Increased ablation efficiency in hard and soft tissues using an annular beam

Lasers are commonly employed in surgery for hard and soft tissues due to their precise space-time energy delivery and compatibility with optical fibers for delivery into body cavities, including for treatment of urological diseases. Infrared laser ablation in tissues can result in non-specific heating and thermal injury. Methods that maximize ablation efficiency, or tissue volume removed per unit energy, while minimizing non-specific thermal injury can improve surgerical workflows and outcomes. We report a novel approach for increased ablation efficiency by modifying the beam shape. Specifically, a Ho:YAG laser is shaped into a converging annular beam. Ablation efficiency was measured on a hard tissue phantom (BegoStone) and soft tissue (porcine kidney). An annular beam similar to 800 mu m in diameter was used to ablate each sample at 10 different locations using a single 1 J pulse per location. The procedure was repeated using a circular beam with similar diameter by placing a 200 mu m fiber 1 mm from the tissue surface. Each ablation crater was imaged with optical coherence tomography and the crater volumes calculated from recorded images. For hard tissue phantoms, ablation efficiency increased 183% for annular vs. circular beams (0.065 +/- 0.013 vs. 0.023 +/- 0.003 mm(3)/J). For soft tissue, ablation efficiency increased 69% for annular vs. circular beams (0.098 +/- 0.021 vs 0.058 +/- 0.018 mm(3)/J). Hard and soft tissue ablation with an annular beam is a promising technique for increasing the speed and safety of laser surgery.