Comparison of ablation efficacy of excimer, pulsed-dye, and holmium-YAG lasers relevant to shock waves

Shaping photomechanical effects in tissue ablation using 355 nm laser pulses

We investigate the influence of the cladding diameter of an optical delivery fiber on the ablation dynamics of porcine aorta immersed in tetracycline antibiotic solution using 355 nm nanosecond pulses. We manipulate the pressure transients by enforcing a rear rigid interface (applied by an enlargement of the cladding diameter) to the ablated area, which leads to enhanced ablation efficiency along with a reduction in tissue disruption effects. Numerical simulations, based on the finite elements method, are used to study the propagation of the pressure transients within the suggested scheme. Ultrasonic transducers are used for measuring the increased pressure in front of the fiber's facet and the reduced pressure at the fiber's circumference in the presence of large diameter cladding. The increase and decrease are both found to be by a factor of ˜1.8. The width of the cavitation bubble is measured by high-speed photography. An enlargement of 13.8% is demonstrated, at the expense of backward expansion along the fiber's axis. A histopathological in vitro study demonstrates an average enhancement of 12.27% in the diameter of the ablated crater, as well as significant reduction in the disruption effects. Our study sheds light on the potential to improve the ablation efficiency without additional energy cost, along with attaining improved safety for interventional medical procedures.

Laser-induced shock wave endothelial cell injury

BACKGROUND AND OBJECTIVE

Several laser procedures, extracorporeal lithotripsies (ESWL), and high-velocity missile trauma generate pressure transients that are transmitted through the tissues. Despite several publications demonstrating shock wave-induced tissue injury, little is known about its pathophysiology. This study introduces an in vitro model for studying shock wave effects on endothelial cell (EC) monolayers.

STUDY DESIGN/MATERIALS AND METHODS

A Nd:YAG laser-driven flyer-plate technique was used to generate shock waves. Physical characteristics were determined with a pressure transducer, a high-speed video camera, and sequential photography. Biological effects were studied with phase contrast and lightfield microscopy, computerized morphometry, immunocytochemistry, spectrophotometry, and enzyme-linked immunosorbent assay (ELISA).

RESULTS

The shock waves generated were highly reproducible. Cavitation was verified and quantified, and its extent could be varied in the vials. Exposed cultures exhibited areas with cell membrane damage and cell detachment. Release of LD was elevated (P < 0.01) in exposed vials. The EC lesions were larger (>P < 0.01) in cultures submitted to high vs. low extent of cavitation.

CONCLUSION

The flyer-plate model can be used to subject cell monolayers to defined and reproducible shock waves causing immediate cell injury similar to the previously reported vascular lesions associated with ESWL, pulsed lasers, and blast trauma. With the flyer-plate model, such lesions may be further studied on the cellular and subcellular levels.

In vitro tissue effects of a combined Ho:YAG/Nd:YAG laser: sprinkling of tissue fragments by Ho:YAG laser light may be problematic for oncological interventions

BACKGROUND AND OBJECTIVE

Surgery of soft tissue, for example, of the tongue or the liver, requires a cutting and coagulating device. Therefore, a combined Ho:YAG/Nd:YAG laser providing the laser beam of both systems together in one bare fiber seems to be useful.

STUDY DESIGN/MATERIALS AND METHODS

We studied the effect of such a laser system in vitro on tongues of pigs.

RESULTS

Combined application of both lasers results in vitro in a thicker coagulation zone in soft tissue (tongue). Tissue fragments possibly containing vital cells are sprinkled by the pulsed energy of the Ho:YAG laser up to a distance of 20 cm.

CONCLUSION

Using the pulsed Ho:YAG laser for oncologic interventions seems to be problematic. Combined laser effect in vivo may result in better hemostasis.

The holmium laser in urology

OBJECTIVE

To review the physics related to the holmium laser, its laser-tissue interactions, and its application to the treatment of urological diseases.

SUMMARY AND BACKGROUND DATA

The holmium: YAG laser is a solid-state, pulsed laser that emits light at 2100 nm. It combines the qualities of the carbon dioxide and neodymium:YAG lasers providing both tissue cutting and coagulation in a single device. Since the holmium wavelength can be transmitted down optical fibers, it is especially suited for endoscopic surgery.

METHODS

The authors provide a review of the literature as it relates to the holmium laser and its application to urology.

RESULTS

The holmium wavelength is strongly absorbed by water. Tissue ablation occurs superficially, providing for precise incision with a thermal injury zone ranging from 0.5 to 1.0 mm. This level of coagulation is sufficient for adequate hemostasis. The most common urologic applications of the holmium laser that have been reported include incision of urethral and ureteral strictures; ablation of superficial transitional cell carcinoma; bladder neck incision and prostate resection; and lithotripsy of urinary calculi.

CONCLUSIONS

The holmium: YAG laser is a multi-purpose, multi-specialty surgical laser. It has been shown to be safe and effective for multiple soft tissue applications and stone fragmentation. Its utilization in urology is anticipated to increase with time as a result of these features.

Smooth excimer laser coronary angioplasty (SELCA) and conventional excimer laser angioplasty: Comparison of vascular injury and smooth muscle cell proliferation

Although the excimer laser, which utilizes 'non-thermal ablation effects', has achieved encouraging results in early clinical trials, the long-term results have failed to show any advantage over conventional percutaneous transluminal coronary angioplasty (PTCA).A new system, Smooth Excimer Laser Coronary Angioplasty (SELCA), has been developed to reduce the tissue damage in the vessel wall caused by shock waves and vapour bubbles.SELCA (wavelength 308 nm, pulse duration 115 ns, repetition rate 150 Hz and energy density 50 mJ mm(-2)) lowers the amount of shock wave formation and pressure peak amplitude in the surrounding tissue by about eight times when compared to the conventional 308 nm excimer laser (ELCA). In this preclinical evaluation, this new system was compared to ELCA. Fifty New Zealand White rabbits were stimulated by repeated weak DC impulses for a period of 28 days in order to form an atherosclerotic plaque in the right carotid artery. The vessels were excised 3, 7,14 and 28 days after laser irradiation for immunohistochemical analysis.SELCA and ELCA laser treatment lead to a decrease in maximal intimal wall thickness 3 days after intervention (control: 177+/-4 microm; SELCA: 131+/-22microm; ELCA: 120 +/-33microm). In the period between 3 and 28 days, a moderate increase in intimal wall thickness was observed after SELCA treatment compared to a significant increase after ELCA (28 days after intervention: SELCA: 157+/-22microm; ELCA: 274 +/-28microm). Bromodeoxyuridine (BrdU) was applied 18 and 12 h before excision of the vessels in order to determine the percent of cells undergoing DNA synthesis. The percent of BrdU labelled SMC in the intima (control: 13 +/- 2 cells mm(-2)) increased in both groups after 3 days (SELCA: 248 +/- 107 cells mm(-2); ELCA: 162 +/- 41 cells mm(-2)) and 7 days (SELCA: 162+/- 55 cells mm(-2); ELCA: 279 +/- 119 cells mm(-2)).The present results demonstrate that vascular wall injury and increase in intimal wall thickness following SELCA are reduced in comparison to the results achieved with the conventional technique. Further trials are necessary to assess whether these improvements will lead to more favourable long-term results after excimer laser angioplasty.

Combination of fiber-guided pulsed erbium and holmium laser radiation for tissue ablation under water

Because of the high absorption of near-infrared laser radiation in biological tissue, erbium lasers and holmium lasers emitting at 3 and 2 µm, respectively, have been proven to have optimal qualities for cutting or welding and coagulating tissue. To combine the advantages of both wavelengths, we realized a multiwavelength laser system by simultaneously guiding erbium and holmium laser radiation by means of a single zirconium fluoride (ZrF(4)) fiber. Laser-induced channel formation in water and poly(acrylamide) gel was investigated by the use of a time-resolved flash-photography setup, while pressure transients were recorded simultaneously with a needle hydrophone. The shapes and depths of vapor channels produced in water and in a submerged gel after single erbium and after combination erbium-holmium radiation delivered by means of a 400-µm ZrF(4) fiber were measured. Transmission measurements were performed to determine the amount of pulse energy available for tissue ablation. The effects of laser wavelength and the delay time between pulses of different wavelengths on the photomechanical and photothermal responses of meniscal tissue were evaluated in vitro by the use of histology. It was observed that the use of a short (200-µs, 100-mJ) holmium laser pulse as a prepulse to generate a vapor bubble through which the ablating erbium laser pulse can be transmitted (delay time, 100 µs) increases the cutting depth in meniscus from 450 to 1120 µm as compared with the depth following a single erbium pulse. The results indicate that a combination of erbium and holmium laser radiation precisely and efficiently cuts tissue under water with 20-50-µm collateral tissue damage.

Holmium:YAG laser endoscopic sinus surgery: a randomized, controlled study

Although surgical lasers were introduced to the field of otolaryngology more than 20 years ago, their use in rhinologic surgery has remained relatively limited. With the development of the holmium:yttrium-aluminum-garnet (YAG) laser, a device is now available that offers those features necessary for effective sinus surgery:precise bone ablation, efficient soft tissue coagulation, and fiberoptic transmission. This solid-state laser of 2.1-microns wavelength can be coupled with endoscopic instrumentation for the surgical treatment of sinus disease. This study was conducted to determine the clinical efficacy of the holmium:YAG laser for endoscopic sinus surgery. A microscopic analysis of laser-treated sinus tissue was also performed in an attempt to determine the histologic basis of the observed clinical findings. In a prospective, randomized, controlled, single-blinded study, 32 consecutive patients underwent endoscopic sinus surgery using the holmium:YAG laser on one side of the nose and conventional endoscopic instrumentation on the other side. Patients rated symptoms for each side of the nose at 1 week, 1 month, 3 months, 6 months, and 2 years after surgery. Healing parameters were similarly rated by the surgeon. Until the study was completed, patients were not told which side of the nose had been treated with laser surgery. No intraoperative complications occurred. The mean intraoperative blood loss was 24.6 mL less on the laser-treated side of the nose than on conventionally treated side (P < .001). At 1 week after surgery, there was increased mucosal edema on the laser-treated side (P < .01). Crust formation was greater on the conventionally treated side at 1 week and was greater on the laser-treated side at 1 mouth (P < .01). Improvements in symptoms of pain, congestion, and drainage were equivalent for both treatment modalities (P < .001). Microscopic analysis demonstrated the ability of the holmium:YAG laser to remove tissue in relatively thin layers with ablation depths of 260 +/- 8.2 microns, 286 +/- 9.4 microns, and 341 +/- 20.4 microns per pulse at energy levels of 0.5, 1.0, and 1.5 J, respectively. A zone of thermal necrosis extending up to 1 mm beyond the site of laser impact was thought to correlate with the increased postoperative edema observed on the laser-treated side. Endoscopic sinus surgery with the holmium:YAG laser is as effective as nonlaser techniques in relieving the symptoms of chronic sinusitis. Laser surgery offers improved intraoperative hemostasis, but it causes increased postoperative tissue edema. The holmium:YAG laser provides the surgeon with an additional tool for the performance of safe, effective sinus surgery.

Threshold and ablation efficiency studies of microsecond ablation of gelatin under water

BACKGROUND AND OBJECTIVE

Laser thrombolysis is the selective ablation of thrombus occluding vessels by microsecond pulsed laser irradiation. To achieve efficient ablation of thrombus, the optimal wavelength, spot size, and pulse energy need to be determined.

STUDY DESIGN/MATERIALS AND METHODS

A gelatin-based thrombus model confined in 3 mm inner diameter tubes was ablated under water using a 1 microsecond pulsed dye laser. Wavelength studies were conducted by varying the absorption of the gelatin between 10-2000 cm-1 corresponding to the waveband between 400-600 nm on the absorption spectrum of thrombus. A unique spectrophotometric method was developed to measure the ablated mass. An acoustic method was used to measure ablation thresholds under water as a function of absorption.

RESULTS

The mass removed per pulse per unit energy was nearly equal over an absorption range of 100-1000 cm-1 at pulse energies above threshold. Mass removal increased linearly with pulse energy but did not have a direct relationship with radiant exposure. Ablation thresholds indicate that the gelatin needed to be heated only to 100 degrees C for ablation to commence. Ablation masses measured were an order of magnitude higher than those predicted by thermal ablation models.

CONCLUSION

The results suggest that any wavelength between 410-590 nm can be used for effective thrombolysis. The ablation efficiency depends on the total energy delivered rather than the radiant exposure. The high ablation efficiencies suggest a dominance of the mechanical action of vapor bubbles over thermal ablation in the ablation process.

Penetration of the holmium:YAG laser through fluid

PURPOSE

The 2.1-microns pulsed holmium:yttrium-aluminum-garnet (Ho:YAG) laser combines the properties of transmissibility down a flexible silica fiber, enabling percutaneous or endoscopic use, with high water absorption, suggesting good safety characteristics. Laser attenuation in practice, however, is an extremely complex process. The authors studied its effective penetration through blood, bile, urine, saline, and contrast media.

MATERIALS AND METHODS

With use of a fiberoptically directed beam and a laser power meter, penetration was measured in vitro with the fiber tip separated from the medium by 5 cm (noncontact mode) and with the fiber tip immersed (contact mode). Logarithm of energy falloff was measured against fluid thickness. Attenuation coefficients (mu) and half value layer (HVL) distances (estimated thickness of fluid needed to have power) were measured.

RESULTS

In noncontact mode, power falloff was exponential. Non-sanguinous media had similar values for mu and HVL (mu = 2.24-2.70 mm-1 and HVL = 0.26-0.31 mm) close to theoretical predictions. Blood caused significantly (P < .05) more attenuation (mu = 5.15 mm-1, HVL = 0.13 mm). In contact mode, attenuation was much more complex with "plateau" distances of up to 1.2 mm, below which attenuation was negligible. The HVL distances ranged from 0.9 to 1.8 mm and were up to 14 times higher. The main reason is probably the formation of microcavities around the fiber tip.

CONCLUSION

The effective penetration of this laser when immersed may be several times that predicted, with important clinical implications.

Early and late quantitative angiographic results of vein graft lesions treated by excimer laser with adjunctive balloon angioplasty

BACKGROUND

Percutaneous excimer laser coronary angioplasty (PELCA) has been approved for treatment of diseased saphenous vein bypass grafts. However, detailed and complete quantitative angiographic analysis of immediate procedural and late follow-up results has not been performed.

METHODS AND RESULTS

PELCA using the CVX-300 excimer laser system was performed in 125 bypass lesions (mean graft age, 96 +/- 53 months; range, 2 to 240 months) in 106 consecutive patients at eight centers. Quantitative analyses of the procedural and follow-up angiograms were done with the Cardiac Measurement System. Stand-alone PELCA was done in 21 lesions (17%). Lesions were located at the ostium (20%), body (67%), or distal anastomosis (13%). The graft reference diameter was 3.26 +/- 0.79 mm (mean +/- SD). Minimal lumen diameter increased from 1.09 +/- 0.52 mm before treatment to 1.61 +/- 0.69 mm after laser and 2.18 +/- 0.63 mm after adjunctive balloon dilation (P < .001) but had declined at follow-up to 1.40 +/- 1.17 mm. Dissections were evident in 45% of lesions after laser treatment (types A and B, 27%; types C through F, 18%), including 7% occlusions. Angiographic success (< or = 50% diameter stenosis [% DS]) was 54% after laser and 91% after adjunctive PTCA, with an overall clinical success rate of 89%. In-hospital complications were death, 0.9%; myocardial infarction (Q-wave and non-Q-wave), 4.5%; and bypass surgery, 0.9%. Independent predictors of % DS after laser were reference diameter, lesion length, and minimal lumen diameter before laser. At angiographic follow-up in 83% of eligible patients, the restenosis rate per lesion (DS > 50%) was 52%, including 23 occlusions (24%). The only independent predictor of increased % DS at follow-up was lesion symmetry. Logistic regression indicated that smaller reference diameter was an independent predictor of late occlusion. Overall 1-year mortality was 8.6%. Actuarial event-free survival (freedom from death, myocardial infarction, bypass surgery, or target vessel percutaneous transluminal coronary angioplasty) was 48.2% at 1 year.

CONCLUSIONS

Excimer laser angioplasty with adjunctive balloon angioplasty can be safely and successfully performed in diseased, old saphenous vein bypass graft lesions considered at high risk for reintervention. The extent of laser ablation remains limited by the diameter and effectiveness of the catheters. Late restenosis and, in particular, total occlusion mitigate the early benefits of the procedure. Other approaches such as the routine use of additional anticoagulation (eg, warfarin) should be considered to reduce the risk of late occlusions and restenosis after laser angioplasty of bypass grafts.

Acute complications of excimer laser coronary angioplasty: a detailed analysis of multicenter results. Coinvestigators of the U.S. and European Percutaneous Excimer Laser Coronary Angioplasty (PELCA) Registries

OBJECTIVES

The aim of this study was to document and analyze the incidence and consequences of complications of excimer laser coronary angioplasty.

BACKGROUND

Excimer laser coronary angioplasty has been reported to be a safe and feasible alternative or adjunct to conventional balloon angioplasty, but serious and unique complications have been observed.

METHODS

Data on 1,595 interventions of excimer laser coronary angioplasty in 1,521 patients were analyzed, using a merged data base from the U.S. and European Percutaneous Excimer Laser Coronary Angioplasty (PELCA) registries.

RESULTS

Procedural success was achieved in 89.3% of interventions. Stand-alone laser angioplasty was performed in 17.8% of interventions. Complications included dissection (22.0%), vasospasm (6.1%), filling defects (4.8%), abrupt reclosure (6.1%), embolization (2.3%), perforation (2.4%), arrhythmia (0.7%) and aneurysm formation (0.3%). Major complications were non-Q wave myocardial infarction (2.3%), Q wave myocardial infarction (1.0%), coronary artery bypass grafting (3.1%) and death (0.7%). Logistic regression analysis revealed correlation between dissections and the use of larger catheter size (p = 0.0005), high energy per pulse levels (p = 0.0001 for native vessels), lesion length > 10 mm (p = 0.001) and presence of a side branch (p = 0.01). The incidence of perforations was higher in women (p = 0.004), in treatment of total occlusions (p = 0.02) and in the presence of a side branch (p = 0.03). Fatal complications were correlated with patients with multivessel disease (p < 0.0001), patients with acute myocardial infarction (p = 0.0009) and older patients (> 70 years old, p = 0.004). The incidence of major complications decreased after performance of 50 laser angioplasty procedures at one institution (p = 0.02).

CONCLUSIONS

This analysis defines both the learning curve and the profile of complications for excimer laser angioplasty and provides insight into the selection of appropriate patients and proper performance of the procedure.

Effectiveness of holmium laser-assisted coronary angioplasty. The Holmium Laser Coronary Registry

The efficacy of holmium laser-assisted angioplasty was studied in 365 narrowings in 331 consecutive patients with coronary artery disease. Clinical indications for study were unstable angina pectoris in 140 patients (42%), stable angina in 136 patients (41%), postmyocardial infarction angina in 35 patients (10.5%), silent myocardial ischemia in 11 patients (3%), acute myocardial infarction in 1 patient (0.3%) and undefined in 8 patients (2%). Coronary morphology characteristics by Multivessel Angioplasty Prognosis Study group criteria were type A in 12.6%, type B1 in 34.2%, type B2 in 27.4% and type C in 25.4%. The laser successfully crossed the total length of the narrowing in 85.2%. Procedural success was 94.2%. Laser alone reduced mean percent luminal narrowing from 88 +/- 11% to 57 +/- 22%. Subsequent balloon angioplasty further reduced the mean luminal narrowing to 23 +/- 18%. Major complication rate was 2.7% (death 0.3%, Q-wave myocardial infarction 0.5%, and emergent bypass surgery 2.7%). Six-month angiographic restenosis (> 50% stenosis) rate was 44%.

Acute response of the arterial wall to pulsed laser irradiation

This study was designed to examine the acute response of normal arterial wall to pulsed laser irradiation. Irradiation with an Excimer or a Holmium YAG laser was performed in 15 normal iliac sites of 8 male New Zealand white rabbits. The excimer laser was operated at 308 nm, 25 Hz, 50 mJ/mm2/pulse, and 135 nsec/pulse and the Ho:YAG laser was operated at 2.1 microns, 3.5 Hz, 400 mJ/pulse, 250 microseconds/pulse. The excimer and Ho:YAG laser were coupled into a multifiber wire-guided catheter of 1.4 and 1.5 mm diameter, respectively. The mean luminal diameter increased similarly from 2.01 +/- 0.29 to 2.46 +/- 0.27 mm (P < 0.0005) and from 2.09 +/- 0.53 to 2.45 +/- 0.30 mm (P < 0.005) after excimer and Ho:YAG laser irradiation, respectively. Perforation occurred in 3 of 15 Ho:YAG irradiated sites and 0 of 15 excimer laser irradiated sites. The sites irradiated with excimer or Ho:YAG laser had similar histologic features, consisting of shedding of the endothelium, disorganization of internal elastic lamina, localized necrosis of vascular smooth muscle cells, and fissures in the medial layer. However, the sites irradiated with excimer laser had lower grading scores than those irradiated with the Ho:YAG laser (P < 0.05). Irradiation with excimer or Ho:YAG laser of normal arteries results in: (1) vasodilation of the irradiated artery; (2) localized mechanical vascular injury, and (3) Ho:YAG laser induces more severe damage to the arterial wall than excimer.

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

A modified excimer 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 microns) 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 microns for normal aorta and in a range of 57 +/- 35 to 110 +/- 39 microns 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.

Percutaneous angioscopic evaluation of luminal changes induced by excimer laser angioplasty

Angioscopy has been shown to provide more detailed information on lesion morphology before and after interventional procedures than angiography. Therefore to evaluate the effects of laser angioplasty, angioscopy was performed in five patients with peripheral or coronary vascular disease who underwent excimer laser angioplasty. The excimer laser was operated at 308 nm, 135 nsec, 25 Hz, and 40 to 60 mjoules/mm2 and was coupled into multifiber wire-guided catheters of 1.4 to 2.0 mm diameter for coronary lesions and into catheters of 2.2 mm diameter for peripheral lesions. There were three coronary (one left anterior descending, one circumflex, one right coronary artery) and two peripheral (one common iliac artery, one superficial femoral artery) lesions. Angioscopy was successfully performed before and after laser ablation without any complications in all five lesions. The characteristics of angioscopic findings after excimer laser angioplasty consisted of flaps, fractures of plaques, and abundant tissue remnants. There was no apparent thermal injury. Recanalized channels were small and irregular. These results indicate that (1) angioscopy is effective and safe for evaluation of lesion morphology after laser angioplasty; (2) laser ablation does not result in thermal injury; and (3) irregular channels after recanalization and abundant tissue remnants may explain the suboptimal results after laser angioplasty.

The use of the holmium:YAG laser in coronary disease: the utility of a unique lensed fiber catheter

Alternatives to balloon angioplasty, which has a restenosis rate of 25%-68%, are being sought for patients at high risk for restenosis. The mid-infrared holmium laser may have properties more advantageous for coronary angioplasty than the ultraviolet excimer laser. The Coronary HaloCath (Trimedyne, Inc.) has a diverging lens that allows the beam diameter to approximate the catheter tip diameter, increasing the size of the channel that can be created by laser ablation. This unique ability may make it more efficacious in treating arteriosclerotic disease. This device was used to treat: 1) a chronic total occlusion, creating a recanalized diameter larger than the catheter diameter; and 2) an ostial right coronary stenosis, which opened more readily than would have been expected with balloon angioplasty. The Coronary HaloCath may prove useful in difficult-to-treat coronary lesions.

Tissue ablation with excimer laser and multiple fiber catheters: effects of optical fiber density and fluence

This study was performed to assess the ablational properties of 1.4-, 1.7-, and 2.0-mm wire-guided multiple fiber catheters coupled to a XeCl excimer laser. Samples of postmortem human aorta were irradiated in blood at fluences of 40, 50, and 60 mJ/mm 2. Our results indicate that: 1) an increase in the active irradiation coverage results in an increase in the ablation efficiency; 2) the ablation efficiency is not fluence related using the 1.4- or 1.7-mm multiple fiber catheter but efficiency is fluence dependent using the 2.0-mm multiple fiber catheter; 3) the depth of tissue ablated with a multiple fiber catheter depends primarily upon the proportion of the active irradiation coverage at the catheter tip; 4) the 2.0-mm multiple fiber catheter induces craters surrounded by a larger zone of tissue damage than that observed with the 1.4- or 1.7-mm multiple fiber catheter; and 5) the 2.0-mm multiple fiber catheter should be used cautiously for laser angioplasty because of its high penetration and its risk of arterial wall damage.