Dynamics of tissue optics during laser heating

Propagation of high power laser radiation in tissue, which is guided by tissue optics, could initiate various reactions in tissue such as temperature rise, coagulation, and ablation. Because the properties of the tissue could vary during laser irradiation process, its optical properties may not remain constant. Hence the dynamics of tissue optics during laser irradiation needs to be investigated. Since optical properties are wavelength dependent, tissue can be highly absorbing at one wavelength and highly scattering at another. In this dissertation, dynamics of thermally induced changes in tissue optics were studied at both extremes as well as the middle range of tissue optics. Interactions between light distribution and thermal response were experimentally and theoretically examined. The absorbing media were selected as the starting point of this research.; A pump-probe system was used to study the dynamics of optical behavior of ocular tissue during laser heating. Nonlinear optical avior of ocular tissue during laser heating. Nonlinear optical behavior of cornea and vitreous humor tissue was characterized in vitro by measuring the spot area of the probe laser transmitted through the tissue. Temperature gradients created by absorption of pump laser radiation caused the probe beam to diverge. The spot area of the transmitted probe beam returned to its original size after the tissue was allowed to cool. This reversible change in optical behavior was attributed to the formation of a negative lens owing to thermally induced local gradients in the refractive index of tissue.; A thermal lens model based on the ray equation and the bioheat equation was formed. Simulated results suggested that thermal lensing can be prominent if the temperature gradient was steep. Comparing the model's predictions with experimental measurements, the thermo-optic coefficient (dn/dT) of cornea at 1064 nm was obtained. Both simulated and experimental results show that thermal lensing alters light distribution in the near as well as the far field.; The dynamics of tissue optics of highly scattering tissue samples (i.e., human skin and canine aorta) were studied. Thermally induced changes in the optical properties of tissue caused a decrease in the total transmittance and an increase in the diffuse reflectance of both fresh and coagulated human skin and canine aorta as a result of the pulsed Nd:YAG laser irradiation. For fresh tissue samples, these changes were primarily reversible until the thermally induced coagulation occurred, then both the reversible as well as irreversible components were evident in the measured optical behavior. However, for coagulated tissue the reversible changes in the optical properties were dominate. Results from this study indicated the existence of the nonlinear optics in turbid biological media during pulsed laser heating.; The influence of scatterers on the formation of the thermal lens was examined using the Z-scan technique. Study showed that thermal lensing could take place in a scattering medium but became weak as the scattering coefficient of the medium reached a certain level. This indicated the influence of thermal lensing on the light distribution needs to be considered particularly if the tissue has a medium or low scattering coefficient.