CW laser ablation velocities as a function of absorption in an experimental one-dimensional tissue model

One-dimensional continuous-wave laser tissue ablation models predict that the steady-state ablation velocity is independent of the optical absorption of the tissue and proportional to laser power density. This theoretical result was tested in an experimental model that approximates the one-dimensional situation. The model consists of 3 to 4 mm-diameter polyacrylamide rods irradiated at one end with an argon laser beam with uniform power density. It was found that ablation velocity is indeed independent of the light absorption coefficient in the material and proportional to the incident power density. Analysis of the relation between the experimental velocities and the power density showed that approximately 40% of the incident laser light is reflected or attenuated in the escaping vapors. Furthermore, compared to the actual absorbed laser power, heat loss caused by thermal radiation and convection from the side surface of the rod is considerable during ablation, especially at lower power densities. This heat loss is not present in a true one-dimensional geometry and creates therefore a marked difference between the rod and one-dimensional geometry. Computations show that in the pre-ablation stage the heat losses from the rod can be neglected, compared to the absorbed energy. In this case the rod is a very good approximation of the true one-dimensional situation.