CN118482634A - Doppler optical coherence tomography method and imaging device - Google Patents
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Abstract
The invention discloses a Doppler optical coherence tomography method and an imaging device, wherein the imaging method comprises the following steps: scanning a sample to be detected to obtain an interference spectrum signal containing sample information; performing first signal processing on the interference spectrum signal to obtain a complex signal representing the depth chromatographic information of the sample; extracting phase components of the complex signals, and performing differential processing one by one according to phase information of adjacent acquisition time intervals to obtain Doppler phase difference parts which change along with time and contain samples to be detected; reconstructing the Doppler phase difference fraction and the amplitude fraction into reconstructed tomographic signals; performing second signal processing on the reconstructed chromatographic signal to obtain a reconstructed complex spectrum function; and performing third signal processing on the repeated spectrum function to obtain a full-depth tomographic image and a Doppler image. The invention can obtain more accurate speed sensitivity, wider speed detection range and higher signal to noise ratio, and has important significance for further clinical application of the Doppler OCT technology.
Description
Technical Field
The invention relates to the technical field of optics, in particular to a Doppler optical coherence tomography method and an imaging device.
Background
The OCT (Optical Coherence Tomography, OCT) is an emerging and rapidly developed imaging technology, has the characteristics of high resolution, no damage and real-time imaging capability, and is easy to dock with the existing instrument, and has very wide application prospect. The OCT can realize nondestructive non-contact high-speed detection of depth resolution under the micron-scale resolution, can be used for acquiring a high-resolution three-dimensional image of a measured sample in real time, and can realize the micron-scale resolution in the depth direction.
In recent years, spectral domain Doppler optical coherence tomography (Doppler Optical Coherence Tomography, DOCT) combines OCT technology with Doppler effect, obtains flow velocity information in tissues by calculating the change of the frequency of light received by a detector, can measure the blood flow velocity of a human carrier in a lossless and high-resolution way, and has higher clinical application value.
The complex frequency domain OCT based on sinusoidal phase modulation introduces sinusoidal phase modulation interferometry into frequency domain OCT, records two-dimensional frequency domain interference fringes by using CCD, and reconstructs the two-dimensional complex frequency domain interference fringes by Fourier analysis. The complex frequency domain DOCT technology based on sinusoidal phase modulation can realize full-depth detection, and obtain higher speed detection sensitivity than the common frequency domain DOCT; the complex frequency domain OCT technology based on sinusoidal phase modulation provides a solution for realizing dynamic image measurement of high-speed vibration samples such as cardiovascular and the like, but the signal-to-noise ratio of the system and the modulation frequency of the system are limited, so that higher speed detection sensitivity is difficult to obtain. Because the large amplitude high-speed motion in the sample to be measured can cause aliasing (i.e. mixing) of the time spectrum after fourier transformation when imaging the living body, the traditional sinusoidal phase demodulation method improves the signal-to-noise ratio compared with the traditional fourier domain OCT technology, but is limited by the modulation frequency brought in the fourier analysis process, so that the doppler image of the sample to be measured is difficult to obtain in an environment with various noise frequencies. Compared with linear phase modulation, sinusoidal phase modulation is easy to realize high-speed scanning, so that a sample to be detected is always in a region with highest sensitivity near zero optical path difference, and noise immunity is high, but sinusoidal phase modulation has the defects of high requirement on hardware time sequence synchronization precision, dependence on phase demodulation precision and the like, so that the sample to be detected is difficult to realize production.
Disclosure of Invention
The embodiment of the invention provides a Doppler optical coherence tomography method and an imaging device, when an organism is imaged, the imaging method can not only overcome the frequency mixing phenomenon caused by high-speed large-amplitude movement in a sample to be detected, but also has the characteristics of no need of an additional phase modulation module and high precision and solves the problem that the traditional Doppler optical coherence tomography system is difficult to accurately measure the movement information of the sample moving at high speed and large amplitude, thereby obtaining higher speed detection sensitivity and realizing dynamic imaging of the sample moving at high speed.
According to an aspect of the present invention, there is provided a doppler optical coherence tomography method comprising:
Scanning a sample to be detected by using an optical coherence tomography system to obtain an interference spectrum signal containing sample information;
performing first signal processing on the interference spectrum signal to obtain a complex signal capable of representing depth chromatographic information of a sample;
Extracting phase components of the complex signals, and performing differential processing one by one according to phase information of adjacent signal acquisition time intervals to obtain Doppler phase difference components which change along with time and comprise the sample to be detected;
reconstructing the Doppler phase difference fraction and the amplitude fraction of the complex signal into a reconstructed complex tomographic signal;
performing second signal processing on the reconstructed complex chromatographic signal to obtain a reconstructed complex spectrum function;
And performing third signal processing on the reconstructed complex spectrum function to obtain a full-depth tomographic image and a Doppler image.
Optionally, the optical coherence tomography system comprises a spectral domain optical coherence tomography system or a swept optical coherence tomography system.
Optionally, before the signal processing of the interference spectrum signal, the method further includes:
preprocessing the interference spectrum signal.
Optionally, the preprocessing includes:
Background noise is reduced;
Windowing or spectral shaping;
wavenumber interpolation, spectral calibration, or dispersion compensation.
Optionally, the first signal processing includes fourier analysis, the second signal processing includes windowing processing and fourier analysis, and the third signal processing includes fourier analysis and digital image processing.
Optionally, the fourier analysis includes a fourier transform and an inverse fourier transform, the first signal processing includes a fourier transform, and the second signal processing and the third signal processing include an inverse fourier transform.
Optionally, the performing third signal processing on the reconstructed complex spectrum signal to obtain a full-depth tomographic image and a doppler image includes:
removing a direct current term and a complex conjugate mirror image in the reconstructed complex spectrum function, and obtaining a complex spectrum signal by performing inverse Fourier transform on a part only containing positive complex signals and returning to a time domain;
performing inverse Fourier transform on the complex spectrum signal in the time domain along the wave number domain to obtain a chromatographic signal with amplitude containing full-depth chromatographic information and phase containing Doppler information;
Mapping the amplitude to a gray space, and performing threshold processing to obtain a full-depth three-dimensional tomographic image without conjugate mirror image;
and carrying out Doppler calculation on the phase to obtain Doppler flow velocity in the sample to be detected.
Optionally, a windowing filtering method is used to remove the direct current term and the complex conjugate mirror image in the reconstructed complex chromatographic signal.
According to another aspect of the present invention, there is provided a doppler optical coherence tomography apparatus, including an interference light path module, a data acquisition module, a control module, and a data processing module;
the interference light path module is used for enabling the sample arm to scan the position of the sample to be imaged under the control of the control module;
the data acquisition module is used for acquiring data of interference spectrum signals generated by the interference light path module, wherein the intervals of data sampling points are equal wavelength intervals or equal wave number intervals, and the data are transmitted to the data processing module;
the data processing module is used for executing the Doppler optical coherence tomography method.
Optionally, the interference light path module comprises a spectral domain optical coherence tomography system or a swept optical coherence tomography system.
According to the Doppler optical coherence tomography method provided by the embodiment of the invention, an optical coherence tomography system is utilized to scan a sample to be detected, so that an interference spectrum signal containing sample information is obtained; performing first signal processing on the interference spectrum signal to obtain a complex signal capable of representing the depth chromatographic information of the sample; then extracting phase components of the complex signals, and carrying out differential processing one by one according to phase information of adjacent signal acquisition time intervals to obtain Doppler phase difference parts which change along with time and contain samples to be detected; reconstructing the Doppler phase difference fraction and the amplitude fraction of the complex signal into a reconstructed complex chromatographic signal; performing second signal processing on the reconstructed chromatographic signal to obtain a reconstructed complex spectrum function; and performing third signal processing on the repeated spectrum function to obtain a full-depth tomographic image and a Doppler image. According to the technical scheme, when the living body is imaged through a differential phase analysis algorithm, the frequency mixing phenomenon caused by high-speed and large-amplitude motion in the sample to be detected can be overcome, and the method has the characteristics of no need of an additional phase modulation module, and high accuracy and difficulty in accurately measuring motion information of the high-speed motion sample and the large-amplitude motion sample by a traditional Doppler optical coherence tomography system, so that higher speed detection sensitivity is obtained, and dynamic imaging of the high-speed motion sample is realized.
It should be understood that the description in this section is not intended to identify key or critical features of the embodiments of the invention or to delineate the scope of the invention. Other features of the present invention will become apparent from the description that follows.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for the description of the embodiments will be briefly described below, and it is apparent that the drawings in the following description are only some embodiments of the present invention, and other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.
FIG. 1 is a schematic flow chart of a Doppler optical coherence tomography method according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of a swept-frequency optical coherence tomography system according to an embodiment of the present invention;
FIG. 3 is a schematic diagram of a spectral domain optical coherence tomography system according to an embodiment of the present invention;
FIG. 4 is a schematic flow chart of another Doppler optical coherence tomography method according to an embodiment of the present invention;
Fig. 5 is a schematic structural diagram of a doppler optical coherence tomography device according to an embodiment of the present invention.
Detailed Description
In order that those skilled in the art will better understand the present invention, a technical solution in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in which it is apparent that the described embodiments are only some embodiments of the present invention, not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the present invention without making any inventive effort, shall fall within the scope of the present invention.
It should be noted that the terms "first," "second," and the like in the description and the claims of the present invention and the above figures are used for distinguishing between similar objects and not necessarily for describing a particular sequential or chronological order. It is to be understood that the data so used may be interchanged where appropriate such that the embodiments of the invention described herein may be implemented in sequences other than those illustrated or otherwise described herein. Furthermore, the terms "comprises," "comprising," and "having," and any variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, system, article, or apparatus that comprises a list of steps or elements is not necessarily limited to those steps or elements expressly listed but may include other steps or elements not expressly listed or inherent to such process, method, article, or apparatus.
OCT techniques are classified into time domain OCT techniques and fourier domain OCT techniques. Because the reflected or scattered signal of the deep layer of the biological sample can detect very weak back scattered light returned by the sample, the signal is extremely easy to be interfered by various noises and speckles, the noise can cause false, distortion and deformation of the signal, the extraction of a layered structure is difficult, and the biopsy implementation of the biological sample is difficult due to the tiny movement of the sample and the instability of an interferometer. Thus, the extraction of large dynamic range weak signals is a key part of OCT technology.
Time domain OCT techniques require axial point-by-point scanning and imaging speed is limited. The Fourier domain OCT does not need a motion mechanism to perform axial scanning, and the interference spectrum detected by the detector is transformed into a sample space through Fourier transformation to obtain the depth information of the sample, so that the Fourier domain OCT has the advantages of high sensitivity and high measurement speed, and simultaneously has the point detection advantage of the time domain OCT.
In 1997, researchers combined the doppler principle with OCT techniques and proposed doppler optical coherence tomography (Doppler Optical Coherence Tomography, DOCT) that could obtain both high resolution tissue structure images and motion information within samples. In clinical diagnosis, quantitative detection of blood flow velocity has important application value for differential diagnosis, disease condition monitoring and pathogenesis of diseases, for example, quantitative detection of retinal blood flow can provide diagnosis basis for a plurality of eye diseases. The group Fetcher in 2002 proposed the frequency domain DOCT technique (Fourier Domain Doppler Optical Coherence Tomography, FD-DOCT) and applied a phase analysis algorithm to FD-DOCT, DOCT based on frequency domain phase analysis has attracted considerable attention from researchers.
Doppler frequency shift information is extracted from an OCT signal, two algorithms are commonly used, one is obtained by carrying out Fourier transform processing on an OCT detection signal, and the method has the problem that depth resolution and speed resolution are mutually restricted; another method is to obtain velocity information by solving the phase of the OCT probe, i.e., phase resolution, which has higher velocity sensitivity and velocity resolution independent of depth resolution. In addition, researchers have also proposed various flow rate extraction algorithms to obtain more accurate images of vessel location or blood flow rate, such as: phase variance method, doppler spectrum bandwidth (standard deviation) method, and the like. Over the next decade, frequency domain DOCT technology has evolved rapidly, and a number of different imaging methods have emerged to obtain vascular or blood flow imaging images. The imaging methods commonly used in the current frequency domain DOCT technology include: phase resolution, phase variance, intensity variance, resonant doppler, single pass vessel imaging, joint time-frequency domain techniques, optical microangiography, etc., which are mostly based on doppler effect versus frequency change.
Methods such as resonant doppler, optical microangiography, and the like increase the phase shift by adding a phase modulation element to the system to distinguish between stationary and moving structures, increasing the complexity of the system and often requiring two modulations to obtain a moving image in both directions. The Doppler frequency shift of the sample motion is directly utilized to distinguish the motion from the static structure by combining the methods of time-frequency domain, single-pass blood flow imaging and the like, but the data volume is larger, and the data processing time is longer. Methods such as intensity variance imaging use the interference signal intensity or speckle intensity changes caused by motion to detect velocity information, but such methods use structural images and are susceptible to overall motion and vascular shadows. The phase analysis and phase analysis Doppler variance imaging method does not need to change the system structure, and the data processing is simple, so that the method is the most commonly used method at present. The former can obtain a motion structure and a motion speed at the same time, and is widely used. But the phase-resolved imaging method is susceptible to noise and therefore needs to be used under conditions of higher signal-to-noise ratio in order to obtain higher speed detection sensitivity.
In recent years, DOCT technology is widely applied to the medical diagnosis fields of vascular structure imaging, blood flow velocity measurement, early diagnosis of tumors and the like, so that DOCT imaging technology has higher requirements on higher speed sensitivity, wider speed detection range, better image signal-to-noise ratio and more accurate flow velocity measurement. The development of FD-DOCT technology improves the sensitivity and imaging speed of Doppler imaging, but FD-DOCT has the problem of complex conjugate mirror image, so that the available imaging depth is halved. The complex frequency domain full depth OCT technique can solve this problem, and a full depth tomographic image can be obtained by reconstructing a complex interference spectrum signal and fourier transforming it.
The complex frequency domain OCT technology is proposed, so that various functional imaging technologies are expanded. The complex frequency domain OCT technology is mainly used for reconstructing interference spectrum fringes by two methods of phase-shifting interferometry and heterodyne interferometry. Phase-shifting interferometry is susceptible to phase-shifting accuracy and chromatic dispersion; heterodyne interferometry does not require precise phase shifting and is more suitable for imaging of dynamic objects. A common method of heterodyne interferometry is B-M scanning, i.e. introducing phase modulation at the same time as the lateral scanning. Franck Jaillon et al propose a parabolic B-M scanning technique which introduces parabolic phase modulation by reconstructing a complex analysis signal with a differential signal and filtering it in the frequency domain to obtain a doppler image, which can well eliminate conjugate mirror images, but in the parabolic modulation process the sensitivity and signal-to-noise ratio of the system can be reduced as the distance between the sample and the zero optical path difference increases.
The complex frequency domain OCT based on sinusoidal phase modulation introduces sinusoidal phase modulation interferometry into frequency domain OCT, records two-dimensional frequency domain interference fringes by using CCD, and reconstructs the two-dimensional complex frequency domain interference fringes by Fourier analysis. The imaginary and real parts of the interference spectrum correspond to the first and second harmonics of the phase modulation spectrum, respectively. For sample mirrors with single layer reflecting surfaces, it has been demonstrated that the direct current term, the autocorrelation term, and the complex conjugate mirror can be effectively removed. The sine phase modulation method has the following advantages for complex conjugate inhibition: (1) The sinusoidal phase modulation is used only by driving a sinusoidal signal, so that the signal is easy to acquire; (2) The real and imaginary components of the complex interference signal can be acquired simultaneously with the same detector. Compared with linear phase modulation, sinusoidal phase modulation is easy to realize high-speed scanning, and can enable a measured sample to be always in a region with highest sensitivity near zero optical path difference, so that noise immunity is high. The full-depth detection can be realized by a complex frequency domain DOCT technology based on sinusoidal phase modulation, and the detection sensitivity is higher than that of the common FD-DOCT; the complex frequency domain OCT technology based on sinusoidal phase modulation provides a solution for realizing dynamic image measurement of high-speed vibration samples such as cardiovascular and the like, but the signal-to-noise ratio of the system and the modulation frequency of the system are limited, so that higher speed detection sensitivity is difficult to obtain. Because the large amplitude high-speed motion in the sample to be measured can cause aliasing (i.e. mixing) of the time spectrum after fourier transformation when imaging the living body, the traditional sinusoidal phase demodulation method improves the signal-to-noise ratio compared with the traditional fourier domain OCT technology, but is limited by the modulation frequency brought in the fourier analysis process, so that the doppler image of the sample to be measured is difficult to obtain in an environment with various noise frequencies. Compared with linear phase modulation, sinusoidal phase modulation is easy to realize high-speed scanning, can enable a measured sample to be always in a region with highest sensitivity near zero optical path difference, has strong noise immunity, but has the defects of high requirement on hardware time sequence synchronization precision, dependence on phase demodulation precision and the like, so that the sinusoidal phase modulation is difficult to realize production.
In order to obtain more accurate speed sensitivity, wider speed detection range and higher signal-to-noise ratio, the embodiment of the invention provides a differential phase analysis complex frequency domain Doppler optical coherence tomography technology, which has great significance for further clinical application of the Doppler OCT technology.
The complex frequency domain DOCT technology based on differential phase analysis provided by the embodiment of the invention can reconstruct a complex frequency domain chromatographic signal without adding a special phase modulation module, and the basic idea is as follows:
After interference chromatographic signals are obtained by carrying out Fourier transform on the interference spectrum signals, complex chromatographic signals are reconstructed by utilizing differential phases of adjacent interference chromatographic signals, and full-depth chromatographic images and Doppler images can be obtained by carrying out Fourier analysis on the reconstructed complex chromatographic signals.
The technique can improve the speed detection range and the signal-to-noise ratio of the complex frequency domain DOCT technique. The differential phase analysis technology provided by the embodiment of the invention has great significance for the DOCT technology in further clinical application.
Specific implementations of embodiments of the invention are described below. Fig. 1 is a schematic flow chart of a doppler optical coherence tomography method according to an embodiment of the present invention. Referring to fig. 1, the doppler optical coherence tomography method provided by the embodiment of the present invention includes:
s110, scanning the sample to be detected by using an optical coherence tomography system to obtain an interference spectrum signal containing sample information.
The sample to be detected can be a biological sample to be detected with high-speed motion and large vibration amplitude, such as living animals, human bodies and the like, and the optical coherence tomography system can optionally comprise a spectral domain optical coherence tomography system or a sweep frequency optical coherence tomography system.
In the specific implementation, the structure of the complex frequency domain optical coherence tomography system is not limited, for example, the complex frequency domain optical coherence tomography system can be a sweep frequency OCT system or a spectral domain OCT system, the optical fiber probe can be controlled to move during scanning, the transmission direction of the light beam can be modulated by using devices such as a galvanometer, and the like, and the complex frequency domain optical coherence tomography system can be selected according to practical situations in the specific implementation.
Fig. 2 is a schematic diagram of a swept-frequency optical coherence tomography system according to an embodiment of the present invention, and fig. 3 is a schematic diagram of a spectral-domain optical coherence tomography system according to an embodiment of the present invention. Referring to fig. 2, the swept optical coherence tomography system includes a swept optical source 101, a first coupler 102, a second coupler 103, a first circulator 104, a second circulator 105, an optical delay unit 106, an optical fiber retroreflector 107, a balance detector 108, and an optical probe 109, where the swept optical source 101 splits laser light to a first circulator 104 at a sample arm end and a second circulator 105 at a reference arm end according to a ratio of optical power (optical fiber beam splitting devices of different ratios such as 99:1, 90:10, 80:20, or 50:50, etc.) by the first coupler 102; the laser is transmitted from the first circulator 104 to the optical fiber probe 109 to realize scanning imaging of the sample 50 to be detected, and the back scattered light of the sample 50 to be detected is transmitted to the second coupler 103 (can be a 50:50 optical fiber combiner) through the optical fiber probe 109 and the first circulator 104; the split laser passes through the second circulator 105 of the reference arm and then is transmitted to the optical delay unit 106, the laser returns to the optical delay unit 106 through the optical fiber retroreflector 107 (or optical elements such as a reflector capable of realizing laser retroreflection) in an original path, then reaches the second circulator 105 through the optical delay unit 106, and is output from the second circulator 105 to the second coupler 103 to interfere with the back scattered light returned in the sample arm, and an interference signal is detected by the balance detector 108.
Referring to fig. 3, the spectral domain optical coherence tomography system includes a spectral domain light source 110, an optical isolator 111, a coupler 112, an optical delay unit 113, a fiber retroreflector 114, a fiber probe 115, and a spectrometer 116, the spectral domain light source 110 (e.g., an SLD or other broadband light source) passing through the optical isolator 111 (preventing laser light from retroreflecting to the laser damaging the laser) and reaching the coupler 112; the laser is divided into a sample arm end and a reference arm end according to the proportion of optical power (optical fiber beam splitter devices with different proportions such as 99:1, 90:10, 80:20, 50:50 and the like); the laser is transmitted to the optical fiber probe 115 from the sample arm to realize scanning imaging of the sample 50 to be detected, and the back scattered light of the sample 50 to be detected returns to the coupler 112 through the optical fiber probe 115; the split laser is output to the optical delay unit 113 by an optical fiber in the reference arm, the laser returns to the optical delay unit 113 in an original path through the optical fiber retroreflector 114 (or an optical element such as a reflector capable of realizing laser retroreflection), then reaches the coupler 113 through the optical delay unit 113 to interfere with the back scattered light returned in the sample arm, and an interference spectrum signal is collected and detected by the spectrometer 116.
For example, in one embodiment, the vibration of the sample to be measured may be denoted as z 10(t)＝z0+bcos(2πf0t+θ0, and the sample to be measured may be scanned using a two-dimensional galvanometer (B-Scan).
The two-dimensional interference spectrum signals I (k, t) acquired by the detector after scanning are as follows:
wherein I 0 (k) is an autocorrelation term in an interference spectrum signal, S (k) is a light source spectral density function, R Sn is the reflectivity of an n-th layer reflecting surface of a sample to be detected, R R is the reflectivity of a reference mirror, t represents the time corresponding to different transverse detection points of the sample to be detected when a detection light beam is scanned, and 2k (z 0+bcos(2πfot+θ0)) is the phase difference between the n-th layer reflecting surface of the sample to be detected and the reflecting surface of the reference mirror.
S120, performing first signal processing on the interference spectrum signals to obtain complex signals capable of representing the depth chromatographic information of the sample.
Wherein the first signal processing may comprise a fourier analysis, wherein the fourier analysis comprises a fourier transform and an inverse fourier transform, which may be a fourier transform.
S130, extracting phase components of the complex signals, and performing differential processing one by one according to phase information of adjacent signal acquisition time intervals to obtain Doppler phase difference components which change along with time and comprise samples to be detected.
The two-dimensional chromatographic phase difference term which changes with time and simultaneously comprises a sinusoidal phase modulation difference term and a Doppler phase difference term of a sample to be detected is as follows:
Wherein t m is the signal acquisition time when the probe beam scans to the mth transverse scanning point of the sample to be detected, and k 0 is the central wave number corresponding to the central wavelength of the light source spectrum;
The phase difference of the signals obtained by the adjacent acquisition time intervals is calculated accurately as follows:
wherein Deltat represents the time interval between two adjacent signals collected by the detector, and is determined by the signal collection frequency of the detector, namely
S140, reconstructing Doppler phase difference parts and amplitude parts of complex signals into reconstructed complex chromatographic signals.
S150, performing second signal processing on the reconstructed chromatographic signal to obtain a reconstructed complex spectrum function.
Wherein the second signal processing may comprise windowed filtering and fourier analysis, where the fourier analysis may be an inverse fourier transform.
The differential phase term only containing the Doppler phase of the sample to be detected is as follows:
Wherein, two-dimensional differential chromatography signal is:
wherein, The Doppler information of the sample to be measured is obtained, and the Doppler information is subjected to a series of image processing to obtain the flow velocity of the sample to be measured.
The two-dimensional complex analytic signal is:
The direct current term and the complex conjugate mirror image can be removed by common signal processing methods such as windowing filtering and the like.
S160, performing third signal processing on the reconstructed spectrum function to obtain a full-depth tomographic image and a Doppler image.
The third signal processing may include fourier analysis, which may be an inverse fourier transform, and digital image processing, among others.
Optionally, performing third signal processing on the reconstructed spectrum signal to obtain a full-depth tomographic image and a doppler image, including:
removing a direct current term and a complex conjugate mirror image in the reconstructed complex spectrum function, and carrying out inverse Fourier transformation on a part only containing positive complex signals to obtain complex spectrum signals;
Performing inverse Fourier transform on the complex spectrum signal in the time domain along the wave number domain to obtain a chromatographic signal with amplitude containing full-depth chromatographic information and phase containing Doppler information;
Mapping the amplitude to a gray space, and performing threshold processing to obtain a full-depth three-dimensional tomographic image without conjugate mirror images;
And carrying out Doppler calculation on the phase to obtain Doppler flow velocity in the sample to be detected.
Alternatively, a windowed filtering method may be used to remove the dc term and complex conjugate mirror from the reconstructed complex analytic signal.
According to the technical scheme, when the living body is imaged through a differential phase analysis algorithm, the frequency mixing phenomenon caused by high-speed and large-amplitude motion in the sample to be detected can be overcome, and the method has the characteristics of no need of an additional phase modulation module, and high accuracy and difficulty in accurately measuring motion information of the high-speed motion sample and the large-amplitude motion sample by a traditional Doppler optical coherence tomography system, so that higher speed detection sensitivity is obtained, and dynamic imaging of the high-speed motion sample is realized.
In another embodiment, optionally, before the signal processing of the interference spectrum signal, the method further includes:
The interference spectrum signal is preprocessed.
By adding the step of preprocessing the interference signal, the accuracy of the imaging resolution can be improved.
Fig. 4 is a schematic flow chart of another doppler optical coherence tomography method according to an embodiment of the present invention. Referring to fig. 4, the doppler optical coherence tomography method includes:
S210, scanning a sample to be detected by using an optical coherence tomography system to obtain an interference spectrum signal containing sample information.
S220, preprocessing the interference spectrum signal.
After pretreatment, the chromatography signal after Fourier transformation can have optimal longitudinal resolution, so that an A-line (point scanning collected signal) signal with high contrast and high longitudinal resolution is obtained. Optionally, the preprocessing includes:
background noise is reduced, and the influence of direct current noise can be eliminated;
The direct current in the interference signal is background noise which is the same in all positions, and the existence of the background noise can reduce the contrast of the image and can be removed during preprocessing. The interference spectrum signals returned by the detection of a group of reference arms are added and averaged, so that the interference spectrum signals can be subtracted from background noise. The method is simple and easy to operate, but increases the acquisition time and is easily influenced by the fluctuation of the light source intensity. The method comprises the following specific steps: the average of the horizontal scanning direction is carried out on a frame of B-scan signal, so that an average A-scan signal is obtained, and is taken as background noise to be subtracted from each A-scan signal.
The windowing treatment or the spectrum shaping can reduce the spectrum leakage in the Fourier transform process;
The windowing process mainly comprises the following steps: the first step is to divide the signal into a number of small blocks, i.e. windows; the second step is to set an appropriate window function for each window; the third step is to window the signal according to the appointed window function; the final step is to recombine the processed signals into the original signals.
The spectral shaping method of spectral domain OCT is as follows:
Imaging an inclined reflecting surface and scanning in the direction of inclination of the reflecting surface can obtain a scan signal. Averaging the data of the same wavelength of the acquired B-scan signal to obtain an average energy spectrum; the obtained average energy spectrum is square-root to obtain the spectral density s (k), the spectral density s (k) is filtered, the Gaussian curve sg (k) with the same half-width as the light source is obtained through fitting the spectral density s (k), and the spectral shaping coefficient is obtained. Multiplying the original interference spectrum signal by a spectrum shaping coefficient to obtain a new interference spectrum signal.
General spectral shaping can shape a non-gaussian beam into a gaussian or other bell-shaped curve to reduce sidebands, but it can sometimes be shaped into a saddle-like spectral shape for higher longitudinal resolution.
The digital sampling interval of the interference spectrum signal can be consistent with the actual wave number distribution interval of the interference light beam by wave number interpolation, spectrum calibration or dispersion compensation.
Wave number interpolation: since the interference signal is an a-line signal with respect to depth obtained by fourier transforming the wave number k, in order to obtain a signal of z-equidistant, it is necessary to convert the interference signal of the original wavelength equidistant into a distribution of k-equidistant. The higher longitudinal resolution can be obtained by global interpolation of the original signal. The method mainly comprises the following steps: and carrying out Fourier transform on the original interference spectrum signals, carrying out zero padding on two sides of the complex signal in the z domain, and then carrying out inverse Fourier transform on the complex signal in the z domain. And finally, performing cubic spline interpolation on the interference spectrum signal subjected to zero padding. The interference spectrum signal obtained after the processing can be directly subjected to signal processing such as Fourier transform or an algorithm to obtain a Doppler image.
Spectral calibration: in the spectral domain OCT system, the signal collected on the CCD is a signal whose intensity corresponds to the pixel, and the signal corresponding to the intensity and the spectrum can be obtained by performing spectral correction on the signal. The spectrum correction is to determine the corresponding relation between the pixels and the wavelengths in the spectrum data measured by the spectrometer, namely the calibration of the spectrometer. If the spectral correction is inaccurate, the point spread function widens with increasing depth, which reduces the longitudinal resolution and signal-to-noise ratio of the system. The embodiment of the invention simultaneously uses a characteristic spectral line method and an automatic spectral correction method to calibrate the wavelength of the spectrometer.
Characteristic spectral line method
The method adopts a correction light source with characteristic spectral lines to calibrate the wavelength of the spectrometer. At least 4-6 characteristic spectral lines of the correction light source are in the spectrum detection range of the spectrometer. The characteristic spectral lines are detected by the linear array CCD, the pixel coordinates of the linear array CCD corresponding to each spectral line are recorded, and the wavelengths corresponding to all pixels of the linear array CCD are determined through three-time polynomial fitting. The correspondence of wavelength λ and pixel x can be expressed by the following equation:
λ(x)＝λ1+a1x+a2x2+a3x3。
Wherein lambda 1 is the wavelength corresponding to the first pixel, a 1-a3 is the 1-3 level spectral correction coefficient, and the spectral correction is to determine the four coefficients. The wavelength distribution on the linear array CCD can be determined by fitting and determining the spectral correction coefficient. This method is simple and accurate, but requires an additional correction light source.
An example: the spectrometer was calibrated using a commercial mercury-argon (Hg-Ar) lamp. And performing polynomial fitting for three times by using seven characteristic spectral lines of the mercury-argon lamp, so that a wavelength distribution curve on the whole linear array CCD can be determined.
Automatic spectral correction
The specific method is to collect 1000 groups of interference signals by taking a reflecting mirror or a glass sheet as a sample, and remove the influence of noise by average. To obtain the phase of the interference signal of the sample reflecting surface, the averaged signal is subjected to Fourier transform, the interference peak is obtained by utilizing band-pass filtering, and the phase part P1 of the interference signal can be separated after the inverse Fourier transform. The mirror is moved a distance D and the above procedure is repeated to obtain the phase P2 of the second set of interference signals. To remove the effect of dispersion on phase in the device, the two sets of phases are subtracted to obtain the phase difference P. The initial distribution K of wave numbers is obtained by using the initial distribution W of the wavelength of the spectrometer along with the pixels. And carrying out global interpolation on the phase P by using K to obtain a change curve P (K) of the phase along with the equidistant wave number. And (3) performing polynomial fitting on P (k) for three times to extract nonlinear phase parts in the polynomial fitting:
σ(k)＝a3k3+a2k2+a0。
If the initial wavenumber is accurate with the pixel distribution K, then the interpolated phase should vary linearly with the equally spaced wavenumber K according to the upward equation. The nonlinear variation of the phase obtained above is caused by the error of the wave number distribution K. Thus removing the nonlinear phase from the original wavenumber distribution yields a new wavenumber distribution:
K′＝K+σ(k)z(peak)。
where z (Peak) =2pi×peak_index (K max-kmin) represents the distance between the sample reflection surface and the reference surface, peak_index is the pixel difference between the two measured Peak positions, and K max and K min are the maximum and minimum values of K, respectively. Using the set wavenumber maximum K m, K 'is translated according to the formula K' =k '- (K' (1) -K m). The above procedure was repeated with the obtained K' as the initial wave number, and the cycle was repeated 3 times. The final wave number is taken into the formula W ' =2pi K ' to obtain the corrected spectral distribution W '. The calculated spectral distribution W' is present in a file for direct recall when running the image generation algorithm.
The preset once term coefficient when the phase is subjected to three term fitting can lead the fitting coefficient to have small deviation from the actual size. The first order coefficient is set as z (peak) calculated last time. The first fit may be set to the distance D the mirror moves at the time of the two measurements.
S230, performing first signal processing on the interference spectrum signal to obtain a complex signal capable of representing the depth chromatographic information of the sample.
S240, extracting phase components of the complex signals, and performing differential processing one by one according to phase information of adjacent signal acquisition time intervals to obtain Doppler phase difference components which change along with time and comprise samples to be detected.
S250, reconstructing Doppler phase difference parts and amplitude parts of complex signals into reconstructed complex chromatographic signals.
S260, performing second signal processing on the reconstructed chromatographic signal to obtain a reconstructed complex spectrum function.
S270, performing third signal processing on the reconstructed spectrum function to obtain a full-depth tomographic image and a Doppler image.
Fig. 5 is a schematic structural diagram of a doppler optical coherence tomography device according to an embodiment of the present invention. Referring to fig. 5, the doppler optical coherence tomography apparatus includes an interference light path module 10, a data acquisition module 20, a control module 30, and a data processing module 40; the interference light path module 10 is used for enabling the sample arm to scan the position of the sample 50 to be detected to be imaged under the control of the control module 30; the data acquisition module 20 is configured to acquire data of the interference spectrum signal generated by the interference light path module, where an interval of data sampling points is an equal wavelength interval or an equal wave number interval, and transmit the data to the data processing module 40; the data processing module 40 is configured to perform any of the doppler optical coherence tomography methods provided in the above embodiments.
In specific implementation, the interference light path module 10 may include a light source, a beam splitting unit, a light delay unit, a scanning unit, etc. for implementing scanning of the sample 50 to be tested, where the interference light path module 10 is the core of the whole hardware, and determines the main performance of the system. Alternatively, the interference light path module 10 includes a spectral domain optical coherence tomography system or a swept optical coherence tomography system, and the specific structure thereof may refer to fig. 3 and fig. 2 of the foregoing embodiments, respectively. The data acquisition module 20 may include a photoelectric detection unit (e.g., balance detector, CCD, etc.) and a data acquisition card, and the control module 30 can control data acquisition, display and storage of interference spectrum signals in doppler imaging experiments, and the data processing module 40 implements data processing.
The above embodiments do not limit the scope of the present invention. It will be apparent to those skilled in the art that various modifications, combinations, sub-combinations and alternatives are possible, depending on design requirements and other factors. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of the present invention.
Claims (10)
1. A doppler optical coherence tomography method, comprising:
Scanning a sample to be detected by using an optical coherence tomography system to obtain an interference spectrum signal containing sample information;
performing first signal processing on the interference spectrum signal to obtain a complex signal capable of representing depth chromatographic information of a sample;
Extracting phase components of the complex signals, and performing differential processing one by one according to phase information of adjacent signal acquisition time intervals to obtain Doppler phase difference components which change along with time and comprise the sample to be detected;
reconstructing the Doppler phase difference fraction and the amplitude fraction of the complex signal into a reconstructed complex tomographic signal;
performing second signal processing on the reconstructed complex chromatographic signal to obtain a reconstructed complex spectrum function;
And performing third signal processing on the reconstructed complex spectrum function to obtain a full-depth tomographic image and a Doppler image.
2. The doppler optical coherence tomography method of claim 1, wherein the optical coherence tomography system comprises a spectral domain optical coherence tomography system or a swept optical coherence tomography system.
3. The doppler optical coherence tomography method of claim 1, further comprising, prior to signal processing the interference spectrum signal:
preprocessing the interference spectrum signal.
4. A doppler optical coherence tomography method as recited in claim 3, wherein the preprocessing comprises:
Background noise is reduced;
Windowing or spectral shaping;
wavenumber interpolation, spectral calibration, or dispersion compensation.
5. The doppler optical coherence tomography method of claim 1, wherein the first signal processing comprises fourier analysis, the second signal processing comprises windowed filtering and fourier analysis, and the third signal processing comprises fourier analysis and digital image processing.
6. The doppler optical coherence tomography method of claim 5, wherein the fourier analysis comprises a fourier transform and an inverse fourier transform, the first signal processing comprises a fourier transform, and the second and third signal processing comprises an inverse fourier transform.
7. The doppler optical coherence tomography method of claim 1, wherein performing third signal processing on the reconstructed complex spectrum signal to obtain a full depth tomographic image and a doppler image comprises:
Removing a direct current term and a complex conjugate mirror image in the reconstructed complex chromatographic signal, and carrying out inverse Fourier transformation on a part only containing positive complex signals to obtain a complex spectrum signal;
performing inverse Fourier transform on the complex spectrum signal in the time domain along the wave number domain to obtain a chromatographic signal with amplitude containing full-depth chromatographic information and phase containing Doppler information;
Mapping the amplitude to a gray space, and performing threshold processing to obtain a full-depth three-dimensional tomographic image without conjugate mirror image;
and carrying out Doppler calculation on the phase to obtain Doppler flow velocity in the sample to be detected.
8. The doppler optical coherence tomography method of claim 7, wherein a windowed filtering method is used to remove direct current terms and complex conjugate images from the reconstructed complex tomography signal.
9. The Doppler optical coherence tomography device is characterized by comprising an interference light path module, a data acquisition module, a control module and a data processing module;
the interference light path module is used for enabling the sample arm to scan the position of the sample to be imaged under the control of the control module;
the data acquisition module is used for acquiring data of interference spectrum signals generated by the interference light path module, wherein the intervals of data sampling points are equal wavelength intervals or equal wave number intervals, and the data are transmitted to the data processing module;
the data processing module is used for executing the Doppler optical coherence tomography method of any one of claims 1 to 8.
10. The doppler optical coherence tomography instrument of claim 9, wherein the interference optical path module comprises a spectral domain optical coherence tomography system or a swept optical coherence tomography system.
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