Analysis of thermal relaxation during laser irradiation of tissue

✍ Scribed by Bernard Choi; Ashley J. Welch

• Publisher: John Wiley and Sons
• Year: 2001
• Tongue: English
• Weight: 221 KB
• Volume: 29
• Category: Article
• ISSN: 0196-8092
• DOI: 10.1002/lsm.1128

✦ Synopsis

Background and Objective: Thermal relaxation time (t 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 multiplepulse irradiation, even if the interpulse time is considerably longer than t 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 laserinduced 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 t eff required for the peak temperature (T peak ) to decrease to values slightly over baseline (ÁT base ). Temperature values included T peak 65 and 100 C, and ÁT 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 pro®les were also considered. Results: 2-D model calculations of t eff demonstrated that t eff (2-D) was as much as 40 times longer than t r . For a given combination of T peak and ÁT base , a linear relationship was calculated between t eff (1-D) and t 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 t 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 t eff for known values of t r , T peak and Á T base . The parameter t eff may be a better estimate than t r of tissue thermal relaxation during multiple-pulse laser irradiation.