Measuring the Effects of Temperature on Optical Propagation in Heated Tissues Using Point Radiance Spectroscopy

Laser interstitial thermal therapy (LITT) is a minimally invasive therapy in which light is delivered using optical fibers inserted into tissue to treat malignant tumours. Heating tissue above 55°C causes tissue coagulation, creating non-viable tissue. Previous work has demonstrated that radiance measurements are sensitive to heat-induced changes in tissue optical properties. This study investigates the use of radiance measurements to differentiate permanent temperature-induced changes in optical propagation, which reflect thermal damage, from any transient changes in optical propagation. Experiments in water using our white-light point radiance spectroscopy (PRS) technique demonstrate that PRS is sensitive to detect optical absorption and temperature-dependence in the optical absorption of water, and a change in the acceptance cone with temperature. Experimental results using PRS in heated ex vivo porcine tissue show that the optical signal mainly represents permanent thermal damage and only a small part of the signal represent a temperature-dependent change due to water.