The physical mechanisms that achieve tissue removal through the delivery of short pulses of high-intensity infrared laser radiation, in a process known as laser ablation, remain obscure. The thermodynamic response of biological tissue to pulsed infrared laser irradiation was investigated by measuring and analyzing the stress transients generated by Q-sw Er:YSGG (lambda = 2.79 microns) and TEA CO2 (lambda = 10.6 microns) laser irradiation of porcine dermis using thin-film piezoelectric transducers. For radiant exposures that do not produce material removal, the stress transients are consistent with thermal expansion of the tissue samples. The temporal structure of the stress transients generated at the threshold radiant exposure for ablation indicates that the onset of material removal is delayed with respect to irradiation. Once material removal is achieved, the magnitude of the peak compressive stress and its variation with radiant exposure are consistent with a model that considers this process as an explosive event occurring after the laser pulse. This mechanism is different from ArF- and KrF-excimer laser ablation where absorption of ultraviolet radiation by the collagenous tissue matrix leads to tissue decomposition during irradiation and results in material removal via rapid surface vaporization. It appears that under the conditions examined in this study, explosive boiling of tissue water is the process that mediates the ablation event. This study provides evidence that the dynamics and mechanism of tissue ablation processes can be altered by targeting tissue water rather than the tissue structural matrix.

The ablation rate and thermal damage in skin produced by a superpulsed CO2 laser operating at pulse repetition rates between 1 and 900 Hz was measured. When delivering a fixed number of pulses (20 or 30) of equal energy, a 55-60% increase in the amount of tissue ablated was observed when the pulse repetition rate rose from 10 to 200 Hz. At pulse repetition rates greater than 200 Hz no further increase was seen. Under identical conditions, an 80% increase in the zone of thermal damage was observed when the pulse repetition rate was increased from 1 to 60 Hz. The large increases in tissue ablation and tissue damage may indicate the existence of a layer of mixed-phase (i.e., liquid and vapor) or metastable liquid which can store significant amounts of thermal energy between pulses. The data suggest that CO2 lasers should be operated at relatively low repetition rates for optimal performance.

The physical mechanisms that enable short pulses of high-intensity ultraviolet laser radiation to remove tissue, in a process known as laser ablation, remain obscure. The thermodynamic response of biological tissue to pulsed laser irradiation was investigated by measuring and subsequently analyzing the stress transients generated by pulsed argon fluorine (ArF, lambda = 193 nm) and krypton fluorine (KrF, lambda = 248 nm) excimer laser irradiation of porcine dermis using thin-film piezoelectric transducers. For radiant exposures that do not cause material removal, the stress transients are consistent with rapid thermal expansion of the tissue. At the threshold radiant exposure for ablation, the peak stress amplitude generated by 248 nm irradiation is more than an order of magnitude larger than that produced by 193 nm irradiation. For radiant exposures where material removal is achieved, the temporal structure of the stress transient indicates that the onset of material removal occurs during irradiation. In this regime, the variation of the peak compressive stress with radiant exposure is consistent with laser-induced rapid surface vaporization. For 193 nm irradiation, ionization of the ablated material occurs at even greater radiant exposures and is accompanied by a change in the variation of peak stress with radiant exposure consistent with a plasma-mediated ablation process. These results suggest that absorption of ultraviolet laser radiation by the extracellular matrix of tissue leads to decomposition of tissue on the time scale of the laser pulse. The difference in volumetric energy density at ablation threshold between the two wavelengths indicates that the larger stresses generated by 248 nm irradiation may facilitate the onset of material removal. However, once material removal is achieved, the stress measurements demonstrate that energy not directly responsible for target decomposition contributes to increasing the specific energy of the plume (and plasma, when present), which drives the gas dynamic expansion of ablated material. This provides direct evidence that ultraviolet laser ablation of soft biological tissues is a surface-mediated process and not explosive in nature.

We describe a spectroscopic technique called interferometric photothermal spectroscopy (IPTS) that can measure the absorption coefficient of pulsed laser radiation in nonscattering tissue samples. The technique is suitable for measuring effective absorption coefficients from 10(3) to 10(5) cm(-1). IPTS is particularly attractive because it requires minimal disturbance of the sample. These features indicate potential use for in vivo measurements of tissue absorption coefficients. To validate the technique, the absorption coefficient of pulsed Q-switched Er:YSGG (2.79-microm) radiation in pure water was measured to be 5200 (+/-500) cm(-1) when IPTS was used, in agreement with other published values. IPTS was also used to measure the absorption coefficient of pulsed ArF excimer laser radiation (193 nm) in bovine corneal stroma (in vitro), giving a value of 1.9 (+/-0.4) x 10(4) cm(-1).

A thermal model to predict the effects of laser parameters on the zone of thermal injury produced by laser ablation of biological tissue is presented. A dimensionless parameter based on the ablation velocity and the optical and thermal properties of the target is key in determining the resulting zone of thermal injury. The zone of thermal injury is minimized when this parameter, known as the Peclet number (Pe), is much larger than one. This occurs because the rapid movement of the ablation front prevents the diffusion of energy beyond the laser that absorbs the laser radiation. For Pe less than one, the slow movement of the ablation front allows for diffusion of energy away from the region of energy deposition and leads to larger zones of thermal injury. The model predictions are compared with data available in the literature. Deviations between the model predictions and published data are discussed and potential effects of pyrolysis, temporally varying pulse shapes and pulse repetition rates are explored.

Time-resolved measurement of the stress transients generated by pulsed infrared irradiation and ablation of tissue has demonstrated that these processes are governed primarily by photothermal processes. For ablation of porcine dermis at 2.79 micrometer (Q-sw Er:YSGG) and 10.6 micrometer (CO 2), the onset of material removal has been shown to be delayed with respect to irradiation and the magnitude of the generated stress transients are consistent with a model for explosive material removal. Upon consideration of the threshold radiant exposure for ablation, it appears that the mechanism and dynamics of these processes are controlled by explosive boiling as the tissue water is likely to be significantly superheated. To examine this issue further, we employed time- resolved optical interferometry to measure the surface displacement generated by Q-sw Er:YSGG laser irradiation of pure water for radiant exposures below the ablation threshold. This was done to directly measure the dynamic thermal expansion and interphase mass transfer generated by pulsed laser heating. These results are compared to a model which computes the dynamic thermal field within a semi-infinite pool of water undergoing pulsed irradiation while subject to a surface heat flux condition given by kinetic theory. We find that the measured mass fluxes exceed that predicted by simple kinetic theory arguments. The implications of the experimental and model results to pulsed laser ablation of tissue are discussed. ©2003 Copyright SPIE - The International Society for Optical Engineering.

The mechanisms responsible for the generation of stresses by pulsed-laser energy deposition in solids are elucidated with special attention given to laser-tissue interactions. These mechanisms include thermal expansion, subsurface cavity formation, ablative recoil and plasma formation and expansion. Scaling laws are presented for the magnitude of the stresses generated by each of these processes. The effect of laser parameters and material properties on the magnitude and temporal behavior of the stress transients is considered. The use of these scaling laws in conjunction with measurement of stress transients induced by pulsed laser sources may be a powerful tool in determining the physical processes which control the response of materials to pulsed energy deposition. In addition, the controlled generation and accurate measurement of acoustic transients may have important diagnostic and therapeutic applications.