CN113092589A - Multilayer irregular medium synthetic aperture imaging method based on sound velocity inversion - Google Patents

Multilayer irregular medium synthetic aperture imaging method based on sound velocity inversion Download PDF

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CN113092589A
CN113092589A CN202110401842.8A CN202110401842A CN113092589A CN 113092589 A CN113092589 A CN 113092589A CN 202110401842 A CN202110401842 A CN 202110401842A CN 113092589 A CN113092589 A CN 113092589A
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他得安
史凌伟
李义方
石勤振
顾美琳
宋小军
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Abstract

The invention provides a multilayer irregular medium synthetic aperture imaging method based on sound velocity inversion, which is characterized by comprising the following steps of: firstly, acquiring a full-matrix ultrasonic signal of a medium to be imaged by utilizing two linear array transducers arranged in parallel; then, acquiring the propagation time of the first arrival wave from the full-matrix ultrasonic signal by using a first arrival wave propagation time acquisition algorithm; then, obtaining a sound velocity distribution model by using an inversion algorithm and a ray tracing method; then, acquiring a reconstructed image of the medium to be imaged by utilizing a Fourier domain full-matrix synthetic aperture reconstruction algorithm; and finally, fusing the reconstructed images to obtain an imaging result. The synthetic aperture imaging method can obtain clear and accurate imaging results.

Description

Multilayer irregular medium synthetic aperture imaging method based on sound velocity inversion
Technical Field
The invention belongs to the field of image analysis, and particularly relates to a multilayer irregular medium synthetic aperture imaging method based on sound velocity inversion.
Background
In recent years, ultrasonic imaging technology has been widely used in many fields, such as industrial nondestructive testing, medical clinical B-mode ultrasonic imaging, doppler imaging, and the like, due to its advantages of low price, simple operation, and the like.
However, since many critical structural components in the industrial field have multiple layers of irregular media formed by complex curved surface structures, stress concentration often occurs in the practical application process of the ultrasonic imaging technology, so that various defects are easily generated, and hidden danger and harm are caused to the structural safety.
In addition, in the ultrasonic detection process, in order to ensure that the acoustic wave is effectively incident into the curved surface member, coupling is usually performed in a water immersion or wedge block manner, and the coupling manner enables the acoustic wave to propagate in a double-layer medium composed of a coupling medium and a detected member, so that the conventional Synthetic Aperture Focusing Technology (SAFT) based on the single-layer constant acoustic velocity medium assumption is difficult to apply. Although the SAFT can be combined with Fermat's law or ray tracing to perform imaging detection on the curved surface component, the method needs to know the surface function of the curved surface component in advance, and has complex algorithm and slow imaging speed. In addition, conventional echo patterns present great difficulties in accurately imaging them.
Disclosure of Invention
In order to solve the problems, the invention provides an aperture imaging method for improving the detection efficiency of an irregular layered medium, which adopts the following technical scheme:
the invention provides a multilayer irregular medium synthetic aperture imaging method based on sound velocity inversion, which is characterized by comprising the following steps of: step S1, placing a medium to be imaged between two linear array transducers arranged in parallel, wherein array elements in the linear array transducers transmit ultrasonic pulse signals to the medium to be imaged in sequence, and the two linear array transducers receive corresponding ultrasonic pulse echo signals in a full-matrix form to serve as full-matrix ultrasonic signals; step S2, acquiring the propagation time of the first arrival wave from the full-matrix ultrasonic signal by using a preset first arrival wave propagation time acquisition algorithm; step S3, obtaining a sound velocity distribution model according to the propagation time by using a preset inversion algorithm and a preset ray tracing method; step S4, based on the sound velocity distribution model and the full matrix ultrasonic signals, utilizing a Fourier domain full matrix synthetic aperture reconstruction algorithm to reconstruct images so as to obtain reconstructed images of the medium to be imaged in the direction of the two linear array transducers; and step S5, fusing the reconstructed images to obtain fused images, and taking the fused images as imaging results.
The multilayer irregular medium synthetic aperture imaging method based on sound velocity inversion provided by the invention can also have the technical characteristics that the inversion algorithm is a travel-time inversion algorithm, and the travel-time inversion algorithm comprises Gihonov regularization and total variation mixed regularization.
The multilayer irregular medium synthetic aperture imaging method based on sound velocity inversion provided by the invention can also have the technical characteristics that the step S3 comprises the following sub-steps: step S3-1, setting an initial slowness S0(ii) a Step S3-2, the initial slowness S0An equation with an equation function:
Figure BDA0003020665460000031
in the formula (I), the compound is shown in the specification,
Figure BDA0003020665460000032
in the nth iteration process, the signal transmitted by the ith transmitting array element in the linear array transducer arrives at the (x, y) position, and then the receiving time of the receiving array element in the jth linear array transducer, N is the number of the transducer elements, omega is the space of a plane to be imaged, and then the fast speed is utilizedThe travel algorithm solves the equation of the equation to obtain the receiving time
Figure BDA0003020665460000033
Step S3-3, according to the receiving time
Figure BDA0003020665460000034
Establishing an acoustic path J using ray tracingn(Sn) (ii) a Step S3-4, setting auxiliary variable m and according to sound wave path Jn(Sn) The travel time inversion algorithm establishes a loss function E (S)n，m)：
Figure BDA0003020665460000035
Figure BDA0003020665460000036
In the formula, TobsTo the propagation time, | m | | non-calculationTVFor total variation blend regularization, Γ (S)n-m) is the Gihonov regularization, σ, ε, ξ are the regularization parameters; step S3-5, solving the loss function E (S) by using a conjugate gradient algorithmnM) to obtain a slowness Sn +1Wherein the search direction p of the conjugate gradient algorithmnComprises the following steps:
ρn＝-gn+βnρn-1
Figure BDA0003020665460000037
wherein, when n is 0, gn-1＝0、ρn-10; step S3-6, determining slowness Sn+1Whether the loss function E (S) is satisfiedn +1M) < delta (delta is an expected error), and if yes, the slowness S is judgedn+1As an optimal solution
Figure BDA0003020665460000038
The flow proceeds to step S3-7, where it is judged NO. Will slow down Sn+1Taking the next iteration as a new S, and entering the step S3-2; step S3-7, the optimal solution is obtained
Figure BDA0003020665460000041
The reciprocal of (d) is used as a sound speed distribution model.
The multilayer irregular medium synthetic aperture imaging method based on sound velocity inversion provided by the invention can also have the technical characteristics that the first arrival wave propagation time acquisition algorithm is an akabane information content criterion algorithm.
The multilayer irregular medium synthetic aperture imaging method based on sound velocity inversion can also have the technical characteristics that the sound velocity distribution model is a soft tissue-cortical bone-soft tissue five-layer medium model.
Action and Effect of the invention
According to the multilayer irregular medium synthetic aperture imaging method based on sound velocity inversion, the two linear array transducers are used for acquiring the corresponding full matrix ultrasonic signal from a medium to be imaged, then the propagation time of the first arrival wave is acquired based on the full matrix ultrasonic signal, further, a sound velocity distribution model is obtained by using a preset inversion algorithm and a preset ray tracing method, a reconstructed image is obtained by using a Fourier domain full matrix synthetic aperture reconstruction algorithm, and finally the reconstructed image is fused to obtain an imaging result. Therefore, clear and accurate high-quality imaging results can be generated, and the defects of the traditional method for imaging the multilayer irregular medium are overcome.
The multilayer irregular medium synthetic aperture imaging method based on the sound velocity inversion can be applied to multilayer objects with irregular media, so that clear and accurate high-quality imaging results are obtained, and the defects of the prior art such as nondestructive testing, B-type ultrasonic imaging or Doppler imaging are overcome.
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FIG. 1 is a flow chart of a multilayer irregular medium synthetic aperture imaging method based on sound velocity inversion according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of the positions of a linear array transducer and a medium to be imaged according to an embodiment of the present invention;
FIG. 3 is a longitudinal sectional view of an embodiment of the present invention;
FIG. 4 is an imaging result diagram of a multilayer irregular medium synthetic aperture imaging method based on sound velocity inversion according to an embodiment of the present invention;
fig. 5 is a graph showing an imaging result of a conventional synthetic aperture imaging method according to an embodiment of the present invention.
Detailed Description
In order to make the technical means, the creation features, the achievement purposes and the effects of the invention easy to understand, the method for imaging the multilayer irregular medium synthetic aperture based on the acoustic velocity inversion of the invention is specifically described below with reference to the embodiments and the accompanying drawings.
< example >
Fig. 1 is a flowchart of a multilayer irregular medium synthetic aperture imaging method based on sound velocity inversion according to an embodiment of the present invention.
As shown in fig. 1, the multilayer irregular medium synthetic aperture imaging method based on sound velocity inversion includes the following steps:
and step S1, placing the medium to be imaged between two parallel linear array transducers, wherein array elements in the linear array transducers transmit ultrasonic pulse signals to the medium to be imaged in sequence, and the two linear array transducers receive corresponding ultrasonic pulse echo signals in a full-matrix form as full-matrix ultrasonic signals.
Fig. 2 is a schematic diagram of the positions of a linear array transducer and a medium to be imaged according to an embodiment of the present invention.
As shown in fig. 2, a medium 10 to be imaged (cortical bone phantom) is placed between two linear array transducers 20, each linear array transducer 20 has 128 array elements with effective width of 0.1mm and spacing of 0.2mm, and 256 array elements in total transmit ultrasonic pulse signals to the medium to be imaged in turn, and receive ultrasonic pulse echo signals generated by the medium to be imaged in a full matrix manner through all the array elements.
The ultrasonic pulse signal is a Gaussian envelope sine wave with two periods, the center frequency of the sine wave is 3.5MHz, and the sampling rate is 25 MHz.
Step S2, acquiring the propagation time of the first arrival wave from the full-matrix ultrasonic signal by using a predetermined first arrival wave propagation time acquisition algorithm.
The first arrival wave propagation time acquisition algorithm is an akachi pool information content criterion algorithm (AIC algorithm for short):
AIC＝(2k-1L)/n (1)
in the formula, k is the number of parameters in the propagation time, L is a log-likelihood value, and n is the number of observed values.
Step S3, a sound velocity distribution model is obtained according to the propagation time by using a predetermined inversion algorithm and a predetermined ray tracing method.
The inversion algorithm is a travel-time inversion algorithm, and the travel-time inversion algorithm includes Gihonov regularization and total variation hybrid regularization.
In this embodiment, the ray tracing method is an existing ray tracing method. The sound velocity distribution model is a five-layer medium model of soft tissue-cortical bone-soft tissue.
Wherein, step S3 includes the following substeps:
step S3-1, setting an initial slowness S0；
Step S3-2, the initial slowness S0An equation with an equation function:
Figure BDA0003020665460000071
in the formula (I), the compound is shown in the specification,
Figure BDA0003020665460000072
in the nth iteration process, the signal transmitted by the ith transmitting array element in the linear array transducer arrives at the (x, y) position, the receiving time of the receiving array element in the jth linear array transducer is further obtained, N is the number of the transducer array elements, omega is the space of a plane to be imaged, and then the fast marching algorithm is utilized to solve the equation of the engineering function so as to obtain the receiving time
Figure BDA0003020665460000073
Step S3-3, according to the receiving time
Figure BDA0003020665460000074
Establishing an acoustic path J using ray tracingn(Sn)；
Step S3-4, setting auxiliary variable m and according to sound wave path Jn(Sn) The travel time inversion algorithm establishes a loss function E (S)n，m)：
Figure BDA0003020665460000075
In the formula, TobsTo the propagation time, | m | | non-calculationTVFor total variation blend regularization, Γ (S)n-m) is the Gihonov regularization, σ, ε, ξ are the regularization parameters;
step S3-5, solving the loss function E (S) by using a conjugate gradient algorithmnM) to obtain a slowness Sn+1Wherein the search direction p of the conjugate gradient algorithmnComprises the following steps:
Figure BDA0003020665460000076
wherein, when n is 0, gn-1＝0、ρn-1＝0；
Step S3-6, determining slowness Sn+1Whether the loss function E (S) is satisfiedn+1M) < delta (delta is an expected error), and if yes, the slowness S is judgedn+1As an optimal solution
Figure BDA0003020665460000081
The flow proceeds to step S3-7, and if it is determined not to be the case, the slowness S is setn+1Taking the next iteration as a new S, and entering the step S3-2;
step S3-7, the optimal solution is obtained
Figure BDA0003020665460000083
Is taken as a sound velocity distribution model (i.e., sound velocity distribution model)
Figure BDA0003020665460000084
)。
And step S4, based on the sound velocity distribution model and the full matrix ultrasonic signals, carrying out image reconstruction by utilizing a Fourier domain full matrix synthetic aperture reconstruction algorithm so as to obtain a reconstructed image of the medium to be imaged in the direction of the two linear array transducers.
The Fourier domain full matrix synthetic aperture reconstruction algorithm comprises the following steps:
Figure BDA0003020665460000082
wherein i (x, z) is the imaging result, Pr(x, z, omega) is the result of inverse Fourier transform of the wave field received by the r-th receiving array element at the focus point, Se(x, z, omega) is the inverse Fourier transform result of the wave field emitted by the e-th emitting array element at the same focus point, and represents conjugation.
And step S5, fusing the first reconstructed image and the second reconstructed image to obtain a fused image as an imaging result.
FIG. 3 is a longitudinal sectional view of an embodiment of the present invention.
In order to verify the effectiveness of the multilayer irregular medium synthetic aperture imaging method based on sound velocity inversion, experimental comparison is carried out by comparing a simulation imaging result (obtained by using simulation data) with an imaging result (obtained by using the synthetic aperture imaging method of the invention aiming at a cortical bone phantom) of the invention. Specifically, the method comprises the following steps:
fig. 3(a) is a longitudinal sectional view of a simulation model, in which an abscissa x (mm) is a horizontal position of an image under a probe, an ordinate z (mm) is an image depth, a rightmost V (m/s) is a sound velocity, simulation data of the simulation model is obtained by a k-space pseudo-spectral method, and each simulation parameter is specifically:
the simulated sound velocities of the cortical bone and the soft tissue are respectively 2900 +/-100 m/s and 1550 +/-50 m/s; the simulated densities of the cortical bone and the soft tissue are 1850 +/-100 kg/m respectively3、1050±20kg/m3(ii) a The grid size dz ═ dx ═ 12.5 μm, and the simulation time step dt ═ 2.5 ns.
Fig. 3(b) is a longitudinal sectional view of a cortical bone mimetic, which is obtained by μ CT scanning. The specific parameters are as follows: the sound velocity of the cortical bone phantom is 2680m/s, and the density is 1150kg/m3The sound velocity of water is 1500m/s, and the density is 1000kg/m3。
Fig. 4 is an imaging result diagram of a multilayer irregular medium synthetic aperture imaging method based on sound velocity inversion in the embodiment of the invention.
Fig. 4(a) shows the simulation imaging result obtained by using the simulation model, and it can be seen from fig. 4(a) that the boundary between the inside and outside of the simulation imaging result is clear, and the shape and position of the boundary are highly overlapped with the boundary of the model medium used for the simulation of fig. 3 (a).
Fig. 4(b) is an imaging result of a cortical bone mimic. As can be seen from fig. 4(b), the inside and outside boundaries of the imaging result of the cortical bone phantom are clear, and the shape and position of the boundary substantially coincide with the medium boundary shown in the μ CT scan image of the cortical bone phantom used in fig. 3 (b). Because there is noise when processing to the imitative body of cortex bone, therefore, it is reasonable that there is certain difference between the imitative body imaging result of cortex bone and the emulation imaging result.
Therefore, no matter the data of the medium to be imaged is simulation data or cortical bone imitation data, the multilayer irregular medium synthetic aperture imaging method based on sound velocity inversion has a good effect, and the imaging result conforms to the corresponding longitudinal section diagram.
Fig. 5 is a graph showing an imaging result of a conventional synthetic aperture imaging method according to an embodiment of the present invention.
In addition, this embodiment is also experimentally compared with the conventional synthetic aperture imaging method.
Fig. 5(a) is a graph of imaging results obtained using a conventional synthetic aperture imaging method based on simulation data. Fig. 5(b) is a graph of imaging results from data of cortical bone mimetics using conventional synthetic aperture imaging methods.
In both fig. 5(a) and fig. 5(b), the difference between the thickness of the dielectric layer and the actual situation of the longitudinal section in fig. 3(a) and fig. 3(b) is large, the shape of the boundary of the dielectric layer is completely different from the actual situation, and the boundary position is also inaccurate, thereby illustrating that the conventional synthetic aperture imaging method cannot obtain accurate imaging of the multilayer irregular medium.
In conclusion, the multilayer irregular medium synthetic aperture imaging method based on sound velocity inversion is superior to the traditional synthetic aperture imaging method.
Examples effects and effects
According to the multilayer irregular medium synthetic aperture imaging method based on sound velocity inversion provided by the embodiment, two linear array transducers are used for acquiring a corresponding full matrix ultrasonic signal from a medium to be imaged, then the propagation time of a first arrival wave is acquired based on the full matrix ultrasonic signal, further, a sound velocity distribution model is obtained by a preset inversion algorithm and a preset ray tracing method, a reconstructed image is obtained by a Fourier domain full matrix synthetic aperture reconstruction algorithm, and finally the reconstructed image is fused to obtain an imaging result. Therefore, clear and accurate high-quality imaging results can be generated, and the defects of the traditional method for imaging the multilayer irregular medium are overcome.
The above-described embodiments are merely illustrative of specific embodiments of the present invention, and the present invention is not limited to the description of the above-described embodiments.

Claims (5)

1. A multilayer irregular medium synthetic aperture imaging method based on sound velocity inversion is characterized by comprising the following steps of:
step S1, placing the medium to be imaged between two linear array transducers which are arranged in parallel, wherein array elements in the linear array transducers sequentially transmit ultrasonic pulse signals to the medium to be imaged, and the two linear array transducers receive corresponding ultrasonic pulse echo signals in a full-matrix form to serve as full-matrix ultrasonic signals;
step S2, acquiring the propagation time of a first arrival wave from the full-matrix ultrasonic signal by using a preset first arrival wave propagation time acquisition algorithm;
step S3, obtaining a sound velocity distribution model according to the propagation time by using a preset inversion algorithm and a preset ray tracing method;
step S4, based on the sound velocity distribution model and the full matrix ultrasonic signals, carrying out image reconstruction by utilizing a Fourier domain full matrix synthetic aperture reconstruction algorithm so as to obtain a reconstructed image of the medium to be imaged in the direction of the two linear array transducers;
and step S5, fusing the reconstructed images to obtain fused images, and taking the fused images as the imaging results.
2. The acoustic velocity inversion-based multilayer irregular medium synthetic aperture imaging method according to claim 1, characterized in that:
the inversion algorithm is a travel-time inversion algorithm, and the travel-time inversion algorithm comprises Gihonov regularization and total variation hybrid regularization.
3. The acoustic velocity inversion-based multilayer irregular medium synthetic aperture imaging method according to claim 2, characterized in that:
wherein the step S3 includes the following sub-steps:
step S3-1, setting an initial slowness S0；
Step S3-2, the initial slowness S0An equation with an equation function:
Figure FDA0003020665450000021
in the formula (I), the compound is shown in the specification,
Figure FDA0003020665450000022
in the nth iteration process, the ith transmitting array element in the linear array transducer is sentThe transmitted signal arrives at the position (x, y), the receiving time of a receiving array element in the jth linear array transducer is obtained, N is the number of transducer elements, omega is the plane space to be imaged, and then the fast marching algorithm is utilized to solve the equation of the function of the equation, so that the receiving time is obtained
Figure FDA0003020665450000023
Step S3-3, according to the receiving time
Figure FDA0003020665450000024
Establishing a sound path J using the ray tracing methodn(Sn)；
Step S3-4, setting an auxiliary variable m and setting the auxiliary variable m according to the sound wave path Jn(Sn) The time-of-flight inversion algorithm establishes a loss function E (S)n，m)：
Figure FDA0003020665450000025
Figure FDA0003020665450000026
In the formula, TobsCalculating the propagation time | m | | non-calculationTVFor the total variation mixture regularization, Γ (S)n-m) is the Gihonov regularization, σ, ε, ξ are regularization parameters;
step S3-5, solving the loss function E (S) by using a conjugate gradient algorithmnM) to obtain a slowness Sn+1Wherein the search direction p of the conjugate gradient algorithmnComprises the following steps:
ρn＝-gn+βnρn-1
Figure FDA0003020665450000031
wherein, when n is 0, gn-1＝0、ρn-1＝0；
Step S3-6, judging the slowness Sn+1Whether the loss function E (S) is satisfiedn+1M) < delta (delta is an expected error), and if yes, the slowness S is judgedn+1As an optimal solution
Figure FDA0003020665450000032
The flow proceeds to step S3-7, and if it is determined not to be the case, the slowness S is setn+1Taking the next iteration as a new S, and entering the step S3-2;
step S3-7, the optimal solution is obtained
Figure FDA0003020665450000033
The inverse of (d) is taken as the sound speed distribution model.
4. The acoustic velocity inversion-based multilayer irregular medium synthetic aperture imaging method according to claim 1, characterized in that:
and the first arrival wave propagation time acquisition algorithm is a Chichi information amount criterion algorithm.
5. The acoustic velocity inversion-based multilayer irregular medium synthetic aperture imaging method according to claim 1, characterized in that:
wherein the sound velocity distribution model is a five-layer medium model of soft tissue-cortical bone-soft tissue.
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