A three-zone model of soft-tissue damage by a CO2 laser

CO2 laser damage to soft tissue may be divided into three zones--an outer layer of carbonised material, a zone of vacuolation where cavities have been formed by the explosive conversion of water to steam and a coagulated zone which has been formed at temperatures below 100 degrees C. A model is proposed which predicts the maximum (i.e. dynamic equilibrium) depths of each of these layers. The maximum effective thickness of the vacuolated zone is shown to depend upon the absorption coefficient of the tissue for CO2 laser radiation, but not on the incident irradiance. The thickness of the carbonised zone is shown to decrease with increasing irradiance, and the depth of the sub-boiling coagulated zone also decreases with increasing irradiance, but depends to some extent on the penetration of laser radiation through the soft tissue. Redistribution of heat within these zones after extinction of the laser radiation is considered, as is the effect of pulsed laser radiation. The predictions of the model could be tested by suitable experiments.