Modeling of thermal injury produced by laser ablation of biological tissue

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

Modeling of thermal injury produced by laser ablation of biological tissue

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

Comparison of pulsed CO2 laser ablation at 10.6 microm and 9.5 microm.

Background and objectiveThe pulsed CO2 laser has received attention because of its successful application to dermatologic surgery and burn debridement surgery. Despite impressive results, tissue removal using pulsed CO2 laser irradiation has not been optimized. We examined the ablation processes by performing mass removal and thermal injury experiments at wavelengths where tissue water is the primary absorber (10.6 microm), and where water and collagen have comparable absorption (9.5 microm).Study design/materials and methodsSamples of porcine reticular dermis were irradiated with 180-ns laser pulses at either wavelength. Tissue removal was measured using a digital balance. Thermal injury was assessed using a microscope with a calibrated reticle after hematoxylin and eosin staining.ResultsTissue removal using 10.6-microm radiation resulted in a heat of ablation of 3,740 J/g, an ablation threshold of 1.15 J/cm2, and a zone of thermal injury of 53 microm. By contrast, tissue removal using 9.5-microm radiation resulted in a heat of ablation of 3,330 J/g, an ablation threshold of 1.47 J/cm2, and a zone of thermal injury of 34 microm. The differences in ablation threshold and thermal injury were statistically significant.ConclusionPulsed CO2 laser irradiation at 9.5 microm removes tissue more efficiently and with a smaller zone of thermal injury than at 10.6 microm

Comparison of pulsed CO2 laser ablation at 10.6 microm and 9.5 microm.

Background and objectiveThe pulsed CO2 laser has received attention because of its successful application to dermatologic surgery and burn debridement surgery. Despite impressive results, tissue removal using pulsed CO2 laser irradiation has not been optimized. We examined the ablation processes by performing mass removal and thermal injury experiments at wavelengths where tissue water is the primary absorber (10.6 microm), and where water and collagen have comparable absorption (9.5 microm).Study design/materials and methodsSamples of porcine reticular dermis were irradiated with 180-ns laser pulses at either wavelength. Tissue removal was measured using a digital balance. Thermal injury was assessed using a microscope with a calibrated reticle after hematoxylin and eosin staining.ResultsTissue removal using 10.6-microm radiation resulted in a heat of ablation of 3,740 J/g, an ablation threshold of 1.15 J/cm2, and a zone of thermal injury of 53 microm. By contrast, tissue removal using 9.5-microm radiation resulted in a heat of ablation of 3,330 J/g, an ablation threshold of 1.47 J/cm2, and a zone of thermal injury of 34 microm. The differences in ablation threshold and thermal injury were statistically significant.ConclusionPulsed CO2 laser irradiation at 9.5 microm removes tissue more efficiently and with a smaller zone of thermal injury than at 10.6 microm

Physical mechanisms controlling the generation of laser-induced stresses

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