Continuous wave laser ablation of tissue: analysis of thermal and mechanical events

Thermal and mechanical events during continuous wave (CW) laser ablation of biological and phantom media were investigated. Porcine aortae, collagen fibers, and polyacrylamide control samples were subjected to argon laser irradiation while infrared and high-speed (240 images/s) video cameras were used to monitor their surfaces. Subsequent analysis of simultaneous changes in surface temperature and physical features correlated thermal and mechanical events. Video images recorded prior to ablation onset of tissue slabs clearly revealed two distinct phases: 1) progressive growth of a surface dehydration zone, and 2) surface deformation, implying subsurface bubble formation. Surface temperature recordings and video imaging revealed that the onset of CW ablation of soft biological media often initiated with a violent explosion, surface tearing, and tissue ejection. Histological inspection revealed intense coagulation in superficial layers near the irradiation site, whereas chiefly mechanical disruption was noted at the base of the crater. Ablation characteristics were consistent with theoretical calculations which indicate subsurface temperature peaks that increase in magnitude and surface proximity as energy deposition rates are increased. Results also suggested that mechanical properties of target media strongly influenced the extent of pressure built up, the nature of ablation onset, and the characteristics of the overall ablation pathway.