Quantification of forward scattering based on the analysis of double‐pass images in the frequency domain

Abstract

PurposeTo assess forward intraocular scattering by means of a new parameter (Frequency Scatter Index, FSI3) based on the analysing double‐pass retinal images in the frequency domain, which minimizes the impact of aberrations on the results.MethodsA prospective observational study was carried out in the Department of Ophthalmology, Hospital Universitari Mútua de Terrassa (Spain) on a group of 19 patients diagnosed with nuclear cataracts of various LOCSIII grades and a control group (CG) with nine healthy eyes. We recorded double‐pass retinal images with a custom set‐up based on a high‐sensitivity digital camera. The FSI3 was then computed using spatial frequencies below three cycles per degree. A preliminary validation of the FSI3 was performed on an artificial eye and two eyes of volunteers with and without commercial diffusers, and under defocus.ResultsThe FSI3 was hardly affected by defocus values up to 2.50 D. The mean (and standard deviation) FSI3 values were as follows: for the CG, 1.19 (0.21); and for LOCSIII grades nuclear opacity 1, 2 and 3, 1.30 (0.12), 1.62 (0.21) and 1.85 (0.21), respectively. Eyes with cataracts showed FSI3 values significantly different than healthy eyes (p = 0.001). A good correlation (ρ = 0.861, p < 0.001) was found between the FSI3 and objective scatter index provided by a commercial instrument.ConclusionSince aberrations have little impact on the FSI3, the light scatter assessment becomes less dependent on the patient's refractive error compensation and the presence of higher‐order aberrations. The FSI3 can further the clinical and scientific understanding of forward intraocular scattering.

Implementation of the Frequency Scatter Index in Clinical Commercially Available Double-pass Systems

“…During the calculation of the FSI, the area in which aberrations have a greater effect on the double-pass image is filtered and therefore such undesired effects are minimized. Previous works showed that an induced defocus on the double-pass image above 1 D results in a considerable overestimation of scattering as measured by OSI 16,21 . Our results show that even a certain increase in aberrations as a result of daily life situations, such as the change in pupil size or peripheral vision, can affect the doublepass image significantly and therefore the value of the OSI while having little impact on the FSI when measuring scattering (Fig.…”

“…To overcome the limitation of the OSI for the quantification of intraocular scattering under the presence of uncorrected refractive errors and higher-order aberrations, the FSI 3 was recently proposed 21 . In the present work, we have studied the suitability of the proposed method to quantify forward intraocular scattering using conventional doublepass instruments currently available in clinical environments, which are equipped with 8-bits cameras, instead of the scientific-grade camera with 14-bits used during the definition of the FSI 3 .…”

“…It was proposed 21 to analyze the effects of intraocular scattering in the frequency domain using the ratio between a pair of modulation transfer functions (MTF) obtained from the same image, but considering two concentric square regions of interest of radii r1 (MTF DP [r1] ) and r2 (MTF DP[r2] ), with r1 < r2:…”

“…However, the intraocular scattering quantification may be compromised under the presence of uncorrected refractive errors and higher-order aberrations 19,20 . To overcome such a limitation, the frequency scatter index3 (FSI 3 ) has been proposed 21 .…”

Visual simulations of presbyopic corrections through cataract opacification

“…The OSI is calculated from the aerial retinal image as the ratio of the intensity at an eccentric location (between 0.2 degrees and 0.33 degrees) and the central part. 26 The higher the OSI value, the greater the level of intraocular scattering.…”