Patent Application
20200080969
The present invention relates to a method for acquiring signals by ultrasound probing, for example in order to carry out imaging or adaptive and selective focusing. It also relates to a corresponding computer program and ultrasound probing device.
The invention applies in particular to the field of non-destructive testing via ultrasounds, wherein the acquisition of ultrasound signals allows to detect and/or to display defects in structures, but it can also apply to any type of ultrasound echographic detection or imaging, in particular to the medical field for the inspection of zones of interest in the human or animal body.
It relates more particularly to a method for acquiring ultrasound signals operating in the following manner:
Such an acquisition is generally carried out using a probing device with a multielement sensor, wherein each transducer is both an emitter and receiver, and switching between these two modes can be controlled electronically. The sensor can be placed in contact with the object to be probed or at a distance, but in the latter case it must be submerged in order to ensure the transmission of the ultrasound waves into the object to be probed. This sensor can be linear (1D) or matrix (2D), with rigid or flexible elements.
The matrix [MR(t)] of time signals obtained by this type of acquisition can then be subjected to processing, in particular for providing an image of the zone of interest inspected or for the extraction of parameters signifying structural defects in the zone of interest inspected. Given the current calculation capacities of processors, this processing can be on board in the control instruments for real-time processing.
In practice, the control of the L emission transducers for the M successive emissions of ultrasound waves towards the zone of interest can be encoded using an encoding matrix [MC], each coefficient MCi,j of this matrix representing a multiplication factor applied to a common excitation time signal e(t) for its emission by the i-th emission transducer at the time of the j-th emission. Delay laws can further be applied to the successive emissions.
When [MC] is the identity matrix and no delay law is applied, the ultrasound acquisition previously defined is qualified as FMC acquisition (from “Full Matrix Capture”). It consists of emitting an ultrasound wave by exciting the first emission transducer and receiving the echoes of this emission with all of the N reception transducers, then electronically switching to all of the emission transducers in order to successively excite them. When it is the same transducers that carry out the functions of emission and of reception, a matrix [MR(t)], noted as [K(t)], of ultrasound time signals having a size of N×N is obtained.
In imaging, the ultrasound time signals forming the coefficients of the matrix [K(t)] are used to carry out synthetic focusing of the “total focusing method” type which allows to obtain an image with optimal resolution throughout the zone of interest. However, in the presence of strong electronic or structural noise, the reconstruction via the total focusing method can provide images of lesser quality compared to the conventional echographic methods. Indeed, in the latter, all the transducers emit simultaneously via the application of a predetermined delay law in such a way as to focus on a given point. However, according to the FMC acquisition method generally implemented to then carry out the reconstruction via synthetic focusing, each emission is carried out by a single transducer which limits the energy transmitted and the depth of penetration of the waves into the part inspected. Finally, this manifests itself as a degradation of the Signal-to-Noise Ratio (SNR) since the amplitudes of the echo signals can be comparable to the level of electronic or structural noise. This degradation of the SNR is even greater when the attenuation of the ultrasound waves is high (viscoelastic attenuation, or attenuation by diffusion due to heterogeneities in the medium), making the detection and the characterisation of possible defects difficult.
A partial solution to this problem of degradation of the SNR is provided in the article by Karaman et al, entitled “Synthetic aperture imaging for small scale systems”, published in IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, vol. 42, No. 3 (May 1995), pages 429-442.
It involves using, for each emission, not a transducer but a plurality of adjacent transducers. A delay law is applied to the emission transducers used in order for them to transmit, into the medium, a spherical ultrasound wave, close to that which would be emitted by a virtual source located at a certain distance from the sensor. The ultrasound wave thus emitted by the virtual source is more intense since its energy is proportional to the square root of the number of emission transducers forming this source. The SNR is improved as much, when supposing that the noise generated is mainly uncorrelated electronic noise. The corresponding encoding matrix [MC] comprises a plurality of non-zero coefficients in each of its columns, precisely defining the number of adjacent transducers used simultaneously at each emission and the delay laws optionally applied.
However, in the case of inspected parts having very significant noise, the improvement in the quality of the images finally obtained by the synthetic total focusing is more limited, the increase in the SNR is lesser and the impact on the detection is not as positive as could be expected. This solution partly compensates for the aforementioned problem but does not eliminate it. Indeed, the increase in the SNR is proportional to the square root of the number of transducers forming each virtual source, and this number is much smaller than the total number of transducers N of the sensor. Moreover, emitting via virtual sources does not allow the problem that can be posed by the reconstruction artefacts substantially caused by the parasite echoes such as the geometry echoes or the complex echoes that include multiple reflections on the borders of the object and mode conversions to be eliminated.
Another solution to this problem of degradation of the SNR is provided in the article by Lopez Villaverde et al, entitled “Ultrasonic imaging in highly attenuating materials with Whalsh-Hadamard codes and the decomposition of the time reversal operator”, published for the conference IEEE International Ultrasonics Symposium which took place in Tours (FR) from 18 to 21 Sep. 2016. It consists of encoding the emissions using a Hadamard matrix [MC] noted as H−). It has been demonstrated that with this type of emission encoding, a matrix [MR(t)] of time signals equivalent to [K(t)] after decoding is obtained with an improved SNR. But in order to obtain an SNR significantly better than in the previous solutions, it generally imposes a large number of successive emissions, for example 2N-1 if the transducers are used in emission using the encoding matrices HN− and HN−.
It may thus be desired to provide a method for acquiring ultrasound signals that allows to overcome at least a portion of the aforementioned problems and constraints.
A method for acquiring signals via ultrasound probing, comprising the following steps, is therefore proposed:
Thus, starting from an encoding matrix [MC′] that can be one of those indicated above or another, in particular an encoding matrix ensuring a satisfactory SNR according to the intended use and the later processing planned, the invention allows to reduce the number of emissions by eliminating one or more emissions on the basis of a criterion that is relevant since it is related to calculations of acoustic fields. This manner of operating allows in particular to reduce the number of the emissions by limiting as much as possible the effect of this reduction on the SNR of the matrix [MR(t)].
Optionally, the matrix [MR(t)] of ultrasound time signals is decoded in order to obtain a decoded matrix [MR′(t)] calculated via matrix product in the following manner:
[MR′(t)]=[MR(t)]·[MC]T·([MC]·[MC]T)−1,
where “T” is the symbol of matrix transposition.
A matrix having a size of N×L, on the basis of which the same processing as with a matrix of time signals obtained by conventional acquisition of the FMC type is possible, is thus obtained.
Also optionally, the calculation of acoustic field carried out for each of the M′ successive initial emissions comprises the calculation of a simplified field model Em′(f,θ) defined for each column having the index m′ of the initial encoding matrix [MC′] in the following manner:
where:
Also optionally, the calculation of acoustic field carried out for each of the M′ successive initial emissions further comprises the calculation of an integrated field value Am′(θ) on the basis of each simplified field model Em′(f,θ) in the following manner:
where fmin and fmax are respectively a minimum and maximum frequency of a bandwidth of the common excitation time signal e(t).
Also optionally, the selection criterion applied to the M′ calculations of acoustic fields comprises an amplitude threshold below which the contributions of the acoustic field are considered to be negligible.
Also optionally, each column of the initial encoding matrix [MC′] producing an initial emission, the calculation of acoustic field of which does not provide a value greater than or equal to the amplitude threshold, is eliminated.
Also optionally, the selection criterion applied to the M′ calculations of acoustic fields further comprises angular thresholding involving removing any contribution of the acoustic field outside of a predetermined angular sector.
Also optionally, the initial encoding matrix [MC′] is a Hadamard matrix or obtained from a Hadamard matrix.
A computer program that can be downloaded from a communication network and/or is recorded on a medium readable by computer and/or can be executed by a processor, comprising instructions for the execution of the steps of a method for acquiring signals according to the invention, when said program is executed on a computer, is also proposed.
An ultrasound probing device is also proposed, comprising:
The invention will be better understood via the following description, given only as an example and made in reference to the appended drawings in which:
In reference to
The object 102 is for example a mechanical part that it is desired to examine via non-destructive testing or, in a medical context, a human body part that it is desired to inspect non-invasively. In the embodiment of
The transducers 1081, . . . ,108N are designed to emit ultrasound waves in the direction of the object 102 in response to control signals identified by the general reference C, in main directions parallel to each other, indicated by dotted arrows in
The transducers 1081, . . . , 108N are further designed to detect echoes of the ultrasound waves reflecting on and in the object 102 and to provide measurement signals identified by the general reference S and corresponding to these echoes. Thus, in the non-limiting example of
The probing device 100 further comprises an electronic circuit 112 for control of the transducers 1081, . . . , 108N of the probe 104 and for processing of the measurement signals S. This electronic circuit 112 is connected to the probe 104 in order to transmit to it the control signals C and in order to receive the measurement signals S. The electronic circuit 112 is for example that of a computer. It has a central processing unit 114, such as a microprocessor designed to emit, to the probe 104, the control signals C and to receive, from the probe 104, the measurement signals S, and a memory 116 in which a computer program 118 is recorded.
The computer program 118 comprises first of all instructions 120 for defining M′ successive initial emissions using an initial encoding matrix [MC′] having a size of L×M', that is to say having a size of N×M' in the non-limiting example in question. Each coefficient MC′i,j of this matrix represents a multiplication factor applied to an excitation time signal e(t), common to all the transducers 1081, . . . , 108N, for its emission by the i-th emission transducer at the time of the j-th emission. This multiplication factor can include a delay of a delay law applied to the j-th initial emission in question. The initial encoding matrix [MC′] can be predetermined and recorded in memory, chosen using the instructions 120 out of a set of initial encoding matrices recorded in memory, defined via a man-machine interface using the instructions 120, etc.
As indicated above, the initial encoding matrix [MC′] can be the identity matrix for M′=L=N successive initial emissions compliant with FMC acquisition.
It can also be a matrix, the non-zero coefficients of which including delay laws are located around its main diagonal, for M′<L=N successive initial emissions compliant with an acquisition as taught in the aforementioned article by Karaman et al.
It can also be a square Hadamard matrix HN, for M′=L=N successive initial emissions compliant with an acquisition as taught in chapter II.A of the aforementioned article by Lopez Villaverde et al. In this case, the number N must be a power of 2.
It can also be a matrix obtained from a Hadamard matrix, for M′ successive initial emissions compliant with an acquisition as taught in chapter III-B of the aforementioned article by Lopez Villaverde et al. For example, this can be a horizontal concatenation of two matrices HN+ and HN− resulting from the matrix HN (HN+=½(JN+HN) and HN−=½(JN−HN), where JN is the matrix having a size of N×N, all the coefficients of which are at 1), for M′=2L−1=2N−1 successive initial emissions. This can also simply be the matrix HN+, for M′=L=N successive initial emissions.
Other initial encoding matrices [MC′] are possible for a person skilled in the art according to the concrete intended uses.
The computer program 118 further comprises instructions 122 for executing a calculation of acoustic field for each of the M′ successive initial emissions defined in the initial encoding matrix [MC′]. This acoustic field is dependent on the transducers themselves, in particular on the materials in which they are designed and on their size, on their positioning and on the medium in which the acoustic waves are emitted.
It is within the reach of a person skilled in the art to carry out a calculation of acoustic field given their general knowledge in the field, but a particularly ingenious calculation, aiming to reduce the calculation times, is provided below. It involves proposing a simplified field model Em′(f,θ) defined for each column having the index m′ of the initial encoding matrix [MC′] in the following manner:
where:
Optionally but advantageously, the instructions 122 continue the calculation of the M′ preceding acoustic fields by integrating the results over the frequency bandwidth of the excitation time signal e(t). The minimum and maximum frequency of this bandwidth are labelled fmin and fmax, respectively. This thus gives an integrated field value Am′(θ) that now depends only on θ, defined for each column having the index m′ of the initial encoding matrix [MC′] in the following manner:
The diagram of
Indeed, it is noted that A1(0) is the maximum value that Am′(θ) can have as a function of m′ and θ.
The computer program 118 further comprises instructions 124 for removing M′−M column(s) of the initial encoding matrix [MC′] (M<M′), this/these removed column(s) corresponding to M′−M initial emission(s) removed on the basis of a selection criterion applied to the M′ calculations of acoustic fields executed by the instructions 122. These instructions allow to obtain a reduced encoding matrix [MC] having a size of L×M, only comprising the non-removed columns of the initial encoding matrix [MC′] and defining the M emissions thus selected.
The computer program 118 further comprises instructions 126 for generating the signals C for control of the transducers 1081, . . . , 108N in such a way as to:
inspection, N measurement signals measuring in particular echoes caused
by reflections of the emission in question in the zone of interest.
The set S of the N×M measurement signals thus transmitted by the transducers 1081, . . . , 108N is sent back by the probe 104 to the central processing unit 114.
The computer program 118 thus further comprises instructions 128 for constructing a matrix [MR(t)] of ultrasound time signals having a size of N×M, each coefficient MRi,j(t) of this matrix representing the measurement signal received by the transducer 108i in response to the j-th emission.
Optionally, the computer program 118 further comprises instructions 130 for carrying out temporal filtering of the matrix [MR(t)], this filtering aiming to remove any information located at times of flight excluded from the zone of interest in the object 102.
Finally, the computer program 118 comprises instructions, designated by the general reference 132, for processing the matrix [MR(t)]. The processing carried out by the instructions 132 can include:
It can in particular be demonstrated that the matrix [MR′(t)], obtained by the following decoding:
[MR′(t)]=[MR(t)]·[MC]T·([MC]·[MC]T)−1,
is a matrix that can be used in the same manner as the matrix of time signals obtained by conventional acquisition of the FMC type. It is noted that in general it has a size of N×L, that is to say square having a size of N×N when the transducers are emitters and receivers. It is, however, clearly less noisy than that obtained by conventional acquisition.
As an experiment, a simulation was carried out for the example of the previous figures, i.e.: the transducers 1081, . . . , 108N are aligned; L=N=64; the initial encoding matrix [MC′] is the Hadamard matrix H64; the inter-element step d is equal to 0.6 mm; the propagation medium is such that the velocity c is equal to 2.3 mm/μs; the function sl(f) is chosen as taking the form of a Gaussian signal having a central frequency at 5 MHz with a bandwidth of 60% at −6 dB; the function Dl(f, θ) is defined according to the teaching of the aforementioned article by Fan et al for a common width of the transducers of 0.5 mm. Moreover, as shown by
By carrying out the selection as defined above and by thus removing seventeen columns from the sixty-four in the Hadamard matrix H64, the comparative results of
If digital images are reconstructed by synthetic focusing of the “total focusing method” type on each of the preceding results, the images of
In reference to
During a step 202, the processing unit 114 executing the instructions 120 defines M′ successive initial emissions using an initial encoding matrix [MC′] having a size of L×M', that is to say a size of N×M' in the non-limiting example in question.
During a step 204, the processing unit 114 executing the instructions 122 carries out a calculation of acoustic field for each of the M′ successive initial emissions defined in the initial encoding matrix [MC′], for example according to the example illustrated by
During a step 206, the processing unit 114 executing the instructions 124 removes M′−M column(s) from the initial encoding matrix [MC′] (M<M′), this or these removed columns corresponding to M′−M initial emissions removed on the basis of a selection criterion applied to the M′ calculations of acoustic fields executed by the instructions 122, in order to obtain a reduced encoding matrix [MC] having a size of L×M, that is to say having a size of N×M in the non-limiting example in question. The selection criterion applied is for example that illustrated by
During a step 208, the processing unit 114 executing the instructions 126 controls the sequences of emissions and of receptions of the transducers 1081, . . . , 108N using the reduced encoding matrix [MC] for the acquisition of the matrix [MR(t)]. After each firing, the signals are received on all of the N transducers, digitized and transmitted to the electronic circuit 112.
During a step 210, the processing unit 114 executing the instructions 128 constructs the matrix [MR(t)], each coefficient MRi,j(t) of this matrix representing the measurement signal received by the transducer 108i in response to the j-th emission, this signal being digitized in order to facilitate its later processing.
During an optional step 212, the processing unit 114 executing the instructions 130 carries out temporal filtering of the matrix [MR(t)], this filtering aiming to remove any information located at times of flight excluded from the zone of interest. The goal of this step 206 is to facilitate the later processing, in particular when the defects to be imaged are close to a strongly echogenic interface, like a bottom of a part. It allows to limit the zone to be imaged to the close vicinity of the defects by excluding in particular the disturbing echogenic interfaces. It is very advantageous in the imaging of cracks forming from the bottom of the object.
Finally, during a last step 214, the processing unit 114 executing the instructions 132 carries out one or more of the processing cited above: optional decoding according to the reduced encoding matrix [MC] used to obtain the matrix [MR′(t)] defined above, noise reduction, adaptive and selective focusing, reconstruction of a digital image of the zone of interest in the object 102, etc.
It is clear that a probing device such as that described above, implementing the acquisition method described in detail above, allows to simplify the acquisition of the ultrasound signals by reducing the number of the emissions, while limiting as much as possible the effect of this reduction on the SNR of the matrix of time signals obtained.
Moreover, it is noted that the invention is not limited to the embodiment described above. Indeed, it is clear to a person skilled in the art that various modifications can be made to the embodiment described above, in light of the teaching that has just been disclosed to them.
In particular, at least a portion of the computer program instructions 120, 122, 124, 126, 128, 130 and 132 could be replaced by microprogrammed or micro-wired electronic circuits, dedicated to the functions carried out during the execution of these instructions.
Moreover, the results and calculations of
Also moreover, in the embodiment described in detail above, L=N=M′. But there is no particular reason for these three parameters to be equal in general.
In general, in the following claims, the terms used must not be interpreted as limiting the claims to the embodiment disclosed in the present description, but must be interpreted to include all the equivalents that the claims aim to cover by their formulation and the providing of which is within the reach of a person skilled in the art by applying their general knowledge to the implementation of the teaching that has just been disclosed thereto.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1662523 | Dec 2016 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FR2017/053390 | 12/5/2017 | WO | 00 |
