Comparison of Er:YAG and 9.6-microm TE CO(2) lasers for ablation of skull tissue.

BACKGROUND AND OBJECTIVE: Craniotomy by using a drill and saw frequently results in fragmentation of the skull plate. Lasers have the potential to remove the skull plate intact, simplifying the reconstructive surgery. STUDY DESIGN/MATERIALS AND METHODS: Transverse-excited CO(2) lasers operating at the peak absorption wavelength of bone (lambda = 9.6 microm) and with pulse durations of 5-8 microsec, approximately the thermal relaxation time in hard tissue, produced high ablation rates and minimal peripheral thermal damage. Both thick (2 mm) and thin (250 microm) bovine skull samples were perforated and the ablation rates calculated. Results were compared with Q-switched and free-running Er:YAG lasers (lambda = 2.94 microm, tau(p) = 0.5 microsec and 300 microsec). RESULTS: The CO(2) laser produced ablation rates of up to 60 and 15 microm per pulse for thin and thick sections, respectively, and perforated thin and thick sections with fluences of less than 1 J/cm(2) and 6 J/cm(2), respectively. There was no discernible thermal damage and no need for water irrigation during ablation. Pulse durations > or =20 microsec resulted in significant tissue charring, which increased with the pulse duration. Although the free-running Er:YAG laser produced ablation rates of up to 100 microm per pulse, fluences of 10 J/cm(2) and 30 J/cm(2) were required to perforate thin and thick samples, respectively, and peripheral thermal damage measured 25-40 microm. CONCLUSIONS: In summary, the novel 5- to 8-microsec pulse length of the TE CO(2) laser is long enough to avoid a marked reduction in the ablation rate due to plasma formation and short enough to avoid peripheral thermal damage through thermal diffusion during the laser pulse. Furthermore, in vivo animal studies with the TE CO(2) laser are warranted for potential clinical application in craniotomy and craniofacial procedures.

Laser ablation of the pulmonary veins by using a fiberoptic balloon catheter: implications for treatment of paroxysmal atrial fibrillation.

BACKGROUND AND OBJECTIVE: Focal sources of paroxysmal atrial fibrillation may be treatable by electrical isolation of the pulmonary veins from the left atrium. A new fiberoptic balloon catheter was tested as an alternative to radiofrequency catheter ablation for creation of circumferential thermal lesions at the pulmonary vein orifice. STUDY DESIGN/MATERIALS AND METHODS: In vitro and in vivo experiments were conducted in canine hearts to demonstrate efficacy and optimize ablation dosimetry. Continuous-wave, 1.06-microm, Nd:YAG laser radiation was delivered radially through diffusing optical fiber tips enclosed in a balloon catheter. During in vivo studies, the catheter was placed at the pulmonary vein orifice through a left atrial appendage sheath under X-ray fluoroscopic guidance during an open-chest procedure. Additionally, circumferential lesions in the left atrial appendage were correlated with epicardial electrograms demonstrating elimination of electrical activity. RESULTS: The pulmonary veins were successfully ablated by using laser powers of 30--50 W and irradiation times of 60--90 seconds. Transmural, continuous, and circumferential lesions were produced in the pulmonary veins in a single application without evidence of tissue vaporization or endothelial disruption. CONCLUSION: Laser ablation by using a fiberoptic balloon catheter may represent a promising alternative to radiofrequency catheter ablation for electrical isolation of the pulmonary veins from the left atrium during treatment of paroxysmal atrial fibrillation. Further development and testing of the fiberoptic catheter is warranted for possible clinical studies.

Potential applications of the erbium:YAG laser in endourology.

The holmium:YAG laser has become the laser of choice in endourology because of its multiple applications in the fragmentation of kidney stones, incision of strictures, and coagulation of tumors. This paper describes the potential use of a new laser, the erbium:YAG laser, for applications in endourology. Recent studies suggest that the Er:YAG laser may be superior to the Ho:YAG laser for precise ablation of strictures with minimal peripheral thermal damage and for more efficient laser lithotripsy. The Er:YAG laser cuts urethral and ureteral tissues more precisely than does the Ho:YAG laser, leaving a residual peripheral thermal damage zone of 30 +/- 10 microm compared with 290 +/- 30 microm for the Ho:YAG laser. This result may be important in the treatment of strictures, where residual thermal damage may induce scarring and result in stricture recurrence. The Er:YAG laser may represent an alternative to the cold knife and Ho:YAG laser in applications where minimal mechanical and thermal insult to tissue is required.

Noninvasive vasectomy using a focused ultrasound clip: thermal measurements and simulations.

INTRODUCTION: Conventional surgical vasectomy may lead to complications including bleeding, infection, and scrotal pain. Noninvasive transcutaneous delivery of therapeutic focused ultrasound has previously been shown to thermally occlude the vas deferens. However, skin burns and inconsistent vas occlusion have presented complications. This study uses bio-heat transfer simulations and thermocouple measurements to determine the optimal ablation dosimetry for vas occlusion without skin burns. METHODS: A 2-rad ultrasound transducer mounted on a vasectomy-clip-delivered ultrasound energy at 4 MHz to the canine vas deferens co-located at the focus between the clip jaws. Chilled degassed water was circulated through an attached latex balloon, providing efficient ultrasound coupling into the tissue and active skin cooling to prevent skin burns. Thermocouples placed at the vas, intradermal, and skin surface locations recorded temperatures during ablation. Procedures were performed with transducer acoustic powers of 3-7 W and sonication times of 60-120 s on both the left and right vas deferens (n = 2) in a total of four dogs (precooling control, 3 W/120 s, 5 W/90 s, 7 W/60 s). Measurements were compared with bio-heat transfer simulations modeling the effects of variations in power and sonication time on tissue temperatures and coagulation zones. RESULTS: Active skin cooling produces a thermal gradient in the tissue during ablation, allowing sufficient thermal doses to be delivered to the vas without skin burns. However, low-power, long-duration heating produced excessive tissue necrosis due to thermal diffusion, while high power and short heating times reduced the therapeutic window and produced skin burns presumably due to direct ultrasound absorption. CONCLUSIONS: Both simulations and experiments suggest that a therapeutic window exists in which thermal occlusion of the vas may be achieved without the formation of skin burns in the canine model (power = 5-7 W, surface intensity = 1.4-1.9 W/cm2, time = 20-50 s). This range of ablation parameters will help guide future experiments to refine incisionless vasectomy using focused ultrasound.

Linear lesions in myocardium created by Nd:YAG laser using diffusing optical fibers: in vitro and in vivo results.

BACKGROUND AND OBJECTIVE: Linear lesions may be necessary for successful catheter ablation of cardiac arrhythmias such as atrial fibrillation. This study uses laser energy delivered through diffusing optical fibers as an alternative to radiofrequency energy for the creation of linear lesions in cardiac tissue in a single application. STUDY DESIGN/MATERIALS AND METHODS: Samples of canine myocardium were placed in a heated, circulating saline bath and irradiated with a 1.06-microm, continuous-wave Nd:YAG laser during in vitro studies. Laser ablation was then performed in vivo on the epicardial surface of the right ventricle during an open-chest procedure by using similar ablation parameters. Laser energy was delivered to the tissue by being diffused radially through flexible optical fiber tips oriented parallel to the tissue surface. Histology and temperature measurements verified transmurality, continuity, and linearity of the lesions. RESULTS: Peak tissue temperatures measured in vitro remained low (51 +/- 1 degrees C at the endocardial surface, 61 +/- 6 degrees C in the mid-myocardium, and 55 +/- 6 degrees C at the epicardial surface) with no evidence of tissue charring or vaporization. Lesion dimensions produced in vitro and in vivo were similar (depth, 6 mm; width, 8-10 mm; length, 16-22 mm), demonstrating that tissue perfusion in vivo did not significantly alter the heating. CONCLUSION: Long linear lesions, necessary for duplication of the surgical maze procedure during catheter ablation of atrial fibrillation, may be created by using laser radiation delivered through flexible diffusing optical fiber tips. Further development of steerable catheters for endocardial atrial ablation and studies correlating thermal damage zones with electrophysiologic indicators of irreversible conduction block are warranted.

Laser skin welding: in vivo tensile strength and wound healing results.

BACKGROUND AND OBJECTIVE: Laser skin welding was investigated as a general model for laser tissue closure. Scanned delivery of near-infrared laser radiation in combination with a dye can produce strong welds with limited thermal damage. STUDY DESIGN/MATERIALS AND METHODS: Two-centimeter-long, full-thickness incisions were made on the backs of guinea pigs. Wounds were closed either by laser welding or sutures and then biopsied at 0, 3, 6, 10, 14, 21, and 28 days postoperatively. Welding was achieved by using continuous-wave, 1. 06-micrometer, Nd:YAG laser radiation scanned over the incisions to produce a dwell time of approximately 80 msec. The cooling time between scans was fixed at 8 seconds. A 4-mm-diameter laser spot was maintained during the experiments, and the power was kept constant at 10 W. The operation time was fixed at 10 minutes per incision. India ink was used as an absorber of the laser radiation at the weld site, and clamps were used temporarily to appose the incision edges. RESULTS: Acute weld strengths of 2.1 +/- 0.7 kg/cm(2) were significantly higher than suture apposition strengths of 0.4 +/- 0.1 kg/cm(2) (P < 0.01), and weld strengths continued to increase over time. Lateral thermal damage in the laser welds was limited to 200 +/- 40 micrometer near the epidermal surface with less thermal damage deeper within the dermis. CONCLUSION: Our welding technique produced higher weld strengths and less thermal damage than reported in previous skin welding studies and may represent an alternative to sutures.

Cryogen spray cooling during laser tissue welding.

Cryogen cooling during laser tissue welding was explored as a means of reducing lateral thermal damage near the tissue surface and shortening operative time. Two centimetre long full-thickness incisions were made on the epilated backs of guinea pigs, in vivo. India ink was applied to the incision edges then clamps were used to appose the edges. A 4 mm diameter beam of 16 W, continuous-wave, 1.06 microm, Nd:YAG laser radiation was scanned over the incisions, producing approximately 100 ms pulses. There was a delay of 2 s between scans. The total irradiation time was varied from 1-2 min. Cryogen was delivered to the weld site through a solenoid valve in spurt durations of 20, 60 and 100 ms. The time between spurts was either 2 or 4 s, corresponding to one spurt every one or two laser scans. Histology and tensile strength measurements were used to evaluate laser welds. Total irradiation times were reduced from 10 min without surface cooling to under 1 min with surface cooling. The thermal denaturation profile showed less denaturation in the papillary dermis than in the mid-dermis. Welds created using optimized irradiation and cooling parameters had significantly higher tensile strengths (1.7 +/- 0.4 kg cm(-2)) than measured in the control studies without cryogen cooling (1.0 +/- 0.2 kg cm(-2)) (p < 0.05). Cryogen cooling of the tissue surface during laser welding results in increased weld strengths while reducing thermal damage and operative times. Long-term studies will be necessary to determine weld strengths and the amount of scarring during wound healing.

Radiometric surface temperature measurements during dye-assisted laser skin closure: in vitro and in vivo results.

BACKGROUND AND OBJECTIVE: A thermal camera was used to measure surface temperatures during laser skin welding to provide feedback for optimization of the laser parameters. STUDY DESIGN/MATERIALS AND METHODS: Two-centimeter-long, full-thickness incisions were made in guinea pig skin in vitro and in vivo. India ink was applied to the incision edges, which were then mechanically apposed. Continuous-wave, 1.06-microm Nd:YAG laser radiation was scanned over the incisions, producing an effective pulse duration of approximately 100 msec. Cooling durations between scans of 1.6, 4.0, and 8.0 sec were studied in vitro. A 5-mm-diameter laser spot was used with the power kept constant at 10 W. Thermal images were obtained at 30 frames per second with a thermal camera detecting 3-5 microm radiation. Surface temperatures were recorded at 0, 1, and 6 mm from the center line of the incision. RESULTS/CONCLUSIONS: Cooling durations of 1.6 and 4.0 seconds in vitro resulted in temperatures at the weld site that remained above approximately 65 degrees C for prolonged periods of time. Cooling durations of 8.0 seconds were sufficient both in vitro and in vivo to prevent a significant rise in baseline temperatures at the weld site over time.

Dye-assisted laser skin closure with pulsed radiation: an in vitro study of weld strength and thermal damage.

Previous laser skin welding studies have used continuous wave delivery of radiation. However, heat diffusion during irradiation prevents strong welds from being achieved without creating large zones of thermal damage. Previously published results indicate that a thermal damage zone in skin greater than 200 µm may prevent normal wound healing. We propose that both strong welds and minimal thermal damage can be achieved by introducing a dye and delivering the radiation in a series of sufficiently short pulses. Two-cm-long incisions were made in guinea pig skin, in vitro. India ink and egg white (albumin) were applied to the wound edges to enhance radiation absorption and to close the wound, respectively. Continuous wave (cw), 1.06 µm, Nd:yttrium-aluminum-garnet laser radiation was scanned over the weld producing ∼100 ms pulses. The cooling time between scans and the number of scans was varied. The thermal damage zone at the weld edges was measured using a transmission polarizing light microscope. The tensile strength of the welds was measured using a tensiometer. For pulsed welding and long cooling times between pulses (8 s), weld strengths of 2.4±0.9 kg/cm2 were measured, and lateral thermal damage at the epidermis was limited to 500±150 µm. With cw welding, comparable weld strengths produced 2700±300 µm of lateral thermal damage. The cw weld strengths were only 0.6±0.3 kg/cm2 for thermal damage zones comparable to pulsed welding. © 1998 Society of Photo-Optical Instrumentation Engineers.