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

Purpose To 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. Methods A prospective observational study was carried out in the Department of Ophthalmology, Hospital Universitari Mutua 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. Results The 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 (rho = 0.861, p < 0.001) was found between the FSI3 and objective scatter index provided by a commercial instrument. Conclusion Since 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.