Dynamic optical-thermal modeling of laser tissue soldering with a scanning source

A transient two-dimensional optical-thermal model that accounts for dynamic changes in optical and thermal properties with temperature was developed to investigate the mechanisms leading to thermal damage during laser tissue soldering. The model was implemented using the electrical circuit simulator simulation program with integrated circuit emphasis (SPICE). Electrical analogies for the optical and thermal behavior of the solder and tissue mere established. With these analogies, light was propagated using a flux representation of the light and the electrical simulator mas used to calculate heat transfer with an algorithm based on the finite difference technique. Thermal damage was calculated using the Arrhenius rate process relation. Temperature-dependent absorption and scattering coefficients, thermal conductivity and thermal diffusivity, mere incorporated in the SPice optical-thermal simulation (SPOTS), as well as the time domain behavior of a scanning laser source. Experimental results from an in vitro study performed using an 808-nm diode laser in conjunction with indocyanine green-doped albumin protein solders to repair bovine aorta specimens compared favorably with numerical results obtained from SPOTS using dynamic optical and thermal properties. The maximum surface temperature was over-estimated by almost 10% when dynamic properties were not taken into account. This difference corresponds to over two orders of magnitude difference in terms of the Arrhenius tissue damage integral. The incorporation of dynamic changes in optical and thermal properties of tissue during laser-induced heating represents a significant advance in computer modeling of laser tissue interactions.