Spectroscopy Laboratory Massachusetts Institute of Technology Cambridge, Massachusetts

ABSTRACT Assuming that the irradiance of the laser beam is much greater than the threshold for ablation, a simple expression is obtained for the depth of ablation crater as a function of incident fluence. It is expected to be valid even where mechanisms other than thermal dominate the ablation process. A high level of irradiance is necessary to avoid excessive heating of adjacent tissue by heat diffusion. Contrary to expectations based on non-abla­ tive heating, short pulses are not necessari­ ly more effective than longer pulses in minimizing heat diffusion to adjacent tissue. Peripheral heating can further be reduced by using a wavelength with high absorption (short penetration depth) in tissue. Under appropriate conditions of wavelength and irradiance, clean, "cold" cuts are possible with the thermal mechanism of ablation. 1. INTRODUCTION It is essential in applications of lasers involving tissue removal, particularly in percutaneous procedures, to have a way of determining the optimum wavelength and dosage parameters (power, spot diameter, duration of exposure) for removing a desired volume of tissue. A major factor affecting this deci­ sion is the extent of heating of the adjacent tissue. A theoretical description of the process is useful not only as a framework for determining the optimal dosage and wave­ length, but also as a guide for better under­ standing the process.The model to be described is designed to give ablation depth L in terms of other relevant parameters when a collimated laser beam is incident on tissue. The geometry is depicted in Fig. 1.