Optical Coherence Tomography System Research Articles

Towards non-invasive tissue hydration measurements with optical coherence tomography.

The attenuation coefficient ( ) measured by optical coherence tomography (OCT) has been used to determine tissue hydration. Previous dual-wavelength OCT systems could not attain the needed precision, which we attribute to the absence of wavelength-dependent scattering of tissue in the underlying model. Assuming that scattering can be described using two parameters, we propose a triple/quadrupole-OCT system to achieve clinically relevant precision in water volume fraction. In this study, we conduct a quantitative analysis to determine the necessary precision of measurements and compare it with numerical simulation. Our findings emphasize that achieving a clinically relevant assessment of a 2% water fraction requires determining the attenuation coefficient with a remarkable precision of 0.01 . This precision threshold is influenced by the chosen wavelength for attenuation measurement and can be enhanced through the inclusion of a fourth wavelength range.

Microscope-integrated optical coherence tomography for in vivo human brain tumor detection with artificial intelligence.

It has been shown that optical coherence tomography (OCT) can identify brain tumor tissue and potentially be used for intraoperative margin diagnostics. However, there is limited evidence on its use in human in vivo settings, particularly in terms of its applicability and accuracy of residual brain tumor detection (RTD). For this reason, a microscope-integrated OCT system was examined to determine in vivo feasibility of RTD after resection with automated scan analysis. Healthy and diseased brain was 3D scanned at the resection edge in 18 brain tumor patients and investigated for its informative value in regard to intraoperative tissue classification. Biopsies were taken at these locations and labeled by a neuropathologist for further analysis as ground truth. Optical OCT properties were obtained, compared, and used for separation with machine learning. In addition, two artificial intelligence-assisted methods were utilized for scan classification, and all approaches were examined for RTD accuracy and compared to standard techniques. In vivo OCT tissue scanning was feasible and easily integrable into the surgical workflow. Measured backscattered light signal intensity, signal attenuation, and signal homogeneity were significantly distinctive in the comparison of scanned white matter to increasing levels of scanned tumor infiltration (p < 0.001) and achieved high values of accuracy (85%) for the detection of diseased brain in the tumor margin with support vector machine separation. A neuronal network approach achieved 82% accuracy and an autoencoder approach 85% accuracy in the detection of diseased brain in the tumor margin. Differentiating cortical gray matter from tumor tissue was not technically feasible in vivo. In vivo OCT scanning of the human brain has been shown to contain significant value for intraoperative RTD, supporting what has previously been discussed for ex vivo OCT brain tumor scanning, with the perspective of complementing current intraoperative methods for this purpose, especially when deciding to withdraw from further resection toward the end of the surgery.

Zonular instability-associated morphologic features in eyes with primary angle closure disease using the swept-source anterior segment – optical coherence tomography system

BackgroundThis study aims to investigate the morphologic features of the crystalline lens in Primary Angle Closure Disease (PACD) patients with zonular instability during cataract surgery using the swept-source CASIA 2 Anterior Segment-Optical Coherence Tomography (AS-OCT) system.MethodsA total of 398 eyes (125 PACD eyes with zonular instability, 133 PACD eyes with zonular stability, and 140 cataract patient controls) of 398 patients who underwent cataract surgery combined or not glaucoma surgery between January 2021 and January 2023 were enrolled. The crystalline lens parameters were measured by CASIA2 AS-OCT. Then, logistic regression was performed to evaluate the risk factors associated with zonular instability.ResultsThe results revealed that PACD eyes had a more anterior lens equator position, a steeper anterior curvature of lens, shorter Axial Length (AL), shallower Anterior Chamber Distance (ACD), higher Lens Vault (LV) and thicker Lens Thickness (LT), when compared to eyes in the cataract control group. Furthermore, PACD eyes in the zonular instability group had steeper front R, front Rs and Front Rf, flatter back Rf, thicker lens anterior part thickness, higher lens anterior-to-posterior part thickness ratios, shallower ACD, and greater LV, when compared to PACD eyes with zonular stability. The logistic regression analysis, which was adjusted for age and gender, revealed that zonular instability was positively correlated with anterior part thickness, lens anterior-to-posterior part thickness ratio, and LV, but was negatively correlated with lens anterior radius and ACD.ConclusionSteeper anterior curvature, increased lens anterior part thickness, higher anterior-to-posterior part thickness ratio, shallower ACD, and greater LV are the anatomic features of PACD eyes associated with zonular instability.

Direct Numerical Modeling as a Tool for Optical Coherence Tomography Development: SNR (Sensitivity) and Lateral Resolution Test Target Interpretation

Optical Coherence Tomography (OCT) is a growing family of biophotonic imaging techniques, but in the literature there is a lack of easy-to-use tools to universally directly evaluate a device’s theoretical performance for a given metric. Modern computing tools mean that direct numerical modeling can, from first principles, simulate the performance metrics of a specific device directly without relying on analytical approximations and/or complexities. Here, we present two different direct numerical models, along with the example MATLAB code for the reader to adapt to their own systems. The first model is of photo-electron shot noise at the detector, the primary noise source for OCT. We use this firstly to evaluate the amount of additional noise present (1.5 dB) for an experimental setup. Secondly, we demonstrate how to use it to precisely quantify the expected shot noise SNR limit difference between time-domain and Fourier-domain OCT systems in a given hypothetical experiment. The second model is used to demonstrate how USAF 1951 test chart images should be interpreted for a given lateral PSF shape. Direct numerical modeling is an easy and powerful basic tool for researchers and developers, the wider use of which may improve the rigor of the OCT literature.

Delineation of three-dimensional tumor margins based on normalized absolute difference mapping via volumetric optical coherence tomography

The extent of surgical resection is an important prognostic factor in the treatment of patients with glioblastoma. Optical coherence tomography (OCT) imaging is one of the adjunctive methods available to achieve the maximal surgical resection. In this study, the tumor margins were visualized with the OCT image obtained from a murine glioma model. A commercialized human glioblastoma cell line (U-87) was employed to develop the orthotopic murine glioma model. A swept-source OCT (SS-OCT) system of 1300 nm was used for three-dimensional imaging. Based on the OCT intensity signal, which was obtained via accumulation of each A-scan data, an en-face optical attenuation coefficient (OAC) map was drawn. Due to the limited working distance of the focused beam, OAC values decrease with depth, and using the OAC difference in the superficial area was chosen to outline the tumor boundary, presenting a challenge in analyzing the tumor margin along the depth direction. To overcome this and enable three-dimensional tumor margin detection, we converted the en-face OAC map into an en-face difference map with x- and y-directions and computed the normalized absolute difference (NAD) at each depth to construct a volumetric NAD map, which was compared with the corresponding H&E-stained image. The proposed method successfully revealed the tumor margin along the peripheral boundaries as well as the margin depth. We believe this method can serve as a useful adjunct in glioma surgery, with further studies necessary for real-world practical applications.

Dynamic refraction and anterior segment OCT biometry during accommodation

Accommodation is the process by which the eye changes focus. These changes are the result of changes to the shape of the crystalline lens. Few prior studies have quantified the relation between lens shape and ocular accommodation, primarily at discrete static accommodation states. We present an instrument that enables measurements of the relation between changes in lens shape and changes in optical power continuously during accommodation. The system combines an autorefractor to measure ocular power, a visual fixation target to stimulate accommodation, and an optical coherence tomography (OCT) system to image the anterior segment and measure ocular distances. Measurements of ocular dimensions and refraction acquired dynamically on three human subjects are presented. The individual accommodative responses are analyzed to correlate the ocular power changes with changes in ocular dimensions.

Handheld contact-type OCT and color fundus system for retinal imaging.

We present proof of concept for a handheld contact-type system capable of simultaneous optical coherence tomography (OCT) imaging of the retina and wide-field digital fundus color photography. The study focuses on demonstrating the feasibility of the proposed approach, particularly for eventual use in pediatric patients during examination under anesthesia in the operating room and in the neonatal intensive care unit. Direct contact of the probe with the cornea allows the photographer to maintain a stable position during imaging, reducing motion artifacts in the OCT images. Additionally, it simplifies the alignment process and increases the field of view of the optics. By integrating OCT and fundus imaging into a single device, the proposed compact modular design eliminates the need for separate, space-consuming systems dedicated to each imaging modality.

Algorithm and software for field distortion correction in a commercial SD-OCT for corneal curvature assessment.

Accurate assessment of corneal curvatures using frequency domain optical coherence tomography (OCT) with galvanometer scanners remains challenging due to the well-known scan field distortion. This paper presents an algorithm and software for correcting the distortion using only two simple measurements in which a readily available standard sphere is positioned in different depths in front of the OCT scanner. This offers a highly accessible and easily reproducible method for the field distortion correction (FDC). The correction was validated by measuring different spherical phantoms and conducting corneal curvature measurements of ex vivo porcine corneas using a commercial spectral-domain OCT system and a clinically approved swept-source OCT as a reference instrument. Thus, the error in radius measurements of spherical phantoms was reduced by >90% and astigmatism by >80% using FDC. In explanted porcine eyes, the error in astigmatism measurements with the Telesto was reduced by 75% for power and 70% for angle. The best fitting sphere radius was determined up to a deviation of 0.4% from the Anterion. This paper describes a correction algorithm for OCT immanent distortion that is applicable to any scanning OCT setup and enables precise corneal curvature measurements. The MATLAB software for the FDC is publicly available on GitHub.

Combined Terahertz Pulsed Imaging and Optical Coherence Tomography Detection Method for Multiple Defects in Thermal Barrier Coatings

While thermal barrier coatings (TBCs) are being sprayed onto aero-engine turbine blades, or while the engine blade is working, high temperatures and strong impact forces will damage TBCs under thermal cycles, resulting in the coating peeling off from the blades. The current method of using ECT, IRT, or another method alone cannot achieve the real-time detection of coating defects with both high precision and high penetration power. Two detection methods, namely, terahertz pulsed imaging (TPI) and optical coherence tomography (OCT), were combined to evaluate typical defects observed in TBCs (including internal debonding cracks, surface high-temperature cracks, and surface etched cracks). The results showed that the OCT system successfully obtained the micron-level axial resolution, but the detection depth of the OCT system was limited. The TPI system achieved a higher penetration depth than OCT—hence, it can be used for the nondestructive detection and evaluation of the internal debonding defects in the sample—but its resolution needs to be improved. Following this conclusion, a method is proposed using TPI and OCT concurrently for the nondestructive testing and quantitative evaluation of TBCs on etched cracks, thus achieving progress both in terms of depth and resolution. In our experiment, defects with a depth of 519 μm and a width of 100 μm were measured. The proposed method is suitable for situations where multiple defects in TBC samples of blades need to be detected simultaneously during the working process. When there are defects deep inside the sample, more small cracks on the surface can be evaluated to achieve a combination of depth and accuracy.

RobOCTNet: Robotics and Deep Learning for Referable Posterior Segment Pathology Detection in an Emergency Department Population.

To evaluate the diagnostic performance of a robotically aligned optical coherence tomography (RAOCT) system coupled with a deep learning model in detecting referable posterior segment pathology in OCT images of emergency department patients. A deep learning model, RobOCTNet, was trained and internally tested to classify OCT images as referable versus non-referable for ophthalmology consultation. For external testing, emergency department patients with signs or symptoms warranting evaluation of the posterior segment were imaged with RAOCT. RobOCTNet was used to classify the images. Model performance was evaluated against a reference standard based on clinical diagnosis and retina specialist OCT review. We included 90,250 OCT images for training and 1489 images for internal testing. RobOCTNet achieved an area under the curve (AUC) of 1.00 (95% confidence interval [CI], 0.99-1.00) for detection of referable posterior segment pathology in the internal test set. For external testing, RAOCT was used to image 72 eyes of 38 emergency department patients. In this set, RobOCTNet had an AUC of 0.91 (95% CI, 0.82-0.97), a sensitivity of 95% (95% CI, 87%-100%), and a specificity of 76% (95% CI, 62%-91%). The model's performance was comparable to two human experts' performance. A robotically aligned OCT coupled with a deep learning model demonstrated high diagnostic performance in detecting referable posterior segment pathology in a cohort of emergency department patients. Robotically aligned OCT coupled with a deep learning model may have the potential to improve emergency department patient triage for ophthalmology referral.

Quantitative evaluation of keyhole stability during laser welding using optical coherence tomography

Online monitoring of keyhole during laser welding is of great importance to evaluate the stability of the live process and the final quality of the weld. Indirect methods based on visual imaging and thermal radiation are lack of strong correlations with keyhole behavior, resulting in a low accuracy. Optical coherence tomography (OCT) shoots beam coaxially with the processing beam into the bottom of the keyhole, enabling direct monitoring of keyhole behavior. However, current OCT keyhole monitoring systems suffer from the harsh noise and are limited to only evaluating the keyhole depth through statistical analysis. In this paper, an enhanced OCT system was built with a capacity of capturing keyhole open/collapse behavior, opening a way of quantitatively evaluating the keyhole behavior. First, the signal was denoised and filled given the nature of fluid dynamics in time domain. Then an algorithm was developed to automatically detect the keyhole open/collapse behavior, based on which the weld status (heat conduction mode and keyhole mode) is determined, and the metrics of interests are characterized quantitatively. The proposed method provides a solid method to quantitatively evaluate the keyhole stability, which is well suited for developing new laser welding process and monitoring laser involved high volume manufacturing.

Long-range optical coherence tomography of pediatric airway during drug induced sleep endoscopy: A preliminary report

ObjectiveDrug induced sleep endoscopy (DISE) is often performed for pediatric obstructive sleep apnea (OSA) when initial diagnostic studies do not provide adequate information for therapy. However, DISE scoring is subjective and with limitations. This proof-of-concept study demonstrates the use of a novel long-range optical coherence tomography (LR-OCT) system during DISE of two pediatric patients. MethodsLR-OCT was used to visualize the airway of pediatric patients during DISE. At the conclusion of DISE, the OCT probe was guided in the airway under endoscopic visual guidance, and cross-sectional images were acquired at the four VOTE locations. Data processing involved image resizing and alignment, followed by rendering of three-dimensional (3D) volumetric models of the airways. ResultsTwo patients were included in this study. Patient one had 18.4%, 20.9%, 72.3%, and 97.3% maximal obstruction at velum, oropharynx, tongue base, and epiglottis, while patient two had 40.2%, 41.4%, 8.0%, and 17.5% maximal obstruction at these regions, respectively. Three-dimensional reconstructions of patients’ airways were also constructed from the OCT images. ConclusionThis proof-of-concept study demonstrates the successful evaluation of pediatric airway during DISE using LR-OCT, which accurately identified sites and degrees of obstruction with respective 3D airway reconstruction.

Optical coherence tomography findings of balloon angioplasty/stenting for in-stent restenosis after carotid artery stenting.

The optimal therapeutic methods for in-stent restenosis (ISR) after carotid artery stenting (CAS) remains controversial. This study aimed to use optical coherence tomography (OCT) to evaluate the in-stent architectures during endovascular angioplasty/stenting for ISR. Six lesions of ISR after CAS were evaluated by OCT during endovascular angioplasty/stenting. In one lesion, the OCT system could not be crossed because of elongation distal to the ISR lesion. In five lesions, pre-procedural OCT clearly revealed neointimal hyperplasia or neoatherosclerosis. The mean in-stent area stenosis was 84%. After regular balloon angioplasty, tissue compression and dissection of various sizes and layers were detected. After balloon angioplasty (with a mean balloon size of 5.4 mm), the minimum lumen area (from 1.7 ± 0.6 to 11.4 ± 5.3 mm2, p < 0.01) and the minimum in-stent area (12.7 ± 2.6 to 21.8 ± 5.0 mm2, p < 0.01) showed a significant increase. Additional stent was placed in one lesion that developed into a flap by dissection after balloon angioplasty. In another lesion in which sufficient dilatation was not achieved by balloon angioplasty, a major stroke occurred by acute occlusion of the ISR lesion 10 months later. OCT can detect the in-stent architecture of ISR lesions after balloon angioplasty and additional stent placement. However, which dissection should be treated by additional stent remain problematic.

Computer-Aided Discrimination of Glaucoma Patients from Healthy Subjects Using the RETeval Portable Device.

Glaucoma is a chronic, progressive eye disease affecting the optic nerve, which may cause visual damage and blindness. In this study, we present a machine-learning investigation to classify patients with glaucoma (case group) with respect to normal participants (control group). We examined 172 eyes at the Ophthalmology Clinic of the "Elpis" General Hospital of Athens between October 2022 and September 2023. In addition, we investigated the glaucoma classification in terms of the following: (a) eye selection and (b) gender. Our methodology was based on the features extracted via two diagnostic optical systems: (i) conventional optical coherence tomography (OCT) and (ii) a modern RETeval portable device. The machine-learning approach comprised three different classifiers: the Bayesian, the Probabilistic Neural Network (PNN), and Support Vectors Machines (SVMs). For all cases examined, classification accuracy was found to be significantly higher when using the RETeval device with respect to the OCT system, as follows: 14.7% for all participants, 13.4% and 29.3% for eye selection (right and left, respectively), and 25.6% and 22.6% for gender (male and female, respectively). The most efficient classifier was found to be the SVM compared to the PNN and Bayesian classifiers. In summary, all aforementioned comparisons demonstrate that the RETeval device has the advantage over the OCT system for the classification of glaucoma patients by using the machine-learning approach.

Development of a field-ready optical coherence tomography system for assessing 2,4-D dimethylammonium-based herbicide exposure in African catfish

Development of a field-ready optical coherence tomography system for assessing 2,4-D dimethylammonium-based herbicide exposure in African catfish

Virtual Hall sensor triggered multi-MHz endoscopic OCT imaging for stable real-time visualization.

Circumferential scanning in endoscopic imaging is crucial across various disciplines, and optical coherence tomography (OCT) is often the preferred choice due to its high-speed, high-resolution, and micron-scale imaging capabilities. Moreover, real-time and high-speed 3D endoscopy is a pivotal technology for medical screening and precise surgical guidance, among other applications. However, challenges such as image jitter and non-uniform rotational distortion (NURD) are persistent obstacles that hinder real-time visualization during high-speed OCT procedures. To address this issue, we developed an innovative, low-cost endoscope that employs a brushless DC motor for scanning, and a sensorless technique for triggering and synchronizing OCT imaging with the scanning motor. This sensorless approach uses the motor's electrical feedback (back electromotive force, BEMF) as a virtual Hall sensor to initiate OCT image acquisition and synchronize it with a Fourier Domain Mode-Locked (FDML)-based Megahertz OCT system. Notably, the implementation of BEMF-triggered OCT has led to a substantial reduction in image jitter and NURD (<4 mrad), thereby opening up a new window for real-time visualization capabilities. This approach suggests potential benefits across various applications, aiming to provide a more accurate, deployable, and cost-effective solution. Subsequent studies can explore the adaptability of this system to specific clinical scenarios and its performance under practical endoscopic conditions.

A dual-channel visible light optical coherence tomography system enables wide-field, full-range, and shot-noise limited human retinal imaging

Visible light optical coherence tomography (VIS-OCT) is an emerging ophthalmic imaging method featuring ultrahigh depth resolution, retinal microvascular oximetry, and distinct scattering contrast in the visible spectral range. The clinical utility of VIS-OCT is hampered by the fundamental trade-off between the imaging depth range and axial resolution, which are determined by the spectral resolution and bandwidth, respectively. To address this trade-off, here we developed a dual-channel VIS-OCT system with three major advancements including the first linear-in-K VIS-OCT spectrometer to decrease the roll-off, reference pathlength modulation to expand the imaging depth range, and per-A-line noise cancellation to remove excess noise, Due to these unique designs, this system achieves 7.2 dB roll-off over the full 1.74 mm depth range (water) with shot-noise limited performance. The system uniquely enables >60° wide-field imaging which would allow simultaneous imaging of the peripheral retina and optic nerve head, as well as ultrahigh 1.3 µm depth resolution (water). Benefiting from the additional near-infrared (NIR) channel of the dual-channel design, this system is compatible with Doppler OCT and OCT angiography (OCTA). The comprehensive structure-function measurement enabled by this dual-channel VIS-OCT system is an advance towards adoption of VIS-OCT in clinical applications.

MCOCT: an experimentally and numerically validated, open-source Monte Carlo simulator for optical coherence tomography.

Here, we present MCOCT, a Monte Carlo simulator for optical coherence tomography (OCT), incorporating a Gaussian illumination scheme and bias to increase backscattered event collection. MCOCT optical fluence was numerically compared and validated to an established simulator (MCX) and showed concordance at the focus while diverging slightly with distance to it. MCOCT OCT signals were experimentally compared and validated to OCT signals acquired in tissue-mimicking phantoms with known optical properties and showed a similar attenuation pattern with increasing depth while diverging beyond 1.5 mm and proximal to layer interfaces. MCOCT may help in the design of OCT systems for a wide range of applications.

Development of handheld optical coherence tomography based on commercial intra‐oral scanner shape for extended clinical utility in dentistry

AbstractThe main objective of this study is to develop a handheld dental optical coherence tomography (OCT) system capable of imaging the target area of the teeth in the oral cavity and demonstrate the applicability of the developed system to in vivo dental disease diagnosis based on a user‐friendly scanner form. The design of the developed system is based on mimicking the shape of a commonly used commercial intra‐oral scanner (IOS) so that the system users do not feel awkward when using it. A microelectromechanical system (MEMS) scanning unit was used to optimize the overall size of the system, and the optical axis was designed in a Z‐shape to maximize the beam size reflected by the MEMS mirror. A pre‐study of in vivo oral scanning was performed to demonstrate the enhanced clinical feasibility of the developed system. Furthermore, the main experiment classified teeth with worn cervical regions into two groups according to the cervical abrasion shape. The developed dental OCT system produced a three‐dimensional image of the scanning area and quantitatively analyzed the margin gap through an A‐scan profile based on cross‐sectional images. This study confirms the effectiveness of the developed handheld dental OCT system in actual dental clinical practice, and we expect our system to be utilized for various dental diseases, not only for diagnosing a cervical abrasions.

Retinal oxygen extraction and hyperemic response in patients with Type II diabetes at different stages of non‐proliferative diabetic retinopathy

Purpose: Diabetic retinopathy (DR) is a worldwide cause of irreversible vision loss. Vascular factors such as reduced oxygen supply and impaired neurovascular coupling play a major role in the development of DR. We therefore investigated retinal oxygen metabolism and functional hyperemia in patients with Type II diabetes at different stages of non‐proliferative DR and healthy controls.Methods: To study retinal oxygen metabolism, a total of 67 patients with Type II diabetes (34 patients with no, 15 with mild and 18 with moderate‐to‐severe DR) and 20 healthy controls were recruited. We used the oxygen module of a Retinal Vessel Analyzer to measure retinal oxygen saturation in arteries and veins, and a custom‐built Doppler optical coherence tomography (DOCT) system to assess total retinal blood flow (TRBF). Values of retinal oxygen saturation and TRBF were then used to calculate retinal oxygen extraction. To measure the hyperaemic response, 29 patients with no, 12 with mild, 15 with moderate‐to‐severe DR and 20 healthy controls were included. Retinal blood flow was assessed before and during visual stimulation by combined measurement of retinal vessel calibres and blood velocity using the Dynamic Vessel Analyzer (Imedos, Germany) and DOCT.Results: Retinal oxygen extraction was lowest in patients with moderate‐to‐severe DR (1.78 ± 0.57 μL O2/min, p = 0.040 vs. healthy controls), followed by mild DR (1.90 ± 0.77 μL O2/min), no DR (2.14 ± 0.6 μL O2/min) and healthy controls (2.24 ± 0.57 μL O2/min). Compared to healthy controls (40.4 ± 27.2%), flicker responses were significantly decreased in patients with DR depending on the stage of DR (no DR 37.7 ± 26.0%, mild DR 26.2 ± 28.2%, moderate‐to‐severe DR 22.3 ± 13.9%; p = 0.035, ANOVA).Conclusions: Our results indicate that retinal oxygen extraction as well as hyperaemic response decreases with increasing severity of DR. Further studies are needed to assess whether these retinal functional changes can be used to identify high‐risk patients.