<title>Principles of optical dosimetry: fluorescence diagnostics</title>

Abstract

To describe the fluorescent intensity behaviour at a certain wavelength or waveband that is emitted out of tissue and which is caused by excitation of a shorter wavelength or waveband, requires solving the transport equation of radiative transfer twice. The first time for finding the spatial distribution of the excitation light inside the tissue that is caused by incident irradiance, the second time for finding the spatial distribution of fluorescent light where, in this case, the excitation light energy fluence rate spatial distribution is the source of producing fluorescent photons. It is assumed that a fluorescent photon, when created, has an equal probability of propagating in any direction. Conse quently, when the incident irradiance consists of a waveband of light, the resulting spatial distribution of the fluence rate inside the tissue may not be identical for all wavelengths within the band. In addition to this, when fluorescence occurs at several wavelengths, there may be differences in propagation behaviour for the different wavelengths. Diagnostic methods that use tissue fluorescence should therefore be used with great care because the absorption and scattering behaviour of the tissue can substantially complicate the interpre tation of the fluorescence signal. Recently, this problem was solved with a Monte Carlo numerical method by Keijzer et al. [1] for a finite laser beam that was perpendicularly incident on the tissue. Such a problem cannot be solved analytically and, as a result, does not produce great insight into the influence of e. g. penetration behaviour of excitation light on the final fluorescence outcome. In this report a simpler approach is chosen in which the problem of fluorescence caused by laser excitation is fully solvable in analytical form. This produces insight into the influence of changes in optical parameters on the fluorescence behaviour albeit at the cost of a loss in reality. To solve the problem in an analytical form requires chosing a strictly one-dimensional tissue.