Basic mechanisms in laser lithotripsy. 1: Optoacoustic-mechanical analysis

Extensive studies on laser lithotripsy of urinary and gall calculi using a microsecond pulsed dye laser have yielded information on the mechanism of plasma initiation, expansion and shock wave fragmentation relevant to the determination of optimum fragmentation conditions. The key to plasma formation lies in the initial absorption of laser energy by the calculus surface to produce a sufficient density of localized vapour to absorb strongly laser energy. The subsequent ionization of the vapour by the further absorption of laser energy leads to plasma formation. The absorbance of several calculi pigmentations have been measured giving an indication of the individual susceptibility to laser induced breakdown. The plasma threshold is measured to be fluence dependence, which is caused by the vaporization and the further heating of the vapour. The transient expansion of the plasma imparts an impulse on stone and induces a sharp shock wave with peak pressure as high as 10 kilobar leading to the fragmentation of calculi.     

Relation between skin surface temperature and minimal blanching during argon,Nd-YAG 532, and CW dye 585 laser therapy of port-wine stains

Laser photocoagulation has proven to be valuable in the treatment of port-wine stains. In this application, the minimal blanching technique is used as an indicator of suitable dosage since it has been demonstrated that the immediate appearance a white mark is required to achieve permanent blanching a few months later. The objective of the investigations undertaken in this study was to correlate the temperature attained at the surface of port-wine stains with immediate blanching, upon irradiation with different laser fluences. A comparative study was performed using an argon laser (all lines), a 532 nm Nd:YAG and a 585 nm argon pumped dye laser. Surface temperature was studied using an infrared camera. Temperature was measured on 10 different port-wine stains using different fluences. Whitening threshold fluence was related to surface temperature. It appeared that whitening threshold fluence corresponded to a surface temperature of 53°C (±3°C). The whitening threshold fluence was dependent on port-wine stains and wavelength. However, whitening threshold fluence remained lower for 532 nm and 585 nm and it correlated to the absorption curve of hemoglobin. © 1993 Wiley-Liss, Inc.     

Quantitative and ultrastructural studies of excimer laser ablation of the cornea at 193 and 248 nanometers

Excimer laser radiation at 193 nm and 248 nm was used to create linear etch perforations of enucleated calf corneas. The etch depth per pulse was determined for various exposures, and specimens were examined by light and transmission electron microscopy. Compared to 248 nm, excimer laser ablation at 193 nm was found to have a lower threshold for onset of ablation, less increase in etch depth per pulse at increasing fluences, and less structural alteration in adjacent cornea. For 193 nm, structural alterations were minimal, confined to an area less than 0.3 micron wide, and did not increase with increasing fluence. These studies suggest that clinical strategies for excimer laser refractive surgery will employ the 193-nm wavelength, with fluence chosen depending on surgical strategy. Ablation exposures above 600 mJ/cm2 at 193 nm may give the most repeatable etch depth.     

Effects of varying argon ion laser intensity and exposure time on the ablation of atherosclerotic plaque

Using continuous wave (CW) argon ion laser light, a total of 253 laser exposures of varying power (1.5, 3, 5, 8 or 10 W) and duration (20-1,333 ms) were delivered to four segments of human atheromatous aorta obtained at autopsy. Exposure conditions were controlled by using an optically shielded laser catheter that provided a 500 micron spot of light of known power. Two thresholds for consistently reproducible ablation could be defined-an intensity threshold at 25.5 W/mm2 and a fluence threshold at 3.2 J/mm2. Above threshold, a fluence of 5.1 J/mm2 was found to produce the most efficient ablation, ie, removed the greatest volume (mm3) per energy delivered (J) compared to other fluence levels employed (p less than 0.0001). Between aortic segments, however, considerable variability in efficiency (mm3/J) was observed, possibly owing to different optical properties and/or plaque composition. Low-intensity laser radiation produced inconsistent ablation and extensive coagulation effects to surrounding tissue. When a fluence of 5.1 J/mm2 was constructed with a high-intensity laser beam and a short exposure time, consistent and efficient tissue removal resulted without histologic evidence of coagulation necrosis.     

Experimental Cholelitholysis with the Pulsed Tunable Dye Laser

This study evaluates the pulsed tunable dye laser with wavelength 504 nm, frequency 10 Hz, and pulse width 1.2 μs for cholelitholysis. Power of 10–40 kW was directed through a 250-pm quartz fiber optic to ablate 55 gallstones (removed from 14 patients). The fiber was positioned in direct contact with the stones under saline. Power delivery was begun at 10 kW and increased in 10-kW increments until litholysis began. The range of power and energy necessary to fragment the gallstones was evaluated on four common bile ducts fresh autopsy specimens). Following fragmentation, all stones were analyzed. There were 35 cholesterol stones (3 calcified) and 20 bilirubin stones (4 calcified). Size ranged from 0.012 to 7.56 cm³ (mean 0.96 ± 1.41 cm³). Energy necessary for fragmentation ranged from 0.4 to 11.2 J (exposure time 1.0–28 s). Power necessary for fragmentation was 20 kW for 2/55 stones and 40 kW for 53/55 stones. At 40 kW (40 mJ/pulse), common bile duct perforation occurred within 1.1 ± 0.1 s (0.44 ± 0.04 J). The pulsed tunable dye laser can fragment gallstones of all compositions. The threshold for fragmentation is 40 kW, but common bile duct perforation occurs at this power. We conclude that laser radiation sufficient to fragment gallstones can injure the common bile duct.     

Ultra-violet and infra-red laser ablation studies of biocompatible polymers

Polytetrafluroethylene (PTFE) and other biocompatible polymers have been extensively used for sutures, vascular grafts and bone, and other hard tissue replacements. The use of surgical lasers for intervention on teflon-tissue interfaces has attracted a great deal of interest, as both the high intensity pulsed lasers and prosthetic biomaterials are in increasing use. The study of the ablational behaviour of PTFE films with three surgical lasers (CO₂, Nd-YAG and XeCl) have been undertaken for assessing the optimal laser parameters for ablation (e.g. the absorption coefficient and the relevant threshold fluence) from ablation rate measurements.     

Spectral evaluation of laser-induced cell damage with photothermal microscopy

BACKGROUND AND OBJECTIVES: Determining cell photo-damage is important for laser medicine and laser safety standards. This work evaluated the potential of photothermal (PT) technique for studying invasive laser-cell interaction, with a focus on PT evaluation of spectral dependence of laser-induced damage in visible region at single intact cell level. STUDY DESIGN/MATERIALS AND METHODS: PT is based on irradiation of a single intact cells with a tunable pump laser pulse (420-570 nm, 8 nanoseconds, 0.1-300 microJ) and monitoring of temperature-dependent variations of the refractive index with a second, collinear probe beam in pulse (imaging) mode (639 nm, 13 nanoseconds, 10 nJ), and continuous (integrated PT response) mode (633 nm, 2 mW). The local and the integrated PT responses from the individual living red blood cells, lymphocytes, and cancer cells (K562) in vitro were obtained at different pump laser fluence and wavelength and compared with data obtained by conventional viability tests (Annexin V--propidium iodide). RESULTS: The cell damage with pump pulse lead to specific change in PT response's temporal shape and PT image's structure. The photodamage thresholds varied in the range of 0.5-5 J/cm2 for red blood cells, 4.4-42 J/cm2 for lymphocytes, and 36-90 J/cm2 for blast cells in the pump wavelength range of 417-555 nm. CONCLUSION: Damage threshold at different wavelength depends on absorption spectra of cells. Spectral evaluation of laser-damage thresholds can be done in two supplements for each PT mode--PT imaging and integrated PT response. The correlation between specific change of PT parameters and cell damage permits using PT technique to rapidly estimate the invasive conditions of the laser-cell interactions.     

Erbium-YAG and holmium-YAG laser ablation of the lens

Er-YAG (2.9 μm, 200 JJLS pulsewidth) and Ho-YAG (2.12 μm, 250 μS pulsewidth) lasers were used to irradiate bovine crystalline lenses. Mass ablated increased with increasing fluence for both lasers and was greater for the Er-YAG than the Ho-YAG laser at all fluences. The mass loss vs. fluence curve was nonlinear for the Er-YAG and linear for the Ho-YAG laser. Ablation threshold was lower for the Er-YAG. Grossly, the Er-YAG laser produced less charring and expressed fewer, smaller tissue pieces than the Ho-YAG. Scanning electron microscopy indicated that the Er-YAG produced smoother walled craters. The Ho-YAG laser produced more dessicated and disrupted craters. The Er-YAG radiation, delivered by fiberoptic probes, may provide a method of performing minimally invasive cataract surgery. © 1994 Wiley-Liss, Inc.     

Does low penetration of human skin by the normal mode ruby laser account for poor permanent depilatory success rates?

Studies reported to date have shown a good depilatory response from patients treated with the normal mode ruby laser (NMRL) over 12 weeks, but a low response over a time period greater than this. Previous publications have suggested that this could be accounted for by the apparently poor skin penetration of laser light and so this study attempted to assess whether this was indeed the case. Skin samples of varying thicknesses were taken from six Caucasian patients and their depths measured. Each was laid individually on an energy meter before having pulses from an NMRL compatible with clinical doses (4.75 J/cm², 9.24 J/cm² and 13.41 J/cm²) fired on the epidermis. Several samples had the laser fired repetitively on the surface to assess whether this caused any change in laser/skin fluence depth profiles. Repetitive firing of the NMRL on the epidermis of skin samples did not alter the energy recorded by the meter beneath. The fluence/depth profiles were constructed showing the majority of energy was lost within the first 1 mm of the skin surface (50%) which then further reduced over distance but at a much slower rate. The maximum depth of penetration was 14.8 mm (SD±0.478) which appeared to be a function of wavelength and not fluence. The results suggest that laser penetration of skin should be adequate for generating enough heat at the hair bulge and bulb, potentially causing permanent damage. The implications of this study are that it is probably the presence of the correct chromophore in large enough amounts which is required for successful permanent depilation to occur. Paper received 23 December 1999; accepted 7 December 2000.     

Subjects' Dissimilarity in the Analysis of Laser–Tissue Dose–Response Experiments: A Potential Cause for Puzzling Results

The in vivo dose–response laser-tissue experiment is usually performed on several subjects, each exposed several times at different tissue sites. The collected data are then unified into a single statistical batch and analysed under the tacit assumption that the tolerances of all the subjects are similar. However, if this assumption is incorrect the data unification may lead to very biased results. This study reanalyses a raw data set measured by the US Army Medical Research Detachment Walter Reed Army Institute of Research (USAMRD-WRAIR), which was used to study the influence of the laser wavelength on the energy threshold of retinal injury. The USAMRD-WRAIR report reveals a significant variation of threshold with small changes in wavelength. Despite an extensive study, performed by the USAMRD-WRAIR researchers, which included possible lasers variations, many aspects of experimental technique and biological absorption properties of the eye, a cause for the threshold variation was not found. Our current results suggest that unaccounted specimen's dissimilarity might be the cause for this unclear threshold variations. Paper received 4 September 2000; accepted after revision 27 March 2001.     

Excimer laser angioplasty: Quantitative comparison in vitro of three ultraviolet wavelengths on tissue ablation and haemolysis

The effects of three ultraviolet excimer laser wavelengths on normal and atheromatous human cadaver aortic wall are presented. Ultraviolet radiation successfully ablates vessel wall; the dose response is greatest at 249 nm followed by 193 nm and 351 nm, where the effect is negligible for the equivalent energy dose. Wavelengths 249 and 193 nm have a selective effect on fibrous atheroma; ablation proceeds at a higher rate in this tissue. Non-linear effects observed at 249 nm may be due to thermal as well as photoablative mechanisms. In addition, blood samples were exposed to all three wavelengths and potassium concentrations were measured; the dose of energy required to produce tissue ablation may also produce significant haemolysis and hyperkalaemia at 249 and 193 nm but not with 351 nm. This finding may be important for coronary angioplasty but less so for peripheral work. The wavelength that produces the strongest dose response is 249 nm; this is the wavelength for which a fibre-optic delivery system should be developed.     

Effect of pulse duration on selective ablation of atherosclerotic plaque by 480- to 490-nanometer laser radiation

The effect of varying pulse duration on selective ablation of human arterial plaque was evaluated by measuring ablation thresholds and ablation efficiencies on fresh human fibrofatty plaque, calcified plaque, normal intima, deep media, and adventitia using 480- to 490-nm laser radiation at 1, 8, and 50-microseconds pulse durations. For all tissues examined, the ablation threshold energy increased with increasing pulse duration, but the threshold for normal tissue ablation was approximately twice that of plaque at all pulse durations. Ablation efficiency was studied at a fluence that was twice the ablation threshold for plaque. For those fluences, normal intima had detectable ablation only at 50 microseconds. The ablation efficiency of fibrofatty plaque was minimally reduced with prolongation of pulse duration; however, there was a substantial decrease in the ablation efficiency of calcified plaque when the pulse duration was lengthened from 8 to 50 microseconds. These data show significant selective ablation of fibrofatty and calcified plaque at 1, 8, and 50 microseconds and suggest that the optimal pulse width for selective ablation of all plaque in the 480- to 490-waveband is less than 50 microseconds.     

Ultrafast imaging of tissue ablation by a XeCl excimer laser in saline.

To determine the temporal evolution of laser induced tissue ablation, arterial wall specimens with either hard calcified or fatty plaques and normal tissue were irradiated in a 0.9% saline solution using a XeCl excimer laser (wavelength 308 nm, energy fluence 7 J/cm2, pulse width 30 ns) through a 600 microns fused silica fiber pointing perpendicular either at a 0.5 mm distance or in direct contact to the vascular surface. Radiation of a pulsed dye laser (wavelength 580 nm) was used to illuminate the tissue surface. The ablation process and the arising bubble above the tissue surface were recorded with a CCD camera attached to a computer based image-processing system. Spherical cavitation bubbles and small tissue particles emerging from the irradiated area have been recorded. The volume of this bubble increased faster for calcified plaques than for normal tissue.     

Fragmentation of biliary stones with a 308 nm excimer laser

The use of a XeCl excimer laser (308 nm) for biliary stone fragmentation is reported. Laser energy is delivered via UV grade fused silica fibers to the target stones immersed in normal saline solution. Sixty biliary calculi--pigment (n = 40), and cholesterol (n = 20)--were fragmented in vitro. The total energy delivered per unit mass of the stone is kept constant. Two energy fluences (80 and 110 mJ/mm2) at two repetition rates (5 and 20 Hz) delivered through fibers of two core sizes (300 and 600 microns) are utilized to study the effect of different laser parameters on the fragmentation process. Although both pigment and cholesterol stones are susceptible to excimer laser fragmentation, higher fragmentation efficiency is obtained for the pigment stones than for the cholesterol stones. Our study suggests that higher energy fluence and larger fiber core size result in higher fragmentation efficiency for pigment stones. Fragmentation thresholds at stone surface for a variety of biliary calculi of known composition were measured. The threshold energy fluence is approximately 3 mJ/mm2 and 17 mJ/mm2 for pigment and cholesterol stones, respectively. Our study indicates that the 308 nm excimer laser may be effective as a laser lithotriptor with low threshold and good efficiency for biliary stone fragmentation.     

Therapeutic response during pulsed laser treatment of port-wine stains: Dependence on vessel diameter and depth in dermis

Selective photothermolysis with pulsed lasers is presumably the most successful therapy for port-wine stain birthmarks (flammeus nevi). Selectivity is obtained by using an optical wavelength corresponding to high absorption in blood, together with small absorption in tissues. Further on, the pulse length is selected to be long enough to allow heat to diffuse into the vessel wall, but simultaneously short enough to prevent thermal damage to perivascular tissues. The optical wavelength and pulse length are therefore dependent on vessel diameter, vessel wall thickness and depth in dermis. The present work demonstrates that in the case of a 0.45 ms long pulse at 585 nm wavelength, vessels of 40–60m require minimum optical fluence. Smaller vessels require higher fluence because the amount of heat needed to heat the wall becomes a substantial fraction of the absorbed optical energy. Larger vessels also require a higher dose because the attenuation of light in blood prevents the blood in the centre of the lumen from participating in the heating process. It is shown that the commonly used optical dose in the range of 6–7 J cm^–2 is expected to inflict vessel rupture rather than thermolysis in superficially located vessels. The present analysis might serve to draw guidelines for a protocol where the optical energy, wavelength and pulse length are optimized with respect to vessel diameter and depth in dermis.     

Effect of the 1,450 nm diode non-ablative laser on collagen expression in an artificial skin model

BACKGROUND AND OBJECTIVE: The 1,450-nm Smoothbeam Laser is a diode laser equipped with a cryogen cooling spray. Primary objectives were to evaluate the effects of this non-ablative laser on Apligraf (bioengineered skin-substitute) and to document its use as a model for non-ablative procedures. We also measured the effects of laser fluence levels on collagen and elastin expression. STUDY DESIGN/MATERIALS AND METHODS: Three sheets of Apligraf were used for this study. Each received six separate laser applications at 4J, 6J, 8J, 10 J, 12J, and 14J. The sheets were then incubated with 10% CO(2) at 37 degrees C and samples were collected and analyzed 3 days later, using RT-PCR and immunofluorescent staining. RESULTS: Collagen III expressions significantly increased in both mRNA and protein levels at approximately 12 J. CONCLUSIONS: There appears to be a threshold effect where there is very little increased collagen III mRNA and protein expression until the laser fluence reaches around 12J.     

Experimental Study of Different Laser Systems for PMMA Extraction Within the Scope of Revision Hip Arthroplasty

. Cement removal at revision hip arthroplasty forms a critical step for a successful operation. The removal of polymethymethacrylate (PMMA) with curet and chisel can cause major damage to the femoral shaft. The use of ultrasound or lithotripsy can cause perforation and microfractures to bone tissue. The goal of our study was to evaluate the application and practicability of different laser systems for cement removal. We examined and compared the effects of a diode laser (wavelength λ=800 nm), a Nd:YAG laser (λ=1064 nm), and an Er:YSSG laser (λ=2780 nm) on PMMA and the PMMA–bone interface. Whereas the Nd:YAG laser with a high ablation rate led to severe bone damage with extensive carbonisation, the Er:YSSG laser with a low ablation rate produced a defined cut at the PMMA–bone interface (max. depth 3.70 mm at 6.0 W laser power). Using the diode laser a defined high quality ablation of PMMA at the PMMA–bone interface was possible without any visible damage of adjacent tissue (max. depth 2.75 mm at 2.9 W laser power). However, sufficient ablation in an adequate operating time could not be realised with this power. The use of a laser tool for cement removal is insufficient. It only facilitated the manual removal of the remaining cement with chisel and curet. Paper received 5 November 1999; accepted after revision 27 April 2000.     

Diffusing Fibre Tip for the Minimally Invasive Treatment of Liver Tumours by Interstitial Laser Coagulation (ILC): An Experimental Ex Vivo Study

A newly developed diffusing laser applicator was examined for interstitial laser coagulation (ILC) of liver tumours. The applicator consisted of a matted quartz core and a quartz glass dome, also matted on its inner surface and sealed to the fibre. The applicator provided a homogeneous light intensity distribution over an active length of about 20 mm. Lesions were created in an ex-vivo porcine liver model using a Nd-YAG laser comparing the new diffusing tip with a Ringmode^?-ITT applicator in order to find optimal laser parameters and damage thresholds. The lesions were investigated using macroscopic size measurement, volume calculation and histological examination (H&E, NADPH-dehydrogenase). The damage threshold of the diffusing tip was 6 W at 14 min exposure time whereas the Ringmode^?-ITT applicator had its limit at 5 W and 12 min exposure. Comparing various exposure times showed that treatment over a time of more than 840 s did not significantly increase the lesion volume. At 5 W and 720 s the mean lesion volume was 6.9±1.1 cm³ with the diffusing tip and 6.3±0.6 cm³ with the Ringmode^?-ITT applicator, both having a slight ellipsoidal shape. Hence, the created lesions were not significantly different for both applicators when the same laser parameters were applied. On the other hand, the new diffusing tip had a higher damage threshold and was therefore capable of producing maximal coagulation volumes of up to 7.9±0.5 cm³ at 5 W and 20 min. The experiments showed that lesions with a dimension of 31×22 mm can be achieved with the diffusing applicator which seem suitably sized for treating small human liver metastases in a single laser session. Paper received 25 April 1997; accepted after revision 13 March 1998.     

Detection of calcified atherosclerotic plaque by laser-induced plasma emission.

The use of fluorescence spectroscopy to discriminate atherosclerotic from normal tissue is limited by a lower sensitivity for calcified than noncalcified atherosclerotic plaque (65% vs. 93%, respectively). To evaluate plasma emission as a means to detect calcified plaque, 325 normal and atherosclerotic cadaveric aortic sites were irradiated through a 100-micron silica fiber in blood by a pulsed holmium laser (lambda = 2.1 microns, fluence = 4 J/mm2). A photodiode positioned near the proximal end of the fiber detected plasma emission during a laser pulse. Plasma emission was detected at 0% (0/110) of normal, 0% (0/107) of noncalcified atherosclerotic tissue, and 91% (98/108) of calcified atherosclerotic sites. Spectroscopic analysis confirmed the presence of calcium lines in the plasma emission from calcified atherosclerotic plaque. Although ablative fluences (greater than 3 J/mm2) were required for plasma generation, a single laser pulse ablated only to a depth of 67 +/- 16 microns in normal tissue. In an additional 10 calcified atherosclerotic sites, laser ablation was continued as long as plasma emission was detected. In all cases, plaque ablation was terminated before arterial perforation. Furthermore, the adjunctive use of plasma detection improved the accuracy of fluorescence spectroscopic classification of normal and atherosclerotic tissue. In conclusion, plasma detection has a high sensitivity (91%) and specificity (100%) for calcified atherosclerotic plaque and may be a useful adjunct for laser angioplasty guidance. Furthermore, plasma detection can be implemented both simply and inexpensively.     

Treatment of nevus of Ota with the Q-switched alexandrite laser.

BACKGROUND AND OBJECTIVE: Q-switched laser systems have been shown to be useful for removal of nevus of Ota, a pigmented lesion on the face. The purpose of this study was to evaluate the efficacy of Q-switched alexandrite laser in the treatment of nevus of Ota in 13 patients. STUDY DESIGN/MATERIALS AND METHODS: A spot test was made at 6.0, 7.0, and 8.0 J/cm(2) energy fluence, and the best energy fluence was selected after a 2-month postoperative evaluation. Q-switched alexandrite laser with a mean fluence of 7.290.46 J/cm(2) was used at 8-week intervals. Total treatment ranged from 1 to 15 sessions (mean 7.1 +/- 5.1 sessions). The single shot technique was used. RESULTS: More than 75% lightening was achieved in seven patients, between 51% and 75% in three, less than 50% in one, and less than 25% in one. In two patients with excellent clearance, a very light gray macula in the lower eyelid persisted after 8 and 14 treatment sessions, respectively. Mild transient hypopigmentation that subsided spontaneously after 2 months was observed in one patient. No repigmentation was seen. CONCLUSIONS: The Q-switched alexandrite laser seems to be an effective and safe modality for the treatment of nevus of Ota.