A method and system for low-artifact laser reflection tomography image reconstruction

Abstract

This invention proposes a low-artifact laser reflection tomography image reconstruction method and system. This invention relates to the field of computational imaging. The system employs a time-division multiplexing structure to separately acquire laser reference waveforms and echo signal waveforms of each target at each angle, and performs discrete digital storage. By introducing correlation calculations between the laser reference waveform and the target echo signal waveforms, real-time correlation waveforms of the echo signals at each angle are obtained. Based on the real-time correlation waveforms of the echo signals at each angle, filtered back-projection image reconstruction is performed to obtain a low-artifact, high-resolution tomographic image of the target. This invention, through the waveform implementation method of time-division multiplexing acquisition and correlation calculation, effectively suppresses noise in the laser echo waveform, preserves the target characteristic waveform with high precision, and solves the problem of suppressing strong noise in low signal-to-noise ratio laser echo signal waveforms.

Description

Technical Field

[0001] This invention belongs to the field of computational imaging technology, and in particular relates to a method and system for low-artifact laser reflection tomography image reconstruction. Background Technology

[0002] In the field of computational imaging, reflective tomography lidar is a laser-based active detection method. Reflective tomography lidar acquires multi-angle range echo information of a target and uses corresponding reconstruction algorithms to realize a two-dimensional contour image of the target. Typically, the image resolution of reflective tomography lidar is mainly determined by the pulse width of the emitted laser source, the system's detection bandwidth, and the signal-to-noise ratio of the echo waveform, making it an effective technique for achieving long-range super-resolution imaging.

[0003] Currently, traditional laser reflection tomography image reconstruction methods utilize filtered backprojection to reconstruct measured waveforms from various angles. However, due to the influence of detector inherent factors and external environmental factors (detector operating principle, response bandwidth, sensitivity, and various photoelectric noises), the measured waveform data of echo signals from various angles exhibit waveform distortion and significant floor noise. This results in severe artifacts in the reconstructed images using traditional filtered backprojection algorithms, limiting image clarity and affecting resolution. Under weak signal-to-noise ratio conditions, the original waveform information is easily submerged by noise, causing the reconstructed image contours to be obscured by artifact noise. When using pulsed light signals to trigger the data acquisition module and receive echo signals, a photodetector module is used to detect the trigger reference signal and the laser echo waveform signal, respectively. The differences in response sensitivity, bandwidth, and noise between the two photodetector modules lead to distortion in the measured laser trigger waveform and signal echo waveform, affecting image reconstruction accuracy and resolution. Furthermore, the system is complex and costly. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention proposes a low-artifact laser reflection tomography image reconstruction method and system.

[0005] The first aspect of this invention discloses a low-artifact laser reflection tomography image reconstruction system, the system comprising a laser, a beam splitter, a transmitting optical component, a receiving optical component, a beam combiner, a photodetector, a data acquisition unit, and a control processor; wherein,

[0006] The control processor is electrically connected to the laser and is used to control the laser's output pulsed laser.

[0007] After the pulsed laser is split by a beam splitter, it forms a reference beam and a signal beam; among which,

[0008] The reference light is transmitted to the photodetector via the beam combiner. The photodetector responds by outputting the time-domain waveform of the reference light, which in turn triggers the collector to perform digital waveform acquisition in order to obtain the laser reference waveform.

[0009] Photodetectors possess the capability of high-bandwidth, high-speed response time-domain signals;

[0010] The signal light is emitted through the transmitting optical component and reflected by the target to form the target echo signal;

[0011] After the target echo signal is received by the receiving optical component, it is transmitted to the photodetector through the beam combiner. The same photodetector responds and outputs the time-domain waveform of the target echo signal, which is then digitally acquired by the acquisition unit triggered by the reference signal to obtain the target echo signal waveform.

[0012] The control processor is also electrically connected to the data acquisition unit to read the laser reference waveform and target echo signal waveform of the same emitted laser pulse, and then calculate the correlation waveform of the target echo signal in real time, and use the correlation waveform of the target echo signal to calculate the target reconstructed image.

[0013] Optionally, the laser is a narrow-pulse laser.

[0014] Optionally, the combiner is a 1×2 multimode fiber combiner, which includes a fiber reference end, a fiber signal end, and a fiber output end, and the length difference between the fiber signal end and the fiber reference end is greater than or equal to one laser pulse width time-of-flight distance.

[0015] Optionally, the emitting optics are used to compress the laser beam divergence angle.

[0016] The second aspect of this invention discloses a low-artifact laser reflection tomography image reconstruction method, which is implemented by the low-artifact laser reflection tomography image reconstruction system described in any of the preceding claims;

[0017] The method includes:

[0018] S1, the control processor controls the laser to emit pulsed laser light; wherein, after the pulsed laser light is split by the beam splitter, it forms a reference beam and a signal beam;

[0019] S2, the reference light is transmitted to the photodetector via the beam combiner, and the photodetector responds by outputting the time-domain waveform of the reference light, which in turn triggers the collector to perform digital waveform acquisition in order to obtain the laser reference waveform at the current target angle;

[0020] S3, the signal light is emitted through the emitting optical component and reflected by the target at the current target angle to form the target echo signal at the current target angle;

[0021] S4, the target echo signal at the current target angle is received by the receiving optical component and transmitted to the photodetector via the beam combiner. The same photodetector responds and outputs the time-domain waveform of the target echo signal at the current target angle. Then, the acquisition unit triggered by the reference signal performs digital waveform acquisition to obtain the target echo signal waveform at the current target angle.

[0022] S5, the control processor reads the laser reference waveform and the target echo signal waveform at the current target angle, then calculates the real-time correlation waveform of the echo signal at the current target angle, and filters the real-time correlation waveform of the echo signal at the current target angle;

[0023] S6, repeat S1-S5 n times, and calculate the real-time correlation waveform filtering results of the echo signals of each angle with an equal rotation angle of Δφ of the target, and then calculate the target reconstruction image; where Δφ·n=360°.

[0024] Optionally, in step S5, the real-time correlation waveform p(r,φ) of the target echo signal is obtained by calculating according to the following formula:

[0025]

[0026] Where r is the distance from the target to the rotation center at the current target angle φ; p ref The laser reference waveform for the current target angle; p sig This is the waveform of the target echo signal at the current target angle.

[0027] Optionally, in step S5, the actual waveform of the real-time correlation waveform of the current target angle echo signal is obtained after filtering. for:

[0028]

[0029] Where, φ i The target angle corresponding to the current real-time relevant waveform. and Forward and inverse Fourier transforms, k is the filter function.

[0030] Optionally, in step S6, the target reconstructed image g(x,y) is:

[0031]

[0032] Where x and y are the two-dimensional coordinates of the target reconstructed image.

[0033] In summary, the solution proposed in this invention has the following technical effects:

[0034] By introducing the real-time emitted laser waveform and performing aliasing correlation with the laser echo waveform, the noise of the laser echo waveform is effectively suppressed through correlation calculation. The target feature waveform is preserved with high precision, solving the problem of strong noise suppression of low signal-to-noise ratio laser echo signal waveform. At the same time, it solves the problem of high image artifact interference in traditional laser reflection tomography image reconstruction methods.

[0035] By introducing a time-division multiplexing optical path into a low-artifact laser reflection tomography image reconstruction system, the output signal waveform difference caused by the bandwidth and sensitivity differences of multiple detectors can be solved by using a single photodetector to detect the waveform of the emitted laser pulse and the echo waveform of the laser pulse in a time-division manner. This results in obtaining more accurate correlation waveforms, improving the imaging accuracy of the system, simplifying the system, improving system integration and stability, and reducing system cost. Attached Figure Description

[0036] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0037] Figure 1 This is a structural diagram of a low-artifact laser reflection tomography image reconstruction system according to an embodiment of the present invention;

[0038] Figure 2 This is a flowchart of a low-artifact laser reflection tomography image reconstruction method according to an embodiment of the present invention;

[0039] Figure 3 Traditional filtering backprojection is used to reconstruct images of cross-shaped targets;

[0040] Figure 4 The image shown is a reconstructed image obtained by a low-artifact laser reflection tomography image reconstruction method according to an embodiment of the present invention. Detailed Implementation

[0041] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0042] The first aspect of this invention discloses a low-artifact laser reflection tomography image reconstruction system. It employs a time-division multiplexing structure to acquire the echo signal waveforms of the target at various angles and the emitted laser waveform at the current angle, and performs discrete digital storage. By introducing the correlation calculation of the measured emitted laser pulse waveform and the laser echo signal, the accurate correlation waveform of the laser echo signal is obtained. Based on the correlation waveform of the laser echo signal, a filtered back-projection image reconstruction is performed to obtain a tomographic image of the target with low artifacts and high resolution.

[0043] Specifically, the system includes a laser, a beam splitter, transmitting optical components, receiving optical components (receiving telescope), a beam combiner, a photodetector, a data acquisition unit (e.g., a high-speed data acquisition card), and a control processor (e.g., an industrial computer); wherein,

[0044] The control processor is electrically connected to the laser and is used to control the laser's output pulsed laser.

[0045] After the pulsed laser is split by a beam splitter, it forms a reference beam and a signal beam; among which,

[0046] The reference light is transmitted to the photodetector via the beam combiner. The photodetector responds by outputting the time-domain waveform of the reference light, which in turn triggers the collector to perform digital waveform acquisition in order to obtain the laser reference waveform.

[0047] The photodetector has the function of high bandwidth and high speed response time domain signal, and the bandwidth is at least sufficient to respond to the laser pulse width waveform;

[0048] The signal light is emitted through the transmitting optical component and reflected by the target to form the target echo signal;

[0049] After the target echo signal is received by the receiving optical component, it is transmitted to the photodetector through the beam combiner. The same photodetector responds and outputs the time-domain waveform of the target echo signal, which is then digitally acquired by the acquisition unit triggered by the reference signal to obtain the target echo signal waveform.

[0050] The control processor is also electrically connected to the data acquisition unit to read the laser reference waveform and target echo signal waveform of the same emitted laser pulse, and then calculate the correlation waveform of the target echo signal in real time, and use the correlation waveform of the target echo signal to calculate the target reconstructed image.

[0051] Optionally, please see Figure 1 The laser is a narrow-pulse laser. The beam combiner is a 1×2 multimode fiber beam combiner, which includes a fiber reference end, a fiber signal end, and a fiber output end, and the length difference between the fiber signal end and the fiber reference end is greater than or equal to one laser pulse width time-of-flight distance. The transmitting optical component is a laser beam expander, used to compress the laser beam divergence angle.

[0052] The system operates as follows: A narrow-pulse laser emits a high-energy narrow-pulse laser beam; after passing through a beam splitter, it enters a laser beam expander to further compress the laser beam divergence angle, radiating towards the target; the beam splitter allows most of the energy to be transmitted into the laser beam expander, while a very small portion is reflected into one end of a 1×2 multimode fiber (fiber reference end), where it is responded to by a high-speed detector, which outputs an electrical signal to trigger a high-speed acquisition card and acquire the emitted pulse waveform as the laser reference waveform; a receiving telescope receives the laser signal scattered by the target and couples it into the other end of the 1×2 multimode fiber (fiber signal end); the echo signal from the fiber signal end is responded to by the same high-speed detector, and the echo waveform is acquired by the high-speed acquisition card as the laser signal waveform; the fiber signal end of the 1×2 multimode fiber combiner is longer than the fiber reference end, with a length difference of not less than one laser pulse width time-of-flight distance; the laser reference waveform and signal waveform of the same emitted pulse laser are digitally stored by an industrial control computer. While coordinating the control of the narrow-pulse laser and the high-speed acquisition card, the industrial control computer calculates the relevant waveforms of the laser echo signal in real time, achieving high-precision laser reflection tomography image reconstruction.

[0053] In this embodiment, by introducing a time-division multiplexing optical path into the low-artifact laser reflection tomography image reconstruction system, the output signal waveform caused by the differences in bandwidth and sensitivity of multiple detectors is solved by using a single photodetector to detect the waveform of the emitted laser pulse and the echo waveform of the laser pulse in a time-division manner. This results in a more accurate correlation waveform, improves the imaging accuracy of the system, simplifies the system, improves the system integration and stability, and reduces the system cost.

[0054] The second aspect of this invention discloses a low-artifact laser reflection tomography image reconstruction method, which is implemented by the low-artifact laser reflection tomography image reconstruction system described in any of the preceding claims;

[0055] Please see Figure 2 The method includes:

[0056] S1, the control processor controls the laser to emit pulsed laser light; wherein, after the pulsed laser light is split by the beam splitter, it forms a reference beam and a signal beam;

[0057] Optionally, the laser emits a narrow laser pulse, which is split into two paths by a beam splitter. One path has high energy and is transmitted into a laser beam expander to irradiate the target in front. The other path has a weaker laser pulse and is coupled into one end (the fiber reference end) of a 1×2 multimode fiber combiner. The high-speed photodetector responds by outputting the time-domain waveform of the emitted pulse, and at the same time triggers the high-speed acquisition card to perform digital waveform acquisition.

[0058] S2, the reference light is transmitted to the photodetector via the beam combiner, and the photodetector responds by outputting the time-domain waveform of the reference light, which in turn triggers the collector to perform digital waveform acquisition in order to obtain the laser reference waveform at the current target angle;

[0059] S3, the signal light is emitted through the emitting optical component and reflected by the target at the current target angle to form the target echo signal at the current target angle;

[0060] S4, the target echo signal at the current target angle is received by the receiving optical component and transmitted to the photodetector via the beam combiner. The same photodetector responds and outputs the time-domain waveform of the target echo signal at the current target angle. Then, the acquisition unit triggered by the reference signal performs digital waveform acquisition to obtain the target echo signal waveform at the current target angle.

[0061] Optionally, the fiber length (fiber signal end) at the other end of the 1×2 multimode fiber combiner is greater than the fiber reference end length, exceeding the transmission distance of not less than one laser pulse width within the fiber; the receiving telescope receives the target's current angle echo signal and couples it into the fiber signal end of the 1×2 multimode fiber combiner, which is then responded to by the same high-speed photodetector and outputs the scattered laser time-domain waveform at the target's current angle, which is then entered into a high-speed acquisition card for digital acquisition.

[0062] The same high-speed photodetector, based on the time-division multiplexing method of optical path, responds to the emitted laser pulses at different time periods with the emitted waveform p. re f and target scattered echo signal waveform p sig And the same acquisition channel of the high-speed acquisition card sequentially performs p re f and p sig Digital discrete sampling and storage of electrical signals. Based on time-division multiplexing of the optical path and photoelectric detection using the same detector, waveform differences caused by variations in bandwidth and sensitivity of different high-speed photodetectors can be eliminated, facilitating the calculation of accurate correlation waveforms in step three. Simultaneously, a single-channel acquisition card can process two electrical signals (p... re f and p sig Accurate data collection improves system integration and reduces costs.

[0063] Optionally, in step S5, the real-time correlation waveform p(r,φ) of the target echo signal is obtained by calculating according to the following formula:

[0064]

[0065] Where r is the distance from the target to the rotation center at the current target angle φ; p ref The laser reference waveform for the current target angle; p sig This is the waveform of the target echo signal at the current target angle.

[0066] Optionally, in step S5, the actual waveform of the real-time correlation waveform of the current target angle echo signal is obtained after filtering. for:

[0067]

[0068] Where, φ i The target angle corresponding to the current real-time relevant waveform. and Forward and inverse Fourier transforms, k is the filter function, typically the Ramp-Lak filter.

[0069] Traditional filtering back projection algorithms are designed for echo signal waveforms p sig In this invention, the echo signal correlation waveform p(r,φ) calculated in step S5 is subjected to filtering processing using the classical filtering back projection algorithm to obtain the actual waveform of the echo signal correlation waveform at the current angle after filtering. It effectively suppresses noise in the laser echo waveform, preserves the target characteristic waveform with high precision, and solves the problem of strong noise suppression in low signal-to-noise ratio laser echo signal waveforms.

[0070] S5, the control processor reads the laser reference waveform and the target echo signal waveform at the current target angle, then calculates the real-time correlation waveform of the echo signal at the current target angle, and filters the real-time correlation waveform of the echo signal at the current target angle;

[0071] S6, repeat S1-S5 n times, and sequentially obtain the real-time correlation waveform filtering results of the echo signals of each angle with an equally spaced target rotation angle of Δφ, and then calculate the target reconstruction image; where Δφ·n=360°.

[0072] Optionally, in step S6, the target reconstructed image g(x,y) is:

[0073]

[0074] Where x and y are the two-dimensional coordinates of the target reconstructed image.

[0075] In summary, the solution proposed in this invention has the following technical effects:

[0076] Laser reflection tomography image reconstruction was achieved by utilizing the correlation waveform data of the echo signal. Since this technique introduces the correlation operation between the measured emitted laser pulse waveform and the echo waveform of the pulse, the signal correlation waveform carrying target feature information is obtained. The image reconstruction work is realized by the filtering back projection method based on the correlation waveform. This is a brand-new and significant optimized image reconstruction algorithm in the field of computational tomography.

[0077] By introducing the real-time emitted laser waveform and performing aliasing correlation with the laser echo waveform, the noise of the laser echo waveform is effectively suppressed through correlation calculation, the target characteristic waveform is preserved with high precision, and the problem of strong noise suppression of low signal-to-noise ratio laser echo signal waveform is solved.

[0078] This method utilizes a filtered back-projection approach based on the correlated waveform of the laser echo signal to directly reconstruct and output images with low artifacts. This improves image resolution and contour sharpness, and further enhances the visibility of image features, especially under low signal-to-noise ratio echo conditions. The imaging results are shown below. Figure 3 and Figure 4 , Figure 3 For cross-shaped targets, traditional filtering backprojection reconstructs the image. Figure 4 The image reconstructed for this application shows that, upon comparison, this application effectively reduces background artifact noise.

[0079] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein, and such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A low-artifact laser reflection tomography image reconstruction system, characterized in that, The system includes a laser, a beam splitter, transmitting optical components, receiving optical components, a beam combiner, a photodetector, a data acquisition unit, and a control processor; wherein... The control processor is electrically connected to the laser and is used to control the laser's output pulsed laser. After the pulsed laser is split by a beam splitter, it forms a reference beam and a signal beam; among which, The reference light is transmitted to the photodetector via a beam combiner. The photodetector responds by outputting the time-domain waveform of the reference light, which in turn triggers the collector to perform digital waveform acquisition to obtain the laser reference waveform. The bandwidth of the photodetector must be at least sufficient to respond to the laser pulse width waveform; The signal light is emitted through the transmitting optical component and reflected by the target to form the target echo signal; After the target echo signal is received by the receiving optical component, it is transmitted to the photodetector through the beam combiner. The same photodetector responds and outputs the time-domain waveform of the target echo signal, which is then digitally acquired by the collector triggered by the reference light to obtain the target echo signal waveform. The control processor is also electrically connected to the data acquisition unit to read the laser reference waveform and target echo signal waveform of the same emitted laser pulse, and then calculate the correlation waveform of the target echo signal in real time, and use the correlation waveform of the target echo signal to calculate the target reconstructed image.

2. The low-artifact laser reflection tomography image reconstruction system according to claim 1, characterized in that, The laser is a narrow pulse laser.

3. The low-artifact laser reflection tomography image reconstruction system according to claim 1, characterized in that, The combiner is a 1×2 multimode fiber combiner, which includes a fiber reference end, a fiber signal end, and a fiber output end, and the length difference between the fiber signal end and the fiber reference end is greater than or equal to one laser pulse width time-of-flight distance.

4. The low-artifact laser reflection tomography image reconstruction system according to claim 1, characterized in that, The emitting optics are used to compress the laser beam divergence angle.

5. A method for reconstructing low-artifact laser reflection tomography images, characterized in that, The method is implemented using a low-artifact laser reflection tomography image reconstruction system according to any one of claims 1-4; The method includes: S1, the control processor controls the laser to emit pulsed laser light; wherein, after the pulsed laser light is split by the beam splitter, it forms a reference beam and a signal beam; S2, the reference light is transmitted to the photodetector via the beam combiner, and the photodetector responds by outputting the time-domain waveform of the reference light, which in turn triggers the collector to perform digital waveform acquisition in order to obtain the laser reference waveform at the current target angle; S3, the signal light is emitted through the emitting optical component and reflected by the target at the current target angle to form the target echo signal at the current target angle; S4, the target echo signal at the current target angle is received by the receiving optical component and transmitted to the photodetector via the beam combiner. The same photodetector responds and outputs the time-domain waveform of the target echo signal at the current target angle, which is then digitally acquired by the collector triggered by the reference light to obtain the target echo signal waveform at the current target angle. S5, the control processor reads the laser reference waveform and the target echo signal waveform at the current target angle, then calculates the real-time correlation waveform of the echo signal at the current target angle, and filters the real-time correlation waveform of the echo signal at the current target angle; S6, repeat S1-S5 n times, and sequentially obtain the real-time correlation waveform filtering results of the echo signals of each angle with an equally spaced target rotation angle of Δφ, and then calculate the target reconstruction image; where Δφ·n=360°.

6. The low-artifact laser reflection tomography image reconstruction method according to claim 5, characterized in that, In step S5, the real-time correlation waveform p(r,φ) of the target echo signal is obtained by calculating according to the following formula: Where r is the distance from the target to the center of rotation at the current target angle φ; p ref The laser reference waveform for the current target angle; p sig This is the waveform of the target echo signal at the current target angle.

7. The low-artifact laser reflection tomography image reconstruction method according to claim 6, characterized in that, In step S5, the actual waveform of the real-time correlation waveform of the current target angle echo signal after filtering. for: Where, φ i The target angle corresponding to the current real-time relevant waveform. and Forward and inverse Fourier transforms, k is the filter function, typically the Ramp-Lak filter.

8. The low-artifact laser reflection tomography image reconstruction method according to claim 7, characterized in that, In step S6, the target reconstructed image g(x,y) is: Where x and y are the two-dimensional coordinates of the target reconstructed image.

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