Proceedings Volume 4245

Ophthalmic Technologies XI

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Proceedings Volume 4245

Ophthalmic Technologies XI

View the digital version of this volume at SPIE Digital Libarary.

Volume Details

Date Published: 7 June 2001
Contents: 8 Sessions, 34 Papers, 0 Presentations
Conference: BiOS 2001 The International Symposium on Biomedical Optics 2001
Volume Number: 4245

Table of Contents

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Table of Contents

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  • Ophthalmic Instruments I: Surgery
  • Ocular Aberrations: Modeling, Measurement, and Correction
  • Ophthalmic Optics
  • Ophthalmic Diagnostics
  • Ocular Surgery: Virtual, Experimental, and Clinical
  • Ophthalmic Instruments II: Diagnostics
  • Light Interaction with Ocular Tissues and Implants
  • Poster Session
  • Light Interaction with Ocular Tissues and Implants
  • Ocular Aberrations: Modeling, Measurement, and Correction
  • Ophthalmic Diagnostics
Ophthalmic Instruments I: Surgery
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Excimer laser ophthalmic system optical coherence tomography
Alexander M. Razhev, Sergei N. Bagayev, Valentin M. Gelikonov, et al.
The excimer laser ophthalmic system Medilex including the ArF (193-nm) excimer laser with optical delivery system and optical coherence tomograph (OCT) for correction of the refraction anomalies is presented. The new optical system has been developed. The possibility of the controlling the corneal surface with micron accuracy by OCT apparatus during the process of UV laser ablation is shown. OCT device is able to measure surface profile, thickness of the cornea and depth of removed tissue during process of UV ablation. Preliminary results on the determination of the surface profile and thickness of the non-modified by UV laser cornea of the alive human eye performed by OCT device with accuracy about 5 micrometers is shown. The results of the experiments on the measurement of the removed material depth on polymer film and cadaverous human eyes simultaneously with ablation process are described.
Scanning confocal slit photon counter measurements of post-PRK haze in two-year study
John Taboada, David Gaines, Mary Alice Perez, et al.
In our study, a group of 80 United States Air Force, non- flying personnel will undergo photorefractive corneal surgery for moderate levels of myopia (< 6 diopters) and 20 will serve as controls. As of this report, approximately 56 have had the treatment. Of these, only about 59% of the treated eyes showed even a trace (.5) level of clinically assessed haze at any time. We report on the use of a recently developed instrument designed for the objective measurement of these low levels of haze in treated corneas. The sensitivity of the instrument is derived from the use of a scanning confocal slit photon counter. The use of a physical standard for calibration secures accuracy and reproducibility over an extensive period of time. Our haze measurements in this study revealed a very low level increase from baseline values for these patients. The typical increase over baseline was of the same magnitude as the variability in the observations, although the inherent variability in the measurements was approximately 0.25 times the value of the patient's haze variability.
Irradiation control parameters for computer-assisted laser photocoagulation of the retina
Espen Naess, Torstein Molvik, Steven F. Barrett, et al.
A system for robotically assisted retinal surgery has been developed to rapidly and safely place lesions on the retina for photocoagulation therapy. This system provides real- time, motion stabilized lesion placement for typical irradiation times of 100 ms. The system consists of three main subsystems: a global, digital-based tracking subsystem; a fast, local analog tracking subsystem; and a confocal reflectance subsystem to control lesion parameters dynamically. We have reported on these subsystems in previous SPIE presentations. This paper concentrates on the development of the second hybrid system prototype. Considerable progress has been made toward reducing the footprint of the optical system, simplifying the user interface, fully characterizing the analog tracking system and using measurable lesion reflectance growth parameters to develop a noninvasive method to infer lesion depth. This method will allow dynamic control of laser dosimetry to provide similar lesions across the non-uniform retinal surface. These system improvements and progress toward a clinically significant system are covered in detail within this paper.
Ocular Aberrations: Modeling, Measurement, and Correction
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Is the human eye a perfect optic?
Michael Mrochen, Maik Kaemmerer, Peter Mierdel, et al.
The aim of this work was to study the optical aberrations of higher order in a normal population and to answer the question on the optical quality of the human eye. Therefore, the optical aberrations of 130 eyes (90 individuals) have been measured by means of a wavefront measuring devices of Tscherning's-type. The pupil of each measured eye was dilated to at least 7 mm in diameter and wavefront sensing was performed with respect to the line of sight. The optical aberrations are expressed in terms of Zernike coefficients up to the 6th order, root-means-squared wavefront errors and critical pupil size. The main finding of this paper is that the `average eye' has only minimal wavefront errors indicating that the construction of the human eye, in principle, provides excellent optics exceeding the Marechal- criterion only by a factor of 2.1 (pupil diameter 5 mm). However, such minimal aberrations are achieved in only 6% of the individual eyes examined in this study. In conclusion, the `averaged human' eye has nearly a perfect optic, but the individual eye provides poor optical quality.
Refractive surgery and supervision: physical and biological constraints to an exciting perspective
Recent technological advancements have opened new perspectives to photorefractive surgery: the target is going to broaden from the simple correction of defocusing errors in ametropic eyes to the compensation of high order optical aberrations on an individual basis, even in emmetropic subjects. In this way, optimization of visual performance may result in `supervision', namely a visual acuity equal to or better than logMAR equals -0.3 (Snellen: 20/10). Assuming that all monochromatic aberrations can be completely nulled through corneal sculpting, several factors remain to limit the ultimately achievable visual acuity: diffraction, photoreceptor spacing, internal noise, chromatic aberration. All these factors have been included in a numerical model based on first principles and best fitting of experimental data, providing estimates on visual acuity and contrast sensitivity. In an aberration-free human eye, the maximum achievable visual acuity is logMAR equals -0.24 (Snellen: 20/11.5) at 2.7 mm pupil size. Diffraction turns out to be the limiting factor up to 2 mm pupil size, chromatic aberration from 2 to 5 mm, and internal noise dominates above 5 mm. The maximum visual acuity gain attainable with a `super-vision' procedure appears thus restricted to only 2 lines above the `mean' logMAR equals 0 level (20/20). Chromatic aberration requires thorough consideration in all photo-refractive procedures.
Problems with wavefront aberrations applied to refractive surgery: developing standards
Refractive surgery is evolving rapidly. A recent development uses wavefront aberration information to improve the surgical outcome. Before the wavefront information can be used effectively a number of problems must first be resolved. One of the main concerns is the presence of halos during night driving conditions. It seems clear that this problem occurs when the pupil is large enough to overlap the ablation transition zone. Several questions associated with the transition zone are examined. Shape descriptors to characterize the transition zone are discussed. Better ways of quantitatively characterizing the transition zone and predicting its properties are needed to help specify the ablation. Many of the issues associated with improving refractive surgery can be addressed by establishing a standards committee that includes basic researchers and clinicians. This committee can become a forum for developing techniques to assess visual outcomes, it can make recommendations for developing a database that would allow researchers to compare the intended outcome of ablation with the actual outcome. In order for this enterprise to be successful increased openness about surgery parameters and surgery outcome would be helpful.
Calculation of the primary spherical aberration of an eye model using paraxial ray tracing with matrix optics
The purpose of this study was to develop a technique to calculate the effects of laser corneal reshaping on the spherical aberration of a model eye. Our model indicates that spherical corrections of high myopia (> 5 D) by laser corneal reshaping are expected to significantly increase the amount of primary spherical aberration of the eye.
Theoretical study of aberration correction in eyes using scanning-spot lasers
To investigate the effect of laser spot size on the outcome of aberration correction with scanning laser corneal ablation.
Selective corneal optical aberration (SCOA) for customized ablation
Benedikt J. Jean M.D., Thomas Bende
Wavefront analysis still have some technical problems which may be solved within the next years. There are some limitations to use wavefront as a diagnostic tool for customized ablation alone. An ideal combination would be wavefront and topography. Meanwhile Selective Corneal Aberration is a method to visualize the optical quality of a measured corneal surface. It is based on a true measured 3D elevation information of a video topometer. Thus values can be interpreted either using Zernike polynomials or visualized as a so called color coded surface quality map. This map gives a quality factor (corneal aberration) for each measured point of the cornea.
Patient treatment based on wavefront-guided CustomCornea correction with the Alcon Summit Autonomous LADARVision excimer laser system
Ezra I. Maguen M.D., James J. Salz M.D., Marguerite B. McDonald, et al.
A study was undertaken to assess whether results of excimer laser vision correction with the LADARVision 193 nm excimer laser (Alcon Summit Autonomous) can be improved with the use of wavefront analysis generated by a proprietary system utilizing a Hartman-Shack sensor and expressed using Zernicke polynomials.
Ophthalmic Optics
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Corneal modeling using conic section fits of PAR corneal topography system measurements
Stanley Zipper, Fabrice Manns, Viviana Fernandez, et al.
The purpose of this study was to measure the average shape and variability of human corneas and to develop a tool for analyzing, height, curvature, and aberrations based on a conic section model. Fresh Eye Bank Eyes were placed in Dextran until the corneal thickness reached a physiological value. The eyes were placed in a custom made holder and measured using an intraoperative PAR Corneal Topography System (CTS) mounted on an operation microscope. Topography was measured before and after removal of the epithelium. A series of MATLAB functions were written to analyze the raw-z (height) data in polar coordinates. The functions fit conic sections to the PAR CTS data along hemi-meridians at 5 degree(s) intervals. The conic shape factor and apical radius were used to calculate and display the curvature. The dependence of these parameters with meridional position was examined.
Measurement of the topography of human cadaver lenses using the PAR corneal topography system
Viviana Fernandez, Fabrice Manns, Stanley Zipper, et al.
To measure the radius of curvature and asphericity of the anterior and posterior surfaces of crystalline lenses of human Eye-Bank eyes using the PAR Corneal Topography System. The measured values will be used in an optical model of the eye for lens refilling procedures.
Feasibility of simultaneous correction of ametropia by varying gel refractive index with phaco-ersatz
Arthur Ho, Paul Erickson, Fabrice Manns, et al.
Recent developments in surgical procedures for restoring accommodation saw the availability of Phaco-ersatz as a feasible procedure returning near vision to the presbyopies. In Phaco-ersatz, the presbyopic crystalline lens is extracted and replaced by a flexible polymer gel in the intact lens capsule. The ability to simultaneously correct ametropia while restoring accommodation with this procedure is seen to be extremely attractive. One strategy by which this may be achieved within the Phaco-ersatz procedure is by making use of polymer gel of different refractive indices. We assessed the feasibility of simultaneously correcting ametropia while restoring accommodation using phaco-ersatz using this approach. Two model eyes (Gullstrand and Navarro) were used to evaluate this approach. Computation results using paraxial equations and ray tracing indicated that while this approach might be feasible for the hypermetrope, its usefulness for correcting myopia is limited due to significant reductions in the resultant amplitude of accommodation. A number of practical considerations may also influence the applicability of this approach.
Ophthalmic Diagnostics
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Noninvasive and remote detection of cataracts during space exploration with dynamic light scattering
Rafat R. Ansari, Frank Giblin, James F. King, et al.
Outer space is filled with high-energy ionizing radiation. Continuous radiation exposure or slight increase (e.g., due to flares) in it can be hazardous to astronaut health. Therefore cataractogenesis is a possible risk factor. In this paper we apply the sensitive technique of dynamic light scattering to investigate the simulated effects of X-ray exposure to the ocular tissue of non-human animals. An internet web based system integrates photon correlation data and controls the hardware for animal monitoring at a remote site in real time.
Minimizing the influence of fundus pigmentation on retinal vessel oximetry measurements
Matthew H. Smith, Kurt R. Denninghoff, Arthur Lompado, et al.
The goal of making calibrated oxygen saturation measurements of blood in retinal arteries and veins via a noninvasive spectroscopic technique has nearly been realized. Semi-continuous advancement in the field of retinal vessel oximetry over the last three decades has resulted in several technologies that seem poised for commercialization. In this paper, we present our instrumentation and technique for making well-calibrated saturation measurements of the blood in retinal vessels. The Eye Oximeter (EOX) is a confocal scanning laser ophthalmoscope capable of acquiring multi-spectral images. Analysis of these spectral vessel images allows spectroscopic determination of the oxygen saturation of blood within each vessel. The primary emphasis of this paper is to illustrate the effect of fundus pigmentation on these oximetric measurements. We show that decreasing fundus reflectivity is mathematically similar to decreasing the vessel thickness. The apparent decreased vessel thickness is a direct consequence of scattering by red blood cells. We present in vitro and in vivo measurements that demonstrate an instrument calibration that is nearly independent of vessel diameter and fundus reflectivity.
Raman detection of carotenoid pigments in the human retina
Werner Gellermann, Igor V. Ermakov, Maia R. Ermakova, et al.
We have used resonant Raman scattering as a novel, non- invasive, in-vivo optical technique to measure the concentration of macular carotenoid pigments in the living human retina of young and elderly adults. Using a backscattering geometry and resonant molecular excitation in the visible, we measure the Raman peaks originating from the single- and double-bond stretch vibrations of the (pi) - electron conjugated molecule's carbon backbone. The Raman signals scale linearly with carotenoid content while the required laser excitation is well below safety limits for macular exposure. Measured macular pigment levels decline significantly with increasing age. The Raman technique is objective and quantitative and may lead to a new method for rapid screening of carotenoid pigment levels in large populations at risk for vision loss from age-related macular degeneration, the leading cause of blindness in the elderly in the United States.
Performance of amplitude-modified second-order Gaussian filter for the detection of retinal blood vessel
Gang Luo, Opas Chutatape, Sanjay M. Krishnan
Automated analysis of vessel morphology is valuable in diagnosis since it can provide measurable abnormalities in diameter and tortuosity that reflect systemic or local ocular disease. Although the center point of vessel can be easily detected with matched filter, the diameter cannot be obtained from the readily available convolution result in the process of location detection. An amplitude modified second-order Gaussian filter is presented in this paper. Simulation shows that a profile of vessel matches only one amplitude modified second-order Gaussian filter, with which the convolution peak reaches its maximum. Therefore, the width specified parameter of vessel model can be figured out by matching operation with a group of modified Gaussian filters. A vessel diameter measurement concept based on calibration is presented in order to avoid the burden of finding the exact edges of vessels, which has caused several different width definitions for the endless Gaussian curve. The real vessel width can be determined simply according to the relationship built through a calibration procedure in advance after the width specified parameter of Gaussian curve is obtained. Experiments of blood vessel detection of color fundus image are given. The results of the vessel diameter measurement show good agreement with visual inspection.
Ocular Surgery: Virtual, Experimental, and Clinical
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Computer-simulated phacoemulsification
Carl-Gustaf Laurell M.D., Leif Nordh, Eva Skarman, et al.
Phacoemulsification makes the cataract operation easier for the patient but involves a demanding technique for the surgeon. It is therefore important to increase the quality of surgical training in order to shorten the learning period for the beginner. This should diminish the risks of the patient. We are developing a computer-based simulator for training of phacoemulsification. The simulator is built on a platform that can be used as a basis for several different training simulators. A prototype has been made that has been partly tested by experienced surgeons.
Ophthalmic Instruments II: Diagnostics
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Celestial and terrestrial tele-ophthalmology: a health monitoring helmet for astronaut/cosmonaut and general public use
A goggles-like head-mounted device equipped with several non-invasive techniques for quantitative medical evaluation of the eye, skin, and brain is envisioned for monitoring the health of astronauts and cosmonauts during long-term space travel and exploration. Real-time non-invasive evaluation of the different structures within these organs will provide indices of the health of these organs, as well as the entire body. The techniques such as dynamic light scattering (for the early detection of cataracts to evaluate effects of cosmic radiation), corneal autofluorescence (to assess extracellular matrix biology (e.g., diabetes), optical polarization (of aqueous fluid to evaluate serum chemistry), laser Doppler velocimetry (of retinal, optic nerve, and choroidal blood flow to assess ocular as well as central nervous system blood flow), reflectometry/oximetry (for oxygen metabolism), optical coherence tomography (for retinal microstructure), and possibility scanning laser technology for intraocular imaging and scanning will be integrated into this compact device.
Video injection for slit lamp augmented reality
Bojidar Madjarov M.D., Jeffrey W. Berger M.D.
Real-time tracking and overlay of previously stored photographic and angiographic images directly onto the real- time slitlamp biomicroscopic or surgical microscopic eye fundus image may facilitate real-time image comparison, measurement, and correlation for diagnosis, management and treatment of retinal diseases. Very high resolution and brightness is necessary to inject the overlayed image into the optical path of the diagnostic or surgical instrument. A binocular slit-lamp biomicroscope interfaced to a CCD camera, framegrabber board, and PC allows for posterior segment examination with a hand-held contact or non-contact lens, synchronous with acquisition and digitization of slitlamp fundus images.
Integrated instrument for dynamic light scattering and natural fluorescence measurements
Over the past two decades, great efforts have been made in ophthalmology to use optical techniques based on dynamic light scattering and tissue natural fluorescence for early (at molecular level) diagnosis of ocular pathologies. In our previous studies, the relationship between the corneal AF and DLS decay widths of ocular tissues were established by performing measurements on diabetes mellitus patients. In those studies, corneal AF mean intensities were significantly correlated with DLS decay width measurements for each diabetic retinopathy grade in the vitreous and in the cornea. This suggested that the quality of the diagnosis could be significantly improved by properly combining these two powerful techniques into a single instrument. Our approach is based on modifying a commercial scanning ocular fluorometer (Fluorotron Master, Ocumetrics Inc., CA, USA) to include both techniques in the same scanning unit. This configuration provides both DLS and AF real time measurements from the same ocular volume: they can be located in each section of the optical axis of the eye from the cornea to the retina. In this paper, the optical setup of the new system is described and preliminary in-vitro and in-vivo measurements are presented.
Zoom ophthalmoscope
George G. Smith, John Farmer, Ian P. Powell
We describe the basic principles behind the design of a zoom ophthalmoscope. The purpose of such a device is to replace two instruments which are currently used to examine the retina; the small field but high magnification direct ophthalmoscope and the large field but low magnification indirect ophthalmoscope. The desired field-of-view and zoom ratio cover the fields-of-view and magnifications of these two instruments.
Thickness and topographic inspection of RPG contact lenses by optical triangulation
Optical triangulation as a non-destructive test method extensively proved its usefulness on the dimensional and topographic inspection of a large range of objects and surfaces. In this communication the issue of microtopographic and thickness inspection of hard contact lenses (RPG) is addressed. The use of optical triangulation is discussed based on the results of the application of our MICROTOP.03.MFC microtopographer to this kind of tasks will be presented.
Light Interaction with Ocular Tissues and Implants
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Maximum tolerable dose (MTD): a new index for ultraviolet radiation toxicity in the lens
Per G. Soderberg, Stefan Lofgren, Marcelo Ayala, et al.
The maximum tolerable dose (MTD2.3:16) for avoidance of cataract on exposure to UVR-300 nm in the rat was currently estimated to 3.65 kJ/m2. For this, Sprague-Dawley rats were unilaterally exposed to UVR in the 300 nm wavelength region, generated with a high pressure mercury arc source. The intensity of forward light scattering was measured one week after exposure. MTD allows estimation of toxicity for continuous response events with small sample experiments. Current safety standards for avoidance of cataract after exposure to UVR are based on a binary response event. It has however recently been shown that UVR-induced cataract is a continuous dose-dependent event. MTD provides a statistically well defined criterium of toxicity for continuous response events.
Hydrophilic treatment of porous PTFE for intractable glaucoma implant devices
Masataka M. Murahara, Yuji Sato, Viviana Fernandez, et al.
Intractable glaucoma results from hindrances in the eyeball aqueous humor pathways that increase the intraocular pressure above normal physiological levels (over 20 mmHg). In this study porous PTFE membranes were made hydrophilic with a photochemical method that use ethyl alcohol and water for the chemical solution.
Nonpenetrating glaucoma surgery using the CO2 laser: experimental studies in human cadaver eyes
Ehud I. Assia M.D., Irina S. Barequet, Mordechai Rosner M.D., et al.
Non-penetrating trabeculectomy (NPT) is a potential replacement to conventional trabeculectomy, as it eliminates the necessity of penetrating the eye which is the cause of most of the complications entailed by the latter operation. NPT however, requires considerable surgical skill, is time consuming and entails complications of its own. We have shown that it can be easily performed by using the CO2 laser to ablate the sclera and corneoscleral tissues to the required depth. The use of the CO2 laser eliminates the danger of inadvertent perforation, a common complication of NPT as the tissue ablation ceases when the end-point of the operation, the aqueous humor percolation, is reached. Our experiments, performed on animal and human cadaver eyes showed that CO2 laser NPT rapid is easily mastered and performed rapid and eliminates almost completely the risk of complications.
Computer modeling of transpupillary thermotherapy for choroidal neovascularization
Jeffrey W. Berger M.D.
Thermal effects of visible and near-infrared laser pulses were modeled in order to guide future pre-clinical and clinical studies for transpupillary thermotherapy for choroidal neovascularization. A finite-difference, thermophysical model of the native fundus was developed, and contributions were added to consider the effects of incident visible and near-infrared laser irradiation, natural pigment heterogeneity, an absorptive CNV complex, as well as subretinal and sub-RPE blood. Calculations were performed with and without choroidal convective cooling.
Optimization of the parameters for intrastromal refractive surgery with ultrashort laser pulses
Focussing femtosecond laser pulses into a transparent media, such as corneal tissue, leads to optical breakdown, generation of a micro-plasma and, thus, a cutting effect inside the tissue. To proof the potential of fs-lasers in refractive surgery, three-dimensional cutting within the corneal stroma was evaluated. With the use of ultrashort laser pulses within the LASIK procedure (laser in situ keratomileusis) possible complications in handling of a mechanical knife, the microkeratome, can be reduced by using the treatment laser as the keratome itself. To study woundhealing effects, animal studies were carried out in rabbit specimen. The surgical outcome was analyzed by means of histological sections, as well as light and scanning electron microscopy. Dependencies on the dispersion caused by focussing optics were evaluated and optimized. Thus, pulse energies well below 1 (mu) J were sufficient to perform the intrastromal cuts. The laser pulses with a duration of 180 fs and energies of 0.5-100 (mu) J were provided by a modelocked frequency doubled erbium fiber-laser with subsequent chirped pulse amplification in a titanium sapphire amplifier at up to 3 kHz.
Poster Session
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Automatic multidiagnosis system for slit lamp
Liliane Ventura, Caio Chiaradia, Andre Marcio Vieira Messias, et al.
We have developed a system for several automatic diagnose in Slit Lamp in order to provide 04 additional measurements to the biomicroscope: (1) counting of the endothelial cells of donated corneas; (2) automatic keratometry; (3) corneal ulcer evaluation; (4) measurement of linear distances and areas of the ocular image. The system consists in a Slit Lamp, a beam-splitter, some optical components, a CCD detector, a frame grabber and a PC. The optical components attached to the beam-splitter are the same for all the functions, except for 1. For function 1, we have developed an optical system that magnifies the image 290X and a software that counts the cells interactively and automatically. Results are in good agreement with commercial specular microscopes (correlation coefficient is 0,98081). The automatic keratometry function is able to measure cylinders over 30 di and also irregular astigmatisms. The system consists of projecting a light ring at the patient's cornea and the further analysis of the deformation of the ring provides the radius of curvature as well as the axis of the astigmatism. The nominal precision is 0,005 mm for the curvature radius and 2 degree(s) for the axis component. The results are in good agreement with commercial systems (correlation coefficient of 0,99347). For function 3, the ulcer is isolated by the usual clinical ways and the image of the green area is automatically detected by the developed software in order to evaluate the evolution of the disease. Function 4 simply allows the clinician do any linear or area measurement of the ocular image. The system is a low cost multi evaluation equipment and it is being used in a public hospital in Brazil.
In-vitro corneal transparency measuring system
A system for measuring the average corneal transparency of preserved corneas has been developed in order to provide a more accurate and standard report of the corneal tissue. The donated cornea transparency is one of the features to be analyzed previously to its indication for the transplant. The small portable system consists of two main parts: the optical and the electronic parts. The optical system consists of a white light, lenses and pin-holes that collimate white light beams that illuminates the cornea in its preservative medium. The light that passes through the cornea is detected by a resistive detector and the average corneal transparency is shown in a display. In order to obtain just the tissue transparency, the electronic circuit was built in a way that there is a baseline input of the preservative medium, previous to the measurement of the corneal transparency. Manipulating the system consists of three steps: (1) Adjusting the zero percentage in the absence of light (at this time the detectors in the dark); (2) Placing the preservative medium in the system and adjusting the 100% value (this is the baseline input); (3) Preserving the cornea and placing it in the system. The system provides the tissue transparency. The system is connected to an endothelium evaluation system for Slit Lamp, that we have developed, and statistics about the relationship of the corneal transparency and density of the endothelial cells will be provided in the next years. The system is being used in a public Eye Bank in Brasil.
Clinical application of red and green laser retinometer for cataracts of various etiology using speckle technologies
Garif G. Akchurin, Valery V. Bakutkin, Dmitry A. Zimnyakov, et al.
Measurement of retinal visual acuity in 230 patient with different types of cataract (senile, posterior capsular, nuclear, complicated) before and after extraction has shown high correlation between these values. Double `red' (He-Ne (lambda) equals 633 nm or diode laser GaAlAs (lambda) equals 630 - 650 nm) and `green' (YAG:Nd laser (lambda) equals 532 nm) retinometer has allowed to determinate the angle of resolution of the retina practically for all types of cataract. Applied speckle-technologies have permitted the range of retinal angular resolution to be estimated with the help of laser retinometer at the stage of preoperative cataract diagnosis. Speckle-modulated laser fields arising at in vitro measurements of different types of human cataracts lenses have been investigated experimentally. Computer analysis of digital images has allowed to estimate of destruction of the spatial coherence of a laser beam scattered by a turbid lens. Effect of saturation of the average speckles size in multiple-scattering biotissues gives good opportunities to determine preoperative retinal visual acuity for any type of cataracts in vivo.
Light Interaction with Ocular Tissues and Implants
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Accumulated IR-laser ablation of endothel at the inner wall of Schlemm's canal
Holger Specht, Hans Eberhard Voelcker, Reinhard O. W. Burk
The typical approach for tissue ablation in laser microsurgery is to use a fiber tip in the contact-mode. An example is the laser trabecular ablation ab interno. The purpose of this paper is to describe the trabecular meshwork ablation with an Er:YAG laser in the none contact mode from inside Schlemm's canal using cavitations bubbles.
Ocular Aberrations: Modeling, Measurement, and Correction
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Effect of lenslet resolution on the accuracy of ocular wavefront measurements
The most critical element in a ocular Shack-Hartmann wavefront sensor is the micro-optic lenslet array. This array largely determines the accuracy of the wavefront measurement and the dynamic range of the measurements. This paper discusses the details of how the density of the lenslet array affects the accuracy of the wavefront measurement. We briefly discuss wavefront reconstruction, which is the mathematical process that takes the output from the lenslet array and reconstructs the input wavefront. We compare the two primary methods of reconstruction, the zonal fit and the modal fit. We also show how a denser array can be designed to have a better dynamic range.
Ophthalmic Diagnostics
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Imaging the primate retina using polarization-sensitve optical coherence tomography
Jason D. Marsack, Mathieu G. Ducros, Sapun H. Parekh, et al.
We report results of a study using polarization sensitive optical coherence tomography (PSOCT) to measure physical properties of the retina and to create images of retinal microstructure. Our instrument incorporates a mode-locked Ti:Al2O3 laser and achromatic polarization optics to record high resolution images. High-resolution B scans (two-dimensional images) of the in-vivo rhesus monkey retina have been recorded in the optic disk, peripapillary area and macula. Images of the peripapillary area allow measurement of the retinal nerve fiber layer (RNFL) thickness and calculation of the Stokes parameters of light back-scattered from the retina. Results of our study indicate: 1) PSOCT may be utilized to measure RNFL thickness; 2) PSOCT may be used to measure areas of birefringent tissue in the retina; and 3) selection of a scan pattern surrounding the optic nerve should account for the relatively large radial RNFL thickness gradient. Moreover, since glaucoma manifests in a destruction of the RNFL, PSOCT may be useful as a screening and diagnostic modality.