Pressure generation during inertially confined laser ablation of biological tissue

Abstract

A Monte Carlo calculation of the laser energy density actually deposited in tissue at the onset of pulsed laser ablation revels that, over a wide range of wavelengths and tissue types, it is an order of magnitude lower than that needed for vaporization. An understanding of the thermodynamics of water reveals that under appropriate conditions of laser pulse duration and penetration depth, tremendous pressure can be generated in the tissue at energy densities well below the heat of vaporization and temperatures below 100 °C. The pressure generated in the tissue by the absorption of laser light then plays a significant role in the ablation process. For example, in ablation of aorta using a pulsed excimer laser (a pulse width of 30 nanoseconds at a wavelength of 308 nanometers), we calculate that the instantaneous pressure generated in the tissue will exceed 700 bars. Ablation occurs when these high pressures lead to stresses which exceed the structural properties of the tissue. A survey of results from the literature shows that the characteristic time for the relaxation of these high pressures in soft tissue is much longer than predicted by a simple speed of sound calculation and a more appropriate characteristic time is proposed. These concepts also help in understanding pulsed laser ablation in hard tissue. Some results of a study of hard tissue ablation are described, including some stroboscopic measurements. An experimental determination of the pressure generated by the absorption of short pulsed laser light, using an interferometric monitoring technique, agrees with theoretical predictions of the above theory. We include some relevant information on the properties of biological tissue and conclude with a cautionary note to the new practitioner.