Comparison of single, dual, and staircase temporal pulse profiles for reducing stone retropulsion during thulium fiber laser lithotripsy in an in vitro stone phantom model

Abstract

Dual pulse mode has recently been integrated into Holmium:YAG laser systems to reduce stone retropulsion. This study explores similar pulse shaping approaches with Thulium fiber laser (TFL). A TFL at 1940 nm wavelength produced three temporal pulse profiles: (1) square pulse, (2) dual pulse, with low energy initial pulse followed by higher energy second pulse, and (3) ascending staircase pulse shape. Energies of 0.1 - 2.0 J, pulse rates of 5 - 200 Hz, average power of 10 and 20 W, and laser irradiation time of 5 s were used (n=5 per group). Stone phantoms (6-mmdiameter, 200-mg-mass) were placed on a horizontal, v-shaped trough, submerged in water, and then irradiated with TFL using a 200-μm-core optical fiber. Dual pulse stone displacements using pulse energies of 0.1, 0.2, 0.5, 1.0, and 2.0 J, measured 65%, 75%, 100%, 100%, and 110% of square pulse displacement at 10 W, and 65%, 60%, 60%, 90%, and 105% of square pulse displacement at 20 W. Staircase pulse stone displacements measured ~ 85% of square pulse stone displacement at 1.0 and 2.0 J for both 10 and 20 W. At lower energies (0.1 - 0.5 J), staircase profile produced a suction effect, resulting in the stone being pulled back to the fiber. Dual pulse mode only reduced stone retropulsion at lower energy settings, possibly due to excessive energy in initial pulse at higher settings. Low power (10 W) square, dual, and staircase pulse shapes ablated uric acid stones at rates of 1.7 ± 0.4, 1.9 ± 0.5, and 1.7 ± 0.5 mg/s, respectively. High power (20 W) square, dual, and staircase pulse shapes ablated stones at a rate of 2.6 ± 0.6, 3.0 ± 0.4, and 2.7 ± 0.7 mg/s, respectively. Future studies will utilize optical imaging of vapor bubble formation as a function of temporal pulse profile to optimize laser parameters for reducing stone retropulsion and enhancing TFL stone ablation rates.