Free Electron Laser Ablation of Soft Tissue: The Effects of Chromophore and Pulse Characteristics on Ablation Mechanics

Abstract

The Vanderbilt University Free Electron Laser (FEL) is a tunable source of pulsed infrared radiation with pulse characteristics unlike those of most laser systems. A primary objective of the research presented in this dissertation is to investigate the effects of chromophore and pulse characteristics in the ablation of soft tissues with the (FEL). The working hypothesis of the research project is that results of ablation of soft tissues with the FEL cannot be solely explained by the selective absorption of protein components in the tissue, and that the pulse characteristics of the laser play an important role.

Three related studies are presented in this dissertation. First, the ablation depth and ablation threshold of rat dermis irradiated with the FEL at many wavelengths were measured and analyzed to reveal gross effects of the ablation process. Second, acoustic transients generated during the ablation of rat dermis and gelatin samples were measured and analyzed to reveal the effects of protein absorption and mechanical strength in the ablation process. Finally, numerical modeling of the ablation process was employed to investigate the effect of the temporal pulse structure of the laser and the effect of dynamic absorption of water on the ablation process.

The results of the studies presented here led to the following conclusions. First, the ablation of soft tissues irradiated with the FEL is largely described by a steady-state ablation model, indicating that the mechanism of ablation is predominantly photothermal in nature. Second, the ablation of soft tissues with infrared FEL radiation is a surface-mediated process, similar to that of traditional ultraviolet laser tissue ablation. Third, the dynamic absorption of water plays a significant role in the process. Finally, protein absorption of the incident radiation results in the targeted destruction of the tissue structural matrix at wavelengths where the absorption of protein represents a significant fraction of the overall absorption cross-section.