CN112782280B - Linear polarization microwave thermoacoustic imaging method and device - Google Patents

Abstract

The invention proposes a linearly polarized microwave thermoacoustic measurement method and device for quantitatively detecting the dielectric anisotropy of a target. By using four linearly polarized microwaves as excitation sources, the newly proposed parameter values above microwave absorption are between 0 and 1. In between, the microscopic anisotropy degree of the target can be quantified with the newly proposed parameter value 0 to 1 according to the microwave absorption characteristics. The feasibility of this method is validated with dielectric anisotropy samples, which provides an efficient and straightforward strategy for tissue polarization measurements, presetting a great potential for bioimaging and material inspection. And it also provides an effective method for microwave thermoacoustic measurement to detect the microscopic anisotropy of the target.

Description

Linear polarization microwave thermoacoustic imaging method and device

technical field

The invention belongs to the technical field of microwave thermoacoustic imaging, and particularly relates to a linear polarization microwave thermoacoustic tomography imaging device and a method thereof.

Background technique

Microwave thermoacoustic imaging technology is a new type of non-destructive testing technology with wide application potential and good application prospects. Microwave thermoacoustic imaging technology combines the advantages of high resolution of ultrasonic imaging and high contrast of microwave imaging. This technology can perform high-resolution and high-contrast imaging of the distribution of dielectric properties of biological tissue. When radiated to biological tissue, the biological tissue will absorb microwave energy and convert it into internal energy. The instantaneous temperature rise of the tissue causes thermal expansion on the body. During the thermal expansion process, a pressure wave will be generated, that is, the ultrasonic wave will diffuse around the tissue, and the generated ultrasonic wave will be absorbed by the tissue. The ultrasonic transducer detects and intercepts, and through the acquisition of ultrasonic signals, an image of the difference in microwave absorption inside the tissue is inverted. Typically unpolarized pulsed microwaves are used as the excitation source, assuming that the target is dielectric isotropy. However, some biological tissues have been reported to display pronounced dielectric anisotropy along the long axis. Conventional thermoacoustic imaging is not sufficient to represent the anisotropic properties of this tissue, which limits its application in biological and materials fields. In this work, we propose a polarized microwave thermoacoustic imaging system (PMTA) capable of quantifying the degree of microscopic anisotropy of a target with newly proposed parameter values from 0 to 1 according to its microwave absorption properties. Using acoustic waves that absorb polarized microwaves provides a straightforward strategy for tissue polarization measurements.

SUMMARY OF THE INVENTION

The primary purpose of the present invention is to overcome the shortcomings and deficiencies of the existing methods, and to provide a PMTA method for quantitatively detecting the microscopic anisotropy of the target. In order to solve the above-mentioned technical problems, the technical scheme of the present invention is: by applying four linearly polarized microwaves As an excitation source, the microscopic anisotropy of the target is quantitatively detected based on vector microwave absorption. According to its microwave absorption properties, parameter values from 0 to 1 obtained by the newly proposed formula are used to quantify the degree of microscopic anisotropy of the target, which can provide a direct strategy for tissue polarization measurements by absorbing acoustic waves of polarized microwaves.

Another aspect of the present invention also provides a device system that can be used for linearly polarized microwave thermoacoustic imaging.

Description of drawings:

FIG. 1 is a schematic structural diagram of a linearly polarized microwave thermoacoustic tomography apparatus according to the present invention. The figure includes a computer, a data acquisition system, a preamplifier, an ultrasonic transducer, and a microwave excitation source. Microwave transmitting antenna.

FIG. 2 is an experimental diagram for verifying the feasibility of PMTA according to the present invention.

Detailed ways:

取向之间的角度分别为0，45，90，135，分别进行热声信号采集，通过应用四个线偏振微波作为激发源，基于矢量微波吸收定量检测目标的微观各向异性，利用MATLAB软件程序通过画弧投影-最大值投影算法得到所测样品的微波吸收重建图像，而本发明也首次提出重建图像所采用的公式是：

得到该公式的主要原理过程为：当介电各向异性靶被线偏振微波激发时，通过假设靶是单轴的，微波和各向异性靶之间的相互作用强烈地取决于线偏振微波取向φ和靶的目标轴

取向之间的角度。这导致平行于和垂直于目标轴的方向之间的微波吸收的明显差异，偏振取向依赖的微波吸收，然后可以将介电各向异性靶的与偏振方向相关的微波吸收系数写成As shown in FIG. 1 , the present invention provides a linearly polarized microwave thermoacoustic imaging device, which includes a computer connected in sequence, a microwave excitation source, a coaxial cable, a microwave antenna that is a dipole antenna, a sample oil tank to be tested, an ultrasonic The transducer, signal amplifier, and data acquisition system are also high-speed data acquisition cards. The computer has working software for controlling the pulsed microwave generator system, as well as image reconstruction software, as well as data acquisition control program software Labview, and the image reconstruction software is MATLAB software; A linearly polarized microwave with a pulse width of 550ns, a center frequency of 3GHz, a repetition frequency of 10Hz, and a peak power of 70Kw is used as the microwave excitation source, and the emitted microwave is coupled to a dipole antenna through a coaxial cable, and a linearly polarized microwave is emitted, and the dipole antenna Length 60mm, width 60mm, height 45mm; ultrasonic transducer aperture size is 120mm, main frequency is 5MHz, 60% bandwidth, sampling rate of high-speed data acquisition card is 50MHz, with 2 channels of data, directly above the dipole antenna is the oil tank, the oil tank There is an ultrasonic coupling liquid in it. When measuring the thermoacoustic signal of the sample, the sample is immersed and fixed in the ultrasonic coupling liquid in the oil tank, and the microwave pulse antenna is fixed directly under the sample; the pulsed microwave emitted by the dipole antenna is radiated on the sample; The energizer is immersed in the ultrasonic decoupling fluid to receive the thermoacoustic signal excited by the pulsed microwave. The ultrasonic coupling fluid is transformer oil at room temperature; the LABVIEW software in the computer can control the data acquisition platform, and the MATLAB software program performs data processing. ; The thermoacoustic signal generated by the detection pulse collected by the high-speed digital acquisition card, and by rotating the dipole antenna, linearly polarized microwaves with similar energy densities in different polarization directions are obtained, and the microwave polarization orientation φ and sample axis are changed.

The angles between the orientations are 0, 45, 90, and 135, respectively, and the thermoacoustic signals are collected respectively. By applying four linearly polarized microwaves as excitation sources, the microscopic anisotropy of the target is quantitatively detected based on vector microwave absorption, and the MATLAB software program is used. The microwave absorption reconstruction image of the measured sample is obtained by the arc projection-maximum projection algorithm, and the present invention also proposes for the first time that the formula used to reconstruct the image is:

The main principle process for obtaining this formula is: when a dielectric anisotropic target is excited by a linearly polarized microwave, by assuming that the target is uniaxial, the interaction between the microwave and the anisotropic target strongly depends on the linearly polarized microwave orientation φ and the target axis of the target

The angle between orientations. This results in a clear difference in microwave absorption between directions parallel to and perpendicular to the target axis, polarization orientation dependent microwave absorption, and the polarization orientation dependent microwave absorption coefficient for a dielectric anisotropic target can then be written as

Here, ε _|| and _ε⊥ are the absorption coefficients in the directions parallel and perpendicular to the target axis, respectively. For an isotropic target satisfying ε _|| =ε _⊥ , equation (1) is simplified to the PMTA imaging principle, and the absorption constant is

因此，线偏振微波激发的各向异性目标的TA信号幅度可写为To simplify the problem, we define

Therefore, the TA signal amplitude of an anisotropic target excited by a linearly polarized microwave can be written as

Here, F(φ) is the energy density of linearly polarized microwaves, Γ is the Green Eisen parameter, and η is the thermal conversion efficiency. The above formula shows that for incident line-polarized microwaves with a certain microwave polarization direction, the TA signal amplitude is highly correlated with θ , which can be used to extract structural features of anisotropic materials; to quantify the anisotropy of the target, a new parameter is proposed in PMTA by applying four linearly polarized microwaves as excitation sources. According to the Stokes form, the polarization state of the microwave can be described by the Stokes vector as

Among them, four Stokes parameters are represented by six microwave intensities with different orientations. I _H and _IV are horizontal (0°) and vertical (90°) linear polarizations; _IP and _IM are 45° and -45° linear polarizations; _IR and _IL are left and right circular polarizations. Then, the degree of linear polarization (DLP) of the microwave can be defined as

Similar to Stokes formalism, and assuming that the target is uniaxial, the degree of anisotropy (DOA) of the target can be defined as

where Q _TA =I _H-TA -I _V-TA , U _TA = _IP-TA -I _M-TA , and I _TA =I _H-TA +I _V-TA . _IH-TA , _IV-TA , _IP-TA and _IM-TA correspond to the TA signal amplitudes excited by linearly polarized microwaves with polarization directions of 0°, 90°, 45° and 135°, respectively. For single-axis targets, DOA has a value between 0 and 1. Therefore, the anisotropy of the target can be quantified by DOA. Through the aforementioned detection and calculation principle, the linearly polarized microwave thermoacoustic measurement method for quantitatively detecting the microscopic anisotropy of the target of the present invention comprises the following steps:

(1) The linearly polarized microwave transmits a linearly polarized pulsed microwave with a center frequency of 3 GHz and a repetition frequency of 10 GHz through a coaxial cable coupling antenna, and the linearly polarized pulsed microwave is adjusted and output through a dipole antenna with a length of 60 mm, a width of 60 mm and a height of 45 mm;

(2) The linearly polarized microwave can achieve a microwave pulse output repetition frequency of 10HZ through the LabView control panel;

取向之间的角度为0度,样品受热膨胀，激发热声信号；(3) Set microwave polarization orientation φ and sample axis

The angle between the orientations is 0 degrees, the sample is thermally expanded, and the thermoacoustic signal is excited;

(4) The thermoacoustic signal is detected by the ultrasonic transducer through the coupling liquid, then amplified by the signal amplifier, and then collected by the high-speed digital acquisition card and saved to the computer. During this process, the computer controls the stepper motor to move the sample point by point, and each time the stepper motor moves, the high-speed digital acquisition card performs a signal acquisition.

取向之间的角度分别为45，90，135，重复(4)进行热声信号采集。(5) Change the microwave polarization orientation φ and the sample axis

The angles between the orientations were 45, 90, and 135, respectively, and (4) was repeated for thermoacoustic signal acquisition.

(6) Process and calculate the collected data on the computer. The first step of the calculation method is to use arc projection for each row, and in the second step, each row is projected to the maximum value to obtain a thermoacoustic reconstruction image, as shown in Figure 2(b) shown. Finally, use the obtained data of 0, 45, 90, 135 degrees with the formula

其中，Q_TA＝I_H-TA-I_V-TA，U_TA＝I_P-TA-I_M-TA，和I_TA＝I_H-TA+I_V-TA。I_H-TA，I_V-TA，I_P-TA和I_M-TA对应于线性偏振微波激发的TA信号幅度，偏振方向分别为0°，90°，45°和135°。

where Q _TA =I _H-TA -I _V-TA , U _TA = _IP-TA -I _M-TA , and I _TA =I _H-TA +I _V-TA . _IH-TA , _IV-TA , _IP-TA and _IM-TA correspond to the TA signal amplitudes excited by linearly polarized microwaves with polarization directions of 0°, 90°, 45° and 135°, respectively.

Get the image of the degree of anisotropy, that is, the DOA image.

偏振方向，图2b为线偏振微波角度分别为0度,45度,90度,135度,180度的热声重建图，图中可以看出，样品的热声重建随着角度的变化而有所变化，图2(c)为图2(b)的热声信号统计结果，图2c的5个点的热声信号统计值分别对应图2b的5个角度的热声重建图，从结果上更是清楚看出不同线偏振角度下的热声幅值的变化差异，图2d的各向异性度图是由图2b的0度,45度,90度,135度的热声图像重建图用我们推导出的各向异性度参数公式

得到，从图中可以看出碳纤维的各向异性度为0.75。In order to prove the feasibility, the present invention also provides imaging of carbon fibers with strong dielectric anisotropy under the excitation of a series of linearly polarized microwaves with unpolarized orientations. Figure 2(a) shows a sample photo. The black dotted line is the principal axis direction of the carbon fiber sample and the double arrows refer to the electric field vector of the incident ray polarized microwave

Polarization direction, Figure 2b is the thermoacoustic reconstruction diagram of the linearly polarized microwave angles of 0 degrees, 45 degrees, 90 degrees, 135 degrees, and 180 degrees. It can be seen from the figure that the thermoacoustic reconstruction of the sample varies with the angle. Figure 2(c) shows the statistical results of the thermoacoustic signals in Figure 2(b). The statistical values of the thermoacoustic signals at the five points in Figure 2c correspond to the thermoacoustic reconstruction maps of the five angles in Figure 2b, respectively. From the results It is even more clear to see the difference in the thermoacoustic amplitudes under different linear polarization angles. The anisotropy map in Figure 2d is reconstructed from the thermoacoustic images of 0, 45, 90, and 135 degrees in Figure 2b. The anisotropy parameter formula we derived

As a result, it can be seen from the figure that the degree of anisotropy of the carbon fiber is 0.75.

In summary, we propose a polarized microwave thermoacoustic imaging (PMTA) to quantitatively detect the microscopic anisotropy of a target by applying four linearly polarized microwaves as excitation sources. Compared with conventional microwave thermoacoustic imaging setups that treat the target as an isotropic absorber, PMTA allows us to quantitatively detect the anisotropic features of the target, with the newly proposed parameter values between 0 and 1 on top of microwave absorption, capable of The newly proposed parameter values 0 to 1 are used to quantify the degree of microscopic anisotropy of the target. The feasibility of this method is verified by dielectric anisotropy samples, and the PMTA method provides an efficient and straightforward strategy for tissue polarization measurements, presetting a great potential for bioimaging and material inspection.

The above description is only a preferred embodiment of the present invention, but the protection scope of the present invention is not limited to this. The equivalent replacement or change of the inventive concept thereof shall be included within the protection scope of the present invention.

Claims (9)

1. A method for detecting microscopic anisotropy of a sample by linear polarization microwave thermoacoustic imaging, the method comprising the steps of:

(1) the computer controls a microwave excitation source to emit linear polarization pulse microwaves with the center frequency of 3GHz, the pulse width of 550ns, the repetition frequency of 10Hz and the peak power of 70Kw, the linear polarization pulse microwaves are transmitted to a dipole antenna through a coaxial cable, and the linear polarization pulse microwaves are adjusted by the dipole antenna with the length of 60mm, the width of 60mm and the height of 45mm and then output radiation to a sample to be detected placed in an oil tank; ultrasonic coupling liquid is arranged in the oil groove, linear polarization pulse microwaves irradiate on sample tissues, and the sample expands under heating and is excited to generate a thermoacoustic signal;

(2) controlling the polarization orientation of linearly polarized pulsed microwaves

And sample axis orientation

The angle between the two is 0 degree, the sample expands under heating, and a thermoacoustic signal is excited, and the thermoacoustic signal P (theta) is expressed by the following formula:

wherein epsilon_||And ε_⊥The microwave absorption coefficients of the sample in the directions parallel and perpendicular to the sample axis respectively,

，

is the energy density of linearly polarized pulsed microwaves, Γ is the glehnson parameter,η _th is the heat conversion efficiency;

(3) the thermoacoustic signals are detected by the ultrasonic transducer through the coupling liquid, the ultrasonic transducer transmits the signals into the signal amplifier, the signals are amplified by the signal amplifier and then collected by the high-speed digital collection card and stored in the computer, the computer controls the stepping motor to move the sample point by point, and the high-speed digital collection card collects the signals once when the stepping motor moves every time;

(4) changing linear polarization pulse microwave orientation

And sample axis orientation

The angles between the two are respectively 45 degrees, 90 degrees and 135 degrees, and the step (3) is repeated to collect thermoacoustic signals;

(5) processing and calculating the acquired data on a computer, obtaining thermoacoustic reconstruction images of the sample by using MATLAB through arc projection and maximum projection algorithms, respectively extracting thermoacoustic signal amplitudes at different angles, and calculating to obtain an anisotropy figure of the sample, wherein the anisotropy is calculated by adopting the following formula:

wherein Q_TA＝I_H-TA-I_V-TA，U_TA＝I_P-TA-I_M-TAAnd I and_TA＝I_H-TA+I_V-TA；I_H-TA，I_V-TA，I_P-TAand I_M-TARespectively corresponding to the microwave orientation of linearly polarized pulses

And sample axis orientation

At angles of 0 °,90 °,45 ° and 135 ° therebetweenAmplitude of thermoacoustic signals excited by linearly polarized pulsed microwaves; for uniaxial samples, the value of DOA is between 0 and 1.

2. The method of claim 1, wherein: the microwave pulse output repetition frequency of 10Hz in the step (1) is realized by a LabView control panel.

3. The method of claim 1, wherein: the signal acquisition in step (3) adopts a real-time acquisition system controlled by LABVIEW software, and can correspond the excited area to the data acquired in the area one by one.

4. A linearly polarized microwave thermoacoustic imaging device implementing the method of any of claims 1-3, characterized by: the device comprises a main control computer, a 3GHz microwave excitation source and a dipole antenna, wherein the dipole antenna is positioned right below a sample in an oil tank, ultrasonic coupling liquid is arranged in the oil tank, the sample is immersed in the ultrasonic coupling liquid and is fixed above the dipole antenna, and an ultrasonic transducer invades into the coupling liquid and contacts with the sample; the high-speed double-channel digital acquisition card and the main control computer form a thermoacoustic signal acquisition and data processing component, the dipole antenna can be rotated, linear polarization microwaves with similar energy densities in different polarization directions are obtained by rotation, and the polarization orientation of the microwaves is changed

And sample axis orientation

The angles between the two are respectively 0 degrees, 45 degrees, 90 degrees and 135 degrees, and the thermo-acoustic signal acquisition is respectively carried out.

5. The apparatus of claim 4, wherein: the aperture size of the ultrasonic transducer is 120mm, the main frequency is 5MHz, and the bandwidth is 60%.

6. The apparatus of claim 4, wherein: the sampling rate of the high-speed double-channel digital acquisition card is 50MHz, and the high-speed double-channel digital acquisition card is provided with 2 data channels.

7. The apparatus of claim 4, wherein: the acquisition control program and the signal processing program installed on the main control computer are compiled by Labview and Matlab programs.

8. The apparatus of claim 4, wherein: the center frequency of pulse microwaves emitted by the microwave excitation source is 3GHz, the pulse width is 550ns, and the repetition frequency is 10 Hz.

9. The apparatus of claim 4, wherein: the ultrasonic coupling liquid filled in the oil groove is transformer oil at room temperature.

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