A New System to Estimate Modulation Transfer Function in a Human Eye with Aftercataract by Analyzing a Digital Transillumination Photograph

Abstract

Abstract: : Purpose: To develop a new system to estimate modulation transfer function (MTF) in a human eye with aftercataract by analyzing a digital transillumination photograph. Methods: Four pseudophakic eyes with aftercataract were included in this study. Each transillumination photo was analyzed for its variance in each 5 x 5 = 25 pixels in the diameter of 3 or 6 mm, and each part was divided into two groups according to the value of the variance: the transmission portion (low variance value) and the scattering portion (high variance value). Point spread function (PSF) estimated from a digital transillumination photo (PSFim) was defined as the sum of PSF1 and PSF2, with PSF1 defined as the PSF of the transmission portion calculated as the square of Fourier transform of the transmission portion, and PSF2 defined as the assumed distribution of the scattering light on the optical axis, which is calculated assuming that the intensity of the scattering increases in proportion to the area of the scattered parts and conversely that the intensity on the optical axis decreases according to the distance from the pupil center. MTF was calculated as Fourier transform of PSFimMTFim. MTFim was modified to MTFim+ considering the optical function changes resulting from aging in human cornea. In the same subject, PSF was measured by PSF sensor using the same size artificial pupil, and the MTF was calculated from the result of the measurement (MTFme). MTFim and MTFim were compared with MTFme. Results: Although there was a great discrepancy between MTFim and MTFme, MTFim+ was almost consistent with MTFme. Conclusions: We developed a new system to estimate MTF in a human eye with aftercataract by analyzing a digital transillumination photograph. Further investigations are necessary not only to determine the accuracy of the method but also to compare it with subjective visual function. This system may have some clinical applications, among others the objective evaluation of the grade of cataract and aftercataract and the indication for YAG laser posterior capsulotomy.

Keywords: imaging/image analysis: clinical • optical properties • cataract