Investigation of the stretched exponentials method for an application in fluorescence lifetime imaging ophthalmoscopy (FLIO)

Fluorescence lifetime imaging ophthalmoscopy (FLIO) is a new imaging technique for measuring the time-resolved in vivo autofluorescence generated by endogenous fluorophores in the ocular fundus. The aim is to assess the metabolism of the retina. Based on the fluorescence lifetime, different fluorescent compounds in the eye can be distinguished. There are a number of different approaches available to approximate the fluorescence lifetime in the time-resolved fluorescence data, such as the widely used multi-exponential approach. In this work, the stretched exponential approach is analyzed for applicability to FLIO data. Therefore, one, two and three fluorophores are simulated with different numbers of photons. Data sets with the following numbers of photons have been simulated: 5,000; 10,000; 25,000; 50,000 and 500,000. The software FLIMX (available at http://www.flimx.de) is utilized to determine the fluorescence lifetimes using the multi-exponential model and the stretched exponential model. The stretched exponential method is well suited to describe the fluorescence signals with one or two exponentials and less suited for three exponentials. Furthermore data from a healthy volunteer is analyzed with both approaches. Despite the simulation results, stretched exponential models seem to be applicable to time-resolved fluorescence data from the human fundus.