Intraocular Scattering Gradation from Double-Pass MTF Analysis

Abstract

Purpose: : To objectively estimate the intraocular scattering by means of a new index (Frecuency Scattering Index, FSI), which is computed by using the whole double-pass (DP) image. The index proposed takes into account the maximum variation of the slope of the modulation transfer function (MTF) obtained from the DP image recorded.

Methods: : We selected a group of 50 patients with different grades of nuclear cataracts, including from early to moderate stages according to the LOCS III classification system (from NO1 to NO3). A control group of 10 young normal eyes was also evaluated with the same procedure. DP images (size: 72 arc min) were obtained using a clinical instrument (OQAS, Visiometrics SL, Spain). From those images the FSI was computed by means of the corresponding MTF images and the analysis of their maximum slope. The FSI index was then compared with the OSI values (Objective Scatter Index) in terms of correlation. The OSI was provided by the OQAS instrument (Artal et al Plos 2011). The influence of low-order aberrations on the FSI was also analyzed by performing measurements on 8 healthy young subjects with induced defocus from -1.50 to +1.50 D in 0.25 D steps. These results were compared with those obtained by using the OSI parameter.

Results: : For the control group and patients with different cataract grades, a statistically significant correlation (r=0.726, p<0.001) was found between the FSI and LOCS III classification. Mean FSI (±SD) was of 0.49±0.12 for the control group; 1.86±1.02 for the NO1 group; 2.47±1.12 for the NO2 group; and 3.35±1.67 for the NO3 group. FSI and OSI values also showed a good and significant correlation (r =0.783, p<0.001). Furthermore, none of the 8 subjects with induced defocus showed a variation in the FSI index higher than 0.7 within the tested range. In the case of the OSI, the range within which that was accomplished was between -1.50 and +1.00 D (Martinez-Roda et al. Clin Exp Optom 2011).

Conclusions: : In this study a new index (FSI) to objectively estimate intraocular scattering by analyzing the slope variation in the MTF computed from the DP images is proposed. This index is computed using the complete DP image recorded and performs very well in discriminating different grades of nuclear cataracts. It has good correlations with the LOCS III clinical classification system and with the OSI parameter. FSI shows a high robustness regarding the induced defocus. Therefore, the FSI index could be a complementary and powerful tool in the objective assessment of intraocular scattering and thus for the improvement of cataract detection and surgery scheduling.

Keywords: cataract • intraocular lens • optical properties