Fourier-domain Optical Coherence Tomography System Research Articles

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

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

Rapid imaging and product screening with low-cost line-field Fourier domain optical coherence tomography

Fourier domain optical coherence tomography (FD-OCT) is a well-established imaging technique that provides high-resolution internal structure images of an object at a fast speed. Modern FD-OCT systems typically operate at speeds of 40,000–100,000 A-scans/s, but are priced at least tens of thousands of pounds. In this study, we demonstrate a line-field FD-OCT (LF-FD-OCT) system that achieves an OCT imaging speed of 100,000 A-scan/s at a hardware cost of thousands of pounds. We demonstrate the potential of LF-FD-OCT for biomedical and industrial imaging applications such as corneas, 3D printed electronics, and printed circuit boards.

Developing a New Dimension for Fourier Domain Optical Coherence Tomography Images by Simultaneous Measurement of the Refractive Index and Thickness.

Introduction: Fourier domain Optical coherence tomography (OCT) is a widely used high-resolution optical imaging technique. It is useful for various applications in medical imaging, such as ophthalmology (e.g. retinal imaging for diagnosing complications like glaucoma or macular degeneration), dermatology, oncology, and cardiology. The ability to noninvasively measure both the refractive index and thickness of biological tissues could have various medical applications and enable earlier disease detection. For example, observing changes in the refractive index can help distinguish between tissues with normal or abnormal function. Methods: In this study, the theoretical framework for simultaneous measurement of the refractive index and physical thickness of multilayer systems is proposed and tested for two different samples, each having three layers, a glass/NaCl solution/glass sample and a glass/sugar solution/glass sample. The whole signal processing procedure and the experimental setup are described. Results: The refractive index and thickness of salt water and sugar water samples in the Fourier-domain OCT (FD-OCT) system were obtained. The resulting data were compared with reference measurements and showed a deviation of about 1% for the samples. Conclusion: We tested the proposed framework for the simultaneous extraction of the refractive index and thickness of multilayer systems of salt water and sugar water from its FD-OCT data. We showed that the measured parameters were in agreement with reference amounts.

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.

Optical flow optical coherence tomography for determining accurate velocity fields.

Determining micron-scale fluid flow velocities using optical coherence tomography (OCT) is important in both biomedical research and clinical diagnosis. Numerous methods have been explored to quantify the flow information, which can be divided into either phase-based or amplitude-based methods. However, phase-based methods, such as Doppler methods, are less sensitive to transverse velocity components and suffer from wrapped phase and phase instability problems for axial velocity components. On the other hand, amplitude-based methods, such as speckle variance OCT, correlation mapping OCT and split-spectrum amplitude-decorrelation angiography, focus more on segmenting flow areas than quantifying flow velocities. In this paper, we propose optical flow OCT (OFOCT) to quantify accurate velocity fields. The equivalence between optical flow and real velocity fields is validated in OCT imaging. The sensitivity fall-off of a Fourier-domain OCT (FDOCT) system is considered in the modified optical flow continuity constraint. Spatial-temporal smoothness constraints are used to make the optical flow problem well-posed and reduce noises in the velocity fields. An iteration solution to the optical flow problem is implemented in a graphics processing unit (GPU) for real-time processing. The accuracy of the velocity fields is verified through phantom flow experiments by using a diluted milk powder solution as a scattering medium. Velocity fields are then used to detect flow turbulence and reconstruct flow trajectory. The results show that OFOCT is accurate in determining velocity fields and applicable to research concerning fluid dynamics.

Robust wavenumber and dispersion calibration for Fourier-domain optical coherence tomography.

Many Fourier-domain optical coherence tomography (FD-OCT) systems sample the interference fringes with a non-uniform wavenumber (k) interval, introducing a chirp to the signal that depends on the path length difference underlying each fringe. A dispersion imbalance between sample and reference arms also generates a chirp in the fringe signal which, in contrast, is independent of depth. Fringe interpolation to obtain a signal linear in k and compensate dispersion imbalance is critical to achieving bandwidth-limited axial resolution. In this work, we propose an optimization-based algorithm to perform robust and automated calibration of FD-OCT systems, recovering both the interpolation function and the dispersion imbalance. Our technique relies on the fact that the unique function that correctly linearizes the fringe data in k space produces a depth-independent chirp. The calibration procedure requires experimental data corresponding to a single reflector at various depth locations, which can easily be obtained by acquiring data while moving a sample mirror in depth. We have tested both spectral domain OCT and swept source OCT systems with various nonlinearities. Results indicate that the proposed calibration method has excellent performance on a wide range of data sets and enables nearly constant resolution at all imaging depths. An implementation of the algorithm is available online.

Full resolution Fourier domain optical coherence tomography

The complex conjugate ambiguity in Fourier domain optical coherence tomography (FDOCT) is a major roadblock that prevents full-detector resolution for the depth scan in single shot operation for the individual A-scan. Current techniques to eliminate this problem involve changing the experimental set-up, usually complicating the OCT system. In this work we show that the standard FDOCT spectrum data when resampled appropriately can be cast exactly in terms of type-1 discrete cosine transform (DCT). Additionally, a sparse reconstruction method in the DCT domain enables image recovery with full-detector resolution, thus effectively doubling the depth scan resolution. In a realistic simulation study we demonstrate full-detector resolution for a discrete reflective target by successfully resolving closely spaced reflective peaks that cannot be separated using the standard Fourier transform based reconstruction. Experimental results on reflective glass sheet targets further validate the methodology. The results of the proposed technique suggest that full resolution FDOCT systems may be implemented practically without additional hardware costs and system complexity.

Altered Anterior Segment Biometric Parameters in Mice Deficient in SPARC.

Secreted protein acidic and rich in cysteine (SPARC) and Hevin are structurally related matricellular proteins involved in extracellular matrix assembly. In this study, we compared the anterior chamber biometric parameters and iris collagen properties in SPARC-, Hevin- and SPARC-/Hevin-null with wild-type (WT) mice. The right eyes of 53 WT, 35 SPARC-, 56 Hevin-, and 63 SPARC-/Hevin-null mice were imaged using the RTVue-100 Fourier-domain optical coherence tomography system. The parameters measured were anterior chamber depth (ACD), trabecular-iris space area (TISA), angle opening distance (AOD), and pupil diameter. Biometric data were analyzed using analysis of covariance and adjusted for age, sex, and pupil diameter. Expression of Col1a1, Col8a1, and Col8a2 transcripts in the irises was measured by quantitative polymerase chain reaction. Collagen fibril thickness was evaluated by transmission electron microscopy. Mice that were SPARC- and SPARC-/Hevin-null had 1.28- and 1.25-fold deeper ACD, 1.45- and 1.53-fold larger TISA, as well as 1.42- and 1.51-fold wider AOD than WT, respectively. These measurements were not significantly different between SPARC- and SPARC-/Hevin-null mice. The SPARC-null iris expressed lower Col1a1, but higher Col8a1 and Col8a2 transcripts compared with WT. Collagen fibrils in the SPARC- and SPARC-/Hevin-null irises were 1.5- and 1.7-fold thinner than WT, respectively. The Hevin-null iris did not differ from WT in these collagen properties. SPARC-null mice have deeper anterior chamber as well as wider drainage angles compared with WT. Therefore, SPARC plays a key role in influencing the spatial organization of the anterior segment, potentially via modulation of collagen properties, while Hevin is not likely to be involved.

In vivo three-dimensional imaging of human corneal nerves using Fourier-domain optical coherence tomography

We have employed Fourier-domain optical coherence tomography (FD-OCT) to achieve corneal nerve imaging, which could be useful in surgical planning and refractive surgery. Because the three-dimensional (3-D) images of the corneal nerves were acquired in vivo, unintentional movement of the subject during the measurement led to imaging artifacts. These artifacts were compensated for with a series of signal processing techniques, namely realigning A-scan images to flatten the boundary and cross-correlating adjacent B-scan images. To overcome the undesirably large signal from scattering at the corneal surface and iris, volume rendering and maximum intensity projections were performed with only the data taken in the stromal region of the cornea, which is located between 200 and 500???m from the corneal surface. The 3-D volume imaging of a 10×10??mm2 area took 9.8 s, which is slightly shorter than the normal tear breakup time. This allowed us to image the branched and threadlike corneal nerve bundles within the human eye. The experimental results show that FD-OCT systems have the potential to be useful in clinical investigations of corneal nerves and by minimizing nerve injury during clinical or surgical procedures.

3D homogeneity study in PMMA layers using a Fourier domain OCT system

Micro-metallic particles embedded in polymers are now widely used in several industrial applications in order to modify the mechanical properties of the bulk. A uniform distribution of these particles inside the polymers is highly desired for instance, when a biological backscattering is simulated or a bio-framework is designed. A 3D Fourier domain optical coherence tomography system to detect the polymer's internal homogeneity is proposed. This optical system has a 2D camera sensor array that records a fringe pattern used to reconstruct with a single shot the tomographic image of the sample. The system gathers the full 3D tomographic and optical phase information during a controlled deformation by means of a motion linear stage. This stage avoids the use of expensive tilting stages, which in addition are commonly controlled by piezo drivers. As proof of principle, a series of different deformations were proposed to detect the uniform or non-uniform internal deposition of copper micro particles. The results are presented as images coming from the 3D tomographic micro reconstruction of the samples, and the 3D optical phase information that identifies the in-homogeneity regions within the Poly methyl methacrylate (PMMA) volume.

Optical module to extend any Fourier-domain optical coherence tomography system into a polarisation-sensitive system

This article presents a theoretical study on an optical module (OM) that can be inserted between an object under investigation and a Fourier-domain optical coherence tomography system, transforming the latter into a polarisation-sensitive optical coherence tomography optical coherence tomography (OCT) system. The module consists of two electro-optic modulators, a Faraday rotator, a linear polariser and a quarter-wave plate. A detailed description on how the module can be used to extract both the net retardance and the fast axis orientation of a linear birefringent sample is presented. This is achieved by taking two sequential measurements for different values of retardance produced by the electro-optic modulator. The module keeps measurements free from undesired polarimetric effects due to birefringence in the single-mode optical fibre and diattenuation in fibre-based couplers within OCT systems. Simulations have been carried out in order to evaluate the effects of chromatic behaviour of the components within the OM.

Subclinical keratoconus detection by pattern analysis of corneal and epithelial thickness maps with optical coherence tomography

To screen for subclinical keratoconus by analyzing corneal, epithelial, and stromal thickness map patterns with Fourier-domain optical coherence tomography (OCT). Four centers in the United States. Cross-sectional observational study. Eyes of normal subjects, subclinical keratoconus eyes, and the topographically normal eye of a unilateral keratoconus patient were studied. Corneas were scanned using a 26,000 Hz Fourier-domain OCT system (RTVue). Normal subjects were divided into training and evaluation groups. Corneal, epithelial, and stromal thickness maps and derived diagnostic indices, including pattern standard deviation (PSD) variables and pachymetric map-based keratoconus risk scores, were calculated from the OCT data. Area under the receiver operating characteristic curve (AUC) analysis was used to evaluate the diagnostic accuracy of the indices. The study comprised 150 eyes of 83 normal subjects, 50 subclinical keratoconus eyes of 32 patients, and 1 topographically normal eye of a unilateral keratoconus patient. Subclinical keratoconus was characterized by inferotemporal thinning of the cornea, epithelium, and stroma. The PSD values for corneal (P < .001), epithelial (P < .001), and stromal (P = .049) thickness maps were all significantly higher in subclinical keratoconic eyes than in the normal group. The diagnostic accuracy was significantly higher for PSD variables (pachymetric PSD, AUC = 0.941; epithelial PSD, AUC = 0.985; stromal PSD, AUC = 0.924) than for the pachymetric map-based keratoconus risk score (AUC = 0.735). High-resolution Fourier-domain OCT could map corneal, epithelial, and stromal thicknesses. Corneal and sublayer thickness changes in subclinical keratoconus could be detected with high accuracy using PSD variables. These new diagnostic variables might be useful in the detection of early keratoconus. Oregon Health and Science University (OHSU) and Drs. Li, Tan, and Huang have a significant financial interest in Optovue, Inc. These potential conflicts have been reviewed and managed by OHSU. Dr. Brass receives research grants from Optovue, Inc. Drs. Chamberlain and Weiss have no financial or proprietary interest in any material or method mentioned.

Comparative study of the fourier-domain optical coherence tomography structural changes and visual field loss for early detection of glaucoma

Objective To evaluate the macular ganglion cell complex (GCC) and retinal nerve fiber layer (RNFL) measurements with Fourier-domain optical coherence tomography (FDOCT) for the detection of early glaucoma and to compare the results with visual field (VF) defects. Design This was a cross-sectional prospective diagnostic study. Patients and methods This study included 10 normal eyes and 48 eyes of patients suspected of having glaucoma on the basis of the appearance of the optic disc. All eyes had normal VFs by standard automated perimetry before the imaging session. A FDOCT system was used to map the macula and peripapillary regions of the retina. Short-wave automated perimetry was performed. Areas under the receiver operating characteristic curves (AUC) were calculated to summarize the diagnostic accuracies of the parameters. Results The AUCs for the VF parameters were 0.819 for the mean deviation and 0.911 for the pattern standard deviation. AUCs for the RNFL parameters ranged from 0.625 for the superior average thickness to 0.693 for the inferior average RNFL thickness. AUCs for the macular parameters ranged from 0.467 for the GCC–global loss volume to 0.556 for the inferior GCC thickness. The AUC of the VF parameter with the largest AUC, pattern standard deviation, was significantly higher than that of the RNFL and macular parameters (0.911 vs. 0.693 and 0.556). Conclusion GCC measurements with FDOCT were similar to the RNFL parameters, but were inferior to VF deficits on short-wave automated perimetry for early detection of glaucoma.

Study of dual-source Fourier-domain optical coherence tomography

Optical coherence tomography (OCT) is a very high-resolution imaging technique whose resolution depends on source bandwidth. Improving resolution is an important topic of research in OCT. Thus, to improve resolution, the bandwidth of the source must be increased. Practical sources have finite bandwidth. Therefore, it is suggested to use more sources. Previous work expected that resolution will be inevitably improved without mentioning to what extent it will be improved and without any referencing to the effect of spectral separation between the sources. We study the resolution of a Fourier-domain OCT (FD-OCT) system based on two sources. First, we show to what extent resolution is improved and we show that this improvement not only depends on the spectral widths of the sources but also on spectral separation of the sources. Second, we show that in most cases resolution will become poorer and discuss mathematically the origin of resolution worsening and reveal the problems encountered in such a system. Third, we propose two techniques to overcome these problems. One of them is by shifting spectral interferograms and the other is by multiplying the spatial interferogram. Then, we clarify their advantages, disadvantages, major drawbacks, and limitations.

Miniaturized magnetic-driven scanning probe for endoscopic optical coherence tomography.

We designed and implemented a magnetic-driven scanning (MDS) probe for endoscopic optical coherence tomography (OCT). The probe uses an externally-driven tiny magnet in the distal end to achieve unobstructed 360-degree circumferential scanning at the side of the probe. The design simplifies the scanning part inside the probe and thus allows for easy miniaturization and cost reduction. We made a prototype probe with an outer diameter of 1.4 mm and demonstrated its capability by acquiring OCT images of ex vivo trachea and artery samples from a pigeon. We used a spectrometer-based Fourier-domain OCT system and the system sensitivity with our prototype probe was measured to be 91 dB with an illumination power of 850 μW and A-scan exposure time of 1 ms. The axial and lateral resolutions of the system are 6.5 μm and 8.1 μm, respectively.

Corneal Epithelial Remodeling after LASIK Measured by Fourier-Domain Optical Coherence Tomography.

Purpose. To quantify corneal epithelial thickness changes after myopic LASIK by OCT. Methods. Epithelial thickness before and after myopic LASIK were measured by a Fourier-domain OCT system. Average central (within 1 mm diameter) and paracentral epithelial thickness (5~6 mm diameter) before and after LASIK were compared. Correlation between central epithelial thickness change and laser spherical equivalent setting was evaluated. An epithelial smoothing constant was estimated based on a mathematical model published previously. Results. Nineteen eyes from 11 subjects were included in the study. Eyes had myopic LASIK ranging from −1.69 D to −6.75 D spherical equivalent. The average central epithelial thickness was 52.6 ± 4.1 μm before LASIK and 56.2 ± 4.3 μm 3 months after LASIK (p = 0.002). The average paracentral epithelial thickness was 51.6 ± 6.6 μm before LASIK and 54.8 ± 4.3 μm 3 months after LASIK (p = 0.007). The change in average central epithelial thickness was correlated with laser spherical equivalent (R 2 = 0.40, p = 0.028). The epithelial smoothing constant was estimated to be 0.46 mm. Conclusions. Corneal epithelial thickens centrally and paracentrally after myopic LASIK. The extent of epithelial remodeling correlated with the amount of LASIK correction and could be predicted by a mathematical model.

Long-range Fourier domain optical coherence tomography of the pediatric subglottis

BackgroundAcquired subglottic stenosis (SGS) most commonly results from prolonged endotracheal intubation and is a diagnostic challenge in the intubated child. At present, no imaging modality allows for in vivo characterization of subglottic microanatomy to identify early signs of acquired SGS while the child remains intubated. Fourier domain optical coherence tomography (FD-OCT) is a minimally invasive, light-based imaging modality which provides high resolution, three dimensional (3D) cross-sectional images of biological tissue. We used long-range FD-OCT to image the subglottis in intubated pediatric patients undergoing minor head and neck surgical procedures in the operating room. MethodsA long-range FD-OCT system and rotary optical probes (1.2mm and 0.7mm outer diameters) were constructed. Forty-six pediatric patients (ages 2–16 years) undergoing minor upper airway surgery (e.g., tonsillectomy and adenoidectomy) were selected for intraoperative, trans-endotracheal tube FD-OCT of the subglottis. Images were analyzed for anatomical landmarks and subepithelial histology. Volumetric image sets were rendered into virtual 3D airway models in Mimics software. ResultsFD-OCT was performed on 46 patients (ages 2–16 years) with no complications. Gross airway contour was visible on all 46 data sets. Twenty (43%) high-quality data sets clearly demonstrated airway anatomy (e.g., tracheal rings, cricoid and vocal folds) and layered microanatomy of the mucosa (e.g., epithelium, basement membrane and lamina propria). The remaining 26 data sets were discarded due to artifact, high signal-to-noise ratio or missing data. 3D airway models were allowed for user-controlled manipulation and multiplanar airway slicing (e.g., sagittal, coronal) for visualization of OCT data at multiple anatomic levels simultaneously. ConclusionsLong-range FD-OCT produces high-resolution, 3D volumetric images of the pediatric subglottis. This technology offers a safe and practical means for in vivo evaluation of lower airway microanatomy in intubated pediatric patients. Ultimately, FD-OCT may be applied to serial monitoring of the neonatal subglottis in long-term intubated infants at risk for acquired SGS.

Chemically Etched Axicon Fiber as a Sensor Probe of Optical Coherence Tomography

In this paper, we investigated a high lateral resolution common-path Fourier domain OCT system with the use of a chemically etched single mode fiber. In our experiments, single mode optical fiber for 840nm was used for preparing the tapered tips. Our system used a conical microlens that was chemically etched by selective chemical etching technique using an etching solution of buffered hydrofluoric acid (BHF). From experimental results, we verified that our proposed optical coherence tomography system could operate as a common-path Fourier domain OCT (FD-OCT) system. Furthermore, we experimentally investigated the chemical etching process of silica-based optical fiber probes by controlling the etching time when applying chemical etching techniques using hydrofluoric acid. We evaluated the beam profile from fiber probe. From these investigations, it was found that etching time was important parameter for sensor probe of optical coherence tomography.

Dual-band Fourier domain optical coherence tomography with depth-related compensations

Dual-band Fourier domain optical coherence tomography (FD-OCT) provides depth-resolved spectroscopic imaging that enhances tissue contrast and reduces image speckle. However, previous dual-band FD-OCT systems could not correctly give the tissue spectroscopic contrast due to depth-related discrepancy in the imaging method and attenuation in biological tissue samples. We designed a new dual-band full-range FD-OCT imaging system and developed an algorithm to compensate depth-related fall-off and light attenuation. In our imaging system, the images from two wavelength bands were intrinsically overlapped and their intensities were balanced. The processing time of dual-band OCT image reconstruction and depth-related compensations were minimized by using multiple threads that execute in parallel. Using the newly developed system, we studied tissue phantoms and human cancer xenografts and muscle tissues dissected from severely compromised immune deficient mice. Improved spectroscopic contrast and sensitivity were achieved, benefiting from the depth-related compensations.

Fourier-Domain Optical Coherence Tomography Imaging in Keratoconus: A Corneal Structural Classification

To study corneal morphologic changes in a large keratoconic population and to establish a structural optical coherence tomography (OCT) classification. Cross-sectional, observational study. A total of 218 keratoconic eyes from 218 patients and 34 eyes from 34 normal subjects. A Fourier-domain OCT system with 5-μm axial resolution was used. For each patient, 3 high-resolution scans were made across the keratoconus cone. All scans were analyzed by keratoconus specialists who were not given access to patients' clinical and topographic data, and who established an OCT classification. The reproducibility of the classification and its correlation with clinical and paraclinical characteristics of patients with keratoconus were evaluated. The OCT examinations were performed every 4 months to follow up structural corneal changes. Evaluation of the structural corneal changes occurring in keratoconus cases with various stages of severity based on OCT findings. Fourier-domain OCT classification containing 5 distinct keratoconus stages is proposed. Stage 1 demonstrates thinning of apparently normal epithelial and stromal layers at the conus. Stage 2 demonstrates hyperreflective anomalies occurring at the Bowman's layer level with epithelial thickening at the conus. Stage 3 demonstrates posterior displacement of the hyperreflective structures occurring at the Bowman's layer level with increased epithelial thickening and stromal thinning. Stage 4 demonstrates pan-stromal scar. Stage 5 demonstrates hydrops; 5a, acute onset: Descemet's membrane rupture and dilaceration of collagen lamellae with large fluid-filled intrastromal cysts; 5b, healing stage: pan-stromal scarring with a remaining aspect of Descemet's membrane rupture. The reproducibility of the classification was very high between the corneal specialist observers. Clinical and paraclinical characteristics of keratoconus, including visual acuity, corneal epithelium and stromal thickness changes, corneal topography, biomechanical corneal characteristics, and microstructural changes observed on confocal microscopy, were concordant with our OCT grading. Optical coherence tomography provides an accurate assessment of structural changes occurring in keratoconus eyes. These changes were correlated with clinical and paraclinical characteristics of patients. The established classification not only allows structural follow-up of patients with keratoconus but also provides insight into the pathogenesis of keratoconus and treatment strategies for future research.