Changes in optical properties of rat skin during thermal coagulation

Abstract

Thomsen, M. D., Helene Vijverberg, Ruiping Huang, Ph. D. andJon Schwartz, M.E.ELaser Biology Research LaboratoryUniversity ofTexas M. D. Anderson Cancer Center1515 HolcombeHouston, Texas 77030ABSTRACTThermal coagulation of albino rat skin heated in vitro results in prominent changes of lightscattering but relatively little in light absorption based on measurements using an integrating spherespectrometer. The reduced scattering coefficients, p(1-g), gradually increase as temperaturesincrease from room temperature to 55° C then rapidly decrease to plateau after 70°C is reached.The differences among the .t5(1-g) values for the different wavelengths were greater at the lowertemperatures than at higher temperatures. The absorption coefficient, changed very little overthe test temperature range (room temperature to 90°C) and then only at higher temperatures and forlonger wavelengths. The optical property changes were associated with thermally induced lightmicroscopic and ultrastructural changes in the dermal collagen, a major tissue component of skin.INTRODUCTION AND BACKGROUNDMany types of tissues and biological materials are visibly altered when heated tocoagulation temperatures. The opacification and whitening of cooked egg white and thetransformation of raw red beef steak to brown meat when grilled are familiar examples toeveryone. Thermal coagulation of tissues during laser irradiation results in many morphologicalchanges that could relate to changes in optical properties of those tissues both during and afterirradiation. 1,2We tested the hypotheses that the optical properties of albino rat skin will be altered as aresult of heating and that the optical property changes could be related to morphological changes ofthermally coagulated dermal collagen. 3MATERIALS AND METHODSOur experimental rationale included the following considerations: 1) the optical propertychanges would be compared to temperature with the heating time held constant 2) the opticalproperty changes would be reflected by measuring p.5(1-g) and p.: 3) albino rat skin wouldeliminate the absorption problems introduced with pigmented skin; 4) the dermal skeletal musclelayer would be dissected away to eliminate the influence of myoglobin and 5) the epidermis anddermis of rat skin would be measured with the light directed to the epitheial surface. 4Fresh pieces of albino rat skin were removed from male Sprague-Dawley rats (285-350gms) and cut into 3 x 3 cm squares. These were wrapped in water-tight aluminum foil packets andsubmerged in an equilibrated water bath: the packets were weighted to prevent their floating on thesurface. The packets were heated for 20 minutes at the following temperatures, 37, 50, 55, 60,65,70 and 90° C. The packets were placed on wet ice immediately after heating then allowed to