Measurement of thermal effects on the optical properties of prostate tissue at wavelengths of 1,064 and 633 nm

Abstract

Background and Objective The extent of thermal injury during laser prostatectomy is dependent on the light distribution in laser-irradiated tissue. As tissue is irradiated, the optical properties change as a function of temperature due to an alteration of molecular and cellular structure. The purpose of the present study was to determine how the exposure of both fresh and previously frozen canine prostate tissue to elevated temperatures affects the optical properties. Study Design/Materials and Methods Optical properties were measured by using a double integrating sphere spectrophotometer with an inverse adding-doubling algorithm. Measurements were made at two wavelengths (1,064 nm and 633 nm) on samples heated in a waterbath in 5°–10° increments for 10 min through a 50°C temperature range. Results Upon coagulation, the absorption coefficient of fresh tissue decreased from the baseline measurement for both wavelengths (0.027 ± 0.003 to 0.019 ± 0.002 for λ = 1,064 nm; 0.073 ± 0.007 to 0.061 ± 0.006 for λ = 633 nm). However, the scattering coefficient increased sharply from the baseline measurement following coagulation (3.06 ± 0.26 to 6.05 ± 0.29 for λ = 1,064 nm; 4.89 ± 0.23 to 7.22 ± 0.30 for λ = 633 nm). Thermal coagulation occurred during exposure to temperatures between 60°C and 70°C. Conclusion Data obtained in this study indicate that thermal coagulation of tissue alters the optical properties. The extent to which these changes occur was found to be dependent on wavelength and freshness of tissue. These results are significant because they suggest how thermally induced changes in the optical properties may limit the depth of light penetration in tissue thus compromising treatment. Lasers Surg. Med. 24:38–47, 1999. © 1999 Wiley-Liss, Inc.