CN105342558B - A kind of bearing calibration based on phase error in Optical coherence tomography - Google Patents
A kind of bearing calibration based on phase error in Optical coherence tomography Download PDFInfo
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Abstract
The bearing calibration of phase error in a kind of Optical coherence tomography based on swept light source disclosed by the invention, it includes (1) interference spectrum and does discrete Fourier transform；The method being modulated to Optical coherence tomography original interference signal.(2) skew of original interference spectral signal is detected using the phase distribution of the modulated signal to Optical coherence tomography original interference signal.(3) method for correcting the skew of Optical coherence tomography original interference spectral signal.The present invention be it is a kind of it is new have the use of feasibility and validity as k clocks that swept light source provides as in the SS OCT systems of sampling clock to phase error real-time correction method caused by being shaken as electronic equipment internal, provided safeguard for accurate measurement tissue CBF, important booster action is played for medical clinic applicationses and treatment, the accuracy of OCT angiographic images quality and blood flow velocity measurement based on Phase-Resolved Analysis is improved, while improves the removal effect of fixed pattern noise.
Description
Technical field
The present invention relates to a kind of correction of phase error in Optical coherence tomography based on swept light source
Method, more particularly to use optical coherence tomography system in biological tissue and the imaging of human body medium vessels.
Background technology
Optical coherence tomography (optical coherence tomography, OCT) is a kind of low-coherent light
Learn interference imaging technology.It can carry out three-dimensional imaging to the diffusing structure of the biological tissues such as human body.The technology is on detection eyeground
There are very big potentiality in the disease of retina, gi system, arteries and skin etc..At present OCT technology development compared with
Function for maturation is that in addition, OCT is also equipped with others and prolongs raw function, wherein studying to static diffusing structure imaging
It is that the blood flow imaging in the biological tissues such as human body, normally referred to as optical coherence blood vessel are made that more one, which prolongs raw functional imaging,
Shadow (optical coherence angiogr aphy, OCA).Realize that OCA method can be divided mainly into correlation method and phase
Position analytic method.Compared to the former, the method for Phase-Resolved Analysis possesses the potentiality that quantitative analysis is carried out to VPV.But phase solution
Analysis method proposes higher requirement to the phase stability of OCT systems, and any system phase error of itself can be all introduced into
Final blood vessel imaging, influence image quality and the accuracy of later stage quantitative analysis.In addition, phase error can also influence OCT into
Fixed pattern noise removal effect as in.
OCT has evolved to the third generation now, based on swept light source Optical coherence tomography (swept-source
OCT, SS-OCT), first and second generation was time domain OCT and spectral coverage OCT respectively.Nominal is in Fu again by spectral coverage OCT and swept light source OCT
Leaf domain OCT, swept light source OCT compared to spectral coverage OCT, the k clocks that it is provided using swept light source as sampling clock so that
Obtained interference spectrum must be measured to be distributed in wave number space linear homogeneous, thus directly the interference spectrum measured can be carried out from
Fourier's change is dissipated to obtain depth reflectance profile, enormously simplify the signal transacting in later stage, and spectral coverage OCT then needs calculating
Resampling is carried out to the spectrum that measurement obtains to cause spectrum to be distributed in wave number space linear homogeneous before discrete Fourier transform.
In SS-OCT systems, phase instability is mainly reflected in interference spectrum signal skew before and after wave number space of measurement,
This phenomenon is mainly the relative instability between the signal acquisition trigger signal and k clock signals provided by swept light source
Caused by, data collecting card circuit may also can cause skew recited above in itself.
At present, two kinds of post-processing approach are shared for the phase instability in elimination system in swept light source OCT systems.
A kind of method is corrected by creating preferable calibration signal-Mach Zehnder interferometry (MZI) signal in systems,
Resampling first is carried out to the interference spectrum collected, sampled signal is uniformly distributed in linear wave number space, then passes through ratio
Front and rear offset direction and offset are judged compared with the correlation between two interference spectrums, so as to correct to eliminate swept light source
All artifacts in OCT image.(for example, seeing Boy Braaf et al. " Phase-stabilized OFDI at 1- μm for
(steady phase optimal frequency domain imaging technique is micro- 1 by the measurement of blood flow in the human choroid
Measurement mankind Choroidal blood flow in rice) ", Optics express (optics news flash), 2011；19(21):20886-903.) another
A kind of outer method be the interference pattern by triggering shake compared with reference chart, by compare the different amplitude that draws or
Phase determines whether fixed pattern noise and does corresponding swept light source Optical coherence tomography (SS-OCT) system
The phase stability of system.(for example, seeing Gangjun Liu et al. " Postprocessing algorithms to
minimize fixed-pattern artifact and reduce trigger jitter in swept source
(fixed mode artifact is minimized and reduced optical coherence tomography in frequency sweep optical coherence tomography
Trigger the post-processing algorithm of shake) ", Optics express (optics news flash), 2015；23(8):9824-34.)
The content of the invention
In view of the shortcomings of the prior art, the present invention seeks to be to propose that a new method is carried out real time correction and swept
The correction side of phase error in phase error in frequency light source OCT, the i.e. Optical coherence tomography based on swept light source
Method, so as to improve the accuracy of OCT angiographic images quality and blood flow velocity measurement based on Phase-Resolved Analysis, while improve solid
The removal effect of mould-fixed noise.
To achieve the above object, technical scheme is as follows：
The present invention is the modulation by carrying out specific frequency to OCT original interferences spectral signal, and by analyzing the modulation
The phase distribution of signal detects offset direction recited above and offset, then calculates corresponding phase error, most
The phase error detected is corrected in spectral domain afterwards.It is based on phase in Optical coherence tomography (OCT) and missed
The bearing calibration of difference mainly includes three steps：
(1) interference spectrum does discrete Fourier transform：First to Optical coherence tomography (OCT) original interference spectrum
Signal carries out the modulation of specific frequency, then does discrete Fourier transform to obtain the phase information of modulated signal；
(2) modulating signal phase is analyzed：The phase of modulated signal acquired in step (1) is analyzed, phase Nogata
It is that the interference spectrum corresponding to other phase values then thinks that there occurs inclined correctly without skew that most phase values is distributed in figure
Move, offset direction and offset can be determined by the difference of its phase value and correct phase value；
(3) interference spectrum skew is corrected：Calculated by the offset direction of the spectrum obtained by step (2) and offset each
Phase difference in frequency, it then will calculate signal of the phase difference of gained directly from step (1) after discrete Fourier transform
In subtract.
Present invention firstly provides a kind of new k clocks that there is the use of feasibility and validity to be provided by swept light source
As, to phase error real-time correction method caused by being shaken as electronic equipment internal, being defined in the SS-OCT systems of sampling clock
Really measurement tissue CBF is provided safeguard, and important booster action is played for medical clinic applicationses and treatment.This method
In change very little of the hardware aspect to system, amount of calculation is small in terms of software algorithm, it is easy to accomplish real time correction.
Brief description of the drawings
Describe the present invention in detail with reference to the accompanying drawings and detailed description；
Fig. 1 is the optical coherence tomography system schematic diagram of the present invention；
500th Interferogram of B-scan collection of Fig. 2 (a) present invention；
Fig. 2 (b) is that all interference spectrums of a B-scan collection are averaged rear image；
Fig. 3 is the amplitude spectrogram after interference spectrum of the present invention is fourier transformed；
Phase distribution figure before Fig. 4 (a) corrections；
Fig. 4 (b) is the phase distribution figure after correction；
Function of the phase error on frequency caused by Fig. 5 (a) left and right sidesing shiftings；
Fig. 5 (b) is functional arrangement of the minimum phase difference of from ± 1 to ± 3 point of movement on Fig. 5 (a) on modulating frequency.
Fig. 6 is the histogram that 61,000 spectrum offset points under scanning triggering and light trigger condition respectively；
Gray level image before the correction of Fig. 7 (a) human body skins image phase；
Doppler image before the correction of Fig. 7 (c) human body skins image phase；
Gray level image after the correction of Fig. 7 (b) human body skins image phase；
Doppler image after the correction of Fig. 7 (d) human body skins image phase.
Embodiment
To be easy to understand the technical means, the inventive features, the objects and the advantages of the present invention, with reference to
Embodiment, the present invention is expanded on further.
In SS-OCT, swept light source provides the trigger signal and k clocks (sampling clock) of signal acquisition.With the second generation
The maximum difference of spectral coverage OCT is sampling clocks of the SS-OCT using k clocks as signal acquisition so that what is collected is dry
Relate to spectral signal linear homogeneous and be distributed in wave number space, this can greatly simplify the signal transacting in later stage.Still further aspect, use
K clocks are the relative instability between trigger signal and k clocks, the company of being embodied in as the major defect of sampling clock
In wave number space relative skew occurs for the interference spectrum of continuous collection, and this skew can reach several clock cycle.
Above-mentioned skew is the main source of phase error in SS-OCT imagings, and the present invention mainly detects and corrected above-mentioned
Offset so as to reduce SS-OCT systematic phase error, there is provided a kind of SS-OCT systems are that the optical coherence based on swept light source breaks
The real-time correction method of phase error in layer scanning imagery, it has the use of feasibility and validity by swept light source for new
The k clocks of offer as in the SS-OCT systems of sampling clock to the real-time school of phase error caused by being shaken as electronic equipment internal
Correction method, concrete principle are the modulation by carrying out specific frequency to OCT original interferences spectral signal, and by analyzing the modulation
The phase distribution of signal detects offset direction recited above and offset, then calculates corresponding phase error, most
The phase error detected is corrected in spectral domain afterwards.(for example, seeing Jianhua Mo et al. " Correction of
Phase-error for Phase-resolved k-clocked Optical Frequency Domain Imaging (bases
In the correction for the phase error that the optimal frequency domain of Phase-Resolved Analysis is imaged) ").
To describe the present invention in detail, the specific implementation method of the present invention is described in further detail below.
Referring to figure, the invention mainly comprises 3 steps：SS-OCT system buildings, data acquisition and procession, phase error school
Normal operation method.It is described in detail below：
(1) SS-OCT system buildings；
SS-OCT systems designed by the present invention are mainly by swept laser source, sample arm, reference arm, X-Y vibration mirror scanning instrument
Formed with data collecting card, as shown in Figure 1.Light source center wavelength is 1310 nanometers, and wave-length coverage is received from 1249.4 to 1359.6
Rice.Laser average output power is 20 milliwatts.Light from swept laser source out separates two by 90/10 fiber coupler
Shu Guang, sample arm (90%) and reference arm (10%) are entered by optical fiber circulator respectively, via reflective surface in two-arm
The back-scattering light returned is interfered by 50/50 fiber coupler, and interference signal is detected to obtain by balanced detector.B
Scanning drives X-Y scanning galvanometers to realize raster scanning by the High Speed Analog output device of 16.
(2) data acquisition and procession；
The interference signal for detecting to obtain by balanced detector first passes through low pass filter to remove frequency higher than 90MHz's
Signal, then recorded by the data collecting card of 14 enter in computer.The outside k clocks that data collecting card is provided by lasing light emitter drive
It is dynamic, Fourier transform calculating spectrum sample to linear wave number space (K spaces) and is done by k clocks.Data acquisition program is built
Stand in LabVIEW platform, for gathered data and control X-Y scanning galvanometers, and carry out data processing with LabVIEW can be with
Show Interferogram in real time.
(3) phase error correction algorithm
Phase error correction algorithm mainly includes 3 steps：Including：(a) interference spectrum does discrete Fourier transform；To light
Learn the method that Coherence Tomography original interference signal is modulated.(b) modulating signal phase is analyzed；Using to optics phase
The phase distribution of the modulated signal of dry Tomography original interference signal detects the skew of original interference spectral signal.
(c) interference spectrum skew is corrected；The method for correcting the skew of Optical coherence tomography original interference spectral signal.The tool
Body is as follows：
(a) interference spectrum does discrete Fourier transform；
B-scan drives X-Y scanning galvanometers to realize raster scanning by the High Speed Analog output device of 16 in the present invention.One
Secondary B-scan includes 12000 interference spectrums, each spectrally there is 1024 points, one 1024*12000's of these spectral compositions
Packet.The interference spectrum of B-scan collection is as shown in Figure 2.Before interference signal is sent to detector, to interference spectrum
Apply a perfect sinusoidal modulation signal.Spectral modulation signal in the light path of one of arm before balanced detector by inserting
Enter a POLYCARBONATE SHEET to produce.The modulated signal is expressed as one in the amplitude spectrum after interference spectrum carries out Fourier transformation
Highly stable peak, as shown in Figure 3.Identify by analyzing the phase of stable modulated signal and quantify the inclined of interference spectrum
Move.
(b) modulating signal phase is analyzed
Mode proposed by the present invention is the modulation by carrying out a certain specific frequency to OCT primary signals, should by analysis
The phase of modulated signal come detect the skew of interference spectrum shake and to detect shake caused by phase error be corrected.
Specifically, data acquisition is carried out in same point, such as repeated acquisition 10,000 times, in this point, its phase distribution can be presented
Discrete state, is not necessarily equally distributed, may have 5000 points in certain, 4200 points be present at another place, its
800 points of remaininging are then to be present in other at one, but total are added up or 10000 points.Then its phase distribution is counted, with point
A most phase value of distribution is as correct phase.Based on the amplitude spectrum finished in step (a) after Fourier transformation, at peak
Three relative peaks are taken to do Orient Phase to three reference signals as reference signal at value, and with wherein amplitude maximum
Point is used as correct phase, and other compared with theoretical value by relatively determining skew digit a little and offset direction and doing ± 1
To ± 3 points of shift correction.The front and rear result of correction is as shown in Figure 4.
(c) interference spectrum skew is corrected
The correcting mode that the present invention takes is that modulated signal offset is directly subtracted in the basis of signals to shift
Theoretical value is so as to obtaining correct phase.First, when starting gathered data, the phase error as caused by a number of point shake
Can theoretically it be calculated.Point offset is as follows in the formula that frequency domain is corrected：
φc(k)=φo(k)-2πkm/1024,(1)
Here φoAnd φcThe preceding phase with after correction of correction is represented respectively, and k is depth factor, and m is skew points (positive sign
Expression moves to right, and negative sign represents to move to left), 1024 represent that the data of each spectral scan are counted out.As shown in Fig. 5 (a).
Furthermore, it is contemplated that the problem of picture depth, find out the frequency of correct phase, continue to optimize the frequency of modulated signal.
As shown in Fig. 5 (a), to each frequency, the mutual phase difference of from ± 1 to ± 3 point of skew is calculated, then takes phase difference most
The corresponding frequency of big value is the optimal frequency (Fig. 5 (b)) of modulated signal.In this invention, selection modulating frequency is about
60MHz, its reason have two：(1) modulated signal peak value is remote enough from the picture depth that we are concerned about in an A sweep；(2)
In theory, without having before overcorrect in the phase gamut of ± 1 to ± 3 points of skews, 60MHZ or so can be by best
Separate (being not in overlapping phenomenon when doing Orient Phase), rather than in 27.9MHz or so.
Then, judge it is left avertence or right avertence during spectra collection by the theoretical value of modulated signal, be specifically offset by it is several
Individual point.It is corrected on the basis of offset direction and offset is aware of.(4) spectral shift reason
The reason for inquire into spectral shift, the present invention test the phase stability under light trigger condition.Light triggering letter
Number by the light for sending light source separate sub-fraction import bragg grating (FBG), what Bragg grating reflection was returned
Light gets converted to electric impulse signal by optical receiver detection, in this, as the trigger signal of OCT data acquisition.By Prague light
The centre wavelength position for the light that fine optical grating reflection is returned be fixed i.e. each fiber grating to optical signal position be it is fixed,
So light trigger signal with k clocks are basically identical that relative unstable shake is not present.With the histogram under scanning trigger condition
Distribution is compared, and both are very similar, as shown in fig. 6, its be 61,000 spectrum respectively scanning triggering and light trigger condition under
Offset the histogram of points.When skew points are ± 2,100 times are exaggerated in order to preferably visualize.Each of the above Nogata bar
On be all labelled with actual points.
It is above-mentioned to mean that light trigger signal is shaken with basically identical be not present of k clocks, and can draw cause shake maximum
Factor is the conclusion of the electronic equipment on data collecting card.Because the quantity of offset point is extremely limited, it is proposed that
Force a stable spectral modulation signal on interference spectrum and its phase that is used is missed to correct the phase caused by shake
Difference.Specific method refers to algorithm steps (a), (b).
(5) experimental verification
In addition, the present invention demonstrates phase error correction algorithm on human body skin.Evaluation phasing effect has three
Standard：1) phase distribution of modulated signal；2) Dopplergram based on Phase-Resolved Analysis；3) fixed pattern noise removal effect.School
Positive result is as shown in Figure 7.
The phase distribution effect of modulated signal finds out difference before and after correction from step (b).Before correction, its phase is presented
Go out discrete and uneven distribution, after algorithm corrects, its phase distribution eliminates phase error than more uniform and concentrate very much
Effect it is fairly obvious.
Fig. 7 is described it can be seen that in Fig. 7 (a) respectively, not by the structure chart of phase error correction, gray level image
Top half still have the vertical elements of some thin grey, in same picture depth, in the latter half of gray level image
There is a thick black line (being caused by sinusoidal modulation signal).This shows that fixed pattern noise is not completely eliminated.With
This by contrast, in Fig. 7 (b), after being corrected by phase error, the above fixed pattern noise row is almost disappeared completely
Except.This shows the elimination that can improve fixed pattern noise after phase calibration error.As for doppler image, phase calibration misses
It is difficult to recognize Doppler signal before poor.It is that Doppler signal is shown it is clear that such as Fig. 7 (c) and Fig. 7 after phase calibration error
(d) shown in.
These results show the fixed mode that our phase error correction approach improves after phase calibration error, correction
Noise remove effect and based on Phase-Resolved Analysis optimal frequency domain imaging Doppler signal contrast on have good effect
Fruit.
The general principle and principal character and advantages of the present invention of the present invention has been shown and described above.The technology of the industry
Personnel are it should be appreciated that the present invention is not limited to the above embodiments, and the simply explanation described in above-described embodiment and specification is originally
The principle of invention, without departing from the spirit and scope of the present invention, various changes and modifications of the present invention are possible, these changes
Change and improvement all fall within the protetion scope of the claimed invention.The claimed scope of the invention by appended claims and its
Equivalent thereof.
Claims (7)
1. a kind of bearing calibration based on phase error in Optical coherence tomography, it is as follows that it includes step：
(1) original interference spectral signal carries out discrete Fourier transform：First to Optical coherence tomography original interference light
The modulation that spectrum signal carries out a certain frequency obtains modulated signal, then again by discrete Fourier transform to obtain modulated signal
Phase information；
(2) modulating signal phase is analyzed：The phase of modulated signal acquired in step (1) is analyzed, in Orient Phase
It is that the interference spectrum corresponding to other phase values is then thought to be offset, partially correctly without skew to be distributed most phase values
Direction and offset are moved by the difference of its phase value and correct phase value to determine；
(3) interference spectrum skew is corrected：Calculated by the offset direction of the spectrum obtained by step (2) and offset in each frequency
On phase difference, then will calculate and subtract in signal of the phase difference directly from step (1) after discrete Fourier transform of gained
Go.
2. bearing calibration according to claim 1, it is characterised in that be first to establish optics phase first in the step (1)
Dry dislocation scanning and imaging system, then carry out data acquisition and procession；
The optical coherence tomography system includes swept laser source, sample arm, reference arm, X-Y vibration mirror scannings instrument and
Data collecting card, the light from swept laser source out separates two-beam by fiber coupler, respectively by optical fiber circulator
Sample arm and reference arm are entered, the back-scattering light returned via reflective surface in sample arm and reference arm passes through optical fiber coupling
Clutch is interfered, and interference signal is detected to obtain by balanced detector.
3. bearing calibration according to claim 2, it is characterised in that the X-Y vibration mirror scannings instrument passes through the high speed of 16
Analogue output unit driving X-Y scanning galvanometers realize raster scanning.
4. bearing calibration according to claim 2, it is characterised in that the light source center wave-length coverage of swept laser source is
1249.4~1359.6 nanometers.
5. bearing calibration according to claim 2, it is characterised in that the data acquisition and procession is visited by balanced detector
The interference signal measured first passes through low pass filter to remove the signal that frequency is higher than 90MHz, then passes through data acquisition
Card is recorded enter in computer；The outside k clocks that data collecting card is provided by lasing light emitter drive, by k clocks spectrum sample to line
Property wave number space simultaneously does Fourier transform calculating.
6. bearing calibration according to claim 5, it is characterised in that the program of the data acquisition is established in LabVIEW
On platform, for gathered data and control X-Y scanning galvanometers, and with LabVIEW platform carry out data processing show in real time it is dry
Relate to spectrum picture.
7. bearing calibration according to claim 1, it is characterised in that the correcting mode of above-mentioned steps (3) is to occur partially
The theoretical value of modulated signal offset is directly subtracted in the basis of signals of shifting so as to obtain correct phase；Comprise the following steps that：
First, when starting gathered data, the phase error as caused by a number of point shake is theoretically calculated, point
Offset is as follows in the formula that frequency domain is corrected：
φc(k)=φo(k)-2πkm/1024,
Wherein, φoAnd φcThe phase before correcting and after correction is represented respectively, and k is depth factor, and m is that skew is counted, and positive sign represents
Move to right, negative sign represents to move to left, and 1024 represent that the data of each spectral scan are counted out；
Then, according to picture depth, the frequency of correct phase is found out, continues to optimize the frequency of modulated signal；To each frequency, meter
The mutual phase difference of skew is calculated, then takes the optimal frequency that the frequency corresponding to phase difference maximum is modulated signal；
Finally, judge it is left avertence or right avertence during spectra collection by the theoretical value of modulated signal, be specifically that offset by several points；
So as to be corrected on the basis of offset direction and offset.
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