A novel strategy for monitoring laser thermal therapy based on changes in optothermal properties of heated tissues

Laser thermal therapy uses near-infrared optical energy to heat and thereby treat diseased tissues such as solid tumors. A method to monitor the progress of laser thermal therapy by detecting temperature-induced changes in optical propagation has been developed. The advantage of a point optical monitoring strategy over a conventional point temperature monitoring approach is that optical intensity measurements are indicative of a larger 'sampling volume' of optothernial events. In porcine kidney ill vivo, the optical intensity at 5 mm from a laser-coupled heating fiber decreased by 49% after 2.5 min of heating at 3 W In bovine tissue ex vivo, the optical intensity at 8 mm from the heating fiber decreased by 62-83% during laser irradiation at 5 W. This substantial decrease in optical penetration is consistent with increased optical scattering by thermally damaged tissue (i.e., kidney and liver) around the heating fiber. This damage was not detected by a temperature sensor placed at the same distance from the heating fiber in kidney or liver. Furthermore, in the porcine kidney experiment, smoke production occurred, which is normally concomitant with high-temperature tissue charring around the heating fiber. This was observed as a complete loss in optical intensity but was not detectable in the temperature data. The measurements in this work indicate that point optical intensity may have a greater sensitivity to important optothermal events than do point temperature measurements for monitoring laser heating in tissues.