Group index of the human cornea at 1.3-microm wavelength obtained in vitro by optical coherence domain reflectometry

Noncontact Acoustic Micro-Tapping Optical Coherence Elastography for Quantification of Corneal Anisotropic Elasticity: In Vivo Rabbit Study

Purpose

The purpose of this study was to demonstrate accurate measurement of corneal elastic moduli in vivo with noncontact and noninvasive optical coherence elastography.

Methods

Elastic properties (in-plane Young's modulus, E, and both in-plane, μ, and out-of-plane, G, shear moduli) of rabbit cornea were quantified in vivo using noncontact dynamic acoustic micro-tapping optical coherence elastography (AµT-OCE). The intraocular pressure (IOP)-dependence of measured mechanical properties was explored in extracted whole globes following in vivo measurement. A nearly incompressible transverse isotropic (NITI) model was used to reconstruct moduli from AµT-OCE data. Independently, cornea elastic moduli were also measured ex vivo with traditional, destructive mechanical tests (tensile extensometry and shear rheometry).

Results

Our study demonstrates strong anisotropy of corneal elasticity in rabbits. The in-plane Young's modulus, computed as E = 3μ, was in the range of 20 MPa to 44 MPa, whereas the out-of-plane shear modulus was in the range of 34 kPa to 261 kPa. Both pressure-dependent ex vivo OCE and destructive mechanical tests performed on the same samples within an hour of euthanasia strongly support the results of AµT-OCE measurements.

Conclusions

Noncontact AµT-OCE can noninvasively quantify cornea anisotropic elastic properties in vivo.

Translational Relevance

As optical coherence tomography (OCT) is broadly accepted in ophthalmology, these results suggest the potential for rapid translation of AµT-OCE into clinical practice. In addition, AµT-OCE can likely improve diagnostic criteria of ectatic corneal diseases, leading to early diagnosis, reduced complications, customized surgical treatment, and personalized biomechanical models of the eye.

Estimation of the full shape of the crystalline lens in-vivo from OCT images using eigenlenses

Quantifying the full 3-D shape of the human crystalline lens is important for improving intraocular lens power or sizing calculations in treatments of cataract and presbyopia. In a previous work we described a novel method for the representation of the full shape of the ex vivo crystalline lens called eigenlenses, which proved more compact and accurate than compared state-of-the art methods of crystalline lens shape quantification. Here we demonstrate the use of eigenlenses to estimate the full shape of the crystalline lens in vivo from optical coherence tomography images, where only the information visible through the pupil is available. We compare the performance of eigenlenses with previous methods of full crystalline lens shape estimation, and demonstrate an improvement in repeatability, robustness and use of computational resources. We found that eigenlenses can be used to describe efficiently the crystalline lens full shape changes with accommodation and refractive error.

Optical coherence tomography refraction and optical path length correction for image-guided corneal surgery

Optical coherence tomography (OCT) may be useful for guidance of ocular microsurgeries such as deep anterior lamellar keratoplasty (DALK), a form of corneal transplantation that requires delicate insertion of a needle into the stroma to approximately 90% of the corneal thickness. However, visualization of the true shape of the cornea and the surgical tool during surgery is impaired in raw OCT volumes due to both light refraction at the corneal boundaries, as well as geometrical optical path length distortion due to the group velocity of broadband OCT light in tissue. Therefore, uncorrected B-scans or volumes may not provide an accurate visualization suitable for reliable surgical guidance. In this article, we introduce a method to correct for both refraction and optical path length distortion in 3D in order to reconstruct corrected OCT B-scans in both natural corneas and corneas deformed by needle insertion. We delineate the separate roles of phase and group index in OCT image distortion correction, and introduce a method to estimate the phase index from the group index which is readily measured in samples. Using the measured group index and estimated phase index of human corneas at 1060 nm, we demonstrate quantitatively accurate geometric reconstructions of the true cornea and inserted needle shape during simulated DALK surgeries.

Accurate In Vivo Bowman's Thickness Measurement Using Mirau Ultrahigh Axial Resolution Line Field Optical Coherence Tomography

Purpose

The purpose of this study was to assess the accuracy, repeatability, and performance limits of in vivo Mirau ultrahigh axial resolution (UHR) line field spectral domain (LF-SD) optical coherence tomography (OCT) for the measurement of Bowman's and epithelial thickness, and to provide a reference range of these values for healthy corneas.

Methods

Volunteers with no history and evidence of corneal disease were included in this study. An in vivo graph search image segmentation of the central cornea was obtained at the normal interface vector orientation. The Mirau-UHR-LF-SD-OCT system used has an axial resolution down to 2.4 µm in air (1.7 µm in tissue), with an A-scan speed of 204.8 kHz and a signal to noise ratio (sensitivity) of 69 (83) dB.

Results

Nine volunteers were included, one of whom wore contact lenses. The repeatability of mean Bowman's and epithelial thicknesses were 0.3 and 1.0 µm, respectively. The measured 95% population range for healthy in vivo thickness was 13.7 to 19.6 µm for the Bowman's layer, and 41.9 to 61.8 µm for the epithelial layer.

Conclusions

The measured thicknesses of Bowman's layer and the corneal epithelium using the Mirau-UHR-LF-SD-OCT were both accurate, with the range for healthy in vivo thicknesses matching prior confocal and OCT systems of varying axial resolutions, and repeatable, equaling the best value prior reported.

Translational Relevance

T1. Development of a commercially viable clinical UHR OCT technology, enabling accurate measurement and interpretation of Bowman's and epithelial layer thickness in clinical practice.

Delineating Corneal Elastic Anisotropy in a Porcine Model Using Noncontact OCT Elastography and Ex Vivo Mechanical Tests

Purpose

To compare noncontact acoustic microtapping (AμT) OCT elastography (OCE) with destructive mechanical tests to confirm corneal elastic anisotropy.

Design

Ex vivo laboratory study with noncontact AμT-OCE followed by mechanical rheometry and extensometry.

Participants

Inflated cornea of whole-globe porcine eyes (n = 9).

Methods

A noncontact AμT transducer was used to launch propagating mechanical waves in the cornea that were imaged with phase-sensitive OCT at physiologically relevant controlled pressures. Reconstruction of both Young's modulus (E) and out-of-plane shear modulus (G) in the cornea from experimental data was performed using a nearly incompressible transversely isotropic (NITI) medium material model assuming spatial isotropy of corneal tensile properties. Corneal samples were excised and parallel plate rheometry was performed to measure shear modulus, G. Corneal samples were then subjected to strip extensometry to measure the Young's modulus, E.

Main Outcome Measures

Strong corneal anisotropy was confirmed with both AμT-OCE and mechanical tests, with the Young's (E) and shear (G) moduli differing by more than an order of magnitude. These results show that AμT-OCE can quantify both moduli simultaneously with a noncontact, noninvasive, clinically translatable technique.

Results

Mean of the OCE measured moduli were E = 12 ± 5 MPa and G = 31 ± 11 kPa at 5 mmHg and E = 20 ± 9 MPa and G = 61 ± 29 kPa at 20 mmHg. Tensile testing yielded a mean Young's modulus of 1 MPa - 20 MPa over a strain range of 1% to 7%. Shear storage and loss modulus (G'/G'') measured with rheometry was approximately 82/13 ± 12/4 kPa at 0.2 Hz and 133/29 ± 16/3 kPa at 16 Hz (0.1% strain).

Conclusions

The cornea is confirmed to be a strongly anisotropic elastic material that cannot be characterized with a single elastic modulus. The NITI model is the simplest one that accounts for the cornea's incompressibility and in-plane distribution of lamellae. AμT-OCE has been shown to be the only reported noncontact, noninvasive method to measure both elastic moduli. Submillimeter spatial resolution and near real-time operation can be achieved. Quantifying corneal elasticity in vivo will enable significant innovation in ophthalmology, helping to develop personalized biomechanical models of the eye that can predict response to ophthalmic interventions.

CLEAR - Scleral lenses

Scleral lenses were the first type of contact lens, developed in the late nineteenth century to restore vision and protect the ocular surface. With the advent of rigid corneal lenses in the middle of the twentieth century and soft lenses in the 1970's, the use of scleral lenses diminished; in recent times there has been a resurgence in their use driven by advances in manufacturing and ocular imaging technology. Scleral lenses are often the only viable form of contact lens wear across a range of clinical indications and can potentially delay the need for corneal surgery. This report provides a brief historical review of scleral lenses and a detailed account of contemporary scleral lens practice including common indications and recommended terminology. Recent research on ocular surface shape is presented, in addition to a comprehensive account of modern scleral lens fitting and on-eye evaluation. A range of optical and physiological challenges associated with scleral lenses are presented, including options for the clinical management of a range of ocular conditions. Future applications which take advantage of the stability of scleral lenses are also discussed. In summary, this report presents evidence-based recommendations to optimise patient outcomes in modern scleral lens practice.

Anterior segment optical coherence tomography scanning protocols and corneal thickness repeatability

PURPOSE

To examine the influence of anterior segment optical coherence tomography imaging protocols on the intraobserver and intrasession repeatability of epithelial, stromal, and total corneal thickness measurements.

METHODS

Repeated anterior segment optical coherence tomography (AS-OCT) images (Spectralis, Heidelberg) were obtained from 15 adults using single 8.3 mm wide horizontal line scans with an average of 2, 10, 20, 30, 50 and 100 B-scans. Volumetric scans consisting of nine 8.3 mm horizontal line scans encompassing a 1.3 mm vertical region were also captured (with 20 B-scans per line scan). Single point thickness measures (at the normal to the tangent of the anterior corneal surface) were compared with thickness measures averaged over the central 6 mm. The impact of B-scan averaging and intraobserver variability were examined for single line scans. For volumetric scans, the impact of the number of line scans upon intraobserver and intrasession variability were calculated.

RESULTS

Intraobserver repeatability did not vary significantly as a function of the number of averaged B-scans per line scan, but was lowest for 20-30 averaged B scans. For volumetric scans, increasing the number of line scans did increase scan duration (p < 0.001), with minimal impact upon the average scan quality index (p = 0.06). Averaging more than 3 line scans did not significantly improve intraobserver or intrasession repeatability for either single point or average thickness measurements.

CONCLUSION

AS-OCT volumetric scans with 3 lines each consisting of 20 B-scans with measurements averaged over a central 6 mm of the cornea provide highly repeatable measures of epithelial, stromal and total corneal thickness (95 % LoA ≤ ±3.2 μm for intraobserver repeatability and ≤ ±3.7 μm for intrasession repeatability). This scanning protocol can provide reliable information when monitoring subtle changes in corneal thickness.

Corneal epithelial thickness profile in dry-eye disease

BACKGROUND/OBJECTIVES

To characterize and evaluate the use of corneal epithelial profile maps generated by an ultrahigh-resolution optical coherence tomography (UHR-OCT) in the diagnosis and management of dry-eye disease (DED).

SUBJECTS/METHODS

This prospective, interventional case-control study included 115 eyes of 71 subjects (52 DED and 19 controls) imaged using an UHR-OCT. Average, maximum, and minimum, range of corneal epithelial thicknesses were extracted from epithelial profile maps. Surface regularity was quantified using the range and variance of the epithelial thickness measured along a horizontal UHR-OCT scan. The variance of thickness measurements along a scan was named epithelial irregularity factor (EIF). Symptoms of 31 DED patients (55 eyes) were quantified by questionnaire and correlated to epithelial profile findings, fluorescein staining, tear breakup time, and Schirmer's test. Twenty-one DED eyes were administered autologous serum drops and follow-up UHR-OCT images were captured.

RESULTS

DED patients had a highly irregular corneal epithelial surface compared with controls. Epithelial thickness profile variance (EIF) and range were significantly higher in DED as compared with controls (5.79 vs. 0.77, p < 0.001 and 7.6 vs. 4.6 μm, p < 0.001). Both parameters were highly significantly correlated with questionnaire scores (EIF: r = 0.778; p < 0.001, range: r = 0.737; p < 0.001). Follow-up showed a statistically significant reduction in epithelial thickness profile variance and range of treated patients ( p < 0.001).

CONCLUSIONS

DED patients have irregular epithelial surface that can be quantified using UHR-OCT generated CEP maps. Epithelial thickness profile range and EIF correlate accurately with patients' symptoms and could be used to follow-up patients and response to treatment.

Wide-field whole eye OCT system with demonstration of quantitative retinal curvature estimation

Current conventional clinical OCT systems image either only the anterior or the posterior eye during a single acquisition. This localized imaging limits conventional OCT's use for characterizing global ocular morphometry and biometry, which requires knowledge of spatial relationships across the entire eye. We developed a "whole eye" optical coherence tomography system that simultaneously acquires volumes with a wide field-of-view for both the anterior chamber (14 x 14 mm) and retina (55°) using a single source and detector. This system was used to measure retinal curvature in a pilot population and compared against curvature of the same eyes measured with magnetic resonance imaging.

Optical coherence tomography and scleral contact lenses: clinical and research applications

Anterior segment optical coherence tomography (OCT) provides a wealth of opportunities for modern contact lens practice. OCT imaging has numerous clinical and research applications related to the tear film, cornea, conjunctiva, sclera and ocular adnexae, in addition to soft, rigid, and hybrid contact lenses. This review summarises the potential use of OCT imaging in modern scleral contact lens practice including initial lens selection, assessing the scleral contact lens fit with respect to the cornea and sclera, and accurately quantifying the ocular response to lens wear. Recent advances in the understanding of anterior segment metrics including scleral thickness, curvature, toricity, and the anatomy of the corneoscleral limbal junction are also discussed.

Intraocular Pressure and Big Bubble Diameter in Deep Anterior Lamellar Keratoplasty: An Ex-Vivo Microscope-Integrated OCT With Heads-Up Display Study

PURPOSE

To investigate the relationship between intraocular pressure (IOP) and big bubble (BB) formation in a model of deep anterior lamellar keratoplasty (DALK).

DESIGN

Ex-vivo.

METHODS

Corneoscleral buttons from human donors were loaded onto an artificial anterior chamber connected to a column of balanced salt solution. A surgeon-in-training learned to perform DALK via the BB technique using swept-source microscope-integrated optical coherence tomography (SS-MIOCT) with heads-up display (HUD). DALK procedures were performed at 6 different IOPs (5, 10, 15, 20, 30, or 40 mm Hg; n = 6 per group) in a randomized fashion, with the surgeon-in-training masked to the pressure and guided by SS-MIOCT with HUD. For a subset of corneas within each pressure group, DALK was performed on matching donor tissue at a control IOP. BB diameter was recorded, and a diameter exceeding the trephine diameter was considered optimal.

RESULTS

Wilcoxon rank sum test showed a difference in BB diameter among the different pressure groups (mean ± SD of 7.75 ± 1.60, 8.33 ± 1.99, 10.9 ± 0.92, 9.08 ± 1.07, 6.67 ± 3.33, and 3.42 ± 3.77 mm in the 5, 10, 15, 20, 30, and 40 mm Hg groups, respectively; P = 0.0014). Per Tukey test, this difference was attributable to comparisons between the 40 mm Hg group and the 5, 10, 15, or 20 mm Hg groups (P = 0.04, 0.02, 0.0001, 0.004, respectively).

CONCLUSIONS

In this ex-vivo model of DALK, the BB technique guided by SS-MIOCT with HUD yielded bubbles of optimal diameters only at physiologic pressures (10‒20 mm Hg). Extremely high IOP (40 mm Hg) resulted in BBs of significantly smaller diameter than BBs obtained at physiologic and low (5 mm Hg) IOPs.

In Vivo Characteristics of Corneal Endothelium/Descemet Membrane Complex for the Diagnosis of Corneal Graft Rejection

PURPOSE

To evaluate the utility of endothelial/Descemet membrane complex (En/DM) characteristics in diagnosing corneal graft rejection.

DESIGN

Diagnostic reliability study.

METHODS

One hundred thirty-nine eyes (96 corneal grafts post penetrating keratoplasty or Descemet stripping automated endothelial keratoplasty: 40 clear, 23 actively rejecting, 24 rejected, and 9 nonimmunologic failed grafts; along with 43 age-matched control eyes) were imaged using high-definition optical coherence tomography. Images were used to describe En/DM and measure central corneal thickness (CCT) and central En/DM thickness (DMT). En/DM rejection index (DRI) was computed to detect the relative En/DM thickening to the entire cornea.

RESULTS

In actively rejecting grafts, DMT and DRI were significantly greater than controls and clear grafts (28, 17, and 17 μm and 1.5, 1 and 1, respectively; P < .001). Rejected grafts had the highest DMT and DRI compared to all groups (59 μm and 2.1; P < .001). DMT and DRI showed excellent accuracy, significantly better than that of CCT, in differentiating actively rejecting from clear grafts (100% and 96% sensitivity; 92.5% and 92.5% specificity), actively rejecting from rejected grafts (88% and 83% sensitivity; 91% and 83% specificity), and nonimmunologic failed from rejected grafts (100% and 100% sensitivity; 88% and 100% specificity). DMT correlated significantly with rejection severity (P < .001).

CONCLUSIONS

In corneal grafts, in vivo relative thickening of the En/DM is diagnostic of graft rejection as measured by DMT and DRI. These indices have excellent accuracy, sensitivity, and specificity in detecting graft immunologic status, superior to CCT. DMT is a quantitative index that correlates accurately with the severity of rejection.

Needle Depth and Big-Bubble Success in Deep Anterior Lamellar Keratoplasty: An Ex Vivo Microscope-Integrated OCT Study

PURPOSE

To examine big-bubble (BB) formation success rates in deep anterior lamellar keratoplasty (DALK) at various corneal depths using real-time guidance from swept-source, microscope-integrated optical coherence tomography (SS-MIOCT).

METHODS

The DALK procedure was performed ex vivo with 34 human donor corneoscleral buttons on pressurized artificial anterior chambers using the BB technique employed by Anwar and Teichmann. We inserted a needle under controlled ex vivo conditions to corneal depths ranging from 40% to ≥90% using real-time guidance from SS-MIOCT and injected air. BB success was then determined for each injection.

RESULTS

The average needle depth for successful full BB formation was 79.9% ± 3.0% compared with 66.9% ± 2.6% for partial BB formation and 49.9% ± 3.4% for no BB formation (P < 0.0001). Expressed as stroma below the needle tip, this corresponded to 123.9 ± 20.0 μm for successful full BB formation compared with 233.7 ± 23.8 μm for partial BB formation and 316.7 ± 17.3 μm for no BB formation (P < 0.0001). All other variables tested (sex, race, age, endothelial cell density, air injected, needle angle, and central corneal thickness) did not significantly affect BB formation success rates.

CONCLUSIONS

BB formation in DALK is more successful if needle insertion and air injection occur at deeper corneal depth. However, ≥90% corneal depth was not necessary in this ex vivo model of DALK. SS-MIOCT can be used to accurately guide the needle in real time.

Value of corneal epithelial and Bowman's layer vertical thickness profiles generated by UHR-OCT for sub-clinical keratoconus diagnosis

Ultra-high resolution optical coherence tomography (UHR-OCT) can image the corneal epithelium and Bowman’s layer and measurement the thicknesses. The purpose of this study was to validate the diagnostic power of vertical thickness profiles of the corneal epithelium and Bowman’s layer imaged by UHR-OCT in the diagnosis of sub-clinical keratoconus (KC). Each eye of 37 KC patients, asymptomatic fellow eyes of 32 KC patients, and each eye of 81 normal subjects were enrolled. Vertical thickness profiles of the corneal epithelium and Bowman’s layer were measured by UHR-OCT. Diagnostic indices were calculated from vertical thickness profiles of each layer and output values of discriminant functions based on individual indices. Receiver operating characteristic curves were determined, and the accuracy of the diagnostic indices were assessed as the area under the curves (AUC). Among all of the individual indices, the maximum ectasia index for epithelium had the highest ability to discriminate sub-clinical KC from normal corneas (AUC = 0.939). The discriminant function containing maximum ectasia indices of epithelium and Bowman’s layer further increased the AUC value (AUC = 0.970) for sub-clinical KC diagnosis. UHR-OCT-derived thickness indices from the entire vertical thickness profiles of the corneal epithelium and Bowman’s layer can provide valuable diagnostic references to detect sub-clinical KC.

Calculation of ophthalmic viscoelastic device-induced focus shift during femtosecond laser-assisted cataract surgery

PURPOSE

To assess if a change in refractive index of the anterior chamber during femtosecond laser-assisted cataract surgery can affect the laser beam focus position.

METHODS

The index of refraction and chromatic dispersion of six ophthalmic viscoelastic devices (OVDs) was measured with an Abbe refractometer. Using the Gullstrand eye model, the index values were used to predict the error in the depth of a femtosecond laser cut when the anterior chamber is filled with OVD. Two sources of error produced by the change in refractive index were evaluated: the error in anterior capsule position measured with optical coherence tomography biometry and the shift in femtosecond laser beam focus depth.

RESULTS

The refractive indices of the OVDs measured ranged from 1.335 to 1.341 in the visible light (at 587 nm). The error in depth measurement of the refilled anterior chamber ranged from -5 to +7 μm. The OVD produced a shift of the femtosecond laser focus ranging from -1 to +6 μm. Replacement of the aqueous humor with OVDs with the densest compound produced a predicted error in cut depth of 13 μm anterior to the expected cut.

CONCLUSIONS

Our calculations show that the change in refractive index due to anterior chamber refilling does not sufficiently shift the laser beam focus position to cause the incomplete capsulotomies reported during femtosecond laser-assisted cataract surgery.

Monitor corneal epithelial healing under bandage contact lens using ultrahigh-resolution optical coherence tomography after pterygium surgery

OBJECTIVE

To monitor corneal epithelial healing under bandage contact lens (BCL) using ultrahigh-resolution optical coherence tomography (UHR-OCT) after pterygium surgery.

METHODS

Thirty-nine eyes of 39 patients undergoing pterygium excision and conjunctival autografting were randomly allocated into 2 groups: 20 eyes were covered with silicone hydrogel BCLs at the end of surgery and 19 eyes served as a control. Ultrahigh-resolution optical coherence tomography scans of the cornea were performed sequentially at 1, 2, and 3 days, and then every other day until the end of re-epithelialization after surgery. Complete epithelial healing was verified with fluorescein staining examination after removal of BCLs. Postsurgery pain was evaluated using the visual analog scale.

RESULTS

All BCLs were successfully fit without any contact lens-related complications. Ultrahigh-resolution optical coherence images clearly revealed the architectural features of postoperative cornea with BCL in situ and showed epithelial healing process. Ultrahigh-resolution optical coherence tomography imaging agreed with the fluorescein staining examinations in detecting corneal epithelial defect. Based on the assessment by UHR-OCT, the average time course of re-epithelialization in the BCL group was 3.9±1.2 days, whereas in the control group, it was 5.7±1.8 days (P=0.001). Visual analog scale scoring showed lower pain levels in the BCL group comparing with the control group at each point of time (all P<0.05).

CONCLUSIONS

Ultrahigh-resolution optical coherence tomography is an excellent tool in monitoring corneal epithelial healing under BCLs and determining the appropriate time for lens removal. Silicone hydrogel BCL is recommended as an adjuvant therapy after pterygium surgery for its efficacy in improving re-epithelialization and postoperative comfort.

Quantitative optical inspection of contact lenses immersed in wet cell using swept source OCT

We demonstrate swept source optical coherence tomography (OCT) imaging of contact lenses (CLs) in a wet cell and comprehensive quantitative characterization of CLs from volumetric OCT datasets. The approach is based on a technique developed for lens autopositioning and autoleveling enabled by lateral capillary interactions between the wet cell wall and the lens floating on the liquid surface. The demonstrated OCT imaging has enhanced contrast due to the application of a scattering medium and it improves visualization of both CL interfaces and edges. We also present precise and accurate three-dimensional metrology of soft and rigid CLs based on the OCT data. The accuracy and precision of the extracted lens parameters are compared with the manufacturer's specifications. The presented methodology facilitates industrial inspection methods of the CLs.

Entire thickness profiles of the epithelium and contact lens in vivo imaged with high-speed and high-resolution optical coherence tomography

PURPOSE

To test the feasibility of measuring the entire thickness profiles of the epithelium and contact lens (CL) in vivo, using high-speed and high-resolution spectral-domain optical coherence tomography (SD-OCT).

METHODS

A custom-built, long scan depth SD-OCT was developed based on a complementary metal oxide semiconductor (CMOS) camera, and the axial resolution was approximately 5.1 μm in tissue. Five eyes of five subjects were imaged twice across the horizontal meridian before and while wearing one CL. Semiautomatic measurement was done to yield the entire thickness profiles of the epithelium, total cornea, and CL after correcting for optical distortion.

RESULTS

The full width and depth of the epithelium, ocular surface, and CL were clearly visualized. The epithelial thickness at the center was 51.9±3.5 μm; it remained at this thickness across the central 7 mm diameter and then increased at both temporal and nasal peripheries. The CL profile showed the thinnest point at the center with thickness of 100.3±4.9 μm. The thickness increased toward the midperiphery and then decreased at the edge.

CONCLUSIONS

This pilot study demonstrated the feasibility of using high-speed CMOS-based OCT to evaluate the entire thickness profiles of the epithelium and CL in vivo. Further development will be needed to extend the scanning from 2D to 3D with a robust automatic image processing ability.

The use of Bowman's layer vertical topographic thickness map in the diagnosis of keratoconus

PURPOSE

To evaluate the use of Bowman's layer (BL) vertical topographic thickness maps in diagnosing keratoconus (KC).

DESIGN

Prospective, case control, interventional case series.

PARTICIPANTS

A total of 42 eyes: 22 eyes of 15 normal subjects and 20 eyes of 15 patients with KC.

INTERVENTION

Bowman's layer 2-dimensional 9-mm vertical topographic thickness maps were created using custom-made ultra high-resolution optical coherence tomography.

MAIN OUTCOME MEASURES

Bowman's layer average and minimum thicknesses of the inferior half of the cornea, Bowman's ectasia index (BEI; defined as BL minimum thickness of the inferior half of the cornea divided by BL average thickness of the superior half of the cornea multiplied by 100), BEI-Max (defined as BL minimum thickness of the inferior half of the cornea divided by BL maximum thickness of the superior half of the cornea multiplied by 100), keratometric astigmatism (Ast-K) of patients with KC, and average keratometric (Avg-K) readings.

RESULTS

In patients with KC, BL vertical thickness maps disclosed localized relative inferior thinning of the BL. Inferior BL average thickness (normal = 15±2, KC = 12±3 μm), inferior BL minimum thickness (normal = 13±2, KC = 7±3 μm), BEI (normal = 91±7, KC = 48±14), and BEI-Max (normal = 75±8; KC = 40±13) all showed highly significant differences in KC compared with normal subjects (P< 0.001). Receiver operating characteristic (ROC) curve analysis showed excellent predictive accuracy for BEI and BEI-Max with 100% sensitivity and specificity (area under the curve [AUC] of 1) with cutoff values of 80 and 60, respectively. The AUC of inferior BL average thickness and minimum thickness were 0.87 and 0.96 with a sensitivity of 80% and 93%, respectively, and a specificity of 93% and 93%, respectively. Inferior BL average thickness, inferior BL minimum thickness, BEI, and BEI-Max correlated highly to Ast-K (R = -0.72, -0.82, -0.84, and -0.82, respectively; P< 0.001) and to Avg-K (R = -0.62, P< 0.001; R = -0.59, P = 0.001; R = -0.60, P< 0.001; and R = -0.59, P = 0.001, respectively).

CONCLUSIONS

Bowman's layer vertical topographic thickness maps of patients with KC disclose characteristic localized relative inferior thinning. Inferior BL average thickness, inferior BL minimum thickness, BEI, and BEI-Max are qualitative and quantitative indices for the diagnosis of KC that accurately correlate with the severity of KC. In our pilot study, BEI and BEI-Max showed excellent accuracy, sensitivity, and specificity in the diagnosis of KC.

Phacoemulsification induced transient swelling of corneal Descemet's Endothelium Complex imaged with ultra-high resolution optical coherence tomography

Purpose

Thickness changes of corneal sub-layers after phacoemulsification were investigated by spectral domain ultra-high resolution optical coherence tomography (UHR-OCT).

Methods

The corneas (n = 26) of 26 age-related cataract surgery patients were studied. UHR-OCT was used to evaluate the thickness of Descemet’s Endothelium Complex (DEC), stroma, Bowman’s layer, epithelium, and full cornea at the center (CCT) before, one day after, and one week after surgery. Non-contact specular microscopy measured CCT, endothelial cell density, and morphology.

Results

The DEC, stroma, Bowman’s layer, and epithelium were visualized by UHR-OCT. Before surgery, the DEC in all cases appeared as a translucent space between two smooth opaque lines. One day after surgery, the posterior corneal surfaces in half of the eyes were wavy and irregular. Compared to the baseline, one day after surgery the thickness increases of the DEC, stroma, and CCT were 4.3 ± 2.6 µm, 25.5 ± 24.9 µm, and 32.1 ± 26.6 µm, respectively (P < 0.001). The morphology of the DEC and the CCT recovered to baseline one week after surgery (P > 0.05), but endothelial cell density was 8.7% less than at baseline. There were no significant changes in Bowman’s layer and epithelium after the operation. The pre-operative DEC thickness was positively correlated with the decreased endothelial cell density at 1 day after surgery (r = 0.55, P = 0.003).

Conclusions

The DEC showed edematous thickening and different degrees of morphological changes after phacoemulsification. The DEC deformation and corneal edema recovered by one week after surgery, which indicated recovery of endothelial function. UHR-OCT is a useful tool to evaluate function of the DEC after phacoemulsification. Pre-operative DEC thickness may indicate the integrity of the endothelium and could be used for predicting endothelial cell loss after phacoemulsification.

Optical coherence tomography and ultrasound biomicroscopy imaging of opaque corneas

PURPOSE

The purpose was to determine the interchangeability of ultrasound biomicroscopy (UBM) and anterior segment optical coherence tomography (AS-OCT) for corneal opacity depth measurement.

METHODS

Twenty-six eyes of 26 consecutive patients with corneal opacities were examined by both AS-OCT and UBM. The corneal thickness and the corneal opacity depth were measured and compared. The interchangeability was determined by Bland-Altman plotting.

RESULTS

The difference in the full corneal thickness and in the corneal opacity depth between OCT and UBM was 5 ± 7 μm and -1 ± 8 μm, respectively. There were strong correlations and no significant differences between the paired parameters (all r > 0.99, P < 0.01). The limits of agreement were 5 ± 14 μm for the corneal thickness and -1 ± 14.8 μm for the corneal opacity depth.

CONCLUSIONS

AS-OCT and UBM may be used interchangeably for measuring both full corneal thickness and corneal opacity depth in patients with corneal opacity.

Tear meniscus evaluation by anterior segment swept-source optical coherence tomography

PURPOSE

To investigate the repeatability of tear meniscus measurements using anterior segment swept-source optical coherence tomography (OCT) and the relationship of tear meniscus measurements with tear film breakup time and Schirmer test results.

DESIGN

Prospective, observational, cross-sectional study.

METHODS

We enrolled 26 healthy subjects (26 eyes; 20 men and 6 women; mean age, 36.5 ± 6.8 years) at the University of Tokyo School of Medicine, Tokyo, Japan, in this study. Examinations were conducted in the following sequence: anterior segment vertical raster scans by swept-source OCT, tear film breakup time, and Schirmer test. Each subject underwent OCT measurements twice by the same grader, and the central upper and lower tear menisci height and area and the lower tear meniscus volume were examined. Each OCT image was evaluated by 2 masked graders using the software calipers.

RESULTS

The average upper and lower tear meniscus heights were 231 ± 78 μm and 256 ± 57 μm, respectively, and the average upper and lower tear meniscus areas were 18 829 ± 7823 μm(2) and 21 903 ± 8173 μm(2), respectively. The average tear meniscus volume was 0.1327 ± 0.051 mm(3). The intergrader intraclass correlations for all the parameters were more than 95%. The OCT tear meniscus measurements and the Schirmer test scores were correlated significantly (P < .05, Spearman nonparametric correlation analysis). However, tear film breakup time was not correlated significantly with any of the parameters of tear menisci (P > .05).

CONCLUSIONS

Anterior segment swept-source OCT is a noninvasive and practical method that can be used for quantitative evaluation of tear fluid.

Diagnosis of ocular surface lesions using ultra-high-resolution optical coherence tomography

PURPOSE

To assess the use of ultra-high-resolution (UHR) optical coherence tomography (OCT) in the diagnosis of ocular surface lesions.

DESIGN

Prospective, noncomparative, interventional case series.

PARTICIPANTS

Fifty-four eyes of 53 consecutive patients with biopsy-proven ocular surface lesions: 8 primary acquired melanosis lesions, 5 amelanotic melanoma lesions, 2 nevi, 19 ocular surface squamous neoplasia lesions, 1 histiocytosis lesion, 6 conjunctival lymphoma lesions, 2 conjunctival amyloidosis lesions, and 11 pterygia lesions.

INTERVENTION

Ultra-high-resolution OCT imaging of the ocular surface lesions.

MAIN OUTCOME MEASURES

Clinical course and photographs, UHR OCT image, and histopathologic findings.

RESULTS

Ultra-high-resolution OCT images of all examined ocular surface lesions showed close correlation with the obtained histopathologic specimens. When clinical differential diagnosis of ocular surface lesions was broad, UHR OCT images provided optical signs indicating a more specific diagnosis and management. In cases of amelanotic melanoma, conjunctival amyloidosis, and primary histiocytosis and in 1 case of ocular surface squamous neoplasia, UHR OCT was instrumental in guiding the diagnosis. In those cases, UHR OCT suggested that the presumed clinical diagnosis was incorrect and favored a diagnosis that later was confirmed by histopathologic examination.

CONCLUSIONS

Correlations between UHR OCT and histopathologic findings confirm that UHR OCT is an adjunctive diagnostic method that can provide a noninvasive means to help guide diagnosis and management of ocular surface lesions.

FINANCIAL DISCLOSURE(S)

The author(s) have no proprietary or commercial interest in any materials discussed in this article.

Anterior chamber cell grading by optical coherence tomography

PURPOSE

To quantify cells in the ocular anterior chamber (AC) by optical coherence tomography (OCT).

METHODS

A time-domain anterior segment OCT system was used to image latex microsphere suspensions in vitro and the AC of uveitis and normal subjects in vivo. The OCT scan pattern, consisting of 2- and 4-mm-diameter concentric circular scans, was divided into central, superior, and inferior regions. A computer algorithm was developed to automatically identify particles in OCT images. A uveitis specialist used slit-lamp biomicroscopy to grade the AC cells on a scale of 0 to 4+.

RESULTS

Latex microspheres and ac cells were visualized as reflective spots in oct images. OCT latex microsphere concentration measurements were highly correlated to known particle concentrations (r = 1.000) and had an efficiency of 0.72. in 30 nongranulomatous and 12 granulomatous eyes, the OCT cell counts correlated well with slit-lamp grades in all three regions (Spearman's rho coefficient: >0.63). The average OCT cell count was 3.7 cells/grade in nongranulomatous eyes and 2.0 cells/grade in granulomatous eyes. OCT revealed significant amounts of inferior AC cells in 5 of 16 quiescent uveitis eyes (mean ± SD: 19.9 ± 7.4 cells). OCT captured rare cells in normal eyes (1.1 ± 1.1 cells centrally).

CONCLUSIONS

OCT provided quantitative information on AC inflammatory cells. The OCT cell counts correlated well with clinical grading, and particles in the inferior AC that were missed by slit-lamp examination were detected by OCT. OCT could be a valuable tool for the diagnosis and management of anterior uveitis.

Characterizing refractive index and thickness of biological tissues using combined multiphoton microscopy and optical coherence tomography

We present a noninvasive method for characterizing the refractive index (RI) and thickness distribution in biological tissues using a combined multiphoton microscopy (MPM) and optical coherence tomography (OCT) system. Tissue layers are distinguished by the MPM and OCT images, and the RI and thickness of each layer are determined by analyzing the co-registered MPM and OCT images. The precision of this method is evaluated on four standard samples which are water, air, immersion oil and cover glass. Precision of within ~1% error compared to reference values is obtained. Biological tissue measurement is demonstrated on fish cornea. Three layers are detected, which are identified as the epithelium and stroma I and II of the cornea. The corresponding RI of each layer is measured to be ~1.446-1.448, 1.345-1.372, and 1.392-1.436, respectively. The difference of RI in the three layers correlates with the tissue compositions including cells in epithelium, large collagen fiber bundles in stroma I, and small collagen fibers in stroma II. The combined MPM/OCT technique is shown to be able to distinguish tissue layers through biochemically specific contrasts and measure RI and thickness of tissue layers at different depths.

Tear meniscus measurement in nasolacrimal duct obstruction patients with Fourier-domain optical coherence tomography: novel three-point capture method

PURPOSE

Fourier-domain optical coherence tomography (FD ODT) for the evaluation of marginal tear film has not been performed in patients with watery eye or in a controlled study. We used FD OCT to evaluate the height of the lower lid tear film at three points in normal adults and compared it with two watery eye groups.

METHODS

We prospectively evaluated with FD OCT 25 normal subjects and 44 patients with a watery eye. Twenty-three patients with primary acquired nasolacrimal duct obstruction (PANDO) and 21 patients with functional nasolacrimal duct obstruction (FNLDO) were enrolled. Three images were obtained by three vertical FD OCT scans centred on the lower eyelid and inferior cornea, the medial limbus and the lateral limbus. The tear meniscus height (TMH), tear meniscus depth (TMD) and tear meniscus area (TMA) were measured with computer calipers and compared at three points between the two groups.

RESULTS

Watery eyes have significantly greater median TMHs at three points (medial: 584 μm, central: 592 μm, lateral: 470 μm) than controls (274, 291, 205 μm, p < 0.001). Medial and central TMHs were higher than lateral TMH in controls and watery eyes. TMD and TMA also increased significantly in watery eyes (p < 0.001). There was a significant increase in central TMH compared to medial TMH in FNLDOs than in PANDOs (p < 0.05).

CONCLUSION

Fourier-domain OCT is a valuable clinical tool in the evaluation of TMH in normal and watery eyes. TMH at three points can be a useful clinical parameter that investigates changes of lower tear meniscus pattern in both PANDO and FNLDO groups.

Vertical and horizontal corneal epithelial thickness profiles determined by ultrahigh resolution optical coherence tomography

PURPOSE

To measure vertical and horizontal thickness profiles of the central and peripheral corneal epithelium and determine if daytime changes occur.

METHODS

Forty eyes of 20 normal subjects were imaged by ultrahigh resolution spectral domain optical coherence tomography to profile the corneal epithelial thickness from the edge of Bowman layer to the central cornea across the vertical and horizontal meridians. Measurements were made at 10:00 AM and again at 6 and 8 hours later.

RESULTS

The baseline vertical meridional epithelial thickness was thinnest, 42.9 ± 4.1 μm, at the edge of Bowman layer in the superior region. It increased in thickness (P < 0.01), toward the central cornea. The central epithelium averaged 52.5 ± 2.4 μm, becoming thickest, 55.2 ± 2.5 μm, in the inferior pericentral region. It thinned toward the inferior periphery, reaching 51.3 ± 5.1 μm at the edge of Bowman layer (P < 0.01). Along the horizontal meridian, the epithelium was thickest at the nasal side, 58.6 ± 5.1 μm, and temporal side, 59.3 ± 6.6 μm, near the edges of Bowman layer. It thinned toward the central cornea. There were no significant changes in the epithelial thickness at any location over 8 hours.

CONCLUSIONS

Epithelial thickness varied over the horizontal and vertical meridians and appeared stable during the daytime.

Distributed scanning volumetric SDOCT for motion corrected corneal biometry

We present a method, termed distributed scanning OCT (DSOCT), which reduces the effects of patient motion on corneal biometry utilizing current-generation clinically available spectral domain optical coherence tomography (SDOCT) systems. We first performed a pilot study of the power spectrum of normal patient axial eye motion based on repeated (M-mode) SDOCT. Using DSOCT to reduce the effects of patient motion, we conducted a preliminary patient study comparing the measured anterior and posterior corneal curvatures and the calculated corneal power to both corneal topography and Scheimpflug photography in normal subjects. The repeatability for the measured radius of curvature of both anterior and posterior surfaces as well as calculated corneal refractive power using DSOCT was comparable to those of both topography and Scheimpflug photography.

Correlation of anterior segment optical coherence tomography measurements with graft trephine diameter following descemet stripping automated endothelial keratoplasty

Background

To assess repeatability of the Zhongshan Assessment Program (ZAP) software measurement of Anterior Segment Optical Coherence Tomography (ASOCT) images and correlate with graft trephine diameter following Descemet Stripping Automated Endothelial Keratoplasty (DSAEK)

Methods

Retrospectively evaluated interventional case series. 121 consecutive eyes undergoing DSAEK over a 26 month period underwent ASOCT imaging 1month after their surgery. ASOCT images were processed using ZAP software which measured the graft and cornea parameters including anterior and posterior graft arc length and cord length, posterior cornea arc length (PCAL) and anterior chamber width.

Results

The graft measurements showed good repeatability on ASOCT using ZAP with high intra class coefficient and small variation in the coefficient of variation. On ASOCT, the mean recipient PCAL was 12.99+/−0.69mm and the anterior chamber width was 11.16+/−0.57mm. The mean Graft anterior arc length was 9.69+/−0.66mm and the mean Graft anterior cord length was 8.92+/−2.94mm. The mean graft posterior arc length was 9.24+/−0.75mm and the mean graft posterior cord length was 8.15+/−0.57mm. Graft posterior arc length (rho=0.788, p< 0.001) correlated best with intra-operative graft trephine diameter. The mean ratio of posterior graft arc length to PCAL was 0.712 +/− 0.056.

Conclusions

We have validated the repeatability of the ZAP software for DSAEK graft measurements from ASOCT images and shown that the graft arc length parameters calculated from the ASOCT images correlate well with the intra-operative graft trephine diameter. This software may help surgeons determine the optimal DSAEK graft size based on pre-operative ASOCT measurements of the recipient eye.

Correlation between optical coherence tomography-derived assessments of lower tear meniscus parameters and clinical features of dry eye disease

PURPOSE

To measure the correlation between subjective symptom score, conventional clinical tests, and Fourier-domain optical coherence tomography (FD-OCT) of lower tear meniscus parameters in patients with dry eye disease.

METHODS

Eighteen patients with dry eye disease requiring medical therapy and/or punctal occlusion were recruited for this prospective, nonrandomized, observational case series. Severity of symptoms of dry eye disease was assessed using the Indiana University Dry Eye Questionnaire 2002. Clinical assessments were completed using slit-lamp biomicroscopy, rose bengal dye staining, fluorescein tear breakup time (TBUT), and a 5-minute Schirmer test with topical anesthesia. The lower tear meniscus was imaged using an FD-OCT system with 5-μm axial resolution and measured manually by a masked grader using computer calipers. Correlation was assessed using Spearman correlation coefficient (ρ).

RESULTS

The mean (±SD) scaled symptom score was 58 ± 21, with a range of 0 to 100. Vital staining test averaged 1.7 ± 3.4, TBUT averaged 4.4 ± 1.8 seconds, and Schirmer tests averaged 10.2 ± 8.1 mm. As determined by optical coherence tomography, the meniscus height was 228 ± 153 μm, depth was 127 ± 79 μm, and cross-sectional area was 0.018 ± 0.021 mm. Optical coherence tomography meniscus area was negatively correlated with the symptom questionnaire score (P < 0.01) and positively correlated with Schirmer test results (P < 0.01). There was no significant correlation between symptom score and rose bengal staining, TBUT, or Schirmer test results (P > 0.01).

CONCLUSIONS

Lower tear meniscus measurement with FD-OCT is an objective noninvasive test that correlates well with symptoms of dry eye disease and the Schirmer test.

Corneal biometry from volumetric SDOCT and comparison with existing clinical modalities

We present a comparison of corneal biometric values from dense volumetric spectral domain optical coherence tomography (SDOCT) scans to reference values in both phantoms and clinical subjects. We also present a new optically based "keratometric equivalent power" formula for SDOCT that eliminates previously described discrepancies between corneal power form SDOCT and existing clinical modalities. Phantom objects of varying radii of curvature and corneas of normal subjects were imaged with a clinical SDOCT system. The optically corrected three-dimensional surfaces were used to recover radii of curvature and power as appropriate. These were then compared to the manufacturer's reference values in phantoms and to measurements from topography and Scheimpflug photography in subjects. In phantom objects, paired differences between SDOCT and reference values for radii of curvature were not statistically significant. In subjects, there were no significant paired differences between SDOCT and reference values from the other modalities for anterior radius and corneal keratometric power. In contrast to other studies, we found that dense volumetric scans with available SDOCT can be used to recover corneal biometric values-including power-that correspond well with existing clinical measurements.

Atypical presentation of Salzmann nodular degeneration diagnosed with ultra-high-resolution optical coherence tomography

A 59-year-old woman presented with bilateral, peripheral, circular corneal infiltrates. There was a clear zone separating the outer margin of the degeneration from the limbus in both eyes. The inner margins were indistinct. Ultra-high-resolution optical coherence tomography (UHR-OCT) imaging demonstrated subepithelial infiltrations with epithelial thinning and corneal surface elevation. The infiltrate was accompanied by significant stromal scarring, which reached deep layers of the corneal stroma. UHR-OCT findings were consistent with Salzmann nodular degeneration. UHR-OCT can be used as an optical biopsy to diagnose atypical corneal degenerations without tissue sampling.

Assessment of corneal dynamics with high-speed swept source optical coherence tomography combined with an air puff system

We present a novel method and instrument for in vivo imaging and measurement of the human corneal dynamics during an air puff. The instrument is based on high-speed swept source optical coherence tomography (ssOCT) combined with a custom adapted air puff chamber from a non-contact tonometer, which uses an air stream to deform the cornea in a non-invasive manner. During the short period of time that the deformation takes place, the ssOCT acquires multiple A-scans in time (M-scan) at the center of the air puff, allowing observation of the dynamics of the anterior and posterior corneal surfaces as well as the anterior lens surface. The dynamics of the measurement are driven by the biomechanical properties of the human eye as well as its intraocular pressure. Thus, the analysis of the M-scan may provide useful information about the biomechanical behavior of the anterior segment during the applanation caused by the air puff. An initial set of controlled clinical experiments are shown to comprehend the performance of the instrument and its potential applicability to further understand the eye biomechanics and intraocular pressure measurements. Limitations and possibilities of the new apparatus are discussed.

Topographic thickness of Bowman's layer determined by ultra-high resolution spectral domain-optical coherence tomography

PURPOSE

To characterize the thickness profile of the corneal epithelium and the Bowman's layer across the horizontal meridian.

METHODS

Forty-four eyes of 22 healthy subjects were investigated in this study. Ultra-high resolution anterior segment spectral domain-optical coherence tomography (SD-OCT) was used to assess the topographic thickness of the epithelium and the Bowman's layer across the cornea. Thicknesses at five locations, including the center, midperiphery, and periphery close to the limbus, on both the nasal and the temporal sides along the horizontal meridian, were analyzed.

RESULTS

Mean epithelial thickness at the central cornea was 52.5 ± 2.4 μm. It increased gradually from the center to the periphery (P < 0.001). There was no significant difference between the nasal side and the temporal side for epithelial thickness. The central Bowman's layer thickness was 17.7 ± 1.6 μm, and it remained constant from the center to the midperiphery (P > 0.05). However, thicknesses at the nasal and temporal periphery, 20.0 ± 1.9 μm and 19.8 ± 2.2 μm, respectively, were significantly greater than the central and midperipheral thicknesses (P < 0.001). Nasal and temporal thicknesses were similar on either side of the center.

CONCLUSIONS

The epithelium and the Bowman's layer were not evenly distributed across the horizontal meridian of the cornea. SD-OCT provided useful information about topographic thickness of the different corneal layers in vivo.

In vivo morphologic characteristics of Salzmann nodular degeneration with ultra-high-resolution optical coherence tomography

PURPOSE

To examine the in vivo morphologic properties of Salzmann nodular degeneration with ultra-high-resolution optical coherence tomography (OCT).

DESIGN

Interventional case series.

METHODS

SETTING

Single-center academic practice.

STUDY POPULATION

Nineteen eyes of 12 patients with Salzmann nodular degeneration were recruited to participate in the study.

OBSERVATIONAL PROCEDURE

Subjects were imaged using novel, custom-built ultra-high-resolution OCT. Images were used to describe in vivo characteristics of subepithelial nodules. Morphometric measurements were made with custom-built software. Ultra-high-resolution OCT findings were compared with histopathologic findings in 3 patients.

MAIN OUTCOME MEASURES

Identifiable in vivo morphologic characteristics of Salzmann nodular degeneration with ultra-high-resolution OCT.

RESULTS

Ultra-high-resolution OCT images demonstrate intraepithelial fibrous nodules with epithelial thinning and corneal surface elevation. The Bowman layer could be differentiated in 9 of 12 patients. The difference between the mean thickness of epithelium above the nodule and the thickness of normal epithelium was statistically significant (P < .0001). The correlation between thickness of the epithelium and thickness of the nodule was statistically significant (r = -0.48; P < .0001). The correlation between thickness of the nodule and total surface thickness (thickness of the epithelium + thickness of the nodule) was statistically significant (r = 0.98; P < .0001). Ultra-high-resolution OCT findings were consistent with histopathologic results.

CONCLUSIONS

Ultra-high-resolution OCT can be used to noninvasively image the cornea in Salzmann nodular degeneration. This new imaging technique helps us to demonstrate different in vivo morphologic characteristics of Salzmann nodular degeneration.

3D refraction correction and extraction of clinical parameters from spectral domain optical coherence tomography of the cornea

Capable of three-dimensional imaging of the cornea with micrometer-scale resolution, spectral domain-optical coherence tomography (SDOCT) offers potential advantages over Placido ring and Scheimpflug photography based systems for accurate extraction of quantitative keratometric parameters. In this work, an SDOCT scanning protocol and motion correction algorithm were implemented to minimize the effects of patient motion during data acquisition. Procedures are described for correction of image data artifacts resulting from 3D refraction of SDOCT light in the cornea and from non-idealities of the scanning system geometry performed as a pre-requisite for accurate parameter extraction. Zernike polynomial 3D reconstruction and a recursive half searching algorithm (RHSA) were implemented to extract clinical keratometric parameters including anterior and posterior radii of curvature, central cornea optical power, central corneal thickness, and thickness maps of the cornea. Accuracy and repeatability of the extracted parameters obtained using a commercial 859nm SDOCT retinal imaging system with a corneal adapter were assessed using a rigid gas permeable (RGP) contact lens as a phantom target. Extraction of these parameters was performed in vivo in 3 patients and compared to commercial Placido topography and Scheimpflug photography systems. The repeatability of SDOCT central corneal power measured in vivo was 0.18 Diopters, and the difference observed between the systems averaged 0.1 Diopters between SDOCT and Scheimpflug photography, and 0.6 Diopters between SDOCT and Placido topography.

Use of ultra-high-resolution optical coherence tomography to detect in vivo characteristics of Descemet's membrane in Fuchs' dystrophy

PURPOSE

To demonstrate the capability of ultra-high-resolution (UHR) anterior segment optical coherence tomography (OCT) to image Descemet's membrane (DM) and measure its thickness in vivo. (2) To evaluate the use of DM characteristics and thickness in the diagnosis of Fuchs' dystrophy.

DESIGN

Case-control study.

PARTICIPANTS

Twenty eyes of 12 Fuchs' dystrophy patients, 20 eyes of 13 young normal, and 20 eyes of 15 elderly normal subjects.

METHODS

Subjects were imaged using novel, custom-built UHR-OCT. Images were used to describe the characteristics of DM. Custom-made software was used to measure DM thickness and central corneal thickness (CCT). Specimens of DM obtained from Fuchs' dystrophy patients who underwent endothelial keratoplasty (EK) were histopathologically examined. Regression analyses were used to assess the correlation of DM thickness measured by UHR-OCT in vivo and by light microscopy and to determine the intergroup correlations between age, CCT, and DM thickness.

MAIN OUTCOME MEASURES

We assessed DM characteristics and thickness, CCT, and age.

RESULTS

Using UHR-OCT, the DM seemed in normal young subjects as a single, opaque, smooth line and in normal elderly subjects as a band of 2 smooth opaque lines with a translucent space in between. In Fuchs' dystrophy, DM appeared as a thickened band of 2 opaque lines; the anterior line was smooth whereas the posterior line had a wavy and irregular appearance with areas of localized thickenings. The DM thickness measured in vivo by UHR-OCT correlated significantly with that measured by light microscopy in 5 Fuchs' dystrophy eyes that underwent EK. The average central thicknesses of DM in normal young, in normal elderly and in Fuchs' dystrophy eyes were 10+/-3, 16+/-2, and 34+/-11 microm, respectively (P<0.001). There was a significant correlation between age and DM thickness only in normal groups. In Fuchs' dystrophy patients, there was a significant correlation between CCT and DM thickness that was not significant for normal groups.

CONCLUSIONS

Ultra-high-resolution OCT is an innovative technique for the in vivo imaging of DM. Determining DM characteristics and thickness by UHR-OCT could be a new approach for the diagnosis of Fuchs' dystrophy.

Ultrahigh-resolution measurement by optical coherence tomography of dynamic tear film changes on contact lenses

Purpose. To determine the dynamic pre- and postlens tear film (PLTF and PoLTF) thicknesses by using optical coherence tomography (OCT). Methods. Ultrahigh-resolution OCT was used to image the tear film of 22 subjects before and after contact lens wear. A soft lens with 1 drop of artificial tears on its concave surface was inserted onto one randomly selected eye. OCT images were taken before insertion, immediately afterward, and every 2 minutes for 10 minutes. For the contralateral eye, the lens inserted was not prewetted on the concave surface. OCT images were taken before insertion, immediately afterward, and at 3 minutes. Then another drop was instilled, and images were taken immediately afterward and every 2 minutes for 10 minutes. Images were processed by custom software to yield tear film thickness. Results. The thickness of precorneal tear film (PCTF) was 1.9 +/- 0.9 mum. The PoLTF was visualized clearly in all cases immediately after lens insertion, with 1 drop on the lens concave surface. Through the first 6 minutes after insertion, the PoLTF was greater than the PCTF. The PLTF (n = 12) and PoLTF (n = 9) were visualized immediately after lens insertion. After 3 minutes, the PLTF in most subjects and PoLTF in all subjects were invisible. The thickness of the PLTF increased after the instillation of artificial tears, whereas the PoLTF did not at any checkpoint for 10 minutes. Conclusions. Ultrahigh-resolution OCT is a promising tool for measuring the tear film. The PoLTF did not increase after instillation of artificial tears.

Reproducibility of tear meniscus measurement by Fourier-domain optical coherence tomography: a pilot study

BACKGROUND AND OBJECTIVE

To study the reproducibility of tear meniscus measurement with high-speed high-resolution Fourier-domain optical coherence tomography (FD-OCT).

PATIENTS AND METHODS

Twenty normal participants were enrolled in this prospective study. The lower tear meniscus in the right eye of each subject was imaged by vertical scans centered on the inferior cornea and the lower eyelid using an FD-OCT system (RTVue; Optovue, Inc., Fremont, CA) with a corneal adaptor. The system performs 26,000 axial scans per second and has a 5-micron axial resolution. Each subject was examined at two visits 30 to 60 days apart. Each eye was scanned twice on each visit. The scans were taken 2 seconds after a blink. The lower meniscus height, depth, and cornea-meniscus angle were measured with a computer caliper. The cross-sectional area was calculated using a two-triangle approximation.

RESULTS

The between-visits coefficient of variation was 17.5%, 18.0%, 35.5%, and 12.2% for meniscus height, depth, area, and angle, respectively. The intraclass correlations for these parameters were 0.605, 0.558, 0.567, and 0.367, respectively.

CONCLUSION

FD-OCT measures lower tear meniscus dimensions and area with higher between-visits reproducibility than previous OCT instruments. FD-OCT may be a useful way to measure dry eye severity and treatment effectiveness.

Filtering Bleb Evaluation with Slit-Lamp–Adapted 1310-nm Optical Coherence Tomography

PURPOSE

Investigation of slit-lamp-adapted 1310-nm optical coherence tomography (OCT) as an in vivo imaging device in the postoperative course of glaucoma surgery.

METHODS

Postoperative images of filtering blebs and deep sclerectomies and their healing processes were qualitatively evaluated with a slit-lamp-adapted anterior segment OCT (AS-OCT; Heidelberg Engineering, Heidelberg, Germany) in 28 patients. Ophthalmologic examinations included slit-lamp examination, applanation tonometry, and slit-lamp photography. The OCT scans were qualitatively correlated with the morphologic and functional outcome of the filtering bleb.

RESULTS

1310-nm OCT was able to demonstrate the internal structure and the dimensions of filtering blebs, as well as the scleral flap and the deep sclerectomy location including Descemet membrane. Functioning filtering blebs showed a low OCT signal, small fluid-filled cysts, superficial microcystic layer, and a slack internal texture. High internal reflectivity indicated an earlier scarring of the filtering bleb. Nonfunctioning filtering blebs delivered a high OCT signal, no or few cysts, and a dense internal texture. These different OCT patterns correlated with the clinical outcome.

CONCLUSIONS

Slit-lamp-adapted 1310-nm OCT allowed the noncontact observation and documentation of the postoperative healing course of filtering blebs after glaucoma surgery. Internal structures of the filtering bleb and deep sclerectomies could be visualized. Functioning and dysfunctioning filtering blebs delivered different OCT pattern and correlated with the clinical outcome. This could be a new way to assess the postoperative healing process with the possibility of earlier intervention in cases of impending scarring.

Comparison of two partial coherence interferometers for corneal pachymetry in high myopia and after LASIK

PURPOSE

We aimed to compare the Haag-Streit optical low-coherence reflectometry (OLCR) pachymeter and the Zeiss Anterior Chamber Master (ACMaster) for measuring central corneal thickness (CCT) in high myopes and after laser in situ keratomileusis (LASIK) for myopia.

METHODS

Central corneal thickness was measured in 55 eyes of 30 myopic subjects (spherical equivalent refraction of - 5.25 D to - 10.75 D, maximal astigmatism of - 2 D), and in 37 eyes of 21 patients 3 months after LASIK for myopia (preoperative spherical equivalent refraction of - 6.0 D to - 10.75 D, maximal astigmatism of - 2 D). All measurements were performed with the Haag-Streit OLCR pachymeter and the Zeiss ACMaster, using group refractive indices of 1.376 and 1.3851, respectively. Thickness measurements were compared using paired t-tests, Pearson's correlation, linear regression and Bland-Altman plots.

RESULTS

In myopic subjects, CCT measured 531 +/- 28 microm and 533 +/- 27 microm with the OLCR pachymeter and the ACMaster, respectively (p < 0.01); all measurements correlated closely (r = 0.99, p < 0.01). In LASIK-treated eyes, CCT measured 472 +/- 24 microm using the OLCR pachymeter and 475 +/- 23 microm using the ACMaster (p < 0.01), again with close correlation between the two instruments (r = 0.99, p < 0.01).

CONCLUSIONS

Measurements of CCT in high myopes and after myopic LASIK were very similar with the Haag-Streit OLCR pachymeter and the Zeiss ACMaster. Using the current group refractive indices, the observed difference between the two instruments of < 3 microm is of little clinical importance. Thus, it would seem safe to use the OLCR pachymeter and the ACMaster interchangeably for CCT measurements in myopia as well as after myopic LASIK.

Optical coherence tomography of clear corneal incisions for cataract surgery

PURPOSE

To study the architecture of clear corneal incisions for phacoemulsification cataract surgery using optical coherence tomography (OCT).

SETTING

Doheny Eye Institute and Department of Ophthalmology, Keck School of Medicine of the University of Southern California, Los Angeles, California, USA.

METHODS

This prospective study comprised 20 eyes of 20 patients 1 month after cataract surgery performed by 1 of 2 experienced surgeons. Temporal clear corneal single-plane incisions were made with 3.0 mm metal keratomes. Each eye was scanned before and 1 month after surgery with a prototype high-speed anterior segment OCT system (Carl Zeiss Meditec, Inc.). The OCT scans were repeated 3 times during the same visit. The length of the corneal incision, thickness of the cornea, and position of the incision (distance from external wound edge to scleral spur) were measured using a computer caliper. The angle of the incision relative to the corneal surface was then calculated.

RESULTS

The mean corneal incision length was 1.81 mm +/- 0.27 (SD), the mean corneal thickness at the incision was 747 +/- 67 microm, and the mean distance between the incision and the scleral spur was 1.46 +/- 0.24 mm. The mean angle of the incision was 26.8 +/- 5.5 degrees. The measurements were repeatable to within +/-0.072 mm (pooled standard deviation) for the incision length, +/-11 microm for the corneal thickness, and +/-0.042 mm for the position of the incision.

CONCLUSIONS

Optical coherence tomography allowed excellent evaluation of corneal incisions in cataract surgery postoperatively. Measurements of wound dimensions using OCT were highly repeatable.