Patent Publication Number: US-9903842-B2

Title: Method for processing signals acquired by ultrasonic probing, corresponding program and ultrasonic probing device

Description:
The present invention relates to a method for processing signals acquired by ultrasonic probing, particularly for performing ultrasonic imaging. It also relates to a corresponding computer program and ultrasonic probing device. 
     The invention is particularly applicable to the field of ultrasonic non-destructive testing, wherein the acquisition of ultrasonic signals makes it possible to view and detect defects in structures, but it may also be applied to any type of ultrasonic echographic imaging, particularly in the medical field for inspecting areas of interest in the human or animal body. 
     It relates more particularly to a processing method acquiring ultrasonic signals as follows:
         controlling a plurality of emission transducers for L successive emissions of ultrasound waves to an area of interest,   controlling N reception transducers so as to simultaneously receive and for a predetermined time, for each successive emission, N measurement signals, particularly measuring noisy echoes due to reflections of the emission in question in the area of interest,   obtaining an array of ultrasonic time signals of the size L×N, each coefficient K i,j (t) of this array representing the measurement signal received by the j-th reception transducer due to the i-th emission.       

     Such an acquisition is generally performed using a multielement sensor probing device, wherein each transducer is both transmitter and receiver, where switching between these two modes may be controlled electronically. The sensor may be placed in contact with the object to be probed or at a distance, but in the latter case it should be submerged to ensure the transmission of the ultrasonic waves in the object to be probed. This sensor may be linear (1D) or array-based (2D), with rigid or flexible elements. 
     The array of time signals obtained using this type of acquisition may then be the subject of processing, particularly for furnishing an image of the area of interest inspected or for retrieving significant structural defect parameters in the area of interest inspected. In view of the computing capability of the processors, this processing may be embedded in the monitoring instruments for real-time processing. 
     In practice, the ultrasonic acquisition defined above, generally referred to as FMC (“Full Matrix Capture”), consists of emitting an ultrasonic wave by exciting the first emission transducer and receiving the echoes of this emission with the set of N reception transducers, then electronically switching in the set of emission transducers to successively excite these emission transducers. The emission and reception transducers may be located on two separate sensors, but when the same transducers carry out the emission and reception functions, an array K(t) of ultrasonic time signals of the size N×N is obtained. 
     In the article by C. Holmes et al, entitled “Post-processing of the full matrix of ultrasonic transmit-receive array data for non-destructive evaluation”, published in NDT&amp;E International 38 (available online on Jun. 15, 2005), pages 701-711, the coefficients of the array K(t) are used to perform a “total focusing” type synthetic aperture focusing making it possible to obtain a high-resolution image of the area of interest. 
     More specifically, this synthetic focusing consists of computing for each point of the area of interest the time of flight T i,j  corresponding to the travel time between each emission transducer (index i) and each reception transducer (index j) via the point in question (L×N times of flight for each point). Synthetic focusing is performed by summing, for each point of the area of interest, the amplitudes retrieved from the signals K i,j (t) at the times t=T i,j . The amplitude A at a point P of the image may thus be expressed as follows: 
     
       
         
           
             
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     Reconstruction using total focusing may be performed according to various known ultrasonic inspection modes: direct mode where the associated times of flight are described above, and other more complex modes where the times of flight include multiple reflections on the boundaries of the structure along with mode conversions. For a detailed explanation of these other more complex modes, reference may particularly be made to the article by A. Fidahoussen et al, entitled “Imaging of defects in several complex configurations by simulation-helped processing of ultrasonic array data”, published in Review of Quantitative Nondestructive Evaluation, vol. 29 (2009), pages 847-854. 
     However, applied to the imaging of noisy parts, reconstruction using total focusing may furnish images of lower quality compared to conventional echographic methods. Indeed, in the latter, all the transducers emit simultaneously by applying a predefined delay sequence so as to focus at a given point. However, according to the FMC acquisition method generally implemented to subsequently conduct reconstruction by means of synthetic aperture focusing, each emission is performed by a single transducer which limits the energy transmitted and the penetration depth of the waves in the inspected part. This is finally conveyed by a degradation of the Signal-To-Noise Ratio (SNR) as the amplitudes of the echo signals may be weaker than the electronic noise. This SNR degradation increases if the object has a high level of structural noise, making the detection and characterization of any defects difficult. 
     One partial solution to this SNR degradation problem is provided in the article by M. 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 consists of using, for each emission, no longer one transducer but M adjacent transducers. A delay sequence is applied to the M emission transducers activated so that they transmit a spherical ultrasonic wave in the medium, similar to that which would be emitted by a virtual source situated at a certain distance from the sensor. The ultrasonic wave thus emitted by the virtual source is more intense since the energy thereof is proportional to the square root of the number of transducers forming this source. The SNR is enhanced accordingly, on the assumption that the noise generated is essentially uncorrelated electronic noise. 
     However, in the case of inspected parts having a high level of structural noise, the enhancement of the quality of the images finally obtained by synthetic total focusing is more limited, the increase in the SNR is lower and the impact on detection is not as positive as may have been hoped. This solution compensates in part for the problem mentioned above but does not eliminate it. Moreover, emitting by means of virtual sources does not help do away with the problem which may be posed by reconstruction artifacts essentially due to parasitic echoes such as geometric echoes or complex echoes including multiple reflections on the boundaries of the object and the mode conversions. Finally, it is noted that for a sensor having N emission/reception transducers, the array K(t) of ultrasonic time signals obtained has the reduced size L×N, where 1≦L≦5. N−M+1, M being the number of transducers activated simultaneously to form the virtual source (1≦M≦N). 
     In the article by C. Prada et al, entitled “Eigenmodes of the time reversal operator: a solution to selective focusing in multiple-target media”, published in Wave Motion 20 (1994), pages 151-163, the array K(t) is used to perform a decomposition of the time reversal operator defined as being the conjugate product (T indicating the complex conjugate) K(ω)·K T (ω), where K(ω) corresponds to the Fourier transform of the array K(t). The principle described in this article is that of determining the invariants of this time reversal operator. For this purpose, a singular value decomposition of the frequency signal array K(ω) obtained using the Fourier transform of the time signal array K(t) is performed following the FMC acquisition. It has indeed been observed that, as a general rule, in the distribution of the singular values obtained, the number of high singular values (i.e. having significant amplitudes) is equal to the number of defects in the part inspected, provided that the defects are sufficiently small and at a distance from each other. Each singular vector thus furnishes the response from a defect (i.e. the Green function thereof) that can be used to focus thereon without needing precise information on the geometric and acoustic properties of the object. This is referred to as the autofocusing principle. The order of the singular vectors is dependent on the more or less scattering nature of the defect. In this way, the first singular vector corresponds to the greatest scatterer, for example the defect closest to the sensor, and so on. 
     However, this time reversal operator decomposition method is essentially a detection method by retrieving significant parameters demonstrating the limitations thereof in the presence of a high level of structural noise, or when the defects are close to a boundary of the inspected object, for example in the case of a defect close to a part base. Indeed, in these specific cases, no actual separation of a singular value in relation to the others is observed. It is thus in fact difficult to match a singular value to a possible defect. Detection is thus more difficult. Furthermore, unlike the synthetic aperture focusing methods mentioned above, this method does not furnish an image, but merely an indication of the presence of a defect or not. The location and characterization of a defect requires the addition of an imaging algorithm. 
     It may thus be sought to provide a method for processing ultrasonic signals suitable for doing away with at least some of the problems and constraints mentioned above. 
     It is therefore proposed a method for processing ultrasonic signals acquired by ultrasonic probing comprising the following steps:
         controlling a plurality of emission transducers for L successive emissions of ultrasound waves to an area of interest,   controlling N reception transducers so as to simultaneously receive and for a predetermined time, for each successive emission, N measurement signals, particularly measuring noisy echoes due to reflections of the emission in question in the area of interest,   obtaining an array of ultrasonic time signals of the size L×N, each coefficient Ki,j of this array representing the measurement signal received by the j-th reception transducer due to the i-th emission,   performing a singular value decomposition of a frequency signal array obtained by transforming this time signal array,
 
comprising a step for denoising the ultrasonic time signal array by:
   removing some of the singular values and associated singular vectors obtained from said singular value decomposition, and   reconstructing a denoised time signal array from the unremoved singular values and singular vectors.       

     In this way, the singular value decomposition of the frequency signal array obtained by the transform of the acquired time signal array is used astutely to denoise the latter, since it has been observed that some of these singular values are in fact directly correlated with structural noises from the probed part and/or parasitic echoes (interface echoes) from ultrasonic acquisition by means of a multielement sensor. By thus reconstructing a denoised time signal array, it is then possible to continue conventional processing of the latter, for example to obtain a superior ultrasonic image of the probed area of interest, on the basis of higher quality data. 
     Optionally, the denoising step is performed by:
         transform of the time signal array into a frequency signal array,   singular value decomposition of the frequency signal array,   removing some of the singular values and associated singular vectors according to a predetermined criterion for distinguishing between singular values associated with defects and singular values associated with noise,   reconstructing a denoised frequency signal array on the basis of unremoved singular values and singular vectors,   inverse transform of this denoised frequency signal array into a denoised time signal array.       

     Also optionally, the predetermined criterion for distinguishing between singular values associated with defects and singular values associated with noise is a criterion relating to successive amplitude differences between singular values in a decreasing series of amplitudes of the singular values determined on the basis of the frequency signal array, for example a curvature change or slope change criterion in this decreasing series of singular value amplitudes. 
     Also optionally, the transform and inverse transform are discrete Fourier transforms. 
     Also optionally, a method for processing ultrasonic signals according to the invention may comprise, before the denoising step, a step for filtering the time signal array by deleting any data situated at times of flight excluded from the area of interest. 
     Also optionally, a method for processing ultrasonic signals according to the invention may comprise a step for reorganizing frequency components of the singular values and singular vectors of the frequency signal array on the basis of an optimization of correlations between frequency occurrences of the singular vectors so as to optimize a correspondence between singular values and defects in the area of interest and thus optimize noise filtering. 
     Also optionally, at each successive emission, M adjacent emission transducers are activated and a delay sequence is applied to these M emission transducers so as to emit a spherical wave from a virtual source situated at a predetermined distance from said plurality of emission transducers. 
     Also optionally, each reception is performed by L′ virtual reception transducers, each virtual reception transducer consisting of M′ adjacent reception transducers to which a delay sequence is applied. 
     Also optionally, a method for processing ultrasonic signals according to the invention may comprise an additional step for reconstructing a digital image of the area of interest from the denoised time signal array, particularly by means of synthetic total focusing processing. 
     It is also proposed a computer program downloadable from a communication network and/or saved on a computer-readable medium and/or executable by a processor, comprising instructions for executing steps of a method for processing ultrasonic signals according to the invention, when said program is executed on a computer. 
     It is also proposed an ultrasonic probing device comprising:
         a probe comprising a plurality of ultrasonic emission transducers and a plurality of ultrasonic reception transducers, and   transducer control and processing means designed to implement a method for processing ultrasonic signals according to the invention.       

    
    
     
       The invention will be understood more clearly using the description hereinafter, given merely by way of example and with reference to the appended drawings wherein: 
         FIG. 1  schematically represents the general structure of an ultrasonic probing device according to one embodiment of the invention. 
         FIG. 2  illustrates the successive steps of a method for acquiring and processing ultrasonic signals implemented by the device in  FIG. 1 , according to one embodiment of the invention, 
         FIGS. 3 and 4  illustrate an ultrasonic wave emission/reception principle implemented by the device in  FIG. 1 , 
         FIGS. 5 and 6  illustrate, using diagrams, singular value distributions obtained from intermediate steps of the method in  FIG. 2 , 
         FIGS. 7 and 8  comparatively illustrate ultrasound images obtained without and with full application of the method in  FIG. 2 . 
     
    
    
     With reference to  FIG. 1 , a device  100  for probing an object  102  according to one embodiment of the invention comprises an ultrasonic probe  104  having a housing  106 , i.e. a non-deformable structural element acting as a reference attached to the probe  104 , wherein, for example in a linear fashion or according to an array, N fixed or movable transducers  108   1 , . . . ,  108   N  are arranged. 
     The object  102  is for example a mechanical part to be examined by means of non-destructive testing or, in a medical context, a part of the human body to be monitored non-invasively. In the embodiment in  FIG. 1 , the object  102  is submerged in a liquid, such as water  110 , and the probe  104  is kept at a distance from the object  102  so that water  110  separates the two. However in a further equivalent embodiment, the probe  104  could be in direct contact with the object  102 . 
     The transducers  108   1 , . . . ,  108   N  are designed to emit ultrasonic waves toward the object  102  in response to control signals identified under the general reference C, along main directions parallel with each other, indicated by the dotted arrows in  FIG. 1 , and in a main plane which is that of the figure. 
     The transducers  108   1 , . . . ,  108   N  are further designed to detect echoes of ultrasonic waves reflected on or in the object  102  and to supply measurement signals identified under the general reference S and corresponding to these echoes. In this way, in the non-limiting example in  FIG. 1 , the transducers  108   1 , . . . ,  108   N  fulfill both an emission and reception function, but receivers different from the emitters could also be provided in different independent housings while remaining in compliance with the principles of the invention. 
     The probing device  100  further comprises an electronic circuit  112  for controlling the transducers  108   1 , . . . ,  108   N  of the probe  104  and for processing the measurement signals S. This electronic circuit  112  is connected to the probe  104  in order to transmit thereto 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  wherein a computer program  118  is saved. 
     The computer program  118  firstly comprises instructions  120  for generating the control signals C for the transducers  108   1 , . . . ,  108   N  so as to:
         activate the transducers  108   1 , . . . ,  108   N  as emitters for L successive emissions of ultrasonic waves to an area of interest of the object  102 ,   activate the transducers  108   1 , . . . ,  108   N  as receivers to, following each successive emission, simultaneously receive, via these N receivers and for a predetermined duration of the sought inspection depth, N measurement signals particularly measuring the noisy echoes due to reflections of each emission in the area of interest.       

     The set S of the L×N measurement signals transmitted by the transducers  108   1 , . . . ,  108   N  is returned by the probe  104  to the central processing unit  114 . 
     The computer program  118  further comprises instructions  122  for constructing an array K(t) of ultrasonic time signals of the size L×N, each coefficient K i,j (t) of this array representing the measurement signal received by the transducer  108   j  in response to the i-th emission. 
     Optionally, the computer program  118  further comprises instructions  124  for performing time filtering of the array K(t), this filtering being intended to delete any data situated at times of flight excluded from the area of interest in the object  102 . 
     The computer program  118  further comprises instructions  126  for transforming the array K(t) into a frequency signal array K(ω) by means of a Fourier transform, advantageously by means of a discrete Fourier transform after time sampling of the coefficients of the array K(t), or, more advantageously, by means of FFT (“Fast Fourier Transform”) computation if the number of samples of each coefficient of the array K(t) permits. 
     The computer program  118  further comprises instructions  128  for decomposing the frequency signal array K(ω) into singular values over a frequency band so as to diagonalize said array. This known operation makes it possible to estimate the arrays U, S and V such that: 
                     K   ⁡     (   ω   )       =       ⁢     U   .   S   .     V   T                     =       ⁢       ∑     i   =   1     L     ⁢           ⁢         λ   i     ⁡     (   ω   )       ·       u   i     ⁡     (   ω   )       ·       v   i   T     ⁡     (   ω   )             ,       where   ⁢           ⁢   U     =     [         u   1     ⁡     (   ω   )       ,   …   ⁢           ,       u   L     ⁡     (   ω   )         ]                       ⁢       and   ⁢           ⁢   V     =     [         v   1     ⁡     (   ω   )       ,   …   ⁢           ,       v   N     ⁡     (   ω   )         ]                   
are orthogonal arrays of the respective sizes L×L and N×N, containing the singular vectors in reception and emission, i.e. the invariants in reception and emission of the time reversal operator, where S is a diagonal array of the size L×N containing the L singular values λ i (ω) of the array K(ω), ordered in decreasing fashion at a given reference frequency λ 1 (ω)≧ . . . ≧λ L (ω)≧0.
 
     Optionally, the computer program  118  further comprises instructions  130  for reorganizing, according to the frequency, the array K(ω) into an array  K (ω) by reorganizing the frequency components of the singular values and singular vectors thereof. Indeed, if the echo of a defect is situated at a time of flight close to that of an interface of the object  102  (for example, a defect close to the base of the part), or if this echo has an amplitude similar to the structural noise, the same singular value of the array S may correspond equally to the defect, to the interface and to the structural noise according to the frequencies in question in the spectral bandwidth of the probe. This may advantageously merit a reorganization of the frequency components of the singular values and the corresponding frequency occurrences of the associated singular vectors, so as to optimize the correspondence between singular values and defects. The reorganized eigenvalues are annotated  λ   1 (ω)≧ . . . ≧ λ   L (ω)≧0. 
     The computer program  118  further comprises instructions  132  for reducing the rank of the array  K (ω) (or that of the array K(ω) if the optional instructions  130  are not executed), optionally reorganized, by removing some of the singular values  λ   i . This removal is performed according to a criterion for distinguishing between singular values associated with a defect and singular values associated with noise, the first having greater amplitudes than the second. Given that  λ   1 ≧ . . . ≧ λ   L ≧0, it is necessary to find the value P between 1 and L such that  λ   1 , . . . ,  λ   P  may be considered to be associated with defects to be detected in the object  102  and  λ   P+1 , . . . ,  λ   L  may be removed as they are considered to be associated with noise. In practice, P is determined by studying the singular value amplitude decline curve and more specifically by studying the successive amplitude differences thereof (i.e.  λ   2 − λ   1 , . . . ,  λ   N − λ   N-1 ) at a reference frequency, for example the central frequency of the frequency spectrum of the array  K (ω). By way of non-limiting example, P may be equal to the index associated with the singular value for which the singular value decline curve exhibits a change of curvature, more specifically a change of slope, indicating a significant variation in the successive amplitude differences between singular values. Such a determination of P may be performed in a manner known per se by linear regression on the assumption of a two-stage linear decline. In the case of small defects ideally spaced out in respect of each other, P is equal to the number of defects present in the area of interest inspected. Reducing the rank of the array  K (ω) thus consists of only retaining Kf(ω) in the following equation:
 
   K   (ω)= Kf (ω)+ Kb (ω), where:
 
     
       
         
           
             
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     The array Kf(ω) reconstructed in this way is a denoised frequency signal array, the noise subspace represented by the array Kb(ω) having been removed. 
     The computer program  118  further comprises instructions  134  for transforming the array Kf(ω) into a denoised time signal array Kf(t) by means of an inverse Fourier transform, advantageously by means of an inverse discrete Fourier transform, or, more advantageously, by IFFT (“Inverse Fast Fourier Transform”) computation if the number of samples of each coefficient of the array Kf(ω) permits. 
     Finally, the computer program  118  comprises instructions  136  for performing synthetic total focusing as defined in the article mentioned above by C. Holmes et al on the denoised array Kf(t). A digital image of the area of interest is thus reconstructed wherein the quality is better than if the synthetic focusing had been carried out on the non-denoised array K(t). In particular, the SNR is enhanced. 
     With reference to  FIG. 2 , a method  200  for acquiring and processing ultrasonic signals implemented by the device  100  in  FIG. 1  will now be described. 
     During a step  202 , the processing unit  114  executing the instructions  120  controls the emission and reception sequences of the transducers  108   1 , . . . ,  108   N  for acquiring the array K(t). 
     These sequences are L in number, an integer between 1 and N−M+1, where M, an integer between 1 and N, is the number of adjacent transducers forming the emitting sub-aperture moving along the housing  106  of the probe  104  in intervals of at least one transducer. The choice of the number M is dependent on the quality sought of the spherical wave emitted by the sub-aperture. After each round, the signals are received on all of the N transducers, digitized and transmitted to the electronic circuit  112 .  FIG. 3  illustrates these emission and reception sequences, successively referenced E 1  and R 1 , E 2  and R 2 , E 3  and R 3 , . . . , EL and RL, where the activated transducers are represented in gray tone. 
     In the case where M≧2, predetermined delay sequences are applied to the transducers forming the sub-aperture of M transducers. They enable focusing of the waves emitted at a point O situated at F mm in depth under the probe  104 . The wavefront emitted does not stop at the point O. A wave diverges from this point and is propagated in the medium. For an observer situated at a depth greater than F, it is as if the divergent wave were from a virtual source located at  0 . The virtual source created does not have a perfectly omnidirectional directivity such as that of a point source but has an angular directivity having a relatively wide angle θ. This directivity may be adjusted by modifying the delays applied to the transducers of the sub-aperture such that the wave emitted by the virtual source is directed in a preferred direction in the object  102 . This enhances the detection of the defects in this area.  FIG. 4  illustrates the virtual source principle according to two delay sequences provided by way of example. 
     During a step  204 , the processing unit  114  executing the instructions  122  constructs the array K(t), each coefficient K i,j (t) of this array representing the measurement signal received by the transducer  108   j  in response to the i-th emission, this signal being digitized to facilitate the subsequent processing thereof. 
     During an optional step  206 , the processing unit  114  executing the instructions  124  performs time filtering of the array K(t), this filtering being intended to delete any data situated at times of flight excluded from the area of interest. The aim of this step  206  is to subsequently facilitate the separation of the two subspaces represented by the arrays Kf(ω) and Kb(ω), in particular when the defects to be imaged are close to a significantly echoic interface, such as a base of a part. It makes it possible to limit the area to be imaged to a region close to the defects by particularly excluding the disturbing echoic interfaces. It is of particular interest in imaging cracks formed from the base of the object. 
     During a step  208 , the processing unit  114  executing the instructions  126  performs a discrete Fourier transform of the array K(t) to obtain the frequency signal array K(ω). 
     During a step  210 , the processing unit  114  executing the instructions  128  diagonalizes the array K(ω) by decomposing same into singular values, as above. 
     During an optional step  212 , the processing unit  114  executing the instructions  130  reorganizes the array K(ω) into an array  K (ω) by reorganizing the frequency components of the singular values and singular vectors of the decomposition arrays S(ω), U(ω) and V(ω) into new decomposition arrays  S (ω),  U (ω) and  V (ω). 
     According to a first alternative embodiment for reorganizing the frequency components of the singular values and singular vectors, for each singular value λ i (ω), 1≦i≦L:
         a reference frequency occurrence of a singular vector associated with λ i (ω) is chosen, for example the singular vector of the array U, u i (ω), this reference frequency occurrence being annotated u i   ref =u i [ω ref ] (this generally consists of the central frequency of the frequency spectrum of K(ω) for which a separation of the highest singular value is observed),   the phase of this reference frequency occurrence u i   ref  is computed, this phase is normalized in the interval [0,1], and then   for each frequency ω of the frequency spectrum of K(ω):
           the phases of the frequency occurrences u k [ω] of the other singular vectors of the array U are computed and these phases are normalized in the interval [0,1],   the correlation between the normalized phase of u i   ref  and the normalized phase of each u k [ω] is computed,   the value j of k for which the correlation is greatest is determined, and   the value of λ j [ω] is assigned to λ i [ω], the value of u j [ω] to u i [ω], and the value of v j [ω] to v i [ω].   
               

     This gives new reorganized arrays  S (ω),  U (ω) and  V (ω) and thus a new reorganized array  K (ω)= U · S · V   T . 
     According to a second alternative embodiment for reorganizing the frequency components of the singular values and singular vectors, for each singular value λ i (ω), 1≦i≦L:
         a reference frequency occurrence of a singular vector associated with λ i (ω) is chosen, for example the singular vector of the array U, u i (ω), this reference frequency occurrence being annotated u i   ref =u i [ω ref ] and corresponding to a maximum frequency occurrence of the singular value λ i (ω) (this generally also consists of the central frequency of the frequency spectrum of K(ω)),   the phase of this reference frequency occurrence u i   ref  is computed, and then   by defining a basic increment Δω for iteratively scanning the frequency spectrum of K(ω):
           the phases of the frequency occurrences u k [ω ref ±Δω] of the other singular vectors of the array U are computed, the correlation between the phase of u i   ref  and the phase of each u k [ω ref ±Δω] is computed, the value j of k for which the correlation is greatest is determined, and the value of λ j [ω ref ±Δω] is assigned to λ i [ω ref ±Δω], the value of u j [ω ref ±Δω] to u i [ω ref ±Δω], and the value of v j [ω ref ±Δω] to v i [ω ref ±Δω],   as the new reference frequency occurrence, that of the singular vector having maximized the correlation with the previous step at the frequency ω ref ±Δω is adopted and the correlation computations of the previous step at ω ref +2Δω are repeated,   the study of K(ω) in the spectral bandwidth of the probe is thus continued step by step until the limits thereof.   
               

     New reorganized arrays  S (ω),  U (ω) and  V (ω) and therefore a new reorganized array  K (ω)= U · S · V   T  are thus obtained. 
     The reorganized array  K (ω) is thus now decomposed into singular values each having singular vectors optimizing the correlations thereof at all frequencies, either in relation to a selected constant reference frequency (first alternative embodiment), or step by step (second alternative embodiment). In this way, after reorganizing the array  K (ω), a defect is associated with the same singular value for all the frequencies in the spectral band of the probe. An example of frequency distributions in respect of amplitude (A) of L eigenvalues is illustrated in  FIG. 5 . The normalized amplitude decline of these eigenvalues, either on average in the frequency spectrum of the array  K (ω), or at a selected central frequency, is for example illustrated in  FIG. 6 : in this example, a significant difference in amplitude is observed between the first and second singular values. 
     During a step  214 , the processing unit  114  executing the instructions  132  reduces the rank of the array  K (ω) (or that of the array K(ω) if the previous optional step was not executed) only retaining 
     
       
         
           
             
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     During a step  216 , the processing unit  114  executing the instructions  134  performs a discrete inverse Fourier transform of the array Kf(ω) to obtain the denoised time signal array Kf(t). 
     Finally, during a final step  218 , the processing unit  114  executing the instructions  136  reconstructs a digital image of the effective area of interest by synthetic focusing on the basis of the denoised array Kf(t). By way of comparison,  FIGS. 7 and 8  illustrate examples of reconstructed digital images, either directly after the step  204  for constructing the array K(t) for  FIG. 7 , or after executing all the steps  202  to  216  for  FIG. 8 . 
     It should be noted that, in concrete terms, the examples illustrated in  FIGS. 5 to 8  were obtained by experimenting on a noisy part made of austenoferritic steel, 70 mm in thickness wherein an artificial 2 mm diameter Generatrix Hole (GH) is machined, at a depth of 40 mm. It sought to image the defect using a 1.1 MHz central frequency sensor consisting of 64 emitting/receiving transducers. Given the inter-element interval (i.e. the width of a transducer in addition to the space between two adjacent transducers) of the sensor (1.4 mm) and the depth of the defect, only the 32 central elements are activated. By way of an example of application, this sensor is placed on the object  102  45° from the GH defect. To enhance the acquisition in relation to a conventional FMC acquisition, the emissions were performed using a sub-aperture consisting of M=7 transducers (38 central elements are then activated for 32 successive emissions) and the delay sequence applied was defined for 7 transducers and such that the waves emitted are essentially oriented at 45°. 
     It appears clearly that a method and a device such as those detailed above are suitable for effectively denoising the ultrasonic signals acquired in the form of an array K(t) as defined above. 
     It should further be noted that the invention is not limited to the embodiment described above. It would be obvious to those skilled in the art that various modifications may be made to the embodiment described above, in the light of the teaching disclosed herein. 
     In particular, the computer program instructions could be replaced by electronic circuits dedicated to the functions carried out during the execution of these instructions. 
     As a general rule, in the claims hereinafter, the terms used should not be interpreted as limiting the claims to the embodiment disclosed in the present description, but should be interpreted to include any equivalents intended to be covered by the claims due to the wording thereof and which can be envisaged by those skilled in the art by applying their general knowledge to the implementation of the teaching disclosed herein.