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.     

Dependence of the XeCl laser cut rate of plaque on the degree of calcification, laser fluence, and optical pulse duration

A XeCl laser with an optical pulse duration of 35 ns was used to determine the cut depth per laser pulse of postmortem human aorta as a function of laser fluence for four main categories of plaque development. The data indicate that the cut depth per pulse progressively decreases as the degree of calcification increases even at very high (100 mJ/mm2) laser fluences. A comparison was made between the XeCl laser cut rate data obtained using the 35-ns duration laser pulses to data obtained using 200-ns duration pulses for each of the four plaque types. As the degree of tissue calcification increased higher XeCl laser fluences were required for the long pulse case to achieve the same cut depth per pulse as that observed using the shorter pulse duration.     

Effect of force on ablation depth for a XeCl excimer laser beam delivered by an optical fiber in contact with arterial tissue under saline.

The effect of force applied to a 430 micron single fiber, delivering 60 pulses of 308 nm XeCl laser radiation at 20 Hz, on the ablation depth in porcine aortic tissue under saline has been investigated. Energy densities of 8, 15, 25, 28, 31, 37, and 45 mJ/mm2 were used. Force was applied by adding weights from 0 to 10 grams to the fiber. The fiber penetration was monitored by means of a position transducer. At 0 grams, the ablation depth increased linearly with incident energy density, but the fiber did not penetrate the tissue; with any weight added, the fiber penetrated the tissue at energy densities above 15 mJ/mm2. The fiber did not penetrate during the first several pulses, possibly due to gas trapped under the fiber. After these first pulses, a smooth linear advancement of the fiber began, which lasted until the pulse train stopped. The ablation depth increased with increasing energy densities and weights. This effect was largest above 25 mJ/mm2 where the ablation efficiencies (unit mm3/J), with weights added to the fiber, were substantially larger than values found in 308 nm ablation experiments described in the literature, which were conducted with either a focused laser beam or a fiber without additional force. The results imply that in 308 nm excimer laser angioplasty, force must be applied to the beam delivery catheter for efficient recanalization, and that experiments performed with a focused beam or without actual penetration of the fiber do not represent the situation encountered in excimer laser angioplasty.     

Change in laser-induced arterial fluorescence during ablation of atherosclerotic plague

Analysis of the change in arterial fluorescence during plaque ablation may provide the basis for developing a fluorescence-guided ablation system capable of selective plaque ablation without risk of vessel perforation. Accordingly, fluorescence spectra were recorded from 91 normal and 91 atherosclerotic specimens of cadaveric human aorta. The ratio of the laser-induced fluorescence intensity at 382 nm to 430 nm (LIF ratio) was capable of classifying these specimens with an 89% accuracy with a threshold value of 1.8 (atherosclerotic greater than or equal to 1.8, normal less than 1.8). To characterize the change in fluorescence during plaque ablation, mechanical plaque ablation with a cold microtome was performed on 16 atherosclerotic aortic specimens. Fluorescence spectra were recorded serially after each 100 microns of plaque ablation; recordings revealed a change in fluorescence spectra from atherosclerotic to a normal pattern. With an LIF ratio of 1.8 to signal termination of plaque ablation, 15 of the atherosclerotic plaques had a residual plaque thickness less than 200 microns; one specimen had a residual plaque thickness of 300 microns. No specimen demonstrated ablation of the media. There was a statistically significant correlation between LIF ratio and plaque thickness (r = .73, P less than .001), but considerable variation in LIF ratio existed at each thickness. Therefore, laser-induced fluorescence spectroscopy is capable of discriminating atherosclerotic from normal aorta and of signaling completion of plaque ablation.     

In vitro evaluation of ablation parameters of normal and fibrous aorta using smooth excimer laser coronary angioplasty

A modified exeimer laser energy delivery system was used to irradiate 100 segments of normal and fibrous aorta in vitro. The laser beam was scanned into 8 fiber bundles consisting of 50 fibers each resulting in a reduction of the applied pulse energy. The total repetition rate was increased to 150 Hz in order to keep the repetition rate per fiber bundle close to 20 Hz and to minimize thermal injury. The results demonstrate that effective ablation (etch rate per 8 pulses > 2.0 μm) occurred at an energy fluency of 50 mJ/mm² in both normal and fibrous aorta. Tissue damage (carbonization, tissue separation, fissures, cracks, and vacuolization) was in a range of 100 ± 28 to 152 ± 30 μm for normal aorta and in a range of 57 ± 35 to 110 ± 39 μm for fibrous aorta. We conclude that effective ablation of normal and fibrous human aorta can be achieved by the application of smooth excimer laser coronary angioplasty. This improvement of excimer laser technology may result in a reduction of shock wave- and cavitation-induced damage leading to a reduction of tissue injury. However, this awaits further in vitro and in vivo confirmation. © 1993 Wiley-Liss, Inc.     

Correlation of fluorescence emission with the plaque content and intimal thickness of atherosclerotic coronary arteries

The use of fluorescence emission for guidance during laser angioplasty may be limited by the complexity of the emission from the broad range of atherosclerotic plaques normally encountered in disease coronary arteries. Fatty, fibrous, and calcific plaque content as well as maximal intimal thickness were measured and correlated with fluorescence intensity ratios from the emission spectra for a broad cross-section of atherosclerotic plaques from human necropsy specimens. Multiple and stepwise regression analysis was used to analyze intensity ratios of 13 wavelengths between 390 and 600 nm corresponding to regions of observed spectral structure. The level of correlation of the intensity ratios with fatty and calcific plaque content was found to be dependent on the complexity of the atherosclerotic lesion. The fluorescence emission was found to correlate well with both fibrous plaque content and intimal thickness, allowing the differentiation between normal and atherosclerotic samples. In conclusion, plaque characteristics can be assessed by fluorescence emission, although the successful implementation of spectroscopic guidance is dependent on the level of prediction error which may vary with tissue type.     

Design and evaluation of a fiberoptic fluorescence guided laser recanalization system

Current angioplasty techniques for recanalization of totally occluded arteries are limited by the inability to cross the occlusion and by the risk of perforation. A fiberoptic fluorescence guided laser recanalization system was developed and evaluated in vitro for recanalization of 17 human femoral or tibial totally occluded arterial segments (length 1.9-6.8 cm, diameter 2.5-6.0 mm). A 400 or 600 micron silica fiber was coupled to a helium-cadmium laser (lambda = 325 nm) for fluorescence excitation and to a holmium: YAG laser (lambda = 2.1 micron) for tissue ablation. Fluorescence was recorded during recanalization after every other holmium laser pulse. During recanalization, each arterial segment was bent 30-90 degrees with respect to the fiber to simulate arterial tortuosity. Ablation continued with fiber advancement as long as the fluorescence confirmed that the target tissue was atherosclerotic. Arterial spectra were classified as normal or atherosclerotic by an on-line computerized fluorescence classification algorithm (sensitivity 93%, specificity 95%). Normal fluorescence necessitated redirection of the fiber greater than 30 times per segment to continue recanalization. Fifteen of 17 totally occluded arteries had multiple recanalization channels created following total energy delivery of 40-1,016 Joules per segment with no angiographic or histologic evidence of laser perforation. Two heavily calcified arterial occlusions were not recanalized due to inhibition of holmium: YAG laser ablation by the recording of normal fluorescence spectra. Therefore, this fluorescence guided laser recanalization system appears safe and effective for recanalization of totally occluded arteries and merits in vivo evaluation. However, the lower sensitivity of fluorescence detection of heavily calcified plaques may limit the efficacy (but not safety) of fluorescence guided recanalization of heavily calcified occlusions.     

Improved criteria for the recognition of atherosclerotic plaque using fluorescence spectroscopy

To determine the fluorescence pattern for distinguishing normal (N) from calcified and fibrous plaque (P), fluorescence spectra of cadaveric aorta were measured with a spectrofluorometer. Emission (Em) and excitation (Ex) spectra corrected for instrumental response were obtained from 200 to 1000 nm. Specimens from 50 patients were measured less than 24 h after autopsy and then examined histologically. Spectra from 25 specimens demonstrated that the ratio of fluorescence intensity 460 nm/385 nm with Ex=337 nm provided separation of N from P (1.53±29 vs 0.82±0.25,p<0.01) and that a ratio of 1.25 correctly identified all N and P. A prospective test of this ratio on an additional 25 specimens yielded a significant difference between N and P (1.70±0.37 vs 0.87±0.23,p<0.0001) with a value of 1.25 correctly identifying all (10/10) N and 93% (14/15) P. Prospective analysis of previously proposed fluorescence ratios (600 nm/580 nm at Ex=480 nm; 530 nm/550 nm at Ex=459 nm; 448 nm/514 nm and 538 nm/514 nm at Ex=337 nm) all resulted in poor separation of N from P. The ratio of 460 nm/385 nm with Ex=337 nm is superior to previously reported criteria for distinguishing N from P and may be useful for guiding laser angioplasty systems.     

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.     

Characteristics of 308 nm excimer laser activated arterial tissue photoemission under ablative and non-ablative conditions

The present study was designed to assess the characteristics of tissue photoemission obtained from normal and atherosclerotic segments of human postmortem femoral arteries by 308 nm excimer laser irradiation of 60 ns pulsewidth. Three ablative (20, 30, and 40 mJ/pulse) and three non-ablative (2.5, 5, and 10 mJ/pulse) energy fluences were employed. Both the activating laser pulses and the induced photoemission were guided simultaneously over one and the same 1,000 micron core optical fiber that was positioned in direct tissue contact perpendicular to the vascular surface. The spectral lineshape of normal arterial and noncalcified atherosclerotic structures was characterized by a broad-continuum, double-peak emission of relevant intensity between wavelengths of 360 and 500 nm, with the most prominent emission in the range of 400-415 (407 nm peak) and 430-445 nm (437 nm peak). Fibrous and lipid atherosclerotic lesions, however, exhibited a significantly reduced intensity at 437 nm compared to normal artery layers (P less than 0.001), expressed as a 407/437 nm ratio of 1.321 +/- 0.075 for fibrous and 1.392 +/- 0.104 for lipid lesions. Normal artery components presented with approximately equal intensity at both emission peaks (407/437 nm ratio: intima, 1.054 +/- 0.033; media, 1.024 +/- 0.019; adventitia, 0.976 +/- 0.021). Comparison of spectral lineshape obtained under various energy fluences within a group of noncalcified tissues disclosed no substantial difference using the 407/437 nm ratio (P greater than 0.05). In contrast, calcified lesions revealed high-intensity multiple-line (397, 442, 461, and 528 nm) emission spectra under ablative energy fluences, whereas a low-intensity broad-continuum, single-peak spectrum resulted from irradiation beyond the ablation threshold. Thus, these findings suggest fluorescence phenomena for broad-continuum spectra, and plasma emission for multiple-line spectra as an underlying photodynamic process. Regardless of the activating energy fluence, spectral analysis of 308 nm activated photoemission provides accurate information about the laser target under standardized in vitro conditions. It is demonstrated that direct contact ablation and simultaneous spectral imaging of the target tissue via the same optical fiber is feasible.(ABSTRACT TRUNCATED AT 400 WORDS)     

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.     

Fluorescence spectroscopy guidance of laser ablation of atherosclerotic plague

Laser-induced fluorescence (LIF) spectroscopy can only be used for laser angioplasty guidance if high-power laser ablation does not significantly alter the pattern of tissue fluorescence. Although the spectra of normal and atherosclerotic arteries differ, the change in fluorescence spectra following laser angioplasty has not been well studied. Therefore, the purpose of this study was to assess whether laser-induced fluorescence spectroscopy could guide selective laser ablation of atherosclerotic plaque and, if so, to develop a quantitative LIF score that could be used to control a "smart" laser angioplasty system. Baseline LIF spectroscopy of 50 normal and 50 atherosclerotic human aortic specimens was performed using an optical fiber coupled to a He-Cd laser and optical multichannel analyzer. LIF was then serially recorded during erbium:YAG laser ablation of 27 atherosclerotic specimens. Laser ablation was terminated when the arterial LIF spectrum visually appeared normal. Histologic analysis revealed a mean initial plaque thickness of 1,228 +/- 54 microns and mean residual plaque thickness of 198 +/- 27 microns. Ablation of the media occurred in only three specimens. A discriminant function was derived to discriminate atherosclerotic from normal tissue for computer guidance of laser angioplasty. The LIF score, derived from stepwise multivariate linear regression analysis of the LIF spectra, correctly classified 93% of aortic specimens. The spectra obtained from the atherosclerotic specimens subjected to fluorescence-guided laser revealed a change in score from "atherosclerotic" to "normal" following plaque ablation. Seven atherosclerotic specimens were subjected to laser angioplasty with on-line computer control using the LIF score. Mean initial plaque thickness was 1,014 +/- 86 microns, and mean residual plaque thickness was 78 +/- 29 microns. There was no evidence of ablation of the media. Therefore, LIF guidance of laser ablation resulted in minimal residual plaque without arterial perforation. These findings support the feasibility of an LIF-guided laser angioplasty system for selective atherosclerotic plaque ablation.     

Evaluation of a fluorescence feedback system for guidance of laser angioplasty

Background and Objective: Laser-induced fluorescence spectroscopy (LIFS) may be capable of guiding laser angioplasty by discriminating normal and atherosclerotic artery and by determining catheter-tissue environment. Previous optical multichannel analyzer based LIFS systems have been expensive and cumbersome. To simplify LIFS, a system based on photomultiplier tubes was developed and evaluated. Study Design/Materials and Methods: Tissue fluorescence was induced by a helium cadmium laser (wavelength = 325 nm, power = 0.2–0.5 mW), collected by clinical multifiber laser angioplasty catheters and directed through one of two filters (10 nm bandpass, 380 nm or 440 nm peak transmission) to a photomultiplier tube. An LIFS ratio was defined as the relative intensity at 380:440 nm after calibration with an elastin fluorescence spectrum; 157 coronary artery cadaveric specimens were evaluated spectroscopically and histologically. To evaluate the utility of LIFS to optimize catheter position by determining catheter–tissue contact, by determining saline dilution of blood, and by orienting eccentric multifiber catheters a new variable, the total fluorescence intensity (TFI) was defined as the sum of arterial fluorescence intensities at 380 nm and 440 nm. TFI was recorded in vitro through multifiber catheters from 20 arterial specimens in vitro in blood and evaluated as a function of the catheter-to-tissue distance (d) over a range from 0 to 400 μ. Results: Defining normal specimens as those with an intimal thickness ≤200 μ, and atherosclerotic as those with an intimal thickness <200 μ, 47/50 (94%) normal and 85/107 (79%) atherosclerotic specimens were correctly classified using a threshold LIFS ratio of 2.0. Mean (±SE) normal ratio was 1.76±0.02 and mean atherosclerotic ratio was 2.78±0.08 (P ≤ 0.01). The classification accuracy of atherosclerotic specimens increased with intimal thickness so that 95% of atherosclerotic specimens (69/73) with intimal thickness ≥400 μ were correctly classified. TFI was capable of determining catheter-tissue contact as maximal TFI was recorded with the catheter in contact with the tissue (d = 0 μ) and decreased markedly with distance (to 52 ± 6% at d = 100 μ, 19 ± 4% at d = 200 μ, and 3 ± 1% at d = 300 μ). TFI was recorded from ten arterial specimens in blood/saline mixtures ranging in hematocrit from 0% (saline) to 50% (whole blood). TFI was capable of detecting saline hemodilution of blood as TFI decreased markedly at higher hematocrits such that TFI could only by recorded at hematocrits <10 % for catheter-to-tissue distances ≥300 μ. TFI was recorded through eccentric multifiber catheters from 25 arterial specimens and evaluated as a function of the degree of catheter-tissue overlap. TFI was capable of detecting maximal catheter-tissue overlap as TFI correlated linearly with the area (A) of overlap (TFI = 1.12 A + .07, r = 0.92). Conclusions: By discriminating atherosclerotic from normal tissue and by confirming catheter-tissue contact and saline hemodilution, fluorescence feedback should minimize irradiation of normal tissue and/or blood and enhance the safety and efficacy of laser angioplasty. © 1995 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.     

Laser-induced fluorescence: III. Quantitative analysis of atherosclerotic plaque content

Background and Objective: Laser-induced fluorescence (LF) spectroscopic analysis of the chemical composition of atherosclerotic plaque was examined. Study Design/Materials and Methods: The intima of 18 dog aortas was injected with chemical compounds found in atherosclerotic plaque. Spectra were recorded in air prior to and after injection of collagens I, III and IV, elastin, cholesterol, triglyceride, and β-nicotinamide adenine dinucleotide (NADH). Results: Significant changes in LF intensity were detected after injection of collagens I and III, cholesterol and elastin in thoracic aorta (P < 0.001), but not with triglyceride or NADH. Minor changes were detected in abdominal aorta. Multiple regression analysis of LF intensity ratios demonstrated a clear correlation with the quantity of injected collagens I (R² = 0.90–0.99) and III (R² = 0.84–1.0), cholesterol (R² = 0.72–0.76), and triglyceride (R² = 0.68–0.80) in both thoracic and abdominal aorta. The correlation between LF and atherosclerotic plaque composition was confirmed in a rooster model of atherosclerosis where multiple regression analysis predicted the measured aortic cholesterol (R² = 0.78) and triglyceride content (R² = 0.96). Conclusions: (1) Fluorescence spectra recorded from dog aorta were significantly altered by injection of collagens I and III, cholesterol, and elastin. (2) LF may allow quantitative assessment of plaque chemical content. © 1995 Wiley-Liss, Inc.     

XeCl laser ablation of atherosclerotic aorta: optical properties and energy pathways.

The energetics of 308-nm excimer laser irradiation of human aorta were studied. The heat generation that occurred during laser irradiation of atherosclerotic aorta equaled the absorbed laser energy minus the fraction of energy for escaping fluorescence (0.8-1.6%) and photochemical decomposition (2%). The absorbed laser energy is equal to the total delivered light energy minus the energy lost as specular reflectance (2.4%, air/tissue) and diffuse reflectance (11.5-15.5%). Overall, about 79-83.5% of the delivered light energy was converted to heat. We conclude that the mechanism of XeCl laser ablation of soft tissue involves thermal overheating of the irradiated volume with subsequent explosive vaporization. The optical properties of normal wall of human aorta and fibrous plaque, both native and denatured were determined. The light scattering was significant and sufficient to cause a subsurface fluence (J/cm2) in native aorta that equaled 1.8 times the broad-beam radiant exposure, phi o (2.7 phi o for denatured aorta). An optical fiber must have a diameter of at least 800 microns to achieve a maximum light penetration (approximately 200 microns for phi o/e) in the aorta along the central axis of the beam.     

Potential use of holmium lasers for angioplasty: evaluation of a new solid-state laser for ablation of atherosclerotic plaque

Tissue effects of the mid-IR Holmium laser (emitting at a wave-length of 2130 nm) were evaluated. This wavelength is attractive because it combines high water absorption and easy transmission through standard optical fibres. The laser was pulsed with pulse durations in the range of 100 microseconds and repetition rates between 2 and 6 Hertz. For all experiments a repetition rate of 2 Hertz was used. The laser beam was coupled into waterfree quartz fibers with core diameters of 200 and 800 microns with an efficiency of 70 and 80%, respectively. Ablation of atherosclerotic plaque has been performed at an ablation threshold of 10J/cm2 for the 800 microns and 40J/cm2 for the 200 microns fibre. Removal of calcified plaque was possible. Ablation efficiency increased in a non-linear fashion with increasing pulse energies. The ablation rate per pulse was approximately 2 mm at energy fluences of 1000J/cm2 for the 200 microns fibre and 1.25 mm at energy fluences of 70J/cm2 for the 800 microns fibre; a further increase in energy densities did not result in higher ablation rates. On macroscopic examination only very limited thermal 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 histologic specimens revealed zones of thermal damage extending 100 up to 1000 microns lateral into adjacent tissue. Thermal damage increased with increasing radiant exposures and depended on the medium used.     

Effect of blood upon the selective ablation of atherosclerotic plaque with a pulsed dye laser

Laser angioplasty systems with laser energy preferentially absorbed by atherosclerotic plaque may offer a safe method of plaque removal. This study evaluated the effect of blood upon selective energy absorption using a pulsed dye laser at 480 nm. Intra-arterial laser irradiation of normal rabbit femoral arteries demonstrated a perforation threshold energy with blood perfusion of 13.1 mJ per pulse compared to 87.9 mJ with saline (P less than .0001), indicating a deleterious effect in the presence of blood. An adverse effect upon arterial healing at 3 days was noted in sheep following intra-arterial irradiation during blood but not saline perfusion. Normal and atherosclerotic human aorta ablation thresholds differed significantly (P less than .0002) under saline (plaque: 20 mJ and normal: 120 mJ) but the difference under blood (plaque: 5 mJ and normal: 20 mJ) was not significant. We conclude that absorption of laser energy by blood can reduce the effect of differential absorption by endogenous chromophores in normal and pathologic vascular tissues and, therefore, removal of blood may be a prerequisite for selective ablation of atherosclerotic plaques.