Excimer laser irradiation of non-calcified and calcified human atheroma (248 and 308nm wavelengths)

In order to develop a system of peripheral arterial angioplasty, we carried out an in vitro study to define the quantitative, thermal and morphological characteristics of human-atheroma ablation by excimer laser. A multigas ‘Sopra’ laser was used. The study was performed by using 248nm, krypton fluoride (KrF), then 308nm, xenon chloride (XeCl) wavelengths. The delivered energy was up to 150 mJ pulse⁻¹, pulse duration was 25ns, and the repetition rate could be adjusted to up to 20Hz. Irradiated tissue segments of the superficial femoral and external iliac arteries were obtained in man during surgical procedures and were both calcified and non-calcified atherosclerotic lesions. Quantitative measurements showed a linear increase of ablated tissue mass depending on the energy delivered. For the same energy, the loss of mass was greater with the 248nm wavelength than with the 308nm. The maximum temperature rise measured at the site of irradiation was 6°C at 248nm and 25°C at 308nm. Histological analysis of the irradiated segments revealed neat and precise ablation without thermal injury of adjacent tissue. At 248nm, this phenomenon was observed for calcified as well as non-calcified atheromas. It is concluded that quantitative, thermal and morphological characteristics of in vitro ablation of calcified and non-calcified human atheroma by excimer laser are compatible to clinical requirements. The results observed at 248nm were experimentally more satisfactory.     

The pulsed dye laser and atherosclerotic vascular disease

The use of a pulsed dye laser to ablate atheromatous tissue obtained from post-mortem human aortic specimens is reported. Laser energy was delivered with a 600 micron quartz fibre, at a wavelength of 504 nm and a pulse length of 1 microseconds. Pulse energy was varied from 30-140 mJ, producing peak pulse powers of the order of 100 kW. With these parameters the laser ablated fatty, fibrous and calcified plaques. At this wavelength atheroma is vaporized but there is minimal damage to normal vessel wall, due to preferential absorption of the laser light. Light microscopy shows that by microsecond pulsing, thermal damage to surrounding tissues associated with continuous wave lasers is avoided. Transmission electron micrographs reveal a sharp demarcation between a laser crater and the adjacent vessel wall with little ultrastructural disruption. Scanning electron micrographs show the crater walls to be smooth. The pulsed dye laser may therefore be effective in the treatment of occlusive peripheral vascular disease without undue risk of vessel perforation.     

Sapphire probe laser ablation of human arteries with CO2 gas and saline perfusion: The effect of flow rate,lasing power and lasing time

To evaluate flow rate dependence of CO₂ gas and saline perfusion for sapphire probe ablation, all together 204 human arterial specimens of atheroma and normal vessel were irradiated with Nd-YAG laser, in an experimental circulation-occlusion model within 37°C flowing whole blood. During lasing procedures, various flow rates of CO₂ gas (0.2–2.01 min⁻¹) and saline (2.0–20.0 ml min⁻¹), and various lasing powers (7, 12 and 17 W) and lasing time (1–20 s) were used. Histological changes of all specimens irradiated were microscopically examined. The results showed that the laser ablation area enlarged with increasing CO₂ flow rates and decreasing saline flow rates. Relative ablation efficiency on atheromatous plaque, in comparison to those on normal vessel wall and surrounding tissue site, increased slightly with increasing lasing power and lasing time. In this experimental setting, the mode of action of the sapphire probe ablation on human arterial atheroma seems to be more satisfactory with CO₂ gas perfusion than with saline perfusion.     

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.     

Pulsed versus continuous wave Nd-YAG laser-induced necrosis: Comparison in the rat liver in vivo

It is assumed that a pulsed Nd-YAG laser will avoid heat diffusion in tissue and thus produce more predictable and less deep lesions. The aim of this study was to compare lesions induced in an homogeneous tissue by the pulsed wave (PW) and the continuous wave (CW) modes of the Nd-YAG laser. Single laser shots were delivered to the liver surface of anaesthetized rats after laparotomy, in vivo. In experiment 1, the quartz fibre was handled close to the liver surface. Energies of 10, 20, 40 J were applied. In experiment 2, the quartz fibre was fixed at a distance from the liver and the laser beam was focused through a handpiece, to obtain a spot of 3 mm in diameter at the liver surface. Energies of 20, 40, 80 J were applied. In both studies, four or five shots were performed for each parameter with each laser mode. After excision of the liver, the maximal depth and width of the crater, the necrotic area and the total affected tissue were measured for each lesion. In experiment 1, there was no difference in any dimension of lesion between the two modes. In experiment 2, the only statistical difference was observed at the fluence of 566 J cm^–2 where the necrotic area as well as the total lesion were deeper with the pulsed mode. This difference was not observed for the crater. In this experimental model depths and widths of the different layers of the lesion induced by the PW mode were comparable to those obtained with the CW mode. This PW mode of Nd-YAG laser does not prevent heat diffusion in tissue.     

Nd-YAG laser photocoagulation of canine myocardium with the transparent contact probe

Laser photocoagulation of myocardium is an alternative to surgical resection in the treatment of drug resistant ventricular tachycardia. In certain areas, of the heart, however, bare fibre delivery of laser energy involves a risk of unintentional damage to nearby structures. The purpose of the study was to determine whether Nd-YAG laser irradiation, delivered with the transparent contact probe, would produce adequate laser photocoagulation of the canine myocardium in comparison to bare fibre delivery of the laser energy. In nine mongrel dogs, continuous wave Nd-YAG laser irradiation with and without a transparent contact probe was directed at the epicardium. Pulse power was 10, 15 and 20 W, pulse duration 5, 7 and 10 s, and spot size was 1 mm. A total of 178 lesions were analyzed microscopically. After a 200 J pulse energy delivered by the contact probe, the lesion depth was 4.9±0.5 mm (mean±s.d.), which is usually adequate to ablate arrhythmia sites. Bare fibre delivery of laser energy did not produce deeper lesions. There was no difference between the bare fibre and the transparent probe in the occurrence of major arrhythmias (4/86 bare fibre, 3/92 transparent probe). We conclude that the transparent contact probe allows safe and effective laser irradiation of sites of origin of ventricular arrhythmias.     

Autofluorescence spectroscopy using a XeCl excimer laser system for simultaneous plaque ablation and fluorescence excitation

Laser–induced fluorescence may be used to guide laser ablation of atherosclerotic lesions. This study was performed to evaluate arterial autofluorescence spectroscopy in vitro using a single XeCl excimer laser (308 nm) for simultaneous tissue ablation and fluorescence excitation. The laser beam was coupled to a 600-μm silica fiber transmitting 40–50 mJ/mm² per pulse. The fluorescence radiation emanating retrogradely from the fiber was collected by a concave mirror for spectroscopic analysis over a range of 321–657 nm. The arterial media (n = 26), lipid plaques (n = 26), and calcified lesions (n = 27) of aortic specimens from ten human cadavers were investigated in air, saline, and blood. Whereas the spectrum of calcified lesions changed with the surrounding optical medium, the other spectra remained constant. In air and blood, the spectra of arterial media, lipid plaques, and calcified lesions could be differentiated qualitatively and quantitatively (P < 0.0001). In saline, there was no clearcut spectroscopic difference between lipid plaques and calcified lesions. However, normal arterial media and atherosclerotic lesions (lipid plaques plus calcified lesions) could still be discriminated. Thus spectroscopy and plaque ablation can be combined using a single XeCl excimer laser. These encouraging results should stimulate further studies to determine the potential use of this approach to guide laser angioplasty in humans. © 1994 Wiley-Liss, Inc.     

Occurrence,extent, and implications of pressure waves during excimer laser ablation of normal arterial wall and atherosclerotic plaque

Ablation of atherosclerotic plaque and normal arterial wall was performed using a Xenon-Chloride Excimer laser with a wavelength of 308 nm and a pulse duration of 115 ns. The light was transmitted via a 600 μm bare fibre and adjusted to an energy density of 3.5J/cm². The acoustic signals generated by the laser pulse were measured with two types of hydrophones consisting of polyvinylidenefluoride with active diameters of 0.3 mm and 0.5 mm and recorded on a dual channel digital storage oscilloscope using either a 0.5 m coaxial cable or a broadband fibre-optic transmission system. Tissue was retrieved from nine cadaver human aortas and macroscopically classified as either normal or calcified atherosclerotic plaque. Histological analysis (Haematoxylin eosin, elastica van Gieson, and immunohistochemical staining) was carried out after the experiments to verify the macroscopic diagnosis and to correlate the acoustic responses with the tissue characteristics. For normal arterial wall, maximum peak pressure was 1.28 MPa ± 0.85 MPa, rise time 163 ns ± 43 ns, and pressure increase 8,2k Pa ± 5,4k Pa/ns. For calcified, atheromatous segments, a maximum peak pressure of 2,02 MPa ± 1,16 MPa, a rise time of 69,9 ns ± 25,8 ns, and a pressure increase of 32,3 kPa ± 21,3 kPa/ns was found. Statistical analysis showed a significant shorter rise time (P < 0.0001) and a higher pressure increase (P < 0.0001) for calcified tissue in comparison to normal arterial wall, whereas maximum pressures alone did not allow a differentiation of tissue characteristics. Several hundred kPa are generated during Excimer laser ablation. The results suggest that focal tissue fragmentation is one mechanism of plaque ablation. A differentiation of tissue characteristics is possible by analysis of rise time and pressure increase, potentially providing the possibility of acoustic ablation control. © 1993 Wiley-Liss, Inc.     

Continuous wave (CW) and pulsed laser effects on vascular tissues and occlusive disease in vitro

Human arterial segments with occlusive defects and acute dog hearts were exposed, in vitro, to high-energy pulsed and continuous wave (CW) laser beams at argon (514 nm) and Nd-YAG (1,064 nm) wavelengths, using various pulse powers, durations and pulse repetition rates. The laser effects included vaporization of plaques in the arterial segments and penetration of the pericardial sac, evaporation of pericardial fluid, and discoloration of tissue with crater-like lesions in the impact zone, all as a result of vaporization of heart muscle tissues. The areas affected and depth of penetration depended on the wavelength, power, pulse duration, and mode of energy deposition. Focused nanosecond Nd-YAG laser pulses at repetition rates of 40-50 Hz caused ablation or vaporization of hard plaques and kidney stones in air and saline. Picosecond (mode-locked) argon laser pulses at repetition rates of 3.8 MHz--average power 6.5 W, peak power of 230 W--caused effective vaporization of hard plaques and kidney stones in air and saline. Picosecond argon laser pulses--average power 1 W, peak power 250 W--were not effective in vaporization. Transmission characteristics of the various types of laser pulses through fiber optic waveguides were determined. The energy and power density required to vaporize fatty and hard plaques and kidney stones were tabulated as a function of laser wavelength, pulse energy, duration, and repetition rates.     

Dependence of laser photocoagulation on interstitial delivery parameters

Photocoagulation was performed ex vivo between tissue slabs by delivering continuous-wave laser energy from an optical fiber either directly, or by depositing the energy into a 2.4 mm diameter steel sphere at the fiber tip. The dependence of photocoagulation lesions on the following variables was assessed: (1) energy source: Nd:YAG-532 nm, 1,064 nm ± steel sphere, (2) tissue type: porcine muscle (light), bovine muscle (dark), (3) delivered power: P = 1.5–3.0 W (porcine), 1.0–2.5 W (bovine), (4) exposure duration: T = 300–1500 s. The resulting cross-sectional photocoagulation lesions are summarized as follows: 532 nm: elongated; central charring in all cases; 1,064 nm: circular; central charring only in bovine for P ? 2.0 W, T ? 500 s; sphere: circular; central charring in bovine for P ? 1.5 W and porcine for P ? 2.0 W. These experiments suggest photocoagulation lesion size decreases as optical penetration increases. The results indicate that interstitial laser photocoagulation lesions >10 mm diameter can be made without charring in both lightly and heavily pigmented tissues ex vivo by delivering 1,064 nm laser energy at sufficiently low power for at least 1,000 s from well-polished optical fibers. © 1994 Wiley-Liss, Inc.     

The effect of pulsed holmium-YAG laser on in vitro and in vivo atherosclerotic plaque

Laser application for atherosclerotic ablation is still limited. The pulsed Holmium-YAG (HO-YAG) laser has physical characteristics which may improve vascular recanalization. We therefore examined the effect of this laser on cadaver human atherosclerotic aortae, human amputated legs and atherosclerotic rabbits in vivo. The pulsed HO-YAG laser successfully ablated calcific and fibrotic aortic segments. Totally occluded arteries in amputated legs including calcified atherosclerotic lesions were successfully recanalized using 165–350 pulses of 0.35–0.4 J energy per pulse transmitted through commercially available fibre optics. Percutaneous delivery of laser energy to the descending aorta of atherosclerotic rabbits was not traumatic to the arterial wall. These results demonstrate the advantages of the pulsed HO-YAG laser to ablate fibrotic and calcific atheroma and to safely recanalize occluded arteries.     

Ablation of porcine ligamentum flavum with Ho:YAG,q‐switched Ho:YAG,and quadrupled Nd:YAG lasers

Background and Objectives

Ligamentum flavum (LF) is a tough, rubbery connective tissue providing a portion of the ligamentous stability to the spinal column, and in its hypertrophied state forms a significant compressive pathology in degenerative spinal stenosis. The interaction of lasers and this biological tissue have not been thoroughly studied. Technological advances improving endoscopic surgical access to the spinal canal makes selective removal of LF using small, flexible tools such as laser‐coupled fiber optics increasingly attractive for treatment of debilitating spinal stenosis. Testing was performed to assess the effect of Ho:YAG, Q‐switched Ho:YAG, and frequency quadrupled Nd:YAG lasers on samples of porcine LF. The objective was to evaluate the suitability of these lasers for surgical removal of LF.

Study Design/Materials and Methods

LF was resected from porcine spine within 2 hours of sacrifice and stored in saline until immediately prior to laser irradiation, which occurred within an additional 2 hours. The optical absorbance of a sample was measured over the spectral band from 190 to 2,360 nm both before and after dehydration. For the experiments using the Ho:YAG (λ = 2,080 nm, t_p = 140 µs, FWHM) and Q‐Switched Ho:YAG (λ = 2,080 nm, t_p = 260 ns, FWHM) lasers, energy was delivered to the LF through a laser‐fiber optic with 600 µm core and NA = 0.39. For the experiment using the frequency quadrupled Nd:YAG laser (λ = 266 nm, t_p = 5 ns FWHM), rather than applying the laser energy through a laser‐fiber, the energy was focused through an aperture and lens directly onto the LF. Five experiments were conducted to evaluate the effect of the given lasers on LF. First, using the Ho:YAG laser, the single‐pulse laser‐hole depth versus laser fluence was measured with the laser‐fiber in direct contact with the LF (1 g force) and with a standoff distance of 1 mm between the laser‐fiber face and the LF. Second, with the LF remaining in situ and the spine bisected along the coronal plane, the surface temperature of the LF was measured with an IR camera during irradiation with the Ho:YAG laser, with and without constant saline flush. Third, the mass loss was measured over the course of 450 Ho:YAG pulses. Fourth, hole depth and temperature were measured over 30 pulses of fixed fluence from the Ho:YAG and Q‐Switched Ho:YAG lasers. Fifth, the ablation rate and surface temperature were measured as a function of fluence from the Nd:YAG laser. Several LF staining and hole‐depth measurement techniques were also explored.

Results

Aside from the expected absorbance peaks corresponding to the water in the LF, the most significant peaks in absorbance were located in the spectral band from 190 to 290 nm and persisted after the tissue was dehydrated. In the first experiment, using the Ho:YAG laser and with the laser‐fiber in direct contact with the LF, the lowest single‐pulse fluence for which LF was visibly removed was 35 J/cm². Testing was conducted at 6 fluences between 35 and 354 J/cm². Over this range the single‐pulse hole depth was shown to be near linear (R² = 0.9374, M = 1.6), ranging from 40 to 639 µm (N = 3). For the case where the laser‐fiber face was displaced 1 mm from the LF surface, the lowest single‐pulse fluence for which tissue was visibly removed was 72 J/cm². Testing was conducted at 4 energy densities between 72 and 180 J/cm². Over this range the single‐pulse hole depth was shown to be near linear (R² = 0.8951, M = 1.4), ranging from 31 to 220 µm (N = 3). In the second experiment, with LF in situ, constant flushing with room temperature saline was shown to drastically reduce surface temperature during exposure to Ho:YAG at 5 Hz with the laser‐fiber in direct contact with the LF. Without saline, over 1 minute of treatment with a per‐pulse fluence of 141 mJ/cm², the average maximum surface temperature measured 110°C. With 10 cc's of saline flushed over 1 minute and a per‐pulse laser fluence of 212 mJ/cm², the average maximum surface temperature was 35°C. In the third experiment, mass loss was shown to be linear over 450 pulses of 600 mJ from the Ho:YAG laser (212 J/cm², direct contact, N = 4; 108 J/cm², 1 mm standoff, N = 4). With the laser‐fiber in direct contact, an average of 53 mg was removed (R² = 0.996, M = 0.117) and with 1 mm laser‐fiber standoff, an average of 44 mg was removed (R² = 0.9988, M = 0.097). In the fourth experiment, 30 pulses of the Ho:YAG and Q‐Switched Ho:YAG lasers at 1 mm standoff, and 5 Hz produced similar hole depths for the tested fluences of 9 J/cm² (151 and 154 µm, respectively) and 18 J/cm² (470 and 442 µm, respectively), though the Ho:YAG laser produced significantly more carbonization around the rim of the laser‐hole. The increased carbonization was corroborated by higher measured LF temperature. In all tests with the Ho:YAG and Q‐Switched Ho:YAG, an audible photo‐acoustic affect coincided with the laser pulse. In the fifth experiment, with the frequency quadrupled Nd:YAG laser at 15 Hz for 450 pulses, ablation depth per pulse was shown to be linear for the fluence range of 0.18 – 0.73 J/cm² (R² = 0.989, M = 2.4). There was no noticeable photo‐acoustic effect nor charring around the rim of the laser‐hole.

Conclusion

The Ho:YAG, Q‐Switched Ho:YAG, and frequency quadrupled Nd:YAG lasers were shown to remove ligamentum flavum (LF). A single pulse of the Ho:YAG laser was shown to cause tearing of the tissue and a large zone of necrosis surrounding the laser‐hole. Multiple pulses of the Ho:YAG and Q‐Switched Ho:YAG lasers caused charring around the rim of the laser‐hole, though the extent of charring was more extensive with the Ho:YAG laser. Charring caused by the Ho:YAG laser was shown to be mitigated by continuously flushing the affected LF with saline during irradiation. The Nd:YAG laser was shown to ablate LF with no gross visible indication of thermal damage to surrounding LF. Lasers Surg. Med. 47:839–851, 2015. © 2015 The Authors. Lasers in Surgery and Medicine Published by Wiley Periodicals, Inc.     

Comparative study of Nd-YAG laser versus electrocautery for liver metastatic resection in the rat

The thermal, hemostatic and lymphostatic effects of the Nd-YAG laser suggest a benefit in the treatment of multiple liver metastases. The aim of this work was to evaluate experimentally this hypothesis in a comparative study with conventional electrocautery resection of liver metastases. The original animal model was represented by syngeneic BDIX rats inoculated under the liver capsule with 1.5×10⁶ DHD/K12 tumour cells originating from a clone of a 1,2 dimethyl hydrazine induced rat colon cancer. One hundred and ten rats bearing three liver metastases were randomly treated by laser volatilization or electrocautery enucleation. The first group of 60 rats was used as a survival group: the laser treated rats survived significantly longer than rats treated by cautery (49.9 days vs 28.9 days) (mean values;p<0.015). In this group, the temperature elevation during operation at the edge of the treated lesion was found higher in the laser group than in the cautery group (56±4.6°C vs 8±3°C) (mean values±s.d.;p<0.001). Nd-YAG laser was also a faster procedure than cautery resection (21±4.2 s vs 57±6.1 s) (mean values±s.d.;p<0.001). At the time of autopsy, the infection rate with laser was found lower than in the cautery group (p<0.025) while no bile leakage was evident. A peritoneal tumour dissemination with ascites was noted in the majority of dead rats. In the second group of 50 rats, the metastatic recurrence was assessed. At day 7, no metastases were found in the laser treated rats while a mean number of 6.5 was found in the cautery group (p<0.001). At the 15th day, more metastases were present in the cautery group. There was a significant correlation between the total number of metastases and the time of death. Those findings suggest that the Nd-YAG laser destruction of experimental liver metastases by its specific effects on tumour cells delayed the recurrence of metastases when compared to the electrocautery resection, contributing to a longer survival of the laser treated rats.     

Comparison of continuous-wave and pulsed excitation for interstitial neodymiurn-YAG laser-induced hyperthermia

To investigate the effect of pulsing on neodymium-YAG laser-induced hyperthermia we have exposed rat liver to low-power Nd-YAG laser light delivered via an interstitially inserted fibre. This was either continuous wave excitation or pulsed excitation at 10 or 40 Hz (pulse duration 100s) with an average power of 1W and exposure durations of 400 s. No differences were seen with respect to overall diameter of the histological damage, diameter of the central cavitation, or intrahepatic temperatures, as measured by an embedded array of microthermocouples. We conclude that with 100-s pulses, within the range of parameters studied, the pulsing rate does not influence the nature or the extent of damage seen after low-power interstitial Nd-YAG laser hyperthermia.     

Alteration of pulse configuration affects the pain response during diode laser photocoagulation

Background and Objective: The shape of the treatment pulse of the diode laser (810 nm) can be easily altered electronically in contrast to ion laser photocoagulators. We investigated whether changes in laser pulse shape influenced the subjective pain response in patients undergoing retinal photocoagulation when only topical anesthesia was used. Study Design/Materials and Methods: Twenty consecutive patients required peripheral retinal photocoagulation for proliferative diabetic retinopathy or extensive retinal breaks. Three diode pulse waveforms including a square wave, shaped-wave, and an envelope of micropulses were compared to one another. Power was adjusted so that each waveform delivered the same total energy. The patients subjectively ranked the intensity of any pain they experienced for each group of lesions. Responses were compared to one another using an analysis of variance. Results: 40% of patients found the standard square wave pulse to be significantly more painful (P < 0.05) than the shaped pulse mode and 30% found the square wave significantly more painful (P < 0.05) than the micropulse mode. Conclusion: Modification of the laser pulse waveform may ameliorate pain induced by diode laser photocoagulation of the retinal periphery. © 1995 Wiley-Liss, Inc.     

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.     

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.     

Formation of pressure waves during in vitro excimer laser irradiation in whole blood and the effect of dilution with contrast media and saline

Pressure waves during excimer laser ablation of vascular tissue may be responsible for complications of coronary excimer laser angioplasty. In this experimental study, pressure waves were measured during excimer laser irradiation in blood and contrast media using a polyvinilidenefluoride hydrophone. At a distance of 4 mm lateral to the tip of a 1.7 mm multifiber laser catheter, excimer laser irradiation in blood resulted in a linear increase of peak pressures from 1,365 ± 165 kPa at 30 mJ/mm² to 2,866 ± 404 kPa at 60 mJ/mm². In contrast media, peak pressure increased from 3,172 ± 573 kPa (30 mJ/mm²) to 5,763 ± 467 kPa (60 mJ/mm²). Contrast media and saline were added to blood. At a concentration of 60% contrast in blood, a 3.4 fold increase of peak pressures was documented as compared to pure blood. Further increase of the concentration did not result in higher pressure waves. Concentrations of saline in blood of 90% and 96% reduced the peak pressures by 16% and >50%, respectively, as compared to pure blood. © 1994 Wiley-Liss, Inc.     

Atherosclerotic tissue analysis by time-resolved XeCl excimer laser reflectometry

The temporal modification of XeCl laser pulses reflected from human aorta tissue immersed in saline has been studied. Dynamic tissue reflectivity of both normal and atherosclerotic tissues has been examined for various incident pulse fluences between 0.7 and 6.5 J/cm². Changes in reflected pulse duration are observed for fluences at or above 2.6 J/cm² with normal tissue targets and 3.0 J/cm² with calcified plaque. Such reflected pulse analysis may prove useful in identifying tissue targets for ablation during laser angioplasty. © 1993 Wiley-Liss, Inc.

1 This article is a US Government work and, such, is in the public domain in the United States of America.

     

Effects of pulse width on erbium: YAG laser photothermal trabecular ablation (LTA)

An erbium (Er):YAG laser can remove trabecular meshwork (TM) by photothermal ablation with minimal contiguous thermal damage. A variable pulse width Er:YAG laser was used to investigate the effect of varying pulse width on ablation of human TM. Trabecular photothermal ablation was performed on tissue obtained from eye bank eyes at pulse widths of 50, 150, and 250μs, with energy held constant at 4 mJ. At this energy, a single laser pulse was sufficient for full-thickness ablation of TM. Laser energy was delivered through a 200-μm diameter optical fiber held in apposition to the tissue sample, which was immersed in physiologic saline. High-speed photography of the resultant steam bubbles also was performed. Light microscopy and scanning electron microscopy of TM ablated at 50 μs revealed the greatest variability in size (0–140 μm) of the full-thickness ablated areas and demonstrated blast effects, tissue shredding and ?10 μm thermal damage. At 150 μs, the full-thickness ablated areas were more consistent in size (115–120 μm), showed no blast effects and 10 to 20 μm thermal damage. At 250 μs, the largest ablations were found (180–220 μm) and showed no blast damage; however, a significant amount of thermal damage (?50 μm) was evident. The steam bubbles produced by the laser energy were largest at 50 μs and did not begin to collapse until well over twice the original pulse interval. At 150 and 250 μs, the steam bubbles were successively smaller and dissipated at the end of the laser pulse. In single pulse Er:YAG photothermal laser trabecular ablation, a pulse width (total energy of 4 mJ) around 150 μs appears to be optimal. The resultant acoustic shock wave from steam bubble formation is smaller, its duration does not exceed the laser pulse width and tissue thermal damage is minimal. © 1993 Wiley-Liss, Inc.