阅读说明：本技术 一种基于切比雪夫多项式的复合窗变迹超声波束形成方法 (Composite window apodization ultrasonic beam forming method based on Chebyshev polynomial ) 是由 王平 梁家祺 王慧悦 何理 柳学功 周瑜 李锡涛 何峰宇 武超 阎鑫龙 陈靖翰 于 2021-08-18 设计创作，主要内容包括：本发明涉及一种基于切比雪夫多项式的复合窗变迹超声波束形成方法,属于超声成像技术领域。该方法首先对超声阵列接收到的回波信号进行处理,得到所需要的超声阵列回波信号；然后输入不同切比雪夫参数分别构造两个切比雪夫窗函数；再将两个切比雪夫窗函数按一定权重进行叠加,得到复合变迹窗,并用复合窗对超声阵列回波信号进行加权；根据近场和远场各自的成像效果分别调整近场窗和远场窗的切比雪夫参数,直到符合预期要求,然后进行后续波束形成和最终成像；该方法能够根据探测深度自行调节谱加权的旁瓣抑制能力,能在近场有效抑制伪像,并改善远场分辨率,从而提高超声成像的整体质量。(The invention relates to a composite window apodization ultrasonic beam forming method based on Chebyshev polynomial, belonging to the technical field of ultrasonic imaging. Firstly, processing an echo signal received by an ultrasonic array to obtain a required ultrasonic array echo signal; then inputting different Chebyshev parameters to respectively construct two Chebyshev window functions; superposing the two Chebyshev window functions according to a certain weight to obtain a composite apodization window, and weighting the ultrasonic array echo signals by using the composite window; respectively adjusting Chebyshev parameters of a near field window and a far field window according to respective imaging effects of the near field and the far field until the Chebyshev parameters meet expected requirements, and then performing subsequent beam forming and final imaging; the method can automatically adjust the side lobe suppression capability of the spectral weighting according to the detection depth, can effectively suppress the artifact in the near field, and improves the far field resolution, thereby improving the overall quality of the ultrasonic imaging.)

1. A method for forming a composite window apodized ultrasonic beam based on a Chebyshev polynomial is characterized in that: the method comprises the following steps:

s1: amplifying, AD converting and time-delay focusing processing are carried out on echo signals received by the ultrasonic array elements to obtain ultrasonic array echo data x (k), x (k)Is expressed as x (k) ═ x₁(k),x₂(k),…,x_N(k)]Wherein N represents the number of array elements of the ultrasonic array, and k represents the sampling time corresponding to the sampling depth d;

s2: inputting different Chebyshev parameters A according to the Chebyshev polynomial property_S-nearAnd A_S-farSeparately constructing two Chebyshev window functions w_near，w_far；

S3: superposing the two Chebyshev windows according to a certain weight, wherein the superposed weight is changed along with the detection depth to obtain a composite apodization window w_comp(d) Wherein d represents the detection depth, and the beam forming and imaging are carried out after the apodization of x (k) by the composite window;

s4: adjusting the near field window w according to the near field imaging effect_nearUntil the desired chebyshev parameters are met;

s5: adjusting a far field window w according to a far field imaging effect_farUntil the desired chebyshev parameters are met;

s6: and finishing the adjustment of the composite window, and performing beam forming and final imaging.

2. The chebyshev polynomial-based composite window apodization ultrasound beamforming method according to claim 1 wherein: in S2, different chebyshev parameters a are input according to the chebyshev polynomial property_S-nearAnd A_S-farSeparately constructing two Chebyshev window functions w_near，w_farThe method specifically comprises the following steps:

s21: inputting Chebyshev parameter A_SIn dB; defining the ratio of the main beam maximum to the side lobe level as R, then:

s22: construction of (N-1) order Chebyshev polynomial T_N-1(α), the expression of which is:

wherein N is the number of receiving array elements of the ultrasonic probe;

s23: the ratio of the main beam maximum to the side lobe level is equal to T according to the polynomial properties_N-1(α₀) A value of (a), wherein₀>1, resolving to alpha₀And scale-transformed to obtain ω:

the beam pattern B (ψ) is then expressed in ψ space as:

where the factor 1/R is used to normalize the beam pattern, ψ is a spatial variable such that B (0) is 1;

s24: finding the original zero psi of the beam pattern_poThe positions are as follows:

it is expressed in α space as:

transforming the scale to ω -space yields:

finally obtaining the zero point psi in psi space_p：

S25: constructing an N multiplied by N array manifold matrix V (psi) whose expression is:

wherein v (ψ) represents an array manifold vector, the expression being:

the final chebyshev parameter a_SThe weight corresponding to the Chebyshev window is:

s26: inputting different Chebyshev parameters A according to the steps_S-nearAnd A_S-farSeparately constructing two Chebyshev window functions w_near，w_farIt is necessary to satisfy A in parameter selection_S-near<A_S-farThe specific numerical values will be adjusted in the following steps.

3. The chebyshev polynomial-based composite window apodization ultrasound beamforming method according to claim 1 wherein: in the S3, the two Chebyshev windows are overlapped according to a certain weight, and the overlapped weight is changed along with the detection depth to obtain a composite apodization window w_comp(d) And performing beam forming after apodizing x (k) by using a composite window, specifically comprising the following steps:

s31: compounding the two Chebyshev windows according to a certain proportion to obtain a composite apodization window w_comp(d) The expression is as follows:

wherein d is_maxAnd d_minRespectively representing the maximum and minimum depths of the detection zone;

s32: and (3) apodizing x (k) by using the constructed composite window, and then performing beam forming, wherein the beam forming signal expression is as follows:

wherein S is_DASRepresenting a beamformed signal, x_i(k) Echo data representing the corresponding depth on the ith array element, w_comp,iIndicating the apodization weights on the ith array element.

4. The chebyshev polynomial-based composite window apodization ultrasound beamforming method according to claim 1 wherein: in the step S4, the near field window w is adjusted according to the near field imaging effect_nearUntil the Chebyshev parameters meet the expected requirements, the method specifically comprises the following steps:

s41: selecting a plurality of near-field target points in primary imaging to obtain side lobe peak values V of the near-field target points under respective depths_PSLCalculating its left boundary value V with the dynamic imaging range_DRLDifference value V of_n,errorAnd setting an acceptable threshold V_near；

S42: if V_n,errorAbove 0, the near field Chebyshev parameter A should be reduced_S-nearIf the difference is less than the threshold value V_nearThen A should be increased_S-nearRepeating the steps S3 and S4 after changing the parameter value until the condition V is satisfied_near<V_n,error<0, then the current w_near,adjI.e. the final near-field chebyshev window function.

5. The chebyshev polynomial based composite window apodization ultrasonic beam shaping of claim 1The method is characterized in that: in the S5, the far-field window w is adjusted according to the far-field imaging effect_farUntil the Chebyshev parameters are in accordance with expectations, the method specifically comprises the following steps:

s51: selecting the target point in the farthest field of the last imaging to obtain the amplitude A of the target point_M-farSetting a desired value V of the amplitude of the far field_farThe expected value V_farShould be slightly larger than the left boundary value V of the dynamic imaging range_DRLSatisfy V_DRL<V_far<1.05V_DRL；

S52: if the target amplitude A_M-far>V_farThen A is_S-farIncrease by 10%; if A_M-far<V_farThen A is_S-farDecrease by 10%, repeat steps S3 and S5 until the target amplitude A_M-farAnd an expected value V_farSatisfies 0.95| V_far|<|A_M-far-V_far|<1.05|V_farIf, then the current w_far,adjI.e. the final near-field chebyshev window function.

6. The chebyshev polynomial-based composite window apodization ultrasound beamforming method according to claim 1 wherein: in S6, generating a composite window, and performing beamforming and final imaging after apodizing the echo signal x (k) of the ultrasound array, specifically including the following steps:

s61: recombining the adjusted distance field and near field Chebyshev windows to obtain w_comp,adj(d) The expression is as follows:

s62: and (3) apodizing the echo signals x (k) of the ultrasonic array by using the adjusted composite window, and then performing beam forming and final imaging, wherein the expression of the beam forming signal is as follows:

Technical Field

The invention belongs to the technical field of ultrasonic imaging, and relates to a composite window apodization ultrasonic beam forming method based on Chebyshev polynomial.

Background

Ultrasonic imaging is a nondestructive testing means widely applied to the medical and industrial fields, and a delay superposition beam former is widely applied to ultrasonic imaging instruments on the market due to the advantages of simple structure and easy realization. However, the beam obtained by the traditional delay superposition method has the defect of high side lobe level, the side lobe is the main reason for generating artifacts, and the high side lobe level reduces the ultrasonic imaging quality.

The technical approach of suppressing the side lobe is to adopt amplitude apodization, and the realization method is to adopt a spectrum weighting technology to the receiving array element, and the spectrum weighting technology utilizes the property of Fourier transform to change the side lobe. Common window functions adopted by the traditional amplitude apodization include a triangular window, a Hanning window, a Hamming window and the like, and can effectively inhibit the height of side lobes and prevent the generation of artifacts. However, the introduction of amplitude apodization brings a new problem that the increase of the main beam width, especially in far-field imaging, causes a decrease of the lateral resolution which greatly reduces the final imaging quality. Meanwhile, due to the existence of ultrasonic attenuation, the requirement of far-field imaging on the apodization window artifact suppression capability is not as high as that of near-field imaging, and the transverse resolution is the main factor influencing the far-field imaging quality. Therefore, some researchers have proposed depth-wise segmentation of the detection region, with apodization using different window functions for different segments to improve far-field resolution. However, the conventional window function is difficult to flexibly change the sidelobe suppression capability of the window function, cannot be well adapted to the sectional apodization method, and only the existing window function is filled into the section to test the imaging effect, and then one window function with relatively good effect is selected from the existing window functions to serve as the window function of the section. There is also a lack of systematic methods for the construction and selection of different piecewise window functions.

The Chebyshev window is an apodized window originally applied to radar array positioning and proposed by Doherty, the construction process of the Chebyshev window is based on the property of a Chebyshev polynomial, and the Chebyshev window is proved to be capable of obtaining the minimum main lobe width under the given side lobe height. However, as with other window functions, the chebyshev window still suffers from the tradeoff between main beam width and side lobe height, i.e., a low side lobe height is accompanied by a larger main lobe width. After the Chebyshev window is introduced into the ultrasonic imaging, due to the existence of an ultrasonic focusing link, the actually obtained echo response and a theoretical beam pattern have deviation and need to be adjusted and corrected by a standard method.

In summary, it is urgently needed to invent a high-efficiency beam forming algorithm which can effectively suppress the artifacts in the near field and keep a higher lateral resolution in the far field, so as to improve the overall ultrasonic imaging quality.

Disclosure of Invention

In view of the above, the present invention is directed to a method for forming a complex window apodized ultrasonic beam based on chebyshev polynomials. The method can automatically adjust the side lobe suppression capability of the spectral weighting according to the detection depth, can effectively suppress the artifact in a near field and improve the resolution in a far field, thereby improving the overall quality of the ultrasonic imaging.

In order to achieve the purpose, the invention provides the following technical scheme:

a Chebyshev polynomial-based composite window apodization ultrasonic beam forming method comprises the following steps:

s1: amplifying, AD converting and time-delay focusing processing are carried out on echo signals received by the ultrasonic array elements to obtain ultrasonic array echo data x (k), wherein x (k) is expressed as x (k) [ x ]₁(k),x₂(k),…,x_N(k)]Wherein N represents the number of array elements of the ultrasonic array, and k represents the sampling time corresponding to the sampling depth d;

s2: inputting different Chebyshev parameters A according to the Chebyshev polynomial property_S-nearAnd A_S-farSeparately constructing two Chebyshev window functions w_near，w_far；

S3: superposing the two Chebyshev windows according to a certain weight, wherein the superposed weight is changed along with the detection depth to obtain a composite apodization window w_comp(d) Wherein d represents the detection depth, and the beam forming and imaging are carried out after the apodization of x (k) by the composite window;

s4: adjusting the near field window w according to the near field imaging effect_nearUntil the desired chebyshev parameters are met;

s5: adjusting a far field window w according to a far field imaging effect_farUntil the desired chebyshev parameters are met;

s6: and finishing the adjustment of the composite window, and performing beam forming and final imaging.

Optionally, in S2, different chebyshev parameters a are input according to the chebyshev polynomial property_S-nearAnd A_S-farSeparately constructing two Chebyshev window functions w_near，w_farThe method specifically comprises the following steps:

s21: inputting Chebyshev parameter A_SIn dB; defining the ratio of the main beam maximum to the side lobe level as R, then:

s22: construction of (N-1) order Chebyshev polynomial T_N-1(α), the expression of which is:

wherein N is the number of receiving array elements of the ultrasonic probe;

s23: the ratio of the main beam maximum to the side lobe level is equal to T according to the polynomial properties_N-1(α₀) A value of (a), wherein₀>1, resolving to alpha₀And scale-transformed to obtain ω:

the beam pattern B (ψ) is then expressed in ψ space as:

where the factor 1/R is used to normalize the beam pattern, ψ is a spatial variable such that B (0) is 1;

s24: finding the original zero psi of the beam pattern_poThe positions are as follows:

it is expressed in α space as:

transforming the scale to ω -space yields:

finally obtaining the zero point psi in psi space_p：

S25: constructing an N multiplied by N array manifold matrix V (psi) whose expression is:

V(ψ)＝［ν(0) ν(ψ₁) ... ν(ψ_p) ... ν(ψ_N-1)]

wherein v (ψ) represents an array manifold vector, the expression being:

the final chebyshev parameter a_SThe weight corresponding to the Chebyshev window is:

w＝[V^H(ψ)]^-1·[1 0 ... 0]^T

s26: inputting different Chebyshev parameters A according to the steps_S-nearAnd A_S-farSeparately constructing two Chebyshev window functions w_near，w_farIt is necessary to satisfy A in parameter selection_S-near<A_S-farThe specific numerical values will be adjusted in the following steps.

Optionally, in S3, the two chebyshev windows are superimposed according to a certain weight, and the superimposed weight changes with the detection depth to obtain the composite apodization window w_comp(d) And performing beam forming after apodizing x (k) by using a composite window, specifically comprising the following steps:

s31: compounding the two Chebyshev windows according to a certain proportion to obtain a composite apodization window w_comp(d) The expression is as follows:

wherein d is_maxAnd d_minRespectively representing the maximum and minimum depths of the detection zone;

s32: and (3) apodizing x (k) by using the constructed composite window, and then performing beam forming, wherein the beam forming signal expression is as follows:

wherein S is_DASRepresenting a beamformed signal, x_i(k) Echo data representing the corresponding depth on the ith array element, w_comp,iIndicating the apodization weights on the ith array element.

Optionally, in S4, the near field window w is adjusted according to the near field imaging effect_nearUntil the Chebyshev parameters meet the expected requirements, the method specifically comprises the following steps:

s41: selecting a plurality of near-field target points in primary imaging to obtain side lobe peak values V of the near-field target points under respective depths_PSLCalculating its left boundary value V with the dynamic imaging range_DRLDifference value V of_n,errorAnd setting an acceptable threshold V_near；

S42: if V_n,errorAbove 0, the near field Chebyshev parameter A should be reduced_S-nearIf the difference is less than the threshold value V_nearThen A should be increased_S-nearRepeating the steps S3 and S4 after changing the parameter value until the condition V is satisfied_near<V_n,error<0, then the current w_near,adjI.e. the final near-field chebyshev window function.

Optionally, in the S5, the far-field window w is adjusted according to the far-field imaging effect_farUntil the Chebyshev parameters are in accordance with expectations, the method specifically comprises the following steps:

s51: selecting the target point in the farthest field of the last imaging to obtain the amplitude A of the target point_M-farSetting a desired value V of the amplitude of the far field_farThe expected value V_farShould be slightly larger than the left boundary value V of the dynamic imaging range_DRLSatisfy V_DRL<V_far<1.05V_DRL；

S52: if the target amplitude A_M-far>V_farThen A is_S-farIncrease by 10%; if A_M-far<V_farThen A is_S-farDecrease by 10%, repeat steps S3 and S5 until the target amplitude A_M-farAnd an expected value V_farIs satisfied with 0.95|V_far|<|A_M-far-V_far|<1.05|V_farIf, then the current w_far,adjI.e. the final near-field chebyshev window function.

Optionally, in S6, generating a composite window, performing beamforming and final imaging after apodizing the echo signal x (k) of the ultrasound array, specifically including the following steps:

s61: recombining the adjusted distance field and near field Chebyshev windows to obtain w_comp,adj(d) The expression is as follows:

s62: and (3) apodizing the echo signals x (k) of the ultrasonic array by using the adjusted composite window, and then performing beam forming and final imaging, wherein the expression of the beam forming signal is as follows:

the invention has the beneficial effects that: the invention provides two Chebyshev windows w respectively suitable for near-field imaging_nearAnd w suitable for far field imaging_farProportionally compounding to make the whole compound window follow the depth of detection from w_nearGradually transits to w_farSo as to meet the change of the emphasis point of the window function performance requirement under different depths. Compared with the traditional apodization window functions such as Hanning window, Haining window and the like, the composite window can effectively inhibit the artifact in the near field and can better improve the transverse resolution in the far field, thereby improving the integral ultrasonic imaging quality.

Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the means of the instrumentalities and combinations particularly pointed out hereinafter.

Drawings

For the purposes of promoting a better understanding of the objects, aspects and advantages of the invention, reference will now be made to the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a flow chart of a Chebyshev polynomial based composite window apodization ultrasonic imaging method according to the present invention;

FIG. 2 is a comparison graph of point target simulation imaging results after apodization of different window functions;

FIG. 3 is a slice image of the point target simulation results at depths of 100mm and 200mm after apodization of different window functions;

FIG. 4 is a graph comparing the imaging results of the geabr _0 data after apodization of different window functions.

Detailed Description

The embodiments of the present invention are described below with reference to specific embodiments, and other advantages and effects of the present invention will be easily understood by those skilled in the art from the disclosure of the present specification. The invention is capable of other and different embodiments and of being practiced or of being carried out in various ways, and its several details are capable of modification in various respects, all without departing from the spirit and scope of the present invention. It should be noted that the drawings provided in the following embodiments are only for illustrating the basic idea of the present invention in a schematic way, and the features in the following embodiments and examples may be combined with each other without conflict.

Wherein the showings are for the purpose of illustrating the invention only and not for the purpose of limiting the same, and in which there is shown by way of illustration only and not in the drawings in which there is no intention to limit the invention thereto; to better illustrate the embodiments of the present invention, some parts of the drawings may be omitted, enlarged or reduced, and do not represent the size of an actual product; it will be understood by those skilled in the art that certain well-known structures in the drawings and descriptions thereof may be omitted.

The same or similar reference numerals in the drawings of the embodiments of the present invention correspond to the same or similar components; in the description of the present invention, it should be understood that if there is an orientation or positional relationship indicated by terms such as "upper", "lower", "left", "right", "front", "rear", etc., based on the orientation or positional relationship shown in the drawings, it is only for convenience of description and simplification of description, but it is not an indication or suggestion that the referred device or element must have a specific orientation, be constructed in a specific orientation, and be operated, and therefore, the terms describing the positional relationship in the drawings are only used for illustrative purposes, and are not to be construed as limiting the present invention, and the specific meaning of the terms may be understood by those skilled in the art according to specific situations.

Fig. 1 is a flowchart of an algorithm of the present invention, and the present invention provides a method for forming a complex window apodized ultrasonic beam based on chebyshev polynomial, the method comprising the steps of:

step S1: amplifying, AD converting and time-delay focusing processing are carried out on echo signals received by the ultrasonic array elements to obtain ultrasonic echo data x (k), wherein x (k) is expressed as x (k) [ x ]₁(k),x₂(k),…,x_N(k)]Where N represents the number of array elements of the ultrasound array and k represents the sampling instant corresponding to the sampling depth d.

Step S2: inputting different Chebyshev parameters A according to the Chebyshev polynomial property_S-nearAnd A_S-farSeparately constructing two Chebyshev window functions w_near，w_farThe method specifically comprises the following steps:

s21: inputting Chebyshev parameter A_SIn dB. Defining the ratio of the main beam maximum to the side lobe level as R, then:

s22: construction of (N-1) order Chebyshev polynomial T_N-1(α), the expression of which is:

wherein N is the number of receiving array elements of the ultrasonic probe;

s23: the ratio of the main beam maximum to the side lobe level is equal to T according to the polynomial properties_N-1(α₀) A value of (a), wherein₀>1, resolving to alpha₀And scale-transformed to obtain ω:

the beam pattern B (ψ) can be expressed in ψ space as:

where the factor 1/R is used to normalize the beam pattern, ψ is a spatial variable such that B (0) is 1;

s24: finding the original zero psi of the beam pattern_poThe positions are as follows:

it is expressed in α space as:

transforming the scale to ω -space yields:

finally obtaining the zero point psi in psi space_p：

S25: constructing an N multiplied by N array manifold matrix V (psi) whose expression is:

V(ψ)＝[ν(0) ν(ψ₁) ... ν(ψ_p) ... ν(ψ_N-1)]

wherein v (ψ) represents an array manifold vector, the expression being:

the final chebyshev parameter a_SThe weight corresponding to the Chebyshev window is:

w＝[V^H(ψ)]^-1·[1 0 ... 0]^T

s26: inputting different Chebyshev parameters A according to the steps_S-nearAnd A_S-farSeparately constructing two Chebyshev window functions w_near，w_farIt is necessary to satisfy A in parameter selection_S-near<A_S-farThe specific numerical values will be adjusted in the following steps.

Step S3: superposing the two Chebyshev windows according to a certain weight, wherein the superposed weight is changed along with the detection depth to obtain a composite apodization window w_comp(d) Wherein d represents the depth of investigation, and the beam forming and imaging are performed after x (k) apodization by using the composite window, which specifically comprises the following steps:

s31: compounding the two Chebyshev windows according to a certain proportion to obtain a composite apodization window w_comp(d) The expression is as follows:

wherein d is_maxAnd d_minRespectively representing the maximum and minimum depths of the detection zone;

s32: and (3) apodizing x (k) by using the constructed composite window, and then performing beam forming, wherein the beam forming signal expression is as follows:

wherein S is_DASRepresenting a beamformed signal, x_i(k) Indicating the corresponding depth on the ith array elementEcho data, w_comp,iIndicating the apodization weights on the ith array element.

Step S4: adjusting the near field window w according to the near field imaging effect_nearUntil the Chebyshev parameters meet the expected requirements, the method specifically comprises the following steps:

s41: selecting a plurality of near-field target points in primary imaging to obtain side lobe peak values V of the near-field target points under respective depths_PSLCalculating its left boundary value V with the dynamic imaging range_DRLDifference value V of_n,errorAnd setting an acceptable threshold V_near；

S42: if V_n,errorAbove 0, the near field Chebyshev parameter A should be reduced_S-nearIf the difference is less than the threshold value V_nearThen A should be increased_S-nearRepeating the steps S3 and S4 after changing the parameter value until the condition V is satisfied_near<V_n,error<0, then the current w_near,adjI.e. the final near-field chebyshev window function.

Step S5: adjusting a far field window w according to a far field imaging effect_farUntil the Chebyshev parameters meet the expected requirements, the method specifically comprises the following steps:

s51: selecting the target point in the farthest field of the last imaging to obtain the amplitude A of the target point_M-farSetting a desired value V of the amplitude of the far field_farThe expected value V_farShould be slightly larger than the left boundary value V of the dynamic imaging range_DRLSatisfy V_DRL<V_far<1.05V_DRL；

S52: if the target amplitude A_M-far>V_farThen A is_S-farIncrease by 10%; if A_M-far<V_farThen A is_S-farDecrease by 10%, repeat steps S3 and S5 until the target amplitude A_M-farAnd an expected value V_farSatisfies 0.95| V_far|<|A_M-far-V_far|<1.05|V_farIf, then the current w_far,adjI.e. the final near-field chebyshev window function.

Step S6: and finishing the adjustment of the composite window, and performing beam forming and final imaging, wherein the method specifically comprises the following steps:

s61: recombining the adjusted distance field and near field Chebyshev windows to obtain w_comp,adj(d) The expression is as follows:

s62: and (3) apodizing the echo signals x (k) of the ultrasonic array by using the adjusted composite window, and then performing beam forming and final imaging, wherein the expression of the beam forming signal is as follows:

field II is an ultrasonic experimental simulation platform developed by Denmark university of Engineers based on acoustic principle, and has been widely accepted and used in theoretical research. To verify the effectiveness of the proposed algorithm, Field II was used to image point targets commonly used in ultrasound imaging and the imaging contrast experiment was performed using the geabr _0 experimental data. In a point target simulation experiment, two lines of 40 point targets with the transverse interval of 10mm and the longitudinal interval of 10mm are arranged and uniformly distributed between the depths of 30 mm-200 mm, a transmitting fixed point focusing and receiving dynamic focusing mode is adopted, a transmitting focus is fixed at the position of 60mm, the imaging dynamic range of an image is set to 65dB, and meanwhile, an ultrasonic attenuation parameter is set to 50 dB/m. The central frequency of the array elements adopted by the geobr _0 experiment is 3.33MHz, the number of the array elements is 64, the spacing is 0.2413mm, the sampling frequency is 17.76MHz, the sound velocity is 1500m/s, and the imaging dynamic range is set to be 50 dB. And performing apodization imaging experiments on the two experimental targets by adopting a rectangular window, a Hanning window, a Hamming window and a composite window provided by the invention, wherein the composite window formed by overlapping Chebyshev windows with near fields of-30 dB and far fields of-20 dB is adopted in a point target imaging simulation experiment, and the composite window formed by overlapping Chebyshev windows with near fields of-15 dB and far fields of-3 dB is adopted in a geobr _0 experiment. The center Amplitude (MA) and the Full-Width-at-Half-Maximum (FWHM) of the double-point target are used for evaluating the transverse resolution of the image, and the advantages and the disadvantages of different apodization windows are judged.

Fig. 2 shows a comparison of the results of point target simulation imaging. As can be seen from fig. 2, the image apodized by the rectangular window has an obvious artifact in the near field, the influence of the ultrasonic attenuation is gradually increased with the increase of the detection depth, and the artifact amplitude is gradually attenuated to a dynamic range and disappears from the image; the images of Hanning window apodization and Haining window apodization do not have obvious artifacts in the whole process, but the transverse resolution of the images in a far field is low, so that the images of point targets at the same depth are mixed together and cannot be distinguished; the composite window apodized image has better artifact suppression capability in the near field than the rectangular window results and higher lateral resolution in the far field than the hanning and haining window results. Table 1 shows the center amplitude and full width at half maximum of the apodization results for the four window functions at different depths. As can be seen from table 1, the FWHM of the composite window is 2.404mm on average, 26.6% narrower than 3.280mm of hanning window, and 22.4% narrower than 3.126mm of hamming window, and the MA value of the composite window is smaller than the other three conventional apodization windows at the same depth, which means that the composite window is more advantageous for distinguishing two point targets at the same depth.

TABLE 1 center amplitude MA (dB) and full width at half maximum FWHM (mm) of four apodization window function results at different depths

Fig. 3 shows slice images at a depth of 100mm and a depth of 200mm in the point target simulation imaging result. As can be seen from fig. 3, at a depth of 100mm, the height of the side lobe of the rectangular window apodization result is within the dynamic imaging range, and a valley value exists between the height of the side lobe and the peak value of the main lobe, so that a more obvious artifact can be formed, and the side lobes between the two points are overlapped with each other to form a more obvious artifact, while the peak values of the side lobes of the hanning window, the hamming window and the compound window apodization result are not within the dynamic imaging range, so that no obvious artifact is generated, and the compound window has a narrower width of the main lobe and a lower central amplitude compared with the hanning window and the hamming window; at the depth of 200mm, due to the existence of ultrasonic attenuation, the wave beam amplitude is reduced, and the imaging range of the side lobe height attenuation of the rectangular window result is eliminated, so that the artifact disappears, while the main lobe width of the results of the Hanning window and the Haining window is too wide, so that a new peak value is mixed between two points, the imaging quality is seriously reduced, the result of the composite window still maintains relatively narrow main lobe width, and no new artifact is generated due to main lobe mixing. In summary, in the point target simulation imaging experiment, the composite window provided by the invention can significantly improve the final imaging quality.

Figure 4 presents a comparison of the results of the geabr _0 imaging experiment. When the point objects arranged in the vertical direction are taken as the observation targets and numbered from near to far, the full widths at half maximum of the partial point objects are shown in table 2. As can be seen from fig. 4 and table 2, compared with the apodization results of the rectangular window, the hanning window, and the hamming window, the composite window proposed by the present invention has higher lateral resolution, and the point target has smaller lateral diffusion degree, and the advantages thereof are particularly obvious in the far field.

TABLE 2 half-Width (mm) of target apodization results at lower part of different window functions

Finally, the above embodiments are only intended to illustrate the technical solutions of the present invention and not to limit the present invention, and although the present invention has been described in detail with reference to the preferred embodiments, it will be understood by those skilled in the art that modifications or equivalent substitutions may be made on the technical solutions of the present invention without departing from the spirit and scope of the technical solutions, and all of them should be covered by the claims of the present invention.