A full-polarization photoacoustic imaging method and device based on vector absorption sphere

Abstract

The present invention discloses a full-polarization photoacoustic imaging method and device based on a vector absorption sphere. The method includes the following steps: expressing the light absorption characteristics of a sample by establishing a mathematical model of the vector absorption sphere; measuring the absorption intensity of the sample absorber under different polarization states of excitation light using photoacoustic signals; reconstructing the vector absorption sphere at each point of the sample based on the absorption intensity under different polarization states of excitation light; extracting five structural parameters from the vector absorption sphere at each point of the sample to reconstruct a structural characteristic diagram of the sample; and the full-polarization photoacoustic imaging device based on the vector absorption sphere includes a polarization pulse light modulation excitation module, a signal detection module, a two-dimensional scanning module, and a signal processing module. The method proposed in the present invention overcomes the inability of traditional polarization photoacoustic imaging to fully express the polarization absorption of a sample, and achieves the measurement of full polarization absorption characteristics and the complete structural expression of the corresponding sample within the same optical path.

Description

Technical Field

[0001] The present invention belongs to the technical field of photoacoustic imaging, and in particular relates to a full-polarization photoacoustic imaging method based on a vector absorption sphere and an imaging device thereof. Background Art

[0002] Polarized photoacoustic imaging offers the advantages of high resolution, high contrast, and deep penetration. Polarized light excites photoacoustic signals, enabling the analysis of a sample's structural information. However, conventional polarized photoacoustic imaging cannot fully capture this structural information and cannot measure a sample's absorption characteristics for both linearly and circularly polarized light within the same optical system.

[0003] Traditional polarization photoacoustic imaging can only analyze the linear dichroism or circular dichroism of a sample. However, the internal structure of natural samples, including biological samples and material samples, often exhibits both linear and circular dichroism. Traditional polarization photoacoustic imaging cannot analyze the overall dichroism that includes both, and therefore has limited expression of the sample's structure. Summary of the Invention

[0004] The main purpose of the present invention is to overcome the shortcomings and deficiencies of the existing technology and provide a full-polarization photoacoustic imaging method and imaging device based on a vector absorption sphere. By using eight beams of excitation light in the same optical path system, the absorption characteristics of the sample can be fully reconstructed, which is expected to deeply analyze the interaction between matter and light, and has potential application prospects in multiple fields such as biological imaging and material detection.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] In a first aspect, the present invention provides a method for full-polarization photoacoustic imaging based on a vector absorption sphere, the method comprising the following steps:

[0007] Step S1, establishing a vector absorption sphere mathematical model to express the light absorption characteristics of the sample, and proposing to characterize the structural characteristics of the sample with five structural characteristic parameters: total absorption intensity A, overall order O, circular dichroism CD, linear dichroism LD, and molecular transition dipole moment OTDM;

[0008] Step S2: The pulsed laser generated by the laser is sequentially passed through a linear polarizer and a rotating quarter-wave plate to modulate the laser into excitation light of different polarization states, which is then focused onto a single point on the sample through an objective lens, and the photoacoustic signal intensity corresponding to the excitation light of each polarization state is measured;

[0009] Step S3, performing a two-dimensional scan on the sample, repeating step S2 to obtain the multi-polarization state photoacoustic signal at each scanning point, and reconstructing the vector absorption sphere at each point of the sample based on the signal;

[0010] Step S4, extracting the structural characteristic parameters of the vector absorption sphere at each point, and reconstructing the total absorption intensity distribution map, overall order distribution map, circular dichroism distribution map, linear dichroism distribution map and molecular transition dipole moment distribution map of the sample.

[0011] In a second aspect, the present invention provides a full-polarization photoacoustic imaging device based on a vector absorption sphere, comprising a polarization pulse light modulation excitation module, a signal detection module, a two-dimensional scanning module, and a signal processing module;

[0012] The polarized pulse light modulation excitation module includes a pulse laser, an energy regulator, a collimating beam expander, a linear polarizer, a quarter wave plate, and an objective lens, and the pulse laser, energy regulator, collimating beam expander, linear polarizer, quarter wave plate, and objective lens are connected in sequence;

[0013] The signal detection module includes a transducer, a filter, an amplifier, and an acquisition card, and the transducer, filter, amplifier, and acquisition card are connected in sequence;

[0014] The two-dimensional scanning module includes an FPGA and a two-dimensional displacement platform, the FPGA and the two-dimensional displacement platform are connected in sequence, and the FPGA is also connected to a pulse laser and an acquisition card respectively;

[0015] The signal processing module includes a computer, which is connected to the acquisition card

[0016] As a preferred technical solution, in the polarized pulse light modulation excitation module, the pulse light beam emitted by the pulse laser passes through the energy regulator and the collimating beam expander in sequence to obtain parallel incident light pulse light with a better light spot, the parallel incident light pulse light passes through the linear polarizer to become horizontally polarized light, the horizontally polarized light passes through a rotating quarter-wave plate whose initial optical axis is parallel to it to form excitation light of different polarization states, and eight beams of excitation light of different polarization states are formed, with the angles between the optical axis of the quarter-wave plate and the horizontal polarized light being 0°, 22.5°, 45°, 67.5°, 90°, 112.5°, 135°, and 157.5°, respectively, and are focused by the objective lens to excite the sample;

[0017] The energy controller is composed of a polarizer, a half-wave plate and a polarization beam splitter;

[0018] The collimating beam expander consists of a first lens, a stop and a second lens.

[0019] As an optimal technical solution, in the signal detection module, the photoacoustic signal generated by the excitation light of different polarization states being focused by the objective lens to excite the sample is collected by the transducer, and the sound pressure signal is converted into an electrical signal. The electrical signal is filtered and amplified by the filter and amplifier and then collected and stored by the acquisition card.

[0020] As a preferred technical solution, FPGA controls the pulse laser to emit a pulse beam, and at the same time controls the acquisition card to collect the photoacoustic signal generated by the pulse light excitation sample and the movement of the two-dimensional displacement platform, thereby realizing two-dimensional scanning of the sample.

[0021] As a preferred technical solution, in the signal processing module, a computer processes the photoacoustic signals collected by the acquisition card and reconstructs the vector absorption sphere of each point of the sample and the structural characteristic diagram of the sample.

[0022] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0023] This method intuitively expresses the vectorial nature of the sample's absorption of photons by establishing a vector absorption sphere theoretical model, and proposes five structural characteristic parameters, namely total absorption intensity A, overall order O, circular dichroism CD, linear dichroism LD, and molecular transition dipole moment OTDM, to describe the structural characteristics of the sample. By modulating polarized excitation light with different polarization states, the photoacoustic signal intensity of the sample at that point under different polarization excitation light is obtained, and by two-dimensional scanning, the photoacoustic signal intensity of each point of the sample under different polarization excitation light is obtained, thereby reconstructing the vector absorption sphere of each point of the sample and obtaining a structural characteristic parameter map of the sample. The present invention overcomes the problem that traditional photoacoustic imaging cannot fully express the polarization absorption characteristics of the sample, and proposes full-polarization photoacoustic imaging based on vector absorption sphere, which can fully express the polarization absorption characteristics of the sample to light, providing a new way to completely analyze the absorption process of the sample's interaction with light, and is expected to play an important role in multiple fields such as biological imaging and material testing.

[0024] The present invention has the ability to measure full polarization absorption in the same optical path. The device for implementing the method has a simple structure and is easy to use, and can be widely used in fields such as internal structure detection of organisms and precision machinery. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0026] Figure 1 Schematic diagram of the process of a full-polarization photoacoustic imaging method based on a vector absorption sphere according to Example 1 of the present invention;

[0027] Figure 2 This is a picture of a vector absorption sphere model at a certain point on a polarizer sample in a full-polarization photoacoustic imaging method based on a vector absorption sphere in Example 1 of the present invention;

[0028] Figure 3The corresponding vector absorption sphere images are reconstructed by selecting several groups of characteristic points in a polarizer sample in a full-polarization photoacoustic imaging method based on a vector absorption sphere in Example 1 of the present invention;

[0029] Figure 4 This is a schematic structural diagram of a fully polarized photoacoustic imaging device based on a vector absorbing sphere according to Example 2 of the present invention;

[0030] Explanation of the accompanying figures: 1-1 is a pulsed laser, 1-2 is an energy regulator, 1-3 is a collimating beam expander, 1-4 is a linear polarizer, 1-5 is a quarter-wave plate, 1-6 is an objective lens, 2-1 is a transducer, 2-2 is a filter, 2-3 is an amplifier, 2-4 is an acquisition card, 3-1 is an FPGA, 3-2 is a two-dimensional displacement platform, and 4-1 is a computer. DETAILED DESCRIPTION

[0031] In order to enable those skilled in the art to better understand the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present invention.

[0032] References to "embodiments" in this application mean that a particular feature, structure, or characteristic described in connection with the embodiment may be included in at least one embodiment of the application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor does it constitute an independent or alternative embodiment that is mutually exclusive of other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described in this application may be combined with other embodiments.

[0033] like Figure 1 As shown, this embodiment provides a full polarization photoacoustic imaging method based on a vector absorption sphere, referring to Figure 1 , the present invention comprises the following steps:

[0034] Step S1: Establish a vector absorption sphere mathematical model to express the light absorption characteristics of the sample, specifically:

[0035] Step S11, five structural characteristic parameters, namely, total absorption intensity A, overall order O, circular dichroism CD, linear dichroism LD and molecular transition dipole moment OTDM, are proposed to describe the structural characteristics of the sample.

[0036] Step S2: Excite the sample with pulsed light to generate a photoacoustic signal, and measure the photoacoustic signal intensity at that point of the sample under the current polarized excitation light, specifically:

[0037] Step S21, the pulse laser emitted by the pulse laser is energy-adjusted to generate energy suitable for the sample to absorb and generate a photoacoustic signal;

[0038] The pulsed light emitted by the pulse laser has a wavelength of 532nm and a pulse width of 10ns, which is suitable for stimulating the absorption of polarizer samples. The laser energy is regulated by a combination of a half-wave plate and a polarization beam splitter prism. The half-wave plate changes the polarization state of the light. After the incident light passes through the polarization beam splitter prism, only the linearly polarized light component can pass through. Therefore, energy control can be achieved by changing the polarization state before passing through. The sample is a polarizer sample with strong absorption at a wavelength of 532nm.

[0039] Step S22: After energy control, the light beam is expanded and collimated to obtain a parallel incident high-quality light spot.

[0040] The beam expansion and collimation is achieved by combining two lenses, which can be a combination of two convex lenses or a combination of a concave lens and a convex lens.

[0041] Step S23: The parallel incident pulse light passes through the linear polarizer and quarter wave plate with the same initial optical axis, modulates a specific polarization state, and then passes through the objective lens to focus and excite the polarizer to generate a photoacoustic signal.

[0042] Step S24: The transducer converts the collected sound pressure signal into an electrical signal, which is amplified by a filter amplifier and then sent to an acquisition card for collection and storage.

[0043] Step S3: Rotate the quarter-wave plate to modulate the sample to excite different polarization states. Specifically:

[0044] Step S31: After collecting the photoacoustic signal after excitation of a certain polarization state at this point, rotate the quarter-wave plate 22.5° to modulate another polarization state. At this time, collect another set of photoacoustic signals, and so on to obtain the photoacoustic signals of the polarizer at this point when the quarter-wave plate is rotated by 0°, 22.5°, 45°, 67.5°, 90°, 112.5°, 135°, and 157.5° respectively.

[0045] Step S4: Use FPGA to control the two-dimensional displacement platform to move the polarizer sample to achieve two-dimensional scanning of the polarizer sample, thereby obtaining eight groups of photoacoustic signals at each point of the polarizer, and reconstructing the vector absorption sphere of each point of the polarizer sample.

[0046] Step S5: reconstructing a structural characteristic diagram of the polarizer sample according to the vector absorption spheres at each point of the polarizer sample.

[0047] In the embodiment of the present invention, in actual operation of step S1, researchers first conduct a comprehensive structural analysis of the polarizer sample. Using specialized spectral analysis equipment—a polarization infrared spectrometer—they conduct in-depth research into the sample's molecular structure and optical properties. By detecting differences in the sample's absorption of infrared light with different polarization directions, the polarization infrared spectrometer can effectively obtain information about the sample's molecular orientation and optical anisotropy.

[0048] Specifically, researchers scanned the sample using a polarization infrared spectrometer set to different polarization modes, such as linear and circular polarization. Under different polarization modes, the molecules in the sample interact with infrared light to varying degrees, resulting in differences in the absorption spectrum. By analyzing these differences, five structural characteristic parameters can be precisely determined: total absorption intensity A, overall order O, circular dichroism CD, linear dichroism LD, and molecular transition dipole moment OTDM. For example, by measuring the sample's absorption under different linear polarizations of infrared light and combining it with a specific computational model, total absorption intensity A can be accurately calculated. Circular dichroism CD can be determined by utilizing the sample's absorption differences under different circular polarizations and incorporating molecular orientation theory. Overall order O can be determined through statistical analysis of infrared spectral data from different polarization directions. By combining these parameters with the mathematical model established using the fully polarized photoacoustic imaging method based on the vector absorption sphere, a more accurate representation of the sample's optical absorption characteristics can be established through cross-validation, strongly demonstrating the correctness and reliability of this fully polarized photoacoustic imaging method.

[0049] In step S21, the pulsed laser is connected to the energy regulation device and turned on, setting it to emit pulsed laser light with a wavelength of 532 nm and a pulse width of 10 ns. A half-wave plate and a polarization beam splitter are sequentially installed in the laser output path. By rotating the half-wave plate, the researchers change the polarization state of the light. The polarization beam splitter then controls the laser energy until it reaches the desired energy level, which is absorbed by the polarizer sample and generates the photoacoustic signal.

[0050] In step S22, a beam expansion and collimation device consisting of two lenses is installed along the energy-controlled beam propagation path. Whether using a combination of two convex lenses or a concave and a convex lens, the distance and angle between the lenses must be precisely adjusted. This adjustment gradually expands and collimates the beam during propagation, ultimately resulting in a high-quality, parallel-incident spot, ensuring uniformity and stability in subsequent sample excitation.

[0051] In step S23, a linear polarizer and a quarter-wave plate with the same initial optical axis are installed in the transmission path of the parallel incident pulsed light. By precisely adjusting the position and angle of the linear polarizer and the quarter-wave plate, a specific polarization state is modulated. The modulated pulsed light is then focused onto the polarizer sample using an objective lens, causing the sample to absorb the light energy and generate a photoacoustic signal.

[0052] In step S24, a transducer is installed at a suitable location near the sample to collect the sound pressure signal generated by the sample. The transducer converts the collected sound pressure signal into an electrical signal. The electrical signal then passes through a filter to remove noise interference and is amplified by an amplifier. Finally, the amplified electrical signal is sent to an acquisition card for collection and stored in a computer for subsequent analysis and processing.

[0053] In step S3 above, after the acquisition card completes collecting the photoacoustic signal from a point on the polarizer under a certain polarization state, the researcher manually or through an automated control device rotates the quarter-wave plate by 22.5°. At this point, the acquisition card is triggered again to collect the photoacoustic signal from that point under the new polarization state. This operation is repeated, rotating the quarter-wave plate to 45°, 67.5°, 90°, 112.5°, 135°, and 157.5°, respectively, and collecting the photoacoustic signal from that point under the corresponding polarization state. This results in eight sets of photoacoustic signals at different rotation angles at that point on the polarizer.

[0054] In step S4, the FPGA controller is connected to the two-dimensional displacement stage. A control program is written on a computer to set parameters such as the scanning area and scanning step size. After starting the program, the FPGA controller controls the two-dimensional displacement stage to move the polarizer sample horizontally and vertically according to the preset program. At each new position, steps S2 and S3 are repeated to collect eight sets of photoacoustic signals at that point. By moving the two-dimensional displacement stage point by point and collecting photoacoustic signals, a two-dimensional scan of the entire polarizer sample is achieved, obtaining eight sets of photoacoustic signals at each point on the polarizer, providing data support for the subsequent reconstruction of the vector absorption sphere.

[0055] In step S5, specialized image reconstruction software is used on a computer to import the eight sets of photoacoustic signal data at each point on the polarizer obtained in step S4. Based on the mathematical model of the vector absorption sphere established in step S1, the software analyzes and processes the photoacoustic signal data at each point, calculating the structural characteristic parameters corresponding to each point. Based on these parameters, the vector absorption spheres at each point on the polarizer sample are visualized, ultimately reconstructing an image that intuitively reflects the structural characteristics of the polarizer sample.

[0056] This full-polarization photoacoustic imaging method based on vector absorption spheres has many advantages. First, by introducing five structural characteristic parameters to establish a mathematical model of the vector absorption sphere, it can more comprehensively and accurately describe the light absorption characteristics of the sample, and the analysis of the sample structure is more in-depth and detailed than traditional methods. Secondly, by precisely modulating pulsed light of different polarization states to excite the sample and combining it with two-dimensional scanning technology, it is possible to obtain rich photoacoustic signal information at each point of the sample, greatly improving the resolution and contrast of the imaging, and achieving high-precision imaging of the microstructure of the polarizer sample. In addition, this method can reconstruct an image that intuitively reflects the structural characteristics of the sample.

[0057] As a preferred technical solution, in step S1, the total absorption intensity A, the overall order O, the circular dichroism CD, the linear dichroism LD and the molecular transition dipole moment OTDM are specifically:

[0058] The total absorption intensity A formula is:

[0059] ;

[0060] The overall order O formula is:

[0061] ;

[0062] Circular dichroism CD formula:

[0063] ;

[0064] Linear dichroism LD formula:

[0065] ;

[0066] Molecular transition dipole moment OTDM formula:

[0067] ;

[0068] Among them, P0 represents the sum of the photoacoustic signal absorption components generated by the sample under the excitation of horizontal linear polarized light and vertical linear polarized light; P1 represents the difference between the photoacoustic signal absorption components generated by the sample under the excitation of horizontal linear polarized light and vertical linear polarized light; P2 represents the difference between the photoacoustic signal absorption components generated by the sample under the excitation of 45° linear polarized light and -45° linear polarized light; P3 represents the difference between the photoacoustic signal absorption components generated by the sample under the excitation of right-handed circularly polarized light and left-handed circularly polarized light. The vector photoacoustic matrix of the sample is defined as , by P 0、 P 1、 P 2、 P3 structure, vector photoacoustic matrix There are the following relationships:

[0069] ;

[0070] in 、 、 、 、 、 They represent the absorption component of the sample to the horizontal polarization state excitation light, the absorption component of the sample to the vertical polarization state excitation light, and the absorption component of the sample to the horizontal polarization state excitation light. The absorption component of the polarization state of the excitation light, the sample The absorption component of the excitation light in the direction of polarization state, the absorption component of the sample to the right circular polarization state excitation light and the absorption component of the sample to the left circular polarization state excitation light, is the vector absorption matrix of the sample, 、 、 、 is the Stokes parameter of the incident light; A 11 A represents the contribution coefficient of the incident light Stokes parameter S0 to the sample photoacoustic signal component P0; 12 A represents the contribution coefficient of the incident light Stokes parameter S1 to the sample photoacoustic signal component P0; 13 A represents the contribution coefficient of the incident light Stokes parameter S2 to the sample photoacoustic signal component P0; 14 A represents the contribution coefficient of the incident light Stokes parameter S3 to the sample photoacoustic signal component P0; 21 A represents the contribution coefficient of the incident light Stokes parameter S0 to the sample photoacoustic signal component P1; 22 A represents the contribution coefficient of the incident light Stokes parameter S1 to the sample photoacoustic signal component P1; 23 A represents the contribution coefficient of the incident light Stokes parameter S2 to the sample photoacoustic signal component P1; 24 A represents the contribution coefficient of the incident light Stokes parameter S3 to the sample photoacoustic signal component P1; 31 A represents the contribution coefficient of the incident light Stokes parameter S0 to the sample photoacoustic signal component P2; 32 A represents the contribution coefficient of the incident light Stokes parameter S1 to the sample photoacoustic signal component P2; 33 A represents the contribution coefficient of the incident light Stokes parameter S2 to the sample photoacoustic signal component P2; 34 A represents the contribution coefficient of the incident light Stokes parameter S3 to the sample photoacoustic signal component P2; 41 A represents the contribution coefficient of the incident light Stokes parameter S0 to the sample photoacoustic signal component P3; 42 A represents the contribution coefficient of the incident light Stokes parameter S1 to the sample photoacoustic signal component P3; 43A represents the contribution coefficient of the incident light Stokes parameter S2 to the sample photoacoustic signal component P3; 44 It represents the contribution coefficient of the incident light Stokes parameter S3 to the sample photoacoustic signal component P3.

[0071] By establishing a vector absorption sphere model, five structural characteristic parameters are obtained to reflect the structural characteristics of the sample.

[0072] As a preferred technical solution, in step S2, the pulsed laser generated by the laser is converted into horizontally polarized light by a linear polarizer, and then modulated into excitation light of different polarization states by a rotating quarter-wave plate whose initial optical axis is parallel to the linear polarizer. After being focused by an objective lens to excite the sample, the different photoacoustic signal intensities corresponding to the point of the sample excited by the different polarized excitation light are measured. Specifically, the amplitude of the photoacoustic signal is:

[0073] ;

[0074] in, is the light absorption coefficient of the tissue, is the intensity of the excitation light, is the heat conversion efficiency, is the Grueneisen coefficient. The photoacoustic signal at the sample point under the current polarized excitation light can represent the light absorption intensity of the sample point to the current polarized excitation light. The Stokes parameter of horizontally polarized light is: ;

[0075] The Mueller matrix of the quarter-wave plate with the initial optical axis parallel to the polarization direction of horizontally polarized light is:

[0076] ;

[0077] in is the angle between the optical axis of the quarter-wave plate and the polarization direction of the horizontally polarized light. The eight different angles are 0°, 22.5°, 45°, 67.5°, 90°, 112.5°, 135°, and 157.5°.

[0078] The Stokes parameters of light after passing through the quarter-wave plate are:

[0079] ;

[0080] Change angle , that is, rotating the quarter-wave plate can generate excitation light with different polarization states, and measuring the different photoacoustic signal intensities corresponding to the point of the sample excited by the different polarization excitation light.

[0081] As a preferred technical solution, in step S3, the sample is scanned two-dimensionally to obtain the different photoacoustic signal intensities generated by the sample excited by different polarized excitation light at each point of the sample, and the vector absorption sphere of each point of the sample is reconstructed. Specifically, the two-dimensional displacement platform is controlled by FPGA to realize the two-dimensional scanning of the sample. Determine the absorption intensity of the polarized state of the excitation light for:

[0082] ;

[0083] The absorption intensity in a certain direction is expressed as a measurable value under the excitation of a certain polarized light in the vector expression matrix of the photoacoustic signal. According to the relationship between the vector photoacoustic matrix and the vector absorption matrix of the sample, we have

[0084] ;

[0085] Right now

[0086] ;

[0087] Then there is

[0088] ;

[0089] Substituting the Stokes parameter expression of the light after passing through the quarter-wave plate, the photoacoustic signal generated by the sample excited by different polarization excitation light is:

[0090] ;

[0091] In the above formula, and is a known quantity, is parallel to the optical axis of the quarter wave plate The sample absorption intensity caused by the incident polarized light of the angle is the photoacoustic signal intensity. We use the Fourier analysis method to obtain:

[0092] ;

[0093] Measure each point of the sample exist arrive The photoacoustic signal generated by the eight beams of excitation light uniformly distributed inside the Take 0°, 22.5°, 45°, 67.5°, 90°, 112.5°, 135°, and 157.5° respectively. Through Fourier analysis, we can get Four coefficients

[0094] ;

[0095] ;

[0096] Where N represents the total number of measurements. In this scheme, the excitation light with different polarization states is obtained by rotating the angle of the quarter-wave plate; P An represents the photoacoustic signal intensity generated by the sample under the corresponding polarization state excitation light during the nth measurement; θ n It represents the angle (i.e., the rotation angle) between the optical axis of the quarter-wave plate and the polarization direction of the horizontally polarized light during the nth measurement. From this, the four parameters in the vector photoacoustic matrix of each point of the sample are calculated.

[0097] ;

[0098] As a preferred technical solution, in step S4, the structural characteristic diagram of the sample is reconstructed by extracting the structural characteristic parameters of the vector absorption spheres at each point of the sample, specifically: the total absorption intensity structural characteristic diagram of the sample is reconstructed by extracting the structural characteristic parameters of the vector absorption spheres at each point of the sample, the overall order structural characteristic diagram of the sample is reconstructed by extracting the structural characteristic parameters of the vector absorption spheres at each point of the sample, the total order structural characteristic diagram of the sample is reconstructed by extracting the structural characteristic parameters of the vector absorption spheres at each point of the sample, the circular dichroism structural characteristic diagram of the sample is reconstructed by extracting the structural characteristic parameters of the vector absorption spheres at each point of the sample, the linear dichroism structural characteristic diagram of the sample is reconstructed by extracting the structural characteristic parameters of the vector absorption spheres at each point of the sample, and the molecular transition dipole moment structural characteristic diagram of the sample is reconstructed by extracting the structural characteristic parameters of the vector absorption spheres at each point of the sample, the molecular transition dipole moment structural characteristic diagram of the sample is reconstructed by extracting the structural characteristic parameters of the vector absorption spheres at each point of the sample.

[0099] like Figure 2 and Figure 3 As shown, the light absorption characteristics of the polarizer sample are expressed by the vector absorption sphere. Figure 2 This is a vector absorption sphere model image at a certain point of the polarizer sample; Figure 3 The corresponding vector absorption sphere images are reconstructed from several characteristic points in the polarizer sample; Figure 2 and Figure 3 It can be seen that the vector absorption sphere theory can intuitively express the vector absorption characteristics of the polarizer to light.

[0100] like Figure 4 As shown, this embodiment provides a full-polarization photoacoustic imaging device based on a vector absorption sphere, including a polarization pulse light modulation excitation module for providing polarized excitation light, a signal detection module for detecting the photoacoustic signal generated by the excitation light exciting the sample, a two-dimensional scanning module for realizing two-dimensional imaging, and a signal processing module for image reconstruction.

[0101] The polarized pulse light modulation excitation module includes a pulse laser 1-1, an energy regulator 1-2, a collimating beam expander 1-3, a linear polarizer 1-4, a quarter wave plate 1-5, and an objective lens 1-6. The pulse laser 1-1, the energy regulator 1-2, the collimating beam expander 1-3, the linear polarizer 1-4, the quarter wave plate 1-5, and the objective lens 1-6 are connected in sequence.

[0102] The signal detection module includes a transducer 2-1, a filter 2-2, an amplifier 2-3, and an acquisition card 2-4. The transducer 2-1, the filter 2-2, the amplifier 2-3, and the acquisition card 2-4 are connected in sequence.

[0103] The two-dimensional scanning module includes an FPGA 3-1 and a two-dimensional displacement platform 3-2. The FPGA 3-1 and the two-dimensional displacement platform 3-2 are connected in sequence. The FPGA 3-1 is also connected to the pulse laser 1-1 and the acquisition card 2-4 respectively.

[0104] The signal processing module includes a computer 4 - 1 , which is connected to the acquisition card 2 - 4 .

[0105] Furthermore, the polarization pulse light modulation excitation module is connected to the signal detection module, and the two-dimensional scanning module is connected to both the polarization pulse light modulation excitation module and the signal detection module.

[0106] Furthermore, the polarization pulse light modulation excitation module is specifically composed of a pulse laser, an energy regulator, a collimating beam expander, a linear polarizer, a quarter-wave plate, and an objective lens. Through the polarization pulse light modulation excitation module, a pulse laser with different polarization states that is suitable for the absorption band of the imaging sample and can excite the photoacoustic signal can be obtained.

[0107] In this embodiment, the laser is a solid pulse laser with a wavelength of 532 nm and a pulse width of 10 ns.

[0108] In this embodiment, the first lens is a convex lens with a focal length of 5 mm; the second lens is a convex lens with a focal length of 10 mm.

[0109] In this embodiment, a pulse laser and a polarization pulse light modulation excitation module are used to obtain pulse lasers with different polarization states suitable for photoacoustic signal excitation of the sample:

[0110] Laser light from a pulsed laser first passes through a half-wave plate to produce linearly polarized light in any direction. After passing through a polarizing beam splitter, only the vertically or horizontally polarized light components are transmitted. By rotating the half-wave plate, the component passing through the polarizing beam splitter is controlled, thereby controlling the energy. The beam passes through a first lens, an aperture, and a second lens to obtain a collimated, high-quality parallel beam after expansion.

[0111] A high-quality parallel incident light beam is converted to horizontal polarization by a linear polarizer. It then passes through a quarter-wave plate, whose initial optical axis is parallel to the linear polarizer, to modulate a specific polarization state. Focused by the objective lens, it excites the sample and generates a photoacoustic signal. Rotating the quarter-wave plate by 22.5° modulates another polarization state, at which point another set of photoacoustic signals is collected. This process is repeated for polarizer rotations of 0°, 22.5°, 45°, 67.5°, 90°, 112.5°, 135°, and 157.5°, respectively.

[0112] Those skilled in the art can understand that all or part of the processes in the above-mentioned embodiment methods can be implemented. The technical features of the above-mentioned embodiments can be arbitrarily combined. In order to make the description concise, not all possible combinations of the technical features in the above-mentioned embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification. The above-mentioned embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above-mentioned embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A full polarization photoacoustic imaging method based on a vector absorption sphere, characterized in that: The following steps are involved: Step S1, establish a vector absorption sphere mathematical model to express the light absorption characteristics of the sample, and propose to characterize the structural characteristics of the sample with five structural characteristic parameters: total absorption intensity A, overall order O, circular dichroism CD, linear dichroism LD and molecular transition dipole moment OTDM. When establishing the vector absorption sphere mathematical model in step S1, the vector photoacoustic matrix is ​​defined to be composed of four components, wherein the first component characterizes the total absorption intensity, the second component is related to the horizontal / vertical polarization absorption component, the third component is related to the 45° / -45° polarization absorption component, and the fourth component is related to the left-handed / right-handed circular polarization absorption component; the total absorption intensity A is defined as the first component of the vector photoacoustic matrix; the overall order O is determined by the vector photoacoustic matrix The circular dichroism CD is determined by the ratio of the fourth component to the first component of the vector photoacoustic matrix; the linear dichroism LD is determined by the ratio of the joint mode of the second and third components of the vector photoacoustic matrix to the first component; the molecular transition dipole moment OTDM is determined by the ratio of the second component to the third component of the vector photoacoustic matrix, that is, by obtaining the second component and the third component of the vector photoacoustic matrix, calculating the ratio between the two, and then taking the inverse tangent of the ratio, and half of the result is the angle value of the direction of the molecular transition dipole moment; the vector photoacoustic matrix is ​​constructed by the linear mapping relationship between the absorption components of the sample for the six polarization states of excitation light and the Stokes parameter of the incident light; Step S2, passing the pulsed laser generated by the laser through a linear polarizer and a rotating quarter-wave plate in sequence, modulating it into excitation light of different polarization states, focusing it to a single point of the sample through an objective lens, and measuring the photoacoustic signal intensity corresponding to the excitation light of each polarization state, wherein when modulating the excitation light of different polarization states in step S2, a quarter-wave plate whose initial optical axis is parallel to the direction of the horizontal polarization light is rotated, and its rotation angle is selected at eight specific orientations at intervals of 22.5°; the first component of the Stokes parameter of the horizontally polarized light is 1, the second component is 1, and the remaining components are 0; the polarization modulation characteristics of the quarter-wave plate are described by its Mueller matrix, and the matrix elements are composed of trigonometric functions of the rotation angle θ; each component of the Stokes parameter of the output light after modulating the quarter-wave plate is determined by the cosine square term, sine-cosine cross term and sine term of the rotation angle θ; the photoacoustic signal intensity is jointly determined by the light absorption coefficient, excitation light intensity, thermal conversion efficiency and Grueneisen coefficient of the sample, and its numerical value represents the absorption intensity of the sample to the current polarized excitation light; Step S3, performing a two-dimensional scan on the sample, repeating step S2 to obtain multi-polarization state photoacoustic signals at each scanning point, and reconstructing the vector absorption sphere at each point of the sample based on the signals, wherein when reconstructing the vector absorption sphere at each point of the sample in step S3, the sample scanning is achieved by controlling the two-dimensional displacement platform through FPGA; for each scanning point, four harmonic coefficients are solved by Fourier analysis using the photoacoustic signal intensity data measured at eight sets of specific rotation angles; the four harmonic coefficients are determined by the weighted summation of the eight sets of discrete angle measurement values, and the weighting factors correspond to the constant term, the double frequency sine term, the quadruple frequency cosine term, and the quadruple frequency sine term, respectively; based on the algebraic combination of the four harmonic coefficients, four component parameters of the vector photoacoustic matrix are directly derived; the solved results of the vector photoacoustic matrix components are the mathematical representation of the vector absorption sphere at the scanning point; Step S4, extracting the structural characteristic parameters of the vector absorption sphere at each point, and reconstructing the total absorption intensity distribution map, overall order distribution map, circular dichroism distribution map, linear dichroism distribution map and molecular transition dipole moment distribution map of the sample.

2. The method for full polarization photoacoustic imaging based on a vector absorption sphere according to claim 1, characterized in that: When extracting the structural characteristic parameters in step S4, independent mapping is performed directly based on the five types of parameters predefined in the vector absorption sphere of each point; The total absorption intensity distribution diagram is generated by directly extracting the total absorption intensity parameter A of all scanning points; The overall order distribution graph is generated by directly extracting the overall order parameter O of all scanning points; The circular dichroism distribution diagram is generated by directly extracting the circular dichroism parameters CD of all scanning points; The linear dichroism distribution diagram is generated by directly extracting the linear dichroism parameters LD of all scanning points; The molecular transition dipole moment distribution diagram is generated by directly extracting the molecular transition dipole moment parameters OTDM of all scanning points.

3. A fully polarized photoacoustic imaging device based on a vector absorption sphere, which implements the method according to any one of claims 1 to 2, characterized in that: It includes a polarization pulse light modulation excitation module, a signal detection module, a two-dimensional scanning module and a signal processing module; The polarized pulse light modulation excitation module includes a pulse laser, an energy regulator, a collimating beam expander, a linear polarizer, a quarter wave plate, and an objective lens, and the pulse laser, energy regulator, collimating beam expander, linear polarizer, quarter wave plate, and objective lens are connected in sequence; The signal detection module includes a transducer, a filter, an amplifier, and an acquisition card, and the transducer, filter, amplifier, and acquisition card are connected in sequence; The two-dimensional scanning module includes an FPGA and a two-dimensional displacement platform, the FPGA and the two-dimensional displacement platform are connected in sequence, and the FPGA is also connected to a pulse laser and an acquisition card respectively; The signal processing module includes a computer, and the computer is connected to the acquisition card.

4. The full polarization photoacoustic imaging device based on a vector absorbing sphere according to claim 3, characterized in that: In the polarized pulse light modulation excitation module, the pulse light beam emitted by the pulse laser passes through an energy regulator and a collimating beam expander in sequence to obtain parallel incident light pulse light with a better light spot. The parallel incident light pulse light is converted into horizontal polarized light by a linear polarizer. The horizontal polarized light is converted into excitation light of different polarization states by a rotating quarter-wave plate whose initial optical axis is parallel to the horizontal polarized light. Eight beams of excitation light of different polarization states are formed, with the angles between the optical axis of the quarter-wave plate and the horizontal polarized light being 0°, 22.5°, 45°, 67.5°, 90°, 112.5°, 135°, and 157.5°, respectively. The eight beams are focused by an objective lens to excite the sample. The energy regulator is composed of a polarizer, a half-wave plate and a polarization beam splitter; The collimating beam expander consists of a first lens, a stop and a second lens.

5. The full polarization photoacoustic imaging device based on a vector absorption sphere according to claim 4, characterized in that: In the signal detection module, the photoacoustic signal generated by the excitation light of different polarization states being focused by the objective lens to excite the sample is collected by the transducer, and the sound pressure signal is converted into an electrical signal. The electrical signal is filtered and amplified by the filter and amplifier and then collected and stored by the acquisition card.

6. The full polarization photoacoustic imaging device based on a vector absorption sphere according to claim 5, characterized in that: In the two-dimensional scanning module, the FPGA controls the pulse laser to emit a pulse beam, and at the same time controls the acquisition card to collect the photoacoustic signal generated by the pulse light excitation sample and the movement of the two-dimensional displacement platform, thereby realizing two-dimensional scanning of the sample.

7. The full-polarization photoacoustic imaging device based on a vector absorbing sphere according to claim 6, characterized in that: In the signal processing module, a computer processes the photoacoustic signals collected by the acquisition card and reconstructs the vector absorption spheres at each point of the sample and the structural characteristic diagram of the sample.

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