Background and objective: Thermal relaxation time (tau(r)) 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 degrees C, and DeltaT(base) = 5, 10, and 20 degrees C. 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 1-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 tau(r) of tissue thermal relaxation during multiple-pulse laser irradiation.
Copyright 2001 Wiley-Liss, Inc.