τ mapping of the autofluorescence of the human ocular fundus

Abstract

Changes in the autofluorescence at the living eye-ground are assumed as important mark in discovering of the pathomechanism in age-related macular degeneration. The discrimination of fluorophores is required and also the presentation of their 2D distribution. Caused by transmission of ocular media, a differentiation between fluorophores by the spectral excitation and emission range is limited. Using the laser scanner principle, the fluorescence lifetime can be measured in 2D. Keeping the maximal permissible exposure, only a very weak signal is detectable, which is optimal for application of the time- correlated single photon counting (TCSPC). In an experimental set-up, pulses of an active model locked Ar<SUP>+</SUP> laser (FWHM = 300 ps, reptition rate = 77.3 MHz, selectable wavelengths: 457.9, 465.8, 472.7, 496.5, 501.7, 514.5 nm)excite the eye-ground during the scanning process. A routing module realizes the synchronization between scanning and TCSPC. Investigation of structured samples of Rhodamin 6G and of Coumarin 522 showed that a mono-exponential decay can be calculated with an error of less than 10 percent using only a few hundred photons. The maximum likelihood algorithm delivers the most correct results. A first in vivo tau-image, exhibit a lifetime of 1.5 ns in the nasal part and 5 ns at large retinal vessels.