Fourier-domain Optical Coherence Tomography Research Articles

Simple and robust calibration procedure for k-linearization and dispersion compensation in optical coherence tomography.

.In Fourier-domain optical coherence tomography (FD-OCT), proper signal sampling and dispersion compensation are essential steps to achieve optimal axial resolution. These calibration steps can be performed through numerical signal processing, but require calibration information about the system that may require lengthy and complex measurement protocols. We report a highly robust calibration procedure that can simultaneously determine correction vectors for nonlinear wavenumber sampling and dispersion compensation. The proposed method requires only two simple mirror measurements and no prior knowledge about the system’s illumination source or detection scheme. This method applies to both spectral domain and swept-source OCT systems. Furthermore, it may be implemented as a low-cost fail-safe to validate the proper function of calibration hardware such as k-clocks. We demonstrate the method’s simple implementation, effectiveness, and robustness on both types of OCT systems.

Geometrical characterization of the corneo-scleral transition in normal patients with Fourier domain optical coherence tomography.

To characterize the geometry at the corneo-scleral transition for a normal population and its correlation with other anatomic parameters of the eyeball. Transversal epidemiologic study on a sample of 100 individuals (right eye) in different ethnic groups (Africans and Caucasians). All of them were examined with Fourier domain optical coherence tomography, auto-refractometer, topographer, and biometer to obtain the corneo-scleral angle (CSA) and additional clinical parameters. The dataset was analyzed to determine correlations between different anatomical parameters and nasal (CSAn) and temporal CSA (CSAt) values. The CSAt presents a significant but low correlation with the anterior chamber depth-ACD (r = 0.25; p = 0.024), the white-to-white (W-W) distance (r = 0.27; p = 0.022), and the anterior chamber volume (r = 0.25; p = 0.016). CSAn did not correlate significantly with any clinical variable, with all values being lower than 179° (concave). Ethic groups presented significant differences for pachymetry (Pac) and corneal volume (p = 0.033 and p = 0.014), being greater for Caucasians, and temporal corneo-iridial angle (p = 0.006), being greater for Africans. CSA presented and inverse correlation with age. The CSAn presents a more concave profile for the normal population, whereas the CSAt presents a planar-convex profile with a great influence of age. In particular, the older the patient, the more convex the CSAt is. This age-related evolution of the CSAt and the concavity on the nasal direction must be considered when prescribing scleral contact lenses or when performing limbal incisions during refractive interventions.

In vivo volumetric imaging by crosstalk-free full-field OCT

Cellular resolution imaging of biological structures has always been a challenge due to strong scattering that limits the achievable transverse resolution or imaging penetration depth. Recently, a major advancement toward high-resolution and volumetric imaging was achieved by implementing a parallel detection (i.e., full field) into Fourier-domain optical coherence tomography. The drawback of using parallel detection is that scattered light can travel laterally and get mapped improperly at a camera creating optical crosstalk, which severely impairs the interpretation of subcellular images and limits its use in medical diagnostics. In this work, we demonstrate for what we believe is the first time how to efficiently reduce crosstalk and enable microscopic quality volumetric reconstructions of the scattering tissue-like human skin in vivo, all within less than a half of a second. To minimize crosstalk, we implemented a very fast deformable membrane that introduces random phase illumination. Additionally, the sample is illuminated under variable angles to reduce the contrast of speckles by incoherent summation of the crosstalk-free volumes. Introducing crosstalk and speckle-free OCT will advance imaging prospects closer to the ideal of a noninvasive optical biopsy.

This issue of ACTA

This issue of ACTA

Spectral interferogram prediction in high resolution dual- and multiple-band fourier domain optical coherence tomography

Using dual-band in Fourier domain optical coherence tomography (FD−OCT) aims at improving resolution. Previous work showed that direct application of Fourier transform to the spectral interferogram with gap between the two bands leads in many cases to resolution poorer than using a single band. In this work we propose an algorithm to fill in the gap in the spectral interferogram based on the linear prediction technique. The results showed that in dual-band FD−OCT we can get resolution equivalent to a continuous band whose width is the sum of the widths of the two bands and plus the width of the spectral gap. We got improved resolutions from 20.6 μm and 9.9 μm to less than 5 micrpmeter in air. Then we generalized our proposed algorithm to general multiple-band FD−OCT systems.

En face optical coherence tomography: a technology review [Invited

A review on the technological development of en face optical coherence tomography (OCT) and optical coherence microscopy (OCM) is provided. The terminology originally referred to time domain OCT, where the preferential scanning was performed in the en face plane. Potentially the fastest realization of en face image recording is full-field OCT, where the full en face plane is illuminated and recorded simultaneously. The term has nowadays been adopted for high-speed Fourier domain approaches, where the en face image is reconstructed from full 3D volumes either by direct slicing or through axial projection in post processing. The success of modern en face OCT lies in its immediate and easy image interpretation, which is in particular of advantage for OCM or OCT angiography. Applications of en face OCT with a focus on ophthalmology are presented. The review concludes by outlining exciting technological prospects of en face OCT based both on time as well as on Fourier domain OCT.

Acceleration of OCT Signal Processing with Lookup Table Method for Logarithmic Transformation

Real-time Fourier domain optical coherence tomography (FDOCT) has been widely used in clinical applications. In order to accelerate the imaging processing and display of FDOCT, an alternative lookup table-based strategy for logarithmic transformation was presented. In this paper, real-time and high-quality FDOCT imaging display of biological tissues at an A-line rate of 62 kHz was demonstrated by optimizing the logarithmic calculation.

Determining the Potential Role of Crystalline Lens Rise in Vaulting in Posterior Chamber Phakic Collamer Lens Surgery for Correction of Myopia.

To assess the effect of crystalline lens rise (CLR) on postoperative vault in eyes implanted with a phakic Visian Implantable Collamer Lens (ICL) (STAAR Surgical Company, Monrovia, CA) with a central port for myopic correction. Non-invasive Fourier-domain swept-source anterior segment optical coherence tomography was used for dynamic assessment of the study eyes under changing light conditions. Phakic intraocular lens (IOL) vault, anterior chamber depth (ACD), and CLR were recorded after surgery, and intra-eye differences were analyzed under scotopic and photopic ambient light conditions. Inter-group analysis and regression analysis were performed to investigate any potential correlation between these biometric variables. This retrospective observational study comprised 111 eyes (65 patients) implanted with a myopic Visian ICL. The mean change in CLR from mydriasis to miosis was 59 ± 60 µm (P < .001). The sample was further divided into four groups according to the CLR value in miosis: CLR < 0, 0 to 200, 201 to 350, and > 350 µm. A significant difference in central vault values was observed between the < 0 and > 350 µm groups, the 0 to 200 and 201 to 350 µm groups, and the 0 to 200 and > 350 µm groups (P < .05). Eyes with a high vault value (> 750 µm in mydriasis) had lower CLR values (P < .001) and higher ACD values (P < .001) than eyes with a low vault value (< 100 µm in miosis). The linear correlation observed was negative between CLR and ACD, positive between postoperative vault and ACD, and negative between postoperative vault and CLR (P < .001). CLR significantly affected anterior chamber depth and postoperative ICL vault. [J Refract Surg. 2019;35(3):177-183.].

Predictive Factors for the Rate of Visual Field Progression in the Advanced Imaging for Glaucoma Study

To investigate predictive factors associated with the rate of visual field (VF) loss in open-angle glaucoma. Prospective multicenter cohort study. Perimetric glaucoma patients of the Advanced Imaging for Glaucoma study were selected for analysis if they had 9 completed visits. Confirmed rapid significant progression (CRSP) of VF was defined as a significant (P < 0.05) negative VF index (VFI) slope of-1%/year or a mean deviation slope of-0.5 dB/year, confirmed at 2 consecutive follow-up visits. Slow progression was defined as VFI slope greater than-0.5%/year or a mean deviation slope of-0.25 dB/year. Fourier-domain optical coherence tomography (FD-OCT) measured optic disc, peripapillary retinal nerve fiber layer (NFL), and macular ganglion cell complex (GCC) thicknesses. Logistic regression was used to identify baseline predictors for CRSP and slow progression. Linear regression was used to identify baseline predictors for the VFI and mean deviation slope. Eyes (n= 150) of 103 participants were included. Slow progression was observed in 80 eyes (53.3%) and CRSP in 23 eyes (15.3%). Larger NFL and GCC baseline focal loss volume (FLV), thinner central corneal thickness, and lower VFI were significant (P < 0.05) baseline predictors of more rapid progression on univariate analysis. The predictor with the highest odds ratio (OR) was NFL-FLV, which was also the most significant non-VF predictor in the multivariate analysis. Eyes with NFL-FLV >8.5% had an OR of 2.67 for CRSP and 0.42 for slow progression. Disc hemorrhage during the follow-up was also important, with an OR of 2.61 for CRSP and 0.23 for slow progression for each occurrence. Focal loss measured by FD-OCT or VF along with CCT are strong baseline predictors for the rate of glaucoma progression.

Three-Dimensional Full-Range Dual-Band Fourier Domain Optical Coherence Tomography Accelerated by Graphic Processing Unit

The authors present a graphic processing unit (GPU) accelerated three-dimensional (3-D) full-range dual-band Fourier domain optical coherence tomography (FD-OCT) using a LabVIEW GPU program with GPU-adaptable algorithms. GPU can dramatically reduce the image processing time, but programing on GPU is more complex than that on central processing unit (CPU). This study implements GPU computing interfaced with LabVIEW, using a LabVIEW GPU analysis toolkit, which significantly reduces the programing complexity. The selected algorithms are suitable for GPU computing in multiband FD-OCT. The developed system overcomes the problem of long image processing time relative to data acquisition time in CPU. A 65-Hz frame rate is achieved for acquiring, processing, and displaying 2 × 450 × 800 full-range dual-band FD-OCT images. The high-speed processing made it possible to interactively display 3-D dual-band FD-OCT images in a real time, showing the potential application for improving multinctional OCT systems currently using complex algorithms.

Anterior Segment Optical Coherence Tomography: Applications for Clinical Care and Scientific Research.

Anterior segment optical coherence tomography (AS-OCT) is a non-contact imaging technique that produces high-resolution images and quantitative measurements of the anterior segment and its anatomical structures. There has been rapid development of OCT technology over the past 2 decades, with the transition from time-domain to Fourier-domain OCT devices. By integrating these advancements in OCT technology, AS-OCT devices have evolved into versatile clinical and research tools for studies of the anterior segment and ocular surface. The primary purpose of this article was to review OCT technology and AS-OCT devices as well as applications of AS-OCT for clinical practice and scientific research. We first describe the different types of OCT technology, how they have been adapted for AS-OCT imaging, and differences between various AS-OCT devices. We then review the applications of AS-OCT for characterizing the anatomical structures of the anterior segment and aqueous outflow pathways, including the anterior chamber angle, trabecular meshwork, and Schlemm canal. We also describe glaucoma-related applications of AS-OCT imaging, which include evaluating patients for static and dynamic biometric risk factors of primary angle closure disease and assessing the efficacy of glaucoma interventions, such as laser peripheral iridotomy and glaucoma surgery. Finally, we review other clinical applications of AS-OCT imaging for detection and management of diseases of the ocular surface, cornea, and lens.

Mid-infrared Fourier-domain optical coherence tomography with a pyroelectric linear array.

Optical technology in the mid-infrared wavelength range is currently a rapidly developing field initiated by the availability of novel high-power and spatially coherent sources. Non-destructive testing techniques based on these sources are very promising for industrial and medical applications. However, there are still many engineering problems due to the technical challenges and high prices of the optical elements suitable for the mid-infrared region. In this paper, we report the development and performances of the first mid-infrared Fourier-domain optical coherence tomography based on a supercontinuum source and low-cost pyroelectric detector. The system is designed to operate in the spectral region around 4 μm. Experimental results are demonstrated for detections of embedded microstructures in ceramic materials and subsurface oil paint layers.

Topical Rho-Associated Kinase Inhibitor, Y27632, Accelerates Corneal Endothelial Regeneration in a Canine Cryoinjury Model.

Corneal endothelial cell regeneration varies by species, with nonhuman primates (NHPs) and rabbits displaying low and high proliferative capacities, respectively. Recent studies report that topical application of rho-associated kinase (ROCK) inhibitors accelerates corneal endothelial wound healing in animal models and human patients with endothelial dysfunction. This study determines the regenerative capacity of canine corneal endothelial cells in vivo and their response to a topical ROCK inhibitor, Y27632, after transcorneal freezing. Right eyes of 6 beagles underwent transcorneal freezing; 10 mM ROCK inhibitor Y27632 or vehicle control was applied topically to both eyes at least 4 times daily for 56 days. Endothelial cell density was evaluated by in vivo confocal microscopy, and corneal thickness was measured by Fourier-domain optical coherence tomography (FD-OCT) and ultrasound pachymetry. Transcorneal freezing induced severe central corneal edema in dogs, with restoration of transparency occurring within 4 weeks. Y27632 significantly decreased corneal thickness by FD-OCT and ultrasound pachymetry in the acute phase and significantly increased endothelial cell density at days 28 and 42 post-cryoinjury, suggesting faster restoration of endothelial cell recovery. Canine corneal endothelial function recovers at a similar rate as NHPs but more slowly than rabbits after cryoinjury. Faster corneal endothelial wound healing was observed by in vivo confocal microscopy and FD-OCT in dogs treated with Y27632 versus vehicle controls. Thus, a canine cryoinjury model may be a useful alternative to NHPs in detecting a response to therapies directed at endothelial regeneration.

Observations of in vivo laser tissue ablation in animal models with different chromophores on the skin and modulating duration per laser exposure

Characteristics such as skin tone and pigmentation color vary among patients, but most researches on laser irradiation in laser ablation have revolved around minimizing damage to reduce pain. Chromophores are the most important factors in photon energy absorption, a key principle of laser ablation. We investigated the influences on ablation depth by different chromophores on the target and modulating duration per laser exposure using an Nd:YVO4 nanosecond 532-nm laser. We used a Fourier-domain optical coherence tomography (Fd-OCT) system combined with a 532-nm Nd:YVO4 laser to observe the ablation process. In addition, an external customized shutter and water-based color pens (red, green, blue, black) were used to determine the effects of modulating the duration per laser exposure and coloring chromophores on porcine skin and hairless mouse models. Experiments with modulating duration per laser exposure demonstrated that shorter duration generated shallower craters than longer one. Painted experimental group showed damaged region as craters in the experiments with coloring various chromophores. In this research, we investigated the effects of modulating duration per laser exposure and different chromophores on the target. Coloring chromophores with water-based dyes using pens increased tissue damage without dyeing cells or tissue.

Comparison of ultrasonic pachymetry and Fourier-domain optical coherence tomography for measurement of corneal thickness in dogs with and without corneal disease

Several ultrasonic and Fourier-domain optical coherence tomography (FD-OCT) pachymeters are used to measure corneal thickness in canine patients and research subjects. This study assessed the reliability of and consistency between two ultrasonic pachymetry (USP) devices, Pachette 3 and Accupach VI, as well as automated and manual measurements obtained using FD-OCT in dogs with and without corneal disease. Corneal thickness measurements were compiled from 108 dogs and analyzed using mixed effects linear regression, with Bonferonni adjustments for post-hoc comparisons, to determine the effects of age, weight and disease state. Data are presented as predicted mean±standard error.Canine corneal disease can result in marked increases in thickness that frequently exceed the upper limits of measurement of some pachymetry devices developed for human use. In this study, the corneas of dogs with endothelial disease or injury frequently exceeded the upper limits of quantitation of 999 and 800μm for the Accupach VI and automated FD-OCT pachymeters, respectively. Using values <800μm, the Pachette 3 generated significantly greater values for central corneal thickness (CCT) than the Accupach VI, manual FD-OCT and automated FD-OCT at 625±7.0, 615±7.2, 613±7.2, and 606±7.4μm respectively (P<0.001). Of the two devices where measurements >1000μm were obtained, manual FD-OCT demonstrated less variability than the Pachette 3. Corneal thickness increased linearly with age and weight with an increase of 6.9±1.8μm/year and 1.6±0.8μm/kg body weight (P<0.005 and P=0.038, respectively).

Mathematical method for analysis of the asymmetric retinal vascular networks.

The purpose of this study was to quantitatively investigate the haemodynamics and oxygen transmission of the retina. Considering the effect of Fåhraeus-Lindqvist effect on the apparent viscosity of blood and the actual haematocrit in blood vessels, this study used the currently known retinal parameters (e.g. blood flow obtained by Doppler Fourier domain optical coherence tomography, FD-OCT for short) to construct a retinal blood circulation model consisting of an asymmetric vascular network system. The blood flow velocity and the vascular diameter in the retinal blood vessels satisfied the exponential relationship. The wall shear stress was related to the release of nitric oxide synthase and endothelin-1 by endothelial cells and played an important role in retinal blood flow regulation. In the retinal arteries, the oxygen tension ranged from 98 to 65 mmHg, and the oxygen saturation ranged from 97.3% to 92.2%. In the retinal veins, the oxygen tension was approximately 41.8 mmHg, and the oxygen saturation ranged from 79.2% to 77.3%. The difference in oxygen content of the arteriovenous network was 5.4 (ml O2/dl blood), and the oxygen extraction of the superior temporal arteriovenous network was 86 (μl/min*ml O2/dl blood). Compared with previous relevant experimental data, the numerical model established in this article demonstrates reliability. It also helps advance our understanding of the retinal pathological processes related to hemodynamics and metabolism.

Schlemm's canal measured by optical coherence tomography and correlation study in a healthy Caucasian child population

To measure the Schlemm's canal (SC) invivo by Fourier-domain optical coherence tomography (FD-OCT) in a Caucasian paediatric population. Participants of this cross-sectional study were 290 healthy children. In the right eye of each child, SC cross-sectional diameter and area measurements were made with the FD-OCT instrument RTVue® (Optovue Inc, Fremont, CA, USA) in the nasal and temporal quadrants. These SC variables were then assessed for correlation with the factors age, gender, refractive error, anterior chamber angle and trabecular meshwork (TM) metrics. Finally, the reproducibility of the SC measurements was assessed in 30 of the participants. Mean participant age was 10.7±3.4years (range 3-18). SC diameters could be measured in both quadrants in 70.6% and 70.4% of subjects, respectively. Mean SC diameters were similar (p=0.125) for the temporal and nasal quadrants: 266.7±84.1μm (range 131-509) and 273.2±77.3μm (range 124-486), respectively. Mean SC areas were also similar (p=0.167) for the two quadrants: 9975±3514μm2 (range 4000-23000) versus 9688±3297μm2 (range 3000-24000). No differences were detected in SC measurements according to gender, refractive error or angle and TM measurements (R≤0.116; p≥0.125). The exception was age which was directly correlated with SC size (p≤0.041). The reproducibility of the SC measurements was excellent (intraclass correlation coefficients ≥0.936). FD-OCT allows the identification of the SC in children. Our data indicate an increase in SC size produced with age.

Normative Values of Peripapillary Retinal Nerve Fiber Layer Thickness in a Middle Eastern Population.

Purpose Peripapillary retinal nerve fiber layer (pRNFL) thickness is subject to high variability. Normative values of pRNFL thickness remain undocumented in the Middle East. The aim of our study is to assess the normative values of pRNFL thickness in a Middle Eastern population. Methods A retrospective chart review of 74 patients was conducted. Outpatients who had presented to the ophthalmology clinic at the Jordan University Hospital between January 2016 and July 2018 were consecutively sampled. Measurements had been recorded using Fourier-domain optical coherence tomography. Multivariable regression models were developed to generate predicted normative values with adjustments to candidate confounders. Results The mean global pRNFL thickness was 99 ± 11 μm. The mean quadrantic pRNFL thickness increased from the nasal quadrant (75 ± 16 μm) to the temporal (82 ± 20 μm), superior (114 ± 20 μm), and inferior (125 ± 20 μm) quadrants. Gender and eye sidedness did not contribute to the variability in pRNFL thickness. The relationship between aging and pRNFL thinning is independent of diabetes mellitus type 2 and systemic hypertension. Both systemic conditions significantly predicted pRNFL changes despite negative fundoscopic findings. Conclusions Our set of predicted normative data may be used to interpret measurements of pRNFL thickness in Middle Eastern patients. Our findings suggest that systemic conditions with potential ocular manifestations may require consideration in predictive models of pRNFL thickness, even in the absence of gross fundoscopic findings. Normative data from additional Middle Eastern populations are required to appraise our models, which adjust for common clinical confounders.

Repeatability of central corneal thickness measurements of donor corneas in preservation chamber using Fourier-domain anterior segment optical coherence tomography.

To evaluate the repeatability of central corneal thickness (CCT) measurements in donor corneas using optical coherence tomography (OCT, RTVue-Optovue, Inc., Fremont, CA). Consecutive corneas were measured by a single observer using the RTVue. All corneas were preserved in the Transend chamber and Life4°C media (Numedis, Inc., Isanti, MN/USA) and stored at 4°C. The repeatability was evaluated using a pooled within-subject standard deviation (SD), coefficient of variation (CoV), and intraclass correlation coefficient (ICC). To investigate inter-observer repeatability, a second observer independently measured the CCT for each image scan. CCT was measured in 32 eyes from 18 donors. Measurements were independently repeated by a second observer. The corneas had a mean CCT of 490.99μm ± 65.95 (381-642) as measured by Observer 1. For observer 1, the SD value for the CCT was 2.94μm, the CoV value was 0.597%, and the ICC value was 0.998 (95% CI 0.996, 0.999). For observer 2, the SD value was 5.91μm, the CoV value was 1.21%, and the ICC value was 0.992 (95% CI 0.985, 0.996). The Kappa statistic 21.88% with a p value < 0.001. The Bland-Altman plot shows that the average CCT measurements between the two observers were within 20μm of each other. The CCT measurements of donor corneas in the preservation chamber using Fourier domain OCT is highly repeatable.

Modified wavelength scanning interferometry for simultaneous tomography and topography of the cornea with Fourier domain optical coherence tomography.

Visual acuity is dependent on corneal shape and size. A minor variation in surface geometry can cause a deformation of corneal geometry, which affects its optical performance. In this work we demonstrate an algorithm for the simultaneous measurement of corneal tomography and topography with a traditional point-scanning Fourier domain optical coherence tomography (FD-OCT) system. A modified wavelength scanning interferometry (mWSI) algorithm enabled topographical evaluation of the surface with nanometer-scale resolution, which is superior to the micrometer-scale resolution of traditional OCT structural imaging. We validated the technique with an optically flat mirror, standard roughness gauges, and atomic force microscopy (AFM). The mirror results show nanometer-scale sensitivity (~3.5 nm), and the mWSI measurements were in good agreement (error ~5%) with the specifications of the roughness comparator and AFM, demonstrating the accuracy of the technique. Following validation, the measurements were made on pig corneas in situ at various artificially controlled intraocular pressures (IOP) and before and after cross-linking (CXL). The results show that the mean surface roughness increased by ~65% after removal of the epithelium in preparation for CXL but did not change as a function of IOP. The demonstrated method could be used for simultaneous measurement of tissue tomography with micrometer-precision and topography with nanometer-precision.