Photoacoustic imaging device and method based on structured light detection

Abstract

This invention relates to a photoacoustic imaging device and method based on structured light detection, comprising a photoacoustic laser, a mirror, a first beam expander, a stage, a sample cell, a probe laser, a polarizing beam splitter, a coupling sensor, a data acquisition card, a computer, a second beam expander, a digital micromirror device, a convex lens, and a photodetector. The coupling sensor is a prism with a graphene layer or metal film coated on its bottom. The sample cell contains liquid, has a window at the bottom sealed with an acoustically transparent film, and the film contacts the imaging object through an ultrasonic coupling agent. The digital micromirror device is connected to the computer, the photodetector is connected to the data acquisition card, and the data acquisition card is connected to the computer. The photoacoustic laser is also connected to the computer. This invention eliminates the need for mechanical movement; it only requires multiple changes to the encoding of the digital micromirror device, followed by image reconstruction by the computer's image reconstruction module to calculate a three-dimensional photoacoustic image, thereby significantly improving imaging speed and reducing motion artifacts and noise.

Description

Technical Field

[0001] This invention relates to photoacoustic imaging technology, and more specifically to a photoacoustic imaging device and method based on structured light detection. Background Technology

[0002] In recent years, photoacoustic imaging, as an emerging biological imaging technology, has attracted widespread interest both domestically and internationally. Photoacoustic imaging combines the advantages of optical and ultrasonic imaging, acquiring ultrasonic signals generated by ultrashort pulse excitation light irradiating absorbers within biological tissues and reconstructing them to obtain structural and functional information within the biological tissues. Compared to other imaging methods, photoacoustic imaging offers advantages such as being radiation-free, highly sensitive, having high resolution, and high contrast.

[0003] Because the size of the objects being imaged varies, the frequency of the ultrasonic signals generated by the excitation light covering the range of several MHz to hundreds of MHz. However, traditional piezoelectric ultrasonic detection methods have limited bandwidth, which cannot meet the requirements of broadband detection of photoacoustic signals, resulting in low imaging resolution. Therefore, the development of photoacoustic imaging devices and methods based on broadband ultrasonic signal detection has significant application value for high-resolution photoacoustic imaging. Existing photoacoustic imaging devices and methods based on broadband ultrasonic signal detection employ photoacoustic imaging technology based on optical detection. This technology utilizes the principle of surface plasmon resonance to detect the ultrasonic signals generated by photoacoustic imaging. Specifically, the ultrasonic signal causes a change in the refractive index of the liquid near the graphene layer or metal film coated on the bottom surface of the prism. The intensity of the focused p-polarized light reflected from the bottom surface of the prism changes with the refractive index of the liquid, thus detecting the ultrasonic signal. This method features a wide ultrasonic signal detection bandwidth (>100MHz). However, the p-polarized light in this method is single-point focused light, meaning that only one ultrasonic signal can be acquired at a single location at a time, obtaining one-dimensional ultrasonic information along the depth direction. Two-dimensional scanning is required through mechanical movement to achieve three-dimensional photoacoustic imaging. This method has a slow imaging speed, and mechanical movement can easily introduce motion artifacts, resulting in poor photoacoustic image quality. Moreover, single-point focusing only acquires the ultrasonic signal from one location at a time, resulting in weak signal strength. Due to the influence of ambient light and electronic noise, the signal-to-noise ratio is low and the sensitivity is poor. Summary of the Invention

[0004] To address the aforementioned problems in the existing technology, the present invention provides a photoacoustic imaging device and method based on structured light detection.

[0005] The technical solution adopted by the present invention to solve the aforementioned technical problem is as follows:

[0006] In a first aspect, the present invention provides a photoacoustic imaging device based on structured light detection, comprising a photoacoustic laser 1, a reflector 2, a first beam expander 3, a stage 4, a sample cell 5, a probe laser 7, a polarizing beam splitter 8, a coupling sensor 10, a data acquisition card 14, and a computer 15. The coupling sensor 10 is a prism with a graphene layer or a metal film coated on its bottom. The device further comprises a second beam expander 9, a digital micromirror device 11, a convex lens 12, and a photodetector 13. The sample cell 5 contains liquid and has a window at its bottom sealed with an acoustically transparent film. The acoustically transparent film contacts the imaging object through an ultrasonic coupling agent. The imaging object is placed on the stage 4. The graphene layer or metal film at the bottom of the prism of the coupling sensor is immersed in the liquid in the sample cell 5. The digital micromirror device 11 is connected to the computer 15, the photodetector 13 is connected to the data acquisition card 14, and the data acquisition card 14 is connected to the computer 15. The photoacoustic laser 1 is connected to the computer 15.

[0007] The probe laser 7 emits a probe laser, which is separated into p-polarized light by the polarization beam splitter 8. The p-polarized light is expanded by the second beam expander 9 to form a probe light surface source, which is incident on the bottom of the coupling sensor 10. The incident direction is adjusted to cause surface plasmon resonance in the graphene layer or metal film at the bottom of the coupling sensor. The irradiation area of ​​the probe light surface source at the bottom of the coupling sensor serves as the detection window. The photoacoustic laser 1 emits pulsed excitation light, which is reflected by the mirror 2 and expanded by the first beam expander 3 to form an excitation light surface source that irradiates the imaging object on the stage. The imaging object emits an ultrasonic signal, which is transmitted through the sample cell 5. The sound-transmitting film propagates through the liquid in the sample cell to the bottom detection window of the coupling sensor 10, causing the refractive index of the liquid near the graphene layer or metal film at the detection window to change over time. The change in the refractive index of the liquid near the graphene layer or metal film at the detection window causes a change in the reflectivity of the p-polarized light at the detection window. The detection light reflected from the detection window of the coupling sensor 10 is encoded and modulated by the digital micromirror device 11 to form structured light, and then reflected to the convex lens and focused into the photodetector 13. The photodetector converts the light signal into an electrical signal, which is then acquired by the data acquisition card 14 and sent to the computer 15 for image reconstruction.

[0008] The computer is equipped with an image reconstruction module, which is used to calculate a three-dimensional photoacoustic image. This module includes three parts: demodulation of the reflected light intensity from the detection window, obtaining the ultrasonic signal intensity using the reflected light intensity from the detection window, and reconstruction of the photoacoustic image; a) Demodulation of the reflected light intensity from the detection window:

[0009] Set the plane containing the prism bottom detection window S as the xy plane with z coordinate 0, the center of the detection window S as the origin of the xy plane, and the positive z direction is the direction perpendicular to the plane containing the prism bottom detection window S downward.

[0010] The light intensity signal obtained by photodetector 13 is the sum of the light intensity of the structured light formed by the reflected light from the light source through the detection window and the encoding and modulation by the digital micromirror device. The detection window S is discretized into N×N point reflection units. The electrical signal corresponding to the light intensity of the structured light encoded by the digital micromirror device at time t is denoted as y(m,t). The reflected light intensity reflected by the point reflection unit in the detection window S on the bottom surface of the prism of the coupled sensor at time t is q(x,y,t), where (x,y) are the position coordinates of the point reflection unit. The encoding and modulation matrix of the digital micromirror device at time t is denoted as A(m,x,y,t), where t is the time difference between the signal acquisition time of the data acquisition card and the time of laser emission by the photoacoustic laser.

[0011] The relationship between the electrical signal y(m,t), the coding modulation matrix A(m,x,y,t), and the reflected light intensity q(x,y,t) is given by equation (1):

[0012] y(m,t)=A(m,x,y,t)*q(x,y,t)(1)

[0013] By collecting the time-series electrical signals of the photodetector m times, m known y(m,t) are obtained, t=t0,t1,…,ti,…tn, where n is an integer, y(m,t) is a time sequence, and the maximum value of t, tn, is greater than the maximum value of the distance between the voxel in the three-dimensional imaging region of the photoacoustic imaging object and the point reflection unit in the detection window S divided by the sound speed in the imaging object. The interval of t follows the sampling theorem, and for a signal of hundreds of MHz, it should be less than 5 ns; A(m,x,y,t) is the known encoding modulation matrix of the digital micromirror device. Using the electrical signal y(m,ti) of the photodetector m times at time ti, the equation (1) is solved to obtain q(x,y,ti) at time ti. In this way, using the electrical signal y(m,t) of the photodetector m times at different times t, all times t are obtained. This process is the demodulation of the reflected light intensity of the detection window.

[0014] b) Obtain the ultrasonic signal intensity using the reflected light intensity from the detection window:

[0015] The intensity of reflected light q(x,y,t) of each point reflection unit in the detection window is linearly related to the intensity of ultrasonic signal p(x,y,t) emitted by the imaged object excited by the photoacoustic laser, that is, it satisfies equation (2):

[0016] p(x,y,t)=a*q(x,y,t)+b(2)

[0017] Where a and b are constants, by setting two or more known intensities of p(x,y,t) and measured q(x,y,t), the values ​​of a and b can be obtained by solving equation (2); then, by using equation (2) with known values ​​of a and b, the ultrasonic signal intensity p(x,y,t) can be obtained through the reflected light intensity q(x,y,t);

[0018] c) Reconstruction of photoacoustic images:

[0019] Using all the ultrasonic signals p(x,y,t) at time t obtained after steps a) and b), where t is the time difference between the signal acquisition time of the data acquisition card in step a) and the time of laser emission by the photoacoustic laser, the three-dimensional photoacoustic image can be obtained by filtering and back-projection reconstruction using p(x,y,t).

[0020] Secondly, the present invention provides a photoacoustic imaging method based on structured light detection, characterized in that the specific steps of the imaging method are as follows:

[0021] 1) The probe laser emits a probe laser, which is separated into p-polarized light by a polarization beam splitter. The p-polarized light is then expanded by a second beam expander to form a probe light surface source, which is incident on the graphene layer or metal film on the bottom surface of the prism of the coupling sensor. The irradiated area serves as the detection window, causing a surface plasmon resonance phenomenon, which is then reflected out of the prism. The probe light surface source reflected from the prism is encoded and modulated by a digital micromirror device to form structured light, which is then focused into the photodetector by a convex lens.

[0022] 2) The computer-controlled digital micromirror device achieves one-time encoding and remains unchanged. The encoding retention time of the digital micromirror device should be greater than the time length obtained by dividing the maximum value of the distance between the voxel in the three-dimensional imaging area of ​​the imaging object and the point reflection unit of the detection window by the speed of sound in the imaging object.

[0023] 3) The photoacoustic laser emits an excitation light pulse, which is reflected by a mirror and then expanded by the first beam expander to form an excitation light surface light source that illuminates the imaging object on the stage. The imaging object emits an ultrasonic signal, which passes through the thin film of the sample cell and then propagates through the liquid in the sample cell to the liquid near the graphene layer or metal film at the detection window of the coupling sensor. This causes the refractive index of the liquid to change over time, which in turn causes a change in the intensity of the detection light surface light source reflected by the graphene layer or metal film at the detection window of the coupling sensor. Ultimately, this causes a change in the electrical signal generated by the structured light formed by the encoding and modulation of the digital micromirror device being focused by the convex lens onto the photodetector.

[0024] 4) Simultaneously with the photoacoustic laser emitting the excitation light pulse, the data acquisition card begins to acquire the electrical signal of the photodetector and transmits it to the computer. The acquisition duration is equal to the encoding retention duration of the digital micromirror device.

[0025] 5) Repeat steps 2) to 4) multiple times;

[0026] 6) Demodulate the reflected light intensity of the detection window sequentially using the electrical signals collected multiple times. Based on the reflected light intensity of each point reflection unit of the detection window, and combined with the linear relationship between the reflected light intensity and the ultrasonic signal intensity, obtain the ultrasonic signal intensity. Finally, obtain a three-dimensional photoacoustic image through filtering and back projection reconstruction.

[0027] Compared with the prior art, the beneficial effects of the present invention are:

[0028] While existing photoacoustic imaging based on surface plasmon resonance boasts wide bandwidths (above 100 MHz), the manufacturing of fast-response area array photodetectors is extremely difficult and expensive. These photodetectors are limited to single-pixel detectors, requiring single-point focusing of the probe light, which can only detect the reflected light intensity signal at one location at a time—equivalent to a single-element ultrasonic detector. For three-dimensional photoacoustic imaging, mechanical movement of the single-point focusing probe light device is required for two-dimensional scanning. This not only results in slow imaging speed but also introduces jitter, causing motion artifacts and noise. Furthermore, single-point focusing only acquires the ultrasonic signal from one location at a time, leading to weak signal strength and low signal-to-noise ratio and poor sensitivity due to the influence of ambient light and electronic noise.

[0029] This invention utilizes a probe light source to illuminate the detection window of a coupling sensor. The reflected light is encoded and modulated by a digital micromirror device (DMM) to form structured light. This structured light is then focused by a convex lens onto a single-pixel detector, acquired by a data acquisition card, and finally processed by a computer's image reconstruction module to obtain a photoacoustic image. By combining a coupling sensor, structured light encoded and modulated by a DMM, a fast-response single-pixel photodetector, and a computer's image reconstruction module, rapid-response area array ultrasonic detection is achieved. Compared to existing single-point focusing probe light detection methods, this structured light-based detection method, by acquiring the sum of reflected light intensity signals from multiple locations each time, significantly improves the sensitivity and signal-to-noise ratio of light intensity signal detection, and reduces the influence of ambient light and electronic noise. Moreover, the structured light-based detection method requires no mechanical movement; it only requires changing the encoding of the DMM multiple times, followed by image reconstruction by the computer to calculate a three-dimensional photoacoustic image, thereby greatly improving imaging speed and reducing motion artifacts and noise. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the structure of the device of the present invention.

[0031] 1 is a photoacoustic laser, 2 is a mirror, 3 is the first beam expander, 4 is a stage, 5 is a sample cell, 6 is an imaging object, 7 is a probe laser, 8 is a polarizing beam splitter, 9 is the second beam expander, 10 is a coupling sensor, 11 is a digital micromirror device, 12 is a convex lens, 13 is a photodetector, 14 is a data acquisition card, and 15 is a computer. Detailed Implementation

[0032] The present invention will be further explained below with reference to the embodiments and accompanying drawings, but this is not intended to limit the scope of protection of this application.

[0033] This invention relates to a photoacoustic imaging device based on structured light detection (see...). Figure 1 The system includes a photoacoustic laser 1, a reflector 2, a first beam expander 3, a stage 4, a sample cell 5, a probe laser 7, a polarizing beam splitter 8, a second beam expander 9, a coupling sensor 10, a digital micromirror device 11, a convex lens 12, a photodetector 13, a data acquisition card 14, and a computer 15. The coupling sensor 10 is a prism with a graphene layer or metal film coated on the bottom; the sample cell 5 contains liquid, with a window at the bottom sealed with an acoustically transparent film, which contacts the imaging object through an ultrasonic coupling agent; the imaging object 6 is placed on the stage 4; the graphene layer or metal film at the bottom of the prism of the coupling sensor 10 is immersed in the liquid in the sample cell 5; the digital micromirror device 11 is connected to the computer 15, the photodetector 13 is connected to the data acquisition card 14, and the data acquisition card 14 is connected to the computer 15; the photoacoustic laser 1 is connected to the computer 15; the probe laser 7 emits a probe laser, which is separated into p-polarized light by the polarization beam splitter 8, and the p-polarized light is expanded by the second beam expander 9 to form a probe light surface light source, which is incident on the bottom of the coupling sensor 10. The incident direction is adjusted to cause surface plasmon resonance in the graphene layer or metal film at the bottom of the coupling sensor 10. The detection light source illuminates the area at the bottom of the coupling sensor 10 as a detection window. The computer 15 controls the photoacoustic laser 1 to emit pulsed excitation light, which is reflected by the mirror 2 and expanded by the first beam expander 3 to form an excitation light source that illuminates the imaging object 6 on the stage 4. The imaging object 6 emits an ultrasonic signal, which passes through the acoustically transparent film of the sample cell 5 and propagates through the liquid in the sample cell 5 to the detection window at the bottom of the coupling sensor 10, causing a change in the refractive index of the liquid near the graphene layer or metal film at the detection window over time. This change in the refractive index of the liquid near the graphene layer or metal film at the detection window causes a change in the reflectivity of the p-polarized light at the detection window. The detection light source reflected from the detection window of the coupling sensor 10 is encoded and modulated by the digital micromirror device 11 to form structured light, which is then reflected to the convex lens 12 and focused into the photodetector 13. The photodetector 13 converts the optical signal into an electrical signal, which is then acquired by the data acquisition card 14 and sent to the computer 15 for image reconstruction.

[0034] The coupling sensor 10 of this invention is a prism with a graphene layer or metal film coated on the bottom. When the graphene layer or metal film undergoes surface plasmon resonance, the change in the reflectivity of the p-polarized light reflected by the coupling sensor, i.e., the change in the intensity of the light detected by the photodetector 13, changes with the change in the refractive index of the liquid near the graphene layer or metal film in the detection window caused by the ultrasonic signal, thereby realizing the detection of ultrasonic signals. The extremely shallow penetration depth of the surface plasmon evanescent field allows the ultrasonic detection bandwidth to reach over 100 MHz.

[0035] In this invention, the second beam expander 9 can be used to adjust the size of the probe light spot to form a probe light surface light source.

[0036] The photodetector 13 is a fast-response single-pixel photodetector, and its response bandwidth to changes in light intensity should be greater than 100 MHz to meet the requirements for responding to rapid changes in ultrasonic signals with frequencies above 100 MHz.

[0037] The digital micromirror device 11 is used to encode and modulate the probe light reflected from the detection window of the coupled sensor, forming structured light which is then focused into the photodetector by a convex lens. The digital micromirror device uses Fourier coding, Hadamard coding, or Gray coding, etc.

[0038] Computer 15 is used to control the acquisition of photoelectric signals, the switching of the photoacoustic laser, the encoding and modulation of the digital micromirror device, and the image reconstruction module. The computer, electrically connected to a data acquisition card, controls the data acquisition card to acquire the electrical signals generated by the photodetector. The computer is also electrically connected to the photoacoustic laser to control its switching, i.e., controlling the emission and deactivation of the excitation light. Finally, the computer, electrically connected to the digital micromirror device, controls its encoding and modulation.

[0039] The image reconstruction module is used to calculate a three-dimensional photoacoustic image, which includes three parts: demodulation of the light intensity reflected from the detection window, obtaining the ultrasonic signal intensity using the light intensity reflected from the detection window, and reconstruction of the photoacoustic image.

[0040] a) Demodulation of the intensity of light reflected from the detection window:

[0041] The light intensity signal obtained by photodetector 13 is the sum of the light intensity of the structured light formed by the reflected light from the light source through the detection window (denoted as S) and encoded and modulated by the digital micromirror device. The detection window S is discretized into N×N point reflection units. The electrical signal corresponding to the structured light intensity encoded by the digital micromirror device at time t is denoted as y(m,t). The reflected light intensity of the point reflection unit within the prism bottom detection window S of the coupled sensor at time t is q(x,y,t), where (x,y) are the position coordinates of the point reflection unit. The plane containing the prism bottom detection window S is the xy plane with a z-coordinate of 0. The center of the detection window S is the origin of the xy plane. The direction perpendicular to the plane containing the prism bottom detection window S is downward, which is the positive z-direction. The encoding and modulation matrix of the digital micromirror device at time t is denoted as A(m,x,y,t). t is the time difference between the signal acquisition time of the data acquisition card and the time of laser emission from the photoacoustic laser 1. Then:

[0042] y(m,t)=A(m,x,y,t)*q(x,y,t)(1)

[0043] By acquiring the time-series electrical signals of the photodetector m times, m known y(m,t) (t=t0,t1,…,ti,…tn, n is an integer) are obtained. y(m,t) is a time sequence. The value of t is, for example, the minimum value t0=0ns, and the maximum value tn is slightly greater than the maximum value of the distance between the voxel in the three-dimensional imaging region of the imaging object and the point reflection unit in the detection window S divided by the sound speed in the imaging object. The interval of t values ​​follows the sampling theorem, and for a signal of hundreds of MHz, it should be less than 5ns. A(m,x,y,t) is the known encoding modulation matrix of the digital micromirror device, such as Fourier encoding. Using the electrical signal y(m,ti) of the photodetector m times at time ti, solving equation (1) will yield q(x,y,ti) at time ti. For example, when the encoding modulation matrix is ​​Fourier encoding, q(x,y,ti) can be obtained by performing an inverse Fourier transform on y(m,ti). By using the electrical signals y(m,t) from the photodetector m times at different times t, q(x,y,t) at all times t can be obtained. This process is the demodulation of the intensity of the light reflected from the detection window.

[0044] b) Obtain the ultrasonic signal intensity using the reflected light intensity from the detection window:

[0045] The intensity of the reflected light q(x,y,t) of each point reflection unit in the detection window is linearly related to the intensity of the ultrasonic signal p(x,y,t) emitted by the imaging object excited by photoacoustic laser 1, that is, it satisfies:

[0046] p(x,y,t)=a*q(x,y,t)+b(2)

[0047] Where a and b are constants. By setting two or more known intensities of p(x,y,t) and measured q(x,y,t), these two constants can be obtained by solving equation (2). Since p(x,y,t) and q(x,y,t) are linearly related, for non-quantitative photoacoustic imaging, q(x,y,t) can be used instead of p(x,y,t) to directly reconstruct the photoacoustic image below.

[0048] c) Reconstruction of photoacoustic images:

[0049] Using the ultrasonic signals p(x,y,t) at all times t obtained after steps a) and b), where t is the time difference between the signal acquisition time of the data acquisition card in step a) and the laser emission time of photoacoustic laser 1, and (x,y) are the position coordinates of the point reflection unit in step a), a three-dimensional photoacoustic image can be obtained by filtering and backprojection reconstruction using p(x,y,t).

[0050] Since a high-frequency ultrasonic signal of 100 MHz is a rapidly changing signal, in order to reflect the change of ultrasonic signal by detecting changes in light intensity, the light intensity change response bandwidth of the photodetector must be greater than 100 MHz. However, area array photodetectors that can achieve this response speed are difficult to manufacture or are very expensive. Therefore, only single-pixel photodetectors that respond quickly to changes in light intensity can be used. As a result, the detection light of existing ultrasonic detection devices based on surface plasmon resonance is all single-point focused.

[0051] This invention uses a surface light source incident on the detection window of a coupled sensor. The reflected light is encoded and modulated by a digital micromirror device to form structured light. This structured light is then focused by a convex lens onto a single-pixel photodetector that responds quickly to changes in light intensity. Combined with step a) in the computer's image reconstruction module, the reflected light intensity of the detection window (i.e., the reflected light intensity of each point reflection unit in the detection window) can be obtained, thus realizing the function of a planar photodetector with a light intensity change response bandwidth of over 100 MHz (i.e., rapid response to light intensity changes). Based on the obtained reflected light intensity of each point reflection unit in the detection window, combined with steps b) and c) of the image reconstruction module, a photoacoustic image can be reconstructed. The imaging steps are as follows:

[0052] 7) A probe laser (such as a helium-neon laser) emits a probe laser beam. After being separated into p-polarized light by a polarization beam splitter, the p-polarized light is expanded by a second beam expander to form a probe light surface source. This light is incident on the graphene layer or metal film on the bottom surface of the prism of the coupling sensor (the irradiated area serves as the detection window), causing surface plasmon resonance and reflection out of the prism. The probe light surface source reflected out of the prism is encoded and modulated by a digital micromirror device to form structured light, which is then focused into a photodetector by a convex lens.

[0053] 8) The computer-controlled digital micromirror device (DMD) performs a single encoding and retains the encoding. The encoding retention time of the DMD should be greater than the maximum distance between the voxels in the three-dimensional imaging region of the imaged object and the point reflection units of the probe window, divided by the speed of sound in the imaged object.

[0054] 9) A computer-controlled photoacoustic laser emits an excitation pulse of a specific wavelength (e.g., 532nm). Reflected by a mirror, the pulse is expanded by the first beam expander to form an excitation light source that illuminates the imaging object on the stage. The imaging object emits an ultrasonic signal, which passes through the thin film in the sample cell and then propagates through the liquid in the sample cell. This propagation affects the liquid near the graphene layer or metal film at the detection window of the coupling sensor, causing a change in the refractive index of the liquid over time. This, in turn, causes a change in the intensity of the detection light source reflected by the graphene layer or metal film at the detection window of the coupling sensor. Ultimately, this results in a change in the electrical signal generated when the structured light, encoded and modulated by a digital micromirror device, is focused by a convex lens onto a photodetector.

[0055] 10) Simultaneously with the emission of the excitation light pulse from the photoacoustic laser, the computer-controlled data acquisition card begins acquiring the electrical signals from the photodetector and transmitting them to the computer. The acquisition duration is equal to the encoding retention duration of the digital micromirror device.

[0056] 11) Repeat steps 2) to 4) multiple times to reimplement the new code.

[0057] 12) The computer's image reconstruction module uses the electrical signals acquired multiple times to demodulate the intensity of the light reflected from the detection window, obtain the intensity of the ultrasonic signal and reconstruct the photoacoustic image using the intensity of the light reflected from the detection window, and finally obtain a three-dimensional photoacoustic image.

[0058] Example 1

[0059] like Figure 1 As shown, the photoacoustic laser 1 emits a pulsed excitation light of a certain wavelength (such as 532nm), which is reflected by the mirror 2 and then expanded by the first beam expander 3 to form an excitation light surface light source that illuminates the imaging object 6 on the stage 4. The imaging object 6 emits an ultrasonic signal.

[0060] The stage 4 is used to place and fix the imaging object 6, and is transparent to the excitation light.

[0061] The sample cell 5 contains a liquid (such as water), and has a window at the bottom sealed with an acoustically transparent membrane. The acoustically transparent membrane is in contact with the imaging object 6 through an ultrasonic coupling agent.

[0062] The coupling sensor 10 is a prism with a graphene layer or metal film coated on the bottom. The graphene layer or metal film on the bottom of the prism is immersed in the liquid in the sample cell 5.

[0063] The probe laser 7 (such as a helium-neon laser) emits a probe laser, which is separated into p-polarized light by the polarization beam splitter 8. The p-polarized light is then expanded by the second beam expander 9 to form a probe light surface source, which is incident on the graphene layer or metal film on the bottom surface of the prism of the coupling sensor 10 (the irradiated area is denoted as S), causing a surface plasmon resonance phenomenon and reflecting it out of the prism.

[0064] The probe light source reflected from the prism is encoded and modulated by the digital micromirror device 11 to form structured light, which is then focused into the photodetector 13 by the convex lens 12.

[0065] The photodetector 13 converts the optical signal into an electrical signal, which is then acquired by the data acquisition card 14 and transmitted to the computer 15 for processing and storage. The photodetector 13, the data acquisition card 14, and the computer 15 are electrically connected.

[0066] The digital micromirror device 11 is used to encode and modulate the probe light source reflected by the coupled sensor 10, forming structured light which is then focused into the photodetector 13 by the convex lens 12. Therefore, the light intensity signal obtained by the photodetector 13 is the sum of the light intensities of the structured light formed by the reflected light from the probe light source through the irradiation area S after encoding and modulation by the digital micromirror device 11. The irradiation area S is discretized into N×N point detection units. Through multiple encoding and modulation by the digital micromirror device 11, and then demodulated by the computer 15, the light intensity reflected by each point detection unit in the irradiation area S can be obtained. Since the light intensity and the ultrasonic signal intensity have a linear relationship, the N×N point detection units discretized into the irradiation area S can be equivalent to an N×N ultrasonic array detector. The encoding of the digital micromirror device 11 can use Fourier encoding, Hadamard encoding, or Gray encoding, etc.

[0067] Computer 15 is used to control the acquisition of photoelectric signals, the encoding and modulation of the digital micromirror device 11, and the image reconstruction module. Computer 15, electrically connected to data acquisition card 14, controls the data acquisition card 14 to acquire the electrical signals generated by photodetector 13. Computer 15 is also electrically connected to photoacoustic laser 1 to acquire and control the optical signals generated by photoacoustic laser 1. Computer 15, electrically connected to digital micromirror device 11, is used to control the encoding and modulation of digital micromirror device 11.

[0068] The image reconstruction module of computer 15 is used to calculate three-dimensional photoacoustic images. This module comprises three parts: demodulation of the reflected light intensity from N×N point detection units in the irradiated area S, obtaining the ultrasonic signal intensity using the reflected light intensity from the point detection units, and reconstruction of the photoacoustic image.

[0069] a) Demodulation of the light intensity of the N×N light intensity detection units in the irradiated area S:

[0070] The photodetector 13 converts the optical signal into an electrical signal, and the two are linearly related. The electrical signal corresponding to the intensity of the structured light encoded by the digital micromirror device 11 at time t, acquired by the data acquisition card 14, is denoted as y(m,t). The reflected light intensity reflected by the point detection unit in the prism bottom illumination area S of the coupled sensor 10 at time t is q(x,y,t), where (x,y) are the position coordinates of the point detection unit. The plane containing the prism bottom illumination area S is the xy plane with a z-coordinate of 0. The center of the illumination area S is the origin of the xy plane. The direction perpendicular to the plane containing the prism bottom illumination area S is downward, which is the positive z-direction. The encoding modulation matrix of the digital micromirror device 11 at time t0 is denoted as A(m,x,y,t). t is the time difference between the signal acquisition time of the data acquisition card 14 and the time of laser emission by the photoacoustic laser 1. The relationship between the three is as follows:

[0071] y(m,t)=A(m,x,y,t)*q(x,y,t) (1)

[0072] By collecting the time-series electrical signals of the photodetector m times, m known y(m,t) are obtained, t=t0,t1,…,ti,…tn, where n is an integer, y(m,t) is a time sequence, and the maximum value of t, tn, is greater than the maximum value of the distance between the voxel in the three-dimensional imaging region of the photoacoustic imaging object and the point reflection unit in the detection window S divided by the speed of sound in the imaging object. The interval of t follows the sampling theorem, and for a signal of hundreds of MHz, it should be less than 5 ns; A(m,x,y,t) is the known encoding and modulation matrix of the digital micromirror device. Using the electrical signal y(m,ti) of the photodetector m times at time ti, equation (1) is solved to obtain q(x,y,ti) at time ti. In this way, using the electrical signal y(m,t) of the photodetector m times at different times t, all times t are obtained. This process is the demodulation of the reflected light intensity of the detection window.

[0073] b) Obtain the ultrasonic signal intensity using the reflected light intensity from the point detection unit:

[0074] The intensity of reflected light q(x,y,t) of each point reflection unit in the detection window is linearly related to the intensity of ultrasonic signal p(x,y,t) emitted by the imaged object excited by the photoacoustic laser, that is, it satisfies equation (2):

[0075] p(x,y,t)=a*q(x,y,t)+b (2)

[0076] Where a and b are constants, by setting two or more known intensities of p(x,y,t) and the measured q(x,y,t), the values ​​of a and b can be obtained by solving equation (2); then, using equation (2) with known values ​​of a and b, the ultrasonic signal intensity p(x,y,t) can be obtained through the reflected light intensity q(x,y,t). Since p(x,y,t) and q(x,y,t) are linearly related, for non-quantitative photoacoustic imaging, q(x,y,t) can also be used instead of p(x,y,t) to directly reconstruct the photoacoustic image below.

[0077] c) Reconstruction of photoacoustic images:

[0078] Using all the ultrasonic signals p(x,y,t) at time t obtained after steps a) and b), where t is the time difference between the signal acquisition time of the data acquisition card in step a) and the time of laser emission by the photoacoustic laser, the three-dimensional photoacoustic image can be obtained by filtering and back-projection reconstruction using p(x,y,t).

[0079] The imaging steps are as follows:

[0080] 1) The probe laser 7 (such as a helium-neon laser) emits a probe laser, which is separated into p-polarized light by the polarization beam splitter 8. The p-polarized light is then expanded by the second beam expander 9 to form a probe light surface source, which is incident on the graphene layer or metal film on the bottom surface of the prism of the coupling sensor 10, causing a surface plasmon resonance phenomenon and reflecting off the prism. The probe light surface source reflected off the prism is encoded and modulated by the digital micromirror device 11 to form structured light, which is then focused by the convex lens 12 into the photodetector 13.

[0081] 2) The computer 15 controls the digital micromirror device 11 to complete one encoding.

[0082] 3) The photoacoustic laser 1 emits an excitation light pulse of a certain wavelength (e.g., 532nm), which is reflected by the mirror 2 and expanded by the first beam expander 3 to form an excitation light surface source that illuminates the imaging object 6 on the stage 4. The imaging object 6 emits an ultrasonic signal, which passes through the thin film of the sample cell 5 and then propagates through the liquid in the sample cell 5 to the liquid near the graphene layer or metal film of the coupling sensor 10, causing the refractive index of the liquid to change over time. This, in turn, causes a change in the intensity of the detection light surface source reflected by the graphene layer or metal film of the coupling sensor 10, and ultimately causes a change in the electrical signal generated by the structured light formed by the encoding and modulation of the digital micromirror device 11 being focused by the convex lens 12 to the photodetector 13.

[0083] 4) At the same time as the photoacoustic laser 1 emits an excitation light pulse, the data acquisition card 14 begins to acquire the electrical signal of the photodetector 13 and transmits it to the computer 15.

[0084] 5) Repeat steps 2) to 4) multiple times.

[0085] 6) The image reconstruction module of computer 15 demodulates the electrical signals acquired multiple times, uses the reflected light intensity of the point detection unit to obtain the ultrasonic signal intensity and reconstruct the photoacoustic image, and finally obtains a three-dimensional photoacoustic image.

[0086] In summary, this application, while ensuring broadband detection, uses surface detection to achieve three-dimensional photoacoustic imaging of the dynamic process of light intensity changes caused by ultrasonic signals. This results in higher signal-to-noise ratio and sensitivity, faster imaging speed, and no need to mechanically move the imaging object or ultrasonic detection device.

[0087] Any aspects not covered in this invention are applicable to existing technologies.

Claims

1. A photoacoustic imaging device based on structured light detection, comprising a photoacoustic laser, a mirror, a first beam expander, a stage, a sample cell, a detector laser, a polarizing beam splitter, a coupling sensor, a data acquisition card, and a computer, wherein the coupling sensor is a prism with a graphene layer or a metal film coated on its bottom; characterized in that, The photoacoustic imaging device also includes a second beam expander, a digital micromirror device, a convex lens, and a photodetector. The sample cell contains liquid and has a window at the bottom sealed with an acoustically transparent membrane. The acoustically transparent membrane contacts the imaging object through an ultrasonic coupling agent. The imaging object is placed on a stage. The graphene layer or metal film at the bottom of the prism of the coupling sensor is immersed in the liquid in the sample cell. The digital micromirror device is connected to a computer, the photodetector is connected to a data acquisition card, and the data acquisition card is connected to the computer. The photoacoustic laser is also connected to the computer. The probe laser emits a probe laser beam, which is separated into p-polarized light by a polarization beam splitter. The p-polarized light is then expanded by a second beam expander to form a probe light surface source, which is incident on the bottom of the coupling sensor. The incident direction is adjusted to induce surface plasmon resonance in the graphene layer or metal film at the bottom of the coupling sensor. The area illuminated by the probe light surface source at the bottom of the coupling sensor serves as the detection window. The photoacoustic laser emits pulsed excitation light, which is reflected by a mirror and expanded by a first beam expander to form an excitation light surface source that illuminates the imaging object on the stage. The imaging object emits an ultrasonic signal, which is transmitted through the sample. The sound-transparent film in the sample cell propagates through the liquid in the sample cell to the bottom detection window of the coupling sensor, causing the refractive index of the liquid near the graphene layer or metal film at the detection window to change over time. The change in the refractive index of the liquid near the graphene layer or metal film at the detection window causes a change in the reflectivity of p-polarized light at the detection window. The detection light reflected from the detection window of the coupling sensor is encoded and modulated by a digital micromirror device to form structured light, which is then reflected by a convex lens and focused into a photodetector. The photodetector converts the light signal into an electrical signal, which is then acquired by a data acquisition card and sent to a computer for image reconstruction. The photodetector is a single-pixel photodetector; The computer is equipped with an image reconstruction module, which is used to calculate three-dimensional photoacoustic images. The image reconstruction module includes three parts: demodulation of the light intensity reflected from the detection window, obtaining the ultrasonic signal intensity using the light intensity reflected from the detection window, and reconstruction of the photoacoustic image. a) Demodulation of the intensity of light reflected from the detection window: Set the plane containing the prism bottom detection window S as the xy plane with z coordinate 0, the center of the detection window S as the origin of the xy plane, and the positive z direction is the direction perpendicular to the plane containing the prism bottom detection window S downward. The light intensity signal obtained by the photodetector is the sum of the light intensity of the structured light formed by the reflected light from the light source through the detection window and the encoding and modulation by the digital micromirror device. The detection window S is discretized into N×N point reflection units. The electrical signal corresponding to the light intensity of the structured light encoded by the digital micromirror device at time t is denoted as y(m,t). The reflected light intensity reflected by the point reflection unit in the detection window S on the bottom surface of the prism of the coupled sensor at time t is q(x,y,t), where (x,y) are the position coordinates of the point reflection unit. The encoding and modulation matrix of the digital micromirror device at time t is denoted as A(m,x,y,t), where t is the time difference between the signal acquisition time of the data acquisition card and the time of laser emission by the photoacoustic laser. The relationship between the electrical signal y(m,t), the coding modulation matrix A(m,x,y,t), and the reflected light intensity q(x,y,t) is given by equation (1): y(m,t)=A(m,x,y,t)*q(x,y,t) (1) By collecting the time-series electrical signals of the photodetector m times, m known y(m,t) are obtained, t=t0,t1,…,ti,…tn, where n is an integer, y(m,t) is a time sequence, and the maximum value of t, tn, is greater than the maximum value of the distance between the voxel in the three-dimensional imaging region of the photoacoustic imaging object and the point reflection unit in the detection window S divided by the sound speed in the imaging object. The interval of t follows the sampling theorem, and for a signal of hundreds of MHz, it should be less than 5 ns; A(m,x,y,t) is the known encoding modulation matrix of the digital micromirror device. Using the electrical signal y(m,ti) of the photodetector m times at time ti, the equation (1) is solved to obtain q(x,y,ti) at time ti. In this way, using the electrical signal y(m,t) of the photodetector m times at different times t, all times t are obtained. This process is the demodulation of the reflected light intensity of the detection window. b) Obtain the ultrasonic signal intensity using the reflected light intensity from the detection window: The intensity of reflected light q(x,y,t) of each point reflection unit in the detection window is linearly related to the intensity of ultrasonic signal p(x,y,t) emitted by the imaged object excited by the photoacoustic laser, that is, it satisfies equation (2): p(x,y,t)=a*q(x,y,t)+b (2) Where a and b are constants, by setting two or more known intensities of p(x,y,t) and measured q(x,y,t), the values ​​of a and b can be obtained by solving equation (2); then, by using equation (2) with known values ​​of a and b, the ultrasonic signal intensity p(x,y,t) can be obtained through the reflected light intensity q(x,y,t); c) Reconstruction of photoacoustic images: Using all the ultrasonic signals p(x,y,t) at time t obtained after steps a) and b), where t is the time difference between the signal acquisition time of the data acquisition card in step a) and the time of laser emission by the photoacoustic laser, the three-dimensional photoacoustic image can be obtained by filtering and back-projection reconstruction using p(x,y,t).

2. The photoacoustic imaging device based on structured light detection according to claim 1, characterized in that, The response speed bandwidth of the photodetector to changes in light intensity should be greater than 100 MHz to meet the requirements for responding to rapid changes in ultrasonic signals with frequencies above 100 MHz.

3. The photoacoustic imaging device based on structured light detection according to claim 1, characterized in that, The digital micromirror device is encoded using Fourier coding, Hadamard coding, or Gray coding.

4. A photoacoustic imaging method based on structured light detection, characterized in that, The imaging method uses the device described in claim 1, and the specific steps are as follows: 1) The probe laser emits a probe laser, which is separated into p-polarized light by a polarization beam splitter. The p-polarized light is then expanded by a second beam expander to form a probe light surface source, which is incident on the graphene layer or metal film on the bottom surface of the prism of the coupling sensor. The irradiated area serves as the detection window, causing a surface plasmon resonance phenomenon, which is then reflected out of the prism. The probe light surface source reflected from the prism is encoded and modulated by a digital micromirror device to form structured light, which is then focused into the photodetector by a convex lens. 2) The computer-controlled digital micromirror device achieves one-time encoding and remains unchanged. The encoding retention time of the digital micromirror device should be greater than the time length obtained by dividing the maximum value of the distance between the voxel in the three-dimensional imaging area of ​​the imaging object and the point reflection unit of the detection window by the speed of sound in the imaging object. 3) The photoacoustic laser emits an excitation light pulse, which is reflected by a mirror and then expanded by the first beam expander to form an excitation light surface light source that illuminates the imaging object on the stage. The imaging object emits an ultrasonic signal, which passes through the thin film in the sample cell and then propagates through the liquid in the sample cell to the liquid near the graphene layer or metal film at the detection window of the coupling sensor. This causes the refractive index of the liquid to change over time, which in turn causes a change in the intensity of the detection light surface light source reflected by the graphene layer or metal film at the detection window of the coupling sensor. Ultimately, this causes a change in the electrical signal generated by the structured light formed by the encoding and modulation of the digital micromirror device being focused by the convex lens onto the photodetector. 4) Simultaneously with the photoacoustic laser emitting the excitation light pulse, the data acquisition card begins to acquire the electrical signal of the photodetector and transmits it to the computer. The acquisition duration is equal to the encoding retention duration of the digital micromirror device. 5) Repeat steps 2) to 4) multiple times; 6) Demodulate the reflected light intensity of the detection window sequentially using the electrical signals collected multiple times. Based on the reflected light intensity of each point reflection unit of the detection window, and combined with the linear relationship between the reflected light intensity and the ultrasonic signal intensity, obtain the ultrasonic signal intensity. Finally, obtain a three-dimensional photoacoustic image through filtering and back projection reconstruction.

Images