Estela López-Olazagasti

Objective performance of a set of uncorrected 20/20 normal eyes: clinical reference

In recent years we have been working in the characterization of the objective average performance of a set of uncorrected human eyes with a 20/20 visual acuity, described as the resultant average wavefront aberration function (WA), point-spread function (PSF), modulation transfer function (MTF), and power refractive maps. This objective performance has been used as our clinical reference to analyze the objective pre- and post-operated performance in laser refractive surgery in different situations. We show some of our current results obtained from the application of our clinical reference.

A new model for the anterior corneal surface using higher-order aspheric surfaces and variable eccentricity

Contact lenses manufacturers and ophthalmologists who perform laser surgery to correct visual problems depend on an accurate model of the anterior corneal surface. Several models have been suggested in the past, going from Gullstrands initial idea to the aspheric profiles and to Bonnets profile based on anatomical data. Clinical evidence shows, however, that the anterior corneal surface is characterized by a variable eccentricity, contrary to the hypothesis on which the current models are based. We present, in this work, a new model for the anterior corneal surface in terms of higher-order aspheric surfaces in which the eccentricity at a given point of the cornea is a continuous function of its distance to the optical axis. We also establish the conditions under which the different conic and Bonnets profiles are recovered from our model. Finally, we present our preliminary results using this model.

Comparison of refractive power maps from a reference surface: geometric versus Zernike power polynomials

In recent years there has been many advances in the field of visual optics, such as new technologies to measure and to analyze the wavefront aberration function of the human eye. In this direction, corneal topographers have been extensively used as a tool to obtain related data that can be used to get the refractive power maps of the human cornea in order to characterize the optical function of the eye. On the other hand, it is well known that we can describe the optical aberrations in the human eye as a polynomial expansion of the Zernike polynomials. In this work we present a qualitative comparison of a refractive power map from a reference refractive surface obtained with an alternate form of representation, first proposed in 2007 by Iskander1 et al., of the wavefront aberrations in the dioptric power domain and the usual geometrical representation of the power refractive maps. We present our preliminary results from such comparison.

Manufacture and analysis of a refractive surface with variable asphericity to model the human cornea

The cornea contributes substantially to the performance of the human eye and obtaining the shape of the anterior corneal surface is crucial for ophthalmic applications such as the manufacture of contact lenses and visual laser correction. In this direction, there exist a large amount of theoretical models which describe the shape of the anterior corneal surface. A model of the anterior corneal surface using high-order aspherics has been previously reported in the literature, and one of the main features of this model is that it has been shown to accurately reproduce the clinical data. In this work we have designed a refractive surface with variable asphericity adopting the model mentioned above by means of finite-element software, and once the design was obtained we proceeded to manufacture the optical surface made of a polymer known as PDMS. Also, an interferometric analysis with a Mach-Zehnder interferometer was performed in order to obtain its wavefront aberration function. The main application of this optical surface is to be used as a substitute of a corneal surface within an optomechanical system to mimic the performance of the human eye.

Emmetropic eyes: objective performance and clinical reference

The application of the wavefront sensors to measuring the monochromatic aberrations of the normal human eyes has given a new insight in the objective understanding of its performance. The resultant wavefront aberration function can be applied to evaluate the image quality on the retina, which includes the analysis of the higher-order aberrations. Among others, and due to their well-known mathematical properties for circular apertures, the wavefront aberration function is most commonly represented in terms of the Zernike polynomials. The main idea is to have a clinical reference of the objective performance of a set of normal human eyes. However, the high-order aberrations in normal human eyes are different for each person¸ that can be interpreted as that there are many possible solutions for the objective performance of emmetropic eyes. When dealing with the Zernike coefficients and excluding the spherical aberration, higher-order aberrations have a tendency to have a zero mean value. Different proposals have been suggested in the literature to deal with this feature. Moreover, it has been also shown that there is an ethnic dependency in the magnitude of the aberrations. We present in this work the objective performance of a set of uncorrected Mexican eyes, and compare them with other ethnic results published in the literature.

BICAD: Breast image computer aided diagnosis for standard BIRADS 1 and 2 in calcifications

The Breast Imaging Reporting and Data System (BIRADS) was developed by the American College of Radiologists as a standard of comparison for rating mammograms and breast ultrasound images. It sets up a classification for the Level of Suspicion (LOS) of the possibility of a breast cancer. In this paper we present an automated image analyzing system that finds calcifications based on the standard BIRADS 1 and 2. For our goal, we studied the digital mammography database in DICOM format provided by the Department of Radiology of the Hospital Universitario de Puebla. We used The Difference of Gaussian (DOG) filter to find edges of the forms of the different calcifications and a back-propagation Artificial Neural Network (ANN) for the pattern recognition of the BIRADS 1 and 2 cases. This method allowed us to automate the segmentation of the calcifications with a low computational cost. We achieved the pattern recognition with a high level of sensitivity of 0.9629 and specificity of 0.9920.

Reconstruction of tridimensional objects with two different textures using Gaussian model

We present in this work the use of two Gaussian models that describe reflectance of two different textures for the reconstruction of tridimensional objects made of these textures. The textures of the objects are segmented using a combination of image processing techniques. These segmentations are correlated with the Gaussian model for corresponding texture, for the reconstruction of the tridimensional object. We show our preliminary experimental result and we discuss the advantages and disadvantages of the Gaussian models, which are compared with the traditional representation of the inverse square law of light.

Wavefront aberration function from hard contact lenses obtained with two different techniques

The analysis and measurement of the wavefront aberration function are very important tools in the field of visual optics; they are used to understand the performance of the human eye in terms of its optical aberrations. In recent years, we have compared, through two different methods, the wavefront aberration function of a reference refractive surface of 5 mm in diameter and we have demonstrated its equivalence1. Now, we want to extend these results to a set of hard contact lenses. These hard contact lenses have been subjected to different laser ablation techniques which are typically used in refractive surgery. Our goal is to characterize the resultant ablation profile. We show our results obtained for both, a nonablated hard contact lens and the corresponding ablated samples.

Refractive power maps of the anterior surface of the cornea according to different models

In order to explore and analyze the effect of an ablation performed on the anterior corneal surface, it is useful to calculate the refractive power maps of the original and the treated corneas. The optical characteristics of the anterior corneal surfaces are typically simulated with different models, according to different degrees of simplification. To predict which ablation would improve the refractive power of such cornea, which is directly related with the spherical aberration associated with the shape of the anterior corneal surface, it is important to analyze those simplifications. Such information is displayed in a refractive power map, which yields the true refractive power of the corneal surface, point by point, expressing this power in diopters. The aim of the present work is twofold: different corneal models are simulated so as to compare the spherical aberration produced by each one. On the other hand, simulations are made in such a way that permits to foresee how the visual performance of an eye can be achieved by modifying the anterior surface of its cornea through the corresponding power maps.

Phase retrieval from a single interferometric pattern to determine the profile caused by laser ablation on spherical surfaces

We have been working in the interferometric analysis of the ablation profile obtained with different techniques of refractive surgery, applied directly on hard contact lenses. We have demonstrated qualitatively that different ablations produce different fringe patterns; implying different focal shifts1. These results were obtained by means of a Mach- Zehnder type interferometer, where we used a similar unablated contact lens as a reference. Due to the size of each sample, it is difficult to get different fringe patterns with different phase factors. Therefore, the typical phase shifting methods are not suitable in our case. To determine the corresponding profile caused by the different ablation techniques we applied in this work the interpolation method that provide an analysis of static fringe patterns. This method of phase retrieval allows us to obtain the PSF and MTF related to each profile. The advantage of this procedure is that we can obtain a time invariant performance of the resulting ablated surface.