CN110584613A - Catheter polarization sensitive optical coherence tomography system and demodulation method - Google Patents
Catheter polarization sensitive optical coherence tomography system and demodulation method Download PDFInfo
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Abstract
A catheter polarization-sensitive optical coherence tomography system and a demodulation method are characterized in that a rapid scanning light source is adopted as a light source in the system, a polarization-maintaining optical fiber is adopted in the system to generate orthogonal polarization state delay, polarization diversity acquisition is carried out through a polarization beam splitter, and polarization diversity imaging of two orthogonal input polarization states is presented in one image at the same time. The demodulation method finally realizes the vascular birefringence imaging through a series of steps of polarization leveling, background signal elimination, spectrum shaping, two-state dispersion elimination, interpolation Fourier transform, reference surface selection, polarization resolving, polar coordinate conversion to Cartesian coordinates and the like. The invention can obtain the vascular birefringence information and provide more characteristic information for the cardiovascular and cerebrovascular diseases.
Description
Technical Field
The invention relates to the field of catheter imaging covering cardiovascular and cerebrovascular systems and the like by catheter Optical Coherence Tomography (OCT), in particular to a system and a method for demodulating birefringent information of a sample in a catheter Polarization-sensitive OCT (Polarization-sensitive OCT) image, namely a PS-OCT image, and specifically relates to a catheter Polarization-sensitive Optical Coherence Tomography system and a demodulation method used in an OCT technology.
Background
The catheter OCT imaging technology is a blood vessel imaging method with the highest image resolution at present, particularly the catheter PS-OCT imaging technology, can solve the medical problem that the stability of atherosclerotic plaques is difficult to judge in vivo, in real time and rapidly, and can improve the prevention and treatment effect of atherosclerotic diseases. However, the existing OCT system has reached a level that may determine the property of the tissue plaque in terms of resolution, but is still insufficient in terms of tissue penetration ability, image sharpness, and accuracy of tissue plaque type determination, and using the PS-OCT technology, improving the performance of the related technology is a key direction for development of the OCT system, and is also a necessary way to solve the aforementioned key scientific problems.
In catheter OCT, catheter PS-OCT is an extension of catheter OCT technology, which provides a quantitative measure of tissue birefringence properties. The birefringence of light changes the polarization state of light and can be associated with proteins and biological macromolecules with oriented structures such as collagen, actin, and the like. The enhanced birefringence phenomenon of catheter PS-OCT is closely related to the existence of a large amount of thick collagen fibers or intimal smooth muscle cells, so that the high-resolution detection of catheter PS-OCT imaging can be applied to the enhanced plaque stability measurement. In addition, catheter PS-OCT systems have the potential to assess plaque collagen and differentiate normal intima, fibrous plaque, lipid plaque, and calcified plaque, among others.
Disclosure of Invention
The invention aims to solve the problem that the existing OCT catheter optical coherence tomography system can only acquire blood vessel intensity information, and provides a catheter polarization-sensitive optical coherence tomography system and a demodulation method to acquire blood vessel birefringence information and provide more characteristic information for cardiovascular and cerebrovascular diseases. In the system, a fast scanning light source is adopted as a light source, a polarization-maintaining optical fiber is adopted in the system to generate orthogonal polarization state delay, polarization diversity acquisition is carried out through a polarization beam splitter, and polarization diversity imaging of two orthogonal input polarization states is presented in one image at the same time. The demodulation method finally realizes the vascular birefringence imaging through a series of steps of polarization leveling, background signal elimination, spectrum shaping, two-state dispersion elimination, interpolation Fourier transform, reference surface selection, polarization resolving, polar coordinate conversion to Cartesian coordinates and the like.
One of the technical schemes of the invention is as follows:
a catheter polarization-sensitive optical coherence tomography system comprises a scanning light source 1 and is characterized in that emergent light of the scanning light source 1 enters from a port 1 of an optical fiber coupler 2 and is distributed to a sample arm and a reference arm from ports 22 and 3 of the optical fiber coupler respectively; emergent light of a port 2 of the optical fiber coupler 2 enters a sample arm, light beams entering the sample arm enter a polarization-maintaining optical fiber 4 after entering a polarization (three-ring) controller 3 and enter a port 1 of a first circulator 6, light exits from the port 2 of the first circulator 6, the emergent light enters an imaging guide pipe (11) through a rotating mechanism (8), and the light reflected by the sample returns to the first circulator 6 from the imaging guide pipe (11) and exits through a port 3 of the first circulator 6; emergent light of a 3 port of the optical fiber coupler 2 enters a reference arm, light entering the reference arm enters a single-mode optical fiber 5, emergent light of the single-mode optical fiber 5 enters a 1 port of a second circulator 7, emergent light of the second circulator 7 enters a reflective optical fiber delay line 10 from a 2 port of the second circulator 7, reflected light of the reflective optical fiber delay line 10 enters through the 2 port of the second circulator 7, emergent light of the sample arm passing through the 3 port of the first circulator 6 and emergent light of the reference arm passing through the first polarization controller 9 enter the optical fiber coupler 12 from 1 port and 2 port of the optical fiber coupler 12 respectively to generate interference, and enter a second polarization controller 13 and a fourth polarization controller 14 from 3 port and 4 port of the optical fiber coupler 12 respectively, emergent light enters a first polarization beam splitter 15 and a second polarization beam splitter 16 respectively, the emergent light of the first polarization beam splitter 15 is respectively incident into a first balanced detector 17 and a second balanced detector 18 from the ports 1 and 2, the emergent light of the second polarization beam splitter 16 is respectively incident into the first balanced detector 17 and the second balanced detector 18 from the ports 1 and 2, and the electric signals of the first balanced detector 17 and the second balanced detector 18 are received by an acquisition card 19 and transmitted into a computer 20.
The scanning light source 1 is a rapid scanning light source, a polarization-maintaining optical fiber is adopted to generate orthogonal polarization state delay, polarization diversity acquisition is carried out through a polarization beam splitter, the length of the polarization-maintaining optical fiber depends on birefringence of the polarization-maintaining optical fiber to generate phase delay, and the phase delay is equal to half of OCT imaging depth, so that polarization diversity imaging of two orthogonal input polarization states can be presented in one image at the same time, and possibility is provided for eliminating system birefringence change introduced by catheter rotation subsequently.
The optical fiber coupler 2 is a 1:99 optical fiber coupler.
Emergent light of the scanning light source 1 enters from a port 1 of a 1:99 optical fiber coupler 2 and is distributed to a sample arm and a reference arm from ports 2 and 3 of the optical fiber coupler 2 in a ratio of 1: 99.
The lengths of the polarization maintaining fiber 4 and the single mode fiber 5 are both 18.5 meters.
The optical fiber coupler 12 is a 50:50 optical fiber coupler; the emergent light of the sample arm passing through the 3 ports of the first circulator 6 and the emergent light of the reference arm passing through the polarization controller 9 are respectively incident into the optical fiber coupler 12 from the 1 port and the 2 port of the optical fiber coupler 12 to generate interference, and enter the first polarization controller 13 and the second polarization controller 14 from the 3 port and the 4 port of the optical fiber coupler 12 in a ratio of 50: 50.
The second technical scheme of the invention is as follows:
a demodulation method for catheter polarization sensitive optical coherence tomography is characterized by comprising the following steps:
the method comprises the following steps: on one hand, the light intensity of the reference light in the H path and the V path is equal in polarization diversity; on the other hand, the light of the sample arm is uniformly distributed to the two polarization controllers 13, 14;
step two: background signals are removed, imaging quality, particularly imaging at a base frequency is seriously affected by the background noise and the base frequency, and the background noise and the base frequency need to be removed;
step three: the method comprises the following steps of performing two-state dispersion compensation, namely processing data acquired by using a PS-OCT system, performing Hilbert transformation on each A-Line of a PS-OCT original image of a channel H and a channel V under a polar coordinate, representing a PS-OCT interference signal in the form of complex index, performing numerical dispersion compensation processing on each A-Line subjected to Hilbert transformation, calculating a corrected phase phi (k) through second-order and third-order dispersion coefficients, multiplying the corrected phase phi (k) by an A-Line original phase in the form of e index, and performing numerical dispersion compensation on the A-Line original signal and correcting frequency broadening of the interference signal through phase addition and subtraction;
step four: spectrum shaping, namely shaping the spectrum of the A-Line data by using a cosine cone window in order to prevent the spectrum leakage of the distance domain signal after Fourier transform;
step five: interpolation Fourier transform is carried out, due to depth multiplexing, the number of data points in the depth direction is small, the points need to be supplemented by the interpolation Fourier transform to be consistent with the number of the circumference points, and finally the two-dimensional data information of the PS-OCT is obtained, namely the distance domain information H of the channel H and the channel V which are overlapped in a two-state mode1+H2And V1+V2；
Step six: binary image segmentation, and range domain information H superimposed on channel H and channel V in binary mode1+H2And V1+V2Dividing, performing cross-correlation operation on two sections of data of each row of A-line data after selecting a division point, adjusting the division point through a cross-correlation peak frequency shift value, and finally obtaining four groups of data H1、H2、V1And V2；
Step seven: selecting a reference position before polarization resolving, namely a birefringence phase delay zero position of each row of A-line data, and selecting the outer surface of the conduit sheath as a reference surface which is not influenced by a sample to be detected; selecting the position of the outer surface of the catheter sheath by adopting a data frame, wherein the point with the maximum amplitude intensity in each row of A-line data falling in the data frame is taken as a reference point;
step eight: to H1、H2、V1And V2Substituting the A-line data of each row into a polarization resolving algorithm to obtain birefringence phase delay;
step nine: and (3) performing coordinate interpolation transformation on the birefringent phase retardation image in the polar coordinate, converting the polar coordinate into a Cartesian coordinate, and finally obtaining a catheter PS-OCT sample birefringent image.
The polarization leveling comprises the following steps: disconnecting the sample arm, and firstly, utilizing the polarization controller 9 on the reference arm and the front two polarization controllers of the polarization beam splitter to equalize the light intensity of the reference arm in the polarization diversity in the H path and the V path (the light intensity of the H path and the V path is equalized by monitoring in software); secondly, a sample arm is connected, the polarization controller 3) in front of the polarization maintaining fiber is used for imaging the catheter as reference, and the peak values of the front surface and the rear surface of the two-state imaging catheter on the one-dimensional image are adjusted, so that the light of the sample arm is uniformly distributed to the two polarization controllers.
The background signal removing method comprises the following steps: disconnecting the sample arm, collecting A-scan information of the background noise and fundamental frequency of the reference light, collecting each A-scan of the signal after reconnecting the sample arm, and subtracting the background noise and fundamental frequency of the reference light from each A-scan.
The invention has the beneficial effects that:
the traditional catheter optical coherence tomography system can only acquire blood vessel intensity information, and the catheter polarization-sensitive optical coherence tomography system and the demodulation method can acquire blood vessel birefringence information and provide more characteristic information for cardiovascular and cerebrovascular diseases.
Drawings
FIG. 1 is a schematic diagram of the system structure of the present invention.
Fig. 2 is a flow chart of the demodulation method of the present invention.
FIG. 3 is a catheter PS-OCT chicken breast tissue intensity image.
FIG. 4 is a birefringence image of catheter PS-OCT chicken breast tissue.
Detailed Description
The invention is further described below with reference to the figures and examples.
The first embodiment.
As shown in fig. 1.
A catheter polarization-sensitive optical coherence tomography system is shown in figure 1 and comprises a scanning light source 1, wherein emergent light of the scanning light source 1 enters from a port 1 of a 1:99 optical fiber coupler 2 and is distributed to a sample arm and a reference arm from ports 2 and 3 in a ratio of 1:99 respectively. Emergent light of a port 2 of a 1:99 optical fiber coupler 2 enters a sample arm, light beams entering the sample arm enter a polarization maintaining optical fiber 4 with the length of 18.5 meters after entering a three-ring controller 3, enter a port 1 of a circulator 6, light exits from the port 2 of the circulator 6, the emergent light enters an imaging guide pipe 11 through a rotating mechanism 8, and the light reflected by the sample returns to the circulator 6 from the imaging guide pipe 11 and exits through a port 3 of the circulator 6. Emergent light of a port 3 of the 1:99 optical fiber coupler 2 enters a reference arm, light entering the reference arm enters a single-mode optical fiber 5 with the length of 18.5 meters, the emergent light enters a port 1 of a circulator 7, the emergent light exits from a port 2 and enters a reflective optical fiber delay line, and the reflected light enters through the port 2 of the circulator 7 and exits from the port 3 to a polarization controller 9. Emergent light of a sample arm passing through a port 3 of a circulator 6 and emergent light of a reference arm passing through a three-ring polarization controller 9 are respectively incident into a 50:50 optical fiber coupler 12 from ports 1 and 2 of the optical fiber coupler 12 to interfere with each other, and respectively enter a polarization controller 13 and a polarization controller 14 from ports 3 and 4 in a ratio of 50:50, the emergent light is respectively incident into polarization beam splitters 15 and 16, the emergent light of the polarization beam splitter 15 is respectively incident into balance detectors 17 and 18 from ports 1 and 2, the emergent light of the polarization beam splitter 16 is respectively incident into the balance detectors 17 and 18 from ports 1 and 2, and electric signals of the balance detectors 17 and 18 are received by an acquisition card 19 and transmitted into a computer 20.
The light source adopts a fast scanning light source, a polarization maintaining optical fiber is adopted in the system to generate orthogonal polarization state delay, polarization diversity acquisition is carried out through a polarization beam splitter, and the length of the polarization maintaining optical fiber depends on the birefringence of the polarization maintaining optical fiber to generate phase delay equal to half of the imaging depth of the common OCT. The method ensures that the system can simultaneously present polarization diversity imaging of two orthogonal input polarization states in one image, and provides possibility for eliminating system birefringence change introduced by catheter rotation subsequently.
Example two.
As shown in fig. 2-4.
A demodulation method of catheter polarization-sensitive optical coherence tomography, as shown in fig. 2, comprising the steps of:
the method comprises the following steps: on one hand, the light intensity of the reference light in the H path and the V path is equal in polarization diversity; on the other hand, the light of the sample arm is distributed uniformly to the two polarization states 13 and 14. The method comprises the following specific steps: the sample arm is disconnected, firstly, the polarization controller 9 on the reference arm and the front two polarization controllers of the polarization beam splitter are utilized, the light intensity of the reference arm in the polarization diversity H path and the light intensity of the reference arm in the V path are equal, and the monitoring software monitors that the light intensity of the reference arm in the H path is equal to that of the reference arm in the V path. Secondly, a sample arm is connected, the polarization controller 3 in front of the polarization maintaining fiber is used for imaging the catheter as reference, and the peak values of the front surface and the rear surface of the two-state imaging catheter on the one-dimensional image are adjusted, so that the light of the sample arm is uniformly distributed to two polarization states.
Step two: background signal removal, background noise and fundamental frequency seriously affect imaging quality, particularly fundamental frequency imaging, and the background noise and the fundamental frequency need to be removed. The method comprises the following steps: the sample arm (catheter portion) was disconnected and the a-scan information of the background and fundamental frequencies of the reference light was collected. The signal was acquired after reconnecting the sample arm (catheter section) and each A-scan was subtracted with the background and fundamental frequency of the reference light.
Step three: a dual state dispersion compensation, which processes the data collected by the PS-OCT system and performs PS-OCT on the original channel H and channel V under polar coordinatesHilbert transformation is carried out on each A-Line of the image, a PS-OCT interference signal is represented in a complex exponential mode, numerical dispersion compensation processing is carried out on each A-Line subjected to Hilbert transformation, a correction phase phi (k) is calculated through second-order and third-order dispersion coefficients, the correction phase phi (k) is multiplied by an A-Line original phase in an e-exponential mode, numerical dispersion compensation of the A-Line original signal is achieved through phase addition and subtraction, and frequency broadening of the interference signal is corrected. The propagation constant beta (delta k) is set at the central wave number k during dispersion compensation0And expanding according to Taylor series to obtain a second order term coefficient and a third order term coefficient, respectively substituting the second order coefficient and the third order coefficient of two polarization states into the power spectrum to perform order and numerical traversal calculation to obtain a local optimal dispersion coefficient, and substituting the local optimal dispersion coefficient into a phase compensation calculation formula of the interference signal to realize the two-state dispersion compensation. Using a system to acquire data in a catheter no-load state to obtain interference signals of the reference light and the reflected light of the end face of the catheter probe; taking any one A-Line data in interference signals of an H channel or a V channel, carrying out Hilbert transformation on the data to obtain a real part and an imaginary part of the A-Line data, and expressing the A-Line signal in the form of an amplitude index by an Euler formula:
wherein phi0(k) Is the initial phase of the interference signal;
initial phase phi of A-Line interference signal0(k) At the center wave number k0Expanding according to Taylor series:
φ(k)＝φ(k)-a2·(2πc)2·(k-k0)2-a3·(2πc)3·(k-k0)3… (2)
obtaining the second-order dispersion coefficient a2And third-order dispersion coefficient a3Wherein a is2Is the group velocity delay;
the phase of each input polarization state can obtain a group a2And a3Initializing the magnitude range and the value range of the two undetermined coefficients, and substituting the values into a formula(2) Performing traversal calculation; in an initialized range, a group of compensation phases related to interference terms are calculated, the calculated compensation phases are multiplied by the original interference signal index of an H channel or a V channel in an e index mode, the obtained result is subjected to fast Fourier transform, and the coordinate position of the maximum value of the power spectrum is the order of magnitude or the numerical value of the dispersion coefficient. Selecting the outer surface area of the wrapping layer of the fiber-optic catheter probe, and traversing a in sequence in the current area2And a3The magnitude and the value of the interference signal are determined by solving the maximum value of the interference peak of the interference signal in the traversal process to determine a2And a3Magnitude and value of (a), the result obtained2And a3I.e. the binary dispersion compensation coefficient of the current system, a is randomly selected A-Line data2And a3Actually a locally optimal solution for the bi-state dispersion compensation coefficient.
Step four: and (4) spectral shaping, wherein after Fourier transformation is not prevented, the distance domain signal spectrum leaks. For A-Line data, a cosine cone window is used to shape the spectrum.
Step five: interpolation Fourier transform is carried out, because the depth multiplexing is carried out in the system, the number of data points in the depth direction is less, the points are supplemented by the interpolation Fourier transform to be consistent with the number of the circumference points, and finally the two-dimensional data information of the PS-OCT is obtained, namely the distance domain information H of the channel H and the channel V which are overlapped in a two-state mode1+H2And V1+V2
Step six: binary image segmentation, and range domain information H superimposed on channel H and channel V in binary mode1+H2And V1+V2Dividing, performing cross-correlation operation on two sections of data of each row of A-line data after selecting a division point, adjusting the division point through a cross-correlation peak frequency shift value, and finally obtaining four groups of data H1、H2、V1And V2. The two-state image segmentation method adopts a cross-correlation method to perform cross-correlation operation on the same A-Line data of an H channel and a V channel, wherein the A-Line data comprises two input polarization states, and the initial positions of the two input polarization states are adjusted according to the offset of a cross-correlation peak so as to ensure that the two input polarization states are twoThe first valid data points of the input polarization states are aligned, ultimately ensuring that the four electric field data forming the measured jones matrix are from the same depth position of the same sample in the subsequent polarization solution.
Step seven: the reference position is selected before polarization calculation, namely the birefringence phase delay zero position of each row of A-line data, and the surface of the sample is generally selected as a reference surface. In the system, the outer surface of the catheter sheath is selected as a reference surface, and the system is not influenced by a sample to be detected. The specific selection method comprises the steps that the position of the outer surface of the catheter sheath is selected by adopting a data frame, and the point with the maximum amplitude intensity in each row of A-line data falling in the data frame is taken as a reference point.
Step eight: to H1、H2、V1And V2And substituting the data of each column of A-line into a polarization calculation algorithm to obtain the birefringence phase delay.
Step nine: performing coordinate interpolation transformation on the birefringent phase retardation image under the polar coordinate, converting the polar coordinate into Cartesian coordinate, and finally obtaining the catheter PS-OCT sample birefringent image
Compared with the unprocessed original intensity image, the birefringence information in the image is correctly expressed after the processing of the invention. As shown in fig. 3 and 4.
In specific implementation, a polarization information extraction method commonly used for PS-OCT is a Similar Jones Matrix (SJM) method, which is an improvement on the characteristics of catheter PS-OCT on the basis of desktop PS-OCT, but the actual polarization calculation result of the method is not ideal, and the birefringence information of a biological tissue or a sample cannot be correctly calculated. The core idea of the SMM method is to convert a measured Jones matrix obtained by a catheter PS-OCT imaging system into a Mueller matrix, then carry out multiplicative decomposition on the Mueller matrix to obtain a phase delay matrix, calculate the phase delay of the tissue birefringence by utilizing the trace of the phase delay matrix, and realize the tissue birefringence imaging.
The estimation process is as follows:
defining four electric field components output by the system as a Jones matrix form:
the conversion relation between the Jones matrix and the Mueller matrix is as follows:
whereinRepresenting the kronecker product, U represents the transformation matrix:
according to the conversion relation between the Jones matrix and the Mueller matrix, converting a measurement Jones matrix J output by the system into a measurement Mueller matrix Q:
for measuring Mueller matrix QZThe elements of (a) are subjected to Euler transformation to obtain:
taking a single A-Scan arbitrarily, assuming that the reference point on the A-Line is z in depth in the sample, the measured reference Mueller matrix Q at zZrefCan be expressed as:
the data acquisition card measures the electric signals at the polarization diversity position, and the electric signals are output by the balanced photoelectric detectors of the H channel and the V channel through photoelectric conversionThe collected electric signals are respectively subjected to background noise removal, two-state numerical dispersion compensation, spectrum shaping filtering and Fourier interpolation transformation to obtain two groups of signals H of an H channel and a V channel in a z domain1+H2And V1+V2H is obtained by image segmentation1，H2，V1，V2An electric field signal, constituting four polarization states. Taking a single A-Line as an example, a measurement Jones matrix J (z) obtained by an imaging system at a position with the sample depth of z is constructed as follows:
let the reference point on the A-Line be z at the sample depthrefAt position, its corresponding reference Jones matrix J (z)ref) Comprises the following steps:
in the case of a catheter PS-OCT imaging system, when calculating the polarization information of biological tissues, the reference surface can be selected from the outer surface of the catheter or the front surface of a sample, and the measurement Jones matrix J (z) and the reference Jones matrix J (z) are calculated by using the formula (2-7) and the formula (2-8)ref) Conversion into a measured Mueller matrix S (z) and a reference Mueller matrix S (z)ref)。
Let MSTIs a back-and-forth transport matrix for biological tissue or samples, MinAnd MoutMueller matrices representing the optical path of the system, respectively, reference Mueller matrix S (z) at the position of the sample reference plane measured at polarization diversityref) Expressed as:
the measured mueller matrix s (z) at the sample z position measured at polarization diversity is expressed as:
mixing S (z)ref) And S (z) performing multiplication operation to eliminate the phase influence of the catheter on the tissue birefringence during high-speed rotation, and rearranging the formula (2-14) and the formula (2-15) to obtain a measured Mueller matrix M (z) of the biological tissue or the sample at the depth of zref,z)：
To construct M (z)refZ) and MS,TFor a pair of similar matrices, then Q needs to be satisfiedZrefMoutMust be a reversible matrix, but if QZrefMoutIncluding depolarization effects and two-way attenuation effects, the condition of the similarity matrix does not hold. In practice, reference is made to the Mueller matrix QZrefIs a unitary matrix, but in a catheter PS-OCT imaging system, the high-speed rotation of the optical fiber in the catheter necessarily brings strong depolarization effect and two-way attenuation effect, so MoutThe optical system also necessarily comprises a depolarization effect and a two-way attenuation effect, and the depolarization effect and the two-way attenuation effect in the optical path of the system need to be eliminated through a matrix decomposition method.
Another important link for the SMM polarization calculation method is that the depolarization effect and the two-way attenuation effect are eliminated through matrix decomposition, so that a measured phase delay Mueller matrix is obtained, the phase delay of the biological tissue or the sample is calculated, and the tissue birefringence imaging is realized.
The common decomposition method of the Mueller matrix comprises multiplication decomposition and summation decomposition, wherein the multiplication decomposition is suitable for processing a polarization unit and a system of a priori sequence, and although the polarization unit of the biological tissue is discrete, complex and various and obviously does not have the characteristics of the priori sequence, the L-C Mueller matrix decomposition method is used for obtaining the decomposed Mueller matrix, and the structure of the decomposed Mueller matrix can basically and correctly describe the basic polarization characteristics of the biological tissue. Based on the catheter PS-OCT imaging system, the Mueller matrix can also be decomposed by using a symmetric decomposition method, and the symmetric decomposition method can also accurately express the basic polarization characteristics of the biological tissue under the condition that the Mueller is known to be depolarized. The method adoptsObtaining phase delay Mueller matrix M of measured Mueller matrix by using L-C Mueller matrix decomposition methodR。
The Mueller matrix of the light transmission medium is arbitrarily described, and comprises three polarization effects of the incident light modulated by the target light transmission medium, including two-way attenuation, phase retardation or depolarization. Any mueller matrix can be decomposed into the following forms:
M＝MΔMRMD (2-17)
wherein M is an arbitrary Mueller matrix, MΔA depolarization matrix representing the modulation of incident light by a target light-transmitting medium describes the ability of the target light-transmitting medium to depolarize the incident light. MRThe phase delay matrix for modulating incident light by a target light transmission medium describes that after the incident light passes through a polarizing device, two eigen-polarization states generated by the incident light have different propagation rates in a fast axis and a slow axis, so that time domain signals of the two eigen-polarization states have time delay, and the phase delay is obtained when the time domain signals are converted into a frequency domain. MDA two-way attenuation matrix for expressing the modulation of incident light by a target light transmission medium describes the change relationship of the reflected light intensity of the incident light passing through the light transmission medium along with the polarization state of the incident light.
Phase delay matrix MRCan be expressed as:
wherein m isRIs a phase delay matrix MRA 3 × 3 sub-matrix, which can be calculated by the following equations (2-19):
wherein m' and mΔRespectively, are depolarization matrices MΔAnd the product of the depolarization matrix and the phase delay matrix, M', which may be expressed as:
in the unknown depolarization matrix MΔAnd a phase delay matrix MRIn this case, the attenuation matrix M may be passed throughDCalculated by multiplying the measured Mueller matrix M by a two-way attenuation matrix MDCan be expressed as:
wherein,representing a two-way attenuation vector, mDIs a two-way attenuation matrix MDA 3 x 3 sub-matrix. Two-way attenuation vectorCan be defined as:
whereinBeing the mode of the two-way attenuation vector,is its unit vector, diThe components representing the vector of the two-way attenuation can be calculated by the equations (2-23), in the context of which M is1(i+1)(i ═ 1,2,3) denotes the elements measuring the first row of the mueller matrix M.
Submatrix m of a two-way attenuation matrixDCan be expressed as:
wherein| | · | represents the euclidean vector norm. In the above, the m' matrix can be calculated.
Depolarization matrix MΔCan be expressed as:
whereinRepresenting the polarization vector of a depolarization matrix, the magnitude of which depends on the polarization vectorAnd two-way attenuation vector
In the phase delay matrix MRIn the calculation process, the depolarization matrix M does not need to be solvedΔOnly the sub-matrix m thereof needs to be solvedΔI.e. a sub-matrix m of the depolarization matrixΔIt can be calculated by the formula (2-28):
wherein λi(i ═ 1,2,3) represents the eigenvalues of the submatrix m'.
At this point, a submatrix m of the depolarization matrix is obtainedΔM isΔThe sum M' is substituted into equation (2-19), and the phase delay matrix M can be solved by the simultaneous equations (2-18)R. In the catheter PSIn an OCT imaging System, the Mueller matrix M (z) is to be measuredrefZ) carrying out matrix multiplication decomposition to obtain a Mueller matrix only containing tissue birefringence and a phase delay matrix M with the depolarization effect and the two-way attenuation effect eliminatedR(zrefAnd z) is expressed as:
in which a phase delay matrix M is measuredR(zrefZ) biological tissue or sample transport matrixAnd system transmission output matrixAre all mueller matrices containing only birefringent components, and in this caseIs a unitary matrix, satisfies a similar matrix condition, MR(zrefZ) andi.e. a pair of similarity matrices. The total phase delay amount and the decomposed phase delay matrix MRCan be expressed as:
wherein,the representation is a transmission Mueller matrix which only contains birefringence components and is detected at the position with the depth of z of the biological tissue or the sample, and tr representsThe traces of the matrix.
The specific implementation of the spectral shaping can be realized by adopting the following method, the time-frequency localization is realized by a signal windowing mode, the signal windowing can correct the problem of spectral leakage, and different window functions are selected to finally influence the resolution and the signal-to-noise ratio. The interference signal in the k-domain is windowed by passing it through a Cosine pyramid (Cosine Tapered) window, the windowed signal being represented as:
wherein y represents an output sequence, i.e. a windowed x-sequence, n is the number of elements in the x-sequence, r is the ratio of the total length of the tapered portion to the total length of the signal, and m is rounded downward; when r is less than or equal to 0, the window is equivalent to a Rectangular (Rectangular) window; when r is larger than or equal to 1, the window is equivalent to a Hanning (Hanning) window, and n and r are traversed according to the process by setting the requirement of target resolution.
The parts not involved in the present invention are the same as or can be implemented using the prior art.
Claims (9)
1. A catheter polarization-sensitive optical coherence tomography system comprises a scanning light source (1), and is characterized in that emergent light of the scanning light source (1) enters from a port 1 of an optical fiber coupler (2) and is distributed to a sample arm and a reference arm from ports 2 and 3 of the optical fiber coupler (2) respectively; emergent light of a port 2 of the optical fiber coupler (2) enters a sample arm, light beams entering the sample arm enter a polarization-maintaining optical fiber (4) after entering a polarization (three-ring) controller (3) and then enter a port 1 of a first circulator (6), the light exits from the port 2 of the first circulator (6), the emergent light enters an imaging guide pipe (11) through a rotating mechanism (8), the light reflected by the sample returns to the first circulator (6) from the imaging guide pipe (11) and exits through the port 3 of the first circulator (6); emergent light of a 3-port of the optical fiber coupler (2) enters a reference arm, light entering the reference arm enters a single-mode optical fiber (5), emergent light of the single-mode optical fiber (5) enters a 1-port of a second circulator (7), emergent light of the second circulator (7) exits from a 2-port of the second circulator (7) and enters a reflective optical fiber delay line (10), reflected light of the reflective optical fiber delay line (10) enters through the 2-port of the second circulator (7) and exits from the 3-port of the second circulator (7) to a first polarization controller (9), emergent light of a sample arm passing through the 3-port of the first circulator (6) and emergent light of the reference arm passing through the first polarization controller (9) respectively enter the optical fiber coupler (12) from 1-port and 2-port of the optical fiber coupler (12) to generate interference, and enter a second polarization controller (13) and a fourth polarization controller (14) from the 3-port and a 4-port of the optical fiber coupler (12), emergent light respectively enters a first polarization beam splitter (15) and a second polarization beam splitter (16), emergent light of the first polarization beam splitter (15) respectively enters a first balanced detector (17) and a second balanced detector (18) from ports 1 and 2 of the first polarization beam splitter, emergent light of the second polarization beam splitter (16) respectively enters the first balanced detector (17) and the second balanced detector (18) from ports 1 and 2 of the second polarization beam splitter, and electric signals of the first balanced detector (17) and the second balanced detector (18) are received by an acquisition card (19) and transmitted to a computer (20).
2. The imaging system according to claim 1, characterized in that the scanning light source (1) is a fast scanning light source, polarization-maintaining fiber is used to generate retardation of orthogonal polarization state, polarization diversity acquisition is performed by a polarization beam splitter, and the length of the polarization-maintaining fiber is dependent on its birefringence to generate phase retardation equal to half of the OCT imaging depth, so as to ensure polarization diversity imaging simultaneously presenting orthogonal two input polarization states in one image, and provide possibility for subsequent elimination of system birefringence change introduced by catheter rotation.
3. The imaging system according to claim 1, characterized in that the fiber coupler (2) is a 1:99 fiber coupler.
4. The imaging system according to claim 3, characterized in that the outgoing light of the scanning light source (1) enters from 1 port of a 1:99 fiber coupler (2) and is distributed from 2 and 3 ports of the fiber coupler (2) to the sample arm and the reference arm in a ratio of 1:99, respectively.
5. The imaging system according to claim 1, characterized in that the length of the polarization maintaining fiber (4) and the length of the single mode fiber (5) are both 18.5 meters.
6. The imaging system of claim 1, wherein the fiber coupler (12) is a 50:50 fiber coupler; emergent light of the sample arm passing through the 3 ports of the first circulator (6) and emergent light of the reference arm passing through the polarization controller (9) are respectively incident into the optical fiber coupler (12) from the 1 port and the 2 port of the optical fiber coupler (12) to generate interference, and the interference enters the first polarization controller (13) and the second polarization controller (14) from the 3 port and the 4 port of the optical fiber coupler (12) in a ratio of 50: 50.
7. A demodulation method for catheter polarization sensitive optical coherence tomography is characterized by comprising the following steps:
the method comprises the following steps: on one hand, the light intensity of the reference light in the H path and the V path is equal in polarization diversity; on the other hand, the light of the sample arm is uniformly distributed to two polarization controllers (13, 14);
step two: background signals are removed, imaging quality, particularly imaging at a base frequency is seriously affected by the background noise and the base frequency, and the background noise and the base frequency need to be removed;
step three: the method comprises the following steps of performing two-state dispersion compensation, namely processing data acquired by using a PS-OCT system, performing Hilbert transformation on each A-Line of a PS-OCT original image of a channel H and a channel V under a polar coordinate, representing a PS-OCT interference signal in the form of complex index, performing numerical dispersion compensation processing on each A-Line subjected to Hilbert transformation, calculating a corrected phase phi (k) through second-order and third-order dispersion coefficients, multiplying the corrected phase phi (k) by an A-Line original phase in the form of e index, and performing numerical dispersion compensation on the A-Line original signal and correcting frequency broadening of the interference signal through phase addition and subtraction;
step four: spectrum shaping, namely shaping the spectrum of the A-Line data by using a cosine cone window in order to prevent the spectrum leakage of the distance domain signal after Fourier transform;
step five: interpolation Fourier transform is carried out, due to depth multiplexing, the number of data points in the depth direction is small, the points need to be supplemented by the interpolation Fourier transform to be consistent with the number of the circumference points, and finally the two-dimensional data information of the PS-OCT is obtained, namely the distance domain information H of the channel H and the channel V which are overlapped in a two-state mode1+H2And V1+V2；
Step six: binary image segmentation, and range domain information H superimposed on channel H and channel V in binary mode1+H2And V1+V2Dividing, performing cross-correlation operation on two sections of data of each row of A-line data after selecting a division point, adjusting the division point through a cross-correlation peak frequency shift value, and finally obtaining four groups of data H1、H2、V1And V2；
Step seven: selecting a reference position before polarization resolving, namely a birefringence phase delay zero position of each row of A-line data, and selecting the outer surface of the conduit sheath as a reference surface which is not influenced by a sample to be detected; selecting the position of the outer surface of the catheter sheath by adopting a data frame, wherein the point with the maximum amplitude intensity in each row of A-line data falling in the data frame is taken as a reference point;
step eight: to H1、H2、V1And V2Substituting the A-line data of each row into a polarization resolving algorithm to obtain birefringence phase delay;
step nine: and (3) performing coordinate interpolation transformation on the birefringent phase retardation image in the polar coordinate, converting the polar coordinate into a Cartesian coordinate, and finally obtaining a catheter PS-OCT sample birefringent image.
8. The method of claim 7, wherein said polarization leveling comprises the steps of: disconnecting the sample arm, and firstly, utilizing a polarization controller (9) on the reference arm and the front two polarization controllers of the polarization beam splitter to enable the light intensity of the reference arm in the H path and the light intensity of the reference arm in the V path to be equal in polarization diversity; secondly, a sample arm is connected, the polarization controller (3) in front of the polarization maintaining fiber is used for imaging the catheter as reference, and the peak values of the front surface and the rear surface of the two-state imaging catheter on the one-dimensional image are adjusted, so that the light of the sample arm is uniformly distributed to the two polarization controllers.
9. The method of claim 7, wherein the background signal removing method comprises: disconnecting the sample arm, collecting A-scan information of the background noise and fundamental frequency of the reference light, collecting each A-scan of the signal after reconnecting the sample arm, and subtracting the background noise and fundamental frequency of the reference light from each A-scan.
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