The Optimal Combination of Scanning Speed and Fiber-to-Stone Distance for Effective and Efficient Dusting during Holmium: YAG Laser Lithotripsy

ObjectivesTo investigate mechanistically the effects of fiber scanning speed (vfiber) and fiber tip-to-stone standoff distance (SD) on dusting efficiency during Holmium (Ho): YAG laser lithotripsy (LL) Materials and MethodsPre-soaked BegoStone samples (23 x 23 x 4 mm, W x L x H) were treated in water using a clinical Ho:YAG laser in dusting mode (0.2 J pulse energy delivered at 20 Hz frequency) at three different SDs (0.10, 0.25 and 0.50 mm) with vfiber in the range of 0 to 10 mm/s. Stone damage was quantified by optical coherence tomography, bubble dynamics were captured by high-speed imaging, and associated pressure transients were measured using a needle hydrophone. To compare photothermal ablation vs. cavitation in stone dusting, the experiments were further repeated in air (photothermal ablation only), and in water with the fiber tip advanced at a short (0.25 mm) offset distance (OSD) from a ureteroscope to mitigate the bubble collapse toward the stone surface, thus eliminating cavitation-induced damage. ResultsCompared to the craters produced by a stationary fiber, the damage troughs produced by a scanning fiber after 100 pulses were significantly larger in volume. The optimal vfiber for maximum dusting efficiency was found to be 3.5 mm/s for SD = 0.10 [~] 0.25 mm, resulting in long (17.5 mm), shallow (0.14 - 0.15 mm) and narrow (0.3 - 0.4 mm) troughs. In contrast, the maximum stone damage was produced at an optimal vfiber of 0.5 mm/s for SD = 0.50 mm, which generates much shorter (2.5 mm), yet deeper (0.35 mm) and wider (1.4 mm) troughs. Greater stone damage was produced in water than in air, especially at vfiber = 0 - 3.5 mm/s. With the scope end placed near the fiber tip, stone damage could be significantly reduced in water by 29% - 58% for SD = 0.10 mm, by 51% - 82% for SD = 0.25 mm, and by 66% - 100 % for SD = 0.50 mm, compared to those produced without the scope. Together, these findings suggest that cavitation plays an indispensable role in stone dusting by scanning treatment. Moreover, under clinically relevant vfiber (1 [~] 3 mm/s), dusting at SD = 0.5 mm (i.e., non-contact mode) may leverage higher frequency of the laser (e.g., 40 to 120 Hz) to harvest the full potential of cavitation damage while significantly reducing the procedure time, compared to its counterpart at SD = 0.1 mm (i.e., contact mode) that promotes photothermal ablation. ConclusionDusting efficiency during Ho:YAG LL may be substantially improved by utilizing the optimal combination of vfiber and SD for a given frequency.