Optical coherence tomography system using chromatic aberration, and operation method therefor

Abstract

This optical coherence tomography (OCT) system comprises: a light source unit for generating source light; a sample arm for providing measurement light to a sample and providing the measurement light reflected from the sample to an optical coupler; a reference arm having an optical structure reflecting reference light; the optical coupler which divides the source light into the measurement light and the reference light to provide same to the sample arm and the reference arm, respectively, and combines and outputs the reflected measurement light and the reflected reference light respectively provided from the sample arm and the reference arm; and an optical detection unit for generating an optical coherence tomography image from the combined measurement light provided from the optical coupler, wherein the sample arm comprises a dispersion lens pair for inducing chromatic aberration of the measurement light by allowing the measurement light to have a plurality of different divergence angles according to wavelengths.

Description

Optical coherence tomography system using chromatic aberration and its operating method

[0001] The present invention relates to an optical coherence tomography system using chromatic aberration and an operating method thereof.

[0002] Optical coherence tomography (OCT) is an imaging technique that utilizes the phenomenon of light interference to obtain three-dimensional cross-sectional images of biological tissue. Active development is underway for high-resolution OCT with a resolution of 1 to 3 μm to observe fine tissue structures.

[0003] High-resolution optical coherence tomography (OCT), with a resolution of 1–3 μm, offers significant potential for subcellular diagnostics compared to conventional OCT with a resolution of 10–20 μm. However, the use of high numerical aperture (NA) lenses required for such high resolution inevitably reduces the depth of focus (DOF), limiting the practical applications of OCT. In other words, when a high-NA objective lens is used to increase lateral resolution, the depth of focus (DOF) is limited inversely proportional to the lateral resolution.

[0004] Several hardware- and software-based techniques have been proposed to address the trade-off between lateral resolution and depth of field (DOF).

[0005] A representative hardware-based technique is to form a Bessel beam to increase the depth of field. This is achieved by using specialized optical systems such as an axicon lens. This results in the formation of a Bessel beam, which elongates the focus of all light, regardless of wavelength, ultimately expanding the depth of field.

[0006] However, when using Bessel beam optics, the system signal-to-noise ratio (SNR) decreases inversely with the square of the increased depth of field, and this reduced system signal-to-noise ratio (SNR) significantly limits imaging deep into tissues.

[0007] Additionally, artifacts caused by the side lobes of the Bessel beam degrade the performance of the image.

[0008] Meanwhile, software-based techniques such as Interferometric Synthetic Aperture Microscopy (ISAM) require a very high level of phase stability over a wide range of data, which limits their application in dynamic imaging.

[0009] The present disclosure provides an optical coherence tomography (OCT) system and an operating method thereof that can resolve the tradeoff between lateral resolution and depth of field while maintaining a high system signal-to-noise ratio (SNR) by using a source light of a wide wavelength band, intentionally inducing a chromatic focal shift of a sample arm to cause measurement lights of different wavelengths to focus at different points in the axial direction on the sample, thereby expanding the depth of field (DOF), and using an image reconstruction algorithm appropriate for the system.

[0010] According to one feature, an optical coherence tomography (OCT) system comprises a light source unit that generates source light, a sample arm that provides measurement light to a sample and provides measurement light reflected from the sample to the optical coupler, a reference arm having an optical structure that reflects reference light, an optical coupler that divides the source light into the measurement light and the reference light and provides them to the sample arm and the reference arm, respectively, and combines the reflected measurement light and the reflected reference light provided from the sample arm and the reference arm, thereby outputting combined light including an interference signal between the measurement light and the reference light, and a light detection unit that analyzes the interference signal of the combined light provided from the optical coupler to generate an optical coherence tomography image, wherein the sample arm includes a pair of dispersing lenses that induce chromatic aberration of the measurement light by causing the measurement light to have a plurality of different divergence angles according to a wavelength.

[0011] The above-mentioned optical detection unit can generate the optical coherence tomography image based on a valid interference signal for each depth of the sample by applying a Gaussian window having a different center position depending on the depth of the sample.

[0012] The center position of the above Gaussian window can be determined by the above chromatic aberration.

[0013] The above chromatic aberration is expressed as a relationship between the wavelength and the shift of the focus formed on the sample (Focal shift), and the Gaussian window can be determined as a center position of the wavelength of the point where the focus is formed so that an interference signal having the wavelength of the point where the focus is formed can be used as a valid interference signal.

[0014] The above optical detection unit can generate the optical coherence tomography image through an operation using an interference signal obtained from the optical coupler, a complex function defined to have various frequencies depending on the depth of the sample in the wave domain (k-domain), and the Gaussian window.

[0015] The above reference arm may include a dispersion compensator that compensates for the dispersion difference between the measurement light and the reference light caused by the dispersion lens pair.

[0016] The reference arm may further include a polarization controller arranged at the front end on the path of the reference light and adjusting the degree of polarization between the sample arm and the reference arm, a collimator arranged behind the polarization controller and in front of the dispersion compensator to convert the reference light into parallel light, and a mirror arranged behind the dispersion compensator to reflect the reference light.

[0017] The sample arm may further include a collimator that is positioned at the front end on the path of the measurement light and in front of the dispersing lens pair, converting the measurement light provided from the optical coupler into parallel light, a scanning mirror that reflects the measurement light dispersed from the dispersing lens pair in various directions and provides it to the sample and provides the measurement light reflected from the sample to the dispersing lens pair through scanning, and an objective lens that focuses the measurement light on the sample.

[0018] The above light source unit may include a light source that generates source light in the form of a supercontinuum laser having a broadband wavelength, a long pass filter (LPF) and a short pass filter (SPF) for filtering a target wavelength band from the source light, and a collimator that collects the source light of the target wavelength band that has passed through the long pass filter and the short pass filter and provides the collected light to the optical coupler.

[0019] According to another feature, an operating method of an optical coherence tomography (OCT) system comprises the steps of: splitting source light into measurement light and reference light through an optical coupler and providing them to a sample arm and a reference arm, respectively; allowing the measurement light, in which chromatic aberration is induced, to be incident on a sample as a measurement target through a pair of dispersing lenses that induce chromatic aberration of the measurement light by having a plurality of different divergence angles according to wavelengths through the sample arm; combining the measurement light reflected from the sample through the optical coupler and the reflected reference light provided from the reference arm to generate combined light including an interference signal between the measurement light and the reference light; and analyzing the interference signal through a light detector to generate an optical coherence tomography image.

[0020] The step of generating the optical coherence tomography image may include the step of setting a Gaussian window having a different center position depending on the depth of the sample, and the step of generating the optical coherence tomography image based on a valid interference signal for each depth of the sample by applying the Gaussian window.

[0021] The above Gaussian window may have a center position of the Gaussian window determined by the chromatic aberration defined as the relationship between the wavelength and the focal shift formed on the sample.

[0022] The step of generating the above optical coherence tomography image can generate the optical coherence tomography image based on an interference signal in which a difference in dispersion between the measurement light and the reference light caused by the dispersion lens pair is compensated for through a dispersion compensator of the reference arm.

[0023] The step of generating the above optical coherence tomography image may include calculating a complex coefficient for a specific frequency of the interference signal by multiplying the interference signal by a complex exponential function having different frequencies and the Gaussian window and calculating the sum, repeating the above process for all frequency ranges to generate the entire frequency spectrum of the interference signal, and calculating the magnitude of the entire frequency spectrum to generate the optical coherence tomography image.

[0024] Prior to the step of providing, the step of generating the source light may further be included through a light source that is a supercontinuum laser having a broadband wavelength.

[0025] According to the present disclosure, high resolution and high signal-to-noise ratio (SNR) can be achieved at an extended depth of field (DOF) through an optical coherence tomography (OCT) system and algorithm utilizing chromatic aberration, thereby obtaining high-resolution / high-sensitivity tomographic images.

[0026] FIG. 1 illustrates a configuration diagram of an optical coherence tomography (OCT) system according to one embodiment.

[0027] Figure 2 shows a Fourier transform algorithm for generating a conventional optical coherence tomography (OCT) image.

[0028] Figure 3 shows the position of the focus according to the wavelength generated in the sample arm according to an embodiment of the present invention.

[0029] FIG. 4 illustrates a chromatic gating algorithm (ChG algorithm) for generating an optical coherence tomography (OCT) image according to an embodiment of the present invention.

[0030] Figure 5 shows the imaging results of applying a Gaussian window to the interference signal (fringe) in Figure 4.

[0031] FIG. 6 is a flowchart illustrating the operation of an optical coherence tomography (OCT) system according to one embodiment.

[0032] Figures 7a, 7b, 7c, and 7d show the results of imaging small beads in water using the OCT system and ChG algorithm that induced chromatic aberration of the present disclosure according to experiments.

[0033] Figures 8a, 8b, 8c, and 8d show the results of imaging small beads in water using conventional high-resolution OCT technology and a Fourier transform-based algorithm according to experiments.

[0034] Figure 9 shows the imaging results obtained using a conventional high-resolution OCT system and a Fourier transform-based algorithm according to the experiment.

[0035] Figure 10 is an imaging result obtained using the OCT system and ChG algorithm that induced chromatic aberration of the present disclosure according to an experiment.

[0036] Figure 11 is an imaging result obtained using the OCT system and ChG algorithm that induced chromatic aberration of the present disclosure according to an experiment.

[0037] Figure 12 shows the imaging results obtained using the OCT system that induces chromatic aberration of the present disclosure according to an experiment and the existing Fourier transform-based algorithm.

[0038] Figure 13 is a contrast comparison chart analyzed using the results of Figures 11 and 12.

[0039] Below, with reference to the attached drawings, embodiments of the present invention are described in detail so that those skilled in the art can easily implement the invention. However, the present invention can be implemented in various different forms and is not limited to the embodiments described herein. In addition, in the drawings, parts irrelevant to the description are omitted to clearly explain the present invention, and similar parts are designated with similar reference numerals throughout the specification.

[0040] In the description, when a part is said to "include" a certain component, this does not mean that it excludes other components, but rather that it may include other components, unless otherwise stated.

[0041] Additionally, terms such as “part,” “unit,” and “module” described in the specification mean a unit that processes at least one function or operation, which may be implemented by hardware, software, or a combination of hardware and software.

[0042] In the description, drawing symbols and names are attached for convenience of explanation, and the devices are not necessarily strictly limited to the drawing symbols or names.

[0043] In this specification, “transmitting or providing” may include not only direct transmission or providing, but also indirect transmission or providing via another device or by using a bypass route.

[0044] In this specification, expressions described in the singular may be interpreted as singular or plural, unless explicit expressions such as “one” or “single” are used.

[0045] In this specification, the same drawing numbers refer to the same components regardless of the drawings, and “and / or” includes each and every combination of one or more of the mentioned components.

[0046] In this specification, terms including ordinal numbers, such as "first" and "second," may be used to describe various components, but these components are not limited by these terms. These terms are used solely to distinguish one component from another. For example, without departing from the scope of the present disclosure, a first component could be referred to as a "second component," and similarly, a second component could also be referred to as a "first component."

[0047] In the flowcharts described with reference to the drawings in this specification, the order of operations may be changed, several operations may be merged, some operations may be split, and certain operations may not be performed.

[0048] The device of the present disclosure is a computing device configured and connected such that at least one processor can perform the operations of the present disclosure by executing instructions. The computer program includes instructions that cause the processor to perform the operations of the present disclosure and can be stored on a non-transitory computer-readable storage medium. The computer program can be downloaded over a network or sold in product form.

[0049] Hereinafter, in the description, the configuration of the prior art is described without assigning drawing numbers to distinguish it from the configuration of the present disclosure.

[0050]

[0051] FIG. 1 illustrates a configuration diagram of an optical coherence tomography (OCT) system according to one embodiment, FIG. 2 illustrates a Fourier transform algorithm for generating a conventional optical coherence tomography (OCT) image, FIG. 3 illustrates a position of a focus according to a wavelength generated in a sample arm according to an embodiment of the present invention, FIG. 4 illustrates a chromatic gating algorithm (ChG) for generating an optical coherence tomography (OCT) image according to an embodiment of the present invention, and FIG. 5 illustrates an example of an imaging result in which a Gaussian window is applied to an interference signal (fringe) in FIG. 4.

[0052] Referring to FIG. 1, an optical coherence tomography (OCT) system (100) includes a light source (110), an interferometer (120), a sample arm (130), a reference arm (140), and a light detector (150).

[0053] The light source unit (110) is a component that generates light and may include a light source (111), a long pass filter (LPF) (112), a short pass filter (SPF) (113), and a collimator (114).

[0054] A light source (111) generates source light. As a source light, the light source (111) can generate a supercontinuum laser having a broadband wavelength.

[0055] A long pass filter (LPF) (112) is an optical filter that filters light with a wavelength longer than the target wavelength band in order to filter the target wavelength band.

[0056] A short pass filter (SPF) (113) is an optical filter that filters light with a wavelength shorter than the target wavelength band in order to filter the target wavelength band.

[0057] According to one embodiment, source light provided by a light source (111) may be filtered into source light in a spectral range between 650 nm and 950 nm by passing through a long pass filter (LPF) (112) and a short pass filter (SPF) (113).

[0058] The first collimator (114) is a lens that collects light of a broadband wavelength with high light collection efficiency. That is, the first collimator (114) collects the source light of a broadband wavelength that has passed through the long pass filter (LPF) (112) and the short pass filter (SPF) (113) so as to cause the source light to be incident on the interferometer (120).

[0059] The interferometer (120) is a component that separates light and generates interference light, and may include an optical fiber coupler (121).

[0060] An optical fiber coupler (hereinafter collectively referred to as an “optical coupler”) (121) divides the source light incident from the first collimator (114) into measurement light and reference light, outputs the measurement light to the sample arm (130), and outputs the reference light to the reference arm (140). In addition, the optical coupler (121) generates combined light by combining the measurement light and reference light reflected from the sample arm (130) and the reference arm (140), respectively, and outputs the combined light to the light detection unit (150).

[0061] At this time, the measurement light and the reference light cause interference with each other within the combined light, and the combined light including the interference light is transmitted to the light detection unit (150).

[0062] A 5:5 fiber coupler can be used as the optical coupler (121). That is, the source light can be separated at a split ratio of 50:50 and provided to the sample arm (130) and the reference arm (140).

[0063] The sample arm (130) forms a path that provides measurement light incident from the optical coupler (121) to the sample and outputs measurement light reflected from the sample to the optical coupler (121). The sample arm (130) may include a second collimator (131), a dispersive lens pair (132), a scanning mirror (133), and an objective lens (134).

[0064] The second collimator (131) is a lens that is placed on the path of the measurement light and converts the measurement light incident from the optical coupler (121) into parallel light.

[0065] The dispersion lens pair (132) is composed of two lenses having high dispersive properties. The dispersion lens pair (132) is arranged on the path of the measurement light so that the measurement light incident from the second collimator (131) has multiple different divergence angles depending on the wavelength.

[0066] The scanning mirror (133) is placed on the path of the measurement light, reflects the measurement light dispersed from the dispersion lens pair (132) and emits it onto the sample, and changes the angle of the measurement light reflected from the sample and then emits it again through scanning.

[0067] At this time, the scanning mirror (133) can reflect the measurement light emitted from the dispersion lens pair (132) in various directions and emit it to the sample.

[0068] The objective lens (134) is a lens that focuses the measurement light onto the sample. In other words, the objective lens (134) positions a spot of the measurement light on the sample.

[0069] Normally, in the absence of a dispersion lens pair (132), light of all wavelengths is focused to a single point on the sample. In contrast, in the present invention, measurement light having various divergence angles, i.e., measurement light of various wavelengths, is focused to different points in the axial direction of the sample through the dispersion lens pair (132). Therefore, the measurement light is focused to various depths, which causes chromatic aberration. This chromatic aberration serves to expand the depth of focus (DOF), thereby enabling high-resolution images of the sample to be obtained over a wide depth range.

[0070] The reference arm (140) forms a path for the reference light incident from the optical coupler (121). The reference arm (140) may include a polarization controller (141), a third collimator (142), a dispersion compensator (143), and a mirror (144).

[0071] The polarization controller (141) serves to adjust the polarization degree between the sample arm (130) and the reference arm (140).

[0072] The third collimator (142) is a lens that converts the reference light incident from the optical fiber coupler (121) into parallel light.

[0073] The dispersion compensator (143) compensates for the dispersion difference between the measurement light and the reference light caused by the dispersion lens pair (132). To compensate for this dispersion difference, the dispersion compensator (143) is made of a circular glass plate made of the same material as the dispersion lens pair (132) and having a thickness equal to the sum of the thicknesses of the dispersion lens pair (132).

[0074] The mirror (144) reflects the reference light that has passed through the third collimator (142) and dispersion compensator (143).

[0075] The optical detection unit (150) generates an optical coherence tomography image from the combined light incident from the optical fiber coupler (121). The optical detection unit (150) may include a spectrometer (151) and an analysis device (152).

[0076] The spectrometer (151) detects an intensity signal according to the wavelength of the combined light incident from the optical fiber coupler (121) and outputs the detection signal to the analysis device (152).

[0077] The analysis device (152) interprets the detection signal received from the spectrometer (151) to generate an optical coherence tomography image of the sample.

[0078] The optical coherence tomography operation by the above-described configuration is described as follows.

[0079] The source light of a broadband wavelength emitted from the light source (111) is filtered into source light having a target wavelength band while passing through a long pass filter (112) and a short pass filter (113). The source light of the target wavelength band is collected through a first collimator (114) and provided to an optical coupler (121).

[0080] The optical coupler (121) splits the incident source light into measurement light and reference light. The interferometer (120) provides the measurement light to the sample arm (130) and the reference light to the reference arm (140).

[0081] The measurement light provided to the sample arm (130) is directed to the sample along the sample optical path of 'second collimator (131) → dispersion lens pair (132) → scanning mirror (133) → objective lens (134)'. The measurement light reflected from the sample is provided to the optical coupler (121) along the reverse path of the sample optical path, that is, along the path of 'objective lens (134) → scanning mirror (133) → dispersion lens pair (132) → second collimator (131)'.

[0082] The reference light provided to the reference arm (140) is provided to the mirror (144) along the reference light path of 'polarization controller (141) → third collimator (142) → dispersion compensator (143)'. The reference light reflected from the mirror (144) is provided to the optical coupler (121) along the reverse path of the reference light path, that is, the path of 'dispersion compensator (143) → third collimator (142) → polarization controller (141)'.

[0083] The reflected measurement light and the reflected reference light are combined in an optical coupler (121) and provided to a spectrometer (151). At this time, the combined reflected measurement light and the reflected reference light cause interference with each other. This interference signal is provided to an analysis device (152) in the form of an intensity signal according to wavelength by the spectrometer (151).

[0084] The analysis device (152) performs an operation to convert the interference signal detected by the spectrometer (151) into a tomographic image, and this operation corresponds to the ChG algorithm described below. That is, the analysis device (152) interprets the interference signal in the form of an intensity signal according to wavelength according to the ChG algorithm to generate a tomographic image.

[0085] According to one example, the source light of a supercontinuum laser passes through a long pass filter (112) and a short pass filter (113) and is incident on an optical coupler (121) as broadband light having a spectral range between 650 nm and 950 nm. The source light is split into measurement light and reference light at a split ratio of 50:50 by the optical coupler (121), and the measurement light is provided to a sample arm (130) and the reference light is provided to a reference arm (140).

[0086] In the sample arm (130), the measurement light can pass through a dispersing lens pair (132) consisting of a pair of high-dispersion aspherical lenses, pass through a scanning mirror (133), and reach an objective lens (134) with a focal length of 16.6 mm. This configuration can achieve an effective numerical aperture (NA) of 0.08 and a chromatic aberration of 487 μm with an input light diameter of 2.7 mm.

[0087] A custom-designed dispersion compensator (143) is installed in the reference arm (140). The dispersion compensator (143) is composed of a circular glass plate made of the same material as the dispersion lens pair (132) used in the sample arm (130). The dispersion compensator (143) may be twice as thick as the dispersion lens pair (132) to ensure consistent dispersion characteristics between the sample arm (130) and the reference arm (140).

[0088] The interference signals reflected from the sample arm (130) and the reference arm (140) and provided to the spectrometer (151) through the optical coupler (121) may have a bandwidth of 300 nm from 650 nm to 950 nm. The spectrometer (151) has 4096 pixels, providing a spectral resolution of 0.073 nm per pixel, which allows for an axial imaging range of up to 2.11 mm in air.

[0089] The optical coherence tomography (OCT) system (100) uses a source light of a wide wavelength band and intentionally induces chromatic aberration using a dispersive lens pair (132) and a dispersion compensator (143) in the sample arm (130). Therefore, the optical coherence tomography (OCT) system (100) can expand the depth of field (DOF) by allowing measurement lights of different wavelengths to focus on different points on the sample in the axial direction.

[0090] However, similar to when using a conventional Bessel beam, the system signal-to-noise ratio degradation and artifacts occur. Therefore, the analysis device (152) uses a chromatic gating algorithm (ChG algorithm) to further alleviate the signal-to-noise ratio degradation and remove artifacts.

[0091] The ChG algorithm is a proposed algorithm of the present disclosure that utilizes changes that occur when chromatic aberration is induced in an optical coherence tomography (OCT) system (100).

[0092] When using a typical optical system without chromatic aberration, the interference signal formed in the photodetector has different frequencies in the wavenumber domain (k-domain) depending on the axial depth of the sample, but the envelope of the interference signal has no correlation with the depth of the sample.

[0093] However, when chromatic aberration is present, the envelope of the interference signal also shifts in the wavenumber domain (k-domain) depending on the depth of the sample. That is, in the case of normal dispersion, the intensity of the interference signal generated in the sample close to the optical system appears large in the long wavenumber (k) domain (or short wavelength domain), and as the sample moves away in the axial direction, the intensity of the interference signal in the short wavenumber (k) domain (or long wavelength domain) increases.

[0094] The Fourier transform is a common method for extracting tomographic images from interference signals in optical coherence tomography. This method extracts frequency components contained in the interference signal across all wavenumbers (k). Therefore, this method is suitable for general optical coherence tomography systems that have interference signals of constant intensity across the wavenumber (k) range, regardless of sample depth.

[0095] However, when there is chromatic aberration, interference signals are formed only in a specific part of the wavenumber (k) region depending on the depth of the sample, and the general Fourier transform uses not only the valid signals of the interference signals but also the invalid signals when generating an optical coherence tomography image, which causes unnecessary artifacts and noise.

[0096] Therefore, the ChG algorithm adds a Gaussian window to the portion corresponding to the valid interference signal created by the depth sample corresponding to each frequency during the Fourier transform process. This allows only the valid signals of the interference signal to be used when generating an optical coherence tomography image, and invalid signals are discarded, thereby removing unnecessary noise and artifacts. This increases the signal-to-noise ratio (SNR) and enhances the system sensitivity.

[0097] Figure 2 shows the Fourier transform result of the interference signal.

[0098] The fringe data in Figure 2 is an interference signal provided by a spectrometer, representing the interference signal according to the wavenumber (k). A-Line data, representing the depth profile of the sample, is derived from the fringe data through Fourier transform. This process can be expressed in mathematical equation 1 below.

[0099] [Mathematical Formula 1]

[0100]

[0101] i[z m ] represents the final result of the Fourier transform, i.e., the A-line. The A-line corresponds to the depth profile at a point. z m represents the depth of the sample.

[0102] I[k n ] is a fringe, which represents an interference signal that has undergone dispersion compensation and k-linearization.

[0103] is z in the wave domain (k-domain) m It represents a complex function with different frequencies depending on .

[0104] In the absence of chromatic aberration, interference signals are evenly distributed across the fringes, regardless of the depth of the sample.

[0105] On the other hand, when chromatic aberration is induced, the interference signal from a sample at a certain depth is concentrated in a region of a certain wavelength (or a certain wavenumber (k)).

[0106] Referring to FIG. 3, the position of the focus according to the wavelength generated in the sample arm (130) is shown, the horizontal axis is the wavelength (Wavelength, nm), the vertical axis is the focal shift (Focal shift, ㎛), and the total induced chromatic aberration is 487 ㎛.

[0107] For example, if the 700 nm measurement light is focused at the -200 μm point, the measurement light reflected at the -200 μm point of the sample has a large intensity near 700 nm. Therefore, if a general Fourier transform such as that in Fig. 2 is performed, an invalid signal is used at a specific depth, which causes artifacts and noise. To prevent this problem, a Gaussian window (W[k n ]) is used.

[0108] In order to generate a signal at the -200 μm point in the A-line, it is appropriate to use an interference signal near 700 nm. To this end, when generating a signal at the -200 μm point, a Gaussian window (W[k n ]) is used.

[0109] Similarly, the measurement light reflected at the 0 μm point of the sample has a large intensity near 800 nm. Therefore, when generating a signal at the 0 μm point, a Gaussian window (W[k n ]) is used.

[0110] That is, the Gaussian window (W[k n ]) has different center positions depending on the sample depth.

[0111] Referring to FIG. 4, the analysis device (152) applies the ChG algorithm to the interference signal provided from the spectrometer (151), i.e., the fringe, to derive the A-line. This process can be expressed in mathematical expression 2 below.

[0112] [Equation 2]

[0113]

[0114] i[z m ] represents the A-line, the final result of the ChG algorithm.

[0115] I[k n ], is the same as defined in mathematical expression 1.

[0116] W[k n ] represents a Gaussian window.

[0117] k c represents the center position of the Gaussian window, and z m is a function of . The function λ(z m ) is z m It refers to the wavelength of the measuring light that focuses on the depth.

[0118] The analysis device (152) can be used in various embodiments to determine λ(z m ) can be obtained.

[0119] In one embodiment, the analysis device (152) analyzes the beam profile of the measurement light reflected from the sample arm (130) through simulation of optical modeling software. For example, Optics Studio (Zemax) can be used as the optical modeling software. Such optical modeling software can simulate the output optical characteristics of the sample arm and quantify the focal shift of chromatic aberration.

[0120] The analysis device (152) determines at which location of the sample the measurement light of each wavelength is focused through the optical modeling software to obtain λ(z m ) can be obtained.

[0121] In another embodiment, the operator performs a mirror imaging experiment to determine λ(z m ) can be obtained and stored in a DB to be analyzed in the analysis device (152). The operator can obtain z on the sample m By placing mirrors at each position and checking which wavelength of measurement light is detected most strongly, λ(z m ) can be obtained.

[0122] In another embodiment, the operator manually k c (z m ) to create the clearest result by taking random samples while adjusting the k c (z m ) can be selected, converted into a database, and stored in the analysis device (152).

[0123] σ k represents the width or depth of the Gaussian window.

[0124] In one example, the operator manually σ k σ that produces the clearest results through the process of taking random samples while adjusting k can be selected, converted into a database, and stored in the analysis device (152).

[0125] Gaussian window (W[k) explained with reference to mathematical expression 2 n ]) is shown as an example in Fig. 5.

[0126] Referring to Figure 5, the Gaussian window (W[k n ]) center (k c ) are located at different locations depending on the frequency, and these Gaussian windows (W[k n ]) is applied to derive the A-line from the fringe.

[0127] Gaussian window (W[k n ]) center (k c ) position is determined by chromatic aberration, as described in Fig. 3. Therefore, an optical coherence tomography image can be generated based on the valid interference signal actually reflected at that depth through a Gaussian window.

[0128] As described above, the optical coherence tomography (OCT) system (100) utilizes the advantages of axial chromatic aberration while minimizing its potential drawbacks. First, the depth of field (DOF) can be expanded while maintaining high lateral resolution by utilizing the optical system (131, 132, 133, 134) of the sample arm (130) with chromatic aberration and high NA. Second, high axial resolution is ensured by using a broadband light source (110) and a spectrometer (151). Third, the overall system performance can be optimized by effectively reducing noise and mitigating artifacts.

[0129] A pair of dispersion lenses (132) of a sample arm (130) is arranged in front of a scanning mirror (133), which arrangement allows the measurement light of the central wavelength to remain parallel, while diverging and converging light is generated for longer and shorter wavelengths. This measurement light passes through the scanning mirror (133) and a high NA objective lens (134), thereby creating a narrow and elongated focus by focusing it at various axial positions, thereby setting an effective NA of 0.08 and a chromatic aberration of 487 μm.

[0130] To match the dispersion characteristics between the sample arm (130) and the reference arm (140), a dispersion compensator (143) made of the same material and thickness as the dispersion lens pair (132) of the sample arm (130) is placed in the reference arm (140).

[0131] Additionally, high axial resolution and wide chromatic aberration are achieved by using a light source with a wide bandwidth in the range of 650 to 950 nm.

[0132] The analysis device (152) uses the ChG algorithm that utilizes the correlation between depth and wavenumber (k) as described in FIG. 4.

[0133] In conventional OCT systems without chromatic aberration, the frequency of the interference signal varies with sample depth, but its envelope is not correlated with depth.

[0134] Chromatic aberration, on the other hand, causes this envelope to shift in the wavenumber domain (k-domain) with sample depth. Assuming that chromatic aberration is caused by normal dispersion, the signal of a sample closer to the objective lens (134) exhibits higher intensity in the higher wavenumber (k) (or shorter wavelength) region, and this trend shifts to the lower wavenumber (k) (or longer wavelength) region as the sample distance increases. Outside this region, only noise exists without any significant signal.

[0135] As illustrated in Figure 2, the Fourier transform is commonly used for image reconstruction in OCT, extracting signals across all wavenumber (k) regions regardless of sample depth. While this Fourier transform is suitable for standard OCT systems, in OCT systems with chromatic aberration, the Fourier transform utilizes both valid and invalid signal regions, resulting in unnecessary artifacts and noise.

[0136] As described in FIGS. 3 to 5, the ChG algorithm of the present disclosure can reduce excessive noise by adding a Gaussian window that is variably centered and moves according to the frequency of the harmonic wave, thereby using only the effective wavenumber (k) region that is in focus for image extraction.

[0137] This wavenumber (k)-based filtering mechanism, namely the ChG algorithm, can mitigate three types of noise. First, the ChG algorithm can effectively suppress system-specific noise while minimizing signal loss. Second, the ChG algorithm can mitigate artifacts caused by out-of-focus light, similar to side-lobe artifacts observed in Bessel beam OCT. Third, the ChG algorithm can remove multi-scattered light that interferes with deep-tissue imaging. This noise mitigation can increase the SNR and enhance the visualization of deeper tissue structures.

[0138]

[0139] FIG. 6 is a flowchart illustrating the operation of an optical coherence tomography (OCT) system according to one embodiment, and describes the operation based on the components described in FIGS. 1 to 5.

[0140] Referring to FIG. 6, an optical coherence tomography (OCT) system (100) provides source light (S101) through a light source (111) which is a supercontinuum laser having a broadband wavelength.

[0141] The optical coherence tomography (OCT) system splits the source light into measurement light and reference light through an optical coupler (120) and provides them to a sample arm (130) and a reference arm (140), respectively (S102).

[0142] An optical coherence tomography (OCT) system provides measurement light, in which chromatic aberration is induced, onto a sample to be measured by a dispersive lens pair (132) through a sample arm (130) (S103). Here, the dispersive lens pair (132) induces chromatic aberration in the measurement light by having multiple different divergence angles depending on the wavelength.

[0143] The optical coherence tomography (OCT) system combines the measurement light reflected from the sample through the optical coupler (120) and the reference light reflected from the reference arm (140) to generate combined light containing an interference signal between the measurement light and the reference light (S104).

[0144] The optical coherence tomography (OCT) system sets a Gaussian window having a different center position depending on the depth of the sample through a light detection unit (150), and applies the set Gaussian window to generate an optical coherence tomography image based on a valid interference signal for each depth of the sample (S105).

[0145] A Gaussian window may have a center position of the Gaussian window determined by chromatic aberration defined as the relationship between wavelength and focal shift formed on the sample.

[0146] In S105, the optical coherence tomography (OCT) system can generate an optical coherence tomography image based on an interference signal in which the dispersion difference between the measurement light and the reference light caused by the dispersion lens pair is compensated for through a dispersion compensator of the reference arm.

[0147] At this time, the optical coherence tomography (OCT) system performs a process of multiplying the interference signal by a complex exponential function having different frequencies and a Gaussian window and calculating the sum to obtain a complex coefficient for a specific frequency of the interference signal, and by repeating this process for all frequency ranges, the entire frequency spectrum of the interference signal is generated, and the magnitude of this entire frequency spectrum can be calculated to generate an optical coherence tomography image.

[0148]

[0149] Experimental example

[0150] In the experiment, the imaging results obtained through the OCT system using chromatic aberration of the present disclosure and the ChG algorithm were compared with the imaging results obtained through the conventional high-resolution OCT system by photographing a 1㎛-sized bead, and the axial and transverse profiles and maximum intensity analysis of the bead images were used to analyze the axial / transverse resolution and signal-to-noise ratio of the system by depth. In addition, the effectiveness of the OCT system using chromatic aberration of the present disclosure and the ChG algorithm was compared with the conventional high-resolution OCT system through tissue sample imaging, and the effectiveness between the ChG algorithm and the conventional Fourier transform algorithm in the OCT system using chromatic aberration was compared.

[0151] The optical coherence tomography (OCT) system (100) of the present disclosure can achieve an expanded depth of field (DOF) compared to conventional high-resolution OCTs. This effect is confirmed through FIGS. 7a, 7b, 7c, 8a, 8b, 8c, 9, and 10.

[0152] The ChG algorithm of this disclosure is effective in increasing SNR and removing side-lobe artifacts by removing noise inherent in the system and noise caused by multiple scattering. These effects are confirmed through Figures 11, 12, and 13.

[0153] It was confirmed that the optical coherence tomography (OCT) system (100) of the present disclosure achieves isotropic high resolution of 2 to 3 μm and exhibits up to 8 times improvement in depth of field (DOF).

[0154] Figures 7a, 7b, 7c, and 7d show photographs of the results of imaging small beads in water using the OCT system and ChG algorithm using chromatic aberration of the present disclosure, and Figures 8a, 8b, 8c, and 8d show photographs of the results of imaging small beads in water using the existing Fourier transform-based high-resolution OCT technology.

[0155] To ensure an unbiased comparison, the experimental setup was designed to be easily switchable between the configuration of the OCT system (100) of the present disclosure and a conventional high-resolution OCT system. This switchable design was achieved by selectively including or excluding a pair of dispersion lenses (132) and a dispersion compensator (143). This design allows for a direct and fair comparison of performance metrics between the two systems under identical experimental conditions.

[0156] Figures 7a, 7b, 7c, 7d and Figures 8a, 8b, 8c, 8d show imaging results of polystyrene microbeads with a diameter of about 0.99 μm suspended in water.

[0157] System performance was numerically analyzed based on SNR, lateral resolution, and axial resolution along the imaging depth.

[0158] Figures 7a and 8a show cross-sectional B-scan images of the beads.

[0159] Figures 7b and 8b show the relationship between depth (μm) and SNR (dB), Figures 7c and 8c show the relationship between depth (μm) and lateral resolution, and Figures 7d and 8d show the relationship between depth (μm) and axial resolution.

[0160] Consistent with the observation results of FIGS. 7a and 8a, according to FIGS. 7b and 8b, the OCT of the present disclosure showed a depth of field (DOF) of 450 μm, which is significantly expanded compared to the 50.0 μm result of the conventional OCT.

[0161] According to FIGS. 7c, 7d and 8c, 8d, the OCT of the present disclosure maintained high lateral resolution throughout the enhanced depth of field (DOF) compared to conventional OCT, but the axial resolution was slightly reduced due to a reduced effective bandwidth of the interference signal.

[0162] Nonetheless, by using a light source with sufficient spectral bandwidth, the reduced axial resolution can be maintained at a level similar to the transverse resolution, so that the OCT of the present disclosure can achieve an isotropic resolution of 2 to 3 μm.

[0163] Additionally, the peak SNR of the OCT of the present disclosure is reduced compared to conventional high-resolution OCT, but maintains a high SNR over a significantly extended depth range.

[0164] The results of a comparison of key performance indicators of the OCT of the present disclosure and the existing OCT described above are summarized in Table 1. The performance of each system was evaluated at various imaging depths in terms of SNR, lateral resolution, and axial resolution based on numerical analysis, and Table 1 presents the results of the performance comparison.

[0165] High-resolution OCTChromatic OCTDOF [㎛]50.0450Peak SNR [dB]49.741.8Mean lateral resolution [㎛]2.923.18Mean axial resolution [㎛]1.572.64

[0166] The above experiments demonstrate that the chromatic aberration-based OCT system and ChG algorithm of the present disclosure are more effective than conventional Fourier transform-based high-resolution OCT. High-resolution imaging experiments using biological tissue further demonstrate the advantages of chromatic aberration-based OCT, demonstrating high SNR and minimal artifacts over an extended imaging range.

[0167] To demonstrate the practicality of the OCT system (100) of the present disclosure, an in vitro imaging experiment was performed on a fresh tissue sample, i.e., a lemon sample, using the OCT system (100). The imaging results obtained using the conventional high-resolution OCT and the OCT of the present disclosure for the lemon sample are compared and shown in FIGS. 9 and 10 .

[0168] FIG. 9 and FIG. 10 are cross-sectional images of a lemon sample according to the experiment of the present disclosure, wherein FIG. 9 is an imaging result obtained using a conventional high-resolution OCT, and FIG. 10 is an imaging result obtained using the OCT of the present disclosure.

[0169] Looking at a2 in Fig. 9, in the conventional high-resolution OCT image, the cell wall is clearly defined near the focal plane.

[0170] In contrast, as shown in Figure 9 (a1) and (a3), in conventional high-resolution OCT images, the cell wall appears blurry and broad in areas 250 μm away from the focus. This is highlighted by arrows. This means that while it is clear near the focus, it becomes blurred further away from the focus.

[0171] In contrast, b1, b2, and b3 in Fig. 10 show that the chromatic aberration OCT images consistently display cellular structures clearly throughout, including areas far from the focus plane, as highlighted by arrows. This indicates that high SNR and resolution are maintained not only near the focus but also far from the focus.

[0172] These experiments confirm that the OCT of the present disclosure can acquire images with high SNR and high resolution (lateral resolution & axial resolution) over a wider range of depth of field (DOF).

[0173]

[0174] To demonstrate the practicality of the OCT system (100) of the present disclosure, a pig esophagus was photographed using the OCT system (100) of the present disclosure. The data thus photographed were converted into images using the ChG algorithm of the present disclosure and a conventional Fourier transform algorithm, and are shown in FIGS. 11 and 12 .

[0175] FIG. 11 and FIG. 12 show cross-sectional images of freshly excised porcine esophageal tissue taken using an OCT system utilizing chromatic aberration of the present disclosure, wherein FIG. 11 is an imaging result obtained using the ChG algorithm of the present disclosure, and FIG. 12 is an imaging result obtained using a conventional Fourier transform algorithm.

[0176] Here, ChG means the ChG algorithm of the present disclosure, and FT means the conventional Fourier transform algorithm.

[0177] According to Figure 11, glycogen-rich squamous epithelial cells can be clearly identified. In contrast, Figure 12 shows more noise and lower clarity of cell structures compared to Figure 11.

[0178] That is, it was confirmed that Fig. 11 can image squamous epithelial cells of the tissue more clearly than Fig. 12. In particular, Fig. 11 shows significantly improved image quality in visualizing mature squamous epithelial cells rich in glycogen in the cytoplasm. This improvement is known to effectively suppress multiple scattered light and noise within a highly scattering medium.

[0179] Figure 13 is the result of analyzing the contrast through the difference in internal and external signal intensity of 10 squamous epithelial cells of Figures 11 and 12.

[0180] According to Figure 13, the algorithm of the present disclosure has an average contrast of 8.72 dB (standard deviation 2.19 dB), while the existing FT algorithm has an average contrast of 6.24 dB (standard deviation 2.62 dB). In other words, the algorithm of the present disclosure shows an average contrast increase of 2.48 dB (39.7%) compared to the existing FT algorithm. This is a particularly significant figure considering the contrast of deep tissue images, which is typically limited to a few decibels.

[0181] These results show that the ChG algorithm of the present disclosure effectively removes noise caused by multiple scattering.

[0182]

[0183] The embodiments of the present invention described above are not implemented only through devices and methods, but may also be implemented through a program that realizes a function corresponding to the configuration of the embodiments of the present invention or a recording medium on which the program is recorded.

[0184] Although the embodiments of the present invention have been described in detail above, the scope of the present invention is not limited thereto, and various modifications and improvements made by those skilled in the art using the basic concept of the present invention defined in the following claims also fall within the scope of the present invention.

Claims

1. As an optical coherence tomography (OCT) system, A light source that generates source light, A sample arm that provides measurement light to a sample and provides measurement light reflected from the sample to the optical coupler; A reference arm of an optical structure that reflects the reference light, An optical coupler that divides the source light into the measurement light and the reference light and provides them to the sample arm and the reference arm, respectively, and combines the reflected measurement light and the reflected reference light provided from the sample arm and the reference arm, respectively, to output combined light containing an interference signal between the measurement light and the reference light, and It includes an optical detection unit that analyzes the interference signal of the combined light provided from the optical coupler to generate an optical coherence tomography image, The above sample cancer is, A pair of dispersion lenses that induce chromatic aberration of the measurement light by allowing the measurement light to have multiple different divergence angles depending on the wavelength. An optical coherence tomography system comprising:

2. In paragraph 1, The above light detection unit, An optical coherence tomography system that generates an optical coherence tomography image based on a valid interference signal for each depth of the sample by applying a Gaussian window having a different center position depending on the depth of the sample.

3. In paragraph 2, The center position of the above Gaussian window, An optical coherence tomography system determined by the above chromatic aberration.

4. In paragraph 3, The above chromatic aberration is, It is expressed as the relationship between the wavelength and the focal shift formed on the sample. The above Gaussian window is, An optical coherence tomography system, wherein the wavelength of the point at which the focus is formed is determined as the central position so that the interference signal having the wavelength of the point at which the focus is formed is used as a valid interference signal.

5. In paragraph 4, The above light detection unit, An optical coherence tomography system that generates an optical coherence tomography image by performing an operation using an interference signal obtained from the optical coupler, a complex function defined to have various frequencies depending on the depth of the sample in the wave domain (k-domain), and the Gaussian window.

6. In paragraph 1, The above reference arm is, A dispersion compensator that compensates for the dispersion difference between the measurement light and the reference light caused by the above-mentioned dispersion lens pair. An optical coherence tomography system comprising:

7. In paragraph 6, The above reference arm is, A polarization controller positioned at the forefront on the path of the reference light and adjusting the polarization degree between the sample arm and the reference arm; A collimator arranged behind the polarization controller and in front of the dispersion compensator to convert the reference light into parallel light, and A mirror placed behind the dispersion compensator and reflecting the reference light An optical coherence tomography system further comprising:

8. In paragraph 6, The above sample cancer is, A collimator, which is positioned at the front end on the path of the measurement light and in front of the dispersing lens pair, and converts the measurement light provided from the optical coupler into parallel light; A scanning mirror that reflects the measurement light dispersed from the dispersion lens pair in various directions and provides it to the sample, and provides the measurement light reflected from the sample to the dispersion lens pair through scanning, and An objective lens that focuses the above measurement light onto the sample An optical coherence tomography system further comprising:

9. In paragraph 1, The above light source unit, A light source that generates source light in the form of a supercontinuum laser with a broadband wavelength, A long pass filter (LPF) and a short pass filter (SPF) for filtering the target wavelength band from the above source light, and A collimator that collects source light of the target wavelength band that has passed through the long pass filter and the short pass filter and provides it to the optical coupler. An optical coherence tomography system comprising:

10. An operating method of an optical coherence tomography (OCT) system, A step of dividing the source light into measurement light and reference light through an optical coupler and providing them to the sample arm and reference arm, respectively; A step of causing the measurement light, in which the chromatic aberration is induced, to be incident on the sample to be measured by a pair of dispersing lenses that induce chromatic aberration of the measurement light by having a plurality of different divergence angles according to the wavelength through the sample arm, A step of combining the measurement light reflected from the sample through the optical coupler and the reflected reference light provided from the reference arm to generate combined light including an interference signal between the measurement light and the reference light, and A step of generating an optical coherence tomography image by analyzing the interference signal through a light detection unit. A method comprising:

11. In paragraph 10, The step of generating the above optical coherence tomography image is: A step of setting a Gaussian window having different center positions depending on the depth of the above sample, and A step of generating the optical coherence tomography image based on a valid interference signal for each depth of the sample by applying the Gaussian window. A method comprising:

12. In paragraph 11, The above Gaussian window is, A method having a center position of the Gaussian window determined by the chromatic aberration defined as the relationship between the wavelength and the focal shift formed on the sample.

13. In paragraph 12, The step of generating the above optical coherence tomography image is: A method for generating an optical coherence tomography image based on an interference signal in which the dispersion difference between the measurement light and the reference light caused by the dispersion lens pair is compensated for through a dispersion compensator of the reference arm.

14. In paragraph 13, The step of generating the above optical coherence tomography image is: A method for calculating a complex coefficient for a specific frequency of the interference signal by multiplying the interference signal by a complex exponential function having different frequencies and the Gaussian window and calculating the sum, repeating the process for all frequency ranges to generate the entire frequency spectrum of the interference signal, and generating the optical coherence tomography image by calculating the magnitude of the entire frequency spectrum.

15. In paragraph 10, Prior to the above-mentioned providing step, A step of generating the source light through a light source that is a supercontinuum laser with a broadband wavelength. A method further comprising:

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