Relationship between damaged fraction and reflected spectra of denaturing tissues

Background and Objectives: During thermal therapy of tissue, such as induced by microwave heating, the initiation of denaturation should be monitored for proper thermal dosage. Additionally, denaturation should be confined to the pathologic volume, while preserving surrounding healthy tissue. The relationship between the damaged fraction and reflected spectra of denaturing tissues was investigated for a variation of the temperature of the tissues. Study Design/Materials and Methods: Denaturation of muscle, liver, and milk was studied in vitro by measuring the temperature-varying reflectance spectrum as heating occurs. A high-resolution fiber optic spectrometer was used to measure the reflectance changes. Temperature was monitored using a thermocouple embedded within the tissue along the side of the optical fiber probe. Results: The values of average free energy to initiate denaturation in muscle and liver at about 60 degrees C were 94.8 and 96.3 kJ/mole, respectively. The reflectance spectra increased in amplitude for muscle and liver, and the peak shifted from approximately 700 to 720 nm in accordance with the damage fraction of tissue. The reflectance spectrum for milk was essentially unchanged. Conclusions: Spectral changes from heated muscle and liver reflect denaturation of proteins contained therein. The spectral information at 800 nm can be used to determine the average free energy for the initiation of denaturation.