A method for single-shot imaging of an internal target in a thick scattering medium using near-infrared light

Abstract

The application discloses a method for single-shot imaging of an internal target of a scattering medium based on near-infrared light. The method utilizes statistical characteristics of speckle and a conclusion that a square root of a covariance of the speckle is equal to an amplitude of autocorrelation of the target, realizes single-shot imaging of the internal target of the thick scattering medium based on the near-infrared light, and can realize imaging of an internal target of fresh chicken breast tissue with a maximum of 20 mean free paths and a maximum imaging resolution of 150 microns. The method only needs to shoot a speckle corresponding to the target to clearly reconstruct the image, which greatly reduces a calculation amount in scattering imaging; the near-infrared light is used as a light source to improve imaging depth, the maximum imaging depth can reach 20 mean free paths, and the method provides a new thought for applications in the fields of biomedicine and the like.

Description

Technical Field

[0001] This application relates to the field of scattering imaging technology, specifically a high-resolution single-shot infrared imaging method based on speckle correlation through a thick scattering medium. Background Technology

[0002] In scattering media, the non-uniformity of the refractive index of the medium causes wavefront distortion after plane light waves are incident, especially when the imaging target is located inside the scattering medium, greatly increasing the difficulty and complexity of imaging. How to effectively image targets hidden in scattering media has always been a significant challenge in the field of optical imaging. To address this problem, researchers have proposed various imaging methods. Early studies used ultrafast time-gate technology to locate hidden targets and verified the feasibility of imaging in scattering media. Subsequently, cameras based on single-photon time-of-flight can rapidly image targets in strongly scattering media without requiring long sampling times. Using polarized light detection technology, studies have also achieved the separation of ballistic photons from background scattered light, thus successfully imaging targets in turbid media. However, many methods are limited by thick scattering media and cannot be widely applied. To overcome this limitation, some studies have employed techniques such as fluorescent labeling, compressed sensing, and spatial light modulation to improve imaging resolution and depth, but these methods still require capturing a large number of speckle images and performing complex data processing. Furthermore, techniques combining the Fourier shower curtain effect and deep learning have been proposed to achieve deep imaging in scattering media, but most still rely on acquiring large amounts of speckle images. In recent years, new label-free semi-supervised learning, spatial-temporal coding (STEP), and photoacoustic imaging methods have also been proposed, further advancing the development of scattering medium imaging technology. These methods have improved imaging depth, speed, and resolution to varying degrees, but still face challenges such as high sampling requirements and adaptability to thick scattering media.

[0003] This study proposes an imaging method based on near-infrared light sources and speckle statistics, aiming to solve the problem of single-frame imaging of targets hidden in thick scattering media, especially to achieve deeper penetration and imaging in thicker biological tissues or scattering media. Summary of the Invention

[0004] To achieve high-resolution, rapid imaging of targets within thick scattering media, this invention proposes a single-shot imaging method based on near-infrared light. Through theoretical model deduction, it is determined that the square root of the speckle covariance equals the autocorrelation amplitude of the target. A phase retrieval algorithm (HIO) is then used to reconstruct the image, yielding a high-fidelity reconstructed image. This invention constructs an experimental optical path using near-infrared light as the light source. A 1550 nm near-infrared fiber laser is used as the light source, passing through an isolator and fiber amplifier before being connected to a fiber collimator. The near-infrared light is then irradiated onto the scattering medium containing the target. A camera then captures the speckle pattern of the target at the exit surface of the scattering medium. The captured speckle pattern is read using Matlab and subjected to phase retrieval algorithm calculation based on covariance to obtain the reconstructed image. This invention ultimately achieves the reconstruction of a target embedded in chicken breast tissue with 20 mean free paths, with a maximum resolution of 150 μm.

[0005] A method for single-shot imaging of targets inside a scattering medium based on near-infrared light;

[0006] The theoretical model described is the core of single-shot imaging. Inspired by the book "Speckle Phenomenon in Optics," this model describes how, when a plane light wave illuminates a scattering medium, the light passes through scattering medium one and illuminates the target in the form of speckled illumination. The light intensity distribution on the target surface can be decomposed into multiple point sources. Photons emitted by each point source pass through scattering medium two via random paths. Finally, the photons meet again on the back surface of scattering medium two. Photons with an optical path difference less than the coherence length interfere with each other to form speckles. This process can be viewed as an impulse response. A short-wave infrared camera then receives the speckle pattern. Based on the description of the statistical characteristics of speckle in the book, we deduce that the square root of the speckle covariance is equal to the amplitude obtained from the autocorrelation of the target. Therefore, we can input the speckle covariance into a phase retrieval algorithm and iteratively reconstruct the target embedded in the scattering medium. Thus, based on this theoretical model, we can reconstruct an image using the speckle pattern of a single target.

[0007] A system for single-shot imaging of targets inside a scattering medium based on near-infrared light;

[0008] The system for single-shot imaging of targets inside a scattering medium based on near-infrared light includes a 1550nm fiber laser as the light source. After passing through a fiber isolator and amplified by a fiber amplifier, the laser is then connected to a fiber laser collimator and emitted. The 1550nm near-infrared light is collimated and then illuminates the scattering medium. It then exits from the rear surface of the scattering medium, and the speckle pattern of the target is captured by a short-wave infrared camera.

[0009] The beneficial effects of this invention are:

[0010] This invention utilizes the conclusion that the square root of the speckle covariance equals the amplitude of the target's autocorrelation to achieve single-shot imaging of targets inside thick scattering media based on near-infrared light. It can image targets inside fresh-cut chicken breast tissue with a mean free path of up to 20 μm, with a maximum imaging resolution of 150 μm. This invention requires only one image of the speckle pattern corresponding to the target to clearly reconstruct the image, significantly reducing the computational burden in scattering imaging and providing new insights for its application in biomedicine and other fields. Attached Figure Description

[0011] Figure 1 This is a schematic diagram of the theoretical model.

[0012] Figure 2 The process of reconstructing an image using a single speckle image is as follows: a. The speckle pattern of the target captured by the camera; b. The covariance map of the speckle; c. The amplitude information of the target obtained by taking the square root of the covariance of the speckle and then feeding it into the HIO algorithm to obtain the reconstructed image of the target; d. The original image of the reconstructed image captured by the camera.

[0013] Figure 3 This is an experimental optical path diagram for imaging a target inside a scattering medium.

[0014] Figure 4 To reconstruct different targets embedded within a thin scattering medium.

[0015] Figure 5 Zinc oxide was used as the scattering medium for imaging internal targets.

[0016] Figure 6 For imaging the interior of a thick scattering medium (chicken breast tissue). a. Overall thickness of chicken breast: 3mm, 2mm in front of the target, 1mm behind the target; b. Overall thickness of chicken breast: 4mm, 3mm in front of the target, 1mm behind the target; c. Overall thickness of chicken breast: 5mm, 4mm in front of the target, 1mm behind the target; Bar: 1mm.

[0017] Figure 7 Image of the internal structure of chicken breast tissue. a. Chicken breast thickness is 4mm, 3mm in front of the target and 1mm behind the target; b. Overall chicken breast thickness is 5mm, 3mm in front of the target and 2mm behind the target; c. Overall chicken breast thickness is 6mm, 3mm in front of the target and 3mm behind the target; Bar: 1mm. Detailed Implementation

[0018] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0019] Figure 1.a illustrates the physical model of this method. First, when a beam of plane light shines on Diffuser1, it passes through Diffuser1 and then shines on the target input surface O. in :

[0020] Formula 1

[0021] here The field of the exit surface of Diffuser1, and then the light from the target output surface O. out Launch:

[0022] Formula 2

[0023] The speckle field generated by Diffuser1 is equivalent to speckle illumination, therefore the amplitude A and phase in the above equation are... This is caused by the speckle field. After the light passes through the second scattering medium, the exit surface of the second scattering medium is set as:

[0024] Formula 3

[0025] Then the light is received by the CCD camera.

[0026] Formula 4

[0027] The uniform speckle pattern captured by the camera, such as Figure 1 As shown in .b; therefore, we can express the speckle pattern captured by the camera as follows:

[0028] Formula 5

[0029] Then we perform covariance calculation on the obtained speckle pattern:

[0030] Formula 6

[0031] The autocorrelation of speckle patterns can be written as:

[0032] Formula 7

[0033] Here, C represents the normalized covariance of the speckle, and * represents the autocorrelation operation. According to formulas (6) and (7), we find that the square root of the speckle covariance is equal to the autocorrelation amplitude of the target O. After obtaining the amplitude information of the target, we send it to the phase retrieval algorithm (HIO). After 250 iterations, we obtain the reconstructed target image. The image reconstruction process is as follows: Figure 2 .

[0034] like Figure 3 As shown, Figure 3The experimental optical path diagram of the proposed method is shown. The target used in the experiment was a metal plate with letters and numbers engraved by laser drilling. 1550nm near-infrared light emitted by a fiber laser (Optilab, Benchtop DFB Source) passed through a fiber isolator (Thorlabs, Fiber Isolator, 1550nm, Low Power, FC / APC) and then into a fiber amplifier (Connet, Mars Fiber Amplifier), where the optical power was amplified to 50mW before being connected to a fiber collimator (Thorlabs, Singlemode GRIN Fiber Collimator, 1550nm, FC / APC). The collimated beam then irradiated the scattering medium embedded in the target, generating speckle patterns which were then captured by a short-wave infrared camera (TEKWIN). The system (SC640) was received; the scattering medium embedded in the target in the experiment was made by clamping the target together with two scattering media, and the scattering surfaces of scattering medium one and scattering medium two were in close contact with the target; the scattering media used were 120-particle (Thorlabs, DGUV10-120) and 220-particle circular glass diffusers (DGUV-220), 450μm thick zinc oxide (zinc oxide plated on a 2×2cm thick quartz glass plate), and chicken breast tissue (1mm, 2mm, 3mm, and 4mm respectively); the short-wave near-infrared camera had a resolution of 512×640 pixels, with a single pixel size of 15μm, and the distance between the camera and scattering medium two was 18cm.

[0035] Four sets of experiments were conducted based on the proposed method to verify its effectiveness and efficiency.

[0036] We usually use the mean free path to determine the thickness of a scattering medium. The mean free path is equal to the reciprocal of the scattering coefficient. When the thickness of a scattering medium is greater than the mean free path, we consider the scattering medium to be a thick scattering medium.

[0037] The first group of experiments was to image different targets embedded in thin scattering media using the proposed method; the second, third, and fourth groups of experiments were to image different targets embedded in scattering media of different thicknesses at different distances using the proposed method.

[0038] like Figure 4 As shown in the figure, this figure presents the experimental results of the first group of experiments.

[0039] The first set of experiments used a set of 120-particle and 220-particle circular glass diffusers, with the target sandwiched between the scattering surfaces of the two diffusers and in close contact with each other. The image shows four columns from left to right: the speckle pattern of the target captured by the camera, the covariance of the target speckle, the reconstructed image, and the original image of the target captured by the camera. Four different target shapes were used in the experiment: 2, Y, E, and 37, with a target size of 2×3 mm. All targets were reconstructed from a single speckle pattern of the target, and the shape and details of the reconstructed targets can be clearly distinguished from the image. Therefore, based on this set of experimental data, we have analyzed and verified that our theoretical model can use a single speckle pattern to reconstruct targets embedded within the scattering medium.

[0040] like Figure 5 As shown in the figure, this figure illustrates the imaging results of the second set of experiments, in which zinc oxide was used as a scattering medium to image the internal target.

[0041] The second set of experiments replaced the scattering medium with 450μm zinc oxide (zinc oxide coated on a 2×25×25mm quartz glass plate) and fresh-cut chicken breast tissue of varying thicknesses. First, we used two 450μm zinc oxide plates as scattering media (thus achieving a mean free path of approximately ten for both plates). The zinc oxide-coated side was placed in close contact with the input and output surfaces of the target, and the camera was positioned 17cm away from the second scattering medium to capture the speckle pattern of the target. The four columns in the figure represent the target speckle pattern, the target reconstructed using covariance, and the original image captured by the camera; all images are reconstructed from a single speckle pattern of the corresponding target. Comparing the reconstructed images with those from the experiment using the thinner scattering medium revealed that the sharpness of the reconstructed images was essentially the same.

[0042] like Figure 6 , Figure 7 As shown, the images illustrate the imaging results through chicken breast tissue from the third and fourth sets of experiments.

[0043] In the experiment, the target was embedded in chicken breast tissue with thicknesses of 3mm, 4mm, 5mm, and 6mm. First, we embedded the target at a distance of 1mm from the back surface of the scattering medium. Figure 6 The figure shows three sets of results. The first column of each set is the original image of the target captured by the camera, and the second column is the image reconstructed using a single speckle image of the corresponding target. The thickness of the scattering medium in these three experiments gradually increased in 1mm increments. Analysis of the experimental results shows that relatively clear images can be recovered for targets with single numbers or letters, but for multiple targets like '37', the results are less clear. Figure 6(a) The recovered image can be clearly distinguished from the gradually decreasing recovery results of the remaining two sets of experiments. Overall, when the distance from the front surface of the target to the front surface of the scattering medium reaches 4 mm, the image recovery quality of the optical system has reached its limit, at which point the entire scattering medium has approximately 15 mean free paths.

[0044] In the fourth experiment, we still used chicken breast as the scattering medium, maintaining a thickness of 3 mm between the front surface of the target and the front surface of the scattering medium, and gradually increasing the thickness between the rear surface of the target and the rear surface of the scattering medium in 1 mm increments. The experimental results are as follows: Figure 7 As shown in the figure, three sets of experimental results are presented. The first column of each set is the original image of the target captured by the camera, and the second column is the recovered image. From the experimental results, we can see that a relatively clear image can be recovered for targets with a single letter or number, but for targets with two numbers, only an outline and some details can be recovered.

Claims

1. A method for single-shot imaging of an internal target in a thick scattering medium based on near-infrared light, characterized in that, The application relates to a theoretical model for realizing single-shot image reconstruction and an imaging light path for realizing single-shot imaging, which are used for realizing recovery and reconstruction of an image of an object in a thick scattering medium. The theoretical model comprises application of a covariance theory based on statistical characteristics of speckles in the theoretical model, and the covariance of the speckles is derived through the statistical characteristics of the speckles and the covariance theory, and then the image is reconstructed through a phase retrieval algorithm.

2. The method for single-shot imaging of an object inside a thick scattering medium based on near-infrared light according to claim 1, wherein, In the image reconstruction process, the covariance of the speckles is calculated and then input into the phase retrieval algorithm, and finally the reconstructed image is obtained after 250 iterations, and the whole image recovery process is realized on MATLAB.

3. The method of claim 1, wherein, The imaging light path comprises a fiber isolator, an optical amplifier, a fiber collimator, a scattering medium embedding an object and an infrared short-wave camera.

4. The method for single-shot imaging of an object inside a thick scattering medium based on near-infrared light according to claim 3, wherein, The scattering medium embedding the object is a combination of 120 granularity and 220 granularity scattering sheets with scattering surfaces close to front and back surfaces of the object, or a combination of two pieces of 450 mu m zinc oxide quartz glass sheets plated on the object, or the object is embedded in chicken breast tissue scattering media with thicknesses of 3 mm, 4 mm, 5 mm and 6 mm respectively.

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