title: Dynamics of pulsed holmium:YAG laser photocoagulation of albumen.
abstract: The pulsed holmium:YAG laser (lambda = 2.12 microm, tau(p) = 250 micros) has been investigated as a method for inducing localized coagulation for medical procedures, yet the dynamics of this process are not well understood. In this study, photocoagulation of albumen (egg white) was analysed experimentally and results compared with optical-thermal simulations to investigate a rate process approach to thermal damage and the role of heat conduction and dynamic changes in absorption. The coagulation threshold was determined using probit analysis, and coagulum dynamics were documented with fast flash photography. The nonlinear computational model, which included a Beer's law optical component, a finite difference heat transfer component and an Arrhenius equation-based damage calculation, was verified against data from the literature. Moderate discrepancies between simulation results and our experimental data probably resulted from the use of a laser beam with an irregular spatial profile. This profile produced a lower than expected coagulation threshold and an irregular damage distribution within a millisecond after laser onset. After 1 ms, heat conduction led to smoothing of the coagulum. Simulations indicated that dynamic changes in absorption led to a reduction in surface temperatures. The Arrhenius equation was shown to be effective for simulating transient albumen coagulation during pulsed holmium:YAG laser irradiation. Greater understanding of pulsed laser-tissue interactions may lead to improved treatment outcome and optimization of laser parameters for a variety of medical procedures.
authors.affiliation: Biomedical Engineering Program, The University of Texas at Austin, 78712, USA. jpfefer@heliix.mgh.harvard.edu
authors.forename: T J
authors.initials: TJ
authors.lastname: Pfefer
authors.forename: K F
authors.initials: KF
authors.lastname: Chan
authors.forename: D X
authors.initials: DX
authors.lastname: Hammer
authors.forename: A J
authors.initials: AJ
authors.lastname: Welch
mesh_terms: D000418:Albumins / Q000737:chemistry / Q000528:radiation effects*; D000818:Animals; D055592:Biophysical Phenomena; D001703:Biophysics; D002645:Chickens; D005260:Female; D006695:Holmium; D066298:In Vitro Techniques; D017075:Laser Coagulation / Q000379:methods*; D016014:Linear Models; D017711:Nonlinear Dynamics; D013816:Thermodynamics
chemical_list: D000418:Albumins; D006695:Holmium