Automatic calibration process of an ultrasonic transducer

Abstract

A computer-implemented method for calibrating ultrasonic signals acquired by a multi-element ultrasonic transducer for imaging an area of ​​interest, the method comprising the steps of: Estimating (601) an ultrasonic field emitted from the transducer to any point P of said area, Estimating (601) an ultrasonic field acquired by the transducer from said point P, Determining (602) an elementary signal received by the transducer from the emitted ultrasonic field and the acquired ultrasonic field, Determining (603) from said elementary signal, for each point P of said area, a calibration coefficient to be applied to the signals received by an ultrasonic transducer for imaging said area or to an ultrasonic image obtained from said received signals. Figure 6

Description

Title of the invention: Method for automatic calibration of an ultrasonic transducer

[0001] The invention relates to the field of non-destructive testing by ultrasound which aims to carry out imaging or ultrasound of a part to be inspected, by means of a multi-element ultrasound transducer.

[0002] The invention relates more specifically to a method of automated amplitude calibration of such a transducer prior to its use.

[0003] A general objective of non-destructive testing is to image a part in order to identify defects inside this part. The defects correspond to reflectors on which the waves are reflected and then back-propagated to the transducer which measures them.

[0004] This process implies that the non-destructive ultrasonic testing of a part is based on relative measurements. Indeed, the measured amplitude has an attenuation that depends on the distance between a reflector inside the part and the emitting element of the transducer. This attenuation must be corrected because the objective is to characterize the reflectors independently of their distance from the transducer.

[0005] It is therefore necessary to implement a calibration procedure to ensure that all potential reflectors of a part result in a uniform amplitude on the received signals regardless of the depth of the reflector. To do this, the control device must be calibrated on reference measurements before an inspection of a new part can be carried out.

[0006] State-of-the-art calibration methods are based on the generation of correction curves of the signal amplitudes as a function of time for each element of a transducer and each angle of incidence of the emitted signal. These curves are obtained from measurements made on a reference part comprising artificial reflectors located at predefined locations in the part, at different depths. The measurements require the positioning of the transducer relative to the part according to different positions and orientations in order to obtain complete calibration curves. They therefore involve the intervention of a human operator who must carry out several successive measurements, each time repositioning the transducer relative to the reference part.The signal amplitude correction curves are determined as a function of time so that reflectors of equal size give uniform amplitude responses for all depths and positions of these reflectors.

[0007] This method has the disadvantage of being delicate and long to carry out and of being require the intervention of a human operator. Consequently, the accuracy of the calibration curves obtained depends on the accuracy of the transducer placement by the operator. Furthermore, the curves obtained require interpolation to obtain all the gain values ​​as a function of depth since the number of reflectors in the reference room is limited.

[0008] There is therefore a need for an automated calibration method that is simple and quick to implement, that does not require the intervention of a human operator and that is more precise.

[0009] The invention proposes a new automatic calibration method which is based on the determination of a correction map calculated for any point of an area to be imaged from an ultrasound field model. It thus does not require the intervention of an operator or prior measurements. The invention thus has the advantage of improving the precision and speed of the calibration step. It is compatible with several ultrasound imaging methods, in particular the TFM (Total Focusing Method) and PWI (Plane Wave Imaging) acquisition methods.

[0010] The subject of the invention is a computer-implemented method for calibrating ultrasonic signals acquired by a multi-element ultrasonic transducer to image an area of ​​interest, the method comprising the steps of: - Estimate an ultrasonic field emitted from the transducer towards any point P of said zone, - Estimate an ultrasonic field acquired by the transducer from said point P, - Determine an elementary signal received by the transducer from the emitted ultrasonic field and the acquired ultrasonic field, - Determine from said elementary signal, for each point P of said zone, a calibration coefficient to be applied to the signals received by an ultrasonic transducer to image said zone or to an ultrasonic image obtained from said received signals.

[0011] According to an alternative embodiment, the method according to the invention further comprises the steps of: - Perform ultrasound imaging of the area of ​​interest using the ultrasound transducer, - Correct the signals received by the ultrasonic transducer or the image determined from said signals using the calculated calibration coefficients.

[0012] According to a particular aspect of the invention, the steps of estimating an ultrasonic field comprise the projection of the estimated ultrasonic field onto a polarization vector of the ultrasonic wave.

[0013]

[0014]

[0015]

[0016]

[0017] According to a particular aspect of the invention, - the elementary signal associated with a pair of elements (transmitter, receiver) of the transducer is taken equal to the time convolution product between the ultrasonic field emitted from the transmitting element towards point P and the ultrasonic field acquired by the receiving element from point P, - The calibration coefficient is taken equal to the sum, over all the elements of the transducer, of the values ​​of the elementary signal taken at times equal to the sum between the travel time of an ultrasonic wave between an emitting element of the transducer and point P and the travel time of an ultrasonic wave between point P and a receiving element of the transducer, - The calibration coefficient being applied to the pixel, corresponding to point P, of an ultrasound image obtained via a total focusing method. According to a particular aspect of the invention, the ultrasonic transducer is capable of emitting a plane wave at a given angle of incidence and the ultrasonic field emitted by the transducer at the angle of incidence towards the point P is taken equal to the sum of the values ​​of the ultrasonic fields emitted from an element of the transducer towards the point P, taken at an instant delayed by a predefined delay so that the transducer emits a plane wave with said angle of incidence. According to a particular aspect of the invention, - The elementary signal associated with a couple (angle of incidence, receiving element of the transducer) is taken equal to the time convolution product between the ultrasonic field emitted by the transducer according to the angle of incidence towards point P and the ultrasonic field acquired by the receiving element coming from point P, - The calibration coefficient is taken equal to the sum, over a plurality of angles of incidence and all the elements of the transducer, of the values ​​of the elementary signal taken at times equal to the sum between the travel time of a plane wave emitted with a given angle of incidence towards point P and the travel time of an ultrasonic wave between point P and a receiving element of the transducer, - The calibration coefficient being applied to the pixel, corresponding to point P, of an ultrasound image obtained via a plane wave ultrasound imaging method. According to a particular aspect of the invention, the ultrasound image is a sum of the signals acquired by the elements of the transducer taken at the times of flight corresponding to a point P of the area to be imaged. According to a particular aspect of the invention, - The elementary signal associated with an angle of incidence is taken equal to the time convolution product between the ultrasonic field emitted by the transducer according to the angle of incidence towards a point located on a radius representative of the propagation of the plane wave and the ultrasonic field emitted from said point towards the transducer, - Each ultrasonic signal acquired by the transducer for a given angle of incidence and time of flight is corrected by the calibration coefficient taken equal to the elementary signal taken at an instant equal to the time of flight.

[0018] According to a particular aspect of the invention, the ultrasonic image is defined by a representation of the signals acquired as a function of the angle of incidence and the time of flight.

[0019] The invention also relates to an ultrasonic imaging device comprising a multi-element ultrasonic transducer and a processing unit configured to implement the steps of the calibration method according to the invention.

[0020] The invention also relates to a computer program comprising instructions which cause the device according to the invention to execute the steps of the calibration method according to the invention.

[0021] Other characteristics and advantages of the present invention will appear more clearly on reading the description which follows in relation to the following appended drawings.

[0022] [Fig-1] represents, on a flowchart, the steps of implementing a method calibration applied to a first method of acquiring ultrasonic signals, according to the prior art,

[0023] [Fig.2] represents, on a flowchart, the steps of implementation of a second calibration method applied to a TFM or PWI ultrasound imaging method, according to the prior art,

[0024] [Fig.3] shows an example of an interface of a calibration assistant according to art prior,

[0025] [Fig.4a] shows a first example of experimental calibration curves obtained according to the calibration method of [Fig.l] for different orientations of the transducer,

[0026] [Fig.4b] shows a second example of experimental calibration curves obtained according to the calibration method of [Fig.l] for different orientations of the transducer,

[0027] [Fig.5] represents, on a flowchart, the steps of implementing a method automated calibration applied to TFM or PWI imaging according to an embodiment of the invention,

[0028] [Fig.6] represents, on a flowchart, the steps of implementation of the method of calculating a correction map used by the calibration method of [Fig.5],

[0029] [Fig.7] illustrates the application of the method according to the invention for ul- imaging TFM trasonic,

[0030] [Fig.8] illustrates a schematic of a scanning ultrasound imaging configuration angular,

[0031] [Fig.9] represents, on a flowchart, the steps of implementing a method automated calibration applied to angular scanning ultrasound imaging according to one embodiment of the invention,

[0032] [Fig. 10] represents, on a flowchart, the steps of implementation of the method of calculating the calibration coefficients used by the method of [Fig.9],

[0033] [Fig. 11] represents an illustrative diagram of a ray representative of the propagation of an ultrasonic wave,

[0034] [Fig. 12] represents an example of signals corrected using the method described in Figures 9 and 10,

[0035] [Fig. 13] represents a diagram of an ultrasound imaging system capable of implementing the invention.

[0036] [Fig.l] illustrates a procedure for calibrating an ultrasonic transducer according to the prior art applied to an acquisition of ultrasonic signals.

[0037] A method for acquiring ultrasonic signals is based on the use of a multi-element transducer associated with focusing in transmission and / or reception by applying delay laws to the elements of the transducer. This application may require angular scanning of the ultrasonic signals and focusing at one or more points. A plane wave is generated by the transducer at an angle of incidence in the direction of a part to be inspected. This wave is reflected on reflectors that the part comprises and then propagated towards the transducer acting in reception which carries out an acquisition of the reflected signals.

[0038] As indicated above, the acquired signals require amplitude calibration as a function of the time of flight related to the distance between the transducer and the reflectors in the room.

[0039] This calibration method comprises a first step 101 of constructing gain correction curves for different emission angles of the ultrasonic signals. During a second step 102, ultrasonic signals are measured by a transducer to control a part. The calibration curves are then used to correct (step 103) the acquired signals in amplitude as a function of time in order to ensure that all the reflectors have a uniform amplitude according to the depth. This calibration is therefore carried out for each of the signals received by the transducer.

[0040] Step 101 of constructing the gain correction curves requires a protocol of measurements carried out by an operator. At each step of the protocol, The operator moves the transducer relative to a reference part which has reflectors positioned at different depths. For this, he can be assisted by a software assistant of the type described in [Fig.3].

[0041] In frame 300, instructions to the operator are displayed to indicate the protocol to follow.

[0042] This protocol consists of carrying out several successive acquisitions via a multi-element transducer 301 positioned relative to a reference part 302. The reference part has lateral holes at different depths which act as reflectors for the ultrasonic waves emitted by the transducer.

[0043] At each step, the operator performs an ultrasonic acquisition for a position of the transducer and an angular orientation with respect to the part.

[0044] [Fig.4a] and 4b show an example of several calibration curves obtained by this protocol for different angles of incidence of the plane wave generated by the transducer equal respectively to 46°, 47°, 48°, 60°, 61° and 62°.

[0045] Each diagram in Figures 4a and 4b represents the amplitude of the signal on the abscissa and the time of flight or the distance between the transmitter and a reflector on the ordinate.

[0046] The four amplitude peaks observed correspond to the measurements resulting from the reflections of the ultrasonic signals on four reflectors of a reference part. Curves 401-406 give the calibration gain which is calculated from these measurements.

[0047] These curves are then used to calibrate new signals in step 103 to correct the amplitude of each signal.

[0048] This correction procedure can also be applied directly to images obtained by TFM or PWI imaging as illustrated in [Fig.2].

[0049] In this case, the images are calculated in step 201 from the signals acquired in step 102, then they are corrected directly using the correction curves in step 202.

[0050] The TFM or PWI imaging methods make it possible to form an image by synthetic focusing, in transmission and reception, at all points of an area of ​​interest. The correction step 202 is applied to each column of the image in order to have the same sensitivity for each point of the image.

[0051] As can be seen in Figures 4a and 4b, the different gain correction curves 401-406 have different shapes for small angle differences (1°). The variations of these curves are not linear and sometimes abrupt depending on the depth for very close angles.

[0052] It can thus be seen that this manual calibration procedure leads to imprecise results.

[0053] This is why an automated calibration method is proposed which does not require measurements on a reference part or intervention by a human operator. The proposed calibration method is based on an estimation of the ultrasonic field generated by the transducer and the reflected field at all points of an area to be imaged. It can be applied to different types of ultrasound imaging methods.

[0054] Figures 5, 6 and 7 describe a first embodiment of an automated calibration method applied to a TFM (Total Focusing Method) ultrasound imaging method.

[0055] [Fig.5] describes, on a flowchart, the steps relating to the imaging method integrating a correction obtained by calibration.

[0056] [Fig.6] describes, on another flowchart, the steps of implementing the method for determining a correction map.

[0057] The TFM (Total Focusing Method) imaging method mainly applies to Full Matrix Capture (FMC) type ultrasound acquisitions as described in reference [1]. For an ultrasound transducer comprising N elements, the FMC acquisition consists of recording a set of NxN elementary signals kj N. The index i denotes the number of the emitting element and the index j that of the receiving element.

[0058] The TFM imaging algorithm consists of coherently summing the received signals to obtain constructive interference, and thus amplitude maxima, at the location where the defects causing the detected echoes are actually located. It is mainly based on the use of flight times theoretically evaluated from direct models. This algorithm can then be summarized in three steps: • Definition of a reconstruction zone to be imaged in a given part (position, dimensions and number of points); • For each point P of the image and for each transmitter-receiver pair ij, calculation of the theoretical flight time, tÿ( p ) = t, ( p) + tj( p ), where ( p) corresponds to the travel time of the ultrasonic wave between the transmitter i and the point P and tj(p) corresponds to the travel time of the ultrasonic wave between the receiver j and the point P. • For each point P of the image, summation of the amplitudes extracted from the signals at times t;(p) + tj(p), which can be written: I™(p) -ti(p) + tj(p) ) # (1.1)

[0059] Thus, as shown in Figure 5, step 502 consists of carrying out an FMC type ultrasonic acquisition using a multi-element transducer positioned relative to a part to be imaged. This step 502 leads to the acquisition of the elementary signals s^t), hj = K • • • -

[0060] In step 503, the TFM method is applied to construct the image IypM(p) as indicated above.

[0061] This image is then corrected in step 504 by a calibration factor in the form of a correction map giving, for each pixel p of the image, the correction factor C(p) to be applied: ^TPM^orr^P) = # (1.2)

[0062] The correction mapping C(p) is pre-calculated in step 501 for all the pixels p corresponding to the area to be inspected. The values ​​of C(p) are saved in a memory or a database which is interrogated in step 504 to correct the TFM image.

[0063] [Fig.6] illustrates the calculation steps of the method for determining the correction mapping C(p).

[0064] The first step 601 consists of estimating the ultrasonic displacement field transmitted from one point and observed at another point.

[0065] We note ( p, t ), the displacement field transmitted by the ultrasonic element i and observed at point P. It can be written in the following form:

[0066] Uf(p, t) =qA (p)Ô(t-tj(p))# (1.3) 1 i

[0067] where A^p) is the amplitude of the wave displacement vector at point P, tj(p) is the flight time of the ultrasonic wave between element i and point P, 0 is the phase of the wave and is the polarization vector of the wave. This vector is normalized and represents the direction of displacement of the particles as the wave passes. It depends on the nature of the wave (longitudinal wave or transverse wave) and its direction of propagation. Those skilled in the art can refer to document [2] for further details on the precise calculation of this displacement field.

[0068] We note U^p, t), the projection of the field U,(p, t) on its polarization:

[0069] U^p, t) = UXp, t)-q^ (1.4)

[0070] where the operator " " represents the scalar product and U; ( p, t ), the displacement field transmitted by element i and observed at point P defined by formula (1.3). This projection describes the propagation of the wave from element i to point P.

[0071] Similarly, we denote by U;(p, t) the displacement field of the wave reflected from point p and observed (acquired) by an ultrasonic element of index j.

[0072] The estimates of the ultrasonic fields U^p, t) and Uj{p, t ) can be calculated using elastodynamic wave propagation simulation software, for example the CIVA software, described in publication [5].

[0073] Using the approach proposed in reference [3], we determine, in step 602, the elementary signal received by element j with an emission by element i in the presence of a diffractor assumed to be punctual at a point P of the part. This elementary signal can be approximated as the time convolution product of the two ultrasonic fields corresponding respectively to the wave path between element i and point p then between point p and element j.

[0074] = U^p, t)*Uj(p, t) # (1.5) where * represents the convolution product temporal.

[0075] Finally, in step 603, the correction gain to be applied to a TFM image is determined for each point p via the following relationship:

[0076] = t^p) +tj(p) ) # (1.6)

[0077] where is the flight time of the wave between transmitter i and point P and tj(p} is the flight time of the wave between receiver j and point P.

[0078] [Fig.7] illustrates an example of implementation of the calibration method according to this first embodiment of the invention.

[0079] [Fig.7] schematically represents a multi-element transducer 700 positioned on a part 701 to be inspected which has aligned transverse holes positioned at different depths. Image 702 corresponds to an uncorrected TFM image of the part while image 704 corresponds to the image corrected using the correction gain determined in step 603. Diagrams 703 and 705 represent the intensity of the pixels of the image along a line which corresponds to the alignment of the holes. It can be seen that in diagram 703, the amplitudes are attenuated to different levels depending on the distance from the hole to the transducer. In diagram 705, after correction, these signals are calibrated to be at a substantially identical percentage of full screen height regardless of the distance from the hole (reflector) to the transducer.

[0080] A second embodiment of the invention applied to another type of ultrasound imaging, PWI (Plane Wave Imaging) or plane wave imaging, is now described.

[0081] The PWI plane wave imaging method applies to an ultrasound acquisition carried out with plane waves in emission. The PWI algorithm repeats all the steps of the TFM algorithm, as described in detail in reference [4]. For a transducer with N elements and M plane waves transmitted in the medium, the intensity of the PWI image at the calculation point P is written: 100821 fpw](p) = Yn (tm(p) + Pj(p) ) # (2.1)

[0083] where tm(p) is the time taken by the plane wave with the angle of incidence 0m for reach the focus point P, P / (p} is the time of flight between the focus point and the receiver j, smJ{t) is the signal received by the element j.

[0084] The steps of the corrected PWI imaging method are similar to those of the TFM method described in [Fig.5] although the implementation of each step is adapted to the specificity of plane wave imaging.

[0085] Thus, in step 501, an acquisition of ultrasonic signals integrating an emission of plane waves is carried out.

[0086] Each plane wave is defined by an angle of incidence 0m and is obtained from all the elements of the transducer which are excited with particular emission delay laws.

[0087] To generate these plane waves, it is necessary to first calculate emission delay laws, for each desired propagation direction, in order to apply these delays to the transducer elements.

[0088] For an N-element transducer and a plane wave defined by an angle of incidence 0m, we note by: 1=1,..., N, m= 1, ...,M# (2.2) the delay applied to the emission at an element of index i of the transducer, where M is the number of plane waves transmitted by the translator.

[0089] For common applications, the transducer elements and the area to be inspected are separated by a plane interface. Then, to calculate the delay laws, it is sufficient to emit into the water a plane wave whose angle of incidence a verifies the Snell-Descartes relation) with desired refraction pangy 0. In this case, the delay applied (in transmission and reception) to an element is given by:

[0090] Tj(0) = psinset + ZjCOsa - min ((xjSina + z^osa)^) # (2.3)

[0091] where ci is the propagation speed in the coupling medium located between the transducer and the part to be inspected, c2 is the propagation speed in the inspected part and (xj; z,) are the coordinates of the center of element i.

[0092] These delays are then applied to the elements of the transducer and then an acquisition of ultrasonic signals (sm / t) is carried out where m corresponds to the index of the angle of incidence associated with the emitted plane wave and j corresponds to the index of an element of the transducer acting in reception.

[0093] In step 503, a PWI image is created from the acquired signals to construct the IPWl image (p) given by relation 2.1.

[0094] This image is then corrected in step 504 by a calibration factor in the form of a correction map giving, for each pixel p of the image, the correction factor C(p) to be applied: lpWI.corr{p) = ^(p) # (^-4)

[0095] The correction mapping C(p) is pre-calculated in step 501 for all the pixels p corresponding to the area to be inspected. The values ​​of C(p) are saved in a memory or a database which is queried in step 504 for correct the TFM image.

[0096] The calculation of the correction mapping follows steps similar to those developed for the TFM method with reference to [Fig.6].

[0097] In step 601, the transmitted ultrasonic displacement field is calculated for a plane wave emitted with an angle of incidence 0 and observed at point P.

[0098] The mapping of the ultrasonic field is calculated in transmission (and in reception if the configuration of the transducer is not symmetrical).

[0099] We denote by U, (p, f ), the displacement field transmitted by element i of the transducer and observed at point P defined by formula (1.4).

[0100] We define by k(0) the direction of propagation and by q(0) the polarization of the plane wave propagating with an angle of incidence 8. For longitudinal waves, the polarization q(6*) is collinear with the direction of propagation k(0). And for transverse waves, the polarization q(0) is orthogonal to the direction of propagation k(0).

[0101] We note Uj(p, t), the projection of the elementary field U;-(p, f) on the polarization axis q(d):

[0102]

[0103] where the operator “ ” represents the scalar product.

[0104] Then, the field transmitted for an angle 6 and observed at point P is expressed as the summation of the contributions of the scalar fields, U / ^p, i), of each element i with the delay applied to each element:

[0105] =l^=lUi(p, ) # (2.6)

[0106] Here, 6) is the delay applied to element i. This projection describes the propagation of the plane wave propagating with an angle of incidence 8 towards point P.

[0107] In step 602, the elementary signal received by element j is calculated with emission by a plane wave with the angle of incidence 8fn in the presence of a diffractor assumed to be punctual at a point P of the part by means of the following approximation:

[0108] ^(p. t) = U(p. t, (?„,) *Uj(p, t) #(2.7)

[0109] where U( p, t, 8m) is defined by relation (2.6) and Uj( p, l ) is defined by relation (2.5).

[0110] The estimates of the ultrasonic fields U^p, t) and U(p, t, 8m) can be calculated using elastody-namic wave propagation simulation software, for example the CIVA software, described in publication [5].

[0111] Finally, in step 603, the correction mapping is calculated via the following relationship: 101121 c(p)-O£^(R*(p)+ <Xp))#(i8)*^ the plane wave with the angle of incidence to reach the focal point P, fj(p) is the time of flight between the focus point p and the receiver j.

[0113] A third embodiment of the invention is now described which applies to a so-called conventional ultrasound imaging method based on an angular scan of the area to be imaged by plane waves.

[0114] The principle of an angular scanning inspection consists of successively applying different corresponding delay laws at different angles so as to insonify an angular sector of the part. It provides a sectoral image of the inspected area, commonly called S-Scan. The inspection configuration is presented in [Fig.8] for an example of a part 803 comprising several identical reflectors. The transducer 801 is positioned on a shoe 802 which comes into contact with the part 803. On the S-Scan imagery 804 resulting from an angular scan between 43° and 79° with an angular step of 1°, it is observed that the amplitude responses of the reflectors of identical dimensions decrease as a function of their distance from the translator.

[0115] The calibration method according to the third embodiment of the invention aims to directly correct the signals acquired by the transducer for each angle of incidence.

[0116] [Fig.9] represents, on a flowchart, the steps of implementing an angular scanning imaging method according to the third embodiment of the invention.

[0117] In step 902, a signal acquisition is carried out for each angle of incidence. For this, a preliminary step consists, as in the case of PWI imaging, in calculating delay laws to be applied to the elements of the transducer in order to emit a plane wave in the desired direction.

[0118] The delay laws are applied to the emission and reception of the transducer elements. For a transducer with N elements and an incidence angle of 6m, the delay applied to the emission and reception of an element is written:

[0119] ri(em)=rinP 1, .... M# (3.1)

[0120] where M is the number of plane waves transmitted and received by the transducer. The delay 3 ), i = 1, ..., N is calculated by the formula (2.3).

[0121] In step 903, the acquired signals are corrected via a calibration factor specific to each angular direction, this calibration factor being precalculated in step 901.

[0122] / ntx „ -d ) 9A The set of acquired signals can be represented in the form of an S-SCAN image which gives the amplitude of the signal as a function of time and angle of incidence.

[0123] [Fig. 10] illustrates, on a flowchart, the steps of implementing the method of calculating the calibration factors according to the third embodiment of the invention.

[0124]

[0125]

[0126]

[0127]

[0128]

[0129]

[0130]

[0131]

[0132]

[0133]

[0134]

[0135] In step 1001, the transmitted ultrasonic field is determined for an angle of incidence 6 and observed at a point p along a ray representative of the pro Fonde's pagation with this angle of incidence. Figure 11 shows a diagram of a ray r representative of the propagation of the wave emitted with an angle of incidence G by a transducer 110 arranged on a shoe 1102 in contact with a part to be inspected 1103. Delay laws 1101 are applied to the elements of the transducer 1100 to generate a plane wave of incidence angle G. The radius r is defined by its origin E which is the point of impact of the wave emitted on the interface separating the elements of the translator and the area to be inspected and the propagation direction k(9) = (sin3, cos3) as illustrated in Figure 11. The ray r comes from the center of the translator (point O), refracted in the part with the refraction angle G and respect for the Snell-Descartes law on the interface. The field U{pyt, 3) defined by formula (2.6) is calculated for all points P of radius 1. It can be calculated using propagation simulation software elastodynamic waves, for example the CIVA software, described in publication [5]. The coordinates of the point P = (pÿP^ are defined by the equation # (3.2) where (e,, are the coordinates of the impact point E. In step 1002, the elementary signal received by the transducer is determined with an emission by a plane wave of incidence G in the presence of a diffractor assumed to be punctual at a point P of the part, said point p traveling along the ray r via the following relation: §( ta(p) + t£p\ 3)-U(p,t- tc^p\ 3) *U(p, t-tr(p), 3), where t^p) is the time taken by the plane wave with the angle of incidence G to reach the point p and tf(p} the time taken by the plane wave coming from the point p to reach the transducer. The elementary signal s(ta(p) + lAp)- 3) corresponds to a round trip between the transducer and point p. In step 1003, the correction coefficient of the acquired signal for incidence 0 is determined, as C(6n^ — 5( 3m ) with variation over the measurement times, ti^Up) + ^P> In the case where the configuration is symmetric, we have ta(p) = tr(p)- [Fig. 12] illustrates, using an example, the result obtained using this calibration method for a room comprising several reflectors of the same dimensions arranged at different depths. Diagrams 1200,1201,1203 show amplitude as a function of time and of an angle of incidence.

[0136] Diagram 1200 shows a mapping of the calibration gain determined by the method of [Fig. 10].

[0137] Diagram 1201 shows an S-SCAN map of an acquisition of ultrasonic signals without correction. Diagram 1202 shows the amplitudes of the signals corresponding to the reflectors of the room. It can be seen that the amplitude varies according to the depth (equivalent to time).

[0138] Diagram 1203 shows the map 1201 corrected by the calibration gain 1200. Diagram 1204 shows the same amplitudes as diagram 1202 but after correction by the calibration gain. It can be seen that the calibration makes it possible to adjust the amplitude of the signals to the same level for all the reflectors regardless of their depth in the room.

[0139] [Fig. 13] shows a diagram of an ultrasonic inspection system configured to implement the invention.

[0140] The system mainly comprises a multi-element ultrasonic transducer TR and a processing unit UT. The transducer TR can have different cutting geometries, for example it can be a linear, matrix, annular or sector sensor. Each element of the ultrasonic transducer can be made by means of a piezoelectric sensor or any other type of sensor capable of emitting and receiving an ultrasonic wave.

[0141] The transducer TR is controlled by the processing unit UT to image a zone F of a structure S to be inspected. For this, the transducer TR can carry out several acquisitions by moving on the surface of the part, for example along a direction of movement D. The inspection system makes it possible to image the structure S at any point P.

[0142] The method for generating calibration coefficients or a correction map according to any of the embodiments of the invention can be implemented by the processing unit UT. For this purpose, it can be executed as a computer program. More generally, this method can be implemented by means of software and / or hardware elements such as a processor and a memory.

[0143] The imaging method, integrating the calibration of the signals acquired by the transducer TR can be implemented by means of the transducer TR and the processing unit UT. Typically, the precalculated calibration coefficients are saved by the processing unit UT which comprises a memory or database and are used to correct the signals acquired by the transducer TR or to correct the image constructed by the processing unit UT from the signals acquired by the transducer TR. References

[0144] [1] C. Holmes, B.W. Drinkwater, P.D. Wilcox, Post-processing of the full matrix of ultrasonic transmit-receive array data for non-destructive évaluation, NDT&E international, Vol. 38, pp. 701-711, 2005.

[0145] [2] El Armani, M., Calmon, P., Roy, O., Royer, D., & Casula, O. (1995). The ul trasonic field of focused transducers through a liquid-solid interface. In Review of Progress in Quantitative Nondestructive Evaluation: Volume 14 (pp. 1075-1082). Boston, MA: Springer US.

[0146] [3] Lingvall, F. (2004). A method of improving overall resolution in ultrasonic array imaging using spatio-temporal deconvolution. Ultrasonics, 42(1-9), 961-968.

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Claims

Claims

1. A computer-implemented method for calibrating ultrasonic signals acquired by a multi-element ultrasonic transducer for imaging an area of ​​interest, the method comprising the steps of: - Estimating (601,1001) an ultrasonic field emitted from the transducer to any point P of said area, - Estimating (601,1001) an ultrasonic field acquired by the transducer from said point P, - Determining (602,1002) an elementary signal received by the transducer from the emitted ultrasonic field and the acquired ultrasonic field, - Determining (603,1003) from said elementary signal, for each point P of said area, a calibration coefficient to be applied to the signals received by an ultrasonic transducer for imaging said area or to an ultrasonic image obtained from said received signals.

2. Calibration method according to claim 1 further comprising the steps of: - Carrying out (502,503,902) an ultrasound imaging of the area of ​​interest by means of the ultrasound transducer, - Correcting (504,903) the signals received by the ultrasound transducer or the image determined from said signals by means of the calculated calibration coefficients.

3. A calibration method according to any one of the preceding claims wherein the steps of estimating (601,1001) an ultrasonic field comprise projecting the estimated ultrasonic field onto a polarization vector of the ultrasonic wave.

4. Calibration method according to any one of the preceding claims in which: - The elementary signal associated with a pair of elements (transmitter, receiver) of the transducer is taken equal (602) to the temporal convolution product between the ultrasonic field emitted from the transmitting element towards the point P and the ultrasonic field acquired by the receiving element coming from the point P, - The calibration coefficient (603) is taken equal to the sum, over all the elements of the transducer, of the values ​​of the elementary signal taken at times equal to the sum between the travel time of an ultrasonic wave between an emitting element of the transducer and point P and the travel time of an ultrasonic wave between point P and a receiving element of the transducer, - The calibration coefficient being applied (504) to the pixel, corresponding to point P, of an ultrasonic image obtained via a total focusing method.

5. Calibration method according to any one of claims 1 to 3 in which the ultrasonic transducer is capable of emitting a plane wave at a given angle of incidence and the ultrasonic field emitted by the transducer at the angle of incidence towards the point P is taken equal (601) to the sum of the values ​​of the ultrasonic fields emitted from an element of the transducer towards the point P, taken at an instant delayed by a predefined delay so that the transducer emits a plane wave with said angle of incidence.

6. Calibration method according to claim 5 in which: - The elementary signal associated with a pair (angle of incidence, receiving element of the transducer) is taken equal (602) to the temporal convolution product between the ultrasonic field emitted by the transducer according to the angle of incidence towards the point P and the ultrasonic field acquired by the receiving element from the point P, - The calibration coefficient is taken equal (603) to the sum, over a plurality of angles of incidence and all the elements of the transducer, of the values ​​of the elementary signal taken at times equal to the sum between the travel time of a plane wave emitted with a given angle of incidence towards the point P and the travel time of an ultrasonic wave between the point P and a receiving element of the transducer, - The calibration coefficient being applied (504) to the pixel, corresponding to the point P, of an ultrasonic image obtained via a plane wave ultrasound imaging method.

7. Calibration method according to any one of the preceding claims in which the ultrasonic image is a sum of the signals acquired by the elements of the transducer taken at the times of flight corresponding to a point P of the area to be imaged.

8. Calibration method according to claim 5 in which: - The elementary signal associated with an angle of incidence is taken equal (1002) to the temporal convolution product between the ultrasonic field emitted by the transducer according to the angle of incidence towards a point located on a radius representative of the propagation of the plane wave and the ultrasonic field emitted from said point towards the transducer, - Each ultrasonic signal acquired by the transducer for a given angle of incidence and a given time of flight is corrected (903) by the calibration coefficient taken equal (1003) to the elementary signal taken at an instant equal to the time of flight.

9. Calibration method according to claim 8 in which the ultrasound image is defined by a representation of the acquired signals as a function of the angle of incidence and the time of flight.

10. An ultrasound imaging device comprising a multi-element ultrasound transducer (TR) and a processing unit (UT) configured to implement the steps of the calibration method according to any one of the preceding claims.

11. A computer program comprising instructions which cause the device according to claim 10 to execute the steps of the calibration method according to any one of claims 1 to 9, when said program is executed on a computer.

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