from the propagation film or the coherent wave propagation film, a focused image of the waves in the ballistic reference frame can be extracted, which is called a "wavefront image" and is calculated using the sound velocity c 0 For each propagation image or coherent wave propagation image, the values ​​(sound pressure values) lying on this arc (i.e., satisfying the above ballistic propagation relation) are extracted with an additional delay δt. Then, a new image called wavefront image is constructed, which represents the evolution of the propagation film or the coherent wave propagation film in the ballistic reference frame. This wavefront image is therefore a wavefront image in the ballistic reference frame.

[0117] According to a first example, the wavefront image is determined indirectly by calculating a propagation film or a coherent wave propagation film and extracting appropriate data from this film as described above to determine the wavefront image over the additive delay range.

[0118] Therefore, the method for ultrasound characterization performed by the computation unit 42 of the system 40 calculates the input virtual transducer TV over the additional delay range. in or reference input virtual transducer TV in,ref , which is determined from: - an image of a propagation film or a coherent wave propagation film, and - δt(Δr out ) = -sign(Δz out )·|Δr out | / c 0 Ballistic propagation relations of this type.

[0119] According to a second example, the wavefront image is obtained by imposing the above ballistic propagation relation, ui (t) is determined directly from

[0120] Therefore, the method for ultrasound characterization performed by the computation unit 42 of the system 40 calculates the input virtual transducer TV over the additional delay range. in , which is determined from: - Focused reflection matrix RFoc(r in ,r out , δt), and - δt(Δr out ) = -sign(Δz out )·|Δr out | / c 0 Ballistic propagation relations of this type.

[0121] In all these examples, the wavefront image is based on the echoes measured by the transducer of the probe, and is expressed as an input virtual transducer TV in or reference input virtual transducer TV in,ref This allows us to estimate the pressure field generated by the wave (response during transmission and reception).

[0122] Note that the signals contained in the wavefront image are submatrices of the focusing reflectance matrix. Therefore, the calculations can be restricted to signals that satisfy the ballistic propagation relations above. In this case, the wavefront image is a submatrix of the focusing reflectance matrix RFoc(r in ,r out , δt).

[0123] The points or pixels of the wavefront image have spatial positions Δr out =r out -r in This has an input virtual transducer TV in Position of r in The coordinates are therefore expressed as the abscissa Δx and ordinate Δz of these images. Wavefront images can also be determined using three-dimensional imaging techniques. Another set of coordinates is then used to represent the wavefront image in various planes.

[0124] 9A shows the amplitude of such a wavefront image, and FIG. 9B shows the real part of the wavefront image. In FIG. 9B, the input virtual transducer TV in The focal point of (in this drawing, the spatial position r of the coordinates Δx = Δz = 0) in ) a phase shift of π radians is observed when passing through the wavefront. This phase shift is known as the Gouy phase shift. The wavefront image clearly shows this phenomenon.

[0125] As is true for the case of propagating images, the input virtual transducer TV in and output virtual transducer TV out The roles of can be reversed, in which case an estimate of the pressure field generated by focusing is obtained as output.

[0126] Determination of composite sound speed The method and system for ultrasonic characterization of a medium according to the present disclosure, executed by the calculation system 42 of the system 40, can also determine the integrated speed of sound at a point in the medium. The integrated speed of sound is an estimate of the average speed of sound between the transducer of the probing device 41 and the point in the medium. More precisely, the integrated speed of sound combines all the local speeds of sound in the areas traversed by the outbound and return paths of the ultrasound.

[0127] In this case, the method comprises the following steps: -Additional delay range over input virtual transducer TV in determining a wavefront image for the sound velocity c in the medium as described above; 0 ) and - Input Virtual Transducer TV in The depth position Δz of the center of the focused spot in the wavefront image (0) determining - Composite sound speed c based on the following equation (8) (1) Steps to calculate:

number

[0128] The "center of the focal spot in the wavefront image" is understood to mean, for example, the position of the maximum of the focal spot in the wavefront image, i.e. the position of the pixel that has the maximum value in the entire wavefront image. It should be noted that since only one focal spot is observable in the wavefront image, its position is unique. Therefore, the position of the focal spot center is also unique and can be determined by the input virtual transducer TV. in The spatial position r in For a point in the medium corresponding to the sound speed c 0 The depth position Δz used to correct (0) (r in )

[0129] For example, the center of the focused spot can be determined by examining the wavefront image for the spatial location of the point of maximum, and the depth position Δz of the center of the focused spot can be determined by (0) is the component along the depth axis Z (corresponding to the axis Δz) at the point of maximum value.

[0130] Depth direction position Δz (0) is the input of each virtual transducer TV taken in the medium. in For each output virtual transducer TV, or vice versa, out Note that the depth position is determined in terms of Δz (0) (r), where r = r in , or r = r out It is.

[0131] In practice, in the image of the propagation film or the coherent wave propagation film, the ultrasonic wave is focused at the time when the additional delay δt is zero (δt=0) by calculating the flight time of the outward trip and the flight time of the return trip, and the focusing reflection matrix RFoc(r in ,r out , δt) and the sound speed c used to calculate the wavefront image via the ballistic propagation relation 0is the transducer 11 of the exploration device 41 and the spatial position r in Input of virtual transducer TV in , and the sound speed corresponds to the correct resultant sound speed for the actual medium between the points in the medium corresponding to

[0132] For example, Figure 10 illustrates this process. Images A, B, and C in Figure 10 are taken at given sound speeds c of 1440 m / s, 1540 m / s, and 1640 m / s, respectively. 0 In these wavefront images, a focused spot is observed moving along the ordinate Δz, i.e., the depth direction (direction Z). The graph indicated by D shows three intensity curves CI of these wavefront images on this ordinate axis Δz (i.e., the axis with Δx=0). A , C.I. B , C.I. C Shows.

[0133] For example, as shown in FIG. 10, the depth direction position Δz (0) (r in ) is obtained by determining the depth position of the maximum value of the wavefront image on the ordinate axis Δz such that Δx = 0. (0) (r in ) is obtained by examining the wavefront image for the location on the ordinate Δz that has a maximum value in the wavefront image, which ordinate Δz corresponds to the abscissa Δx of the wavefront image being zero.

[0134] For example, the intensity curve CI in graph D A For the depth direction position Δz (0) (r in ) is substantially equal to 4.5 mm, and the position r of the selected input virtual transducer TVin in In the first assumed sound speed c (0) A resultant sound speed greater than c (1) (r in), the vertical position of the focused spot in image A along the Δz axis is shifted upwards, towards the origin of the input virtual transducer (Δx = Δz = 0), which corresponds to the input virtual transducer TV in corresponds to a calculated adjustment of the resultant sound speed at this point in the medium.

[0135] As a result, in practice, the sound speed or composite sound speed can be determined by calculating the value of the wavefront image on the Δz axis where Δx=0.

[0136] Thus, the method for ultrasonic characterization of a medium comprises the following steps to determine the resultant sound velocity: - An array 10 of transducers 11 is used to transmit a series of multiple incident ultrasonic waves into a region of the medium. in (wherein the series of multiple incident ultrasonic waves is of emission basis i); - the empirical reflection matrix R defined between the emission basis i and the reception basis u as output ui generating (t); - spatial position r in Input of virtual transducer TV in and spatial position r out Output of Virtual Transducer TV out The focusing reflection matrix RFoc(r in ,r out , δt) (output virtual transducer TV out The response of the input virtual transducer TV in (obtained at a time point shifted by an additional delay δt relative to the time point of the response of -Additional delay range over input virtual transducer TV in determining a wavefront image for a sound velocity c in the medium; 0 and the wavefront image is determined as a function of - Focused reflection matrix RFoc(r in ,r out , δt) and - δt(Δr out ) = -sign(Δz out )·|Δr out | / c 0 Ballistic propagation relations for types is determined based on the formula: δt is the additional delay, |Δr out | is the input virtual transducer TV in and output virtual transducer TV out is the absolute value of the vector between out =r out -r in and Δz out is the spatial position vector Δr out is the component along the depth axis Z); - the depth position Δz of the center of the focused spot in the wavefront image (0) determining; - The synthetic sound speed c from the following equation 9 (1) Steps to calculate:

number

[0137] Optionally, the method can be iterated one or more times as described above, with the previous synthetic sound speed c being calculated by the following iteration formula (10): (n) Determination of the wavefront image obtained by and the depth direction position Δz of the center of the focused spot (n) Determination of and new synthetic sound speed c (n) Based on the calculation of the new synthetic sound speed c (n+1) is calculated:

number

[0138] In practice, this iterative process rapidly converges to an optimal synthetic sound speed, which corresponds to the best synthetic sound speed for the transducer 11 of the probing device and the selected point in the medium (input virtual transducer).

[0139] Alternatively, the method for determining the resultant sound speed may include determining a wavefront image and determining a depth position Δz of the focused spot. (0) (r in ), by performing a step of linearly combining a set of wavefront images corresponding to a given coherence region ZC to improve the wavefront image, each wavefront image in the set being located at a different spatial position r in Selected Input Virtual Transducer TV in and spatial position r out Output of Virtual Transducer TV out between r out = Δr out +r in Then, Δr out is predetermined and the same for all wavefront images in the set, and the selected input virtual transducers are close to each other. Therefore, the reference input virtual transducer TV in,ref An "improved wavefront image" or a coherent wavefront image is obtained in relation to the reference input virtual transducer TV. in,ref is the same relative position Δr out represents the input virtual transducer for that set of wavefront images to be used in conjunction with a coherence region ZC selected for

[0140] For example, as described above for the case of a transmission film, the reference input virtual transducer TV in,ref is an input virtual transducer at a spatial location corresponding to the average of the spatial locations of a selected plurality of virtual input transducers, or is an input virtual transducer at a spatial location corresponding to a weighted average of the spatial locations of a selected plurality of virtual input transducers.

[0141] In summary, in the method of the present disclosure, the following steps are added: - determining the wavefront image and the depth position Δz of the focused spot (0) (r in ), a step of improving the wavefront image by linearly combining a set of wavefront images corresponding to the coherence regions (each wavefront image is located at a different spatial location r in The selected input virtual transducer (TV in ) and spatial position r out Output of virtual transducer (TV out ) and r out = Δr out +r in Then, Δr out is predetermined and the same for all wavefront images in the set, the selected input virtual transducers are close to each other, and the reference input virtual transducer (TV in,ref ) an improved wavefront image is obtained for the reference input virtual transducer TV in,ref characterizes the input virtual transducer of the set of wavefront images used in conjunction with the coherence region ZC); - Depth position Δz (0) (r in In the step of determining the wavefront image, an improved wavefront image is used instead of the wavefront image, and the depth position of the center of the focused spot is determined by the reference input virtual transducer TV in,ref The depth position of the center of this focal spot is relative to the spatial position of the reference input virtual transducer TV in,ref The resultant sound speed c at the spatial position (1) (r in,ref ) can be estimated.

[0142] The improved wavefront image (the coherent wavefront image) is then used (instead of the wavefront image) to determine the axial position of the center of the focused spot. Its distance or depth position Δz (0) (r in,ref ) is a characteristic of the inaccurate model of the sound speed and is the reference input virtual transducer TV in,ref The spatial position rin,ref The resultant sound speed c related to (1) (r in,ref ) can be used to estimate

[0143] According to one embodiment, a singular value decomposition (SVD) of the set of wavefront images is calculated to obtain a singular vector W associated with the singular value with the maximum absolute value of the singular value decomposition. 1 The linear combination is determined by obtaining the singular vector W 1 For the same additional delay δt, the reference input virtual transducer TV in,ref , resulting in an improved wavefront image corresponding to

[0144] The set of wavefront images can be processed by singular value decomposition to combine measurements or experimental results of multiple acoustic turbulences in the region near the input virtual transducer, avoiding turbulence-related fluctuations and improving the contrast and usability of the wavefront images.

[0145] Also, a synthetic sound velocity is calculated as described above, and for a linear combination of a set of wavefront images, a selected input virtual transducer (TV) that substantially covers the entire region of interest in the medium is calculated. in ), it is possible to determine the optimal sound speed of the medium (which is realistic for the medium as a whole). In particular, these selected input virtual transducers may be regularly distributed at predetermined intervals throughout the region of interest of the medium. For example, these selected input virtual transducers may account for more than or equal to 20% of the number of input virtual transducers used to construct an ultrasound image of the medium covering, for example, the region of interest.

[0146] Depth direction distance or position Δz (0) (r in ) and Δz (0) (r in,ref ) is the spatial position r in Or in,refNote that the resultant sound speed can be interpreted as the focusing error of the output due to the aberrations experienced during the backward propagation stage of the echo coming from the input. A measure of the resultant sound speed can also be determined by examining the aberrations experienced by the wavefront during the forward path. This measure is written by reversing the roles of the input and output virtual transducers and reversing the "in" and "out" subscripts in the above equation, resulting in a resultant sound speed c (1) out Another estimate of is obtained.

[0147] In addition, the resultant sound velocity obtained from the aberrations occurring on the outward and / or return paths (i.e., the resultant sound velocity c (1) in and c (1) out ) can be combined to improve the estimate of the composite sound speed.

[0148] The method is then completed by the following steps: - synthetic sound speed c for the output virtual transducer (1) (r out ) by reversing the roles of the input and output virtual transducers to determine - synthetic sound speed c for the input virtual transducer (1) (r in ) and the composite sound velocity c for the output virtual transducer (1) (r out ) to obtain an improved synthetic sound speed.

[0149] Synthetic sound speed image The method for ultrasound characterization performed by the calculation unit 42 of the system 40 can be complemented by constructing one or more "integrated speed of sound images", which are calculated for a spatial position r by at least one calculation of the integrated speed of sound as described above. in Input of virtual transducer TV in A plurality of points in the medium corresponding to the first point P1 are determined.

[0150] Figure 11 shows two examples of such images.

[0151] The first example corresponds to images A1 and A2 of FIG. 11, where the medium has a reference sound velocity substantially equal to c ref = 1542 m / s. Image A1 is a standard ultrasound image, while image A2 is a synthetic sound speed image obtained by the above method. In synthetic sound speed image A2, the mean value of the sound speed in the medium is estimated to be 1544 m / s with a standard deviation of ±3 m / s, which is in perfect agreement with the reference value of the sound speed in this medium.

[0152] The second example corresponds to images B1 and B2 of FIG. 11, where the medium is a layered medium with a thickness of 20 mm, a fibrous structure, and a first layer with a sound speed of approximately 1570 m / s, placed on top of the same phantom-type medium with a sound speed of substantially 1542 m / s, predetermined at the time of construction. Image B1 is a standard ultrasound image of this medium, while image B2 is a synthetic sound speed image obtained by the method described above. The synthetic sound speed image B2 reflects a high sound speed of approximately 1580 m / s in the first layer, and a low sound speed that is less than the predicted sound speed and corresponds to the sound speed of the medium examined in the first example. This effect is due to the fact that the sound speed calculated by the method is a synthetic sound speed, corresponding to the average or synthetic sound speed for the entire round trip (go and return) path of the wave between the transducer 11 and a point in the medium.

[0153] Axial aberration correction The method and system for ultrasonic characterization of a medium performed by the computing unit 42 of the system 40 according to the present disclosure may also determine an "axial correction."

[0154] The method for ultrasonic characterization of a medium to determine the temporal and local characteristics of ultrasonic focusing with axial correction comprises the following steps to obtain the focusing reflection matrix as described above: - A series of multiple incident ultrasonic waves (US) are transmitted into a region of the medium using an array 10 of multiple transducers 11. in(wherein the series of multiple incident ultrasonic waves is of emission basis i); - the empirical reflection matrix R defined between the emission basis i and the reception basis u as output ui generating (t); - spatial position r in Input of virtual transducer TV in and spatial position r out Output of Virtual Transducer TV out The focusing reflection matrix RFoc(r in ,r out , δt) (output virtual transducer TV out The response of the input virtual transducer TV in (obtained at a time point shifted by an additional delay δt relative to the time point of the response of - Additional delay range for input virtual transducer TV in determining a wavefront image of the medium, 0 determined as above as a function of - the depth position Δz of the center of the focused spot in the wavefront image (0) (r in ) step.

[0155] "Center of the focal spot in the wavefront image" is understood to mean, for example, the position of the maximum of the focal spot in the wavefront image, i.e. the position of the pixel with the maximum value in the entire wavefront image. The center and depth position of the focal spot can be found / determined according to any one of the methods described above.

[0156] This method calculates the focusing reflection matrix RFoc(r in ,r out , δt) the focussed reflection matrix RFoc corrected by the translation of the response (1) (r in ,r out , δt). The spatial translation is determined by the previously determined depth position Δz(0) (r in ) function.

[0157] In the first example, the spatial translation is 2·Δz (0) (r in ) correction value Δz corr (r in ) at spatial position r out Output of Virtual Transducer TV out The corrected focusing matrix Roc (1) (r in ,r out , δt) is obtained as follows: RFoc (1) (r in ,r out , δt)=RFoc(r in ,{x out ,z out +Δz corr (r out )},δt) (Equation 11)

[0158] And the corrected focusing reflection matrix RFoc (1) (r in ,r out , δt) to r = r in =r out , δt=0, the corrected ultrasound image I (1) (r in ) can be constructed, resulting in the following equation (12): I (1) (r)=RFoc (1) (r in ,r out =r in ,δt=0) (Equation 12)

[0159] Conversely, spatial translation is 2·Δz (0) (r in ) correction value Δz corr (r in ) at spatial position r in Input of virtual transducer TV in, which corresponds to a spatial translation of the axial component along the depth axis Z of (1) (r in ,r out , δt) is obtained: RFoc (1) (r in ,r out , δt)=RFoc({x in ,z in +Δz corr (r in )},r out ,δt) (Equation 13)

[0160] Depth direction position Δz (0) is the input of each virtual transducer TV taken in the medium. in Note that the aberrations experienced during the return trip are determined for each output virtual transducer TV taken in the medium by reversing the subscripts "in" and "out". out The position Δz characterizes the aberration experienced during the outward journey. (0) (r out ) can be determined. More generally, this depth position depends on the spatial position r of each considered point, where r = r in Or r = r out As Δz (0) = Δz (0) This can be expressed as (r).

[0161] According to the second example, spatial translation is achieved by: - Δz, determined by the following formula (14): (1) The correction value Δz is equal to (r) corr To calculate (r):

number

[0162] This calculation is based on the following equation 15: (1) It can also be expressed as a function of (r):

number

number

[0163] According to a variant of the two above examples, the translation can be performed by calculating a spatial Fourier transform, a phase shift with a phase ramp depending on the correction value and an inverse spatial Fourier transform. This embodiment has the advantage that it makes it possible to combine translation and interpolation for new spatial coordinates.

[0164] A method implemented in this way may, for example, include the following steps: - the focusing reflection matrix RFoc(r in ,r out , δt) in the depth direction, which is the spatial frequency matrix RFreq z (r in ,x out ,k zout , δt) step: RFreq z (r in ,x out ,k z,out , δt)=TF zout [RFoc(r in ,r out ,δt)] (Equation 17) During the ceremony, TF zout is the depth direction Δz out is the spatial Fourier transform in k zout is in the interval [ω - / c 0 ,ω + / c 0 ], the corresponding wave number included in the pulse ω - and ω + is the pulse at the boundary of the ultrasonic bandwidth, x out is the spatial position r out Each output of the virtual transducer TV out is the transverse component in the x-axis direction; - Following the above two examples, 2·Δz (0) (r in ) the depth correction value Δz corr Phase slope and spatial frequency matrix RFreq z (r in ,x out ,kzout , δt) in the same depth direction, the corrected focused reflection matrix RF is determined for each spatial position of the input virtual transducer TVin and applied in the following equation 18: oc (1) (r in ,r out , δt) step:

number

[0165] Δz out The spatial Fourier transform in the direction can be described, for example, by the discrete spatial Fourier transform equation in Equation 19 below:

number

[0166] Other formulas for the Fourier transform and spatial Fourier transform also exist.

[0167] Δz out The inverse spatial Fourier transform in the direction can be described by the inversion formula in Equation 20:

number

[0168] According to the third example, the assumed sound speed c 0 New sound speed c to replace 1 The corrected focusing reflection matrix RF using (r) oc (1) (r in ,r out , δt) to translate the response axially.

[0169] This third example method further comprises the steps of: obtaining an axially corrected focusing reflection matrix: - From the following equation 21, the synthetic sound speed c (1) Steps to calculate (r):

number

[0170] For each of these examples, the corrected focusing reflection matrix RFoc (1) (r in ,r out , δt) is the axial correction of the focusing matrix, i.e. the focusing matrix with corrected axial aberrations. This corrected focusing matrix advantageously makes it possible to construct an ultrasound image with reduced axial aberrations. Thus, the axial distances in the corrected ultrasound image are more accurate, e.g. making it possible to obtain higher quality images.

[0171] Corrected focusing reflection matrix RFoc (1) (r in ,r out , δt) is obtained by a spatial translation, which is the spatial translation of one or both virtual transducers (TV in and / or TV out ) or by changing the sound speed c. These examples are based on the experimental reflection matrix R uiThis allows to improve the beamforming step, which is similar to the process of converting the time information given by the experimental signal (also called RF signal) at (t) into spatial information by the relation t=z / c. Thus, the spatial position of a point in the medium is corrected axially in the depth direction Z, resulting in a more accurate image of its vertical position.

[0172] For example, Figure 12 illustrates this process. The image shown in Figure 12, labeled A, is the sound speed in the phantom, c 0 = Speed ​​of sound c = 1540 m / s 0 corresponds to the ultrasound image obtained in c water = 1480 m / s. Thus, ultrasound image A is usually obtained with degraded resolution and contrast due to the inhomogeneity of the investigated medium. With known aberration correction methods, it is possible to obtain the image shown at B, which is improved in the lateral direction and gives a higher quality image than the conventional method. However, in this image the depth position of the reflective elements is not corrected (see the horizontal arrow between images A and B).

[0173] The image shown with C corresponds to an ultrasound image obtained with the axial correction proposed in the method described above. In this image C, the reflective elements are slightly shifted upwards (towards the outer surface), showing the effect of the reduced sound speed in water compared to the sound speed in the phantom. This axial correction therefore brings the axial position (in depth) of the points in the image closer to the nature of the observed medium and the distances measured in the image closer to their real values.

[0174] Any of the above three example methods can also be improved by determining the sound speed at both the input and output using a combination of a set of composite images as described above in relation to determining the composite sound speed, and by determining an improved wavefront image, for example by singular value decomposition.

[0175] The set of wavefront images are now processed by singular value decomposition to combine measurements or experimental results of acoustic turbulence in the region near the input virtual transducer, allowing for highly advantageous improvements in the contrast and usability of the wavefront images.

[0176] Ultrasound image corrected with axial aberration correction The method for ultrasound characterization performed by the computation unit 42 of the system 40 to determine the axial correction can be complemented by constructing one or more “corrected ultrasound images” that are based on the corrected focusing reflection matrix RFoc (1) (r in ,r out , δt) based on the spatial position r in Input of virtual transducer TV in Calculate ultrasonic intensity values ​​for a plurality of points in the medium, each corresponding to a virtual transducer TV out Input Virtual Transducer TV in (i.e., r in =r out ) is imposed.

[0177] Determination of the preferred direction of anisotropy of scatterers in a medium. The method and system for ultrasonic characterization of a medium performed by the computational unit 42 of the system 40 according to the present disclosure is also capable of locally determining the preferred direction of anisotropy of scatterers in the medium.

[0178] Scatterer anisotropy characterizes the scatterers that can generate echoes in a preferential direction when subjected to ultrasound with a particular incidence direction. This anisotropy is therefore relevant for scatterers with dimensions larger than the wavelength. This is of particular interest for medical imaging of tissues, organ walls, and surgical instruments such as biopsy needles.

[0179] In this case, the method comprises steps similar or identical to those described above up to the following steps: -Additional delay range over input virtual transducer TV in determining a wavefront image for the sound velocity c in the medium as described above; 0 is determined as a function of

[0180] And the method further comprises the steps of: determining the preferred direction of the focused spots in the wavefront image by image processing of the wavefront image;

[0181] For example, Fig. 13 illustrates this process. The image shown in Fig. 13 at A corresponds to an ultrasound image with spatial variation in the anisotropy direction of the tissue. The ultrasound imaged medium corresponds to a muscle of a subject (in this example, a calf), and multiple regions with fibers oriented in various directions are observed. This muscle image application is just one example of an anisotropic medium to which the method is applicable. However, such conventional ultrasound images generally provide distorted information. Prior art ultrasound methods do not allow reliable observation of this anisotropy, which is a local property of the medium, because the propagation of ultrasound in such media is not of constant speed, nor in a straight line direction from the transducer of the probe.

[0182] The images shown in Figure 13 at B, C, D, and E correspond to wavefront images constructed for small regions of the ultrasound image connected by arrows, where these wavefront images are processed by singular value decomposition of multiple virtual transducers in each of those regions to capture or explore experimental acoustic perturbations in that region, improving the contrast of the generated wavefront images and their analysis.

[0183] All these wavefront images B, C, D, and E show focal spots that are elongated in the vertical direction (depth axis direction Δz) but with different tilts. The tilt of the focal spot in the wavefront image is local tilt information that is highly correlated with the actual value of the tilt of the muscle fibers in the considered region. The tilt axis of the focal spot is actually substantially perpendicular to the fiber direction, especially at the center of the image where the incident wave has a direction substantially along the depth direction Z.

[0184] Thus, the method determines the preferred direction of the focused spot in the wavefront image by image processing the wavefront image.

[0185] According to a first example, the method allows for example to extract the outline of the focused spot by a threshold level below the maximum value in the wavefront image (for example 50% or 70% of the maximum value). From this outline, the preferred or main direction (direction of the largest dimension of the focused spot) and the secondary direction (direction of the smallest dimension) can be derived. However, other image processing methods can also be used to extract the preferred direction of the focused spot.

[0186] According to a second example, the method may for example: - the wavefront image U(r in ,Δx out ,Δz out ) to the polar coordinate system U(r in ,Δs out ,Δφ out ) type; - the value of the wavefront image in the polar coordinate reference system is multiplied by a plurality of radial distance deviation values ​​Δs out , and sum up multiple angle values ​​Δφ out The angular sensitivity function f(r in ,Δφ out ) to obtain; - the optimum angle value Δφ corresponding to the maximum of the angular sensitivity function max out (r in ) (optimum angle value Δφ max out (r in) is the input virtual transducer TV in ) which corresponds to the preferred direction of the focusing spot relative to

[0187] Then, the following equation 22 is obtained:

number

[0188] FIG. 14 corresponds to FIG. 13B and shows the angular sensitivity function f(r in ,Δφ out ) where the angular sensitivity function for this illustrative example has been normalized to have a maximum value equal to unity (1). out =-11°, which is the estimated angle of the local preferred direction at the point considered in the medium in this example.

[0189] Optionally, an angle value Δφ corresponding to the angle of view of the point under consideration in the medium as seen by the transducer. out A correction is applied to obtain the angular anisotropy value γ out (r in ) is obtained.

[0190] Here, this estimation is based on the correlation of the output signal. Conversely, other angular anisotropy values ​​γ in (r out ) can also be estimated. Advantageously, two angular anisotropy values ​​γ out (r in ) and γ in (r out ) can be combined to obtain a better local characterization of the anisotropy direction of the medium.

[0191] According to one example, the method can be completed by the following steps: - reversing the input virtual transducer and the output virtual transducer to determine a preferred direction for the output virtual transducer; and - Combining the preferred directions for the input virtual transducer and the preferred directions for the output virtual transducer to obtain an improved preferred direction.

[0192] According to another example, the method can be completed by the following steps: Reverse the roles of the input and output virtual transducers and set the angular anisotropy value γ out (r out ) determining the - the angular anisotropy value γ for the input virtual transducer out (r in ) and the angular anisotropy value γ for the output virtual transducer out (r out ) to obtain an improved angular anisotropy value.

[0193] An example of the calculation of the angular anisotropy value is given by the following Equation 23 (for the first case, angular anisotropy for an input virtual transducer):

number

[0194] Such an angular anisotropy value is calculated, for example, according to the calculation described in Non-Patent Document 3.

[0195] The spatial position r of the input virtual transducer in As a definition of the angle of view of a point, for example, the following type of equation 24 is added:

number

[0196] Other formulas for calculating the angular anisotropy value can be envisioned by those skilled in the art to obtain more realistic values ​​for the preferred direction angle.

[0197] 15 shows an ultrasound image overlaid with lines corresponding to the estimation of preferred directions for a set of points distributed across the ultrasound image. Note that a large consistency between the estimated preferred directions and the underlying structures is visible in the ultrasound image. The proposed method advantageously allows for a good estimation of preferred directions across the entire ultrasound image.

[0198] Measurement of this preferred direction (tilt angle of the focused spot about its largest dimension) is an important parameter for improving the quality of the ultrasound image in that region, and knowing this allows for the selection of incident ultrasound US, for example, by selecting a plane wave with a particular tilt or a wave focused at a particular location. in This also makes it possible to adapt the apodization selected on receive during the beamforming step.

[0199] This measurement of local preferred orientation makes it possible to determine and locate the possible presence of tissue lesions by analyzing the degree of anisotropy over larger regions.

[0200] So, a method for ultrasonic characterization of a medium for locally determining the preferred direction of anisotropy comprises the following steps: - A series of multiple incident ultrasonic waves (US) are transmitted into a region of the medium using an array 10 of multiple transducers 11. in (wherein the series of multiple incident ultrasonic waves is of emission basis i); - the empirical reflection matrix R defined between the emission basis i and the reception basis u as output ui generating (t); - spatial position r in Input of virtual transducer TV in and spatial position r out Output of Virtual Transducer TVout The focusing reflection matrix RFoc(r in ,r out , δt) (output virtual transducer TV out The response of the input virtual transducer TV in (obtained at a time shifted by an additional delay δt relative to the time of the response of -Additional delay range over input virtual transducer TV in determining a wavefront image for a sound velocity c in the medium; 0 and the wavefront image is determined based on: - Focused reflection matrix RFoc(r in ,r out , δt) and - δt(Δr out ) = -sign(Δz out )·|Δr out | / c 0 ) type ballistic propagation relation, During the ceremony, δt is the additional delay, |Δr out | is the input virtual transducer TV in and output virtual transducer TV out is the absolute value of the vector between Δr out =r out -r in and Δz out is the spatial position vector Δr out (is the component along the depth axis Z of determining the preferred direction of the focused spots in the wavefront image by image processing of the wavefront image;

[0201] The proposed method can be improved by determining an improved wavefront image using a combination of a set of wavefront images as described above in the section on determining the composite sound speed, for example by the singular value decomposition method. In this case, the preferred direction obtained from the improved wavefront image corresponds to the selected coherence region and is related to the spatial position r of the reference virtual transducer. in,ref This allows characterizing the anisotropy of the medium due to

[0202] Here, multiple wavefront images from the set can be processed by singular value decomposition to combine multiple measurements or experimental results of acoustic turbulence in the region near the input virtual transducer to improve the contrast and usability of the wavefront images.

[0203] Then, the method can be further modified as follows: Between the steps of determining the wavefront image and determining the preferred direction of the focal spot, a step of improving the wavefront image is carried out, in which a linear combination of a set of wavefront images corresponding to coherence regions is carried out, each wavefront image being located at a different spatial position r in The input virtual transducer (TV in ) and spatial position r out Output of virtual transducer (TV out ) and r out = Δr out +r in Then, Δr out is predetermined and identical for all wavefront images in the set, the selected input virtual transducers are close to each other, and the reference input virtual transducer (TV in,ref ) is obtained, and this reference input virtual transducer TV in,ref characterizes the input virtual transducer of the set of wavefront images used in conjunction with the coherence region ZC, In the step of determining the preferred direction of the focused spot, the improved wavefront image is used instead of the wavefront image. The preferred direction of the focused spot is determined by the reference input virtual transducer TV in,ref is relative to the spatial position of

[0204] Also, input virtual transducer TV in and output virtual transducer TV out By reversing the roles of , i.e., by reversing the subscripts of "in" and "out", we can obtain the spatial location r out Output of Virtual Transducer TV out The preferred direction of the focused spot Δφ max in (r) can be determined. Two preferred directions associated with position r, namely, Δφ max in (r) and Δφ max out By combining (r), the measurement of scatterer anisotropy can be improved.

[0205] Calculation of the preferred directions of these images allows characterizing the anisotropy of the scatterers of the medium, for example anisotropic structures in the medium (such as a needle introduced into a tissue or a wall separating different tissues), where the anisotropy of the scatterers is understood to mean factors larger than the ultrasound wavelength.

[0206] Analysis of the time signal for confocal The method and system for ultrasonic characterization of a medium implemented by the computing unit 42 of the system 40 according to the present disclosure may also perform localized spectral analysis of ultrasonic focusing.

[0207] In such analyses, the confocal response is of particular interest, since it corresponds to the spatial position r in Input of virtual transducer TV in is the spatial position r out Output of Virtual Transducer TV out That is, r in =rout =r.

[0208] Then, with an additional delay δt, these virtual transducers probe the time response of the selected scatterers.

[0209] In this case, the method comprises the following steps as described above to obtain a focused reflection matrix, but applied at the same spatial position, i.e. the confocal position: - A series of multiple incident ultrasonic waves (US) are transmitted into a region of the medium using an array 10 of multiple transducers 11. in (where the set of incident ultrasound waves is of emission basis i); - the empirical reflection matrix R defined between the emission basis i and the reception basis i as output ui generating (t); - spatial position r in Input of virtual transducer TV in and spatial position r out Output of Virtual Transducer TV out determining a focusing reflection matrix RFoc(r, δt) that contains the response of the medium between in =r out = r, and the output virtual transducer TV out The response of the input virtual transducer TV in (obtained at a time point shifted by an additional delay δt relative to the time point of the response of

[0210] The method then comprises the following steps making it possible to carry out a local spectral analysis: - the frequency matrix RFreq of the following equation 25, which is the time Fourier transform of the focused reflection matrix RFoc(r, δt) t determining (r,ω); RFreq t (r,ω)=TF t [RFoc(r,δt)] (Equation 25) During the ceremony, TF tis the time Fourier transform, ω is a pulse with ω=2πf, and f is the frequency corresponding to the pulse.

[0211] The time Fourier transform can be described, for example, by the discrete time Fourier transform equation in Equation 26 below:

number

[0212] Other Fourier transform and time Fourier transform formulations exist, e.g., discrete or continuous, normalized or not, and can also be used.

[0213] RFreq t (r,ω) contains a local estimate of the spectrum of the echoes backscattered by the medium. More precisely, the echoes are from scatterers contained in a monochromatic (monochrome) focused spot centered at position r. In the absence of aberrations, their size is therefore given by the diffraction limit, which is set at the central frequency of the echoes backscattered by the medium.

[0214] The method can therefore be complemented by medical imaging methods based on frequency analysis of the backscattered echoes with the aim of improving the spatial resolution. In particular, the method makes it possible to carry out spatial beamforming in reception for each frequency before carrying out the spectral analysis. The confocal configuration advantageously makes it possible to limit the phenomenon of pulse diffraction.

[0215] For example, the method includes: t This can be complemented by a filtering step that performs frequency filtering of the elements of (r,ω). In particular, low-pass, band-pass, or high-pass frequency filtering can be performed to extract the desired components in the response of the focused reflection matrix depending on the intended application. For example, optionally, the incident ultrasound US inThe frequency filtering can be adapted to extract harmonic components of the fundamental frequency of

[0216] For example, Fig. 16 illustrates this process. The image shown in Fig. 16A shows an ultrasound image of a medium containing an air bubble. The air bubble is a resonant structure in the medium that continues to vibrate after the passage of the incident wave, disturbing the ultrasound image and generating echoes that reach the receiving transducer with a flight time larger than the ballistic flight time, resulting in artifacts in the ultrasound image downstream of the bubble. The image shown in Fig. 16B is a zoomed-in view of image A, where a clear echo of the air bubble is observed at spatial position r = [x,z] = [11,17] mm, and the artifact is located downstream of that position (i.e. in the vertical depth direction). The images shown in Fig. 16C1 and C2 correspond to the amplitude and real parts, respectively, of the propagation image for zero additional delay δt.

[0217] The focused reflection matrix RFoc(r,δt) of the present method allows to study the time signal of this bubble oscillation. The images shown in Fig. 16 D, E and F correspond respectively to the plots of the real part, the amplitude and the frequency spectrum of the response RFoc(r,δt) at a point of spatial position r corresponding to the position of the bubble. In images D and E, a second echo is observed approximately 1.5 μs after the main echo centered at δt=0. Image F shows the first spectral plot excluding this second echo and the second spectral plot including it. The second spectral plot contains a main frequency of approximately 6 Mhz, corresponding to the frequency of the incident wave, and another frequency of approximately 3 MHz, corresponding to the resonant frequency (oscillation) of the bubble.

[0218] The method thus makes it possible to carry out a spectral analysis and identify, for example, the resonant frequencies of gas bubbles or other resonant structures in the observed medium.

[0219] It is then possible to filter the response of the focused reflection matrix, for example by a predefined bandpass filter, and then use the filtered response to calculate an improved ultrasound image, thereby attenuating or eliminating the effects of resonance in the ultrasound image.

[0220] Conversely, by retaining only the resonances of the response of the focused reflection matrix, a resonant frequency image can be constructed. Note that the resonant frequency of a bubble is related to its size and can be used to estimate the local pressure in the medium.

[0221] In the second example, RFreq t Using (r,ω) it is possible to study the attenuation in the medium. In fact, this phenomenon is frequency dependent. Since high frequencies are attenuated more than low frequencies, the attenuation coefficient can be estimated, for example, by comparing the spectra of echoes from two different depths of the medium being observed. The above-described method for estimating the local spectrum of echoes from a given area is therefore ideal for determining the attenuation. To that end, this method can be complemented with a step of determining the depth-averaged spectrum S(z,ω), determined by the average value of the spectrum of the frequency matrix at a given depth z in the medium.

[0222] For example, this depth-averaged spectrum is calculated by the following Equation 27, which is a normalized average value, averaged over a set of spatial locations with the same depth z and abscissa x falling within a given range:

number

[0223] For example, Figure 17 illustrates the calculation of the depth-averaged spectrum, constructing a set of spectral images for all depths of an ultrasound image. The image shown in Figure 17, A, shows an in-vivo ultrasound image of a healthy calf, and the image shown in Figure 17, B, shows the depth-averaged spectrum in grayscale. This depth spectrum image shows the strongest attenuation of high frequencies at large depths.

[0224] Using such images, the evolution of the attenuation as a function of depth can be estimated by using the entire frequency content through a method of adjustment between theoretical and / or experimental models and the images.

[0225] In a third example, the method can also be completed with a step of determining the spectral correlation width δω(r) for a point at a spatial position r, by calculating the full width at half maximum of the autocorrelation of each spectrum of the frequency matrix RFreqt(r,ω), i.e., according to the following equation 28:

number

[0226] matrix RFreq tDue to the spatial resolution of (r,ω), the spectral correlation width δω(r) is a local value that can be used to characterize the properties of scatterers contained in a monochromatic (monochrome) focused spot centered at spatial location r. If the focused spot contains a single non-resonant scatterer, the spectral correlation width δω(r) is on the order of magnitude of the bandwidth of the ultrasound signal. If the focused spot contains multiple randomly distributed scatterers of the same intensity (ultrasonic speckle condition), the value of the spectral correlation width δω(r) will be much smaller than the bandwidth Δω.

[0227] The method may also comprise determining at least one "spectral correlation image", the spectral correlation image being obtained by determining a spectral width δω(r) for a plurality of points of the medium each corresponding to a point in the medium at a spatial location r.

[0228] For example, Figure 18 illustrates this process. The image shown in Figure 18, A, is an ultrasound image of a phantom medium containing several distinct elements (a point-like target and an echogenic cylinder). The corresponding image, B, is a spectral correlation image of a previous ultrasound image obtained by calculating the spectral correlation width δω(r) for a set of points in this medium. In image B, the edge of the cylinder and the point-like target have a larger spectral correlation width δω(r) than the rest of the medium, which is composed of a large number of randomly distributed sub-resolution scatterers.

[0229] Calculation of the spectral correlation width of these images can be used to characterize the nature of targets in the medium, for example to distinguish between distinct speckle spots and single scatterers, which is useful, for example, to identify air bubbles for contrast imaging or to identify microcalcifications that characterize the presence of tumors, especially breast cancer. [Explanation of symbols]

[0230] 10. Array 11 Transducer 20 Medium 21 Scatterer 22 Aberration Layer 40 System for ultrasonic characterization 41 Exploration Device 42 Computational Units 43 Display Devices

Claims

1. 1. A method for ultrasonic characterization of a medium to determine a resultant sound velocity in the medium, comprising: A series of incident ultrasound waves (US) of emission basis (i) are generated using an array (10) of multiple transducers (11). ｉｎ ) in an area of ​​the medium; The empirical reflection matrix R defined between the emission basis (i) as input and the reception basis (u) as output ｕｉ generating (t); spatial position r ｉｎ Input virtual transducer (TV ｉｎ ) and spatial position r ｏｕｔ The output of the virtual transducer (TV ｏｕｔ ) a focusing reflection matrix RFoc(r ｉｎ , r ｏｕｔ , δt), ｏｕｔ ) is the response of the input virtual transducer (TV ｉｎ ) at a time shifted by an additional delay δt relative to the time of the response of The input virtual transducer (TV ｉｎ ) where the wavefront image is determined based on the sound speed c in the medium. ０ and the wavefront image is determined as a function of The focusing reflection matrix RFoc(r ｉｎ , r ｏｕｔ , δt) and δt(Δr ｏｕｔ ) = -sign(Δz ｏｕｔ ) |Δr ｏｕｔ | / c ０ is determined based on the ballistic propagation relation of the type δt is the additional delay, |Δr ｏｕｔ | is the input virtual transducer (TV ｉｎ ) and the output virtual transducer (TV ｏｕｔ ) vector Δr ｏｕｔ = r ｏｕｔ -r ｉｎ is the absolute value of Δz ｏｕｔ is the spatial position vector Δr ｏｕｔ a step, which is a component along the depth axis Z of The depth position Δz of the center of the focused spot in the wavefront image （０） (r ｉｎ ) Synthetic sound speed c （１） (r ｉｎ )を [0010] and calculating based on z ｉｎ is the input virtual transducer (TV ｉｎ ) spatial position vector r ｉｎ is a component along a depth axis Z.

2. The method of claim 1 , wherein the center of the focused spot is determined by finding the spatial location of a point of maximum in the wavefront image.

3. 3. The method according to claim 1, wherein the wavefront image is determined only on the depth axis Z.

4. determining the wavefront image and determining the depth position Δz of the focused spot; （０） (r ｉｎ Between the steps of determining the coherence region (ZC), a step of linearly combining a set of wavefront images corresponding to a given coherence region (ZC) to improve the wavefront image is performed, each wavefront image of the set being located at a different spatial position r ｉｎ The input virtual transducer (TV ｉｎ ) and r ｏｕｔ = Δr ｏｕｔ +r ｉｎ At a spatial position r such that ｏｕｔ The output of the virtual transducer (TV ｏｕｔ ) and Δr ｏｕｔ is predetermined and identical for all wavefront images in the set, and the reference input virtual transducer (TV ｉｎ，ｒｅｆ The input virtual transducers selected to obtain an improved wavefront image for the reference input virtual transducer (TV ｉｎ，ｒｅｆ ) represents the input virtual transducer of the set of wavefront images used for the selected coherence region (ZC); The depth direction position Δz （０） (r ｉｎ In the step of determining the wavefront image, the improved wavefront image is used instead of the wavefront image, and the depth position of the center of the focused spot is determined by the reference input virtual transducer (TV ｉｎ，ｒｅｆ ) and the depth position of the center of the focused spot is relative to the spatial position of the reference input virtual transducer (TV ｉｎ，ｒｅｆ ) the resultant sound speed c at the spatial position （１） (r ｉｎ，ｒｅｆ 4. The method according to claim 1 , wherein the method makes it possible to estimate

5. The linear combination is performed by computing a singular value decomposition (SVD) of the set of wavefront images and finding a singular vector (W １ ) and the singular vector (W １ ) is added to the reference input virtual transducer (TV ｉｎ，ｒｅｆ 5. The method of claim 4, wherein the improved wavefront image corresponds to

6. An input virtual transducer (TV) selected to cover substantially the entire region of interest in the medium for a linear combination of the set of wavefront images. ｉｎ 6. The method of claim 4 or 5, wherein the optimum sound velocity in the medium is determined by calculating a composite sound velocity using a set of wavefront images corresponding to the wavefront images.

7. The focusing reflection matrix RFoc(r ｉｎ , r ｏｕｔ , δt), determining the wavefront image, and determining the depth position Δz of the center of the focused spot in the wavefront image. （ｎ） The step of determining the sound speed c used in the first step or instead of the previously used sound speed ０ Instead of the synthetic sound speed c determined in the previous iteration （ｎ） It is repeated using In the step of calculating the composite sound speed, [0025] Using The synthetic sound velocity value at the point of the medium corresponding to the input virtual transducer is the synthetic sound velocity c calculated in the nth step n of the method. （ｎ） (r ｉｎ 7. The method according to claim 1, wherein step n is determined by a predetermined number of iterations, or by a stopping threshold of the difference between two successive composite sound speed values, or by a combination of both.

8. The roles of the input virtual transducer and the output virtual transducer are reversed to obtain a synthetic sound velocity value c （１） (r ｏｕｔ ) is determined, The synthetic sound speed c for the input virtual transducer （１） (r ｉｎ ) and the synthetic sound speed c for the output virtual transducer （１） (r ｏｕｔ 8. The method of claim 1 , wherein the first and second frequencies are combined to obtain an improved composite sound speed.

9. spatial position r ｉｎ Input virtual transducer (TV ｉｎ ) or spatial position r ｉｎ，ｒｅｆ Reference input virtual transducer (TV ｉｎ，ｒｅｆ 9. The method of claim 1, further comprising determining a synthetic sound speed image by determining a synthetic sound speed for a plurality of points in the medium, each corresponding to a respective one of the plurality of points.

10. The method of claim 9 , wherein in the step of determining the synthetic sound speed image, a value at each point of the synthetic sound speed image is calculated from values ​​of the synthetic sound speed image.

11. In the step of determining a focusing reflection matrix, The input virtual transducer (TV ｉｎ ) is calculated for the spatial position r ｉｎ The emitting base and the input virtual transducer (TV) are arranged to generate an input focused spot at ｉｎ ) using the time of flight of the outward wave between ｕｉ (t), which corresponds to a convergence process at the input based on The output virtual transducer (TV ｏｕｔ ) is calculated for the spatial position r ｏｕｔ The output virtual transducer (TV ｏｕｔ ) and the receiving base (u) transducer using the return flight time of the wave ｕｉ (t) corresponds to a focusing process at the output based on 11. The method of claim 1, wherein the additional delay δt is a time difference added to the outbound and inbound times of flight during the focusing process.

12. The focusing reflection matrix: [0030] It is calculated by N ｉｎ is the number of elements in the emission basis (i), N ｏｕｔ is the number of elements in the receiving basis (u) at the output, R ｕｉ (t) is the empirical reflection matrix, R ｕｉ (u ｏｕｔ , i ｉｎ , τ(r ｉｎ , r ｏｕｔ , u ｏｕｔ , i ｉｎ , δt) is the index i of the emission basis at time τ ｉｎ Following the emission of ｏｕｔ The experimental reflection matrix R recorded by the transducer ｕｉ (t) is an element of τ is the transducer of the emitting base (i) and the spatial position r ｉｎ Input virtual transducer (TV ｉｎ ) the time of flight τ ｉｎ and spatial position r ｏｕｔ The output of the virtual transducer (TV ｏｕｔ ) and the receiving base (u) transducer, the return flight time τ ｏｕｔ and an additional delay δt, τ(r ｉｎ , r ｏｕｔ , u ｏｕｔ , i ｉｎ , δt) = τ ｉｎ (r ｉｎ , i ｉｎ ) + τ ｏｕｔ (r ｏｕｔ , u ｏｕｔ 12. The method of claim 1, wherein the Δt is expressed as Δt + δt.

13. A system (40) for ultrasonic characterization of a medium (20) for determining a local resultant sound velocity value in the medium, comprising: an array (10) of transducers for generating a series of incident ultrasonic waves into a region of a medium and for recording ultrasonic waves backscattered by said region as a function of time; A system comprising: a computing unit (42) connected to the array of transducers for performing the method according to any one of claims 1 to 12.

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