The role of pulse length in limiting distant damage to vascular tissue caused by the Nd-YAG laser

The total damage caused by equivalent doses of energy given to human cadaver vascular tissue over the same time scale from three Nd-YAG lasers of different pulse lengths is quantified. The continuous wave (c.w.) laser produces vacuolation and coagulation around a vaporized crater; the 100 μs pulsed laser produces less surrounding damage and the 10 ns pulsed laser none at all. The areas of damage in five craters made with 10 J energy were measured from histology slides using a digitising platten, and it was found that in each case the total amount of damage was the same, even though the depth of the craters made varied. The dose response for vaporization of the 10 ns pulsed laser was the greatest at 35 μm/J and that of the c.w. laser was least at 8 μm/J. A pulse length of 100 μs may not be the optimum for limiting surrounding tissue damage during laser angioplasty but it produces much less damage than a c.w. laser and unlike the 10 ns pulses is easily transmissible down an optical fibre.     

In vitro Alexandrite laser angioplasty: A study on fibre conduction and tissue effects

Pulsed ultra-violet excimer laser radiation is capable of tissue ablation with only minimal thermal injury of adjacent tissue structures. Since difficult fibre optic coupling of energy was observed, alternative Q-switched laser sources capable of ablation of atherosclerotic plaque are under current investigation. To evaluate tissue effects of Alexandrite laser radiation, 160 arterial segments with macroscopic evidence of atherosclerotic disease were treated. The laser light was transmitted via silica based quartz fibres with different diameters. Using the Q-switched Alexandrite laser at the fundamental wavelength (748 nm) with a pulse duration of 300 ns the energy density threshold for tissue ablation was found to be in the range of 63 to 126 J cm^–2 using a 300m fibre. On macroscopic examination only limited thermal and acoustic injury was found in crater adjacent tissue structures. Crater edges were even and did not reveal signs of crater charring or debris in the crater lumen. However, the histological cross-sections revealed thermal injury extending from 100 up to 200m lateral into adjacent tissue. The crater margins revealed fissuring as a result of shock wave injury. Thermal damage was most evident if irradiation of atherosclerotic tissue was performed in blood.     

Ablation of neural tissue by short-pulsed lasers — a technical report

Summary The basis for most laser applications in neurosurgery is the conversion of laser light into heat when the incident laser beam is absorbed by the tissue. Irradiation of neural tissue with laser light therefore leads to its thermal damage. However, due to the diffusion of heat energy into the surrounding tissue, often there is thermal damage to neural tissue outside the area of the target volume. These are the characteristics of thermal laser/tissue interaction. In this paper we discuss how we used three different short-pulsed lasers to achieve non-thermal ablation of neural tissue.Three different short-pulsed lasers were used to generate ultrashort laser pulses in the picosecond to femtosecond range. The interaction of such laser pulses with tissue was predicted to be nonthermal. The short-pulsed lasers were used for the ablation of neural tissue using an in vitro calf brain model. The histopathological examination of the lesions revealed that the neural tissue had been removed very precisely without any sign of thermal damage to the surrounding tissue.     

Optical fibre delivery and tissue ablation studies using a pulsed hydrogen fluoride laser

We describe an experimental study of the transmission of pulsed, multiline (2.67–2.95 μm) hydrogen fluoride (HF) laser radiation in a fluoride glass fibre. For a 400 ns full-width at half-maximum (fwhm) duration laser pulse transmission measurements and photoacoustic techniques show that non-linear loss at the fibre input surface appears to set a maximum useful input fluence of ∼15 J cm⁻² for the 500 μm core diameter fibre used. However, with modest length fibres, exit fluence levels adequate for the ablation of soft tissue can be obtained with the fibre operating well below this limiting value. Removal rate measurements and scanning electron microscope evaluation of the irradiated site for bovine cornea ablated using the fibre-delivered HF laser pulses are reported.     

Free-Electron Laser (FEL) Ablation of Ocular Tissues

The purpose of the present study was to investigate the ablation characteristics of free-electron laser (FEL) pulses at 5.3 μm and 6.0 μm on ocular tissues. The advanced FEL was developed by the Los Alamos National Laboratory. It operates in the 4–7 μm range, with a macropulse duration of 10 μs and repetition rate of 10 Hz. Each macropulse consists of a train of micropulses that are 15 ps in pulse duration separated by 10 ns. The output energy of macropulses can be adjusted from 5 to 120 mJ. Five transplant grade corneal-scleral buttons preserved in corneal storage medium were used. Wavelengths 5.3 and 6.0 μm were selected based on Fourier-transform infrared absorption spectra of a type I collagen film. Ocular tissue cuts made at 6.0 μm revealed a well-defined ablation boundary with a lateral tissue damage at 10±2 μm. The ablation boundary of the corneal cuts made at 5.3 μm revealed tissue disruption with thermally denatured tissue constituents with loss of organised structure. The lateral dimension of this effect extended up to 220 μm. It was concluded first that Fourier-transform infrared absorption spectra may be useful in selecting laser ablation wavelengths, and second that lasers emitting near 6 μm achieve excellent spatial confinement of laser energy secondary to both protein and water energy partitioning. This wavelength may have potential for cutting ocular tissue such as cornea, sclera, vitreous or lens, because of energy partitioning.     

Stretched nanosecond pulses of a Q-switched Nd-YAG laser: Transmission through optical fibres and in vitro effects on human calcified tissues

Using a new Q-switched Nd-YAG laser prototype (1064 nm), every initial pulse (15 ns or 33 ns duration, 10 Hz repetition rate, 2×E energy, Pmax power) was transformed into two successive pulses (each one with 15 or 33 ns duration, 1×E energy) delayed by 27 ns. With a 15 ns initial pulse, the two 15 ns components (each one with Pmax/2) were well separated and called a 15 ns double pulse. When the duration of the initial pulse was stretched to 33 ns, the overlapping of the two 33ns components (each with Pmax/4) produced only one pulse trapezoidally shaped, of 60 ns duration and called 60 bi-pulse. With double pulses, it was possible to transmit 100 mJ through a silica-silica optical fibre of 400 μm diameter, whereas 155 mJ were transmitted with bi-pulses. Human calcified atheromatous tissues were irradiated with bi-pulses through a 200 μm diameter optical fibre. Craters (0.6–0.7 mm in diameter) were easily obtained in atheromatous aortic segment with 40 mJ energy per bi-pulse. No trace of carbonization was noticed. Fragmentation of urinary and biliary calculi was also obtained.     

Study of mechanical and thermal damage in brain tissue after ablation by Erbium-YAG laser

This work studied the ablation of bovine brain tissue by free-running Erbium-YAG laser pulses. Single-shot interactions were investigated by means of an ultra-fast imaging technique. Thin sections of the treated tissue were processed for histochemical analysis of enzyme activity to assess the extent of thermal/mechanical damage. Thereafter, a scanning beam technique was employed to deliver multiple pulses over a definite region of tissue. An analytical balance was used to measure the removed mass in order to calculate the ablation efficiency. The present quantity has been compared to the amount of the tissue damaged, as assessed by the histochemical analysis. The present work shows that the interaction of the Erbium-YAG laser pulses with a soft tissue may cause a large amount of mechanical damage, while thermal damage is restricted within a thin layer around the ablation crater. A precise control of fluence and operating conditions prevents overwhelming side-effects, and possibly allows the use of the Erbium-YAG laser for the ablation of brain and other soft tissues.     

Mechanisms of intraocular photodisruption with picosecond and nanosecond laser pulses

Nd:YAG laser photodisruption with nanosecond (ns) pulses is an established method for intraocular surgery. In order to assess whether an increased precision can be achieved by the use of picosecond (ps) pulses, the plasma size, the shock wave characteristics, and the cavitation bubble expansion after optical breakdown with ps- and ns-laser pulses were investigated by time-resolved photography and acoustic measurements. Nd:YAG laser pulses with a duration of 30 ps and 6 ns, respectively, were focused into a water-filled glass cuvette. Frequency doubled light from the same laser pulses was optically delayed between 2 ns and 136 ns and used as illumination light source for photography. Since the individual events were well reproducible, the shock wave and bubble wall position could be determined as a function of time. From the slope of these r(t) curves, the shock wave and bubble wall velocities were determined, and the shock wave pressure was calculated from the shock velocity. The plasma size at various laser pulse energies was measured from photographs of the plasma radiation. The breakdown thresholds at 30 ps and 6 ns pulse duration were found to be 15 μJ and 200 μJ, respectively. At threshold, ps-plasmas are shorter than ns-plasmas, but at the same pulse energy they are always ～2.5 times longer. The initial shock pressures were 17 kbar after ps-pulses with an energy of 50 μJ, and 21 kbar after 1 mJ ns-pulses. The pressure amplitude decayed much faster after the ps-pulses. The maximum expansion velocity of the cavitation bubble was 350 m/s after a 50 μJ ps-pulse, but 1,600 m/s after a 1 mJ ns-pulse. The side effects of intraocular microsurgery associated with shock wave emission and cavitation bubble expansion can be considerably reduced by the use of ps-pulses, and new applications of photodisruption may become possible. © 1994 Wiley-Liss, Inc.     

Arterial response to excimer and argon laser irradiation in the atherosclerotic swine

Injury associated with laser-induced tissue ablation may be reduced by using pulsed energy delivery at low repetition rates, as opposed to using continuous wave energy delivery. This study was designed to examine the similarities and differences between these two systems as regards the healing process, and to examine whether one is superior to the other. In order to test this postulate, the healing response of normal and atherosclerotic aorta were examined after exposure in vivo to argon and excimer (XeCl 308 nm) laser radiation in hypercholesterolemic swine. Swine were fed hyperlipidemic diets for eight months following balloon denudation of the descending aorta. Following general anaesthetic, the descending aorta was isolated and laser burns were made on both normal and atherosclerotic intima using a continuous wave argon laser delivered through a 50 diameter quartz fibre, and a XeCl excimer laser carried through a 1 mm diameter fibre. Energy levels of 3 to 5 J were applied with the argon laser. The pulse duration for the excimer laser was 30 ns and craters were produced using 10 to 60 pulses at a repetition rate of 20 Hz and an energy density of 2 J cm^–2.Forty-eight hours after laser application, craters created by both lasers were filled with thrombus material. Argon burns were surrounded by thermal and acoustic injury which was not seen with excimer burns. Three weeks after laser application all crater surfaces were reconstituted. Unlike the excimer burns, argon craters demonstrated necrosis well beyond the crater margins and were characterized by multinucleate giant-cell reaction surrounding char debris. By nine weeks both excimer and argon laser burns were covered by fibrous tissue but could be distinguished by the fact that char debris and subjacent tissue injury arose with the argon burns.The results suggest that both lasers can be used to remove focal atherosclerotic plaque from arteries without inducing excessive thrombogenicity. Rapid healing is observed with both; however, damage to surrounding tissue is significantly greater with a continuous energy delivery laser as opposed to pulsed energy delivery.Work supported in part by: Heart and Stroke Foundation of Ontario, Grant-in-Aid No. 5-17     

Optoacoustic monitoring of cutting efficiency and thermal damage during laser ablation

Successful laser surgery is characterized by a precise cut and effective hemostasis with minimal collateral thermal damage to the adjacent tissues. Consequently, the surgeon needs to control several parameters, such as power, pulse repetition rate, and velocity of movements. In this study we propose utilizing optoacoustics for providing the necessary real-time feedback of cutting efficiency and collateral thermal damage. Laser ablation was performed on a bovine meat slab using a Q-switched Nd-YAG laser (532 nm, 4 kHz, 18 W). Due to the short pulse duration of 7.6 ns, the same laser has also been used for generation of optoacoustic signals. Both the shockwaves, generated due to tissue removal, as well as the normal optoacoustic responses from the surrounding tissue were detected using a single broadband piezoelectric transducer. It has been observed that the rapid reduction in the shockwave amplitude occurs as more material is being removed, indicating decrease in cutting efficiency, whereas gradual decrease in the optoacoustic signal likely corresponds to coagulation around the ablation crater. Further heating of the surrounding tissue leads to carbonization accompanied by a significant shift in the optoacoustic spectra. Our results hold promise for real-time monitoring of cutting efficiency and collateral thermal damage during laser surgery. In practice, this could eventually facilitate development of automatic cut-off mechanisms that will guarantee an optimal tradeoff between cutting and heating while avoiding severe thermal damage to the surrounding tissues.     

Effects of laser parameters on pulsed Er-YAG laser skin ablation

Previous studies demonstrated that pulsed 2.94m Er-YAG laser radiation allows a precise etching of organic tissue with only minimal thermal damage. This makes the Er-YAG laser a promising tool for the careful removal of superficial skin lesions. In order to provide optimized laser parameters for potential clinical use and to enhance our understanding of the mid-infrared ablation process, we measured the ablation rate, temperature profile and damage zones for various pulse numbers, radiant energies and pulse repetition rates. Ablation is very efficient (about 6m J^–1 cm² for high radiant exposure) and the crater depth is exactly (1Hz) or nearly (2 Hz) linearly related to the radiant exposure. In contrast, no significant effects of the laser parameters on the thermal damage of the epidermis and the crater bottom were observed. In conclusion, for a future clinical use high radiant energies should be applicable without the disadvantage of enhanced damage.     

Thermal side effects of fiber-guided XeCl excimer laser drilling of cartilage

We examined thermal effects during ablation of human joint cartilage using two XeCl excimer lasers with pulse durations of ～ 20 ns and 60 ns. An increase in radiant exposure or repetition rate caused a rise in tissue temperature up to 82°C at a 100- μm distance. With increasing distance from the crater edge, the temperature dropped exponentially. Radiant exposures higher than 1.8 J/cm² and repetition rates above 20 Hz lead to a formation of hot gaseous products escaping from the laser crater. When os-teoarthritic cartilage is irradiated, these gases spread inside the tissue causing a temperature rise of > 50°C at a distance of 1 mm from the crater edge. In the contact mode, we found a linear rise of ablation rate with increasing repetition rate both in air or saline. But ablation rates in saline were only half the rates achieved in air. Both phenomenons can be explained by additional thermal effects of excimer lasers working in the range of higher repetition rates and pulse energies. © 1994 Wiley-Liss, Inc.     

Upper limits of laser pulse widths required for melanosome and epidermal specific damage

A critical analysis of thermal relaxation times used as an upper limit of laser pulse widths after the absorption of laser radiation by pigmented epidermal lesions has been made. Only the case in which the radiation is absorbed mainly by melanin-containing melanosomes within the tissue is treated. Two of the known criteria currently used to estimate relaxation times are compared, and relaxation times based on an exact solution of the heat conduction equation are found. Numerical results indicate that the relaxation times needed for the temperature to reach its maximum value at a given distance from the melanosome are in the range 0–5.0 μs when the distance varies in the range 0.5–2.0 μm for a typical melanosome radius of 0.5 μm, and for laser pulse widths much shorter than the corresponding non-zero relaxation times. It is shown that, with visible and ultraviolet laser pulses, it is difficult selectively to damage epidermal lesions that have a low density of melanosomes. It is suggested, therefore, that these lesions can be treated with low-fluence infrared radiation from, for example, a CO₂ laser. This suggestion agrees with experimental results published by other authors.     

Effects of Q-switched Nd:YAG Laser on Calcified Tissues

. This study evaluated the effects of the Q-switched Nd:YAG (1064 nm) laser on four types of calcified tissues (dentine, enamel, bone and cementum) at frequencies of 1, 5 and 10 Hz for irradiation times of 100, 100 and 50 s, respectively. Laser fluences per pulse ranged from 30 to 50 J/cm², and pulse duration was 15 ns. To evaluate morphological modifications after laser irradiation, specimens were examined by scanning electron microscopy. Chemical modifications in residual tissues were studied by Raman spectroscopy, and the depth of craters produced by laser impact was analysed as a function of laser fluences. The results showed that tissues were ablated essentially by a photoacoustic mechanism which produced no carbonisation or high melting zone in residual tissues, even though cracks and fractures appeared around craters. Crater depth per pulse was 1–5 μm/pulse depending on the frequency used. Statistical analysis showed that increasing the number of pulses, thereby increasing crater depth led to a decrease in the ablation rate. Raman spectroscopy showed none of the chemical modifications in residual tissues known to occur in heat-treated enamel, dentine and bone after laser irradiation. Paper received 5 June 1997; accepted following revision 11 February 1999.     

IR Lasers and Application Systems for Myringotomy

. The study examines an Er:YAG laser (2940 nm) and different application systems of the CO₂ laser (10 600 nm) with regard to their suitability for a one-shot laser myringotomy of an adequate perforation size (∼2 mm). The laser–tissue interaction of the Er:YAG laser and the CO₂ laser in fresh tympanic membranes of horses (thickness: 80–100 μm) as well as in formalin-fixed human tympanic membranes (thickness: 100 μm) is studied correlating perforation diameters to the applied power/energy density and the effects demonstrated by light and scanning electron microscopy are analysed. Using the Er:YAG laser with a focused laser beam (spot diameter: 400 μm) or with a maximally defocused laser beam (spot diameter: 1600 μm) perforations of an adequte size (2 mm) can only be achieved with multiple laser pulses. Histological studies disclose only minimal thermic side effects in the adjacent tissue in both specimens. If the CO₂ laser radiation is transmitted via a silver halide polycrystalline fibre (diameter: 900 μm) a maximal perforation diameter of 1300 μm is achieved with significant thermic side effects such as coagulation. Using an Acuspot™ 710 micromanipulator (focused beam diameter: 180 μm) combined with a SilkTouch™ scanner a maximal perforation diameters of 1700 μm can be achieved in horse tympanic membrane with one laser pulse. A prototype of a hand-held CO₂ laser otoscope in combination with the SilkTouch™ scanner is suitable for performing laser myringotomies with a diameter of 2 mm with a single laser pulse in fresh horse tympanic membrane. Paper received July 1999; accepted after revision December 1999.     

Histological evaluation of vertical laser channels from ablative fractional resurfacing: an ex vivo pig skin model

Ablative fractional resurfacing (AFR) represents a new treatment potential for various skin conditions and new laser devices are being introduced. It is important to gain information about the impact of laser settings on the dimensions of the created laser channels for obtaining a safe and efficient treatment outcome. The aim of this study was to establish a standard model to document the histological tissue damage profiles after AFR and to test a new laser device at diverse settings. Ex vivo abdominal pig skin was treated with a MedArt 620, prototype fractional carbon dioxide (CO₂) laser (Medart, Hvidovre, Denmark) delivering single microbeams (MB) with a spot size of 165 μm. By using a constant pulse duration of 2 ms, intensities of 1–18 W, single and 2–4 stacked pulses, energies were delivered in a range from 2–144 mJ/MB. Histological evaluations included 3–4 high-quality histological measurements for each laser setting (n = 28). AFR created cone-shaped laser channels. Ablation depths varied from reaching the superficial dermis (2 mJ, median 41 μm) to approaching the subcutaneous fat (144 mJ, median 1,943 μm) and correlated to the applied energy levels in an approximate linear relation (r² = 0.84, p < 0.001). The dermal ablation width increased slightly within the energy range of 4–144 mJ (median 163 μm). The thickness of the coagulation zone reached a plateau around 65 μm at energies levels above 16 mJ. The calculated volumes of ablated tissue increased with increasing energies. We suggest this ex vivo pig skin model to characterize AFR laser channels histologically.     

Photocoagulation with a copper vapour laser in rabbit liver: A comparative study between the 510 nm and the 578 nm wavelengths

The coagulation effect, penetration depth and healing process of the 510.6 nm (green) and 578.2 nm (yellow) wavelengths of copper vapour laser (CVL) were compared in vivo in rabbit liver (n=15). A pulsed CVL, the Cu 15 from Oxford Laser—pulse repetition 10 kHz, peak-power 70 kW, pulse width 25 ns, and average maximal power 16W—was used connected to a dichroic system. The beam was transmitted through a 1000 μm quartz fibre and focused with a handpiece providing a 2 mm diameter spot size. By means of this delivery system 270 focused lesions are achieved at a power output of 2.65 W (power density 80 W cm⁻²) with irradiation times of 3, 5 and 10 s. The operative and microscopic verifications were achieved at 0 hour, and on days 3, 10, 20 and 30. Immediately after laser application, the lesions were triangular, well demarcated, and characterized by a central vaporization surrounded by four peripheral zones: carbonization; coagulation; oedema; and transition. The penetration depth was noticeably bigger in the yellow wavelength than with green wavelength, evidenced on day 10 by superior size of yellow wavelength photocoagulations and coagulation necrosis. Fibrosis appeared by day 3 and was gaining ground quickly and intensively after yellow wavelength while the fibrotic reaction was delayed on day 10 after green wavelength. The more penetrating effect of yellow wavelength advocates for its use in liver tumour destruction and photoradiation therapy while the green wavelength, inducing less aggressive effect on the surrounding tissue, seems more suitable for liver resection.     

Factors influencing the ablation of atherosclerotic plaque with the argon laser

This paper describes the ablative effect of argon laser light, delivered fibre optically in vitro, on 234 segments of atherosclerotic human aorta. Variables such as energy density, type of atheroma and immersion media were taken into account. All irradiated specimens were subsequently submitted to histological examination and crater volumes in mm³ were derived from micrometer measurements made at light microscopy. Results showed: (1) a linear relationship between energy dose and crater volume in fibrous atheroma; (2) significantly greater surrounding tissue damage in the higher energy dose groups; (3) a lower dose response in calcified tissue than in fatty streaks or fibrous atheroma; (4) immersion of tissue in blood during ablation resulted in a significantly greater dose response than immersion in plasma or saline, and the corresponding surrounding tissue damage was greatest under blood. Thus, argon laser light is both effective and predictable in response when ablating atheromatous tissue, and the efficiency of the process depends on the immersion medium. The degree of surrounding tissue damage depends on the energy dose.     

Mid-IR laser ablation of articular and fibro-cartilage: a wavelength dependence study of thermal injury and crater morphology

BACKGROUND AND OBJECTIVE: The aim of this study was to evaluate areas of collateral thermal injury and crater morphology for evidence of wavelength-dependent effects on the ablation of articular cartilage and fibro-cartilage (meniscus) using selected mid-IR wavelengths produced by a free electron laser. STUDY DESIGN/MATERIALS AND METHODS: Two types of cartilage, articular cartilage and fibro-cartilage were used in the study. The wavelengths (lambda) evaluated were 2.79, 2.9, 6.1, and 6.45 microm generated by a free electron laser (FEL) using a 4 microseconds macropulse configuration. The zone of thermal injury and crater morphology produced by laser ablation were examined by light microscopy following standard histologic processing. RESULTS: The zone of thermal injury and crater morphology created in cartilage by the FEL at selected mid-IR wavelengths were examined as a function of incident radiant exposure. Ablation using lambda = 6.1 microm provided the largest crater size for both articular and fibro-cartilage at all radiant exposures. For the zones of collateral thermal injury in articular cartilage, lambda = 6.1 microm produced the least thermal injury at the radiant exposure of 7.6 J/cm2. When the radiant exposure is increased to 20.4 J/cm2, both lambda = 6.1 and 6.45 microm produced less thermal injury than the ablation using lambda = 2.79 and 2.9 microm. The greatest amount of collateral thermal injury was produced by lambda = 2.79 microm for both tissue types. CONCLUSIONS: The results demonstrate that crater depth and collateral thermal injury produced in articular cartilage and fibro-cartilage are wavelength-dependent with 6.1 microm providing the largest craters at all radiant exposures. The least amount of thermal injury was created in articular cartilage using lambda = 6.1 microm at the radiant exposure of 7.6 J/cm2. Both 6.1 and 6.45 microm wavelengths demonstrated similar amount of thermal injury at 20 J/cm2 that was less than lambda = 2.79 and 2.9 microm at similar fluences. These observations are explained based on the absorption by water and protein in the tissue types studied. It is further observed that the use of crater dimensions may not provide a reliable estimate for the amount of tissue removal provided by an ablation procedure.     

Preserving the Integrity of Oesophageal Stents with Laser Therapy and Argon Plasma Coagulation: An In Vitro Study

. Thermal lasers and argon plasma coagulation are widely used in the treatment of stent overgrowth in patients with advanced oesophageal malignancy. The aim of treatment is to achieve patency while avoiding damage to the prosthesis. This experimental study was designed to determine the power and duration of application that can be safely tolerated by four different types of oesophageal prostheses. Five stents were studied: wall stent; open metal mesh stent (uncovered Ultraflex); covered metal mesh stent (covered Ultraflex); Gianturco (Z-stent); Esophagocoil. Nd-YAG Laser, GaAlAs diode laser and argon plasma coagulation were applied in non-contact mode at gradually increasing power levels and duration and the effects were observed. The use of argon plasma coagulation on Esophagocoil stent seems safe in power settings of 100 W up to 10 s. The diode laser is intermediate in that Gianturco and Esophagocoil stents can withstand pulses of up to 50 W for about 2 s. The Nd-YAG laser is detrimental to most stents at power levels of 20 W. Only the Esophagocoil withstands Nd-YAG pulses of 60 W but only up to 1 s. Wallstent, open and membrane-covered mesh stents perform poorly in that they can only tolerate up to 1.5 s of power at 25 W with the Diode and 1.0 s of power at 20 W with Nd-YAG laser. The use of different thermal modalities on the five stents has indicated safe power limits and duration. Membrane-covered stents are always damaged by thermal laser application unless the membrane is truly transparent. Paper received 23 February 2000; accepted after revision 24 March 2000.