Hollow-waveguide-based nanosecond, near-infrared pulsed laser ablation of tissue

Background and Objective: Short-pulse solid-state lasers have recently received much attention as new coherent light sources for medical applications, but steady transmission of their high-energy output pulses through a solid quartz fiber is difficult because of the onset of laser-induced breakdown. We previously demonstrated that hollow waveguides could be used to deliver nanosecond laser pulses for tissue ablation. The aim of this study was to determine the optimum laser pulse energy and range of defocused distance for obtaining a deep and sharp ablation channel in myocardial tissue with laser pulses transmitted through a hollow waveguide. Study Design/Materials and Methods: Cyclic-olefin-polymer-coated silver hollow waveguides of 1 mm in inner diameter and 1 m in length were used. A vacuum-cored scheme was applied to the waveguides to suppress laser-induced air breakdown. Porcine myocardial tissue was irradiated with 300 laser pulses that were delivered through the waveguide in vitro at various laser energy levels and defocused distances, and depths and diameters of channels were measured. Histological analysis of the ablated tissues was also performed. Results: At an ablation energy of ∼60 mJ/pulse, deep (>4.5 mm) and sharp (depth-to-diameter ratio of >6) channels were created in tissue in the range of defocused distances of -4∼+0.5 mm. Under these conditions, wave-guide bending did not cause a remarkable change in ablation characteristics. Histological analysis of ablated tissue showed limited thermal damage but suggested a certain extent of mechanical effects in the tissue. Conclusion: With near-infrared, nanosecond laser pulses delivered through a cyclic-olefin-polymer-coated silver hollow waveguide, efficient and sharp ablation of myocardial tissue can be achieved, suggesting the usefulness of the hollow waveguide as a new flexible delivery system for high-intensity laser pulses.