Light Optical Coherence Tomography Research Articles

Axial super-resolution optical coherence tomography via complex-valued network

Optical coherence tomography (OCT) is a fast and non-invasive optical interferometric imaging technique that can provide high-resolution cross-sectional images of biological tissues. OCT’s key strength is its depth resolving capability which remains invariant along the imaging depth and is determined by the axial resolution. The axial resolution is inversely proportional to the bandwidth of the OCT light source. Thus, the use of broadband light sources can effectively improve the axial resolution and however leads to an increased cost. In recent years, real-valued deep learning technique has been introduced to obtain super-resolution optical imaging. In this study, we proposed a complex-valued super-resolution network (CVSR-Net) to achieve an axial super-resolution for OCT by fully utilizing the amplitude and phase of OCT signal. The method was evaluated on three OCT datasets. The results show that the CVSR-Net outperforms its real-valued counterpart with a better depth resolving capability. Furthermore, comparisons were made between our network, six prevailing real-valued networks and their complex-valued counterparts. The results demonstrate that the complex-valued network exhibited a better super-resolution performance than its real-valued counterpart and our proposed CVSR-Net achieved the best performance. In addition, the CVSR-Net was tested on out-of-distribution domain datasets and its super-resolution performance was well maintained as compared to that on source domain datasets, indicating a good generalization capability.

Optical Coherence Tomography Angiography (OCT-A) in Uveitis: A Literature Review and a Reassessment of Its Real Role.

The global and precise follow-up of uveitis has become possible with the availability of dual fluorescein (FA) and indocyanine green angiography (ICGA) since the mid-1990s. Progressively, additional non-invasive imaging methods have emerged, bringing value-added precision to the imaging appraisal of uveitis, including, among others, optical coherence tomography (OCT), enhanced-depth imaging OCT (EDI-OCT) and blue light fundus autofluorescence (BAF). More recently, another complementary imaging method, OCT-angiography (OCT-A), further allowed retinal and choroidal circulation to be imaged without the need for dye injection. The purpose of this review was aimed at examining the evidence in published reports indicating whether OCT-A could possibly replace dye angiographic methods, as well as the real practical impact of OCT-A. A literature search in the PubMed database was performed using the terms OCT-angiography and uveitis, OCTA and uveitis and OCT-A and uveitis. Case reports were excluded. Articles were classified into technical reports, research reports and reviews. Articles in the two latter categories were analyzed in a more detailed, individual fashion. Special attention was paid to whether there were arguments in favor of an exclusive rather than complementary use of OCT-A. Furthermore, a synthesis of the main practical applications of OCT-A in the management of uveitis was attempted. Between 2016 (the year of the first articles) and 2022, 144 articles containing the search terms were identified. After excluding case report articles, 114 articles were retained: 4 in 2016, 17 in 2017, 14 in 2018, 21 in 2019, 14 in 2020, 18 in 2021 and 26 in 2022. Seven articles contained technical information or consensus-based terminology. Ninety-two articles could be considered as clinical research articles. Of those, only two hinted in their conclusions that OCT-A could hypothetically replace dye methods. The terms mostly used to qualify the contribution of the articles in this group were "complementary to dye methods", "adjunct", "supplementing" and other similar terms. Fifteen articles were reviews, none of which hinted that OCT-A could replace dye methods. The situations where OCT-A represented a significant practical contribution to the practical appraisal of uveitis were identified. To date, no evidence was found in the literature that OCT-A can replace the classic dye methods; however, it can complement them. Promoting the possibility that non-invasive OCT-A can substitute the invasive dye methods is deleterious, giving the elusive impression that dye methods are no longer inevitable for evaluating uveitis patients. Nevertheless, OCT-A is a precious tool in uveitis research.

Optical coherence tomography refraction and optical path length correction for image-guided corneal surgery.

Optical coherence tomography (OCT) may be useful for guidance of ocular microsurgeries such as deep anterior lamellar keratoplasty (DALK), a form of corneal transplantation that requires delicate insertion of a needle into the stroma to approximately 90% of the corneal thickness. However, visualization of the true shape of the cornea and the surgical tool during surgery is impaired in raw OCT volumes due to both light refraction at the corneal boundaries, as well as geometrical optical path length distortion due to the group velocity of broadband OCT light in tissue. Therefore, uncorrected B-scans or volumes may not provide an accurate visualization suitable for reliable surgical guidance. In this article, we introduce a method to correct for both refraction and optical path length distortion in 3D in order to reconstruct corrected OCT B-scans in both natural corneas and corneas deformed by needle insertion. We delineate the separate roles of phase and group index in OCT image distortion correction, and introduce a method to estimate the phase index from the group index which is readily measured in samples. Using the measured group index and estimated phase index of human corneas at 1060 nm, we demonstrate quantitatively accurate geometric reconstructions of the true cornea and inserted needle shape during simulated DALK surgeries.

Evaluation Through the Optical Coherence Tomography Analysis of the Influence of Non-Alcoholic Fatty Liver Disease on the Gingival Inflammation in Periodontal Patients.

PurposeThe purpose of this ex vivo study is to exhibit the inflammatory changes that occur within the gingival tissue by using optical coherence tomography (OCT) in periodontal patients with non-alcoholic fatty liver disease (NAFLD) and if NAFLD could influence the local periodontal inflammation.Patients and MethodsGingival tissue samples obtained from patients were divided into three groups – P (periodontitis), NAFLD+P (NAFLD+periodontitis) and H (healthy) groups - and were scanned using an OCT light beam, in order to perform a qualitative and quantitative analysis of images. The value of average pixel density has been associated with the degree of inflammation.ResultsThe highest average pixel density was found in patients from the H group, while the lowest value of average pixel density was recorded in gingival tissue samples collected from patients with NAFLD+P. The image assessments from NAFLD+P group delivered lower values of average pixel density than those of P group, suggesting a possible influence of this disease on the inflammatory tissular changes produced by periodontal disease.ConclusionAfter comparing the OCT analysis results obtained for the three groups of patients, we can consider that NAFLD may be an aggravating factor for the inflammation of periodontal disease.

Экспериментальное обоснование кросслинкинга склеры

Purpose. To substantiate the efficacy and safety of scleral collagen cross-linking with riboflavin and ultraviolet radiation. Material and methods. The study was conducted on cadaver porcine eyes and rabbit eyes. Results. It has been experimentally proven that crosslinking with riboflavin / ultraviolet A leads to an increase in the biomechanical strength of the scleral tissue, changing its structure and leading to an increase in the packing density of collagen fibers and an increase in the diameter of collagen fibrils, which confirms the formation of additional cross-links between scleral macromolecules. The absence of any pathological changes in the fibers of the fibrous membrane of the eye was established. The permeability of scleral tissue for ultraviolet radiation of the A range was determined. Based on the data obtained, a formula was proposed for calculating the threshold value of the intensity of ultraviolet radiation during the SCXL procedure. Using the methods of light microscopy, electroretinography and optical coherence tomography, it was revealed that SCXL with riboflavin / ultraviolet A under the studied parameters is safe for the structures of the eye (cornea, retina and optic nerve). A gentle technology for photopolymerization of the sclera with the use of the “UFalink S” device has been developed, the advantage of which is the ability to carry out ultraviolet irradiation in hard-to-reach areas of the eyeball. At the same time, an effective increase in the biomechanical strength characteristics of the scleral tissue was proved and the absence of a damaging effect on the structures of the eye was demonstrated. Conclusion. A clinical study of the scleral crosslinking procedure with riboflavin / ultraviolet A with the proposed parameters: irradiance of 3 mW / cm², irradiation time 30 minutes and preliminary saturation of the sclera with 0.1% aqueous solution of riboflavin for 20 minutes, which in experiments have shown their effectiveness and safety, is recommended. During the procedure, it is recommended to use a water-based photosensitizer riboflavin, without the addition of dextran, which leads to dehydration of the sclera and an increase in its permeability to ultraviolet light, which increases the risk of damage to the inner layers of the eye. Key words: scleral crosslinking, ultraviolet A, riboflavin, experiment, myopia.

Impact of melanin on choroidal measurements

The thickness of the choroid is measured on the optical coherence tomography (OCT) as the distance between the hyperreflective retinal pigment epithelium and the hyperreflective chorio-scleral junction, contributed by the choroidal vessels and choroidal melanocytes. Our hypothesis is that the amount of absorption and reflectance of the infrared wavelength OCT light by the melanin present in the choroidal melanocytes contribute to the visibility of the choroidal structures and choroidal thickness on OCT. We explain this hypothesis via two representative cases with choroidal pigmentary abnormalities. In conclusion, we believe that the optical properties of the melanin in choroid influences choroidal measurements and may lead to inaccurate assessment. Therefore, further understanding about choroidal pigmentation is necessary before we further adapt choroidal biomarkers in our clinical practice.

Real-time co-localized OCT surveillance of laser therapy using motion corrected speckle decorrelation.

We present a system capable of real-time delivery and monitoring of laser therapy by imaging with optical coherence tomography (OCT) through a double-clad fiber (DCF). A double-clad fiber coupler is used to inject and collect OCT light into the core of a DCF and inject the therapy light into its larger inner cladding, allowing for both imaging and therapy to be perfectly coregistered. Monitoring of treatment depth is achieved by calculating the speckle intensity decorrelation occurring during tissue coagulation. Furthermore, an analytical noise correction was used on the correlation to extend the maximum monitoring depth. We also present a method for correcting motion-induced decorrelation using a lookup table. Using the value of the noise- and motion-corrected correlation coefficient in a novel approach, our system is capable of identifying the depth of thermal coagulation in real time and automatically shut the therapy laser off when the targeted depth is reached. The process is demonstrated ex vivo in rat tongue and abdominal muscles for depths ranging from 500 µm to 1000 µm with induced motion in real time.

Development of a broadband superluminescent diode based on self-assembled InAs quantum dots and demonstration of high-axial-resolution optical coherence tomography imaging

We developed a near-infrared (NIR) superluminescent diode (SLD) based on self-assembled InAs quantum dots (QDs) and demonstrated high-axial-resolution optical coherence tomography (OCT) imaging using this QD-based SLD (QD-SLD). The QD-SLD utilized InAs QDs with controlled emission wavelengths as a NIR broadband light emitter, and a tilted waveguide with segmented electrodes was prepared for edge-emitting broadband electroluminescence (EL) spanning approximately 1–1.3 μm. The bandwidth of the EL spectrum was increased up to 144 nm at a temperature of 25 °C controlled using a thermoelectric cooler. The inverse Fourier transform of the EL spectrum predicted a minimum resolution of 3.6 μm in air. The QD-SLD was subsequently introduced into a spectral-domain (SD)-OCT setup, and SD-OCT imaging was performed for industrial and biological test samples. The OCT images obtained using the QD-SLD showed an axial resolution of ~4 μm, which was almost the same as that predicted from the spectrum. This axial resolution is less than the typical size of a single biological cell (~5 μm), and the practical demonstration of high-axial-resolution OCT imaging shows the application of QD-SLDs as a compact OCT light source, which enables the development of a portable OCT system.

Impact of low tissue backscattering by optical coherence tomography on endothelial function after drug-eluting stent implantation.

This study evaluated the impact of optical coherence tomography (OCT)-derived low-backscattered tissue on mid-term coronary endothelial function after drug-eluting stent (DES) implantation. Although OCT enables detailed in vivo evaluation of neointimal tissue characterization after DES implantation, its association with physiological vascular healing response is unclear. Thirty-three stable angina pectoris patients underwent OCT examination and endothelial function testing with intracoronary infusion of incremental doses of acetylcholine 8-month after DES implantation in a single lesion of the left anterior descending artery. Neointimal tissue was classified into two patterns based on the predominant OCT light backscatter: high backscatter and low backscatter. Although the presence of uncovered or malapposed stent strut was not associated with the degree of vasoconstriction, the degree of vasoconstriction was significantly greater in the DES with low-backscattered neointima than in the DES without low-backscattered neointima (- 32.1 ± 25.7 vs. - 4.1 ± 20.1%, p = 0.003). Moreover, there was an inverse linear relationship between low backscatter tissue index and degree of vasoconstriction after acetylcholine infusion (r = 0.50 and p = 0.003). The endothelium-dependent vasomotor response after 8-month of DES was impaired in patients with low neointimal tissue backscatter on OCT imaging. OCT assessment of low-backscattered tissue may be used as surrogate markers for impairment of endothelial function after DES.

Superluminescent diode with a broadband gain based on self-assembled InAs quantum dots and segmented contacts for an optical coherence tomography light source

We report a broadband-gain superluminescent diode (SLD) based on self-assembled InAs quantum dots (QDs) for application in a high-resolution optical coherence tomography (OCT) light source. Four InAs QD layers, with sequentially shifted emission wavelengths achieved by varying the thickness of the In0.2Ga0.8As strain-reducing capping layers, were embedded in a conventional p-n heterojunction comprising GaAs and AlGaAs layers. A ridge-type waveguide with segmented contacts was formed on the grown wafer, and an as-cleaved 4-mm-long chip (QD-SLD) was prepared. The segmented contacts were effective in applying a high injection current density to the QDs and obtaining emission from excited states of the QDs, resulting in an extension of the bandwidth of the electroluminescence spectrum. In addition, gain spectra deduced with the segmented contacts indicated a broadband smooth positive gain region spanning 160 nm. Furthermore, OCT imaging with the fabricated QD-SLD was performed, and OCT images with an axial resolution of ∼4 μm in air were obtained. These results demonstrate the effectiveness of the QD-SLD with segmented contacts as a high-resolution OCT light source.

New Developments in Murine Imaging for Assessing Photoreceptor Degeneration In Vivo.

Optical Coherence Tomography (OCT) is a powerful clinical tool that measures near infrared light backscattered from the eye and other tissues. OCT is used for assessing changes in retinal structure, including layer thicknesses, detachments and the presence of drusen in patient populations. Our custom-built OCT system for the mouse eye quantitatively images all layers of the neural retinal, the RPE, Bruchs' membrane and the choroid. Longitudinal assessment of the same retinal region reveals that the relative intensities of retinal layers are highly stable in healthy tissue, but show progressive increases in intensity in a model of retinal degeneration. The observed changes in OCT signal have been correlated with ultrastructural disruptions that were most dramatic in the inner segments and nuclei of the rods. These early changes in photoreceptor structure coincided with activation of retinal microglia, which migrated vertically from the inner to the outer retina to phagocytose photoreceptor cell bodies (Levine et al., Vis Res 102:71-79, 2014). We conclude that quantitative analysis of OCT light scattering signals may be a useful tool for early detection and subcellular localization of cell stress prior to cell death, and for assessing the progression of degenerative disease over time. Future efforts to develop sensitive approaches for monitoring microglial dynamics in vivo may likewise elucidate earlier signs of cellular stress during retinal degeneration.

Miniature probe integrating optical-resolution photoacoustic microscopy, optical coherence tomography, and ultrasound imaging: proof-of-concept.

In this Letter, we present a novel tri-modal miniature side-view probe, through which optical-resolution photoacoustic microscopy (OR-PAM), optical coherence tomography (OCT), and pulse-echo ultrasound (US) images can be coaxially acquired and displayed simultaneously. The probe consists of a common optical path for OR-PAM (light delivery) and OCT (light delivery/detection), and a 40-MHz unfocused ultrasound transducer for OR-PAM (photoacoustic detection) and US (ultrasound transmission/receiving) with an overall diameter of 2mm. Combining OR-PAM, OCT, and US would provide complementary information including optical absorption (OR-PAM), optical back-scattering (OCT), and deep tissue structures (US) about biological tissue. Based on an integrated imaging system consisting of OR-PAM, time-domain OCT, and US, phantom images and in vivo images of rat ear were acquired to demonstrate the capabilities of the integrated tri-modality imaging probe. The probe yields a lateral resolution of 13.6μm for OR-PAM and OCT, and an axial resolution of 43μm for OR-PAM and US. Currently, for a scanning area of 1 ×1 mm, it took ∼25 min to acquire data for tri-modal volumetric imaging.

Ultra-thin and flexible endoscopy probe for optical coherence tomography based on stepwise transitional core fiber

We present an ultra-thin fiber-body endoscopy probe for optical coherence tomography (OCT) which is based on a stepwise transitional core (STC) fiber. In a minimalistic design, our probe was made of spliced specialty fibers that could be directly used for beam probing optics without using a lens. In our probe, the OCT light delivered through a single-mode fiber was efficiently expanded to a large mode field of 24 μm diameter for a low beam divergence. The size of our probe was 85 μm in the probe's diameter while operated in a 160-μm thick protective tubing. Through theoretical and experimental analyses, our probe was found to exhibit various attractive features in terms of compactness, flexibility and reliability along with its excellent fabrication simplicity.

Lens-free endoscopy probe for optical coherence tomography

We present an ultrathin fiber-optic endoscopy probe for optical coherence tomography (OCT), which is made of a series of fused optical fibers instead of the conventional scheme based on an objective lens. The large-core fiber with a core diameter of 20 μm was utilized for the probe, while a single-mode fiber of core diameter 8.2 μm mainly delivered the OCT light. Those fibers were spliced with a bridge fiber of an intermediate core size. The guided light was stepwise converted to a beam of a large mode-field diameter to be radiated with a larger depth of focus. We obtained a 125 μm thick all-fiber endoscopy probe with a side-viewing capability implemented by an angled fiber end. Successful OCT imaging was demonstrated with a swept-source OCT system and showed the practical applicability of our lens-free endoscopy probe.

Different phenotypes of the appearance of the outer plexiform layer on optical coherence tomography

To present a selected case series of different phenotypes of the normal outer plexiform layer (OPL) visualized by optical coherence tomography (OCT). Five cases were selected to represent the spectrum of appearances of the OPL in this case series. Categorical descriptions of each manifestation were then developed. Additional SD-OCT scans were obtained from a normal volunteer to further support the hypothesis. The inner one-third of the OPL typically appears hyperreflective on OCT, while the outer two-thirds (Henle fiber layer) may have a more varied appearance. Six different phenotypes of Henle fiber layer reflectivity were noted in this series, and classified as: bright, columnar, dentate, delimited, indistinct, and dark. The brightness of the Henle fiber layer appears to depend on the geometric angle between the OCT light beam and the axonal fibers in this portion of the OPL. This angle appears to be a function of the natural orientation of the Henle fiber layer tissue (θN), the existence of subretinal pathology that alters the angle of the neurosensory retina (θ(P)), and the tilt angle of the tissue on the B-scan (θ(T)) due to decentered OCT acquisition. Since accurate interpretation of the OPL/ONL boundary is of vital importance to study the thickness of ONL, location of cystoid lesions, hyperreflective crescents over drusen, et al., our case series may aid better understanding of the OPL appearance in SD-OCT. In the absence of clear delineation, it may be most correct to refer to indistinct OPL and ONL together as the photoreceptor nuclear axonal complex (PNAC).

Miniature probe combining optical-resolution photoacoustic microscopy and optical coherence tomography for in vivomicrocirculation study

Photoacoustic microscopy (PAM) is sensitive to optical absorption, while optical coherence tomography (OCT) is based on optical backscattering. Combining PAM and OCT can provide complementary information about biological tissue. Here we present a combined optical-resolution PAM (ORPAM) and OCT system that is integrated through a miniature probe with an overall diameter of 2.3 mm, suitable for insertion through a standard endoscopic or laparoscopic port during minimally invasive surgery or endoscopic exam. The hybrid probe consists of a common optical path for OCT (light delivery/detection) and ORPAM (light excitation) and a 10 MHz unfocused ultrasound transducer for photoacoustic detection. The combined system yields a lateral resolution of 15 μm for both ORPAM and OCT.

Integrated multimodal endomicroscopy platform for simultaneous en face optical coherence and two-photon fluorescence imaging

We report an all-fiber-optic scanning, multimodal endomicroscope capable of simultaneous optical coherence tomography (OCT) and two-photon fluorescence (TPF) imaging. Both imaging modalities share the same miniature fiber-optic scanning endomicroscope, which consists of a double-clad fiber with a core operating in single mode at both the OCT (1310 nm) and two-photon excitation (1550 nm) wavelengths, a piezoelectric two-dimensional fiber-optic beam scanner, and a miniature aspherical compound lens suitable for simultaneous acquisition of en face OCT and TPF images. A fiber-optic wavelength division multiplexer was employed in the integrated platform to combine the low coherence OCT light source and the femtosecond two-photon excitation laser into the same optical path. Preliminary imaging results of cell cultures and mouse tissue ex vivo demonstrate the feasibility of simultaneous real-time OCT and TPF imaging in a scanning endomicroscopy setting for the first time.

Optimization of Bi2O3–B2O3-based glass phosphor co-doped with Yb3+ and Nd3+ for optical coherence tomography light source

We have optimized Yb2O3 concentration, Nd2O3 concentration, and thickness of a Yb3+, Nd3+ co-doped Bi2O3–B2O3-based glass phosphor to achieve a higher output power with a Gaussian-like shaped luminescence for an optical coherence tomography (OCT) light source. A simple model that includes absorption of excitation light, absorption of luminescent light, and luminescence efficiency was used for estimation of the output power. We have estimated from the model that the highest output power was achieved at Yb2O3 concentration of 3%, Nd2O3 concentration of 0.5%, and the thickness of 1.8 mm. This estimation showed a good agreement with the experimental results. For the optimized glass phosphor, we have observed Gaussian-like shaped luminescence, which is important for the OCT light source.

Reduction of image artifacts in three-dimensional optical coherence tomography of skin in vivo

This paper presents results of in vivo studies on the effect of refractive index-matching media on image artifacts in optical coherence tomography (OCT) images of human skin. These artifacts present as streaks of artificially low backscatter and displacement or distortion of features. They are primarily caused by refraction and scattering of the OCT light beam at the skin surface. The impact of the application of glycerol and ultrasound gel is assessed on both novel skin-mimicking phantoms and in vivo human skin, including assessment of the epidermal thickening caused by the media. Based on our findings, recommendations are given for optimal OCT imaging of skin in vivo.

Non-destructive quantification of pharmaceutical tablet coatings using terahertz pulsed imaging and optical coherence tomography

Optical coherence tomography (OCT) and terahertz pulsed imaging (TPI) are two powerful techniques allowing high quality cross-sectional images from within scattering media to be obtained non-destructively. In this paper, we report experimental results of using OCT and TPI for quantitatively characterizing pharmaceutical tablet coatings in the thickness range of 10–140 μm. We found that the spectral OCT system developed in-house has an axial resolution of 0.9 μm, and is capable of quantifying very thin coatings in the range of 10–60 μm. The upper limit of 60 μm within the tablet coating and core is owed to the strong scattering of OCT light, which has relatively short wavelengths in the range of 0.5–1.0 μm. On the other hand, TPI utilizes terahertz radiation that has substantially long wavelengths in the range of hundreds of microns, and thus is less prone to the scattering problem. Consequently TPI has been demonstrated to be able to quantify thicker coatings in the range of 40–140 μm and beyond. We concluded that OCT and TPI are two complementary analytical techniques for non-destructive and quantitative characterization of pharmaceutical tablet coatings.