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

Linear polarization microwave thermoacoustic imaging method and device Download PDF

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CN112782280B
CN112782280B CN201911085075.3A CN201911085075A CN112782280B CN 112782280 B CN112782280 B CN 112782280B CN 201911085075 A CN201911085075 A CN 201911085075A CN 112782280 B CN112782280 B CN 112782280B
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CN112782280A (en
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覃欢
李昱静
邢达
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South China Normal University
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/04—Analysing solids
    • G01N29/06—Visualisation of the interior, e.g. acoustic microscopy
    • G01N29/0654—Imaging
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/0093—Detecting, measuring or recording by applying one single type of energy and measuring its conversion into another type of energy
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/05—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves 
    • A61B5/0507—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves  using microwaves or terahertz waves
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N22/00—Investigating or analysing materials by the use of microwaves or radio waves, i.e. electromagnetic waves with a wavelength of one millimetre or more
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/22—Details, e.g. general constructional or apparatus details
    • G01N29/24—Probes
    • G01N29/2431—Probes using other means for acoustic excitation, e.g. heat, microwaves, electron beams

Abstract

The invention provides a linear polarization microwave thermoacoustic measurement method and a device for quantitatively detecting the dielectric anisotropy of a target, wherein four linear polarization microwaves are used as excitation sources, the newly proposed parameter value on microwave absorption is between 0 and 1, and the newly proposed parameter value 0 to 1 can be used for quantifying the microscopic anisotropy degree of the target according to the microwave absorption characteristics. The feasibility of the method is verified by a dielectric anisotropy sample, the method provides an effective and direct strategy for tissue polarization measurement, and great potential is preset for biological imaging and material inspection. And also provides an effective method for microwave thermoacoustic measurement of microscopic anisotropy of a detection 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 device and a method thereof.
Background
The microwave thermoacoustic imaging technology is a novel nondestructive testing technology with wide application potential and good application prospect. The microwave thermoacoustic imaging technology has the advantages of high resolution of ultrasonic imaging and high contrast of microwave imaging, the technology performs high resolution and high contrast imaging on dielectric property distribution of biological tissues, the microwave thermoacoustic imaging technology radiates non-ionizing radiation pulse microwaves onto the biological tissues, the biological tissues can absorb the microwave energy and convert the microwave energy into internal energy, the tissue instantaneous temperature rise causes thermal expansion on the body, pressure waves, namely the ultrasonic waves, are generated in the thermal expansion process and spread to the periphery of the tissues, the generated ultrasonic waves can be detected and intercepted by a tissue ultrasonic transducer, and the ultrasonic signal acquisition is used for inverting the microwave absorption difference images in the tissues. Unpolarized pulsed microwaves are commonly used as excitation source, assuming the target is dielectric isotropy. However, it has been reported that some biological tissues exhibit a significant dielectric anisotropy in the long axis. Conventional thermo-acoustic imaging is not sufficient to exhibit the anisotropic nature of such tissue, which limits its application in biological and materials fields. In this work, we propose a polarized microwave thermoacoustic imaging system (PMTA) that can quantify the microscopic degree of anisotropy of a target based on its microwave absorption properties using newly proposed parameter values of 0 to 1. Using acoustic waves that absorb polarized microwaves can provide a straightforward strategy for tissue polarization measurements.
Disclosure of Invention
The invention aims to overcome the defects of the existing method and provide a PMTA method for quantitatively detecting the microscopic anisotropy of a target, and in order to solve the technical problems, the technical scheme of the invention is as follows: the microscopic anisotropy of the target was quantitatively detected based on vector microwave absorption by applying four linearly polarized microwaves as excitation sources. According to the microwave absorption characteristic, the degree of the microscopic anisotropy of the target is quantified by using the parameter values 0 to 1 obtained by a newly proposed formula, and a direct strategy can be provided for tissue polarization measurement by using the sound wave absorbing polarized microwaves.
It is also another aspect of the present invention to provide a device system that may be used for linearly polarized microwave thermoacoustic imaging.
Description of the drawings:
FIG. 1 is a schematic structural diagram of a linear polarization microwave thermoacoustic tomography apparatus according to the present invention. The figure comprises a computer, a data acquisition system, a preamplifier, an ultrasonic transducer and a microwave excitation source. A microwave transmitting antenna.
FIG. 2 is an experimental diagram of the present invention demonstrating the feasibility of PMTA.
The specific implementation mode is as follows:
as shown in FIG. 1, the present invention provides a linearly polarized microwave thermoacoustic imaging device, which comprises a computer, a microwave excitation source, a coaxial cable, a microwave antenna (dipole antenna), and sample oil to be measuredThe computer is provided with working software for controlling a pulse microwave generator system, image reconstruction software and data acquisition control program software Labview, and the image reconstruction software is MATLAB software; linear polarization microwave with the pulse width of 550ns, the center frequency of 3GHz, the repetition frequency of 10Hz and the peak power of 70Kw is used as a microwave excitation source, the emitted microwave is coupled to a dipole antenna through a coaxial cable to emit the linear polarization microwave, and the dipole antenna has the length of 60mm, the width of 60mm and the height of 45 mm; the aperture size of the ultrasonic transducer is 120mm, the main frequency is 5MHz, the bandwidth is 60%, the sampling rate of the high-speed data acquisition card is 50MHz, and the high-speed data acquisition card has 2 channels of data, an oil groove is arranged right above the dipole antenna, ultrasonic coupling liquid is arranged in the oil groove, when the thermoacoustic signal measurement of the sample is carried out, the sample is immersed and fixed in the ultrasonic coupling liquid in the oil groove, and the microwave pulse antenna is fixed right below the sample; pulsed microwaves emitted by the dipole antenna are radiated on the sample; the ultrasonic transducer is immersed in ultrasonic coupling liquid for receiving a thermoacoustic signal excited by pulse microwave of a sample, and the ultrasonic coupling liquid is transformer oil at room temperature; LABVIEW software in a computer can control a data acquisition platform and an MATLAB software program to perform data processing; the thermoacoustic signal generated by the detection pulse acquired by the high-speed digital acquisition card is obtained by rotating the dipole antenna to obtain linearly polarized microwaves with similar energy densities in different polarization directions, and the polarization orientation phi of the microwaves and the sample axis are changed
Figure BDA0002264837310000032
Angles among orientations are respectively 0, 45, 90 and 135, thermoacoustic signal acquisition is respectively carried out, four linear polarization microwaves are used as excitation sources, the microscopic anisotropy of a target is quantitatively detected based on vector microwave absorption, an MATLAB software program is utilized to obtain a microwave absorption reconstruction image of a measured sample through an arc projection-maximum projection algorithm, and the formula adopted for reconstructing the image is also provided for the first time in the invention as follows:
Figure BDA0002264837310000031
obtain the formulaThe main principle process is as follows: when a dielectric anisotropic target is excited by linearly polarized microwaves, the interaction between the microwaves and the anisotropic target strongly depends on the linearly polarized microwave orientation phi and the target axis of the target by assuming that the target is uniaxial
Figure BDA0002264837310000033
The angle between the orientations. This results in a significant difference in microwave absorption between directions parallel and perpendicular to the target axis, polarization orientation dependent microwave absorption, and then the polarization direction dependent microwave absorption coefficient of the dielectric anisotropic target can be written as
Figure BDA0002264837310000041
Here,. epsilon||And ε⊥The absorption coefficients in the directions parallel and perpendicular to the target axis, respectively. For satisfying epsilon||＝ε⊥Equation (1) reduces to the PMTA imaging principle, with an absorption constant of
Figure BDA0002264837310000042
To simplify the problem, we define
Figure BDA0002264837310000043
Therefore, the TA signal amplitude of a linearly polarized microwave excited anisotropic target can be written as
Figure BDA0002264837310000044
Here, F (Φ) is the energy density of the linearly polarized microwaves, Γ is the gleneson parameter, η is the thermal conversion efficiency, the above equation indicates that for incident linearly polarized microwaves having a certain microwave polarization direction, the TA signal amplitude is highly correlated with θ, and can be used to extract the structural features of anisotropic materials; to quantify the anisotropy of the target, a new parameter was proposed in PMTA by applying four linearly polarized microwaves as excitation sources. According to the Stokes form, the polarization state of a microwave can be described by a Stokes vector
Figure BDA0002264837310000045
Wherein the four stokes parameters are represented by six microwave intensities having different orientations. I isHAnd IVAre horizontally (0 °) and vertically (90 °) linear polarizations; i isPAnd IMAre 45 ° and-45 ° linear polarizations; i isRAnd ILIs left-right circular polarization. Then, the linear polarization Degree (DLP) of the microwave can be defined as
Figure BDA0002264837310000051
Similar to the Stokes formalism, and assuming that the target is uniaxial, the degree of anisotropy (DOA) of the target can be defined as
Figure BDA0002264837310000052
Wherein Q isTA＝IH-TA-IV-TA，UTA＝IP-TA-IM-TAAnd I andTA＝IH-TA+IV-TA。IH-TA，IV-TA，IP-TAand IM-TAThe polarization directions correspond to the amplitude of the linearly polarized microwave excited TA signal at 0 °,90 °,45 ° and 135 °, respectively. For a uniaxial target, the value of DOA is between 0 and 1. Thus, the anisotropy of the target can be quantified by DOA. By the detection and calculation principle, the linear polarization microwave thermoacoustic measurement method for quantitatively detecting the microscopic anisotropy of the target comprises the following steps of:
(1) the linear polarization microwave transmits linear polarization pulse microwave with the center frequency of 3GHz and the repetition frequency of 10GHz through a coaxial cable coupling antenna, and the linear polarization pulse microwave is output after being adjusted by a dipole antenna with the length of 60mm, the width of 60mm and the height of 45 mm;
(2) the linear polarization microwave can realize the microwave pulse output repetition frequency of 10HZ through a LabView control panel;
(3) setting microwave polarization orientation phi and sample axis
Figure BDA0002264837310000053
The angle between the orientations is 0 degree, and the sample expands when heated to excite a thermoacoustic signal;
(4) the thermoacoustic signals are detected by the ultrasonic transducer through the coupling liquid, amplified by the signal amplifier, collected by the high-speed digital acquisition card and stored in the computer. In the process, the computer controls the stepping motor to move the sample point by point, and the high-speed digital acquisition card acquires a signal each time the stepping motor moves.
(5) Changing the microwave polarization orientation phi and the sample axis
Figure BDA0002264837310000063
The angles between the orientations are 45, 90 and 135 respectively, and the thermo-acoustic signal acquisition is repeated (4).
(6) The acquired data are processed and calculated on a computer, and the calculation method comprises the steps of performing arc projection on each line in the first step, and performing maximum projection on each line in the second step to obtain a thermoacoustic reconstructed image, as shown in fig. 2 (b). Finally, the obtained data of 0, 45, 90 and 135 degrees is used for formula
Figure BDA0002264837310000061
Wherein Q isTA＝IH-TA-IV-TA，UTA＝IP-TA-IM-TAAnd I andTA＝IH-TA+IV-TA。IH-TA，IV-TA，IP-TAand IM-TAThe polarization directions correspond to the amplitude of the linearly polarized microwave excited TA signal at 0 °,90 °,45 ° and 135 °, respectively.
An image of the degree of anisotropy, namely the DOA image, is obtained.
The invention is to proveWhile the carbon fiber having strong dielectric anisotropy properties is imaged under excitation of a series of linearly polarized microwaves having unpolarized orientation, fig. 2(a) shows a photograph of the sample. The black dotted line is the main axis direction of the carbon fiber sample and the double arrow indicates the electric field vector of the incident linearly polarized microwave
Figure BDA0002264837310000064
Polarization direction, FIG. 2b is thermoacoustic reconstruction graphs with linearly polarized microwave angles of 0 degree, 45 degrees, 90 degrees, 135 degrees and 180 degrees respectively, it can be seen that thermoacoustic reconstruction of a sample changes with the change of the angles, FIG. 2(c) is the statistical result of thermoacoustic signals of FIG. 2(b), the statistical values of thermoacoustic signals of 5 points of FIG. 2c respectively correspond to the thermoacoustic reconstruction graphs of 5 angles of FIG. 2b, from the result, the variation difference of thermoacoustic amplitude values under different linearly polarized angles is clearly seen, and the anisotropy graph of FIG. 2d is the thermoacoustic image reconstruction graph derived from the anisotropy parameter formulas of 0 degree, 45 degrees, 90 degrees and 135 degrees of FIG. 2b
Figure BDA0002264837310000062
As a result, it can be seen from the figure that the degree of anisotropy of the carbon fiber was 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 to conventional microwave thermoacoustic imaging devices, which treat the target as an isotropic absorber, PMTA allows us to quantitatively detect the anisotropic characteristics of the target, with newly proposed parameter values between 0 and 1 above the microwave absorption, which enables quantification of the microscopic degree of anisotropy of the target with newly proposed parameter values of 0 to 1. The feasibility of the method is verified by a dielectric anisotropy sample, and the PMTA method provides an effective and direct strategy for tissue polarization measurement, and has great potential for biological imaging and material inspection.
The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art should be considered to be within the technical scope of the present invention, and the technical solutions and the inventive concepts thereof according to the present invention should be equivalent or changed within the 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
Figure 36099DEST_PATH_IMAGE002
And sample axis orientation
Figure DEST_PATH_IMAGE002
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:
Figure DEST_PATH_IMAGE004
wherein epsilon||And ε⊥The microwave absorption coefficients of the sample in the directions parallel and perpendicular to the sample axis respectively,
Figure DEST_PATH_IMAGE006
，
Figure DEST_PATH_IMAGE008
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
Figure 687660DEST_PATH_IMAGE002
And sample axis orientation
Figure 258618DEST_PATH_IMAGE002
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:
Figure DEST_PATH_IMAGE010
wherein QTA＝IH-TA-IV-TA，UTA＝IP-TA-IM-TAAnd I andTA＝IH-TA+IV-TA；IH-TA，IV-TA，IP-TAand IM-TARespectively corresponding to the microwave orientation of linearly polarized pulses
Figure DEST_PATH_IMAGE003
And sample axis orientation
Figure DEST_PATH_IMAGE011
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
Figure 529714DEST_PATH_IMAGE002
And sample axis orientation
Figure 582283DEST_PATH_IMAGE011
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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