A Model of Tissue Charring During Interstitial Laser Photocoagulation: Estimation of the Char Temperature

Interstitial laser photocoagulation (ELP) was performed ex vivo in lean bovine muscle by delivering 1.5 W of continuous-wave 1064 nm Nd:YAG laser energy from a 400 μm core plane-cut optical fiber. The strategy for determining the char temperature involved measuring temperatures where thermal gradients were reduced, and extracting times at which temperature-time profiles displayed interesting nonlinear changes. These times were used to guide a finite difference thermal model, calculating transient temperatures based on two physical descriptions of tissue charring. Modifications in the optical and thermophysical properties due to tissue coagulation (T ≥ 60 °C) and vaporization of tissue water (T ≥ 100°C), respectively, were considered. By placing measured charring dimensions, 2.0 ± 0.3 mm, on calculated temperature-distance profiles, a tissue charring temperature of 414 ± 92°C was estimated.