Laser-based endoscopic procedures present special challenges to deliver energy for ablation or coagulation of target tissues. When optical fiber–target quasi-contact (< 0.5 mm distance) cannot be maintained or is undesirable, the creation of intervening vapor bubbles and channels provide for the necessary transmission of laser energy to the target. This work investigates the characteristics and the dynamics of vapor channels that directly affect ablation efficiency and ablation rate and are known to effect stone movement, all of which impact procedure efficiency and safety.
Methods
A simplified, experimental model for thulium fiber laser (1940 nm) lithotripsy consists of a water-filled cuvette and a vertically oriented laser fiber (200 μm core diameter) with its tip at 9 mm for “quasi-free” bubble generation and at vapor channel working distances 1–5 mm from and centered on the transparent cuvette bottom simulating a target's surface. Laser power transmission is recorded and synchronized with video frames from a high-speed camera (24,260 frames per second) to capture the induced vapor channels' and bubbles' development.
Results
Laser-induced channel transmission from 0% to 100% for 1, 2, and 3 mm fiber–target distances undergoes oscillations with average periods of 0.32, 0.64, and 1.0 ms, respectively, for 500 W laser output power. For fixed fiber–target distances of 0.5, 1, and 2 mm, the variation of these average oscillation frequencies across laser powers from 500 to 1000 W is much smaller, not exceeding 14%. For fiber–target distances in the range of 1–5 mm, the fraction of the 500 W laser's total pulse energy delivered to the target for 1, 2, and 3 ms pulses linearly decreases from 0.78 to less than 0.2. The channel and bubble dynamics begin with a spherical seed bubble expansion centered on the distal fiber tip that evolves into a pear shape whose surface exhibits periodic irregularities attributable to laser beam interruption by water droplets within the developing bubble.
期刊介绍:
Lasers in Surgery and Medicine publishes the highest quality research and clinical manuscripts in areas relating to the use of lasers in medicine and biology. The journal publishes basic and clinical studies on the therapeutic and diagnostic use of lasers in all the surgical and medical specialties. Contributions regarding clinical trials, new therapeutic techniques or instrumentation, laser biophysics and bioengineering, photobiology and photochemistry, outcomes research, cost-effectiveness, and other aspects of biomedicine are welcome. Using a process of rigorous yet rapid review of submitted manuscripts, findings of high scientific and medical interest are published with a minimum delay.