Objectives

During holmium:yttrium-aluminum-garnet (holmium:YAG) laser lithotripsy to break urinary stones, urologists frequently see flashes of light. As infrared laser pulses are invisible, what is the source of light? Here we studied the origin, characteristics, and some effects of flashes of light in laser lithotripsy.

Methods

Ultrahigh-speed video-microscopy was used to record single laser pulses at 0.2-1.0 J energy lasered with 242 µm glass-core-diameter fibers in contact with whole surgically retrieved urinary stones and hydroxyapatite (HA)-coated glass slides in air and water. Acoustic transients were measured with a hydrophone. Visible-light and infrared photodetectors resolved temporal profiles of visible-light emission and infrared-laser pulses.

Results

Temporal profiles of laser pulses showed intensity spikes of various duration and amplitude. The pulses were seen to produce dim light and bright sparks with submicrosecond risetime. The spark produced by the intensity spike at the beginning of laser pulse generated a shock wave in the surrounding liquid. The subsequent sparks were in a vapor bubble and generated no shock waves. Sparks enhanced absorption of laser radiation, indicative of plasma formation and optical breakdown. The occurrence and number of sparks varied even with the same urinary stone. Sparks were consistently observed at laser energy >0.5 J with HA-coated glass slides. The slides broke or cracked by cavitation with sparks in 63 ± 15% of pulses (1.0 J, N = 60). No glass-slide breakage occurred without sparks (1.0 J, N = 500).

Conclusion

Unappreciated in previous studies, plasma formation with free-running long-pulse holmium:YAG lasers can be an additional physical mechanism of action in laser procedures.

A novel treatment modality incorporating calcium-adhering microbubbles has recently entered human clinical trials as a new minimally-invasive approach to treat urinary stones. In this treatment method, lipid-shell gas-core microbubbles can be introduced into the urinary tract through a catheter. Lipid moities with calcium-adherance properties incorporated into the lipid shell facilitate binding to stones. The microbubbles can be excited by an extracorporeal source of quasi-collimated ultrasound. Alternatively, the microbubbles can be excited by an intraluminal source, such as a fiber-optic laser. With either excitation technique, calcium-adhering microbubbles can significantly increase rates of erosion, pitting, and fragmentation of stones. We report here on new experiments using high-speed photography to characterize microbubble expansion and collapse. The bubble geometry observed in the experiments was used as one of the initial shapes for the numerical modeling. The modeling showed that the bubble dynamics strongly depends on bubble shape and stand-off distance. For the experimentally observed shape of microbubbles, the numerical modeling showed that the collapse of the microbubbles was associated with pressure increases of some two-to-three orders of magnitude compared to the excitation source pressures. This in-vitro study provides key insights into the use of microbubbles with calcium-adhering moieties in treatment of urinary stones.

A novel treatment modality incorporating calcium-adhering microbubbles has recently entered human clinical trials as a new minimally-invasive approach to treat urinary stones. In this treatment method, lipid-shell gas-core microbubbles can be introduced into the urinary tract through a catheter. Lipid moities with calcium-adherance properties incorporated into the lipid shell facilitate binding to stones. The microbubbles can be excited by an extracorporeal source of quasi-collimated ultrasound. Alternatively, the microbubbles can be excited by an intraluminal source, such as a fiber-optic laser. With either excitation technique, calcium-adhering microbubbles can significantly increase rates of erosion, pitting, and fragmentation of stones. We report here on new experiments using high-speed photography to characterize microbubble expansion and collapse. The bubble geometry observed in the experiments was used as one of the initial shapes for the numerical modeling. The modeling showed that the bubble dynamics strongly depends on bubble shape and stand-off distance. For the experimentally observed shape of microbubbles, the numerical modeling showed that the collapse of the microbubbles was associated with pressure increases of some two-to-three orders of magnitude compared to the excitation source pressures. This in-vitro study provides key insights into the use of microbubbles with calcium-adhering moieties in treatment of urinary stones.