Quantitative measurements of pulsed infrared laser tissue ablation

Several new lasers that emit infrared laser radiation are being considered for surgical applications. In order to understand the irradiation and tissue parameters that control the infrared laser ablation process, three different, yet related, experiments were conducted. Ablation of guinea pig skin and bovine aorta, myocardium and liver using a TEA CO₂ laser with a pulse duration of 2 μs was quantified by measuring the mass of tissue removed as a function of incident fluence per pulse. For per pulse fluences greater than 5 J/cm² the ablation rate data are strongly dependent upon the mechanical strength of the tissue thus indicating a potential inadequacy of ablation models that do not consider tissue strength. The ablation of both soft and hard tissues using the normal-spiking-mode Er:YAG laser was quantified by measuring the number of pulses needed to perforate a measured thickness of tissue. The ablation of aorta and skin was more efficient than bone ablation. The ablation craters formed in skin and bone were the same shape as the incident laser beam, i.e. circular. In aorta, elliptical craters were formed at high fluence pulses; the long axis of the ellipse was always oriented perpendicular to the longitudinal axis of the aorta. Again, tissue mechanical properties were shown to be important. Er:YAG laser ablation dynamics were studied using flash photography and optical pump-probe techniques. The velocity of the plume front was found to be approximately Mach 4. It was also shown that each spike in the normal-spiking-mode pulse train was capable of ablating and rapidly ejecting tissue. The combined results of the three experiments indicate that removal of tissue by infrared laser radiation is an explosive process, the speed of which is determined in part by the mechanical strength of the tissue.