Fourier-domain Optical Coherence Tomography Imaging Research Articles

Monte Carlo-based full-wavelength simulator of Fourier-domain optical coherence tomography.

Monte Carlo (MC) simulation has been widely used to study imaging procedures, including Fourier-domain optical coherence tomography (FD-OCT). Despite the broadband nature of FD-OCT, the results obtained at a single wavelength are often used in previous studies. Some wavelength-relied imaging applications, such as spectroscopic OCT (S-OCT), are unlikely to be simulated in this way due to the lack of information from the entire spectrum. Here, we propose a novel simulator for full-wavelength MC simulation of FD-OCT. All wavelengths within the emission spectrum of the light source will be simulated, and the optical properties derived from Mie theory will be applied. We further combine the inverse discrete Fourier transform (IDFT) with a probability distribution-based signal pre-processing to combat the excessive noises in the OCT signal reconstruction, which is caused by the non-uniform distribution of the scattering events at different wavelengths. Proof-of-concept simulations are conducted to show the excellent performance of the proposed simulator on signal reconstruction and optical properties extraction. Compared with the conventional method, the proposed simulator is more accurate and could better preserve the wavelength-dependent features. For example, the mean square error (MSE) computed between the backscattering coefficient extracted by the proposed simulator and the ground truth is 0.11, which is far less than the value (7.67) of the conventional method. We believe this simulator could be an effective tool to study the wavelength dependency in FD-OCT imaging as well as a preferred solution for simulating spectroscopic OCT.

Age-Related Changes in the Lacrimal Punctum Morphology in a Normal Population: Punctum Update (PUP) Study-Paper 1.

To define the clinical and optical coherence tomography (OCT) morphology of the lacrimal punctum and assess the age-related changes across 8 decades of life in a normal population. A total of 1310 high-magnification slit-lamp, Fourier-domain OCT (FD-OCT), and optical coherence tomography angiography (OCTA) images were obtained from 320 puncta of normal asymptomatic individuals representing the Indian population. The punctum and peri-punctal area were defined with the help of 2 rims (inner punctal rim and outer punctal rim ) and 3 zones (white zone [Wz], separation zone [Sz], and peri-punctal vascularity). FD-OCT images were used to measure the external punctal diameters and internal lacrimal punctal diameters and the reflectivity patterns of the 3 punctal layers. OCTA was used to assess the branching and extent of vascular networks. The upper puncta were narrower and more circular than the lower puncta across the decades. The elevation of the punctal papilla began in the upper puncta in the fifth decade, involved the lower puncta in the sixth decade, and gradually became exaggerated by the eighth decade. A typical punctal narrowing in previously wide puncta began to appear in the sixth decade of life, peaks in the seventh decade, and reverses gradually and spontaneously by the end of the eighth decade of life. The third and fourth decades saw a narrowing of the Sz with prominent vascularity crossing the Sz to reach the Wz. The Sz became indistinct in the fifth and sixth decades of life. The FD-OCT showed gradual thickening and dominance of the fibrous layer from the fifth to eighth decade of life. The clinical vascularity in the peri-punctal region increases from the third decade onwards, with vascular networks becoming increasingly dense, intricate, and branched as the age progresses. The present study defined and characterized the involutional changes in a normal population's first-eighth decades of life. Significant morphological changes were noticed across the different age groups with several clinical implications.

Toric Lenticule Implantation for Correction of Hyperopia and Astigmatism Following Small Incision Lenticule Intrastromal Keratoplasty With the Triple Marking Method.

To describe the first use of toric lenticule implantation with the triple marking method to correct hyperopia and hyperopic astigmatism following small incision lenticule intrastromal keratoplasty (SMI-LIKE). Five eyes of four patients with hyperopia (+5.25 to +9.50 diopters sphere) were included. Allogenic toric lenticules extracted using myopic small incision lenticule extraction were implanted and rotated into a femtosecond laser-created stromal pocket in hyperopic recipients with triple marking. The uncorrected (UDVA) and corrected (CDVA) distance visual acuities, manifest refraction, corneal topography, and Fourier-domain optical coherence tomography images were examined 6 months postoperatively. Six months after surgery, 4 eyes (80%) gained two or more lines of UDVA, and 1 eye (20%) had unchanged UDVA. No eyes lost any lines of CDVA. The spherical equivalent decreased from +6.23 ± 1.34 diopters (D) preoperatively to -0.35 ± 0.27 D postoperatively. The mean refractive astigmatism also decreased significantly from -2.15 ± 1.10 D preoperatively to -0.70 ± 0.21 D postoperatively. Of 5 treated eyes, 2 (40%) had postoperative astigmatism within ±0.50 diopters cylinder and 5 (100%) within ±1.00 diopters cylinder. Corneal topography showed a significant increase in the anterior surface keratometry value, whereas corneal astigmatism decreased from 2.62 ± 1.55 to 1.10 ± 0.44 D. Vector analyses suggested good predictability and efficacy for astigmatism correction. Implantation of toric lenticules following SMI-LIKE with the triple marking method is a feasible alternative for correcting hyperopia and hyperopic astigmatism, and the short-term results seem safe and stable. [J Refract Surg. 2022;38(2):82-88.].

Applying a Pix2Pix Generative Adversarial Network to a Fourier-Domain Optical Coherence Tomography System for Artifact Elimination

The presence of artifacts, including conjugate, DC, and auto-correlation artifacts, is a critical limitation of Fourier-domain optical coherence tomography (FD-OCT). Many methods have been proposed to resolve this problem to obtain high-quality images. Furthermore, the development of deep learning has resulted in many prospective advancements in the medical field; image-to-image translation by using generative adversarial networks (GANs) is one such advancement. In this study, we propose applying the Pix2Pix GAN to eliminate artifacts from FD-OCT images. The first experiment results showed that the proposed framework could translate conventional FD-OCT depth profiles into artifact-free FD-OCT depth profiles. In addition, the FD-OCT depth profile and optical distance of translated images matched those of ground truth images. Second experiment verified that the proposed GAN-based FD-OCT can be applied to generate artifact-free FD-OCT image with different parameters of sample refractive index, the front surface of the sample toward the zero-delay position, and the physical thickness of the sample. Third experiment proved that the proposed model could translated the conventional FD-OCT depth profiles with additional Gaussian noises source image into artifacts-free FD-OCT and successfully relieved the noise.

Effect of spectral leakage on the image formation of Fourier-domain optical coherence tomography.

We report on the investigation of spectral leakage's impact on the reconstruction of Fourier-domain optical coherence tomography (FD-OCT). We discuss the shift-variant nature introduced by the spectral leakage and develop a novel spatial-domain FD-OCT image formation model. A proof-of-concept phantom experiment is conducted to validate our model. Compared with previous models, the proposed framework could better describe the image formation process, especially when the fineness of the axial structure approaches the theoretical resolution limit.

A Coincident Thinning Index for Keratoconus Identification Using OCT Pachymetry and Epithelial Thickness Maps.

To develop a coincident thinning (CTN) index to differentiate between keratoconic and healthy corneas using optical coherence tomography (OCT) measurements of pachymetry and epithelial thickness. Pattern deviation maps of pachymetry and epithelial thickness were generated using Fourier-domain OCT images of the cornea. The co-localized thinning of the two maps was quantified using a novel CTN index, which was calculated from Gaussian fits of the regions of maximum relative thinning. The CTN index was validated using k-fold cross-validation, and its classification performance was compared to minimum pachymetry and maximum keratometry. A total of 82 normal eyes and 133 eyes within three groups of keratoconus severity were evaluated. The pattern deviation maps for the keratoconic eyes showed relative thinning that was larger in magnitude and more strongly correlated with the Gaussian function compared to normal eyes (all P < .01). The distance between the pachymetric and epithelial maximum relative thinning locations was significantly smaller for the keratoconic eyes than for the normal eyes (all P < .02). The CTN index was significantly larger for all three keratoconus groups compared to normal eyes (all P < .0001). The CTN index demonstrated a sensitivity of 100% in detecting manifest keratoconus, 100% for subclinical keratoconus, and 56% for forme fruste keratoconus. The overall classification accuracy was better for the CTN index (93%) than for minimum pachymetry (86%) and maximum keratometry (86%). The CTN index is a highly sensitive measure of coincident pachymetric and epithelial thinning. It provides valuable information for detecting and monitoring early to moderate keratoconus. [J Refract Surg. 2020;36(11):757-765.].

Vitreomacular Interphase Disorders in Eyes with Proliferative Sickle Cell Retinopathy

Purpose: To describe the type of vitreomacula interface disorders (VID) seen in eyes with proliferative sickle cell retinopathy, based on optical coherence tomography findings and assess their effect on visual acuity.&#x0D; Patients and Methods: This was a retrospective observational case study. Clinical records and imaging records were reviewed to identify all PSR cases with Fourier-domain OCT imaging showing VIDs at a single academic private-practice office location from January 1, 2015 to July 30, 2018. Identified VIDs were classified as Vitreomacula Adhesions, Vitreomacula Traction, Lamellar Macula Hole, Full Thickness Macula Hole, Epiretina membranes and Macula Pseudohole.&#x0D; Results: Out of a total of 98 eyes of 78 patients with PSR evaluated, only 12 eyes had VIDs; this represents 12.2% of PSR patients. There were 3 (25%) males and 9 (75%) females with ages ranging from 32 to 64 years, a mean age of 45.42yrs [SD 10.27], 9(75%) right eyes were affected. In 9(75%) patients their genotype was SC and 3(25%) with genotype SS. PSR was at stage 3 in 6(50%) eyes, while the remaining 6(eyes) were at stage 4. The most common VID was epiretina membranes (ERM) seen in 7(58.3%)eyes, in 2(16.6%) ERMs eyes were associated with Lamella Macula Holes and in 1(8.3%) the ERM was associated with a pseudohole. VMAs were seen in in 3(25%) eyes and FTMHs were seen in 2 (16.6%) eyes. ERMs were thus seen in 7.1% (7 eyes) of the 98 eyes with PSR that were considered.&#x0D; Conclusion: Epiretina membranes with or without macular holes are the most common vitreomacular interphase disorders seen in our series of eyes with proliferative sickle cell retinopathy. They were associated with mild to moderate impairment in visual acuity.

Fourier-Domain OCT Imaging of the Ocular Surface and Tear Film Dynamics: A Review of the State of the Art and an Integrative Model of the Tear Behavior During the Inter-Blink Period and Visual Fixation.

In the last few decades, the ocular surface and the tear film have been noninvasively investigated in vivo, in a three-dimensional, high resolution, and real-time mode, by optical coherence tomography (OCT). Recently, OCT technology has made great strides in improving the acquisition speed and image resolution, thus increasing its impact in daily clinical practice and in the research setting. All these results have been achieved because of a transition from traditional time-domain (TD) to Fourier-domain (FD) technology. FD-OCT devices include a spectrometer in the receiver that analyzes the spectrum of reflected light on the retina or ocular surface and transforms it into information about the depth of the structures according to the Fourier principle. In this review, we summarize and provide the state-of-the-art in FD-OCT imaging of the ocular surface system, addressing specific aspects such as tear film dynamics and epithelial changes under physiologic and pathologic conditions. A theory on the dynamic nature of the tear film has been developed to explain the variations within the individual compartments. Moreover, an integrative model of tear film behavior during the inter-blink period and visual fixation is proposed.

Application of optical coherence tomography and optical path length method for monitoring corneal thickness and refractive index change during corneal cross-linking.

Corneal cross-linking (CXL) using UVA irradiation with a riboflavin photosensitizer has emerged as a new treatment paradigm for corneal ectatic disorders. The thickness threshold for protection of intraocular structures has often been challenged with ongoing developments, and corneal thinning becomes an important safety concern, especially for patients with thin corneas. In this study with an ex vivo bovine eye model, we monitored corneal thinning and corneal refractive index changes using optical coherence tomography (OCT) integrated with an adaptation of the optical path length method. CXL experiments were performed based on the standard protocol that includes removal of the corneal epithelium to facilitate diffusion of riboflavin into the stroma. The corneal stromal thickness and group refractive index were measured by a 1310nm Fourier-domain OCT imaging system at three critical points of the procedure: immediately after epithelial removal, after 30min riboflavin instillation, and after 30min UVA irradiation with continuing instillation. We found that the refractive index of the bovine cornea changed significantly from epithelial removal to riboflavin instillation and UVA irradiation, increasing from 1.377±0.005 (mean±standard deviation) after de-epithelization to 1.387±0.003 after 30min instillation and 1.388±0.008 after subsequent irradiation. The corneas also underwent a considerable decrease (10%-20%) in stromal thickness with thinning of 95±29 μm (mean±standard deviation) after riboflavin instillation and a further decrease (∼5%) with thinning of 42±19 μm after UVA irradiation. Our study highlights the importance of corneal thickness monitoring during CXL, especially after riboflavin instillation when the decrease is the largest, to avoid delivering endothelial cytotoxic doses. An increase in refractive index heightens the concern for corneal thinning and the need for careful monitoring as a safety precaution.

Energy-Efficient and Calibration-Aware Fourier-Domain OCT Imaging Processor

This paper presents a novel Fourier-domain optical coherence tomography (FDOCT) imaging processor capable of performing all image formation operations, including dc removal, resampling, real-valued fast Fourier transform (RFFT), and display rendering. Optimizing an RFFT unit through complex-valued FFT computation makes it possible to reduce the clock rate required for the RFFT and post-RFFT units by half, thereby enhancing energy efficiency. A novel resource-sharing scheme further enables calibration (i.e., calculation of resampling indices) awareness checking with minimal area overhead. This makes the proposed FDOCT imaging processor ideally suited to portable OCT application. A test chip operating at dual 80/40 MHz was designed using the Taiwan Semiconductor Manufacturing Company 0.18- $\mu \text{m}$ technology. In postlayout simulation of FDOCT imaging mode, the test chip was shown to consume only 283.6 mW, which represents a power saving of 29% compared to an equivalent design without optimization. The proposed design was also verified at the system level using a field programmable gate arrays (FPGA) platform. In evaluation experiments, we could achieve 78-KHz A-line scan and 30-fps $512\times 1000$ OCT image display. The developed FPGA design also supports calibration awareness check using modest hardware resources.

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.

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 Chamber Angle Tissue as an Incidental Optical Coherence Tomography Finding in a Large Healthy Caucasian Child Population.

The aim of this study was to describe the presence of anterior chamber angle (ACA) tissue as an incidental optical coherence tomography (OCT) finding in healthy children. For this cross-sectional study, we reviewed Fourier-domain OCT images obtained in 422 children, who were chosen as candidates for inclusion in a prior study, and who visited our clinic for a routine vision test. The presence of angle tissue was assessed in the nasal and temporal quadrants. On tomographies showing angle tissue, several measurements were made, including ACA width and trabecular meshwork length and size. Hyperreflective tissue was observed in the ACA recess in 14 eyes of 13 children subjected to OCT (3.3%). This tissue was only present in 1 quadrant. The mean age of the affected children was 10.1±2.5 years (range, 7 to 14); 69% were female patients. Spherical refractive error was -0.5 to +1.75 D. ACA width was 45.9±9.1 (24.8 to 57.9) in the nasal quadrant and 44.3±10.1 (23.9 to 60.1) in the temporal quadrant. Trabecular meshwork length and size were 494±94 μm (322 to 732) and 0.064±0.018 mm (0.03 to 0.11), respectively. In all 13 children, intraocular pressure was ≤18 mm Hg, the appearance of the optic nerve head was normal, and none had a family history of glaucoma. We, here, report the incidental observation of angle tissue in a small percentage of healthy Caucasian children. The follow-up of these children in future work could clarify the possible clinical implications of this observation.

Multi-shaping technique reduces sidelobe magnitude in optical coherence tomography.

Shaping methods that are commonly used in Fourier-domain optical coherence tomography (FD-OCT) can suppress sidelobe artifacts in the axial direction, but they typically broaden the mainlobe of the point spread function (PSF) and reduce the axial resolution. To improve OCT image quality without this tradeoff, we have developed a multi-shaping technique that reduces the axial sidelobe magnitude dramatically and achieves better axial resolution than conventional shaping methods. This technique is robust and compatible in various FD-OCT imaging systems. Testing of multi-shaping in three experimental settings shows that it reduced the axial sidelobe contribution by more than 8 dB and improved the contrast to noise by at least 30% and up to three-fold. Multi-shaping enables accurate image analysis and is potentially useful in many OCT applications.

Off-axis reference beam for full-field swept-source OCT and holoscopy.

In numerous applications, Fourier-domain optical coherence tomography (FD-OCT) suffers from a limited imaging depth due to signal roll-off, a limited focal range, and autocorrelation noise. Here, we propose a parallel full-field FD-OCT imaging method that uses a swept laser source and an area camera in combination with an off-axis reference, which is incident on the camera at a small angle. As in digital off-axis holography, this angle separates autocorrelation signals and the complex conjugated mirror image from the actual signal in Fourier space. We demonstrate that by reconstructing the signal term only, this approach enables full-range imaging, i.e., it increases the imaging depth by a factor of two, and removes autocorrelation artifacts. The previously demonstrated techniques of inverse scattering and holoscopy can then numerically extend the focal range without loss of lateral resolution or imaging sensitivity. The resulting, significantly enhanced measurement depth is demonstrated by imaging a porcine eye over its entire depth, including cornea, lens, and retina. Finally, the feasibility of in vivo measurements is demonstrated by imaging the living human retina.

Lower Macular Pigment Optical Density in Foveal-Involved Glaucoma

To evaluate the relationship between macular pigment optical density (MPOD) and structural parameters of the macula and optic nerve head in glaucomatous eyes. A cross-sectional analysis of the baseline data collected during the Macular Pigment and Glaucoma Trial (ISRCTN registry number: 56985060). Eighty-eight subjects (48 male, 40 female) with a diagnosis of open-angle glaucoma and a median age of 67 years (interquartile range, 13; range, 36-84 years) were enrolled in this trial. The MPOD at 0.25°, 0.5°, and 1° retinal eccentricity was measured using a customized heterochromatic flicker photometry technique. Glaucoma-related structural parameters were captured using RTVue Fourier-domain optical coherence tomography (FD-OCT). Statistical significance was set at P < 0.01, and P values ranging from 0.01 to 0.05 were considered borderline significant. The MPOD and its relationship to the macula and optic nerve head topography in glaucomatous eyes. The MPOD peaked centrally at 0.25° of retinal eccentricity (mean ± standard deviation, 0.23±0.14) and decreased at more peripheral eccentricities. For the overall group, borderline significant correlations were observed between MPOD and a range of topographic measures, including inferior peripapillary retinal nerve fiber layer (RNFL) thickness, inferior ganglion cell complex (GCC) thickness, foveal inner retinal thickness, cup-to-disc area ratio, and optic disc rim area. Glaucomatous eyes with GCC loss involving the foveal zone on FD-OCT imaging (n = 52) had lower MPOD at 0.25°, 0.5°, and 1° of retinal eccentricity compared with those without foveal GCC involvement (P < 0.001, for all). Those with foveal GCC loss also had greater glaucoma severity, and this was evident by lower GCC and RNFL thickness, greater cup-to-disc area ratio, and lower optic disc rim area (P < 0.001 for all). Our observations indicate that MPOD is lower in glaucomatous eyes with foveal GCC involvement relative to those without foveal involvement. A longitudinal evaluation of MPOD and structural change among patients with glaucoma is required to elucidate the nature of any causal relationship that might exist between MPOD and foveal damage in glaucoma.

DIRECTIONAL OPTICAL COHERENCE TOMOGRAPHY PROVIDES ACCURATE OUTER NUCLEAR LAYER AND HENLE FIBER LAYER MEASUREMENTS.

The outer nuclear layer (ONL) contains photoreceptor nuclei, and its thickness is an important biomarker for retinal degenerations. Accurate ONL thickness measurements are obscured in standard optical coherence tomography (OCT) images because of Henle fiber layer (HFL). Improved differentiation of the ONL and HFL boundary is made possible by using directional OCT, a method that purposefully varies the pupil entrance position of the OCT beam. Fifty-seven normal eyes were imaged using multiple pupil entry positions with a commercial spectral domain OCT system. Cross-sectional image sets were registered to each other and segmented at the top of HFL, the border of HFL and the ONL and at the external limiting membrane. Thicknesses of the ONL and HFL were measured and analyzed. The true ONL and HFL thicknesses varied substantially by eccentricity and between individuals. The true macular ONL thickness comprised an average of 54.6% of measurements that also included HFL. The ONL and HFL thicknesses at specific retinal eccentricities were poorly correlated. Accurate ONL and HFL thickness measurements are made possible by the optical contrast of directional OCT. Distinguishing these individual layers can improve clinical trial endpoints and assessment of disease progression.

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To evaluate the relationship between macular pigment optical density (MPOD) and structural parameters of the macula and optic nerve head in glaucomatous eyes.A cross-sectional analysis of the baseline data collected during the Macular Pigment and Glaucoma Trial (ISRCTN registry number: 56985060).Eighty-eight subjects (48 male, 40 female) with a diagnosis of open-angle glaucoma and a median age of 67 years (interquartile range, 13; range, 36–84 years) were enrolled in this trial.The MPOD at 0.25°, 0.5°, and 1° retinal eccentricity was measured using a customized heterochromatic flicker photometry technique. Glaucoma-related structural parameters were captured using RTVue Fourier-domain optical coherence tomography (FD-OCT). Statistical significance was set at P < 0.01, and P values ranging from 0.01 to 0.05 were considered borderline significant.The MPOD and its relationship to the macula and optic nerve head topography in glaucomatous eyes.The MPOD peaked centrally at 0.25° of retinal eccentricity (mean ± standard deviation, 0.23±0.14) and decreased at more peripheral eccentricities. For the overall group, borderline significant correlations were observed between MPOD and a range of topographic measures, including inferior peripapillary retinal nerve fiber layer (RNFL) thickness, inferior ganglion cell complex (GCC) thickness, foveal inner retinal thickness, cup-to-disc area ratio, and optic disc rim area. Glaucomatous eyes with GCC loss involving the foveal zone on FD-OCT imaging (n = 52) had lower MPOD at 0.25°, 0.5°, and 1° of retinal eccentricity compared with those without foveal GCC involvement (P < 0.001, for all). Those with foveal GCC loss also had greater glaucoma severity, and this was evident by lower GCC and RNFL thickness, greater cup-to-disc area ratio, and lower optic disc rim area (P < 0.001 for all).Our observations indicate that MPOD is lower in glaucomatous eyes with foveal GCC involvement relative to those without foveal involvement. A longitudinal evaluation of MPOD and structural change among patients with glaucoma is required to elucidate the nature of any causal relationship that might exist between MPOD and foveal damage in glaucoma.

Enhancement of Corneal Visibility in Optical Coherence Tomography Images Using Corneal Adaptive Compensation.

To improve the contrast of optical coherence tomography (OCT) images of the cornea (post processing). We have recently developed standard compensation (SC) algorithms to remove light attenuation artifacts. A more recent approach, namely adaptive compensation (AC), further limited noise overamplification within deep tissue regions. AC was shown to work efficiently when all A-scan signals were fully attenuated at high depth. But in many imaging applications (e.g., OCT imaging of the cornea), such an assumption is not satisfied, which can result in strong noise overamplification. A corneal adaptive compensation (CAC) algorithm was therefore developed to overcome such limitation. CAC benefited from local A-scan processing (rather than global as in AC) and its performance was compared with that of SC and AC using Fourier-domain OCT images of four human corneas. CAC provided considerably superior image contrast improvement than SC or AC did, with excellent visibility of the corneal stroma, low noise overamplification, homogeneous signal amplification, and high contrast. Specifically, CAC provided mean interlayer contrasts (a measure of high stromal visibility and low noise) greater than 0.97, while SC and AC provided lower values ranging from 0.38 to 1.00. CAC provided considerable improvement compared with SC and AC by eliminating noise overamplification, while maintaining all benefits of compensation, thus making the corneal endothelium and corneal thickness easily identifiable. CAC may find wide applicability in clinical practice and could contribute to improved morphometric and biomechanical understanding of the cornea.

Measurement of retinal blood flow in normal Chinese-American subjects by Doppler Fourier-domain optical coherence tomography.

To measure total retinal blood flow (TRBF) in normal, healthy Chinese Americans by using semi-automated analysis of Doppler Fourier-domain optical coherence tomography (FD-OCT) scans. Two hundred sixty-six normal, healthy Chinese-American participants (266 eyes) were enrolled from The Chinese American Eye Study. All participants underwent complete ophthalmic examination, including best-corrected visual acuity, indirect ophthalmoscopy, and Doppler FD-OCT imaging, using the circumpapillary double circular scan protocol. Total retinal blood flow and other vascular parameters (e.g., venous and arterial cross-sectional area and their velocities) were calculated by using Doppler OCT of Retinal Circulation software. Associations between TRBF and other clinical parameters were assessed by using bivariate correlations and linear regression. The mean age of study participants was 57.40 ± 5.60 (range, 50-82) years. The mean TRBF was 49.34 ± 10.08 (range, 27.17-78.08, 95% confidence interval: 25.98-69.10) μL/min. The mean venous area was 0.0548 (±0.0084) mm(2). Superior retinal hemispheric blood flow (25.50 ± 6.62 μL/min) was slightly greater than inferior retinal hemispheric blood flow (23.84 ± 7.19 μL/min, P = 0.008). The mean flow velocity was 15.16 ± 3.12 mm/s. There was a weak but significant negative correlation between TRBF and age (r = -0.15, P = 0.012). No significant correlation was found between TRBF and axial length (r = 0.11, P = 0.08). Retinal blood flow was not significantly correlated with any other clinical parameters, including body mass index, systolic blood pressure, diastolic blood pressure, and intraocular pressure. Normal Doppler OCT-derived total retinal blood values in a Chinese-American population showed considerable variability, some of which was explained by age. These observations should help design future studies evaluating TRBF in populations with eye disease.