A shear wave sparse imaging method based on nonlinear beamforming for ultrasonic phased array

Abstract

The present invention proposes an ultrasonic phased array shear wave sparse imaging method based on nonlinear beamforming, which relates to the field of ultrasonic nondestructive testing technology. The method comprises the following steps: S1, calculating the coordinates of ultrasonic phased array elements based on ultrasonic phased array probe parameters and wedge parameters; S2, randomly selecting M effective array elements from the N array elements of the ultrasonic phased array, discretely gridding the target area, calculating the acoustic field radiation energy of each effective array element to each grid, then superimposing the acoustic field radiation energies to calculate the synthetic acoustic field radiation energy, and calculating the average value based on the synthetic acoustic field radiation energy. The sparse array distribution with the largest average value is taken as the ultrasonic sparse array distribution; S3, collecting the full matrix signal of the ultrasonic sparse array of the test object, and performing nonlinear wavenumber shaping processing on the target area for visual reconstruction. This method, based on a nonlinear beamforming algorithm for shear wave imaging, ensures high-quality imaging while significantly reducing calculation time and improving imaging efficiency.

Description

Technical Field

[0001] The present invention relates to the technical field of ultrasonic nondestructive testing, and in particular to an ultrasonic phased array shear wave sparse imaging method based on nonlinear beamforming. Background Art

[0002] Ultrasonic phased array testing involves controlling the individual elements in the sensor array according to a specific delay time, regularly stimulating and receiving ultrasonic waves to achieve deflection and focusing of the ultrasonic beam, thereby performing nondestructive testing of defects within the workpiece. Ultrasonic phased arrays are widely used for nondestructive testing of key equipment in aerospace, nuclear power, composite materials, and other fields due to their fast detection speed, high sensitivity, and excellent object adaptability. Currently, the use of full matrix capture (FMC) and total focus method (TFM) imaging of full matrix data enables focused imaging at any location within the test area, greatly improving image resolution.

[0003] However, the above-mentioned all-focus imaging algorithm uses a similar linear delay-and-add (DAS) strategy to achieve ultrasonic image reconstruction, without utilizing the spatial coherence of the signal. In recent years, a nonlinear beamforming algorithm called delay-multiply-accumulate (DMAS) that utilizes the spatial coherence of the signal to improve the signal-to-noise ratio has gradually attracted attention. This method has significantly improved the lateral resolution, background noise suppression, and contrast resolution of defect imaging compared to the all-focus imaging method. For example, an ultrasonic plane wave imaging method based on an improved DMAS algorithm is proposed in Chinese Patent Authorization Announcement No.: CN108670304B. The beamforming algorithm can greatly improve the imaging performance. However, the above-mentioned method is for plane wave excitation and is not based on shear wave imaging. In addition, due to the large amount of data and data redundancy caused by the full matrix acquisition of the ultrasonic phased array, an inclined wedge is often set when exciting shear waves, which makes the calculation of the refraction point very time-consuming and the imaging efficiency low, which to a certain extent limits its practical use in industrial non-destructive testing. Summary of the Invention

[0004] In order to overcome the shortcomings of the background technology, the present invention discloses an ultrasonic phased array shear wave sparse imaging method based on nonlinear beamforming, comprising the following steps:

[0005] S1. Calculate the coordinates of ultrasonic phased array elements according to ultrasonic phased array probe parameters and wedge parameters;

[0006] S2. Randomly select M effective array elements from the N array elements of the ultrasonic phased array, discretize the target area into a grid, calculate the sound field radiation energy of each effective array element to each grid, then superimpose the sound field radiation energies to calculate a synthetic sound field radiation energy, and calculate an average value based on the synthetic sound field radiation energy. The sparse array distribution with the largest average value is taken as the ultrasonic sparse array distribution;

[0007] S3. Collect the full matrix signal of the ultrasonic sparse array of the detection object, perform nonlinear beamforming processing on the target area for visual reconstruction.

[0008] Specifically, step S1 is as follows:

[0009] The ultrasonic phased array probe parameters include: the number of array elements N, the array element spacing p, the array element width a and the array element center frequency f c The wedge parameters include: wedge tilt angle β, wedge density ρ1, wedge longitudinal wave speed c1 and first array element height h; with the projection of the center point of the first array element on the surface of the test object as the origin, the horizontal coordinates of the centers of each array element of the ultrasonic phased array are:

[0010] elx(m)=(m-1)pcos(β)

[0011] The vertical coordinate of the center of each element of the ultrasonic phased array is:

[0012] elz(m)=h+(m-1)psin(β)

[0013] Wherein, m is the array element number, m=1, 2, 3, ..., N.

[0014] Specifically, in step S2, the sound field radiation energy of each effective array element to each grid is calculated by the following formula:

[0015]

[0016] Among them, r m is the vector of the mth array element pointing to the detection target point, r1 m is the distance that ultrasound travels in the wedge; is the propagation distance of ultrasound in the test object, the superscript m is the array element number; exp() is the exponential function symbol with the natural exponential e as the base; k1 is the wave number of ultrasound in the wedge, k1 = 2πf c / c1; k2 is the wave number of ultrasound in the detection object, k2=2πf c / c2; c2 is the shear wave velocity of the test object; θ1 is the incident angle of ultrasound at the interface between the wedge and the spacer, θ2 is the refraction angle of ultrasound at the interface between the wedge and the spacer, and the superscript m is the array element number; p m(r m ) is the sound field radiation energy of the array element with serial number m to the detection target point.

[0017] Specifically, the synthetic sound field radiation energy P(r) in step S2 is:

[0018]

[0019] Where Ω is the valid array element number set, and there are M elements in Ω that are different from each other; m (r) is the sound field radiation energy of the array element with serial number m; p n (r) is the sound field radiation energy of the array element with serial number n.

[0020] Specifically, step S3 includes the following steps:

[0021] S31. Collect the full matrix signal of the ultrasonic sparse array according to the distribution of the ultrasonic sparse array; the full matrix signal of the ultrasonic sparse array is a sparse array element full matrix data set s composed of the ultrasonic signals received by all effective array elements when the effective array elements in the ultrasonic sparse array excite the ultrasonic signals one by one. m,n (t),m=1,2,3,…,M; n=1,2,3,…,M;

[0022] S32, according to the sparse full matrix signal substitution formula, s m,n (t) is expressed as s j (t), and perform phase domain analytical signal processing to obtain the phase domain analytical signal S of the sparse full matrix data j (t):

[0023] S j (t) = s j (t)+i*Hilbert(s j (t)),

[0024] Where Hilbert() is the Hilbert transform, j is the index of the signal in the sparse full matrix signal;

[0025] S33, according to the grid points in the imaging area and S j The signal is used to solve the flight time t j (x,z), and derive the image matrix after processing through the nonlinear beamforming algorithm to complete the visual reconstruction of the target area.

[0026] Specifically, the sparse full matrix signal substitution formula in step S32 is:

[0027] subscript=transmitter*M+receiver

[0028] Where transmitter is the transmitting element number, receiver is the receiving element number, and M is the number of valid elements in the sparse array.

[0029] Specifically, the nonlinear beamforming algorithm in step S33 is:

[0030]

[0031] Where I(x,z) is the mapping amplitude of the grid points in the imaging area; S is the phase domain analytical signal of the sparse full matrix data, and the subscripts j and k are the index numbers of the signal in the sparse full matrix signal; S j (t) is a signal segment under the j label; S j (t j (x,z)) is a signal segment labeled j at t=t j The amplitude of (x,z); sgn() is the sign value function, specifically:

[0032]

[0033] Specifically, the flight time t in step S33 j (x,z) is S j The ultrasonic propagation time of the signal to the grid point (x,z) within the imaging area is:

[0034] S j The signal is transmitted from the transmitting array element according to the longitudinal wave of the wedge to the incident refraction point of the wedge (x inc1 ,0), then according to the transverse wave of the detection object to (x,z), then according to the detection transverse wave to the incident refraction point of the detection object (x inc2 ,0), and then according to the wedge longitudinal wave to the receiving array element, the time required is t j (x,z).

[0035] Specifically, in step S33, the flight time t is calculated based on each grid point in the imaging area, the longitudinal wave velocity of the wedge, and the shear wave velocity of the specimen. j (x,z) is specifically:

[0036] S j The signal is replaced by the A-wave signal collected by the effective array element m stimulating the effective array element n. According to Fermat's minimum flight time theorem, the following formula is obtained:

[0037]

[0038] Among them, c 1p is the longitudinal wave speed of ultrasound in the wedge; c 2sis the transverse wave speed of ultrasound in the detection object; elx(m) is the central abscissa of the effective array element m, elz(m) is the central ordinate of the effective array element m, m = 1, 2, 3, ..., M; elx(n) is the central abscissa of the effective array element n, elz(n) is the central ordinate of the effective array element n, n = 1, 2, 3, ..., M.

[0039] The present invention proposes an ultrasonic phased array shear wave sparse imaging method based on nonlinear beamforming, comprising the following steps: S1. Calculating the coordinates of ultrasonic phased array elements based on ultrasonic phased array probe parameters and wedge parameters; S2. Randomly selecting M effective elements from the N elements of the ultrasonic phased array, discretely gridding the target area, calculating the acoustic field radiation energy of each effective element for each grid, then superimposing the acoustic field radiation energies to calculate the composite acoustic field radiation energy, and calculating the average value based on the composite acoustic field radiation energy. The sparse array distribution with the largest average value is taken as the ultrasonic sparse array distribution; S3. Acquiring the full matrix signal of the ultrasonic sparse array of the detection object, performing nonlinear beamforming processing on the target area for visual reconstruction. This method, based on a nonlinear beamforming algorithm for shear wave imaging, significantly reduces calculation time and improves imaging efficiency while ensuring high-quality imaging.

[0040] In addition, the ultrasonic sparse array distribution selection method proposed in the present invention can select a suitable small number of effective array elements to construct a sparse array, reducing the amount of data required for imaging, reducing mutual interference and power consumption between array elements, and effectively improving imaging quality and imaging efficiency.

[0041] In addition, the ultrasonic phased array shear wave sparse imaging method based on nonlinear beamforming proposed in the present invention is simple, easy to implement, low in cost, and easy to promote and use in industrial non-destructive testing. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0043] Figure 1 1 is a flow chart of a method for ultrasonic phased array shear wave sparse imaging based on nonlinear beamforming according to an embodiment of the present invention;

[0044] Figure 2 is a schematic diagram of a detection object provided by an embodiment of the present invention;

[0045] Figure 3 is a geometric diagram of the sound field radiation energy provided by an embodiment of the present invention;

[0046] Figure 4 Schematic diagram of energy distribution of a synthetic sound field provided by an embodiment of the present invention;

[0047] Figure 5 Schematic diagram of imaging results provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0048] The present invention can be explained in detail through the following embodiments. The purpose of disclosing the present invention is to protect all technical improvements within the scope of the present invention. In the description of the present invention, it should be understood that if there are terms such as "upper", "lower", "front", "back", "left", "right", etc. to indicate directions or positional relationships, they only correspond to the drawings of this application for the convenience of describing the present invention, and do not indicate or imply that the device or element referred to must have a specific direction.

[0049] Example 1

[0050] refer to Figure 1 This embodiment provides an ultrasonic phased array shear wave sparse imaging method based on nonlinear beamforming, comprising the following steps:

[0051] S1. Calculate the coordinates of ultrasonic phased array elements according to ultrasonic phased array probe parameters and wedge parameters;

[0052] This embodiment uses the obliquely distributed transverse through-holes of the standard phased array B-type test block as the test object, where the longitudinal wave speed c1 is 5900 m / s and the transverse wave speed c2 is 3230 m / s. Figure 2 shown.

[0053] In this embodiment, the ultrasonic signal data is obtained by using a NOVASCAN portable 32 / 128 phased array detection platform with a sampling frequency of 50 MHz and 4000 sampling points.

[0054] The ultrasonic phased array probe is a 32-element linear array with 0.6mm pitch and a center frequency of 5MHz. The wedge used is model SDP2-N55S-IH, with a tilt angle of 36°, a first element height of 12.80mm, and a wedge longitudinal wave velocity of 2337m / s.

[0055] Therefore, it is possible to propose the number of array elements N = 32, the center frequency of the excitation ultrasonic signal fc = 5 MHz, the longitudinal wave speed of the wedge c1 = 2337 m / s and the shear wave speed of the test object c2 = 3230 m / s, and the density of the wedge ρ1;

[0056] The number of array elements N = 32, the array element spacing p = 0.6 mm and the array element width a = 0.4 mm, the wedge geometric parameters tilt angle β = 36°, the first array element height h = 12.80 mm, obtain the center coordinates of each array element in the array, such as Figure 3 shown.

[0057] The ultrasonic phased array probe parameters include: the number of array elements N, the array element spacing p, the array element width a and the array element center frequency f c The wedge parameters include: wedge tilt angle β, wedge density ρ1, wedge longitudinal wave speed c1 and first array element height h; with the projection of the center point of the first array element on the surface of the test object as the origin, the horizontal coordinates of the centers of each array element of the ultrasonic phased array are:

[0058] elx(m)=(m-1)pcos(β)

[0059] The vertical coordinate of the center of each element of the ultrasonic phased array is:

[0060] elz(m)=h+(m-1)psin(β)

[0061] Wherein, m is the array element number, m=1, 2, 3, ..., N.

[0062] S2. Randomly select M effective array elements from the N array elements of the ultrasonic phased array, discretize the target area into a grid, calculate the sound field radiation energy of each effective array element to each grid, then superimpose the sound field radiation energies to calculate a synthetic sound field radiation energy, and calculate an average value based on the synthetic sound field radiation energy. The sparse array distribution with the largest average value is taken as the ultrasonic sparse array distribution;

[0063] The selection of the target area can be determined according to the specific engineering application scenario. The array distribution of the maximum sound field energy is obtained to improve the defect detection capability. The target area is discretized to obtain discrete grid points. Then, each effective array element solves the coordinates of each grid point to obtain the corresponding sound field radiation energy.

[0064] Specifically, the number of effective array elements M of the ultrasonic phased array is proposed, and M effective array elements are randomly selected from the N array elements of the ultrasonic phased array. The sound field radiation energy of each effective array element to the discretized grid of the target area is calculated, and then the sound field radiation energies are superimposed to calculate the synthetic sound field radiation energy.

[0065] Specifically, in step S2, the sound field radiation energy of each effective array element to each grid is calculated by the following formula:

[0066]

[0067] Among them, r m is the vector of the mth array element pointing to the detection target point, r1 m is the distance that ultrasound travels in the wedge; is the propagation distance of ultrasound in the test object, the superscript m is the array element number; exp() is the exponential function symbol with the natural exponential e as the base; k1 is the wave number of ultrasound in the wedge, k1 = 2πf c / c1; k2 is the wave number of ultrasound in the detection object, k2=2πf c / c2; c2 is the shear wave velocity of the test object; θ1 is the incident angle of ultrasound at the interface between the wedge and the spacer, θ2 is the refraction angle of ultrasound at the interface between the wedge and the spacer, and the superscript m is the array element number; p m (r m ) is the sound field radiation energy of the array element with sequence number m to the detection target point. The detection target point corresponds to the grid point coordinates obtained by discrete gridding of the target area.

[0068] Specifically, the synthetic sound field radiation energy P(r) in step S2 is:

[0069]

[0070] Where Ω is the set of valid array element numbers. Assuming that the number of valid array elements in the array is M, there are M elements in Ω that are different from each other. The two sound field expressions are for the transmitter and receiver, respectively. Due to the reciprocity theorem, the sound field expressions of the transmitter / receiver of the same array element are consistent, so they are not distinguished, that is, p m (r) is the sound field radiation energy of the array element with serial number m; p n (r) is the sound field radiation energy of the array element with serial number n.

[0071] The synthetic sound field radiation energy can be used to obtain the theoretical sound field visualization image of the ultrasonic sparse array through gradient amplitude color mapping. The energy of all grids in the target area is divided by the maximum sound field energy, and the maximum energy of all grids does not exceed 1. The synthetic sound field visualization image is then normalized to facilitate the subsequent evaluation of the sparse array design.

[0072] Specifically, the sparse array distribution in step S2 is obtained by a genetic algorithm or a particle swarm algorithm.

[0073] The ultrasonic sparse array distribution layout is designed by sparse array synthetic sound field radiation energy distribution. Among N array elements, M effective array elements are randomly selected to calculate their synthetic sound field radiation energy. The average energy of each array element arrangement is calculated, and the array element arrangement with the largest average energy is obtained. In this example, the optimal sparse array distribution is efficiently obtained by genetic algorithm as the ultrasonic sparse array distribution. The synthetic sound field visualization image corresponding to the ultrasonic sparse array distribution is as follows: Figure 4 shown.

[0074] The value of the effective number of array elements M can be selected according to actual needs. In this embodiment, the value of M is 4.

[0075] S3. Collect the full matrix signal of the ultrasonic sparse array of the detection object, perform nonlinear beamforming processing on the target area for visual reconstruction.

[0076] S31, collecting ultrasound sparse array full matrix signals according to ultrasound sparse array distribution;

[0077] The ultrasonic sparse array full matrix signal is a sparse array element full matrix data set s composed of ultrasonic signals received by all effective array elements when effective array elements in the ultrasonic sparse array excite the ultrasonic signals one by one. m,n (t),m=1,2,3,…,M; n=1,2,3,…,M;

[0078] S32, according to the sparse full matrix signal substitution formula, s m,n (t) is expressed as s j (t), and perform phase domain analytical signal processing to obtain the phase domain analytical signal S of the sparse full matrix data j (t):

[0079] S j (t) = s j (t)+i*Hilbert(s j (t)),

[0080] Where Hilbert() is the Hilbert transform, j is the index of the signal in the sparse full matrix signal;

[0081] Specifically, the sparse full matrix signal substitution formula is:

[0082] subscript=transmitter*M+receiver

[0083] Where transmitter is the transmitting element number, receiver is the receiving element number, and M is the number of valid elements in the sparse array. j (t) is the subscript conversion form of the sparse array element full matrix data set.

[0084] S33, according to the grid points in the imaging area and S j The signal is used to solve the flight time t j (x,z), and derive the image matrix after processing through nonlinear beamforming algorithm to complete the visual reconstruction of the target area. Figure 5 shown.

[0085] Specifically, the nonlinear beamforming algorithm is:

[0086]

[0087] Where I(x,z) is the mapping amplitude of the grid points in the imaging area; S is the phase domain analytical signal of the sparse full matrix data, and the subscripts j and k in S are the index numbers of the signal in the sparse full matrix signal; S j (t) is a signal segment under the j label; S j (t j (x,z)) is a signal segment labeled j at t=t j The amplitude of (x,z); sgn() is the sign value function, specifically:

[0088]

[0089] The flight time t j (x,z) is S j The ultrasonic propagation time of the signal to the grid point (x,z) within the imaging area is:

[0090] S j The signal is transmitted from the transmitting array element according to the longitudinal wave of the wedge to the incident refraction point of the wedge (x inc1 ,0), then according to the transverse wave of the detection object to (x,z), then according to the detection transverse wave to the incident refraction point of the detection object (x inc2 ,0), and then according to the wedge longitudinal wave to the receiving array element, the time required is t j (x,z).

[0091] The selection of the imaging area can be determined according to the specific engineering application scenario and should be consistent with the actual engineering detection area, that is, consistent with the target area. The grid point coordinates in the imaging area are consistent with the grid point coordinates obtained by discrete gridding of the target area.

[0092] Specifically, in step S33, the flight time t is calculated based on each grid point in the imaging area, the longitudinal wave velocity of the wedge, and the shear wave velocity of the specimen. j (x,z) is specifically:

[0093] S j The signal is replaced by the A-wave signal collected by the effective array element m stimulating the effective array element n. According to Fermat's minimum flight time theorem, the following formula is obtained:

[0094]

[0095] Among them, c 1p is the longitudinal wave speed of ultrasound in the wedge; c 2sis the transverse wave speed of ultrasound in the detection object; elx(m) is the central abscissa of the effective array element m, elz(m) is the central ordinate of the effective array element m, m = 1, 2, 3, ..., M; elx(n) is the central abscissa of the effective array element n, elz(n) is the central ordinate of the effective array element n, n = 1, 2, 3, ..., M.

[0096] This embodiment proposes an ultrasonic phased array shear wave sparse imaging method based on nonlinear beamforming, comprising the following steps: S1. Calculating the coordinates of ultrasonic phased array elements based on ultrasonic phased array probe parameters and wedge parameters; S2. Randomly selecting M effective elements from the N elements of the ultrasonic phased array, discretely gridding the target area, calculating the acoustic field radiation energy of each effective element for each grid, then superimposing the acoustic field radiation energies to calculate the synthetic acoustic field radiation energy, and calculating the average value based on the synthetic acoustic field radiation energy. The sparse array distribution with the largest average value is taken as the ultrasonic sparse array distribution; S3. Acquiring the full matrix signal of the ultrasonic sparse array of the detection object, performing nonlinear beamforming processing on the target area for visual reconstruction. This method performs shear wave imaging based on a nonlinear beamforming algorithm, significantly reducing calculation time and improving imaging efficiency while ensuring high-quality imaging.

[0097] In addition, the ultrasonic sparse array distribution selection method proposed in this embodiment can select a suitable small number of effective array elements to construct a sparse array, reducing the amount of data required for imaging, reducing mutual interference and power consumption between array elements, and effectively improving imaging quality and imaging efficiency.

[0098] In addition, the ultrasonic phased array shear wave sparse imaging method based on nonlinear beamforming proposed in this embodiment is simple, easy to implement, low-cost, and easy to promote and use in industrial non-destructive testing.

[0099] Thus far, the technical solutions of the present invention have been described with reference to examples. However, it will be readily understood by those skilled in the art that the scope of protection of the present invention is clearly not limited to these specific embodiments. Without departing from the principles of the present invention, those skilled in the art may make equivalent changes or substitutions to the relevant technical features, and the technical solutions after such changes or substitutions will fall within the scope of protection of the present invention.

Claims

1. A method for ultrasonic phased array shear wave sparse imaging based on nonlinear beamforming, characterized in that: The following steps are involved: S1. Calculate the coordinates of ultrasonic phased array elements according to ultrasonic phased array probe parameters and wedge parameters; S2. Randomly select M effective array elements from the N array elements of the ultrasonic phased array to form a sparse array, discretize the target area into multiple grids, calculate the sound field radiation energy of each effective array element to each grid, then superimpose the sound field radiation energies to calculate a synthetic sound field radiation energy, and calculate an average value based on the synthetic sound field radiation energy. The sparse array distribution with the largest average value is taken as the ultrasonic sparse array distribution; The sound field radiation energy of each effective array element to each grid is calculated by the following formula: Among them, r m is the vector of the mth array element pointing to the detection target point, is the distance that ultrasound travels in the wedge; is the propagation distance of ultrasound in the test object, the superscript m is the array element number; a is the array element width; exp() is the exponential function symbol with the natural exponential e as the base; k1 is the wave number of ultrasound in the wedge, k1 = 2πf c / c1; k2 is the wave number of ultrasound in the detection object, k2=2πf c / c2; c2 is the shear wave velocity of the test object; θ1 is the incident angle of ultrasound at the interface between the wedge and the spacer, θ2 is the refraction angle of ultrasound at the interface between the wedge and the spacer, and the superscript m is the array element number; p m (r m ) is the sound field radiation energy of the array element with serial number m to detect the target point; The synthetic sound field radiation energy is calculated by superimposing the sound field radiation energy. Specifically, the synthetic sound field radiation energy P(r) is calculated by the following formula: Where Ω is the valid array element number set, and there are M elements in Ω that are different from each other; m (r) is the sound field radiation energy of the array element with serial number m; p n (r) is the sound field radiation energy of the array element with serial number n; S3. Collect the full matrix signal of the ultrasonic sparse array of the detection object, perform nonlinear beamforming processing on the target area for visual reconstruction.

2. The method according to claim 1, characterized in that Step S1 is specifically as follows: The ultrasonic phased array probe parameters include: the number of array elements N, the array element spacing p, the array element width a and the array element center frequency f c The wedge parameters include: wedge tilt angle β, wedge density ρ1, wedge longitudinal wave speed c1 and first array element height h; with the projection of the center point of the first array element on the surface of the test object as the origin, the horizontal coordinates of the centers of each array element of the ultrasonic phased array are: elx(m)=(m-1)pcos(β) The vertical coordinate of the center of each element of the ultrasonic phased array is: elz(m)=h+(m-1)psin(β) Wherein, m is the array element number, m=1, 2, 3, ..., N.

3. The method according to claim 1, characterized in that Step S3 specifically includes the following steps: S31. Collect the full matrix signal of the ultrasonic sparse array according to the distribution of the ultrasonic sparse array; the full matrix signal of the ultrasonic sparse array is a sparse array element full matrix data set s composed of the ultrasonic signals received by all effective array elements when the effective array elements in the ultrasonic sparse array excite the ultrasonic signals one by one. m,n (t),m=1,2,3,…,M; n=1,2,3,…,M; S32, according to the sparse full matrix signal substitution formula, s m,n (t) is expressed as s j (t), and perform phase domain analytical signal processing to obtain the phase domain analytical signal S of the sparse full matrix data j (t): S j (t)=s j (t)+i*Hilbert(s j (t)), Where Hilbert() is the Hilbert transform, j is the index of the signal in the sparse full matrix signal; S33, according to the grid points in the imaging area and S j The signal is used to solve the flight time t j (x,z), and derive the image matrix after processing through the nonlinear beamforming algorithm to complete the visual reconstruction of the target area.

4. The method according to claim 3, characterized in that The sparse full matrix signal substitution formula in step S32 is: subscript=transmitter*M+receiver Where transmitter is the transmitting element number, receiver is the receiving element number, and M is the number of valid elements in the sparse array.

5. The method according to claim 3, characterized in that The nonlinear beamforming algorithm in step S33 is: Where I(x,z) is the mapping amplitude of the grid points in the imaging area; S is the phase domain analytical signal of the sparse full matrix data, and the subscripts j and k are the index numbers of the signal in the sparse full matrix signal; S j (t j (x,z)) is a signal segment labeled j at t=t j The amplitude of (x,z); sgn() is the sign value function, specifically:

6. The method according to claim 3, characterized in that The flight time t in step S33 j (x,z) is S j The ultrasonic propagation time of the signal to the grid point (x,z) within the imaging area is: S j The signal is transmitted from the transmitting array element according to the longitudinal wave of the wedge to the incident refraction point of the wedge (x inc1 ,0), then according to the transverse wave of the detection object to (x,z), then according to the detection transverse wave to the incident refraction point of the detection object (x inc2 ,0), and then according to the wedge longitudinal wave to the receiving array element, the time required is t j (x,z).

7. The method according to claim 6, characterized in that In step S33, each grid point in the imaging area and S j The signal is used to solve the flight time t j (x,z) is specifically: S j The signal is replaced by the A-wave signal collected by the effective array element m stimulating the effective array element n. According to Fermat's minimum flight time theorem, the following formula is obtained: Among them, c 1p is the longitudinal wave speed of ultrasound in the wedge; c 2s is the transverse wave speed of ultrasound in the detection object; elx(m) is the central abscissa of the effective array element m, elz(m) is the central ordinate of the effective array element m, m = 1, 2, 3, ..., M; elx(n) is the central abscissa of the effective array element n, elz(n) is the central ordinate of the effective array element n, n = 1, 2, 3, ..., M.

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