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Analysis of thermal relaxation during laser irradiation of tissue
Author(s) -
Choi Bernard,
Welch Ashley J.
Publication year - 2001
Publication title -
lasers in surgery and medicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.888
H-Index - 112
eISSN - 1096-9101
pISSN - 0196-8092
DOI - 10.1002/lsm.1128
Subject(s) - laser , superposition principle , relaxation (psychology) , thermal , irradiation , base (topology) , atmospheric temperature range , range (aeronautics) , materials science , gaussian , optics , analytical chemistry (journal) , thermodynamics , chemistry , physics , mathematics , composite material , psychology , social psychology , computational chemistry , chromatography , quantum mechanics , nuclear physics , mathematical analysis
Background and Objective Thermal relaxation time (τ 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 τ 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 τ eff required for the peak temperature ( T peak ) to decrease to values slightly over baseline (Δ T base ). Temperature values included T peak =65 and 100°C, and Δ T base = 5, 10, and 20°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 τ eff demonstrated that τ eff (2‐D) was as much as 40 times longer than τ r . For a given combination of T peak and Δ T base , a linear relationship was calculated between τ eff (1‐D) and τ 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 τ 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 τ eff for known values of τ r , T peak and Δ T base . The parameter τ eff may be a better estimate than τ r of tissue thermal relaxation during multiple‐pulse laser irradiation. Lasers Surg. Med. 29:351–359, 2001. © 2001 Wiley‐Liss, Inc.