Marta Dark

Laser-induced thermoelastic deformation: a three-dimensional solution and its application to the ablation of biological tissue.

Under certain conditions, laser light incident on a target material can induce an explosive removal of some material, a process called laser ablation. The photomechanical model of laser ablation asserts that this process is initiated when the laser-induced stresses exceed the strength of the material in question. Although one-dimensional calculations have shown that short pulsed lasers can create significant transient tensile stresses in target materials, the stresses last for only a few nanoseconds and the spatial location of the peak stresses is not consistent with experimental observations of material failure in biological tissues. Using the theory of elasticity, analytical expressions have been derived for the thermoelastic stresses and deformations in an axially symmetric three-dimensional solid body caused by the absorption of laser light. The full three-dimensional solution includes three stresses, radial, circumferential and shear, which are necessarily absent in the simple one-dimensional solution. These stresses have long-lived components that exist for eight orders of magnitude longer in time than the acoustic transients, an important point when the details of dynamic fracture are considered. Many important qualitative features are revealed including the spatial location of the peak stresses, which is more consistent with experimental observations of failure.

Photomechanical basis of laser ablation of biological tissue

The photomechanical model of laser ablation of biological tissue asserts that ablation is initiated when the laser-induced tensile stress exceeds the ultimate tensile strength of the target. We show that, unlike the one-dimensional thermoelastic model of laser-induced stress generation that has appeared in the literature, the full three-dimensional solution predicts the development of significant tensile stresses on the surface of the target, precisely where ablation is observed to occur. An interferometric technique has been developed to measure the time-dependent thermoelastic expansion, and the results for subthreshold laser fluences are in precise agreement with the predictions of the three-dimensional model.

Physical properties of hydrated tissue determined by surface interferometry of laser-induced thermoelastic deformation

Knee meniscus is a hydrated tissue; it is a fibrocartilage of the knee joint composed primarily of water. We present results of interferometric surface monitoring by which we measure physical properties of human knee meniscal cartilage. The physical response of biological tissue to a short laser pulse is primarily thermomechanical. When the pulse is shorter than characteristic times (thermal diffusion time and acoustic relaxation time) stresses build and propagate as acoustic waves in the tissue. The tissue responds to the laser-induced stress by thermoelastic expansion. Solving the thermoelastic wave equation numerically predicts the correct laser-induced expansion. By comparing theory with experimental data, we can obtain the longitudinal speed of sound, the effective optical penetration depth and the Grüneisen coefficient. This study yields information about the laser tissue interaction and determines properties of the meniscus samples that could be used as diagnostic parameters.

Mechanisms of meniscal tissue ablation by short pulse laser irradiation.

A new experimental technique was developed to study short-pulsed laser ablation of biologic tissues (human meniscus and bovine tibial bone), water, and acrylic. The experimental technique was based on interferometric monitoring of the motion of the tissue surface to measure its laser-induced expansion after irradiation. The thermoelastic expansion of these materials after laser irradiation under subablation threshold was examined to determine its role in the initiation of ablation. The experimentally observed surface expansion of cortical bone and acrylic was in agreement with theoretical predictions. The movement of meniscal tissue was similar to that shown by water. The latter 2 materials showed additional features consistent with the growth and collapse of cavitation bubbles. The exact role of cavitation in the irradiation of meniscal tissue by laser light remains unknown, but may represent a clinically important mode of tissue ablation and postirradiation trauma.

Pressure generation during inertially confined laser ablation of biological tissue

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 l...

Short pulse laser ablation is photomechanical, not thermal or chemical

Proposed mechanisms for pulsed laser ablation of biological tissue include photochemical, photothermal and photomechanical models. The principal observed effects which the correct model must explain include the high efficiency of the process, typically an order of magnitude less energy is required than for long pulse or cw ablation, and the minimal thermal damage to surrounding tissue. The photomechanical model postulates that ablation is initiated when the laser-induced stress exceeds the tensile strength of the material. In a version of the photomechanical model, called photospall, one assumes the target occupies an infinite half-space and performs a one dimensional analysis. The required tensile stress is then created by the reflection of the initial compressive stress from the free surface. The predictions of this model are in good agreement with observed spallation in materials like metals where the absorption depth is so small that the one dimensional approximation holds. This model was also suggested as a mechanism for the ablation of biological tissue, however there are serious discrepancies between predictions and experiments.

Physics of laser-induced stress wave propagation, cracking, and cavitation in biological tissue

In the regime where the specific time for propagation of stress waves is longer than the laser pulse duration, but shorter than the heat dissipation time, stress can be one of the governing mechanisms of laser-induced ablation of biological tissue. In such inertially confined regimes, knowing the mechanical properties of biological tissue an the kinetics of cracking (in hard tissue represented by bone) and cavitation (in soft tissue represented by meniscus) are important to understand the ablation process. An experimental technique has been developed to study laser-induced stress generation and mechanical properties of tissue in such regimes. This technique is based on monitoring the tissue surface after laser irradiation, using an interferometer that can measure submicron surface displacements on a nanosecond time scale. The subablation threshold laser-induced surface displacements can be related to the stress within the tissue and mechanical properties of the tissue. The surface movement of aqueous solution and meniscus tissue irradiated by 7.5-ns pulses of 355 nm light was consistent with growth and collapse of cavitation bubble. Bone movement was qualitatively consistent with theoretical predictions obtained by solving the equation of motion both analytically and numerically. In the regime where laser beam radius and optical absorption depth are comparable, it is shown that a full 3D analysis is necessary to understand the observed results.