K 161Laser scanner ophthalmoscope for 2-D time resolved fluorescence measurement at the fundus
D. Schweitzer, A. Kolb, M. Hammer
Purpose: Changes in metabolism are assumed as a cause of age-related macular degeneration. Measurement and discrimination of different intrinsic fluorophores as well as the documentation of their 2-D distribution is a step towards the discovery of the pathomechanism in AMD.
Method: There are serious problems in measurement of autofluorescence at the fundus: The fluorescence signal is about three orders weaker than the reflected light. Specific excitation in the UV is impossible because of the absorption edge of the lens at 400 nm. The age-depended transmission of the ocular media influences the intensity of the fluorescence light. The 2-D detection of different fluorophores is very time-consuming. These problems can be solved by the detection of substance-specific decay times (ns time scale) after short-pulse excitation. The decay time is an intrinsic attribute of the fluorophore and does not depend on the applied intensity. In principle, the decay time can be determined in the frequency- or in the time-domain.
Results: The maximal permissible exposure is the distinctive limiting factor in the selection of the measuring principle. Calculating the number of measurable fluorescence photons after excitation it was found that the conditions for the in vivo measurement of the autofluorescence after pulse excitation correspond optimally to the requirements of the time-correlated single photon counting. For that reason, a conventional SLO was adapted by an active mode-locked Ar laser. The laser delivers pulses of 300 ps with a frequency of 77 MHz at selectable wavelengths between 458 nm and 514 nm. The single fluorescence photons are detected by a special unit for time correlated single photon counting. The synchronization between the scanning process and the photon counting is realized by a routing module. The system scans a field of 20 degree and has a resolution of about 50µm. As only about 300 photons are necessary for the calculation of mono-exponential decay at each investigated location, the measurement is performed within about 3 s. First in vivo images are shown.
Dep. of Ophthalmology, University of Jena, Germany