Hyun Wook Kang

Laser ablation in a liquid-confined environment using a nanosecond laser pulse

Laser ablation of aluminum metal with 1ns, 800nm pulse at low radiant exposures was investigated in air (dry) and water (wet) environments. Compared to dry ablation, an approximately eight times increase in material removal rate was associated with wet ablation. Based on optical reflectance and scanning electron microscope images, bubble formation/collapse was responsible for augmented acoustic pressure and ablation performance. Numerically simulated temperature distributions during wet ablation were consistent with the occurrence of explosive water vaporization near the critical temperature of water. Strong pressure emission during liquid vaporization and jet formation can account for enhanced ablation process. Radial expansion of bubbles minimized the redeposition of debris, leading to improvements in energy coupling to the target and ablation performance.

Photoselective Vaporization Prostatectomy: Experience With a Novel 180 W 532 nm Lithium Triborate Laser and Fiber Delivery System in Living Dogs

PURPOSE We studied vaporization parameters, and anatomical and histopathological outcomes of photoselective vaporization of the prostate with the novel GreenLight™ XPS™ 180 W, 532 nm lithium triborate laser and MoXy™ fiber in a survival model of living dogs. We compared these findings with those of the existing GreenLight HPS™ 120 W 532 nm lithium triborate laser photoselective vaporization of the prostate in living dogs. MATERIALS AND METHODS Eight dogs underwent antegrade photoselective vaporization of the prostate with the 180 W laser delivered through a new 750 μm (vs the existing 600 μm core diameter), 50% larger, spot sized, side firing fiber. Four dogs were sacrificed 3 hours and 8 weeks postoperatively, respectively. We recorded laser energy and time. Prostates were sectioned, measured and histologically analyzed after hematoxylin and eosin, triphenyltetrazolium chloride or Gomori trichrome staining and compared with a normal control. RESULTS Photoselective vaporization of the prostate with the 180 W laser bloodlessly created a 76% larger cavity (mean 11.8 vs 6.7 cm(3), p = 0.014), vaporized tissue at a 77% higher rate (mean 2.3 vs 1.3 cm(3) per minute, p = 0.03) and did so in 37% less time per volume vaporized (0.5 vs 0.8 minutes per cm(3), p = 0.003). Hematoxylin and eosin, and triphenyltetrazolium chloride staining histologically revealed a 33% thicker mean coagulation zone vs that of 120 W laser photoselective vaporization of the prostate (2.0 ± 0.4 vs 1.5 ± 0.3 mm, p <0.005). In prostates healed for 8 weeks postoperatively hematoxylin and eosin, and Gomori trichrome staining showed re-epithelialized cavities with negligible submucosal fibrosis compared with a normal prostate. CONCLUSIONS GreenLight XPS 180 W 532 nm lithium triborate laser photoselective vaporization of the prostate with the MoXy fiber has a significantly higher vaporization rate and speed with a deeper hemostatic coagulation zone but favorable tissue interaction and healing equal to those of HPS 120 W laser photoselective vaporization of the prostate in dogs.

Effect of liquid thickness on laser ablation efficiency

The purpose of this study was to investigate the effect of liquid thickness on laser ablation efficiency. Both Q-switched Nd:YAG (yttrium aluminum garnet) and free-running Er:YAG lasers were used to ablate polymethyl-methacrylate samples in the presence of a water layer. The thickness of the liquid layer varied from 500μmto3mm. Ablation performance as a function of liquid thickness for both lasers was quantitatively measured by optical coherence tomography. For the Q-switched Nd:YAG laser, wet ablation produced up to three times greater ablation volume than dry ablation, and the ablation efficiency decreased with increase of liquid thickness. On the other hand, for the Er:YAG laser, wet ablation with a 500μm layer of water produced comparable ablation volume to dry ablation. Ablation performance decreased as the liquid layer thickness increased. Q-switched laser ablation assisted by a thin liquid layer efficiently augmented material removal, while ablation efficiency of a long-pulsed Er:YAG laser decrease...

Investigations on laser hard tissue ablation under various environments

The purpose of this study was to investigate the effect of liquid environments upon laser bone ablation. A long-pulsed Er,Cr:YSGG laser was employed to ablate bovine bone tibia at various radiant exposures under dry, wet (using water or perfluorocarbon) and spray environmental conditions. Energy loss by the application of liquid during laser irradiation was evaluated, and ablation performance for all conditions was quantitatively measured by optical coherence tomography (OCT). Microscope images were also used to estimate thermal side effects in tissue after multiple-pulse ablation. Wet using water and spray conditions equally attenuated the 2.79 microm wavelength laser beam. Higher transmission efficiency was obtained utilizing a layer of perfluorocarbon. Dry ablation exhibited severe carbonization due to excessive heat accumulation. Wet condition using water resulted in similar ablation volume to the dry case without carbonization. The perfluorocarbon layer produced the largest ablation volume but some carbonization due to the poor thermal conductivity. Spray induced clean cutting with slightly reduced efficiency. Liquid-assisted ablation provided significant beneficial effects such as augmented material removal and cooling/cleaning effects during laser osteotomy.

Enhancement of bovine bone ablation assisted by a transparent liquid Layer on a target surface

The purpose of this study was to investigate the laser-induced ablation of bovine bone assisted by a transparent liquid layer on top of the target surface. A Q-switched Nd:YAG laser was used to ablate bovine tibia at various energy levels. Distilled water was applied to the sample surface in order to examine the role of a transparent liquid layer during the ablation. Plasma generation and transient acoustic waves were monitored to identify dominant mechanisms involved in the ablation process. Ablation efficiency was measured from the cross-sectional tomography acquired by optical coherence tomography (OCT). Ablation with a liquid layer lowered the damage threshold and enhanced both the laser-induced acoustic excitation and the ablation efficiency, which saturated at higher radiant exposures. The enhanced ablation of the liquid-assisted process is primarily due to photomechanical effects associated with explosive vaporization and plasma confinement. The saturation of the pressure amplitude and ablation efficiency was attributed to increased plasma shielding

Laser vaporization of the prostate in vivo: Experience with the 150-W 980-nm diode laser in living canines.

Anatomic, tissue ablation and coagulation, and histopathologic outcomes of the 150‐W 980‐nm diode laser selective light vaporization (SLV™) of the prostate in the first survival study of living canines were analyzed.

Hard tissue ablation with a spray-assisted mid-IR laser

The objective of this study was to understand the dominant mechanism(s) for dental enamel ablation with the application of water spray. A free-running Er,Cr:YSGG (yttrium, scandium, gallium, garnet) laser was used to ablate human enamel tissue at various radiant exposures. During dental ablation, distilled water was sprayed on the sample surface, and these results were compared to ablation without a spray (dry ablation). In order to identify dominant ablation mechanisms, transient acoustic waves were compared to ablation thresholds and the volume of material removed. The ablation profile and depth were measured using optical coherence tomography (OCT). Irregular surface modification, charring and peripheral cracks were associated with dry ablation, whereas craters for spray samples were relatively clean without thermal damage. In spite of a 60% higher ablation threshold for spray associated irradiations owing to water absorption, acoustic peak pressures were six times higher and ablation volume was up to a factor of 2 larger compared to dry ablation. The enhanced pressure and ablation performance of the spray-assisted process was the result of rapid water vaporization, material ejection with recoil stress, interstitial water explosion and possibly liquid-jet formation. With water cooling and abrasive/disruptive mechanical effects, the spray ablation can be a safe and efficient modality for dental treatment.

Photothermal response of superparamagnetic iron oxide nanoparticles

Superparamagnetic iron oxide nanoparticles have been used as MRI contrast agents in medical imaging. The purpose of this study was to explore the photothermal response of superparamagnetic iron oxide nanoparticles for biomedical applications.

Investigation of stone retropulsion as a function of Ho:YAG laser pulse duration

Stone retropulsion during Ho:YAG (λ = 2.12 μm) laser lithotripsy with various pulse durations (τp: 250 ~ 495 μsec) was investigated. Depending on pulse energy, optical pulse durations were divided into two regimes: short pulse (250~350 μsec) and long pulse (315~495 μsec). Retropulsion distance was measured as a function of pulse energy from 0.4 J to 1.2 J. Calculus phantoms made from plaster of Paris were ablated with a free running Ho:YAG laser using various optical fibers (200, 400, 600 μm) in water. In order to examine the ablation efficiency of two different pulse durations, a single pulse was applied, and the dynamics of the recoil action of a calculus phantom was monitored using a high-speed camera. The correlation among laser-induced topography, ablation volume, and retropulsion was evaluated. Higher pulse energy and larger fibers resulted in larger ablation volume and retropulsion. At a given pulse energy, optical pulses with different durations yielded comparable ablation volumes. The shorter duration pulses induced more retropulsion than longer pulses did at the same pulse energy. Larger retropulsion with the shorter pulse is thought to be induced by higher temperature at the vapor-solid interface, subsequently resulting in faster plume ejection with higher recoil momentum. The results suggest that a longer pulse could minimize retropulsion of the stone during lithotripsy.

Defining Optimal Laser-Fiber Sweeping Angle for Effective Tissue Vaporization Using 180 W 532 nm Lithium Triborate Laser

PURPOSE The goal of this study is to identify the most efficient sweeping angle (SA) during photoselective vaporization of the prostate (PVP). MATERIALS AND METHODS Experiments were conducted with GreenLight XPS™ laser at 120 and 180 W. Ten blocks of porcine kidney were used for each SA (0, 15, 30, 45, 60, 90, and 120 degrees). Vaporization efficiency was assessed by the amount of tissue removed per time. The coagulation zone (CZ) thickness was also measured. RESULTS Maximal vaporization rate (VR) was achieved at SA 15 and 30 degrees. Irrespective of power, VR increased and CZ decreased linearly with decreasing SA from 120 to 30 degrees. The CZ was the thinnest at SA 30 degrees. CONCLUSIONS Optimal vaporization occurred at a SA of 15 degrees and 30 degrees with the lowest CZ at 30 degrees. Contrary to a previous recommendation for a wider SA (60 degrees or greater), a narrower SA (30 degrees) achieved the maximal tissue vaporization efficiency.