Analysis of thermal relaxation during laser irradiation of tissue

Background and Objective: Thermal relaxation time (T,) is a commonly-used parameter for estimating the time required for heat to conduct away from a directly-heated tissue region. Previous studies have demonstrated that temperature superposition can occur during multiple-pulse irradiation, even if the interpulse time is considerably longer than tau (r). The objectives of this study were (1) to analyze tissue thermal relaxation following laser-induced heating, and (2) to calculate the time required for a laser-induced temperature rise to decrease to near-baseline values. Study Design/Materials and Methods: One-dimensional (1-D) analytical and numerical and 2-D numerical models were designed and used for calculations of the time tau (eff) required for the peak temperature (T-peak) to decrease to values slightly over baseline (DeltaT(base)). Temperature values included T-peak 65 and 100 degreesC, and DeltaT(base) = 5, 10, and 20 degreesC. To generalize the calculations, a wide range of optical and thermal properties was incorporated into the models. Flattop and gaussian spatial beam profiles were, also considered. Results: 2-D model calculations of tau (eff) demonstrated that tau (eff) (2-D) was as much as 40 times longer than tau (r). For a given combination of T-peak and DeltaT(base), a linear relationship was calculated between tau (eff) (1-D) and tau (r), and was independent of optical and thermal properties. A comparison of 1-D and 2-D models demonstrated that I-D models generally predicted longer values of tau (eff) than those predicted with a 2-D geometry when the laser spot diameter was equal to or less than the optical penetration depth. Conclusion: Relatively simple calculations can be performed to estimate tau (eff) for known values of tau (r), T-peak and DeltaT(base). The parameter tau (eff) may be a better estimate than Tr of tissue thermal relaxation during multiple-pulse laser irradiation. (C) 2001 Wiley-Liss, Inc.