Changes in birefringence as markers of thermal damage in tissues.

Light microscopy using polarized transmission illumination of routinely stained histologic sections shows changes of the native birefringence of certain tissue constituents when heated by laser irradiation or electrosurgical current. The naturally occurring birefringence of cardiac muscle disappears permanently when the muscle is frozen, thawed, and heated to temperatures in excess of 42 degrees C in vitro. This loss of birefringence is produced with temperatures at which other morphologic thermal changes are hard to detect; thus, it is a low-temperature tissue marker which can be used to observe the extent of thermal damage in tissues. Partial loss of the native birefringence of collagen occurs in canine urinary bladder coagulated by laser irradiation and pericardium heated with electrodes. In addition, thermally coagulated collagens have variable birefringence color shifts when compared to the adjacent unaffected collagens in stained histologic sections. The gradual birefringence color changes are seen at tissue temperatures higher than those at which the thermally induced hyalinization (coagulation) of collagen usually occurs (about 60-70 degrees C), but below those at which carbonization is seen (200+ degrees C). Birefringence changes can be measured to test mathematical models of thermal damage necessary for development of dosimetry models in medical applications of laser irradiation.