The thermoelastic basis of short pulsed laser ablation of biological tissue.

Abstract

Strong evidence that short-pulse laser ablation of biological tissues is a photomechanical process is presented. A full three-dimensional, time-dependent solution to the thermoelastic wave equation is compared to the results of experiments using an interferometric surface monitor to measure thermoelastic expansion. Agreement is excellent for calibrations performed on glass and on acrylic at low laser fluences. For cortical bone, the measurements agree well with the theoretical predictions once optical scattering is included. The theory predicts the presence of the tensile stresses necessary to rupture the tissue during photomechanical ablation. The technique is also used to monitor the ablation event both before and after material is ejected. TheoryExperimental results reported in the literature reveal that the energy density required to initiate ablation of biological tissue with nanosecond laser pulses is 10-fold less than that required for vaporization. This holds for a wide range of laser wavelengths (1, 2). When the laser pulse duration is shorter than a characteristic time, the material is "inertially confined"-i.e., it does not have time to expand, and heating takes place at constant volume.For ablation using short-pulsed lasers, there is evidence that photomechanical effects play the most significant role (1, 3). Here it is presumed that, since most materials are weaker in tension than in compression, the material will fail wherever the induced tensile stresses exceed the tensile strength. A onedimensional photomechanical model of laser-induced spallation correctly predicts the reduced energy density observed for nanosecond pulses. However, it also predicts that damage should first occur approximately one absorption depth beneath the surface (4, 5). In fact, ablation occurs at or near the surface. For the lasers and wavelengths used in ablating biological tissue, the optical absorption depth is usually comparable to the transverse laser dimension, and a onedimensional approximation is not appropriate. Onedimensional estimates made by our group did not correctly predict the observed surface movement, although they did account for the order of magnitude decrease in the energy required to reach threshold (1, 6). The various discrepancies can be reconciled by including three-dimensional effects. We have solved the full time-dependent three-dimensional equations, which predict that significant tensile stresses are created on the surface, precisely where ablation is observed to occur. All the time scales considered are much shorter than the time associated with thermal relaxation.When a material absorbs and is heated by laser energy, the resulting nonuniform temperature distribution causes internal forces, which lead to thermoelastic deformation. This deformation in a solid body is determined by the thermoelastic wave equation (7):3(l -2cr) [1] subject to the appropriate boundary and initial conditions, where u is the displacement vector, p is the density, E is Young's modulus, o is Poisson's ratio, X3...

Modeling of Skin Tissue Ablation by Nanosecond Pulses From Ultraviolet to Near-Infrared and Comparison With Experimental Results

“…With similar arguments, one can show that the laser-induced pressure within a chromophore has a similar dissipation time and thus the changes in the chromophore volume can be neglected during a nanosecond pulse. This has been discussed in terms of inertial confinement [3].…”

“…To describe the chromophore ionization in the optical breakdown of the skin epidermis, it is necessary to make several assumptions regarding the optical, electronic, and thermal properties of the tissue and chromophores. We start by assuming that various major chromophores in the skin epidermis can be represented by a mean absorption cross section which is a function of wavelength and relates to the bulk absorption coefficient through (3) where is the number density of the chromophores in the tissue. When a single chromophore is heated to a high temperature by the laser pulse, the electrons are freed from molecules due to thermal excitation.…”

“…Furthermore, the quantitative aspect of the selective photothermolysis model is not sufficient to calculate critical parameters such as the breakdown thresholds and cannot serve as a platform for quantitative comparison with the experimental results. The effect of adiabatic heating on the breakdown threshold in tissue has been quantitatively analyzed in terms of a photomechanical model based on the same photothermal mechanism [3]. In addition, a photochemical model was advanced to qualitatively interpret the corneal ablation in the deep UV region [4].…”

Elastic and Thermal Dynamics in Femtosecond Laser-Induced Structural Change Inside Glasses Studied by the Transient Lens Method

“…Therefore, we simulated the response of the glass (based on elastic dynamics) to sudden generation of a thermal stress. The equation used in the simulation [30,31] is the timedependent elastic equation for an isotropic material given by…”

Thermo-elastic optical coherence tomography

“…Previous studies have described the thermo-elastic deformation in tissue following short-pulse laser absorption [26][27][28][29][30]. The absorbed energy first leads to a nonuniform temperature rise in the sample, which causes a rapid (nanosecond to microsecond depending on the illuminated volume and speed of sound) thermo-elastic expansion that sends out an elastic wave propagating through the tissue.…”