The pulsed dye laser versus the Q‐switched Nd:YAG laser in laser‐induced shock‐wave lithotripsy

Abstract

To date, there are two fairly well-established alternatives for laser-induced shock-wave lithotripsy in clinical practice. The Q-switched Nd:YAG laser is distinguished by the high-stone selectivity of its coupler systems. The necessity of a coupler system and its fairly small conversion rate of light energy into mechanical energy present serious drawbacks. Furthermore, the minimal outer diameter of the transmission system is 1.8 mm. The pulsed-dye laser can be used with a highly flexible and uncomplicated 200-micron fiber. However, the laser system itself is more complicated than the Q-switched Nd:YAG laser and requires a great deal of maintenance. Biological evaluation of damage caused by direct irradiation shows that both laser systems produce minor damage of different degrees. YAG laser lithotripsy with the optomechanical coupler was assessed in 31 patients with ureteral calculi. The instability and limited effectiveness of the fiber application system necessitated auxiliary lithotripsy methods in 14 cases. Dye-laser lithotripsy is currently being tested in clinical application. Further development, such as systems for blind application or electronic feedback mechanisms to limit adverse tissue effects, have yet to be optimized. Nevertheless, laser-induced shock-wave lithotripsy has the potential to become a standard procedure in the endourologic management of stone disease.

Nonlinear absorption: intraocular microsurgery and laser lithotripsy

“…Since crack formation takes time, longer cracks can develop after µs laser pulses than after ns pulses, where the pressure transients are much shorter (Peterson 1972). Stone fragments are therefore larger after irradiation with µs pulses than with ns pulses, where powdery fragments are produced (Thomas et al 1988b, Rink et al 1995 and a larger number of pulses is required to break up the stone (Rink et al 1995). The creation of very small fragments requiring a large amount of acoustic energy is not required to allow excretion of the fragments.…”

“…The primary source of collateral tissue damage is misaiming of the laser pulses leading to plasma formation at the tissue instead of on the stone, often with the consequence of a haemorrhage (Thomas et al 1988b). Tissue damage can also arise from the expanding cavitation bubble, which at very large pulse energies may even rupture the ureter or bile duct (Tidd et al 1976).…”

“…Damage to the tip of the bare fibre is, of course, not desirable either, but will not prevent continuation of the procedure. Since the irradiated spot at the fibre tip has a diameter of 200 µm or more, pulse energies of 40-70 mJ are necessary to surpass the breakdown threshold at the stone surface and efficiently fragment the calculus (Thomas et al 1988b). Nanosecond pulses in this energy range are difficult to transmit through optical fibres without causing damage to the fibre, especially near the fibre tip where microcracks develop due to the pressure transients from the laser plasma.…”

“…Attention will therefore be focused on this parameter range. Other attempts to perform lithotripsy with pulse durations of a few ns (Schmidt-Kloiber et al 1985, Thomas et al 1988b, Shi et al 1990 will not be discussed here in detail.…”

“…If within 500 ns the intensity of the light collected by the fibre does not surpass the required threshold, the rest of the laser pulse is blocked out by means of the Pockels cell. After Engelhardt et al (1988) and Thomas et al (1988b).…”