Applications of the Change in Tissue Optical Properties Caused by the Presence of Photosensitizers

The presence in tissue of small amounts of light absorbing dyes, such as photosensitizers, can significantly change the spatial distribution of light fluence. The main effects are a reduction in penetration of light into the tissue and a decrease in the diffuse reflectance from the surface. This paper describes the application of these perturbations to three problems in the dosimetry of photodynamic therapy (PDT) - the quantitation of photosensitizer concentration in vivo, the determination of tissue optical properties, and the concept of threshold dose. We shall briefly discuss each of these.