System for on- and off-axis volumetric OCT imaging and ray tracing aberrometry of the crystalline lens

Peripheral defocus of monofocal intraocular lenses

PURPOSE

To quantify the angular dependence of monofocal intraocular lens (IOL) power.

SETTING

Ophthalmic Biophysics Laboratory, Kallam Anji Reddy campus, L V Prasad Eye Institute, Hyderabad, India.

DESIGN

Laboratory study.

METHODS

Experiments were performed on IOLs from 2 different manufacturers (APPALENS 207, Appasamy Associates and SN60WF, Alcon Laboratories, Inc.). IOL powers ranged from 17 to 25 diopters (D). The IOLs were mounted in a fluid-filled chamber, and the on-axis and off-axis powers were measured using a laser ray tracing system over the central 3 mm zone with delivery angles ranging from -30 to +30 degrees in 5-degree increments. The position of the best focus was calculated for each IOL at each angle. The angular dependence of IOL power was compared with theoretical predictions.

RESULTS

Peripheral defocus increased significantly with increasing incidence angle and power. The peripheral defocus at ±30 degrees increased from 5.8 to 8.5 D when the power increased from 17.5 to 24.5 D for APPALENS 207 and from 4.9 to 7.4 D when the power increased from 17 to 25 D for SN60WF. The mean difference between the measured and theoretical tangential power at ±30 degrees was 0.50 ± 0.16 D for the APPALENS 207 and -0.40 ± 0.10 D for the SN60WF, independent of IOL power.

CONCLUSIONS

IOLs introduce a significant amount of peripheral defocus which varies significantly with IOL power and design. Given that peripheral defocus is related to lens power, replacement of the crystalline lens (approximately 24 D) with an IOL will produce a significant difference in peripheral defocus profile after surgery.

Estimation of the full shape of the crystalline lens from OCT: validation using stretched donor lenses

Quantifying human crystalline lens geometry as a function of age and accommodation is important for improved cataract and presbyopia treatments. In previous works we presented eigenlenses as a basis of 3-D functions to represent the full shape of the crystalline lens ex vivo. Also, we presented the application of eigenlenses to estimate the full shape of the lens in vivo from 3-D optical coherence tomography (OCT) images, where only the central part of the lens -visible through the pupil- is available. The current work presents a validation of the use of eigenlenses to estimate in vivo the full shape of dis-accommodated lenses. We used 14 ex vivo crystalline lenses from donor eyes (11-54 y/o) mounted in a lens stretcher, and measured the geometry and the power of the lenses using a combined OCT and ray tracing aberrometry system. Ex vivo, the full extent of the lens is accessible from OCT because the incident light is not blocked by the iris. We measured in non-stretched (fully accommodated) and stretched (mimicking in vivo dis-accommodated lenses) conditions. Then, we simulated computationally in vivo conditions on the obtained ex vivo lenses geometry (assuming that just the portion of the lens within a given pupil is available), and estimated the full shape using eigenlenses. The mean absolute error (MAE) between estimated and measured lens' diameters and volumes were MAE = 0.26 ± 0.18 mm and MAE = 7.0 ± 4.5 mm3, respectively. Furthermore, we concluded that the estimation error between measured and estimated lenses did not depend on the accommodative state (change in power due to stretching), and thus eigenlenses are also useful for the full shape estimation of in vivo dis-accommodated lenses.

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.

Combined anterior segment OCT and wavefront-based autorefractor using a shared beam

We have combined an anterior segment (AS) optical coherence tomography (OCT) system and a wavefront-based aberrometer with an approach that senses ocular wavefront aberrations using the OCT beam. Temporal interlacing of the OCT and aberrometer channels allows for OCT images and refractive error measurements to be acquired continuously and in real-time. The system measures refractive error with accuracy and precision comparable to that of clinical autorefractors. The proposed approach provides a compact modular design that is suitable for integrating OCT and wavefront-based autorefraction within the optical head of the ophthalmic surgical microscope for guiding cataract surgery or table-top devices for simultaneous autorefraction and ocular biometry.

Increased crystalline lens coverage in optical coherence tomography with oblique scanning and volume stitching

A three-dimensional optical coherence tomography (OCT) crystalline lens imaging method based on oblique scanning and image stitching is presented. The method was designed to increase OCT imaging volume of crystalline lens in vivo. A long-range swept-source (SS)-OCT imaging system, which can measure the entire anterior segment of eye in a single acquisition, is used to acquire one central volume and 4 extra volumes with different angles between optical axis of OCT instrument and the pupillary axis. The volumes are then stitched automatically by developed software. To show its effectiveness and verify its validity, we scanned the subjects before and after pupil dilation drops and compared the experimental results. By determining the number of voxels representing the signal from the crystalline lens in 3-D OCT images, our method can provide around 17% additional volumetric lens coverage compared with a regular imaging procedure. The proposed approach could be used clinically in early diagnosis of cortical cataract. Wider field of view offered by this method may facilitate more accurate lens biometry in its peripheral zones, which potentially contributes to understanding of lens shape modifications of the accommodating eye.

Age-Dependence of the Peripheral Defocus of the Isolated Human Crystalline Lens

Purpose

To characterize the peripheral defocus of isolated human crystalline lenses and its age dependence.

Methods

Data were acquired on 116 isolated lenses from 99 human eyes (age range, 0.03–61 years; postmortem time, 40.1 ± 21.4 hours). Lenses were placed in a custom-built combined laser ray tracing and optical coherence tomography system that measures the slopes of rays refracted through the lens for on-axis and off-axis incidence angles. Ray slopes were measured by recording spot patterns as a function of axial position with an imaging sensor mounted on a positioning stage below the tissue chamber. Delivery angles ranged from –30° to +30° in 5° increments using a 6 mm × 6 mm raster scan with 0.5-mm spacing. Lens power at each angle was calculated by finding the axial position that minimizes the root-mean-square size of the spot pattern formed by the 49 central rays, corresponding to a 3-mm zone on-axis. The age dependence of the on-axis and off-axis optical power and the relative peripheral defocus (difference between off-axis and on-axis power) of lenses were quantified.

Results

At all angles, lens power decreased significantly with age. Lens power increased with increasing delivery angle for all lenses, corresponding to a shift toward myopic peripheral defocus. There was a statistically significant decrease in the lens peripheral defocus with age.

Conclusions

The isolated human lens power increases with increasing field angle. The lens relative peripheral defocus decreases with age, which may contribute to the age-related changes of ocular peripheral defocus during refractive development.

Isolated human crystalline lens three-dimensional shape: A comparison between Indian and European populations

There have been many studies on lens properties in specific populations (e.g. in China, Europe, Singapore, etc.) some of which suggest there may be differences between populations. Differences could be caused by ethnic or environmental influences or experimental procedures. The purpose of this study is to evaluate if any differences exist between Indian and European populations in the central geometric and full shape properties of human lenses. Two custom-developed spectral domain optical coherence tomography systems were used to acquire the crystalline lens geometry: one in India (69 lenses from 59 donors) and the other in Spain (24 lenses from 19 donors). The steps for obtaining accurate 3-D models from optical coherence tomography raw images comprised of image segmentation, fan and optical distortion correction, tilt removal and registration. The outcome variables were lens equatorial diameter, lens thickness, anterior and posterior lens thicknesses and their ratio, central radius of curvature of the anterior and posterior lens surfaces, lens volume and lens surface area. A mixed effects model by maximum likelihood estimation was used to evaluate the effect of age, population and their interaction (age*population) on lens parameters. After adjusting for age, there were no population differences observed in anterior and posterior radii of curvature, equatorial diameter, lens thickness, anterior and posterior lens thicknesses and their ratio, volume and surface area (all p ≥ 0.08). There was also no effect of the interaction term on anterior and posterior radii of curvature, equatorial diameter, lens thickness, anterior and posterior lens thicknesses and their ratio, volume and surface area (all p ≥ 0.06). All central geometric and full shape parameters appeared to be comparable between the European and Indian populations. This is the first study to compare geometric and full shape lens parameters between different populations in vitro.

Eigenlenses: a new model for full crystalline lens shape representation and its applications

The crystalline lens is an important optical element in the eye, responsible for focusing, and which experiences significant changes throughout life. The shape of the lens is usually studied only in the optical area (central 4 to 6 mm). However, for a great number of applications, a description of the full shape of the crystalline lens is required. We propose a new method for the representation of the full shape of the crystalline lens, constructed from 3-dimensional optical coherence tomography images of 133 isolated crystalline lenses (0-71 y/o), which we have called eigenlenses. The method is shown to be compact and accurate to describe not only the full shape of the crystalline lens, but also the optical zone in comparison with other methods. We also demonstrate its application to the extrapolation of the full shape of the crystalline lens from in-vivo optical images of the anterior segment of the eye, where only the central part of the lens visible through the pupil is available, and in the generation (synthesis) of realistic full lenses of a given age. The method has critical applications, among others, in improving and evaluating myopia and presbyopia treatments.

Age-Related Changes to the Three-Dimensional Full Shape of the Isolated Human Crystalline Lens

Purpose

Studying the full shape crystalline lens geometry is important to understand the changes undergone by the crystalline lens leading to presbyopia, cataract, or failure of emmetropization, and to aid in the design and selection of intraocular lenses and new strategies for correction. We used custom-developed three-dimensional (3-D) quantitative optical coherence tomography (OCT) to study age-related changes in the full shape of the isolated human crystalline lens.

Methods

A total of 103 ex vivo human isolated lenses from 87 subjects (age range, 0–56 years) were imaged using a 3-D spectral-domain OCT system. Lens models, constructed after segmentation of the surfaces and distortion correction, were used to automatically quantify central geometric parameters (lens thickness, radii of curvatures, and asphericities of anterior and posterior surfaces) and full shape parameters (lens volume, surface area, diameter, and equatorial plane position). Age-dependencies of these parameters were studied.

Results

Most of the measured parameters showed a biphasic behavior, statistically significantly increasing (radii of curvature, lens volume, surface area, diameter) or decreasing (asphericities, lens thickness) very fast in the first two decades of life, followed by a slow but significant increase after age 20 years (for all the parameters except for the posterior surface asphericity and the equatorial plane position, that remained constant).

Conclusions

Three-dimensional quantitative OCT allowed us to study the age-dependency of geometric parameters of the full isolated human crystalline lens. We found that most of the lens geometric parameters showed a biphasic behavior, changing rapidly before age 20 years and with a slower linear growth thereafter.

Off-axis optical coherence tomography imaging of the crystalline lens to reconstruct the gradient refractive index using optical methods

Earlier studies have shown that the gradient index of refraction (GRIN) of the crystalline lens can be reconstructed in vitro using Optical Coherence Tomography (OCT) images. However, the methodology cannot be extended in vivo because it requires accurate measurements of the external geometry of the lens. Specifically, the posterior surface is measured by flipping the lens so that the posterior lens surface faces the OCT beam, a method that cannot be implemented in vivo. When the posterior surface is imaged through the lens in its natural position, it appears distorted by the unknown GRIN. In this study, we demonstrate a method to reconstruct both the GRIN and the posterior surface shape without the need to flip the lens by applying optimization routines using both on-axis and off-axis OCT images of cynomolgous monkey crystalline lenses, obtained by rotating the OCT delivery probe from -45 to +45 degrees in 5 degree steps. We found that the GRIN profile parameters can be reconstructed with precisions up to 0.009, 0.004, 1.7 and 1.1 (nucleus and surface refractive indices, and axial and meridional power law, respectively), the radius of curvature within 0.089 mm and the conic constant within 0.3. While the method was applied on isolated crystalline lenses, it paves the way to in vivo lens GRIN and posterior lens surface reconstruction.

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.