Study of the optical effect of cataracts and its correction using wavefront shaping

Abstract

Cataract is one of the main causes of visual impairment in the world. It increases both intraocular scattering and aberrations because microscopic and macroscopic changes of the refractive index in the lens. Consequently, the effect of cataracts is the simultaneous blurring and reduction of the contrast in the retinal images. The intensity of the effect is associated to the amount of straylight, which is a quantification of the impinging light into the eye deviated to large retinal angles (i.e., larger than one degree). The current treatment of cataract is surgery, where the lens is extracted and replaced with an intraocular lens. However, the requirements for its safe practice have not allowed to cover its global demand, especially in developing countries. Therefore, a non-invasive optical technique is appealing, and its development is an open challenge. There is a unique precedent reported in 1973, where the phase of an extracted lens was retrieved using holography for its posterior phase conjugation. Moreover, the performance of current adaptive optics systems is limited by the large amount of scattering that is present in the cataractous eyes. On the other hand, wavefront-shaping (WS) technique was developed for imaging through opaque samples. In this work, the WS technique is proposed to partially correct the effects of cataracts non-invasively in-vivo, being the objective to evaluate its performance for this application. The technique was tested in an optical bench to avoid the influence of the dynamics of ocular phenomena which is inherent in an in-vivo implementation. As first step, a model was developed to reproduce the ocular point spread function (PSF) with different amounts of straylight through the weighted summation of both low and high spatial frequency phase perturbations at the entrance pupil of the eye. The model was initially tested by numerically calculating the following ocular phenomena and their comparison with previously reported experimental and clinical studies: aging effects on high order aberrations and Strehl ratio, the wavelength dependence of straylight and the ciliary corona. The calculated phase maps by the model were displayed on a liquid crystal on silicon (LCoS) device as spatial light modulator to experimentally generate the amounts of straylight, measured by the optical integration method. Moreover, the model was incorporated in a visual simulator to evaluate the effects of straylight on the visual acuity, the contrast sensitivity and the glare sensitivity, in fovea and two eccentricities of the visual nasal field. In this way, the relationships between the amount of straylight and these measurements of the visual function were established. Furthermore, the modest benefits of the cataract surgery with preexisting retinal diseases can be explained from these measurements. An instrument was developed for the experimental evaluation of the WS technique's performance in the correction of simulated cataracts. It simultaneously generates and corrects the effects of cataracts using a double-pass through a single LCoS device. The following process were done for the incorporation of the LCoS device: phase calibration, optimization of the phase modulation, suppression of the unmodulated light, and compensation of its inherent aberrations. Thus, the effects of three levels of cataract, with associated logarithmic amounts of straylight ranged from 1.75 to 2.25, were corrected using seven different spatial resolutions of the correcting phase maps. The quality of the correction was initially evaluated, considering one pass through the affected ocular media, by means of the following objective and subjective metrics: the enhancement of the PSF, the visual Strehl ratio, the correlation coefficients and the visual acuity. According to these measurements, the main effect of the WS technique is deblurring. Finally, the developed instrument was slightly modified to evaluate the performance of the WS technique considering symmetrical and asymmetrical double-passes through the cataractous ocular media. Although the WS technique optimized the double-pass PSFs, it did not mean a benefit in the quality of vision. Therefore, some suggestions are given to obtain a visual benefit from the implementation of WS in double-pass.