Recent advances in infrared laser lithotripsy [Invited]

Abstract

The flashlamp-pumped, solid-state, pulsed, mid-infrared, holmium:YAG laser (λ = 2120 nm) has been the clinical gold standard laser for lithotripsy for over the past two decades. However, while the holmium laser is the dominant laser technology in ureteroscopy because it efficiently ablates all urinary stone types, this mature laser technology has several fundamental limitations. Alternative, mid-IR laser technologies, including a thulium fiber laser (λ = 1908 and 1940 nm), a thulium:YAG laser (λ = 2010 nm), and an erbium:YAG laser (λ = 2940 nm) have also been explored for lithotripsy. The capabilities and limitations of these mid-IR lasers are reviewed in the context of the quest for an ideal laser lithotripsy system capable of providing both rapid and safe ablation of urinary stones.

Fig. 1

Photographs and scanning electron micrographs (SEM) of uric acid (UA) and calcium oxalate monohydrate (COM) stones before and after laser ablation with Thulium fiber laser in air and water mediums. Direct absorption of IR laser energy by stone in air results in a change from native state to more amorphous state with fusion of stone material, while absorption of IR energy by water contained in pores along stone surface may result in thermal expansion and production of cracks (shown by arrows), contributing to ablation.

Fig. 2

Multiple approaches to pulsed laser lithotripsy have been utilized, including (a) short pulse (250-350 µs) for fragmentation, (b) long pulse (up to 1200 µs) to reduce stone retropulsion, (c) double-pulse delivery (2 x 350 µs), (d) delivery of pulse trains for a factor of 2 times increase in ablation rates, and (e) “Moses Tech” involving delivery of a low energy, short duration pulse to create a vapor bubble immediately followed by a higher energy, longer duration pulse for more efficient stone ablation, with reduced stone retropulsion as well.

Fig. 3

Spatial beam profiles of (a) Holmium:YAG laser and (b) Thulium fiber laser. The multimodal Holmium laser beam prevents focusing down to small spots for coupling into fibers less than 200 µm. The TFL beam has been coupled into fibers as small as 50 µm core.

Fig. 4

Comparison of temporal beam profiles of short-pulse, flashlamp-pumped, solid-state, Holmium:YAG laser (red) and the diode-pumped, Thulium fiber laser (blue). The greater amount of energy contained in the initial spike of the Holmium temporal beam profile contributes to the initial vapor bubble expansion and collapse and consequently greater stone retropulsion observed with the Holmium laser than with the Thulium fiber laser.

Fig. 5

(a) Next generation (circa 2016) of compact, air-cooled, tabletop 500 W peak power, 1940 nm Thulium fiber laser sitting on top of (b) first generation, water-cooled 100 W peak power, 1908 nm Thulium fiber laser (circa 2004).