Fluorescence spectroscopy of turbid media: Autofluorescence of the human aorta.

Fluorescence spectra of turbid media depend on the geometry of excitation and collection. The geometry dependence of 476-nm excited fluorescence of the human arterial wall was investigated both experimentally and with a Monte Carlo simulation. Optical properties and the fluorescence yield of each of the three arterial layers were determined. Attenuation of fluorescence by wavelength dependent scattering and reabsorption causes the fluorescence spectra observed at the tissue surface to change with distance from the excitation beam. The ratio of 600-nm fluorescence to 580-nm fluorescence increases significantly beyond the excitation beam. This ratio depends on the amount of oxyhemoglobin in the sample, illustrating how reabsorption can influence autofluorescence measurements. The effects of different excitation/collection geometries on fluorescence spectra are discussed in relation to the design of catheters to differentiate normal and pathologic tissues.