We have formulated the theory of inertially confined mechanism for ablation of biological tissues. The motivation behind this theory is the observation that the energy density (E) absorbed in the tissue subsurface at ablation threshold (for short pulses) is typically ten times smaller than that required for vaporization, and of the order E ~ 200-300 J/cm3. Thus vaporization alone is inadequate to explain pulsed ablation, and other physical mechanisms must be responsible for tissue removal on this short time scale. By studying the steam tables (water is used as a thermodynamic model for soft tissue), one finds that an increase in energy density of 300 J/cm3, under conditions of constant volume, is sufficient to generate a pressure of 900 bars in the tissue. The condition of constant volume is assumed because, for short pulses (less than several hundred nanoseconds), the tissue is inertially confined (it lacks sufficient time to expand). Ablation occurs when the lase r-induced pressure exceeds a threshold value (which is dependent on the structural properties of the tissue) and proceeds through a disassembly of the tissue at the microcracks and subsequent acceleration away from the crater due to the pressure gradient.
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