Analysis of thulium fiber laser induced bubble dynamics for ablation of kidney stones

Abstract

The Thulium fiber laser (TFL) is being explored as an alternative to the Holmium : YAG laser for lithotripsy. TFL parameters differ in several fundamental ways from Holmium laser, including smaller fiber delivery, more strongly absorbed wavelength, low pulse energy/high pulse rate operation, and more uniform temporal pulse structure. High speed imaging of laser induced bubbles was performed at 105,000 frames per second and 10 μm spatial resolution to determine influence of these laser parameters on bubble formation and needle hydrophone data was also used to measure pressure transients. The TFL was operated at 1908 nm with pulse energies of 5-65 mJ, and pulse durations of 200-1000 μs, delivered through 105-μm-core and 270-μm-core silica optical fibers. Bubble dynamics using Holmium laser at a wavelength of 2100 nm with pulse energies of 200-1000 mJ and pulse duration of 350 μs was studied, for comparison. A single, 500 μs TFL pulse produced a bubble stream extending 1200 ± 90 μm and 1070 ± 50 μm from fiber tip, with maximum bubble widths averaging 650 ± 20 μm and 870 ± 40 μm (n = 4), for 105 μm and 270 μm fibers, respectively. These observations are consistent with previous studies which reported TFL ablation stallout at working distances beyond 1.0 mm. TFL bubble dimensions were four times smaller than for Holmium laser due to lower peak power and smaller fiber diameter used. The maximum pressure transients measured 0.6 bars at 35 mJ pulse energy for TFL and 7.5 bars at 600 mJ pulse energy for Holmium laser. These fundamental studies of bubble dynamics as a function of specific laser and fiber parameters may assist with optimization of the TFL parameters for safe and efficient lithotripsy in the clinic. Image of bubble formation during fiber optic delivery of Thulium fiber laser energy in saline (35 mJ, 500 μs).

Thulium fiber laser: the new player for kidney stone treatment? A comparison with Holmium:YAG laser

“…In 2005, the first report on Thulium fiber laser lithotripsy adapted a continuous-wave generator to operate in a pulsed mode and demonstrated the feasibility of lithotripsy on COM and UA stones [59]. Thereafter, fibers with a core diameter as small as 50-150 µm were repeatedly reported to efficiently deliver Thulium fiber laser beam on urinary stones [40,42,54,[60][61][62][63][64][65][66][67][68]. Also, cumulative evidence from a series of studies on distal fiber tip design suggests the muzzle tip design for prevention of stone retropulsion during Thulium fiber laser delivery [60,62,63,67,68].…”

“…An analysis of Thulium fiber laser bubble formation at the distal fiber tip revealed the formation of a bubble stream with multiple bubble expansions and collapses [54]. This phenomenon is reminiscent of the Moses effect, which has been first described in 1988 as a vapor channel resulting from water irradiation by laser and which leaves an open path with low absorption coefficient between the fiber tip and the stone surface [69].…”

Fiber optic muzzle brake tip for reducing fiber burnback and stone retropulsion during thulium fiber laser lithotripsy

“…The experimental thulium fiber laser (TFL) is being studied as an alternative to the current gold standard Holmium:YAG laser for lithotripsy. [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] The TFL has a higher absorption coefficient (μ a ∼ 120 cm −1 ) and shorter optical penetration depth in water, which translates into an ablation threshold four times lower compared with the Holmium laser (μ a ∼ 30 cm −1 ). 7,19,20 This property allows the TFL to ablate tissues at lower pulse energies than with the Holmium laser.…”

Ho:YAG laser lithotripsy in non-contact mode: optimization of fiber to stone working distance to improve ablation efficiency

“…Currently, it is unclear why ablation volume with a long Ho:YAG laser pulse is highest in non-contact mode. Since holmium bubbles are necessary for the thermomechanical contribution to laser ablation [18], the presence of the micro-bubble stream as noticed with the high-speed camera for long pulses may play a role in the ablative effect, as well as the shielding effect of products of ablation during the pulse, and the laser beam scattering and divergence. The vapor channel of the long pulse, characterized by the pear-shaped bubble and chaotic multiple bubble stream, is probably most efficient to conduct the laser energy directly onto the stone surface at a distance of 0.4 mm.…”