Super-resolution ultrasonic holographic ghost imaging device and imaging method based on photoacoustic waves

Abstract

The invention discloses a super-resolution ultrasonic holographic ghost imaging device based on photoacoustic waves and an imaging method thereof, comprising a collimated light source, a filtering beam expansion optical path, a spatial light modulator, a projection imaging optical path, a photoacoustic converter, a water tank located above the photoacoustic converter to press a sample and blocked with a film, and a sound wave detection and acquisition system, wherein the collimated light source emits parallel light, the diameter of the parallel light is enlarged and spatially filtered through the filtering beam expansion optical path, and the light beam with spatial coding is modulated into a light beam with spatial coding through the spatial light modulator, and the light beam with spatial coding is irradiated to the photoacoustic converter through the projection imaging optical path to generate a sound wave with a corresponding intensity distribution, and the sound wave changes in amplitude and phase after passing through the sample, and then enters the water tank through the film to continue to propagate and be received, and the sound wave detection and acquisition system reconstructs a hologram of the transmittance and phase change of the sound wave passing through the sample. The invention can image the ultrasonic hologram of the sample so as to fully understand the composition, morphology and other structural information of the sample.

Description

Technical Field

[0001] The present invention relates to the field of ultrasonic microscopic imaging, and in particular to a super-resolution ultrasonic holographic ghost imaging device based on photoacoustic waves and an imaging method thereof. Background Art

[0002] Ultrasonic imaging is widely used in medical diagnosis, industrial flaw detection, and underwater marine detection due to its non-destructive, non-invasive, and deep imaging characteristics. Ultrasonic microscopy, as a type of ultrasonic imaging, can be used for micron-level non-destructive testing of materials and high-resolution imaging of biological samples due to its high imaging resolution. Existing ultrasonic microscopy requires the use of ultra-high frequency (center frequency above 100MHz) focused ultrasonic transducers to excite sound waves. Due to the huge attenuation of ultra-high frequency sound waves in the medium, the imaging depth drops sharply, and the weaker sound wave signal leads to a decrease in the signal-to-noise ratio.

[0003] Near-field scanning imaging technology can achieve super-resolution imaging at lower frequency sound waves. It has been adopted in the fields of optical imaging, atomic force microscopy and scanning tunneling microscopy, but it also faces two challenges. 1. The resolution of near-field scanning imaging is determined by the size of the detector (or transmitter). However, as the size of the acoustic detector or transmitter decreases, its sensitivity to detecting sound waves and the intensity of the emitted sound waves will drop sharply, resulting in a poor signal-to-noise ratio. At the same time, there are huge technical challenges in processing and manufacturing small-sized acoustic detectors or transmitters. 2. Near-field scanning imaging relies on point-by-point scanning (Raster Scanning). A large number of pixels means huge time costs, which reduces imaging efficiency.

[0004] The photoacoustic effect describes a phenomenon in which light excites sound waves. Its mechanism is that after a substance absorbs the energy of light, it converts the light energy into heat energy. The heat energy causes the substance to expand and produce thermal stress. The release of thermal stress produces sound waves, which are called photoacoustic waves. Currently, the photoacoustic effect has demonstrated its practical value in the fields of photoacoustic spectroscopy gas detection and photoacoustic imaging. It has been confirmed that the technology of using lasers to excite ultrasound waves is mature and can generate fairly strong ultrasound waves even at the spatial scale of hundreds of nanometers.

[0005] At present, the above problems need to be solved urgently. Summary of the invention

[0006] Purpose of the invention: The first purpose of the present invention is to propose a super-resolution ultrasonic holographic ghost imaging device based on photoacoustic waves that can reconstruct a sample holographic image so as to fully understand the sample morphological structure.

[0007] The second object of the present invention is to provide an imaging method for a super-resolution ultrasonic holographic ghost imaging device based on photoacoustic waves.

[0008] Technical solution: To achieve the above objectives, the present invention discloses a super-resolution ultrasonic holographic ghost imaging device based on photoacoustic waves, comprising a collimated light source, a filtering and beam expanding optical path, a spatial light modulator for encoding light illumination patterns, a projection imaging optical path, a photoacoustic converter for placing samples, a water tank located above the photoacoustic converter to press the samples and sealed with a thin film, and an acoustic wave detection and collection system. The collimated light source emits parallel light, which is amplified by the filtering and beam expanding optical path and spatially filtered, and modulated into a light beam with spatial coding by the spatial light modulator. The light beam with spatial coding is irradiated to the photoacoustic converter through the projection imaging optical path to generate an acoustic wave with a corresponding intensity distribution. The acoustic wave changes in amplitude and phase after passing through the sample, and then enters the water tank through the thin film to continue propagating and is received by the acoustic wave detection and collection system. The acoustic wave detection and collection system associates and calculates the detected acoustic wave signal and the coding matrix of the spatial light modulator to reconstruct a hologram of the transmittance and phase change of the acoustic wave passing through the sample.

[0009] Among them, the spatial light modulator is a digital micromirror device based on light intensity regulation, and the projection imaging optical path includes a beam splitter, an objective lens, a tube lens and a CCD camera. The light emitted from the spatial light modulator passes through the beam splitter, and then passes through the tube lens and the objective lens in turn, and is magnified or reduced in a certain proportion and projected onto the focal plane. The light is reflected by the photoacoustic converter and passes through the objective lens and the tube lens to form an image on the CCD camera.

[0010] Preferably, the spatial light modulator is a digital micromirror device based on light intensity regulation, and the projection imaging optical path includes a beam splitter, an objective lens, a sleeve lens and a CCD camera. The light emitted from the spatial light modulator passes through the beam splitter, and then passes through the sleeve lens and the objective lens in sequence, and then is diffracted and projected onto the focal plane. The light is reflected by the photoacoustic converter, passes through the objective lens and the sleeve lens, and then forms an image on the CCD camera.

[0011] Furthermore, the collimated light source is a pulsed laser or a continuous light with modulated intensity.

[0012] Furthermore, the filtering and beam expanding optical path includes a first condensing lens, a filter aperture for filtering high-frequency components, and a second condensing lens which are sequentially arranged along the incident direction of the light.

[0013] Preferably, the photoacoustic converter includes a transparent hard substrate and a film containing a light-absorbing medium and a heat-expanding medium. When a light beam with spatial coding is irradiated onto the photoacoustic converter, the light-absorbing medium converts light energy into heat energy. After the heat diffuses into the heat-expanding medium, thermal expansion occurs, thereby generating sound waves.

[0014] Furthermore, the acoustic wave detection and acquisition system includes an acoustic wave detector, a preamplifier, a data acquisition card and a computer located in the water tank, wherein the acoustic waves detected by the acoustic wave detector are transmitted to the computer through the preamplifier and the data acquisition card in turn, and the computer reconstructs a hologram of the transmittance and phase change of the acoustic wave passing through the sample based on the acoustic wave.

[0015] Furthermore, a beam sampler is provided on the optical path between the filter beam expansion optical path and the spatial light modulator. The beam sampler reflects a small amount of light to a photodetector used for laser intensity calibration. The light intensity signal detected by the photodetector is collected by a data acquisition card.

[0016] The present invention provides an imaging method of a super-resolution ultrasonic holographic ghost imaging device based on photoacoustic waves, comprising the following steps:

[0017] Adjust the height and position of the photoacoustic converter so that when the CCD camera has a clear image, the photoacoustic converter is located on the focal plane of the objective lens;

[0018] The preset sampling pattern set is stored in the spatial light modulator. When the collimated light source is a pulsed laser, the light beam is irradiated onto the photodetector. The light intensity signal detected by the photodetector is collected by the data acquisition card, and a trigger signal is output to the spatial light modulator to trigger the switching of the next projection pattern. After the light beam is irradiated onto the spatial light modulator, it passes through the projection imaging optical path and is projected onto the photoacoustic converter. When the collimated light source is a continuous light with modulated intensity, the computer controls the spatial light modulator to switch the next projection pattern at equal time intervals. After the light beam is irradiated onto the spatial light modulator, it passes through the projection imaging optical path and is projected onto the photoacoustic converter.

[0019] The light beam with spatial coding is irradiated onto the photoacoustic converter, generating sound waves with corresponding intensity distribution. The sound waves with spatial coding propagate through the sample, the film at the bottom of the water tank and the water in the water tank in sequence, and are detected by the sound wave detector. The detected signal is amplified by the preamplifier, collected by the data acquisition card, and transmitted to the computer.

[0020] The pattern generated by the spatial light modulator is changed multiple times, and each pattern generates a corresponding sound wave; the computer uses the sound wave detection signal set and the pattern set generated by the spatial light modulator, and uses correlation calculation or compressed sensing to reconstruct a hologram of the transmittance and phase change of the sound wave passing through the sample.

[0021] Preferably, the computer uses the acoustic wave detection signal set and the pattern set generated by the spatial light modulator to reconstruct a hologram of the transmittance and phase change of the acoustic wave passing through the sample by using correlation calculation or compressed sensing. The specific steps are:

[0022] First, when there is no sample, the spatial light modulator is in the fully open state and the reference signal r(t) is measured; the sampling matrix I encoded by the spatial light modulator is usedn (x, y) sampling, n represents the pattern of the nth sampling, and the detection signal data s of each sampling n (t), after the acquisition is completed, the acoustic wave detection signal set S(t) is obtained, and the pattern set generated by the spatial light modulator is I(x,y);

[0023] Define the time response function after the sound wave passes through the sample as h(x, y, t), and associate the calculated signal set S(t) and the pattern set I(x, y) to obtain the convolution result of the time response function h(x, y, t) and the reference signal;

[0024] Under the premise of knowing the reference signal r(t), after Fourier transformation, the transmittance and phase change of the photoacoustic wave after passing through the sample are calculated. The calculation formula is:

[0025] T(x,y,f)=|F t [h(x,y,t)]|

[0026] P(x,y,f)=argF t [h(x,y,t)]

[0027] Where T(x, y, f) is the transmittance spectrum after the sound wave passes through the sample, P(x, y, f) is the phase change spectrum of the sample, and Ft represents the Fourier transform.

[0028] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages:

[0029] (1) The present invention uses a collimated light source to emit laser light, which is focused into a parallel light beam through a filtering and beam expansion optical path, and modulated into a light beam with spatial coding through a spatial light modulator. The light beam with spatial coding is irradiated to a photoacoustic converter through a projection imaging optical path to generate a sound wave with a corresponding intensity distribution. After passing through the sample, the sound wave undergoes amplitude and phase changes, and then enters the water tank through a thin film to continue propagating and is received by a sound wave detection and collection system. The sound wave detection and collection system reconstructs a hologram of the transmittance and phase change of the sound wave passing through the sample based on the sound wave. The reconstructible sample holographic imaging facilitates a comprehensive understanding of the sample morphology and structure.

[0030] (2) The present invention uses a water tank with a plastic film bottom and presses it on the photoacoustic converter. The sample is pressed against the photoacoustic converter under the pressure of the film, ensuring that the sample is in the near field of the sound source;

[0031] (3) The present invention utilizes a beam splitter, an objective lens, a sleeve lens and a CCD camera to form a projection imaging optical path, operates the ghost imaging principle, and conveniently and quickly adjusts the photoacoustic converter to be in the focal plane position;

[0032] (4) The present invention combines near-field imaging, ultrasound imaging, and ghost imaging, and extends optical ghost imaging to acoustic ghost imaging. Compared with optical ghost imaging, the advantages of acoustic waves such as good biological safety, greater penetration depth, and easy emission and detection show great application prospects;

[0033] (5) The present invention utilizes light to excite ultrasonic waves, thus avoiding the use of piezoelectric transducers to excite sound waves. At the same time, the regulation of light is simple and reliable, and the technology is mature. In addition, light can be focused to an extremely small spatial scale, which is conducive to improving spatial resolution. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a schematic diagram of the structure of Embodiment 1 of the present invention;

[0035] Figure 2 This is an optical microscope image of the aluminum wire sample in Example 1 of the present invention;

[0036] Figure 3 This is a transmittance diagram of the aluminum wire after Hadamard sampling in Example 1 of the present invention;

[0037] Figure 4 This is a phase change diagram of the aluminum wire after Hadamard sampling in Example 1 of the present invention;

[0038] Figure 5 This is a schematic diagram of the structure of Example 2 of the present invention. DETAILED DESCRIPTION

[0039] The technical solution of the present invention is further described below in conjunction with the accompanying drawings.

[0040] Example 1

[0041] like Figure 1As shown, the present invention comprises a collimated light source 1, a first condenser lens 2, a filter pinhole 3, a second condenser lens 4, a beam sampler 5, a spatial light modulator 6, a photodetector 7, a beam splitter 8, a sleeve lens 9, an objective lens 10, a photoacoustic converter 11, a CCD camera 12, a sample 13, an acoustic wave detector 14, a water tank 15, a preamplifier, a data acquisition card and a computer; wherein the first condenser lens 2, the filter pinhole 3 and the second condenser lens 4 constitute a filtering beam expansion optical path, and the beam splitter 8, the objective lens 10, the sleeve lens 9 and the CCD camera 12 constitute a projection The imaging optical path, the acoustic wave detector, the preamplifier, the data acquisition card and the computer constitute the acoustic wave detection and acquisition system. The collimated light source 1, the filtering and beam expansion optical path, the spatial light modulator 6, the projection imaging optical path, the photoacoustic converter 11, the sample 13, the water tank 15 blocked with a film and the acoustic wave detection and acquisition system are arranged in sequence along the incident direction of the light; the photodetector 7 is electrically connected to the data acquisition card, the CCD camera 12 is electrically connected to the computer, the CCD camera 12 is used for focusing imaging, and the acoustic wave detector, the preamplifier, the data acquisition card and the computer are electrically connected in sequence. The collimated light source 1 emits a laser, which is focused into a parallel light beam through a filtering and beam expansion optical path, and is modulated into a light beam with spatial coding through a spatial light modulator 6. The light beam with spatial coding is irradiated to the photoacoustic converter 11 through a projection imaging optical path to generate a sound wave with a corresponding intensity distribution. The sound wave changes in amplitude and phase after passing through the sample 13, and then enters the water tank 15 through a thin film to continue propagating, and is received by the sound wave detection and collection system. The sound wave detection and collection system reconstructs a hologram of the transmittance and phase change of the sound wave passing through the sample based on the sound wave.

[0042] The collimated light source 1 is a pulse laser with a pulse width of 2ns and a wavelength of 532nm. The pulse laser can excite high-frequency pulse sound waves. The energy of a single laser pulse is 100μJ and the repetition frequency is 100Hz. The first condenser lens 2, the filter aperture 3 and the second condenser lens 4 arranged in sequence along the incident direction of the light constitute a filter beam expansion optical path, wherein the focal length of the appropriate condenser lens is selected according to the needs so that the collimated light beam is expanded to an appropriate diameter; the filter aperture 3 is located at the focus of the first condenser lens 2; the filter beam expansion optical path expands the diameter of the laser beam by 5 times, and the filter aperture 3 is placed at the focus of the first condenser lens 2 after the light beam passes through the first condenser lens 2 to filter the high-frequency components of the laser beam, and the divergent light beam is converged into a parallel light beam by the second condenser lens 4 again. After the light beam hits the beam sampler, the beam sampler 5 reflects a small amount of light to the photodetector 7 for measurement. The signal measured by the photodetector 7 is collected by the data acquisition card and used to trigger the spatial light modulator 6 to switch images and calibrate the laser intensity. The remaining light beam hits the spatial light modulator 6, and the spatial light modulator 6 is used to control the intensity or phase of the light beam.

[0043] The spatial light modulator 6 is used to encode the laser illumination pattern. The spatial light modulator 6 uses a digital micromirror device based on light intensity regulation. The projection imaging optical path can reduce the spatial light modulator 6 chip pattern by 20 times. The digital micromirror device is manufactured by Texas Instruments and is a high-resolution 0.65 1080p digital micromirror device with 1920*1080 pixels. The distance between the micromirrors is 7.56μm, the array filling factor is as high as 92%, the micromirror size is 7.56μm×7.56μm, and the micromirror's limit resolution under a 532nm wavelength laser is 721nm; therefore, at least 2×2 digital micromirror device combinations of pixels can be distinguished. The digital micromirror device uses a square area composed of 1024×1024 pixels. After being reduced by 20 times, the actual area size is 387.1μm×387.1μm. The specific combination of micromirrors can be combined according to actual needs. The projection imaging optical path includes a beam splitter 8, an objective lens 10, a sleeve lens 9 and a CCD camera 12, wherein the sleeve lens 9 is located between the beam splitter 8 and the objective lens 10. After the light emitted from the spatial light modulator 6 passes through the beam splitter 8, it passes through the sleeve lens 9 and the objective lens 10 in sequence and is diffracted and projected onto the focal plane. The photoacoustic converter 11 is located on the focal plane. After the light is reflected by the photoacoustic converter 11, it passes through the objective lens 10 and the sleeve lens 9 and forms an image on the CCD camera 12. The CCD camera 12 is connected to a computer and the image is displayed on the computer. The height and position of the photoacoustic converter are adjusted. When the image of the CCD camera 12 is clear and consistent with the preset projection pattern displayed on the computer, it means that the photoacoustic converter 11 is exactly located on the focal plane of the objective lens 10, and focusing is achieved.

[0044] The photoacoustic converter 11 realizes efficient conversion of light energy into sound wave energy. The photoacoustic converter 11 includes a transparent hard substrate and an ultra-thin film. The ultra-thin film is a common carbon composite material and a polymer / metal coating structure, which contains a light-absorbing medium and a thermal expansion medium. After the light beam with spatial coding is irradiated on the photoacoustic converter, the light-absorbing medium converts the light energy into thermal energy. After the heat diffuses to the thermal expansion medium, thermal expansion occurs, thereby generating sound waves. In the photoacoustic converter 11 used in the specific embodiment, the transparent hard substrate is a quartz glass substrate. A layer of PDMS-Cr-PDMS-Cr-PDMS-Al is spin-coated on the quartz glass substrate to form an ultra-thin film, wherein the thickness of Cr is 8nm and the thickness of PDMS is about 100nm. Cr is a light-absorbing medium for absorbing laser light; PDMS is a thermal expansion medium for generating thermal expansion, and Al can reflect excess laser light back to the Cr film. The thickness of the photoacoustic converter 11 does not exceed 500nm.

[0045] The diameter of the laser emitted from the collimated light source 1 is about 3 mm, and a small amount of light is reflected by the beam sampler 5 to the photodetector 7 for triggering signals and laser intensity calibration; then the laser beam is expanded 5 times through the filtering and expanding optical path to a diameter of about 15 mm, which can cover the projection area required by the spatial light modulator; the spatially encoded light beam reflected from the spatial light modulator passes through the sleeve lens 9 and the objective lens 10 in turn, and finally irradiates the photoacoustic converter 11 to generate sound waves with corresponding intensity distribution.

[0046] The acoustic wave detector 14 is used for detecting photoacoustic waves. The acoustic wave detector 14 can be a piezoelectric material transducer or an acoustic wave detection technology based on a Fabry–Pérot interferometer. The acoustic wave detector 14 uses the v316 produced by Olympus, with a focal length of 25.4 mm, a nominal unit diameter of 3.175 mm, a center frequency of 20 MHz, and a -6 dB bandwidth of 11.86-28.75 MHz. The acoustic wave detector 14 is immersed in a water tank 15. The focus of the acoustic wave detector 14 is on the focal plane of the objective lens 10, and the sample 11 is in the center of the focal plane. The laser irradiates the photoacoustic converter 11 to generate acoustic waves. The sample uses an aluminum wire with a diameter of about 32.5 μm. The aluminum wire is straightened and placed on the photoacoustic converter 11, and a drop of clean water is dripped on the photoacoustic converter 11 to immerse the aluminum wire in water. Then, the water tank 15 with a plastic film bottom is sealed and pressed on the photoacoustic converter 11. The aluminum wire will be attached to the photoacoustic converter 11 under the extrusion of the plastic film, ensuring the high resolution of near-field imaging; at the same time, the added clean water will be discharged due to the extrusion, but the very little water left between the plastic film and the photoacoustic converter 11 is used to couple the sound waves through the plastic film into the water tank 15, continue to propagate in the water, and finally be received by the sound wave detector 14. The sound wave signal received by the sound wave detector 14 is amplified 25 times by the preamplifier and then sent to the data acquisition card for sampling, and finally the data is uploaded to the computer, and the computer reconstructs a hologram of the transmittance and phase change of the sound wave passing through the sample according to the sound wave.

[0047] The present invention provides an imaging method of a super-resolution ultrasonic holographic ghost imaging device based on photoacoustic waves, comprising the following steps:

[0048] Adjust the height and position of the photoacoustic converter so that when the CCD camera has a clear image, the photoacoustic converter is located on the focal plane of the objective lens;

[0049] The preset sampling pattern set is stored in the spatial light modulator. When the collimated light source is a pulsed laser, the light beam is irradiated onto the photodetector. The laser is collected by the data acquisition card, and the trigger signal is output to the spatial light modulator to switch to the next projection pattern. After the light beam is irradiated onto the spatial light modulator, it passes through the projection imaging optical path and is projected onto the photoacoustic converter at a certain scale.

[0050] The light beam with spatial coding is irradiated onto the photoacoustic converter, generating sound waves with corresponding pattern intensity distribution. The sound waves with spatial coding propagate through the sample, the film at the bottom of the water tank and the water in the water tank in sequence, and are detected by the sound wave detector. The detected signal is amplified by the preamplifier, collected by the data acquisition card, and transmitted to the computer.

[0051] The pattern generated by the spatial light modulator is changed multiple times, and each pattern generates a corresponding sound wave; the computer uses the sound wave detection signal set and the pattern set generated by the spatial light modulator to reconstruct a hologram of the transmittance and phase change of the sound wave passing through the sample using correlation calculation or compressed sensing;

[0052] The specific steps of using correlation computing or compressed sensing to reconstruct the hologram of the transmittance and phase change of the sound wave passing through the sample are:

[0053] First, the reference signal r(t) is measured without placing sample 13. After passing through the 16384-order Hadamard sampling matrix I n After (x, y) sampling, the image is divided into 128×128 pixels, n represents the pattern of the nth sampling, and the detection signal data s of each sampling n (t), after all data are collected, the acoustic wave detection signal set S(t) is obtained, and the pattern set generated by the spatial light modulator is I(x, y);

[0054] According to the formula Calculate the convolution of the sample's time response function and the reference signal Obtain the known time response function h(x,y,t) of the acoustic detection signal;

[0055] Under the premise of knowing the reference signal r(t), after Fourier transformation, the transmittance and phase change of the photoacoustic wave after passing through the sample are calculated. The calculation formula is:

[0056]

[0057] Where T(x,y,f) is the transmittance of the sample as it changes with frequency, P(x,y,f) is the phase change of the sample, h(x,y,t) is the known time response function of the acoustic detection signal, and Ft represents the Fourier transform.

[0058] The derivation process of formula (4) is:

[0059] The z=0 plane is the plane where the photoacoustic converter 11 is located, the laser flux distribution is I(x,y), and it is assumed that the acoustic wave generated by the unit intensity laser is p(t). When there is no sample, the signal received by the acoustic wave detector 14 is S(t):

[0060]

[0061] Where * is the time convolution, (x, y) is the projected laser area, P is any point in the area, and R is the spatial impulse response of the acoustic wave detector 14 at point P; assuming that the spatial impulse response of the acoustic wave detector 14 in the laser projection area is independent of point P, the reference signal r(t) = p(t)*R(P, t) is defined; formula (1) can be simplified as:

[0062]

[0063] The sample is placed close to the photoacoustic converter 11. After the photoacoustic wave generated by the photoacoustic point source on the photoacoustic converter 11 passes through the sample, it is scattered and absorbed by the sample, resulting in a change in transmittance. At the same time, the different sound velocities of the sample cause the phase of the sound wave to change. The time response function of the sample is defined as h(x, y, t), so the signal detected by the sound wave detector 14 after passing through the sample is S n (t):

[0064]

[0065] Where n is the sampling number, by changing the laser irradiation pattern I n (x, y), and obtain the waveform and amplitude S of the corresponding photoacoustic wave signal n (t); By designing a sampling scheme and combining it with compressed sensing and deep learning, super-resolution ultrasonic microscopy imaging is achieved based on formula (3), and the sampling rate can be greatly reduced.

[0066] When the time response function h(x, y, t) is known, the Fourier transform is used, and the Fourier transform is represented by Ft to obtain the sample's transmittance T(x, y, f) and phase change P(x, y, f) as the frequency changes, as shown in formula (4):

[0067]

[0068] like Figure 2 As shown in the optical microscope image of the aluminum wire sample, Figure 3 The transmittance diagram of aluminum wire after Hadamard sampling is shown as follows: Figure 4 The image shows the phase change of aluminum wire after Hadamard sampling. Then 256×256 pixels were used for sampling, and the resolution reached 1.5μm, achieving super-resolution imaging. The sample image includes the transmittance and phase change after the sound wave passes through the sample, so it is called holographic imaging, which helps to fully understand the morphology and structure of the sample.

[0069] In order to overcome the problem that the sensitivity of detecting sound waves and the intensity of transmitting sound waves in near-field scanning imaging decrease sharply as the size decreases, resulting in a poor signal-to-noise ratio, the present invention uses photoacoustic effect to replace the traditional piezoelectric ceramic transducer for transmitting sound waves. Its advantages are mainly reflected in two points: (1) the use of photoacoustic effect can excite ultrasound waves of a certain intensity in a small space, overcoming the shortcomings of traditional transducers; (2) the intensity, pattern control, and wavelength adjustment of the laser are very mature. In addition, unlike near-field scanning imaging, which requires a lot of time to scan the sample point by point, ghost imaging can directly reconstruct the image of the object by associating the sampling pattern with the measurement signal, which can be effectively combined with compressed sensing sampling and deep learning; under the premise of ensuring the imaging quality, the number of samples can be greatly reduced and the imaging speed can be improved; and the shortcoming of near-field scanning imaging that takes a long time to complete can be overcome.

[0070] Example 2

[0071] like Figure 5 As shown, the structure of Example 2 is the same as that of Example 1, except that: the spatial light modulator 6 is a liquid crystal spatial light modulator based on phase regulation, the projection imaging optical path includes a beam splitter 8, an objective lens 10, a sleeve lens 9 and a CCD camera 12, wherein the sleeve lens 9 is located in front of the optical path of the CCD camera 12, the light emitted from the spatial light modulator 6 passes through the beam splitter 8, and is directly diffracted and projected onto the focal plane through the objective lens 10, the light is reflected by the photoacoustic converter 11, passes through the objective lens 10 and the sleeve lens 9, and forms an image on the CCD camera 12, the CCD camera 12 is connected to a computer, and the image is displayed on the computer. The height and position of the photoacoustic converter are adjusted, and when the image of the CCD camera 12 is displayed on the computer and is clear and consistent with the preset projection pattern, it means that the photoacoustic converter 11 is exactly located on the focal plane of the objective lens 10, and focusing is achieved.

[0072] Example 3

[0073] The structure of Example 3 is the same as that of Example 1, except that the collimated light source is a continuous light with modulated intensity, and the continuous light with modulated intensity can generate sound waves of equal frequency.

[0074] The present invention provides an imaging method of a super-resolution ultrasonic holographic ghost imaging device based on photoacoustic waves, comprising the following steps:

[0075] Adjust the height and position of the photoacoustic converter so that when the CCD camera has a clear image, the photoacoustic converter is located on the focal plane of the objective lens;

[0076] The preset sampling pattern set is stored in the spatial light modulator. The collimated light source is a continuous light with modulated intensity. The computer controls the spatial light modulator to switch the next projection pattern at equal time intervals. After the light beam is irradiated to the spatial light modulator, it passes through the projection imaging optical path and is projected onto the photoacoustic converter.

[0077] The light beam with spatial coding is irradiated onto the photoacoustic converter, generating sound waves with corresponding intensity distribution. The sound waves with spatial coding propagate through the sample, the film at the bottom of the water tank and the water in the water tank in sequence, and are detected by the sound wave detector. The detected signal is amplified by the preamplifier, collected by the data acquisition card, and transmitted to the computer.

[0078] The pattern generated by the spatial light modulator is changed multiple times, and each pattern generates a corresponding sound wave; the computer uses the sound wave detection signal set and the pattern set generated by the spatial light modulator to reconstruct a hologram of the transmittance and phase change of the sound wave passing through the sample using correlation calculation or compressed sensing;

[0079] The specific steps of using correlation computing or compressed sensing to reconstruct the hologram of the transmittance and phase change of the sound wave passing through the sample are:

[0080] First, the reference signal r(t) is measured without placing sample 13. After passing through the 16384-order Hadamard sampling matrix I n After (x, y) sampling, the image is divided into 128×128 pixels, n represents the pattern of the nth sampling, and the detection signal data s of each sampling n (t), after all data are collected, the acoustic wave detection signal set S(t) is obtained, and the pattern set generated by the spatial light modulator is I(x, y);

[0081] According to the formula Calculate the convolution of the sample's time response function and the reference signal Obtain the known time response function h(x,y,t) of the acoustic detection signal;

[0082] Under the premise of knowing the reference signal r(t), after Fourier transformation, the transmittance and phase change of the photoacoustic wave after passing through the sample are calculated. The calculation formula is:

[0083]

[0084] Where T(x,y,f) is the transmittance of the sample as it changes with frequency, P(x,y,f) is the phase change of the sample, h(x,y,t) is the known time response function of the acoustic detection signal, and Ft represents the Fourier transform.

Claims

1. A super-resolution ultrasonic holographic ghost imaging device based on photoacoustic waves, Features: The invention comprises a collimated light source, a filtering and beam expanding optical path, a spatial light modulator for encoding light illumination patterns, a projection imaging optical path, a photoacoustic converter for placing samples, a water tank located above the photoacoustic converter to press the samples and sealed with a thin film, and an acoustic wave detection and collection system. The collimated light source emits parallel light, which is amplified by the filtering and beam expanding optical path and spatially filtered, and modulated into a light beam with spatial coding by the spatial light modulator. The light beam with spatial coding is irradiated to the photoacoustic converter through the projection imaging optical path to generate an acoustic wave with a corresponding intensity distribution. The amplitude and phase of the acoustic wave change after passing through the sample, and then the acoustic wave enters the water tank through the thin film to continue to propagate, and is received by the acoustic wave detection and collection system. The acoustic wave detection and collection system associates and calculates the detected acoustic wave signal and the coding matrix of the spatial light modulator to reconstruct a hologram of the transmittance and phase change of the acoustic wave passing through the sample.

2. The super-resolution ultrasonic holographic ghost imaging device based on photoacoustic waves according to claim 1, Features: The spatial light modulator is a liquid crystal spatial light modulator based on phase regulation, and the projection imaging optical path includes a beam splitter, an objective lens, a tube lens and a CCD camera. The light emitted from the spatial light modulator passes through the beam splitter and is directly diffracted by the objective lens and projected onto the focal plane. The light is reflected by the photoacoustic converter and passes through the objective lens and the tube lens to form an image on the CCD camera.

3. The super-resolution ultrasonic holographic ghost imaging device based on photoacoustic waves according to claim 1, Features: The spatial light modulator is a digital micromirror device based on light intensity regulation. The projection imaging optical path includes a beam splitter, an objective lens, a sleeve lens and a CCD camera. The light emitted from the spatial light modulator passes through the beam splitter, and then passes through the sleeve lens and the objective lens in sequence, and is magnified or reduced in a certain proportion and projected onto the focal plane. The light is reflected by the photoacoustic converter, passes through the objective lens and the sleeve lens, and forms an image on the CCD camera.

4. The super-resolution ultrasonic holographic ghost imaging device based on photoacoustic waves according to claim 1, Features: The collimated light source is a pulsed laser or a continuous light with modulated intensity.

5. The super-resolution ultrasonic holographic ghost imaging device based on photoacoustic waves according to claim 1, Features: The filtering and beam expanding optical path comprises a first condensing lens, a filter aperture for filtering high-frequency components and a second condensing lens which are sequentially arranged along the incident direction of the light.

6. The super-resolution ultrasonic holographic ghost imaging device based on photoacoustic waves according to claim 1, Features: The photoacoustic converter comprises a transparent hard substrate and a film containing a light-absorbing medium and a heat-expanding medium. When a light beam with spatial coding is irradiated onto the photoacoustic converter, the light-absorbing medium converts light energy into heat energy. After the heat diffuses to the heat-expanding medium, thermal expansion occurs, thereby generating sound waves.

7. The super-resolution ultrasonic holographic ghost imaging device based on photoacoustic waves according to claim 1, Features: The acoustic wave detection and acquisition system includes an acoustic wave detector, a preamplifier, a data acquisition card and a computer located in a water tank, wherein the acoustic waves detected by the acoustic wave detector are sequentially transmitted to the computer through the preamplifier and the data acquisition card, and the computer reconstructs a hologram of the transmittance and phase change of the acoustic wave passing through the sample based on the acoustic wave.

8. The super-resolution ultrasonic holographic ghost imaging device based on photoacoustic waves according to claim 7, Features: A beam sampler is arranged on the optical path between the filter beam expansion optical path and the spatial light modulator. The beam sampler reflects a small amount of light to a photodetector used for laser intensity calibration. The light intensity signal detected by the photodetector is collected by a data acquisition card.

9. An imaging method of a super-resolution ultrasonic holographic ghost imaging device based on photoacoustic waves according to any one of claims 1 to 8, It is characterized in that The steps include: Adjust the height and position of the photoacoustic converter so that when the CCD camera has a clear image, the photoacoustic converter is located on the focal plane of the objective lens; The preset sampling pattern set is stored in the spatial light modulator. When the collimated light source is a pulsed laser, the light beam is irradiated onto the photodetector. The light intensity signal detected by the photodetector is collected by the data acquisition card, and a trigger signal is output to the spatial light modulator to trigger the switching of the next projection pattern. After the light beam is irradiated onto the spatial light modulator, it passes through the projection imaging optical path and is projected onto the photoacoustic converter. When the collimated light source is a continuous light with modulated intensity, the computer controls the spatial light modulator to switch the next projection pattern at equal time intervals. After the light beam is irradiated onto the spatial light modulator, it passes through the projection imaging optical path and is projected onto the photoacoustic converter. The light beam with spatial coding is irradiated onto the photoacoustic converter, generating sound waves with corresponding intensity distribution. The sound waves with spatial coding propagate through the sample, the film at the bottom of the water tank and the water in the water tank in sequence, and are detected by the sound wave detector. The detected signal is amplified by the preamplifier, collected by the data acquisition card, and transmitted to the computer. The pattern generated by the spatial light modulator is changed multiple times, and each pattern generates a corresponding sound wave; The computer uses the acoustic wave detection signal set and the pattern set generated by the spatial light modulator, and uses correlation computing or compressed sensing to reconstruct a hologram of the transmittance and phase changes of the acoustic wave passing through the sample.

10. The imaging method of the super-resolution ultrasonic holographic ghost imaging device based on photoacoustic waves according to claim 9, It is characterized in that The specific steps of reconstructing a hologram of the transmittance and phase change of the sound wave passing through the sample by using the computer using the sound wave detection signal set and the pattern set generated by the spatial light modulator by using correlation calculation or compressed sensing are as follows: First, when there is no sample, the spatial light modulator is in the fully open state and the reference signal r(t) is measured; the sampling matrix I encoded by the spatial light modulator is used n (x, y) sampling, n represents the pattern of the nth sampling, and the detection signal data s of each sampling n (t), after the acquisition is completed, the acoustic wave detection signal set S(t) is obtained, and the pattern set generated by the spatial light modulator is I(x,y); Define the time response function after the sound wave passes through the sample as h(x, y, t), and associate the calculated signal set S(t) and the pattern set I(x, y) to obtain the convolution result of the time response function h(x, y, t) and the reference signal; Under the premise of knowing the reference signal r(t), after Fourier transformation, the transmittance and phase change of the photoacoustic wave after passing through the sample are calculated. The calculation formula is: T(x,y,f)=|F t [h(x,y,t)]| P(x,y,f)=argF t [h(x,y,t)] Where T(x, y, f) is the transmittance spectrum after the sound wave passes through the sample, P(x, y, f) is the phase change spectrum of the sample, and Ft represents the Fourier transform.

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