In vivo video rate optical coherence tomography

Emerging Applications of Intraoperative Optical Coherence Tomography in Corneal Surgery: A Narrative Review

Intraoperative OCT (iOCT) is an innovative imaging modality that provides ophthalmic surgeons with real-time cross-sectional views of the surgical field. Recent advances in OCT technology, such as higher acquisition scanning rates, enable real-time video visualization. iOCT systems are widely used in both vitreoretinal and anterior segment surgeries. In corneal surgeries, iOCT imaging aims to optimize efficacy and safety by improving depth perception and enhancing visualization in cases of opaque optical media. iOCT is a valuable tool not only for experienced corneal surgeons, but also for training novice surgeons. This review summarizes the emerging applications of iOCT in corneal surgery, particularly in technically demanding lamellar keratoplasty procedures, as well as in various other corneal diseases and complications that require surgical intervention.

Visualization of CO2 electrolysis using optical coherence tomography

Electrolysers offer an appealing technology for conversion of CO2 into high-value chemicals. However, there are few tools available to track the reactions that occur within electrolysers. Here we report an electrolysis optical coherence tomography platform to visualize the chemical reactions occurring in a CO2 electrolyser. This platform was designed to capture three-dimensional images and videos at high spatial and temporal resolutions. We recorded 12 h of footage of an electrolyser containing a porous electrode separated by a membrane, converting a continuous feed of liquid KHCO3 to reduce CO2 into CO at applied current densities of 50-800 mA cm-2. This platform visualized reactants, intermediates and products, and captured the strikingly dynamic movement of the cathode and membrane components during electrolysis. It also linked CO production to regions of the electrolyser in which CO2 was in direct contact with both membrane and catalyst layers. These results highlight how this platform can be used to track reactions in continuous flow electrochemical reactors.

Robust real-time estimation of non-uniform angular velocity and sub-pixel jitter in images captured with resonant scanners

Images captured with resonant scanners are affected by angular velocity fluctuations that result in image distortion and by poor synchronization between scanning and light detection that creates jitter between image rows. We previously demonstrated that both problems can be mitigated in post-processing by recording the scanner orientation in synchrony with the image capture, followed by data resampling [Opt. Express30, 112 (2022)10.1364/OE.446162]. Here we introduce more robust algorithms for estimation of both angular velocity fluctuation and jitter in the presence of random and deterministic noise. We also show linearization of the scanner oscillation model to reduce calculation times by two orders of magnitude, reaching 65,000 jitter estimations per second when using 2,800 samples per image row, and 500,000 when using only 500 samples, easily supporting real-time generation of jitter-corrected images.

Line-field confocal optical coherence tomography coupled with artificial intelligence algorithms to identify quantitative biomarkers of facial skin ageing

Quantitative biomarkers of facial skin ageing were studied from one hundred healthy Caucasian female volunteers, aged 20-70 years, using in vivo 3D Line-field Confocal Optical Coherence Tomography (LC-OCT) imaging coupled with Artificial Intelligence (AI)-based quantification algorithms. Layer metrics, i.e. stratum corneum thickness (SC), viable epidermal thickness and Dermal-Epidermal Junction (DEJ) undulation, as well as cellular metrics were measured for the temple, cheekbone and mandible. For all three investigated facial areas, minimal age-related variations were observed in the thickness of the SC and viable epidermis layers. A flatter and more homogeneous epidermis (decrease in the standard deviation of the number of layers means), a less dense cellular network with fewer cells per layer (decrease in cell surface density), and larger and more heterogeneous nuclei within each layer (increase in nuclei volume and their standard deviation) were found with significant variations with age. The higher atypia scores further reflected the heterogeneity of nuclei throughout the viable epidermis. The 3D visualisation of fine structures in the skin at the micrometric resolution and the 1200 µm × 500 µm field of view achieved with LC-OCT imaging enabled to compute relevant quantitative biomarkers for a better understanding of skin biology and the ageing process in vivo.

Correction of non-uniform angular velocity and sub-pixel jitter in optical scanning

Optical scanners are widely used in high-resolution scientific, medical, and industrial devices. The accuracy and precision of these instruments are often limited by angular speed fluctuations due to rotational inertia and by poor synchronization between scanning and light detection, respectively. Here we demonstrate that both problems can be mitigated by recording scanner orientation in synchrony with light detection, followed by data resampling. This approach is illustrated with synthetic and experimental data from a point-scanning microscope with a resonant scanner and a non-resonant scanner. Fitting of the resonant scanner orientation data to a cosine model was used to correct image warping and sampling jitter, as well as to precisely interleave image lines collected during the clockwise and counterclockwise resonant scanner portions of the rotation cycle. Vertical scanner orientation data interpolation was used to correct image distortion due to angular speed fluctuations following abrupt control signal changes.

Dynamic wavefront distortion in resonant scanners

Dynamic mirror deformation can substantially degrade the performance of optical instruments using resonant scanners. Here, we evaluate two scanners with resonant frequencies >12kHz with low dynamic distortion. First, we tested an existing galvanometric motor with a novel, to the best of our knowledge, mirror substrate material, silicon carbide, which resonates at 13.8 kHz. This material is stiffer than conventional optical glasses and has lower manufacturing toxicity than beryllium, the stiffest material currently used for this application. Then, we tested a biaxial microelectromechanical (MEMS) scanner with the resonant axis operating at 29.4 kHz. Dynamic deformation measurements show that wavefront aberrations in the galvanometric scanner are dominated by linear oblique astigmatism (90%), while wavefront aberrations in the MEMS scanner are dominated by horizontal coma (30%) and oblique trefoil (27%). In both scanners, distortion amplitude increases linearly with deflection angle, yielding diffraction-limited performance over half of the maximum possible deflection for wavelengths longer than 450 nm and over the full deflection range for wavelengths above 850 nm. Diffraction-limited performance for shorter wavelengths or over larger fractions of the deflection range can be achieved by reducing the beam diameter at the mirror surface. The small dynamic distortion of the MEMS scanner offers a promising alternative to galvanometric resonant scanners with desirable but currently unattainably high resonant frequencies.

Six-pack holographic imaging for dynamic rejection of out-of-focus objects

Six-pack holography is adapted to reject out-of-focus objects in dynamic samples, using a single camera exposure and without any scanning. By illuminating the sample from six different angles in parallel using a low-coherence source, out-of-focus objects are laterally shifted in six different directions when projected onto the focal plane. Then pixel-wise averaging of the six reconstructed images creates a significantly clearer image, with rejection of out-of-focus objects. Dynamic imaging results are shown for swimming microalgae and flowing microbeads, including numerical refocusing by Fresnel propagation. The averaged images reduced the contribution of out-of-focus objects by up to 83% in comparison to standard holograms captured using the same light source, further improving the system sectioning capabilities. Both simulation and experimental results are presented.

Optical Coherence Tomography for Quantifying Human Cutaneous Chronic Graft-versus-Host Disease

Chronic graft-versus-host disease (cGVHD) is the most common cause of nonrelapse mortality after allogeneic hematopoietic cell transplantation (alloHCT). Cutaneous cGVHD is characterized by thickening of the skin and connective tissues, causing discomfort and limited mobility. Current assessment of these skin lesions is based on physical examination of their thickening, pinchability, and movability. Optical coherence tomography (OCT) is a noninvasive, high-resolution technique using near-infrared light to interrogate tissues and image the microstructure without the use of contrast agents. We determined the applicability of OCT to human cutaneous cGVHD. Seven patients with varying degrees of cutaneous cGVHD, including 3 controls who underwent autologous HCT were prospectively examined using the cGVHD Skin (Vienna) Scale and imaged with OCT. Analysis of OCT images and clinical exams revealed that stratum corneum thickness, epidermal thickness, and depth of light transmission were correlated with cutaneous cGVHD severity in the hands, forearms, upper arms, legs, thighs, and upper back (P ≤ .03). Longitudinal OCT changes during cGVHD treatment paralleled clinical changes in the arm and upper back. OCT changes were observed in the absence of clinical changes. OCT imaging reflects the severity of cutaneous cGVHD and can be used to follow these lesions. OCT may facilitate the design of therapeutic trials in cGVHD by providing a quantitative measurement of cGVHD severity. Additional studies are needed.

Dynamic distortion in resonant galvanometric optical scanners

High-speed optical systems are revolutionizing biomedical imaging in microscopy, DNA sequencing, and flow cytometry, as well as numerous other applications, including data storage, display technologies, printing, and autonomous vehicles. These systems often achieve the necessary imaging or sensing speed through the use of resonant galvanometric optical scanners. Here, we show that the optical performance of these devices suffers due to the dynamic mirror distortion that arises from the variation in torque with angular displacement. In one of two scanners tested, these distortions result in a variation of signal-to-noise (Strehl) ratio by an order of magnitude across the field of view, degrading transverse resolution by more than a factor of 2. This mirror distortion could be mitigated through the use of stiffer materials, such as beryllium or silicon carbide, at the expense of surface roughness, as these cannot be polished to the same degree of smoothness as common optical glasses. The repeatability of the dynamic distortion indicates that computational and optical corrective methods are also possible.

Dual-mode line-field confocal optical coherence tomography for ultrahigh-resolution vertical and horizontal section imaging of human skin in vivo

Line-field confocal optical coherence tomography (LC-OCT) is a recently introduced technique for ultrahigh-resolution vertical section (B-scan) imaging of human skin in vivo. This work presents a new implementation of the LC-OCT technique to obtain horizontal section images (C-scans) in addition to B-scans. C-scan imaging is achieved with this dual-mode LC-OCT system using a mirror galvanometer for lateral scanning along with a piezoelectric chip for modulation of the interferometric signal. A quasi-identical spatial resolution of ∼ 1 µm is measured for both B-scans and C-scans. The images are acquired in both modes at a rate of 10 frames per second. The horizontal field of view of the C-scans is 1.2 × 0.5 mm², identical to the vertical field of view of the B-scans. The user can switch between the two modes by clicking a button. In vivo cellular-resolution imaging of human skin is demonstrated in both B-scan and C-scan modes, with the possibility to navigate within the skin tissues in real time.

Synthetic Aperture Radar Imaging for Burn Wounds Diagnostics

The need for technologies to monitor the wound healing under dressing materials has led us to investigate the feasibility of using microwave and millimetre wave radiations due to their sensitivity to water, non- ionising nature, and transparency to dressing materials and clothing. This paper presents synthetic aperture radar (SAR) images obtained from an active microwave and millimetre wave scanner operating over the band 15–40 GHz. Experimental images obtained from porcine skin samples with the presence of dressing materials and after the application of localised heat treatments reveal that SAR images can be used for diagnosing burns and for potentially monitoring the healing under dressing materials. The experimental images were extracted separately from the amplitude and phase measurements of the input reflection coefficient (S₁₁). The acquired images indicate that skin and burns can be detected and observed through dressing materials as well as features of the skin such as edges, irregularities, bends, burns, and variation in the reflectance of the skin. These unique findings enable a microwave and millimetre-wave scanner to be used for evaluating the wound healing progress under dressing materials without their often-painful removal: a capability that will reduce the cost of healthcare, distress caused by long waiting hours, and the healthcare interventional time.

Deep principal dimension encoding for the classification of early neoplasia in Barrett's Esophagus with volumetric laser endomicroscopy

Barrett cancer is a treatable disease when detected at an early stage. However, current screening protocols are often not effective at finding the disease early. Volumetric Laser Endomicroscopy (VLE) is a promising new imaging tool for finding dysplasia in Barrett's esophagus (BE) at an early stage, by acquiring cross-sectional images of the microscopic structure of BE up to 3-mm deep. However, interpretation of VLE scans is difficult for medical doctors due to both the size and subtlety of the gray-scale data. Therefore, algorithms that can accurately find cancerous regions are very valuable for the interpretation of VLE data. In this study, we propose a fully-automatic multi-step Computer-Aided Detection (CAD) algorithm that optimally leverages the effectiveness of deep learning strategies by encoding the principal dimension in VLE data. Additionally, we show that combining the encoded dimensions with conventional machine learning techniques further improves results while maintaining interpretability. Furthermore, we train and validate our algorithm on a new histopathologically validated set of in-vivo VLE snapshots. Additionally, an independent test set is used to assess the performance of the model. Finally, we compare the performance of our algorithm against previous state-of-the-art systems. With the encoded principal dimension, we obtain an Area Under the Curve (AUC) and F₁ score of 0.93 and 87.4% on the test set respectively. We show this is a significant improvement compared to the state-of-the-art of 0.89 and 83.1%, respectively, thereby demonstrating the effectiveness of our approach.

Optical coherence tomography imaging of cardiac substrates

Cardiovascular disease is the leading cause of morbidity and mortality in the United States. Knowledge of a patient's heart structure will help to plan procedures, potentially identifying arrhythmia substrates, critical structures to avoid, detect transplant rejection, and reduce ambiguity when interpreting electrograms and functional measurements. Similarly, basic research of numerous cardiac diseases would greatly benefit from structural imaging at cellular scale. For both applications imaging on the scale of a myocyte is needed, which is approximately 100 µm × 10 µm. The use of optical coherence tomography (OCT) as a tool for characterizing cardiac tissue structure and function has been growing in the past two decades. We briefly review OCT principles and highlight important considerations when imaging cardiac muscle. In particular, image penetration, tissue birefringence, and light absorption by blood during in vivo imaging are important factors when imaging the heart with OCT. Within the article, we highlight applications of cardiac OCT imaging including imaging heart tissue structure in small animal models, quantification of myofiber organization, monitoring of radiofrequency ablation (RFA) lesion formation, structure-function analysis enabled by functional extensions of OCT and multimodal analysis and characterizing important substrates within the human heart. The review concludes with a summary and future outlook of OCT imaging the heart, which is promising with progress in optical catheter development, functional extensions of OCT, and real time image processing to enable dynamic imaging and real time tracking during therapeutic procedures.

Sensitivity-enhanced ultrafast optical tomography by parametric- and Raman-amplified temporal imaging

To overcome the speed limitation of conventional optical tomography, a temporal imaging technique has been integrated with optical time-domain reflectometry to realize ultrafast temporally magnified (TM) tomography. In this Letter, the sensitivity of TM tomography has been further enhanced using optical parametric amplification and distributed Raman amplification, and this technique is named temporally encoded amplified and magnified (TEAM) tomography. As a result, a 78-dB sensitivity has been realized, comparable to ultrafast optical coherence tomography systems. In addition, an 86.7-μm axial resolution can be realized across a 67.5-mm imaging range. To demonstrate the significance of sensitivity improvement, tomographic imaging of a centimeter-thick phantom is provided at an A-scan rate of 44 MHz.

Optical coherence tomography in Optics Express [Invited]

Optical coherence tomography (OCT) is one of the most successful technologies in the history of biomedical optics. Optics Express played an important role in communicating groundbreaking technological achievements in the field of OCT, and, conversely, OCT papers are among the most frequently cited papers published in Optics Express. On the occasion of the 20th anniversary of the journal, this review analyzes the reasons for the success of OCT papers in Optics Express and discusses possible motivations for researchers to submit some of their best OCT papers to the journal.

Rapid scanning optical delay line based on a diffraction grating pair for a low-coherence reflectometer

We present a simple low-coherence time-domain interferometric reflectometer with a rapidly scanning optical delay line (RSODL) based on a non-parallel diffraction grating (DG) pair. The novelty of the solution is that the lightwave in the reference channel is focused on a galvo-mirror in a sub-mm static spot, which allows implementation of fast microelectromechanical systems scan optics. It is shown that the DG pair can be operated as a non-dispersive element that provides dynamic group delay of a reference lightwave. The DG pair system is also capable of tuning the RSODL dispersion from negative to positive values. The experimental depth range in air was obtained as large as 2.5 mm for axial resolution of 20 μm.

Predictive features for early cancer detection in Barrett's esophagus using Volumetric Laser Endomicroscopy

The incidence of Barrett cancer is increasing rapidly and current screening protocols often miss the disease at an early, treatable stage. Volumetric Laser Endomicroscopy (VLE) is a promising new tool for finding this type of cancer early, capturing a full circumferential scan of Barrett's Esophagus (BE), up to 3-mm depth. However, the interpretation of these VLE scans can be complicated, due to the large amount of cross-sectional images and the subtle grayscale variations. Therefore, algorithms for automated analysis of VLE data can offer a valuable contribution to its overall interpretation. In this study, we broadly investigate the potential of Computer-Aided Detection (CADe) for the identification of early Barrett's cancer using VLE. We employ a histopathologically validated set of ex-vivo VLE images for evaluating and comparing a considerable set of widely-used image features and machine learning algorithms. In addition, we show that incorporating clinical knowledge in feature design, leads to a superior classification performance and additional benefits, such as low complexity and fast computation time. Furthermore, we identify an optimal tissue depth for classification of 0.5-1.0 mm, and propose an extension to the evaluated features that exploits this phenomenon, improving their predictive properties for cancer detection in VLE data. Finally, we compare the performance of the CADe methods with the classification accuracy of two VLE experts. With a maximum Area Under the Curve (AUC) in the range of 0.90-0.93 for the evaluated features and machine learning methods versus an AUC of 0.81 for the medical experts, our experiments show that computer-aided methods can achieve a considerably better performance than trained human observers in the analysis of VLE data.

Ultrahigh-speed line-scan SD-OCT for four-dimensional in vivo imaging of small animal models

We report an ultrahigh-speed and high-resolution line-scan spectral-domain optical coherence tomography (SD-OCT) system that integrates a number of mechanisms for improving image quality. The illumination uniformity is significantly improved by the use of a Powell lens; Phase stepping and differential reconstruction are combined to suppress autocorrelation artifacts; Nonlocal means (NLM) is employed to enhance the signal to noise ratio while minimizing motion artifacts. The system is capable of acquiring cross-sectional images at more than 3,500 B-scans per second with sensitivities between 70dB and 90dB. The high B-scan rate enables image post-processing with nonlocal means, an advanced noise reduction algorithm that affords enhanced morphological details and reduced motion artifacts. The achieved axial and lateral resolutions are 2.0 and 6.2 microns, respectively. We have used this system to acquire four-dimensional (three-dimensional space and one-dimensional time) imaging data from live chicken embryos at up to 40 volumes per second. Dynamic cardiac tissue deformation and blood flow could be clearly visualized at high temporal and spatial resolutions, providing valuable information for understanding the mechanical and fluid dynamic properties of the developing cardiac system.

Optical Coherence Tomography for Retinal Surgery: Perioperative Analysis to Real-Time Four-Dimensional Image-Guided Surgery

Magnification of the surgical field using the operating microscope facilitated profound innovations in retinal surgery in the 1970s, such as pars plana vitrectomy. Although surgical instrumentation and illumination techniques are continually developing, the operating microscope for vitreoretinal procedures has remained essentially unchanged and currently limits the surgeon's depth perception and assessment of subtle microanatomy. Optical coherence tomography (OCT) has revolutionized clinical management of retinal pathology, and its introduction into the operating suite may have a similar impact on surgical visualization and treatment. In this article, we review the evolution of OCT for retinal surgery, from perioperative analysis to live volumetric (four-dimensional, 4D) image-guided surgery. We begin by briefly addressing the benefits and limitations of the operating microscope, the progression of OCT technology, and OCT applications in clinical/perioperative retinal imaging. Next, we review intraoperative OCT (iOCT) applications using handheld probes during surgical pauses, two-dimensional (2D) microscope-integrated OCT (MIOCT) of live surgery, and volumetric MIOCT of live surgery. The iOCT discussion focuses on technological advancements, applications during human retinal surgery, translational difficulties and limitations, and future directions.

Intravascular optical coherence tomography [Invited]

Shortly after the first demonstration of optical coherence tomography for imaging the microstructure of the human eye, work began on developing systems and catheters suitable for intravascular imaging in order to diagnose and investigate atherosclerosis and potentially to monitor therapy. This review covers the driving considerations of the clinical application and its constraints, the major engineering milestones that enabled the current, high-performance commercial imaging systems, the key studies that laid the groundwork for image interpretation, and the clinical research that traces intravascular optical coherence tomography (OCT) from early human pilot studies to current clinical trials.

Review of intraoperative optical coherence tomography: technology and applications [Invited]

During microsurgery, en face imaging of the surgical field through the operating microscope limits the surgeon's depth perception and visualization of instruments and sub-surface anatomy. Surgical procedures outside microsurgery, such as breast tumor resections, may also benefit from visualization of the sub-surface tissue structures. The widespread clinical adoption of optical coherence tomography (OCT) in ophthalmology and its growing prominence in other fields, such as cancer imaging, has motivated the development of intraoperative OCT for real-time tomographic visualization of surgical interventions. This article reviews key technological developments in intraoperative OCT and their applications in human surgery. We focus on handheld OCT probes, microscope-integrated OCT systems, and OCT-guided laser treatment platforms designed for intraoperative use. Moreover, we discuss intraoperative OCT adjuncts and processing techniques currently under development to optimize the surgical feedback derivable from OCT data. Lastly, we survey salient clinical studies of intraoperative OCT for human surgery.

Stimulated Raman scattering microscopy and spectroscopy with a rapid scanning optical delay line

Stimulated Raman scattering (SRS) microscopy that is capable of both high-speed imaging and rapid spectroscopy will be advantageous for detailed chemical analysis of heterogeneous biological specimens. We have developed a system based on spectral focusing SRS technology with the integration of a rapid scanning optical delay line, which allows continuous tuning of SRS spectra by scanning a galvo mirror. We demonstrate SRS spectral measurements of dimethyl sulfoxide solution at low concentrations and multi-color imaging of rice pollens and HeLa cells with line-by-line delay tuning to reduce motion artifacts, as well as fast acquisition of SRS spectra at specific regions of interest.

High-speed OCT light sources and systems [Invited]

Imaging speed is one of the most important parameters that define the performance of optical coherence tomography (OCT) systems. During the last two decades, OCT speed has increased by over three orders of magnitude. New developments in wavelength-swept lasers have repeatedly been crucial for this development. In this review, we discuss the historical evolution and current state of the art of high-speed OCT systems, with focus on wavelength swept light sources and swept source OCT systems.

Automated classification of optical coherence tomography images of human atrial tissue

Tissue composition of the atria plays a critical role in the pathology of cardiovascular disease, tissue remodeling, and arrhythmogenic substrates. Optical coherence tomography (OCT) has the ability to capture the tissue composition information of the human atria. In this study, we developed a region-based automated method to classify tissue compositions within human atria samples within OCT images. We segmented regional information without prior information about the tissue architecture and subsequently extracted features within each segmented region. A relevance vector machine model was used to perform automated classification. Segmentation of human atrial ex vivo datasets was correlated with trichrome histology and our classification algorithm had an average accuracy of 80.41% for identifying adipose, myocardium, fibrotic myocardium, and collagen tissue compositions.

In vivo optical interferometric imaging of human skin utilizing monochromatic light source

We have demonstrated tomographic imaging of in vivo human skin with an optical interferometric imaging technique using a monochromatic light source. The axial resolution of this method is determined by the center wavelength and the NA of the objective and is irrelevant to the bandwidth of the light source in contrast to optical coherence tomography. Our imaging system is constructed with low-priced and small-sized compact disk optical pickup components, a laser diode, a high NA objective, and a voice coil actuator. In spite of its low cost and small size, our imaging system can visualize the structure of human skin as clearly as a commercial reflectance confocal microscope.

Optical coherence tomography for embryonic imaging: a review

Embryogenesis is a highly complex and dynamic process, and its visualization is crucial for understanding basic physiological processes during development and for identifying and assessing possible defects, malformations, and diseases. While traditional imaging modalities, such as ultrasound biomicroscopy, micro-magnetic resonance imaging, and micro-computed tomography, have long been adapted for embryonic imaging, these techniques generally have limitations in their speed, spatial resolution, and contrast to capture processes such as cardiodynamics during embryogenesis. Optical coherence tomography (OCT) is a noninvasive imaging modality with micrometer-scale spatial resolution and imaging depth up to a few millimeters in tissue. OCT has bridged the gap between ultrahigh resolution imaging techniques with limited imaging depth like confocal microscopy and modalities, such as ultrasound sonography, which have deeper penetration but poorer spatial resolution. Moreover, the noninvasive nature of OCT has enabled live imaging of embryos without any external contrast agents. We review how OCT has been utilized to study developing embryos and also discuss advances in techniques used in conjunction with OCT to understand embryonic development.

Adaptive optics optical coherence tomography at 1 MHz

Image acquisition speed of optical coherence tomography (OCT) remains a fundamental barrier that limits its scientific and clinical utility. Here we demonstrate a novel multi-camera adaptive optics (AO-)OCT system for ophthalmologic use that operates at 1 million A-lines/s at a wavelength of 790 nm with 5.3 μm axial resolution in retinal tissue. Central to the spectral-domain design is a novel detection channel based on four high-speed spectrometers that receive light sequentially from a 1 × 4 optical switch assembly. Absence of moving parts enables ultra-fast (50ns) and precise switching with low insertion loss (-0.18 dB per channel). This manner of control makes use of all available light in the detection channel and avoids camera dead-time, both critical for imaging at high speeds. Additional benefit in signal-to-noise accrues from the larger numerical aperture afforded by the use of AO and yields retinal images of comparable dynamic range to that of clinical OCT. We validated system performance by a series of experiments that included imaging in both model and human eyes. We demonstrated the performance of our MHz AO-OCT system to capture detailed images of individual retinal nerve fiber bundles and cone photoreceptors. This is the fastest ophthalmic OCT system we know of in the 700 to 915 nm spectral band.

Low-coherence heterodyne interferometry using an achromatic frequency shifter based on a frequency-domain optical delay line

In this paper a high-sensitivity low-coherence heterodyne interferometric system consisting of two polarizing Michelson interferometers arranged in tandem is presented. A compact frequency-domain optical delay line was placed in the first interferometer to produce a 4.4 kHz frequency shift for broadband near-infrared light. The frequency shift was wavelength independent because the scanning delay line had a zero-group-delay configuration. The fringe amplitude and phase of a low-coherence interference signal were detected using a lock-in amplifier. The experimental results show that the signal-to-noise ratio in the proposed technique is more than 30-fold higher than that of conventional low-coherence homodyne interferometry.

High-resolution 1050 nm spectral domain retinal optical coherence tomography at 120 kHz A-scan rate with 6.1 mm imaging depth

We report a newly developed high speed 1050nm spectral domain optical coherence tomography (SD-OCT) system for imaging posterior segment of human eye. The system is capable of an axial resolution at ~10 µm in air, an imaging depth of 6.1 mm in air, a system sensitivity fall-off at ~6 dB/3mm and an imaging speed of 120,000 A-scans per second. We experimentally demonstrate the system's capability to perform phase-resolved imaging of dynamic blood flow within retina, indicating high phase stability of the SDOCT system. Finally, we show an example that uses this newly developed system to image posterior segment of human eye with a large view of view (10 × 9 mm(2)), providing detailed visualization of microstructural features from anterior retina to posterior choroid. The demonstrated system parameters and imaging performances are comparable to those that a typical 1 µm swept source OCT would deliver for retinal imaging.

Morphometric analysis of aqueous humor outflow structures with spectral-domain optical coherence tomography

PURPOSE

To describe morphometric details of the human aqueous humor (AH) outflow microvasculature visualized with 360-degree virtual castings during active AH outflow in cadaver eyes and to compare these structures with corrosion casting studies.

METHODS

The conventional AH outflow pathways of donor eyes (n = 7) and eyes in vivo (n = 3) were imaged with spectral-domain optical coherence tomography (SD-OCT) and wide-bandwidth superluminescent diode array during active AH outflow. Digital image contrast was adjusted to isolate AH microvasculature, and images were viewed in a 3D viewer. Additional eyes (n = 3) were perfused with mock AH containing fluorescent tracer microspheres to compare microvasculature patterns.

RESULTS

Observations revealed components of the conventional outflow pathway from Schlemm's canal (SC) to the superficial intrascleral venous plexus (ISVP). The superficial ISVP in both our study and corrosion casts were composed of interconnected venules (10-50 μm) forming a hexagonal meshwork. Larger radial arcades (50-100 μm) drained the region nearest SC and converged with larger tortuous vessels (>100 μm). A 360-degree virtual casting closely approximated corrosion casting studies. Tracer studies corroborated our findings. Tracer decorated several larger vessels (50-100 μm) extending posteriorly from the limbus in both raw and contrast-enhanced fluorescence images. Smaller tracer-labeled vessels (30-40 μm) were seen branching between larger vessels and exhibited a similar hexagonal network pattern.

CONCLUSIONS

SD-OCT is capable of detailed morphometric analysis of the conventional outflow pathway in vivo or ex vivo with details comparable to corrosion casting techniques.

GPU accelerated real-time multi-functional spectral-domain optical coherence tomography system at 1300 nm

We present a GPU accelerated multi-functional spectral domain optical coherence tomography system at 1300 nm. The system is capable of real-time processing and display of every intensity image, comprised of 512 pixels by 2048 A-lines acquired at 20 frames per second. The update rate for all four images with size of 512 pixels by 2048 A-lines simultaneously (intensity, phase retardation, flow and en face view) is approximately 10 frames per second. Additionally, we report for the first time the characterization of phase retardation and diattenuation by a sample comprised of a stacked set of polarizing film and wave plate. The calculated optic axis orientation, phase retardation and diattenuation match well with expected values. The speed of each facet of the multi-functional OCT CPU-GPU hybrid acquisition system, intensity, phase retardation, and flow, were separately demonstrated by imaging a horseshoe crab lateral compound eye, a non-uniformly heated chicken muscle, and a microfluidic device. A mouse brain with thin skull preparation was imaged in vivo and demonstrated the capability of the system for live multi-functional OCT visualization.

Longitudinal Imaging of Heart Development With Optical Coherence Tomography

Optical coherence tomography (OCT) has great potential for deciphering the role of mechanics in normal and abnormal heart development. OCT images tissue microstructure and blood flow deep into the tissue (1-2mm) at high spatiotemporal resolutions allowing unprecedented images of the developing heart. Here, we review the advancement of OCT technology to image heart development and report some of our recent findings utilizing OCT imaging under environmental control for longitudinal imaging. Precise control of the environment is absolutely required in longitudinal studies that follow the growth of the embryo or studies comparing normal versus perturbed heart development to obtain meaningful in vivo results. These types of studies are essential to tease out the influence of cardiac dynamics on molecular expression and their role in the progression of congenital heart defects.

Full-range swept source optical coherence tomography based on carrier frequency by transmissive dispersive optical delay line

A high speed swept source optical coherence tomography (SS-OCT) system capable of full-range imaging is presented. Wave-number carrier frequency is introduced into the spectral interference signal by a transmissive dispersive optical delay line (TDODL). High carrier frequency in the spectral interference signal corresponding to an equivalent distance-shift is exploited to obtain full-range OCT imaging. Theoretical development is conducted with the instantaneous coherence function introduced for a complete description of a spectral interference signal. Performance advantage of the TDODL-based method over the conventional approach where only one side (positive or negative path length difference) is used for imaging to avoid overlaying mirror artifacts is confirmed by the measured envelopes of spectral interference signal. Feasibility of the proposed method for full-range imaging is validated in a custom-built SS-OCT system by in vivo imaging of a biological sample.

Optical sensing method for screening disease in melon seeds by using optical coherence tomography

We report a noble optical sensing method to diagnose seed abnormalities using optical coherence tomography (OCT). Melon seeds infected with Cucumber green mottle mosaic virus (CGMMV) were scanned by OCT. The cross-sectional sensed area of the abnormal seeds showed an additional subsurface layer under the surface which is not found in normal seeds. The presence of CGMMV in the sample was examined by a blind test (n = 140) and compared by the reverse transcription-polymerase chain reaction. The abnormal layers (n = 40) were quantitatively investigated using A-scan sensing analysis and statistical method. By utilizing 3D OCT image reconstruction, we confirmed the distinctive layers on the whole seeds. These results show that OCT with the proposed data processing method can systemically pick up morphological modification induced by viral infection in seeds, and, furthermore, OCT can play an important role in automatic screening of viral infections in seeds.

3D visualization of aqueous humor outflow structures in-situ in humans

Aqueous humor (AH) exiting the eye via the trabecular meshwork and Schlemm's canal (SC) passes through the deep and intrascleral venous plexus (ISVP) or directly through aqueous veins. The purpose of this study was to visualize the human AH outflow system 360° in three dimensions (3D) during active AH outflow in a virtual casting. The conventional AH outflow pathways of 7 donor eyes were imaged with a modified Bioptigen spectral-domain optical coherence tomography system (Bioptigen Inc, USA; SuperLum LTD, Ireland) at a perfusion pressure of 20 mmHg (N = 3), and 10 mmHg (N = 4). In all eyes, 36 scans (3 equally distributed in each clock hour), each covering a 2 × 3 × 2 mm volume (512 frames, each 512 × 1024 pixels), were obtained. All image data were black/white inverted, and the background subtracted (ImageJ 1.40 g, http://rsb.info.nih.gov/ij/). Contrast was adjusted to isolate the ISVP. SC, collector channels, the deep and ISVP, and episcleral veins were observed throughout the limbus. Aqueous veins could be observed extending into the episcleral veins. Individual scan ISVP castings were rendered and assembled in 3D space in Amira 4.1 (Visage Imaging Inc. USA). A 360-degree casting of the ISVP was obtained in all perfused eyes. The ISVP tended to be dense and overlapping in the superior and inferior quadrants, and thinner in the lateral quadrants. The human AH outflow pathway can be imaged using SD-OCT. The more superficial structures of the AH outflow pathway present with sufficient contrast as to be optically isolated and cast in-situ 360° in cadaver eye perfusion models. This approach may be useful as a model in future studies of human AH outflow.

A clinical instrument for combined raman spectroscopy-optical coherence tomography of skin cancers

BACKGROUND AND OBJECTIVE

The current standard for diagnosis of skin cancers is visual inspection followed by biopsy and histopathology. This process can be invasive, subjective, time consuming, and costly. Optical techniques, including Optical Coherence Tomography (OCT) and Raman Spectroscopy (RS), have been developed to perform non-invasive characterization of skin lesions based on either morphological or biochemical features of disease. The objective of this work is to report a clinical instrument capable of both morphological and biochemical characterization of skin cancers with RS-OCT.

MATERIALS AND METHODS

The portable instrument utilizes independent 785 nm RS and 1,310 nm OCT system backbones. The two modalities are integrated in a 4″ (H) × 5″(W) × 8″(L) clinical probe. The probe enables sequential acquisition of co-registered OCT and RS data sets. The axial response of the RS collection in the skin was estimated using scattering phantoms. In addition, RS-OCT data from patients with cancerous and non-cancerous lesions are reported.

RESULTS

The RS-OCT instrument is capable of screening areas as large as 15 mm (transverse) by 2.4 mm (in depth) at up to 8 frames/second with OCT, and identifying locations to perform RS. RS signal is collected from a 44 µm transverse spot through a depth of approximately 530 µm. RS-OCT data sets from a superficial scar and a nodular BCC are reported to demonstrate the clinical potential of the instrument.

CONCLUSION

The RS-OCT instrument reported here is capable of morphological and biochemical characterization of cancerous skin lesions in a clinical setting. OCT can visualize microstructural irregularities and perform an initial morphological analysis of the lesion. The images can be used to guide acquisition of biochemically specific Raman spectra. The two data sets can then be evaluated with respect to one another to take advantage of the mutually complimentary nature of RS and OCT.

Complete complex conjugate resolved heterodyne swept-source optical coherence tomography using a dispersive optical delay line

Swept-source optical coherence tomography (SSOCT) provides a substantial sensitivity advantage over its time-domain counterpart, but suffers from a reduced imaging depth range due to sensitivity falloff and complex conjugate ambiguity. Heterodyne complex conjugate-resolved SSOCT (HCCR-SSOCT) has been previously demonstrated as a technique to completely resolve the complex conjugate ambiguity, effectively doubling the falloff limited imaging depth, without the reduction in imaging speed associated with other CCR techniques. However, previous implementations of this technique have employed expensive and lossy optical modulators to provide the required differential phase modulation. In this paper, we demonstrate the use of a dispersive optical delay line (D-ODL) as the reference arm of an OCT system to realize HCCR-SSOCT. This technique maintains the existing advantages of HCCR-SSOCT in that it completely resolves the complex conjugate artifact and does not reduce imaging speed, while conferring the additional advantages of being low cost, maintaining system sensitivity and resolution, not requiring any additional signal processing, and working at all wavelengths and imaging speeds. The D-ODL also allows for hardware correction of unbalanced dispersion in the reference and sample arm, adding further flexibility to system design. We demonstrate the technique using an SSOCT system operating at 100kHz with a central wavelength of 1040nm. Falloff measurements performed using a standard OCT configuration and the proposed D-ODL demonstrate a doubling of the effective imaging range with no sensitivity or resolution penalty. Feasibility of the technique for in vivo imaging was demonstrated by imaging the ocular anterior segments of healthy human volunteers.

Slow light with a swept-frequency source

ct: We introduce a new concept for stimulated-Brillouin-scattering-based slow light in optical fibers that is applicable for broadly-tunable frequency-swept sources. It allows slow light to be achieved, in principle, over the entire transparency window of the optical fiber. We demonstrate a slow light delay of 10 ns at 1.55 μm using a 10-m-long photonic crystal fiber with a source sweep rate of 400 MHz/μs and a pump power of 200 mW. We also show that there exists a maximal delay obtainable by this method, which is set by the SBS threshold, independent of sweep rate. For our fiber with optimum length, this maximum delay is ~38 ns, obtained for a pump power of 760 mW.

Image analysis for classification of dysplasia in Barrett's esophagus using endoscopic optical coherence tomography

Barrett's esophagus (BE) and associated adenocarcinoma have emerged as a major health care problem. Endoscopic optical coherence tomography is a microscopic sub-surface imaging technology that has been shown to differentiate tissue layers of the gastrointestinal wall and identify dysplasia in the mucosa, and is proposed as a surveillance tool to aid in management of BE. In this work a computer-aided diagnosis (CAD) system has been demonstrated for classification of dysplasia in Barrett's esophagus using EOCT. The system is composed of four modules: region of interest segmentation, dysplasia-related image feature extraction, feature selection, and site classification and validation. Multiple feature extraction and classification methods were evaluated and the process of developing the CAD system is described in detail. Use of multiple EOCT images to classify a single site was also investigated. A total of 96 EOCT image-biopsy pairs (63 non-dysplastic, 26 low-grade and 7 high-grade dysplastic biopsy sites) from a previously described clinical study were analyzed using the CAD system, yielding an accuracy of 84% for classification of non-dysplastic vs. dysplastic BE tissue. The results motivate continued development of CAD to potentially enable EOCT surveillance of large surface areas of Barrett's mucosa to identify dysplasia.

Non-destructive label-free monitoring of collagen gel remodeling using optical coherence tomography

Matrix remodeling plays a fundamental role in physiological and pathological processes, as well as in tissue engineering applications. In this paper, optical coherence tomography (OCT), a non-destructive optical imaging technology, was used to image collagen gel remodeling by smooth muscle cells (SMCs). The optical scattering properties of collagen-SMC gels were characterized quantitatively by fitting OCT data to a theoretical model. Matrix remodeling over 5 days produced a 10-fold increase in the reflectivity of the collagen gels, corresponding to a decrease in scattering anisotropy from 0.91 to 0.46. The increase in reflectivity was corroborated in confocal mosaic images. Blocking matrix degradation in collagen-SMC gels with doxycycline, a non-specific matrix metalloproteinases (MMPs) inhibitor, impeded the decrease in scattering anisotropy and resulted in few macroscopic signs of remodeling. Causing matrix degradation in acellular gels with a 3 h treatment of MMP-8 (collagenase 2) partially mimicked the decrease in anisotropy measured in collagen-SMC gels after 5 days. These results suggest that the decrease in scattering anisotropy in the collagen-SMC gels was due to MMP activity that degrades collagen fibrils into smaller fragments.

High-speed optical coherence tomography: basics and applications

In the past decade we have observed a rapid development of ultrahigh-speed optical coherence tomography (OCT) instruments, which currently enable performing cross-sectional in vivo imaging of biological samples with speeds of more than 100,000 A-scans/s. This progress in OCT technology has been achieved by the development of Fourier-domain detection techniques. Introduction of high-speed imaging capabilities lifts the primary limitation of early OCT technology by giving access to in vivo three-dimensional volumetric reconstructions on large scales within reasonable time constraints. As result, novel tools can be created that add new perspective for existing OCT applications and open new fields of research in biomedical imaging. Especially promising is the capability of performing functional imaging, which shows a potential to enable the differentiation of tissue pathologies via metabolic properties or functional responses. In this contribution the fundamental limitations and advantages of time-domain and Fourier-domain interferometric detection methods are discussed. Additionally the progress of high-speed OCT instruments and their impact on imaging applications is reviewed. Finally new perspectives on functional imaging with the use of state-of-the-art high-speed OCT technology are demonstrated.

Imaging port wine stains by fiber optical coherence tomography

We develop a fiber optical coherence tomography (OCT) system in the clinical utility of imaging port wine stains (PWS). We use our OCT system on 41 patients with PWS to document the difference between PWS skin and contralateral normal skin. The system, which operates at 4 frames with axial and transverse resolutions of 10 and 9 mum, respectively, in the skin tissue, can clearly distinguish the dilated dermal blood vessels from normal tissue. We present OCT images of patients with PWS and normal human skin. We obtain the structural parameters, including epidermal thickness and diameter and depth of dilated blood vessels. We demonstrate that OCT may be a useful tool for the noninvasive imaging of PWS. It may help determine the photosensitizer dose and laser parameters in photodynamic therapy for treating port wine stains.

Apoptosis- and necrosis-induced changes in light attenuation measured by optical coherence tomography

Optical coherence tomography (OCT) was used to determine optical properties of pelleted human fibroblasts in which necrosis or apoptosis had been induced. We analysed the OCT data, including both the scattering properties of the medium and the axial point spread function of the OCT system. The optical attenuation coefficient in necrotic cells decreased from 2.2 ± 0.3 mm⁻¹ to 1.3 ± 0.6 mm⁻¹, whereas, in the apoptotic cells, an increase to 6.4 ± 1.7 mm⁻¹ was observed. The results from cultured cells, as presented in this study, indicate the ability of OCT to detect and differentiate between viable, apoptotic, and necrotic cells, based on their attenuation coefficient. This functional supplement to high-resolution OCT imaging can be of great clinical benefit, enabling on-line monitoring of tissues, e.g. for feedback in cancer treatment.

Motion artifact suppression in full-field optical coherence tomography

Significant motion artifacts limit the performance of conventional full-field optical coherence tomography (FF-OCT) for in-vivo imaging. We present a theoretical and experimental study of those limitations. A new FF-OCT system suppressing most of artifacts due to sample motions is demonstrated using instantaneous phase shifting with nonpolarizing optics and pulsed illumination. The experimental setup is based on a Linnik-type interferometer illuminated by the superluminescence emission from a Ti:Al(2)O(3) waveguide crystal. En face tomographic images are calculated as a combination of two phase-opposed interferometric images acquired simultaneously by two CCD cameras placed at both outputs of the interferometer, with a spatial resolution of 0.8 microm x 1.6 microm (axial x transverse) and a detection sensitivity of approximately 60 dB.

Quantitative characterization of developing collagen gels using optical coherence tomography

Nondestructive optical imaging methods such as optical coherence tomography (OCT) have been proposed for characterizing engineered tissues such as collagen gels. In our study, OCT was used to image collagen gels with different seeding densities of smooth muscle cells (SMCs), including acellular gels, over a five-day period during which the gels contracted and became turbid with increased optical scattering. The gels were characterized quantitatively by their optical properties, specified by analysis of OCT data using a theoretical model. At 6 h, seeded cell density and scattering coefficient (mu(s)) were correlated, with mu(s) equal to 10.8 cm(-1)(10(6) cells mL). Seeded cell density and the scattering anisotropy (g) were uncorrelated. Over five days, the reflectivity in SMC gels gradually doubled with little change in optical attenuation, which indicated a decrease in g that increased backscatter, but only a small drop in mu(s). At five days, a subpopulation of sites on the gel showed substantially higher reflectivity (approximately a tenfold increase from the first 24 h). In summary, the increased turbidity of SMC gels that develops over time is due to a change in the structure of collagen, which affects g, and not simply due to a change in number density of collagen fibers due to contraction.

Swept-source polarization-sensitive optical coherence tomography based on polarization-maintaining fiber

We present a swept-source polarization-sensitive optical coherence tomography system based on a polarization-maintaining fiber interferometer. The system produces reflectivity and birefringence information along a depth profile with a single sweep of the optical spectrum. Unlike single-mode fiber systems, retardance and relative optical axis orientation images are calculated without compensation. The source is a 45 mW polygon-based swept-source centered at 1290 nm and tuned at a rate of 28 kHz. The interferometer consists of a single polarization-maintaining coupler that utilizes balanced detection for improved performance. Characterization data shows that this system yields accurate measurements with high sensitivity (106.2 dB) comparable to conventional setups. Images of biological tissues with high dynamic range are demonstrated.