Air coupling photoacoustic imaging system and imaging method based on finite aperture model

Abstract

The invention discloses an air coupling photoacoustic imaging system and method based on a finite aperture model, and belongs to the field of biomedical imaging. The system comprises a lens assembly, a reflector assembly, a focusing lens assembly, a three-axis mobile station, a rotating motor, a self-made ultrasonic transducer clamp, an ultrasonic transducer, a self-made sample clamp, a signal amplifier, a data acquisition card and an imaging processing terminal. Compared with the prior art, traditional water immersion or gel coupling is improved into the mode that air is adopted as a coupling medium, a finite aperture model is introduced to improve the finite aperture effect caused by point source mismatching in the imaging process, the imaging effect is further improved, and compared with other imaging methods in the photoacoustic imaging field, the imaging method has the advantages that the imaging efficiency is improved; a reconstructed image with higher resolution can be obtained, artifacts far away from a rotation center are reduced, and the imaging quality is improved.

Description

Technical Field

[0001] The present invention relates to a photoacoustic imaging system and method, and in particular to an air-coupled photoacoustic imaging system and imaging method based on a finite aperture model. Background Art

[0002] With the continuous advancement and development of modern science and technology, photoacoustic imaging (PAI) is an emerging nondestructive testing method with great potential. It is an ultrasound-based functional imaging method that combines the advantages of ultrasound imaging's high penetration depth and high resolution with the high contrast of optical imaging. In PAI, an ultrashort laser pulse is typically used to illuminate the object to be inspected. The object absorbs the laser energy and becomes excited. Then, through a non-radiative excitation process, the absorbed light energy is converted into heat. This causes the material to expand and contract, producing periodic pressure changes, which in turn generates an ultrasound signal, known as a photoacoustic signal. After this signal is received by an ultrasonic transducer, an image reconstruction algorithm is used to generate a photoacoustic image, representing the internal structure of the object to be inspected. PAI is a hybrid, non-invasive imaging method with the advantages of being radiation-free, non-invasive, and multi-scale. It has rapidly developed in recent decades and has played a significant role in biomedical monitoring and diagnosis, as well as industrial nondestructive testing. It has been applied in a range of fields, including weld defect detection, internal defect detection in lithium metal batteries, and damage assessment of carbon fiber composites.

[0003] Photoacoustic Computed Tomography (PACT) is the most widely used imaging modality in photoacoustic imaging. It typically uses water or ultrasound gel as a coupling agent to ensure efficient transmission of ultrasound waves between the object to be inspected and the ultrasound detector. However, this reliance on a coupling medium also presents limitations, particularly in industrial and medical environments. For example, in industrial inspection, when the object to be inspected has a large surface or complex geometry, covering the entire object with coupling agent is impractical. In these cases, traditional coupling PACT methods may not provide adequate resolution or detection performance. Another major challenge facing PACT is the "finite aperture effect." Most photoacoustic imaging reconstruction algorithms use an ideal point transducer model. While this simplifies computations, ideal point transducers have a small effective area and poor sensitivity. In practical applications, non-focused planar ultrasound transducers with larger effective areas are often used to address the low signal-to-noise ratio. This leads to the inaccurate assumption of the ideal point transducer model for image reconstruction, resulting in a proportional decrease in tangential resolution with increasing distance from the rotation center, a phenomenon known as the finite aperture effect.

[0004] Therefore, the existing technical methods for photoacoustic imaging have the above-mentioned problems such as finite aperture effect and limited application of coupling media. Summary of the Invention

[0005] The purpose of the present invention is to provide an air-coupled photoacoustic imaging system and imaging method based on a finite aperture model. This system aims to address the application limitations and finite aperture effects of traditional water immersion or gel coupling media, further improve the imaging effect, and, compared with other imaging methods in the field of photoacoustic imaging, enhance the image reconstruction quality and meet the imaging needs in complex environments.

[0006] In a first aspect, the present invention provides an air-coupled photoacoustic imaging system based on a finite aperture model, wherein the system includes a laser, a lens assembly, a reflector assembly, a focusing lens assembly, a three-axis moving stage, a rotating motor, a homemade ultrasonic transducer fixture, an ultrasonic transducer, a homemade sample fixture, a signal amplifier, a data acquisition card, and an imaging processing terminal;

[0007] The laser is arranged upstream of the lens assembly, and the reflector assembly is arranged between the lens assembly and the focusing lens assembly;

[0008] The rotary motor is arranged upstream of a 3D printed homemade ultrasonic transducer fixture (L-shaped), and the 3D printed homemade ultrasonic transducer fixture (L-shaped) is arranged on a three-axis moving platform;

[0009] The ultrasonic transducer is arranged on a 3D printed homemade ultrasonic transducer fixture (L-shaped), the sample is arranged on a 3D printed homemade sample fixture (H-shaped), the ultrasonic transducer is arranged on the side of the sample, that is, on the circumference with the sample as the rotation center, and the signal amplifier is arranged downstream of the ultrasonic transducer;

[0010] The rotating motor is in communication connection with the data acquisition card, the ultrasonic transducer is in communication connection with the data acquisition card, the laser is in communication connection with the data acquisition card, and the imaging processing terminal is in communication connection with the data acquisition card;

[0011] The laser light generated by the laser is collimated by the lens assembly and the reflector assembly, and then injected into the focusing lens assembly to obtain a uniformly focused light beam, with air as the coupling medium, to fully irradiate the sample;

[0012] The data acquisition card acquires the ultrasonic signal detected by the ultrasonic transducer and outputs it to the imaging processing terminal; the imaging processing terminal processes the ultrasonic signal obtained by scanning through a finite aperture model algorithm to generate a sample image.

[0013] In the air-coupled photoacoustic imaging system based on the finite aperture model, the sample is placed at the focus of the light beam emitted by the focusing lens.

[0014] In the air-coupled photoacoustic imaging system based on the finite aperture model, the laser is a nanosecond pulse laser.

[0015] In the air-coupled photoacoustic imaging system based on the finite aperture model, the homemade ultrasonic transducer fixture is a homemade fixture made of 3D printing materials and is L-shaped.

[0016] In the air-coupled photoacoustic imaging system based on the finite aperture model, the homemade sample fixture is a homemade fixture made of 3D printing materials and is H-shaped.

[0017] In the air-coupled photoacoustic imaging system based on the finite aperture model, the ultrasonic transducer is a single planar non-focused ultrasonic transducer.

[0018] In the air-coupled photoacoustic imaging system based on the finite aperture model, the coupling medium is air.

[0019] In a second aspect, the present invention further provides an air-coupled photoacoustic imaging method based on a finite aperture model, wherein the imaging method is applied to the imaging system as described in the first aspect above, and the imaging method includes:

[0020] Turn on the laser to emit a laser beam, which is collimated by the lens assembly, reflected by the reflector assembly, and focused by the focusing lens to fully illuminate and scan the sample. The sample is excited by the laser beam to generate ultrasonic waves, which are transmitted to the ultrasonic transducer through air coupling;

[0021] The imaging processing terminal sends a signal acquisition signal to synchronize the laser pulse triggering, the rotary motor rotates, and the data acquisition card starts data acquisition;

[0022] The data acquisition card collects the ultrasonic signal received by the ultrasonic transducer and transmits it to the imaging processing terminal in real time;

[0023] The imaging processing terminal performs imaging processing on the scanned ultrasonic signal through a finite aperture model algorithm and generates a reconstructed image.

[0024] The air-coupled photoacoustic imaging method based on the finite aperture model, wherein in the finite aperture model algorithm, the sensor is modeled as a virtual point detector located at a distance L behind the actual detection surface, and the distance L is determined by calculating the acoustic field of the transducer and the flight time of the acoustic wave.

[0025] The acoustic wave flight time is as follows:

[0026]

[0027] Where r' is the location of the sound source, R i is the center position of the plane transducer detection surface, Δ t is the flight time between the transducer and the sound source, (x, y) is the coordinate of the detected point, R is the rotation radius, θ i is the angular coordinate of the i-th transducer, and v represents the speed of sound.

[0028] The present invention provides an air-coupled photoacoustic imaging system and imaging method based on a finite aperture model, belonging to the field of biomedical imaging. The imaging system includes a laser, a lens assembly, a reflector assembly, a focusing lens assembly, a three-axis motion stage, a rotating motor, a self-made ultrasonic transducer fixture, an ultrasonic transducer, a self-made sample fixture, a signal amplifier, a data acquisition card, and an imaging processing terminal. Compared with the existing technology, the present invention improves traditional water immersion or gel coupling by using air as the coupling medium. It also introduces a finite aperture model to ameliorate the finite aperture effect caused by point source mismatch during imaging, further improving imaging quality and reducing artifacts away from the center of rotation. This allows for higher-resolution reconstructed images and enhanced imaging quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 The device structure diagram of the air-coupled photoacoustic imaging system based on the finite aperture model;

[0030] Figure 2 The present invention is a flow chart of a method for air-coupled photoacoustic imaging based on a finite aperture model;

[0031] Figure 3 This is the application effect diagram of the air-coupled photoacoustic imaging method based on the finite aperture model.

[0032] Reference numerals:

[0033] 1. Laser; 2. Lens assembly; 3. Reflector assembly; 4. Focusing lens assembly; 5. Three-axis moving stage; 6. Homemade ultrasonic transducer fixture; 7. Homemade sample fixture; 8. Sample; 9. Ultrasonic transducer; 10. Rotating motor; 11. Imaging processing terminal; 12. Signal amplifier; 13. Signal acquisition card; 301. First reflector; 302. Second reflector; 303. Third reflector.

[0034] (a) Spectrum of the tungsten PA signal under water coupling conditions, (b) Experimental results of tungsten imaging under water coupling conditions, (c) The cross-section indicated by the white dashed line in Figure b and its FWHM are used as the spatial resolution of the system, (d) Spectrum of the tungsten PA signal under air coupling conditions, (e) Experimental results of tungsten imaging under air coupling conditions, (f) The cross-section indicated by the white dashed line in Figure e and its FWHM are used as the spatial resolution of the system. DETAILED DESCRIPTION

[0035] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0036] It will be understood that when used in this specification and the appended claims, the terms “comprises” and “comprising” indicate the presence of described features, integers, steps, operations, elements and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or groups thereof.

[0037] It should also be understood that the terms used in the present specification are only for the purpose of describing particular embodiments and are not intended to limit the present invention. As used in the present specification and the appended claims, the singular forms "a", "an", and "the" are intended to include the plural forms unless the context clearly indicates otherwise.

[0038] It should be further understood that the term "and / or" used in the present description and the appended claims refers to and includes any and all possible combinations of one or more of the associated listed items.

[0039] In this example, see Figure 1. As shown in the figure, an embodiment of the present invention provides an air-coupled photoacoustic imaging system based on a finite aperture model, wherein the system includes 1 laser, 2 lens assembly, 3 reflector assembly, 4 focusing lens assembly, 5 three-axis moving stage, 6 homemade ultrasonic transducer fixture, 7 homemade sample fixture, 8 sample, 9 ultrasonic transducer, 10 rotating motor, 11 imaging processing terminal, 12 signal amplifier, 13 signal acquisition card; the laser 1 is arranged upstream of the lens assembly 2, the reflector assembly 3 is arranged between the lens assembly 2 and the focusing lens assembly 4; the rotating motor 10 is arranged upstream of the 3D printed homemade ultrasonic transducer fixture (L-type) 6, the 3D printed homemade ultrasonic transducer fixture (L-type) 6 is arranged on the 5 three-axis moving stage; the ultrasonic transducer 9 is arranged on the 3D printed homemade ultrasonic transducer fixture (L-type) 6, the sample 8 is arranged on the 3D printed homemade sample fixture (H-type) 7, the The 9 ultrasonic transducers are arranged on the side of the 8 samples, that is, on the circumference with the samples as the rotation center, and the 12 signal amplifiers are arranged downstream of the 9 ultrasonic transducers; the 10 rotating motor is communicatively connected to the 13 data acquisition card, the 9 ultrasonic transducers are communicatively connected to the 13 data acquisition card, the 1 laser is communicatively connected to the 13 data acquisition card, and the 11 imaging processing terminal is communicatively connected to the 13 data acquisition card; the laser generated by the 1 laser is collimated by the 2 lens assemblies and the 3 reflector assemblies, and then injected into the 4 focusing lens assemblies to obtain a uniformly focused light beam, with air as the coupling medium, to fully irradiate the 8 samples; the 13 data acquisition card collects the ultrasonic signals detected by the 9 ultrasonic transducers and outputs them to the 11 imaging processing terminal; the 11 imaging processing terminal processes the scanned ultrasonic signals through a finite aperture model algorithm to generate a sample image.

[0040] Specifically, 1 laser is set upstream of the imaging light path and serves as an excitation light source, 2 lens components collimate the laser beam generated by the excitation light, 3 reflector components further collimate the beam, and the collimated beam is incident on 4 focusing lens components for focusing, which can maximize the uniformity of irradiation to 8 samples. 8 samples are placed on 7 said 3D printed homemade sample fixture (H type). Under the irradiation of the incident light, due to the transient thermoelastic effect, the sample generates ultrasonic waves under the irradiation of the beam. 9 ultrasonic transducer detects the generated ultrasonic signal (8 samples and 9 ultrasonic transducers are air-coupled). The ultrasonic signal is collected by 13 data acquisition card and sent to 11 imaging processing terminal (computer host). 11 imaging processing terminal is used for subsequent data analysis and image reconstruction. 11 imaging processing terminal sends a signal acquisition signal, synchronizes the 1 laser pulse triggering, the 6 rotary motors to rotate, and the 13 data acquisition card starts data acquisition.

[0041] A 3-mirror assembly is set between the 2-lens assembly and the 4-focusing lens assembly, including a first reflector 301, a second reflector 302 and a third reflector 303, to reflect the excited light beam. Through the combination of the first reflector 301, the second reflector 302 and the third reflector 303, the light beam is reflected and the direction and angle of the light beam are adjusted, so that the light beam can be more accurately irradiated to the 4-focusing lens assembly.

[0042] In the above-mentioned imaging system, signals are collected by 360 ultrasonic probes surrounding the target sample. The acquisition length of each aline signal is 20,000, which conforms to the Nyquist sampling theorem and forms the original signal data before data processing. This is equivalent to 360 times of signal acquisition for each target point on the target sample.

[0043] The air-coupled photoacoustic imaging system based on the finite aperture model, wherein the laser is a 1 nanosecond pulse laser (model Dava-200, Beamtech) with a wavelength of 532 nm, a pulse duration of 10 ns, and a repetition frequency of 1-20 kHz as an excitation source.

[0044] In order to present a better imaging effect, the eight samples are placed at the focus of the light beam emitted by the four focusing lenses, which can enable the tissue samples to generate stronger ultrasonic waves at the focus of the irradiation light beam, thereby improving the concentration of the generated ultrasonic waves, that is, the signal strength of the ultrasonic signal can be increased, thereby improving the imaging quality of the sample.

[0045] In a specific embodiment, the self-made ultrasonic transducer fixture 6 is a self-made fixture made of 3D-printed materials and is L-shaped. The self-made sample fixture 7 is a self-made fixture made of 3D-printed materials and is H-shaped. The ultrasonic transducer 9 is a single planar unfocused ultrasonic transducer. The coupling medium is air.

[0046] In the embodiment of the present application, an air-coupled photoacoustic imaging method based on a finite aperture model is also proposed. Figure 2 As shown in the figure, the imaging method includes steps S1 to S4.

[0047] S1 turns on the laser to emit a laser beam which is collimated by the lens assembly, reflected by the reflector assembly, and focused by the focusing lens to fully illuminate the sample. The laser wavelength is 532nm, the pulse duration is 10ns, and the repetition frequency is 1-20kHz.

[0048] The S2 laser fully irradiates the sample, and the sample is excited by the laser beam to generate ultrasonic waves which are then transmitted to the ultrasonic transducer through air coupling.

[0049] The imaging processing terminal issues a signal acquisition signal, synchronizing the laser pulse triggering, the rotary motor rotation, and the data acquisition card to begin data acquisition. Signals are collected by 360 ultrasound probes surrounding the target sample. Each aline signal acquisition length is 20,000, conforming to the Nyquist sampling theorem, forming the raw signal data before data processing. This is equivalent to 360 signal acquisitions for each target point on the target sample.

[0050] The S3 data acquisition card collects the ultrasonic signals received by the ultrasonic transducer and transmits them to the imaging processing terminal in real time.

[0051] The S4 imaging processing terminal uses a finite aperture model algorithm to process the scanned ultrasonic signal and generate a reconstructed image. In this finite aperture model algorithm, the sensor is modeled as a virtual point detector located at a distance L behind the actual detection surface. The distance L is determined by calculating the transducer's acoustic field and the acoustic wave's time of flight. The acoustic wave's time of flight formula is as follows:

[0052]

[0053] Where r' is the location of the sound source, R i is the center position of the plane transducer detection surface, Δ t is the flight time between the transducer and the sound source, (x, y) is the coordinate of the detected point, R is the rotation radius, θ i is the angular coordinate of the i-th transducer, and v represents the speed of sound.

[0054] The air-coupled photoacoustic imaging system and imaging method developed by the present invention have been verified through experiments using a 0.1 mm tungsten wire. The experimental results under different coupling conditions with the same experimental material were analyzed, and the finite aperture model algorithm was used for processing. When the center frequency was 1 MHz, the resolution under air-coupling conditions was 0.15 mm, and the resolution under water-coupling conditions was 0.40 mm. The experimental results show that the resolution under air-coupling conditions is almost three times that under water-coupling conditions.

[0055] The present invention discloses an air-coupled photoacoustic imaging system and imaging method based on a finite aperture model, belonging to the field of biomedical imaging. The imaging system includes a laser, a lens assembly, a reflector assembly, a focusing lens assembly, a three-axis mobile stage, a rotating motor, a self-made ultrasonic transducer fixture, an ultrasonic transducer, a self-made sample fixture, a signal amplifier, a data acquisition card, and an imaging processing terminal. Compared with the existing technology, the present invention improves traditional water immersion or gel coupling to use air as the coupling medium, and introduces a finite aperture model to improve the finite aperture effect caused by point source mismatch during imaging, further improving the imaging effect. Compared with other imaging methods in the field of photoacoustic imaging, it can obtain higher-resolution reconstructed images, reduce artifacts away from the rotation center, improve imaging quality, and meet imaging needs in complex environments.

[0056] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and such modifications or substitutions are intended to be within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be subject to the scope of protection of the claims.

Claims

1. An air-coupled photoacoustic imaging system and imaging method based on a finite aperture model, characterized in that: The system includes a laser, a lens assembly, a reflector assembly, a focusing lens assembly, a three-axis moving stage, a rotating motor, a self-made ultrasonic transducer fixture, an ultrasonic transducer, a self-made sample fixture, a signal amplifier, a data acquisition card and an imaging processing terminal; The laser is arranged upstream of the lens assembly, and the reflector assembly is arranged between the lens assembly and the focusing lens assembly; The rotary motor is arranged upstream of the 3D printed homemade ultrasonic transducer fixture (L-shaped), and the 3D printed homemade ultrasonic transducer fixture (L-shaped) is arranged on a three-axis moving table; The ultrasonic transducer is arranged on a 3D printed homemade ultrasonic transducer fixture (L-type), the sample is arranged on a 3D printed homemade sample fixture (H-type), the ultrasonic transducer is arranged on the side of the sample, that is, on the circumference with the sample as the rotation center, and the signal amplifier is arranged downstream of the ultrasonic transducer; The rotating motor is in communication connection with the data acquisition card, the ultrasonic transducer is in communication connection with the data acquisition card, the laser is in communication connection with the data acquisition card, and the imaging processing terminal is in communication connection with the data acquisition card; The laser light generated by the laser is collimated by the lens assembly and the reflector assembly, and then injected into the focusing lens assembly to obtain a uniformly focused light beam, with air as the coupling medium, to fully irradiate the sample; The data acquisition card acquires the ultrasonic signal detected by the ultrasonic transducer and outputs it to the imaging processing terminal; the imaging processing terminal processes the ultrasonic signal obtained by scanning through a finite aperture model algorithm to generate a sample image.

2. The air-coupled photoacoustic imaging system based on the finite aperture model according to claim 1, characterized in that: The sample is placed at the focus of the light beam emitted by the focusing lens.

3. The air-coupled photoacoustic imaging system based on the finite aperture model according to claim 1, characterized in that: The laser is a nanosecond pulse laser.

4. The air-coupled photoacoustic imaging system based on the finite aperture model according to claim 1, characterized in that: The homemade ultrasonic transducer fixture is a homemade fixture made of 3D printing materials and is L-shaped.

5. The air-coupled photoacoustic imaging system based on the finite aperture model according to claim 1, characterized in that: The homemade sample fixture is a homemade fixture made of 3D printing materials and is H-shaped.

6. The air-coupled photoacoustic imaging system based on the finite aperture model according to claim 1, characterized in that: The ultrasonic transducer is a single planar non-focused ultrasonic transducer.

7. The air-coupled photoacoustic imaging system based on the finite aperture model according to claim 1, characterized in that: The coupling medium is air.

8. An air-coupled photoacoustic imaging method based on a finite aperture model, characterized in that: The imaging method is applied to the imaging system according to any one of claims 1 to 7, and the imaging method comprises: Turn on the laser to emit a laser beam, which is collimated by a lens assembly, reflected by a reflector assembly, and focused by a focusing lens, so as to fully irradiate and scan the sample. The sample is excited by the laser beam to generate ultrasonic waves, which are transmitted to the ultrasonic transducer through air coupling; The imaging processing terminal sends a signal acquisition signal, synchronizes the laser pulse triggering, the rotary motor rotates, and the data acquisition card starts data acquisition; The data acquisition card collects the ultrasonic signal received by the ultrasonic transducer and transmits it to the imaging processing terminal in real time; The imaging processing terminal performs imaging processing on the scanned ultrasonic signal through a finite aperture model algorithm and generates a reconstructed image.

9. The air-coupled photoacoustic imaging method based on the finite aperture model according to claim 8, characterized in that: In the finite aperture model algorithm, the sensor is modeled as a virtual point detector located at a distance L behind the actual detection surface. The distance L is determined by calculating the acoustic field of the transducer and the flight time of the sound wave.

10. The acoustic wave flight time according to claim 9 is as follows: in, r' is the location of the sound source, R i is the center position of the planar transducer detection surface, Δ t is the flight time between the transducer and the sound source, (x, y) is the coordinate of the detected point, R is the rotation radius, θ i is the angular coordinate of the i-th transducer, and v represents the speed of sound.

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