Essenpreis, Matthias; (1993) Thermally induced changes in optical properties of biological tissues. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis describes an optical phenomenon which may occur when biological tissues are irradiated with therapeutic laser light at powers above the thermal denaturation threshold. The scattering coefficient of most tissues changes with thermal denaturation. This may alter both the light distributions within and the light distributions emerging from the tissue during the therapy. The former effect may require a non-linear description of the thermal interaction whereas the latter may offer the opportunity to observe the progress of the therapy. The clinical uses of lasers in medical therapy and diagnosis are reviewed in the first chapter and physical background information on light and heat transport in tissues are presented. Existing reports on changes in optical coefficients with temperature are collected and discussed in the second chapter. In the third chapter the mechanisms involved in the thermal damage to tissues are examined by means of transmission electron microscopy, experimental measurements of the denaturation kinetics of tissue and a theoretical modelling of cell survival. In the fourth Chapter, a stochastic model for the calculation of light distributions in tissues is described and validated against analytical solutions. Subsequently, it is used to study the effect of different combinations of absorption and scattering coefficient on the light distributions within and emerging from the tissue. Experimentally determined values of absorption coefficient, scattering coefficient and single scattering phase function of rat liver, human aorta and human prostate are presented in the fifth chapter. The optical coefficients were measured both on native and thermally damaged tissues. The final chapter suggests a model for the calculation of temperature distributions in tissues during pulsed laser irradiation. This model relies on the light distributions given by the stochastic model and uses a finite element approximation to the heat diffusion equation. Examples of changes in temperature distributions with changes in optical coefficients are presented.

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