Apparatus and method for improving the signal-to-noise ratio of imaging performance in optical coherence tomography.

Abstract

An optical coherence tomography system is provided that can operate in two or more optical bandwidth configurations to trade off between high resolution imaging and high signal-to-noise ratio and deep imaging depth. The system and associated methods can perform both high resolution, shallow penetration depth optical coherence tomography imaging and low resolution, deep penetration depth optical coherence tomography imaging using a single light source. Methods and apparatus are described that allow a single system to dynamically switch between multiple modes or operate in a hybrid mode that can achieve a balance between resolution and SNR / penetration depth.

Description

[Technical Field]

[0001] (Cross-reference of related applications) This application claims priority to U.S. Provisional Patent Application No. 63 / 213,554, filed on 22 June 2021, which is incorporated herein by reference.

[0002] (Statement regarding research funded by the federal government) This invention was made with government funding under grant number FA9550-20-10063, awarded by the National Institutes of Health. The government has certain rights in this invention. [Background technology]

[0003] Optical coherence tomography (OCT) systems construct an image of a sample by receiving and analyzing backscattered light from that sample. The backscattered light is measured at numerous wavelengths, and the axial resolution of the measurement depends on the specific set of wavelengths included in the measurement set and the optical power supplied to the sample at each of these wavelengths. Generally, if the analysis includes a wider set of wavelengths and / or if the optical power is widely distributed across the wavelengths being measured (a wider optical bandwidth is obtained), the axial resolution improves (becomes smaller). If the set of wavelengths included in the analysis is more limited, or if the optical power is concentrated over a narrow range of wavelengths (a narrower or less broad optical bandwidth is obtained), the axial resolution decreases (becomes larger). Since axial resolution is often a critical factor in OCT imaging, many systems are configured to provide broadband optical illumination. Generally, based on imaging performance, there is no reason not to maximize the optical bandwidth. [Overview of the Initiative]

[0004] What is needed is a technology that allows OCT systems to dynamically change their imaging performance in terms of axial resolution and signal-to-noise ratio.

[0005] This disclosure utilizes a second conclusion regarding optical bandwidth for image performance, which has not been previously described: that when imaging diffusely scattered samples such as biological tissue using optical coherence tomography (OCT), optical bandwidth also affects the signal-to-noise ratio (SNR) of the measurement result. This means that two OCT systems operating at the same A-line rate and irradiating the sample with the same average power, but providing light of two different optical bandwidths, can produce images with different signal-to-noise ratio characteristics. Systems using a wider optical bandwidth will have a lower (worse) signal-to-noise ratio. Therefore, the optical bandwidth of an OCT system affects both axial resolution and the signal-to-noise ratio of the measurement result. This creates an opportunity to tune system performance to aim for better axial resolution or better SNR based on control of the optical bandwidth of the imaging light. For example, there are many scenarios in which it is chosen to reduce axial resolution to improve SNR, such as when trying to increase the imaging depth (depth of penetration).

[0006] This effect can be used when designing OCT systems that operate in a specific optical bandwidth chosen to achieve a compromise between competing goals of axial resolution and signal-to-noise ratio (SNR). However, in OCT systems, there are cases where axial resolution is more important, and other cases where SNR is more important. Configuring an OCT system to provide fixed levels of axial resolution and signal-to-noise performance has drawbacks.

[0007] This invention relates to the field of optical coherence tomography. Specifically, the invention relates to a system in which the imaging characteristics of the signal-to-noise ratio and axial resolution of the measurement can be configured through the manipulation of the optical spectrum of the imaging light and / or the configuration of a processing algorithm applied to the acquired data. A light source specially configured for this purpose is used.

[0008] Accordingly, this disclosure provides embodiments of optical coherence tomography (OCT) systems that can operate in two or more optical bandwidth configurations, enabling deeper imaging depths, with the aim of resolving the trade-off between high-resolution imaging and high signal-to-noise ratio (SNR) imaging. This system and related methods enable both high-resolution, shallow-penetration-depth (SNR) and low-resolution, deep-penetration-depth (SNR) OCT imaging to be performed using a single light source. Novel methods and apparatuses are described in which a single system can dynamically switch between modes or operate in hybrid modes to achieve a balance between resolution and SNR / penetration depth. This is considered particularly relevant to the posterior segment OCT market, where high-resolution retinal imaging is required, but deep penetration is also necessary to view optic nerve head features. This could potentially be integrated into commercially available retinal OCT imaging systems. This is also highly relevant to intravascular OCT systems, where imaging depth is a critical factor. In both cases, there are constraints on imaging time and imaging power, so there is no viable way to improve SNR, making the approach described here particularly important. These configurations can be implemented with minimal additional system complexity and can improve performance at a small, perhaps negligible, cost.

[0009] Accordingly, this specification discloses a series of preferred embodiments describing the design and operation of an OCT system capable of dynamically changing axial resolution and SNR by operating in at least two illumination states. These states are distinguished by different optical bandwidths, which affect both axial resolution and SNR. Operating the system in a first state that provides a larger optical bandwidth can produce higher resolution images with a lower signal-to-noise ratio. Operating the system in a second state with a narrower optical bandwidth than the first state results in relatively high signal-to-noise performance at the expense of axial resolution. Of course, the improved signal-to-noise performance can be utilized for a variety of reasons, such as more accurate measurements or imaging to greater depths of the sample. In these preferred embodiments, one or more light sources are used as inputs to the optical coherence tomography system, and the one or more light sources include light sources based on wavelength-swept configurations, continuous broadband configurations, wavelength-stepped configurations, and discretely structured broadband configurations.

[0010] In one configuration, the Disclosure provides an optical coherence tomography (OCT) system comprising: a light source configured to provide first illumination and second illumination to a diffuse scattering sample, wherein the first illumination provides light of a first bandwidth and the second illumination provides light of a second bandwidth narrower than the first bandwidth; an optical coherence tomography (OCT) optical apparatus having a sample optical path and a reference optical path, each optically coupled to the light source, for generating interference data from the sample; a detector optically coupled to the OCT optical apparatus and configured to collect the interference data from the sample; and a controller configured to switch the illumination provided to the OCT optical apparatus between the first illumination and the second illumination, wherein the interference data includes first interference data obtained using the first illumination and second interference data obtained using the second illumination.

[0011] In one configuration, the Disclosure provides an optical coherence tomography (OCT) system comprising: a light source configured to provide first illumination and second illumination to a diffuse scattering sample, wherein the first illumination provides light of a first bandwidth and the second illumination provides light of a second bandwidth narrower than the first bandwidth; an optical coherence tomography (OCT) optical apparatus having a sample optical path and a reference optical path, each optically coupled to the light source, for generating interference data from the sample; a detector optically coupled to the OCT optical apparatus and configured to collect the interference data from the sample as a function of wavelength; and a controller configured to switch the illumination provided to the OCT optical apparatus between the first illumination and the second illumination, wherein the interference data includes first interference data obtained using the first illumination and second interference data obtained using the second illumination, and the ratio of the first bandwidth to the second bandwidth is at least 2.

[0012] In another configuration, the Disclosure provides an optical coherence tomography (OCT) system comprising a light source configured to provide a first illumination and a second illumination to a diffuse scattering sample, wherein the first illumination provides light of a first bandwidth and has a first wavelength-vs-time profile and a first power-vs-time profile, and the second illumination provides light of a second bandwidth narrower than the first bandwidth and has a second wavelength-vs-time profile and a second power-vs-time profile, and an optical coherence tomography (OCT) system comprising a sample optical path and a reference optical path, each optically coupled to the light source, for generating interference data from the sample. T) An optical device comprising: an optical device; a detector optically coupled to the OCT optical device and configured to collect interference data from the sample; and a controller configured to switch the illumination provided to the OCT optical device between the first illumination and the second illumination by controlling at least one of the first wavelength-to-time profile or the first power-to-time profile of the first illumination and at least one of the second wavelength-to-time profile or the second power-to-time profile of the second illumination, wherein the interference data includes first interference data obtained using the first illumination and second interference data obtained using the second illumination.

[0013] In yet another configuration, the Disclosure provides a method for performing optical coherence tomography (OCT), the method comprising: using a light source and providing a first illumination and a second illumination to a diffusely scattered sample, wherein the first illumination provides light of a first bandwidth and the second illumination provides light of a second bandwidth narrower than the first bandwidth; generating interference data from the sample using an OCT optical apparatus having a sample optical path and a reference optical path, each optically coupled to the light source; and using a controller to switch the illumination provided to the OCT optical apparatus between the first illumination and the second illumination, the interference data comprising first interference data obtained using the first illumination and second interference data obtained using the second illumination.

[0014] In yet another configuration, the Disclosure provides a method for performing optical coherence tomography (OCT), the method comprising: providing a first illumination and a second illumination to a diffusely scattered sample using a light source, wherein the first illumination provides light of a first bandwidth and the second illumination provides light of a second bandwidth narrower than the first bandwidth; generating interference data from the sample using an optical coherence tomography (OCT) optical apparatus comprising a sample optical path and a reference optical path, each optically coupled to the light source; collecting the interference data from the sample as a function of wavelength using a detector comprising a spectrometer and optically coupled to the OCT optical apparatus; and switching the illumination provided to the OCT optical apparatus between the first illumination and the second illumination using a controller, wherein the interference data includes first interference data obtained using the first illumination and second interference data obtained using the second illumination, and the ratio of the first bandwidth to the second bandwidth is at least 2.

[0015] In another configuration, the Disclosure provides a method for performing optical coherence tomography (OCT), the method comprising the steps of: providing a first illumination and a second illumination to a diffuse scattering sample using a light source, wherein the first illumination provides light of a first bandwidth and has a first wavelength-vs-time profile and a first power-vs-time profile; and providing light of a second bandwidth narrower than the first bandwidth using the light source and having a second wavelength-vs-time profile and a second power-vs-time profile; and using an optical coherence tomography (OCT) optical apparatus comprising a sample optical path and a reference optical path, each optically coupled to the light source, to obtain interference data from the sample. The process includes the steps of generating, collecting interference data from the sample using a detector optically coupled to the OCT optical device, and controlling the light source to switch the illumination provided to the OCT optical device between the first illumination and the second illumination by controlling at least one of the first wavelength-to-time profile or the first power-to-time profile of the first illumination and at least one of the second wavelength-to-time profile or the second power-to-time profile of the second illumination, wherein the interference data includes first interference data obtained using the first illumination and second interference data obtained using the second illumination.

[0016] The various purposes, features, and advantages of the disclosed subject matter can be better understood by referring to the following detailed description of the disclosed subject matter, when considered in relation to drawings that identify similar elements using similar reference numbers, as follows: [Brief explanation of the drawing]

[0017] [Figure 1] An example of a spectral domain OCT system according to several embodiments described herein is shown. [Figure 2] An example of a wavelength-swept OCT system according to multiple embodiments described herein is shown. [Figure 3]In OCT, it shows the relationship between the reduction of the optical band and the signal-to-noise ratio for a diffuse scattering sample. [Figure 4] It shows an example of the increase in the penetration depth or the signal-to-noise ratio (SNR) caused by the reduction of the optical bandwidth in OCT. [Figure 5] Based on multiple embodiments in the disclosure of this specification, it shows an example of input OCT light modulated by at least controlling the spectral characteristics of light provided to a sample. [Figure 6] Based on multiple embodiments in the disclosure of this specification, it shows an OCT system comprising at least two broadband light sources. [Figure 7] Based on multiple embodiments in the disclosure of this specification, it shows a time-varying optical frequency light source having two or more configurations. [Figure 8] Based on multiple embodiments in the disclosure of this specification, it shows the wavelength and power output by multiple configurations of a wavelength-swept laser. [Figure 9] Based on multiple embodiments in the disclosure of this specification, it shows the wavelength and power output by multiple configurations of a broadband laser light source. [Figure 10] Based on multiple embodiments in the disclosure of this specification, it shows the wavelength and power output by multiple configurations of a step-type wavelength laser light source. [Figure 11] Based on multiple embodiments in the disclosure of this specification, it shows an example of using two independent light sources. [Figure 12] It shows the sampled spectrum measured by an oscilloscope. The numbers in the left column indicate the bandwidth reduction factor (BRF) controlled by the angular velocity of the polygon mirror, and the shaded area indicates the optical band associated with an acquisition period of 20 microseconds. The trigger signal is shifted to maintain the same central wavelength. [Figure 13]The following shows the measured dependence of SNR on optical bandwidth. Panel (a) shows images of diffuse tissue (chicken breast) with different experimental optical bandwidth reduction factors. Panel (b) shows images of diffuse tissue (chicken breast) with different calculated optical bandwidth reduction factors. Panel (c) shows the SNR as a function of the experimental optical bandwidth reduction factor in the diffuse scattering area indicated by the yellow box in panel (a). Panel (d) shows the SNR as a function of the experimental optical bandwidth reduction factor for the Miller signal (OCT image not shown). Panel (e) shows the SNR as a function of the calculated optical bandwidth reduction factor in the diffuse scattering area indicated by the yellow box in panel (a). Panel (f) shows the SNR as a function of the calculated optical bandwidth reduction factor for the Miller signal (OCT image not shown). Note that the SNR in the diffuse scattering area shown in panels (c) and (e) is calculated as the ratio of the average signal power in the yellow box to the average noise power measured on the tissue. [Figure 14] The results of imaging of pig knee cartilage are shown. Panel (a) is a diagram showing the structure of pig knee cartilage. Panel (b) is a color photograph of the imaging area. The red lines indicate the image locations in panels (c) to (e), and the arrows indicate the scanning direction (from left to right in panels (c) to (e)). Panel (c) shows the full-resolution OCT image. Panel (d) shows the computationally generated 9x bandwidth reduction. Panel (e) shows the experimentally generated 9x bandwidth reduction. The yellow arrows indicate tide marks that are only visible when bandwidth is reduced and acquired between the articular cartilage and subchondral bone. [Modes for carrying out the invention]

[0018] Unless otherwise specified, all terms used herein (including technical and scientific terms) have the same meaning as those generally understood by those skilled in the art in which the invention pertains. Furthermore, terms as defined in commonly used dictionaries should be interpreted as having the meaning consistent with their meanings in the context of the relevant art and this disclosure, and should not be interpreted in an idealized or overly formal sense unless explicitly defined herein.

[0019] It should be understood that many techniques and steps are disclosed. Each of these has its own merits, and each can be used in combination with one or more, and possibly all, of the other disclosed techniques. Therefore, for clarity, this specification refrains from unnecessarily repeating every possible combination of the individual steps. Nevertheless, it should be understood that this specification and the claims fully encompass such combinations within the scope of this disclosure and the claims.

[0020] Various embodiments and aspects of this disclosure are described with reference to the reference numerals mentioned below, and the accompanying drawings illustrate various embodiments. The following description and drawings are illustrative of this disclosure and should not be construed as limiting it. Many specific details are described in order to provide a full understanding of the various embodiments of this disclosure. However, in some cases, details of the well-known or prior art are omitted in order to provide a concise description of the embodiments.

[0021] In this specification, the reference numerals in “one embodiment” or “an embodiment” mean that certain features, structures, or characteristics described in relation to an embodiment may be included in at least one embodiment of this disclosure. The phrase “in one embodiment” appearing in various places in this specification does not necessarily mean that all descriptions refer to the same embodiment.

[0022] (Relationship between axial resolution and signal-to-noise ratio in OCT) An OCT system provides measurements to analyze depth from a sample. The OCT system comprises a light source and an interferometer that transmits light from the light source to the sample and a reference optical path within the OCT system. Light reflected from the sample is captured by the OCT system and mixed with a reference field using common OCT techniques. The interference signal is detected, digitized, and provided to a computer arrangement. The computer arrangement generates an image based on at least this digitized signal.

[0023] An exemplary OCT system based on a broadband light source is shown in Figure 1. The broadband light source 101 supplies light to a beam splitter 102, where the light is split into a sample arm and a reference arm. In the sample arm, the light strikes the sample 103 and is captured and guided to interfere with the light from the reference arm 104. The optical interference is guided to a detector 105, such as a spectrometer, which can measure the light as a function of wavelength. This mixing of sample reflection and reference light field can be achieved, for example, using Michelson interferometry (shown in Figure 1) or Mach-Zehnder interferometry. The readout from the spectrometer or similar device is transferred to a computer device 106 for signal / image reconstruction and other processing steps.

[0024] An exemplary OCT system based on a wavelength-swept light source is shown in Figure 1. The wavelength-swept light source 201 supplies light to a beam splitter 202, where the light is split into separate sample arms and a reference arm. In the sample arm, the light is directed towards the sample 203, and the light reflected from the sample is captured and directed to interfere with the light from the reference arm 204. The recombined light is directed to a detector 205 that can measure the light as a function, such as a single-ended or balanced detector. Meanwhile, Figure 2 shows this using a Michelson interferometer, although this can also be achieved using Mach-Zehnder interferometer technology. The readout from the detector or similar device is then transferred to a computer device 206 for signal / image reconstruction and other processing steps.

[0025] In OCT, there is an inverse relationship between optical bandwidth and axial resolution. Acquiring with a larger optical bandwidth results in lower axial resolution. To avoid confusion, note that as axial resolution decreases, the image resolution increases. In other words, high-resolution systems provide measurements at lower resolution. Because of this relationship, it is common practice in this field to design OCT systems that maximize the optical bandwidth of the imaging system to achieve the smallest possible axial resolution.

[0026] This application is based on our observation that the optical bandwidth of an OCT measurement also affects the signal-to-noise ratio (SNR) of the image, a concept that has not been understood or explained before. The relationship between optical bandwidth and SNR varies depending on the sample being imaged. In the case of specular reflection, such as reflection from a mirror, the SNR is independent of the optical bandwidth. In the case of diffusely scattered samples, such as biological tissue, the SNR of the measurement increases as the optical bandwidth decreases, as shown in Figure 3. Given that OCT is primarily used to image diffusely scattered samples and that the SNR of the measurement is an important imaging parameter, this can be utilized to improve imaging performance.

[0027] As an example, consider the design of two sweep-source OCT systems illuminating a sample, following the discussion below. Each system provides the same optical power, operates with the same A-line duration, and is characterized by the same level of relative intensity noise (RIN). The first sweep-source OCT system is configured to sweep the laser over a 100 nm optical bandwidth. The second sweep-source OCT system is configured to sweep the laser over a more limited 10 nm optical bandwidth. The noise performance of each system is determined by the optical noise of the reference arm field, which is the same for both systems (the noise is independent of the sweep bandwidth). The signal power in each measurement where depth is analyzed is inversely proportional to the optical bandwidth. This is because lower optical bandwidths capture the signal over a wider depth range, meaning there are more scatterers contributing to the measurement. For example, the 100 nm system captures scattered light within a depth range of approximately 7 μm. The 10 nm system captures backscattered light within a depth range of approximately 70 μm. Nominally, within a 70 μm range, there are 10 times more scatterers than in a 7 μm voxel (assuming constant reflectivity), the signal from each voxel is 10 times higher, and the SNR (10 dB) is 10 times higher. This discussion, while describing a swept light source OCT system as an example, applies equally to other OCT architectures that utilize light sources with discrete sets of optical frequency lines, including but not limited to spectral domain OCT, optical frequency step systems, frequency comb OCT systems, and circular-ranging OCT.

[0028] Assuming that the reflection boundary is located at the center of each range, it should be noted that since the 70 μm range has the same number of scatterers as the 7 μm range, specular reflection, such as from mirrors, will result in an SNR that is independent of the optical bandwidth.

[0029] One important implication of this phenomenon is that the signal-to-noise ratio (SNR) of OCT measurements is minimally affected by imposing an optical bandwidth narrowing window in post-processing, as is done in split-spectrum angiography, spectral-binning polarization-sensitive OCT, frequency compounding for speckle reduction, and spectroscopic or hyperspectral OCT. Here, windowing narrows the effective optical bandwidth to improve the SNR, but it also reduces the effective imaging power (by discarding photons received outside the window), thus lowering the SNR. These effects cancel each other out, producing measurements with nominally the same SNR. This is important in that it allows these methods to be used without an SNR penalty.

[0030] The effects described in this study have been demonstrated in the near-infrared wavelength range, but they are equally applicable to other wavelength ranges, including, but not limited to, the visible light range (e.g., 380 nm to 780 nm), the ultraviolet light range (e.g., 100 nm to 380 nm), and the infrared light range (e.g., 780 nm to 1 mm).

[0031] (Configure an OCT system that uses a bandwidth-reducing light source to image deeper parts of the sample.) By using a coupling between optical bandwidth and SNR, the SNR of an OCT system can be improved at a specific imaging power and A-line duration. This enhanced SNR allows imaging of deeper regions of the sample. One sweep-source OCT system was used with a fixed optical power of 9 mW and a fixed A-line duration of 20 μs. This system was configured to image with a first optical bandwidth of 100 nm and a second optical bandwidth of 16 nm by adjusting the rotation speed of a polygon mirror placed within the wavelength-swept laser source. Pig knee cartilage was imaged. Figure 4 shows images acquired with each of these two OCT system configurations. Images acquired using the smaller optical bandwidth showed a higher SNR, allowing analysis of deeper boundaries not visible in images acquired with the larger optical bandwidth. The SNR of the ROI was quantitatively measured for both systems and was shown to be 9 dB higher when using the smaller optical bandwidth.

[0032] (OCT imaging using multi-state illumination) In a first set of preferred embodiments, the OCT system is configured to modulate imaging performance indicators such as axial resolution and SNR by controlling at least the spectral characteristics of the light supplied to the sample. Figure 5 illustrates this configuration. The light supplied to the sample (510) is controlled by the OCT system so that this illumination light is supplied in at least one of two possible states. These states are distinguished by supplying the sample with light having at least different spectral characteristics. The spectral characteristics of the light supplied to the sample can be characterized by its power spectral density P(ν).

[0033] The optical bandwidth of this illumination can be calculated in several ways. A common method is to calculate the optical bandwidth using the full-width and half-maximum (FWHM) of the power spectral density. A second common method is to define the root mean square (RMS) bandwidth Δν as shown in the following equation (Equation 1).

number

number

[0034] Alternatively, optical bandwidth can be defined as the total edge-to-edge optical bandwidth of the usable imaging light generated by the light source. Measuring this edge-to-edge optical bandwidth allows for the exclusion of, for example, amplified spontaneous emission background from a laser light source. The edge-to-edge definition is suitable for, for example, wavelength-swept or wavelength-stepped light sources.

[0035] The above illustrates a series of exemplary quantifications of the optical bandwidth of an illumination beam, and it should be understood that this does not preclude further methods for quantifying optical bandwidth.

[0036] The system is configured to guide light to the sample in a first state (511a) characterized by P1(ν), <ν1>, and Δν1, and at least a second state (511b) characterized by P2(ν), <ν2>, and Δν2. In a first set of this preferred embodiment, the light source is configured such that Δν2 = A × Δν1 (A < 1). The first state can be configured to provide better axial resolution and a lower SNR than the second state. In one embodiment, A = 0.5, which improves the imaging SNR by 3 dB. While a 3 dB improvement in SNR is modest, it is meaningful in some cases and, depending on the use case, provides value commensurate with the system configuration complexity of providing two illumination states. In a further embodiment, A can be configured to 0.25, which provides a mechanism to increase the SNR by 6 dB, which is a significant improvement in most imaging scenarios. In a further embodiment, A can be configured to 0.1, which provides a mechanism to increase the SNR by 10 dB, dramatically enhancing the image, which can be used, for example, to view fairly deep parts of a sample or to analyze structures that are slightly below or close to the noise level of the imaging system.

[0037] In some embodiments of the present invention, the illumination states are configured to supply approximately the same average optical power to the sample, although the optical bandwidth may vary. This can be used in scenarios where the average power to the sample is limited by safety requirements. For example, in retinal imaging, the average power is limited to approximately 2 mW by American National Standards Institute (ANSI) documentation. At the same time, the signal-to-noise ratio (SNR) of the imaging is a critical parameter. The present invention provides a mechanism to improve the SNR while maintaining power at the maximum allowable power level and maintaining the imaging speed. This is done by using illumination with reduced optical bandwidth, improving the imaging SNR at the expense of axial resolution. In scenarios where a higher SNR is required but neither the imaging power nor the imaging speed can be changed, the present invention provides a mechanism to increase the SNR at the expense of axial resolution. In some embodiments, the difference in power supplied to the sample may vary somewhat between illumination states, but the change in optical power may be smaller than the change in optical bandwidth between illumination states. For example, if the bandwidth changes by a factor of four (A=0.25), the power of these configurations may change by a factor of less than four, e.g., 10%. These power variations may be unintended consequences, for example, of using light sources in different configurations. Another way to express these relationships is that, in various embodiments, the ratio of the bandwidth of the wider bandwidth illumination to the bandwidth of the narrower bandwidth illumination is at least 2, at least 4, at least 10, etc.

[0038] In various embodiments, the power level of the first illumination state is about 50% of the power level of the second illumination state. In some embodiments, the power level of the first illumination state is about 75%, about 90%, about 95%, or about 99% of the power level of the second illumination state. In some embodiments, the power level of the first illumination state is substantially equal to (e.g., at least 90%) the power level of the second illumination state. This may correspond to embodiments in which the first illumination state provides a wider optical bandwidth than the second state, or embodiments in which the first illumination state provides a narrower optical bandwidth than the second state.

[0039] An OCT system can be configured to control which of the available illumination states is used by a single signal or set of signals provided to the OCT system from a computer device. The computer device may be the same one that receives the image data and the one that generates the image. Control over which illumination state to use can be based on user selection, a pre-programmed sequence, characteristics of previously acquired OCT imaging data, or another source of information. The system can switch illumination states between each OCT depth scan (A-line), between each OCT frame, between each OCT acquisition volume, or a combination of the above. An OCT system can also be configured to apply a specific illumination state to a particular location within a three-dimensional scan. For example, in a retinal imaging experiment, the system can be configured to deploy an illumination state with at least a low optical bandwidth in the region near the optic nerve head where the retinal thickness increases, and the additional SNR is useful for analyzing deeper tissue layers such as the lamina cribosa. The system can be configured to acquire imaging data using multiple illumination states at the same location in the sample, providing imaging data with both high axial resolution and high SNR at that location.

[0040] (Embodiment having a broadband light source) In a preferred embodiment of multi-state illumination, the OCT system comprises at least two broadband spectral light sources 601a and 601b. These light sources may be, for example, amplified spontaneous emission (ASE) light sources, superluminescent diodes (SLDs), supercontinuum light sources, or mode-locked laser light sources. Each illumination state has a corresponding bandwidth, which, as an example, is defined here as the full width at half maximum measured from the power graph as a function of wavelength. Alternative definitions of bandwidth, as described above, may also be used. Interference signals are detected between sample 603 and reference mirror 604 using at least one spectrometer 605. Light sources 601a and 601b provide different spectral outputs 612a and 612b. The power of each light source is controlled by computer device 620 via signals 620a and 620b. The spectral characteristics of the light 610 supplied to sample 603 are determined based on the control signals 620a and 620b. These light sources can be combined within an optical element 611, which may be a passive power combiner (beam splitter), a polarization-based combiner, or an active optical switch. The power provided by each light source can be controlled by signals 620a and 620b to achieve nominally consistent power delivery to the sample and to provide an adjustable optical bandwidth. Alternatively, the light sources can be configured to provide a fixed output power, and variable attenuators 630a, 630b, and 630 can be placed within the OCT system to control the optical power of light 610.

[0041] In another preferred embodiment, the OCT system includes an adjustable spectral filter configured to control the spectral characteristics of the illumination light provided to the sample. This spectral filter can be adjusted for transmittance as a function of wavelength and can be used to control average power, center optical frequency, and optical bandwidth. A computer device may provide control signals for configuring this spectral filter, or it may be configured manually. The spectral filter may be based, for example, on microelectromechanical systems (MEMs), acousto-optic filters, adjustable fiber Bragg gratings, adjustable long-period gratings, or other optical filtering techniques known in the art. Using a spectral filter can reduce the optical bandwidth compared to a less filtered state. When performing such filtering, the spectral filter may also shift the center optical frequency of the light source. For example, a first illumination state may provide light centered at 850 nm with an optical bandwidth (FWHM) of 70 nm, while a second illumination state may provide light centered at 900 nm with an optical bandwidth (FWHM) of 10 nm.

[0042] In another preferred embodiment, the OCT system can utilize a semiconductor-based light source, such as an SLD or semiconductor optical amplifier (SOA), which provides an optical bandwidth partially controlled by the current supplied to the device. In one embodiment, the SOA device may include a quantum dot light source and an amplifier.

[0043] An external gain source, such as an SOA or variable optical attenuator, is used to control the power of the illumination light directed to the sample, separating the optical bandwidth configuration from the illumination power and offsetting changes in optical power caused, for example, by filtering. The external gain source can be gain-modulated to generate a specific set of output powers for each illumination state.

[0044] (Embodiment having a wavelength-swept light source) In a further embodiment of multi-state illumination, the OCT system is constructed using a wavelength-swept light source that provides a sample with light having an optical frequency that changes over time. Each illumination state has a corresponding bandwidth, which here is defined as the inter-edge optical bandwidth described above as an example. Additional measurements of the optical bandwidth can also be applied. The light source can be configured to provide light of a first state characterized by frequency tuning as a function of time over a first bandwidth Δν1 (Figure 7, 701A), and light of at least a second state characterized by frequency tuning as a function of time over a second bandwidth Δν2 701B. The average power of the light delivered to the sample can be configured to be approximately the same for each state. The wavelength sweep rate of the light source can be approximately the same between the two states or can be varied between the two states.

[0045] In one embodiment, wavelength-swept light can be generated by a single laser source that can be configured to provide light of different optical bandwidths. This can be achieved by controlling the signal supplied to a mechanically driven optical filter, based on a laser design that includes such filters. This can be done, for example, by controlling the amplitude between peaks supplied to a Fabry-Perot filter located within a laser cavity, or by controlling the amplitude between peaks supplied to a movable mirror, based on, for example, MEMS, resonant scanners, or galvanometer mirrors.

[0046] In further embodiments, the wavelength-swept light can be generated by a laser source that does not include a mechanical optical filter and consists of a voltage signal or current signal provided by Vernier-tuned distributed feedback sources.

[0047] In further embodiments, the wavelength-swept light can be generated by a set of laser sources, each providing a substantially fixed optical bandwidth, but which can be configured to be switched on or off to control the optical bandwidth provided to the sample. Alternatively, each laser light source can be controlled to provide a bandwidth over a specific range, and these light sources can be used individually or in combination to achieve wider or narrower optical bandwidths.

[0048] In further embodiments, a wavelength-swept light source can be configured to provide at least two illumination states by controlling the sweep profile of the wavelength-swept light source. In this configuration, the wavelength-swept laser operates according to an optical frequency-versus-time curve and an optical power-versus-time curve. The average power and optical bandwidth provided to the sample are both measured on average over the sweep of the light source and are functions of both of these curves. For example, the light source can be configured to provide two illumination states, both following the same optical frequency-versus-time curve but having two different optical power-versus-time curves. In the first state, the instantaneous optical power is constant, equal to, for example, 2mW. In the second state, the instantaneous optical power decreases to less than 2mW at optical frequencies far from the average value and increases to more than 2mW at optical frequencies close to the average value. This embodiment provides the same average optical power (2mW) in the second state as in the first state, but provides an optical bandwidth smaller than the optical bandwidth measured using the RMS optical bandwidth calculated by Equation (Equation 1).

[0049] Alternatively, the optical power-to-time curve can be kept fixed, while the optical frequency-to-time curve can be modified. For example, in the first state, optical frequency-to-time can follow a linear trend. In the second state, the optical frequency can be swept more quickly through optical frequencies further away from the central optical frequency compared to sweeping through optical frequencies at or near the central optical frequency. In this configuration, the average optical power provided by the two states is the same, but the averaged RMS optical bandwidth over the entire sweep is smaller in the second state (because the laser sweep speed at the edge locations is faster, resulting in less energy being supplied at the edges).

[0050] These embodiments described above are examples of a broader range of configurations for modifying the optical power versus time and / or optical frequency versus time curves so that two illumination states with different optical bandwidths are provided.

[0051] In embodiments where the optical frequency-time curve is nonlinear, the OCT system can be configured to digitize these signals at a fixed clock rate determined, for example, by the sampling rate required when the laser is sweeping at its fastest speed. The digitized signals can be processed using signal processing such as digital filtering or moving averages to extract the higher SNR of the slower-swept portions. The filtering provided may vary over time and can be responsive to the optical frequency sweep rate of the laser. For example, the digitized signal of the portion swept at 1 / 10th the speed of the fastest portion of the laser frequency can be processed with a moving average across 10 adjacent digitized samples. This yields measurement data at the same spectral interval but has the advantage of taking advantage of the oversampling provided by the slower sweep rate. The moving average filter can be replaced with a digital signal processing (DSP) filter that offers similar advantages.

[0052] (Embodiment having discrete structure broadband illumination) In further embodiments of multi-state illumination, a broadband light source comprising a set of discrete lines can be used, including but not limited to a frequency comb light source following the approach described in the section “Embodiments Having Broadband Light Sources.” In such configurations, the optical bandwidth can be defined, for example, using the inter-edge bandwidth or RMS bandwidth. The definition of FWHM can be used and specified to be calculated over the envelope of the spectrum, as is common in the art, rather than as a measure of a single discrete line of light.

[0053] (Embodiment having stepped optical frequency illumination) In further embodiments of multi-state illumination, a light source in which steps are formed in the time domain between discrete optical frequencies, including but not limited to the optical frequencies of an equally spaced optical frequency comb, can be used according to the approach described in the section “Embodiments with Wavelength Sweeping Light Sources.” Each illumination state has a corresponding bandwidth, defined, for example, as the inter-edge optical bandwidth. Such measurements can be performed over a time window sufficient to capture all optical frequency components to which a Fourier transform is applied to generate depth analysis data. The time-domain stepped optical frequency source can be based, for example, on phase-coded mode locking, stretched pulse mode locking, or other approaches known to those skilled in the art. The time-domain stepped optical frequency comb light source can be programmed to provide different illumination light to a sample by changing the sequence of optical frequencies. In one embodiment, the light source provides a sequence with different inter-edge optical bandwidths. In another embodiment, the light source provides a sequence that varies the number of times a given optical frequency is repeated, thereby changing the RMS optical bandwidth by giving more power to these repeated optical frequencies. The measurements from repeated optical frequency sampling can be combined to configure the processing to provide lower noise. Using these two configurations in combination can influence the modulation applied to the optical bandwidth provided to the sample.

[0054] (Additional embodiment) Figure 8 shows the wavelengths and power outputs of multiple configurations of a single wavelength-swept laser according to one embodiment of the present disclosure. In this embodiment, a single wavelength-swept laser can sweep across two or more optical bandwidth configurations. Wavelength sweep refers to a change in wavelength as a function of time, where consecutive wavelengths may or may not appear consecutively. Configurations with shorter optical bandwidths output a higher average power per unit wavelength compared to configurations with longer optical bandwidths. In this embodiment, the multiple configurations do not have to operate simultaneously. Switching between two or more configurations can be done manually via hardware, calculated via software, or in an automated, pre-programmed manner. This is sometimes referred to in the art as an implementation of a “swept light source”.

[0055] Figure 9 shows the wavelengths and power outputs of multiple configurations of a single broadband laser according to one embodiment of the present disclosure. In this embodiment, a single broadband laser (including, but not limited to, a superluminescent diode or a supercontinuum laser) has two or more optical bandwidth configurations. In this embodiment, all wavelengths of each configuration are emitted simultaneously. Configurations with shorter optical bandwidths output a higher average power per unit wavelength compared to configurations with longer optical bandwidths. The multiple configurations do not have to operate simultaneously. Switching between two or more configurations can be done manually via hardware, calculated via software, or in an automated, pre-programmed manner. This switching can be performed within the laser or within an optical coherence tomography system. This is sometimes referred to in the art as an implementation of a “spectral domain”.

[0056] Figure 10 shows the wavelengths and power outputs of multiple configurations of a single step-wavelength laser according to one embodiment of the present disclosure. In this embodiment, a single step-wavelength laser can span two or more optical bandwidth configurations. Wavelength stepping refers to a discrete change in wavelength as a function of time, where consecutive wavelengths may or may not appear. Shorter optical bandwidth configurations output a higher average power per unit wavelength compared to longer optical bandwidth configurations. Switching between two or more configurations can be done manually via hardware, calculated via software, or in an automated, pre-programmed manner. This is sometimes referred to in the art as a “circular range” implementation.

[0057] Figure 11 shows an example of the use of two independent light sources according to various embodiments of this disclosure. Any combination of two or more independent light sources, as described in other embodiments, can be used for the purpose of achieving a trade-off between axial resolution and signal-to-noise ratio. Figure 11 shows an example in which the light sources are alternately turned on and off. The on and off periods can follow any pattern, but the two light sources may not operate simultaneously.

[0058] In any embodiment, data may be generated that requires processing to convert the raw data into an image. Such processing may include any typical OCT post-processing procedure known to those skilled in the art. Some of these steps may include resampling to linear wavenumber intervals, variance compensation, chirp compensation, spectral shaping, and Fourier transform.

[0059] In each of the embodiments described above, it may be advantageous to configure the system such that the lateral resolution provided by the imaging system is substantially the same in the first and second illumination states. This may mean that the axial resolution, and therefore the signal-to-noise ratio of the imaging, is modulated between illumination states without significantly altering the lateral resolution. For example, in these embodiments, the same optical apparatus used to focus the illumination light onto the sample and collect the reflected light can be used in both illumination states without changing the optical elements, such as the objective lens or other lenses.

[0060] (Examples) The following provides non-limiting examples of embodiments of the present disclosure.

[0061] In optical coherence tomography (OCT), axial resolution and signal-to-noise ratio (SNR) are typically considered independent parameters. This example demonstrates that this is only true for mirror-like surfaces, and that there is an inherent relationship between axial resolution and measured SNR in diffusely scattered samples such as biological tissue. We explain the origin of this coupling and demonstrate that it can be used to achieve increased imaging penetration depth at the expense of resolution. Finally, we argue that this coupling should be considered during the OCT system design process to balance the competing needs of resolution, sensitivity, and system / light source complexity.

[0062] The inverse relationship between optical bandwidth and axial resolution is a characteristic feature of optical coherence tomography (OCT). This example asks whether optical bandwidth also affects the signal-to-noise ratio (SNR). When asking such a question, we set aside the experimental complexities proportional to the light source bandwidth, such as those related to light source design and interferometer polarization mode dispersion, and focus on the fundamental relationship between optical bandwidth and SNR. In more practical terms, this example asks whether there are scenarios where it is necessary to intentionally limit axial resolution performance in order to improve the imaging SNR.

[0063] We conclude that optical bandwidth / axial resolution definitely affects SNR, but this depends on the nature of the sample. In the case of specular reflection, such as reflection from a mirror, SNR does not depend on optical bandwidth, but in the case of diffuse scattering samples such as biological tissue, there is a fundamental inverse relationship between optical bandwidth and measured SNR. Given that OCT is mainly used for imaging diffuse scattering samples, we argue that this relationship should at least be recognized and should be considered during the design of OCT systems in some applications. We also show that spectral splitting methods in OCT (e.g., spectral binning polarization-sensitive OCT) are effective because of this relationship.

[0064] First, we present a physical argument to support the claim that, for a diffusely scattered sample, the measured SNR is inversely proportional to the optical bandwidth used to perform the measurement. Consider two swept light source OCT systems with equivalent optical power, A-line duration, and relative intensity noise (this argument also applies to spectral domain architectures with limited shot noise). However, one system sweeps an optical bandwidth of 100 nm, while the other sweeps an optical bandwidth of only 10 nm. For brevity, we assume that each system uses a digitizer to capture fringes with a fixed number of samples during the A-line. The detector and detector bandwidths are identical. Their respective noise performance is determined by the optical noise of the reference arm and is therefore equivalent. In other words, the optical bandwidth traced by the laser does not affect the measured noise. Each bin of the signal power within each measured voxel, or the final discrete Fourier transform of the digitized fringe, scales inversely to the optical bandwidth. This is because a lower optical bandwidth captures more scattered photons from the diffusely scattered sample. For example, a 100nm system captures backscattered light within a depth range of approximately 7μm, while a 10nm system captures backscattered light within a depth range of 70μm. Within the 70μm range, there are 10 times more scatterers than in a 7μm voxel (assuming constant reflectivity), resulting in 10 times higher power in the reference arm of each voxel and a 10 times higher SNR (10dB). In short, this argument suggests that, for diffusely scattered samples, the relationship between the measured SNR of a voxel and the measured optical bandwidth (Δν) is... diffuse ∝(Δν) -1 It can be predicted that it will be given as such.

[0065] Applying the same inference to mirrors, specular reflections, or other subaxial resolution boundaries, we find that the SNR is independent of the optical bandwidth, i.e., SNR specular ∝(Δν) 0 This can be predicted. In these samples, all scatterers coexist at a single depth location, so extending the measurement voxel depth range does not add scatterers within the voxel, and therefore the signal power does not increase.

[0066] This relationship can be seen in the conventional mathematical description of OCT if we note that we accurately describe the specular reflection sample and the diffuse scattering sample. We adopt the cross-correlation term of the OCT A-line as the starting point. In the case of the specular sample, the OCT signal is emitted from a single reflector with a field reflectivity r located at the axial position z s and the cross-correlation term i s is restricted by the delta function δ as shown in Equation (3). D★

Equation

[0067] Here, ρ is the detector responsivity, r r is the field reflectivity of the reference arm, assuming that the reference arm is located at z = 0. The parameter γ(z) is the coherence function of the light source, and is normalized to γ(0) = 1 under the assumption that the light source power is kept constant. The width of γ(z) representing the axial resolution may vary. As expected, the measured signal i D★ (2z z ) does not depend on the width of γ(z) and thus does not depend on the optical bandwidth of the light source either.

[0068] In the case of the diffuse scattering sample, the above-mentioned signal is the contribution from all scatterers over depth, each of which is convolved with the coherence function. Considering N scatterers located over a depth range larger than the width of the coherence function, the cross-correlation term becomes as follows in Equation (4).

Equation

[0069] For simplicity, consider the reflectivity r sn of the scatterers as real values both positive and negative. Assume that there are a large number of scatterers within the width of the coherence function, consistent with the diffuse scattering region. Assume that the sample is deep enough within the sample so that there are no surface effects to consider. Then the signal i D★ ​(z) contains the sum of many single scattering events. The number of scatterers contributing to this sum varies depending on the width of the coherence function. A wider coherence function allows for more scatterers. The ensemble average of this signal <i D★ (z)〉 is scaled as the square root of the number of scatterers (random walks), i.e., it is scaled as the square root of the width of γ(z). Corresponding measured intensity 〈|i D★ (z) 2 Since > is directly proportional to the width of γ(z), it is inversely proportional to (Δν). Note that this relationship is only accurate if the spectral shape of the light source (e.g., Gaussian shape) does not change.

[0070] Applying similar mathematical reasoning to the autocorrelation term (not included in equations (3) and (4)), the SNR of the autocorrelation signal intensity is (Δν) 2 It is noteworthy that an interesting prediction is made: that it is inversely proportional to [the original value].

[0071] SNR diffuse ∝(Δν) -1 The relationship is shown on a logarithmic scale in Figure 3. Furthermore, it is beneficial to explicitly include the A-line duration τ in these SNR relationships. Regardless of the sample type, the measured SNR is linearly proportional to τ, and the SNR diffuse ∝(τ)(Δν) -1 and SNR specular ∝(τ)(Δν) 0 It is well known that this can be obtained.

[0072] To verify these relationships, we performed a series of imaging experiments using a sweep-source OCT system that allowed for adjustment of the optical bandwidth while keeping the A-line duration constant and the average power nearly constant. This system was based on a polygon scanning mirror with a center of 1.3 μm. As shown in Figure 12, the angular velocity of the polygon scanning mirror could be adjusted over a wide range, modulating the A-line rate of the light source accordingly. The rotation frequency of the polygon mirror was reduced by coefficients of 1 (full resolution, 50 kHz sweep rate), 3 (17 kHz sweep rate), 5 (10 kHz sweep rate), 7 (7.1 kHz sweep rate), and 9 (5.6 kHz sweep rate). For each acquisition process, we maintained the same digitizer sampling rate and A-line sample count (2048 unique sampling points over τ = 20 μs per A-line). Small fluctuations in optical power were mainly caused by the spectral envelope of the light source between the full bandwidth setting and all bandwidth reduction settings. This variation could be corrected by measuring the spectral shape from an oscilloscope. Laser chirp and system dispersion correction curves were calculated for each bandwidth configuration. A Hann window was applied before the Fourier transform to remove side lobes. A long working distance scanning lens with an effective focal length of 54 mm (LSM54-1310, Thorlabs) was used as the microscope objective lens for the system. This minimized the variation in the measured SNR due to beam focusing.

[0073] Images of chicken breast were acquired, and the SNR performance in the subsurface region was analyzed as representative of diffuse scattering. For comparison, images of the specular surface (as representative of specular reflection) were also acquired and analyzed. SNR was evaluated by varying the optical bandwidth by physically changing the polygon velocity and a constant A-line duration, as described above (see Figure 13(a)). Furthermore, for a fixed polygon rotation frequency (A-line rate of 50 kHz), the fringe duration (τ) was reduced in post-processing to achieve the same optical bandwidth [e.g., spectral windowing, see Figure 13(b)].

[0074] The results of this experiment support the scaling relationship described above. Figure 13(c) shows that the SNR increases in the diffuse scattering region in proportion to the reciprocal of the optical bandwidth. Figure 13(d) shows that the SNR gain seen in Figure 13(c) is derived from the diffuse scattering characteristics of the sample. Importantly, the mirror signal commonly used to characterize the resolution and sensitivity of OCT systems follows the scaling in Figure 13(d) and does not show dependence on the optical bandwidth. The axial resolution of an OCT system and the SNR of the generated image are inseparable, and the measured sensitivity should be used carefully as a predictor of the imaging SNR. Figure 13(e) shows that the SNR in the diffuse region hardly changes even when the optical bandwidth is computationally reduced. This is interesting because it represents a spectral windowing approach in which the processing of narrow portions of the acquired fringes causes a reduction in optical bandwidth and effective A-line duration. These two effects (increase in photons within the coherence gate due to reduced resolution, and decrease in detected photons due to shortened measurement duration) have conflicting effects on the SNR. When the spectral envelope is constant, these effects reliably cancel each other out, and the SNR is unaffected. Although not explicitly stated, this is indeed why these approaches can be used in split-spectrum angiography, spectral binning polarization-sensitive OCT, frequency synthesis for speckle reduction, and spectroscopic or hyperspectral OCT (within diffuse reflectance lazym) without SNR penalty. Figure 13(f) shows that in the case of specular reflectors, an SNR penalty is observed when these windowing methods are applied. Herein lies the SNR / axial resolution trade-off, and we then demonstrate how a reduction in optical bandwidth can be used to achieve a deeper imaging depth at the expense of axial resolution (see Figure 14).

[0075] Porcine knee cartilage is a relatively thick (~2 mm), fairly homogeneous tissue with a cartilage-bone interface, and is often at the limit of OCT detection (see Figure 14(a)). In scenarios where articular cartilage thickness is a critical parameter, our results above suggest that imaging with a significantly reduced bandwidth to prioritize SNR performance may be advantageous. We imaged knee samples using a similar protocol to the one described above (see color photograph in Figure 14(b)). Figure 14(c) shows cross-sectional images acquired at full resolution, while Figures 14(d) and 14(e) show 1 / 9 bandwidth from computationally and experimentally windowed data, respectively. It can be seen that the boundaries of tide marks are best visible when experimentally acquired at 1 / 9 bandwidth. Other potential applications where this may be useful include tumor boundary demarcation and measurement of coating thickness in industrial tablets.

[0076] In another aspect of the SNR resolution optimization process, axial resolution is paramount when the research goal is to investigate the microstructure of materials containing numerous boundary features. In this case, assuming those boundary features are specularly reflected, the SNR of those boundary features remains intact, while the SNR is impaired in areas of more uniform scattering. This phenomenon may also contribute to improved contrast between these boundary and dispersion scatterer signals when measured with high-resolution OCT. A recent paper visualized highly scattered cell bodies located within the nuclear layer of the retina, traditionally less scattered, using a high-resolution (1.2 μm) visible-light OCT system. Our results suggest that these cell bodies appear on the scale of the system's axial resolution, thus indicating their SNR is not affected by higher resolution. In contrast, a decrease in SNR is observed in the background of the nuclear layer, leading to an increased "cell body vs. nuclear layer" intensity ratio when imaged with a high-resolution system. Further research is needed to verify that this effect contributes to the increased contrast.

[0077] With recent advancements in laser technology enabling partially coherent light across a wide spectral bandwidth, there is a growing movement within the OCT community to pursue increasingly higher axial resolution. To date, sensitivity measurements performed on high-resolution systems have been compared to those on lower-resolution systems by equivalent standards. Many systems reporting axial resolution on the order of 1 μm are based on supercontinuum light sources, which are widely accepted to be noisy. Despite these systems approaching shot-noise-limited sensitivity, this study outlines that they continue to produce images with lower image SNRs than comparable products with more limited optical bandwidths.

[0078] There are also several physical limitations to the achievable SNR improvement. As the axial resolution approaches the size of the sample structure under investigation, the dispersion scatterer approximation breaks down, the entire structure falls into a surface boundary region, and the SNR becomes independent of the optical bandwidth / axial resolution. There are also limits to the SNR improvement that can be achieved by reducing the axial resolution by reducing the bandwidth. This limitation may be due to depth-dependent signal attenuation, which reduces the signal from the bottom of the voxel compared to the signal from the top, or the signal from the confocal gate. Of course, the latter is most relevant to optical coherence microscopy systems. Finally, from this study, uncorrected dispersion imbalance or chirp fringes in an interferometer can improve the SNR because both reduce the axial resolution. However, this ignores the attenuation of the point image distribution function associated with the broadening of the PSF due to dispersion / chirp. Due to this attenuation, dispersion / chirp does not increase the number of photons in a single coherence gate, and therefore the SNR is not improved.

[0079] Traditionally, OCT system design has balanced the practical and engineering challenges of using extremely broadband light sources with the advantages of high axial resolution. In this study, we showed that high axial resolution can lead to a decrease in signal-to-noise ratio (SNR). Therefore, depending on the application, it may be advantageous to intentionally reduce axial resolution during imaging.

[0080] While the present invention has been illustrated and described with reference to its preferred embodiments and specific examples, it will be apparent to those skilled in the art that other embodiments and examples can perform similar functions and / or achieve similar results. As a result, all such equivalent embodiments and examples are in the spirit and scope of the present invention and are intended to be covered by the following claims.

Claims

1. A light source configured to provide first and second illumination to a diffuse scattering sample, wherein the first illumination provides light of a first bandwidth and the second illumination provides light of a second bandwidth narrower than the first bandwidth, To generate interference data from the diffuse scattering sample, an optical coherence tomography (OCT) optical apparatus is provided, comprising a sample optical path and a reference optical path, each optically coupled to the light source; A detector configured to be optically coupled to the OCT optical device and to collect the interference data from the diffuse scattering sample, A controller configured to switch the illumination provided to the OCT optical device between the first illumination and the second illumination, Equipped with, The interference data includes first interference data obtained using the first illumination and second interference data obtained using the second illumination. The controller is configured to determine the first signal-to-noise ratio (SNR) of the first interference data and the second signal-to-noise ratio (SNR) of the second interference data. The second signal-to-noise ratio is greater than the first signal-to-noise ratio. The first power level of the first illumination is substantially equal to the second power level of the second illumination. Optical coherence tomography (OCT) system.

2. The controller is configured to determine the first axial resolution of the first interference data and the second axial resolution of the second interference data. The OCT system according to claim 1, wherein the first axial resolution is smaller than the second axial resolution.

3. The OCT system according to claim 1 or claim 2, wherein the ratio of the second bandwidth to the first bandwidth is 0.5 or less.

4. The OCT system according to claim 1 or claim 2, wherein the ratio of the second bandwidth to the first bandwidth is 0.25 or less.

5. The OCT system according to claim 1 or claim 2, wherein the ratio of the second bandwidth to the first bandwidth is 0.1 or less.

6. The OCT system according to claim 1 or 2, wherein at least one of the first bandwidth or the second bandwidth is determined based on determining at least one of the full width at half maximum (FWHM) of the power spectral density, the root mean square (RMS) bandwidth, and the total inter-edge optical bandwidth of the available imaging light of the light source.

7. The OCT system according to claim 1 or claim 2, wherein the light source irradiates with the first illumination during a first period and irradiates with the second illumination during a second period different from the first period.

8. The light source comprises at least one of a sweeping light source, a comb light source, and a step-type light source. The OCT system according to claim 7, wherein the first bandwidth or the second bandwidth is determined based on determining the total inter-edge optical bandwidth of the usable imaging light of the light source.

9. The OCT system according to claim 7, wherein the detector is configured to collect the interference data from the diffuse scattering sample as a function of time.

10. The OCT system according to claim 9, wherein the detector comprises at least one single-ended detector or a balanced detector.

11. The light source comprises a first light source that provides the first illumination and a second light source that provides the second illumination. The OCT system according to claim 1 or claim 2, wherein the controller is configured to switch between conducting the first illumination from the first light source to the diffuse scattering sample and conducting the second illumination from the second light source to the diffuse scattering sample.

12. The OCT system according to claim 11, wherein at least one of the first light source and the second light source comprises a broadband light source.

13. The OCT system according to claim 12, wherein the detector is configured to collect interference data from the diffuse scattering sample as a function of wavelength.

14. The OCT system according to claim 13, wherein the detector comprises a spectrometer.

15. The OCT system according to claim 1 or 2, wherein the lateral resolution of the first interference data is equal to the lateral resolution of the second interference data.

16. A light source configured to provide first and second illumination to a diffuse scattering sample, wherein the first illumination provides light of a first bandwidth and the second illumination provides light of a second bandwidth narrower than the first bandwidth, To generate interference data from the diffuse scattering sample, an optical coherence tomography (OCT) optical apparatus is provided, comprising a sample optical path and a reference optical path, each optically coupled to the light source; A detector configured to be optically coupled with the OCT optical device and to collect the interference data from the diffuse scattering sample as a function of wavelength, The illumination provided to the OCT optical device is switched between the first illumination and the second illumination. A controller configured to enable, Equipped with, The interference data includes first interference data obtained using the first illumination and second interference data obtained using the second illumination. The ratio of the first bandwidth to the second bandwidth is at least 2. The controller is configured to determine the first signal-to-noise ratio (SNR) of the first interference data and the second signal-to-noise ratio (SNR) of the second interference data. The second signal-to-noise ratio is greater than the first signal-to-noise ratio. The first power level of the first illumination is substantially equal to the second power level of the second illumination. Optical coherence tomography (OCT) system.

17. A light source configured to provide first and second illumination to a diffuse scattering sample, wherein the first illumination provides light of a first bandwidth and has a first wavelength-to-time profile and a first power-to-time profile, and the second illumination provides light of a second bandwidth narrower than the first bandwidth and has a second wavelength-to-time profile and a second power-to-time profile, To generate interference data from the diffuse scattering sample, an optical coherence tomography (OCT) optical apparatus is provided, comprising a sample optical path and a reference optical path, each optically coupled to the light source; A detector configured to be optically coupled to the OCT optical device and to collect the interference data from the diffuse scattering sample, A controller configured to switch the illumination provided to the OCT optical device between the first illumination and the second illumination by controlling at least one of the first wavelength-to-time profile or the first power-to-time profile of the first illumination and at least one of the second wavelength-to-time profile or the second power-to-time profile of the second illumination, Equipped with, The interference data includes first interference data obtained using the first illumination and second interference data obtained using the second illumination. The controller is configured to determine the first signal-to-noise ratio (SNR) of the first interference data and the second signal-to-noise ratio (SNR) of the second interference data. The second signal-to-noise ratio is greater than the first signal-to-noise ratio. The first power level of the first illumination is substantially equal to the second power level of the second illumination. Optical coherence tomography (OCT) system.

18. The optical coherence tomography (OCT) system according to claim 17, wherein the controller is configured to control the light source to switch illuminations provided to the OCT optical apparatus in order to provide a higher signal-to-noise ratio (SNR) or lower axial resolution, based on controlling at least one of the first wavelength-to-time profile or the first power-to-time profile of the first illumination and controlling at least one of the second wavelength-to-time profile or the second power-to-time profile of the second illumination.

19. A step of using a light source to provide a first illumination and a second illumination to a diffuse scattering sample, wherein the first illumination provides light of a first bandwidth and the second illumination provides light of a second bandwidth that is narrower than the first bandwidth, Using an OCT optical apparatus, each comprising a sample optical path and a reference optical path that are optically coupled to the light source, the step of generating interference data from the diffusely scattered sample, which includes first interference data acquired using the first illumination and second interference data acquired using the second illumination, The steps include: collecting interference data from the diffuse scattering sample using a detector optically coupled with the OCT optical device; A step of using a controller to determine the first signal-to-noise ratio (SNR) of the first interference data and the second signal-to-noise ratio (SNR) of the second interference data, The process includes the step of using the controller to switch the illumination provided to the OCT optical device between the first illumination and the second illumination, The second signal-to-noise ratio is greater than the first signal-to-noise ratio. The first power level of the first illumination is substantially equal to the second power level of the second illumination. A method for performing optical coherence tomography (OCT).

20. The process further includes the step of determining the first axial resolution of the first interference data and the second axial resolution of the second interference data using the controller, The method according to claim 19, wherein the first axial resolution is smaller than the second axial resolution.

21. The method according to claim 19 or claim 20, wherein the ratio of the second bandwidth to the first bandwidth is 0.5 or less.

22. The method according to claim 19 or claim 20, wherein the ratio of the second bandwidth to the first bandwidth is 0.25 or less.

23. The method according to claim 19 or claim 20, wherein the ratio of the second bandwidth to the first bandwidth is 0.1 or less.

24. The method according to claim 19 or 20, further comprising the step of determining at least one of the first bandwidth or the second bandwidth based on determining at least one of the full width at half maximum (FWHM) of the power spectral density, the root mean square (RMS) bandwidth, and the total inter-edge optical bandwidth of the available imaging light of the light source.

25. The method according to claim 19 or 20, wherein the step of providing the first illumination and the second illumination further includes the step of irradiating with the first illumination during a first period and irradiating with the second illumination during a second period different from the first period.

26. The light source comprises at least one of a sweeping light source, a comb light source, and a step-type light source. The method according to claim 25, further comprising the step of determining the first bandwidth or the second bandwidth based on determining the total inter-edge optical bandwidth of the available imaging light of the light source.

27. The method according to claim 25, wherein the step of collecting the interference data from the diffuse scattering sample further comprises the step of collecting the interference data from the diffuse scattering sample as a function of time.

28. The method according to claim 27, wherein the detector comprises at least one single-ended detector or a balanced detector.

29. The light source comprises a first light source that provides the first illumination and a second light source that provides the second illumination. The step of providing first and second illumination is to provide the first light source to the diffuse scattering sample The method according to claim 19 or claim 20, further comprising the step of switching between conducting the first illumination to a light source and conducting the second illumination from the second light source to the diffuse scattering sample.

30. The method according to claim 29, wherein at least one of the first light source and the second light source comprises a broadband light source.

31. The method according to claim 30, wherein the step of collecting the interference data from the diffuse scattering sample further includes the step of collecting the interference data from the diffuse scattering sample as a function of wavelength.

32. The method according to claim 31, wherein the detector comprises a spectrometer.

33. The method according to claim 19 or 20, wherein the lateral resolution of the first interference data is equal to the lateral resolution of the second interference data.

34. A step of providing a first illumination and a second illumination to a diffuse scattering sample using a light source, wherein the first illumination provides light of a first bandwidth and the second illumination provides light of a second bandwidth that is narrower than the first bandwidth. Using an optical coherence tomography (OCT) optical apparatus, each comprising a sample optical path and a reference optical path that are optically coupled to the light source, the step of generating interference data from the diffusely scattered sample, which includes first interference data acquired using the first illumination and second interference data acquired using the second illumination, The steps include: collecting interference data from the diffuse scattering sample as a function of wavelength using a detector equipped with a spectrometer and optically coupled with the OCT optical device; A step of using a controller to determine the first signal-to-noise ratio (SNR) of the first interference data and the second signal-to-noise ratio (SNR) of the second interference data, The steps include using the controller to switch the illumination provided to the OCT optical device between the first illumination and the second illumination, Equipped with, The ratio of the first bandwidth to the second bandwidth is at least 2. The second signal-to-noise ratio is greater than the first signal-to-noise ratio. The first power level of the first illumination is substantially equal to the second power level of the second illumination. A method for performing optical coherence tomography (OCT).

35. A step of providing a first illumination and a second illumination to a diffuse scattering sample using a light source, wherein the first illumination provides light of a first bandwidth and has a first wavelength-to-time profile and a first power-to-time profile, and the second illumination provides light of a second bandwidth narrower than the first bandwidth using the light source and has a second wavelength-to-time profile and a second power-to-time profile, Using an optical coherence tomography (OCT) optical apparatus, each comprising a sample optical path and a reference optical path that are optically coupled to the light source, the step of generating interference data from the diffusely scattered sample, which includes first interference data acquired using the first illumination and second interference data acquired using the second illumination, The steps include: collecting interference data from the diffuse scattering sample using a detector optically coupled with the OCT optical device; A step of using a controller to determine the first signal-to-noise ratio (SNR) of the first interference data and the second signal-to-noise ratio (SNR) of the second interference data, The steps of controlling the light source to switch the illumination provided to the OCT optical device between the first illumination and the second illumination by controlling at least one of the first wavelength-to-time profile or the first power-to-time profile of the first illumination and at least one of the second wavelength-to-time profile or the second power-to-time profile of the second illumination using the controller, Equipped with, The second signal-to-noise ratio is greater than the first signal-to-noise ratio. The first power level of the first illumination is substantially equal to the second power level of the second illumination. A method for performing optical coherence tomography (OCT).

36. The method according to claim 35, wherein the step of controlling the light source further includes the step of controlling the light source to switch the illumination provided to the OCT optical device to provide a higher signal-to-noise ratio (SNR) or a lower axial resolution, based on controlling at least one of the first wavelength-to-time profile or the first power-to-time profile of the first illumination and controlling at least one of the second wavelength-to-time profile or the second power-to-time profile of the second illumination.

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