As more and more small core fibers (sub 300 μm) become available for high-power laser application (>20 W), fiber distal-tip degradation during ureteroscopic laser lithotripsy (URSL) is critical. Still, it is an overlooked aspect due to its subtle effect on the ablation rate and lack of adequate characterization. The objective of this study is to measure fiber tip degradation in terms of surface modification and output beam profile. The fiber tip is a location of interest not only because fiber material performance at the tip impacts laser beam shape, power, and quality but moreover because this is the primary region where inter-procedural degradation compounds. By characterizing the degradational relationship between laser beam profile and fiber tip damage, the next generation of fiber development could be motivated to improve URSL procedures. A high-magnification microscope was used to investigate the fiber tip surface degradation due to laser dose with or without a stone phantom. A single-mode 660 nm laser beam and an M-square measurement system were used to evaluate the laser beam profile variation through lithotripsy fibers with different distal end degradations. The results show that the laser beam profile through the fiber varies with the core size of the fiber and distal end degradation.
Laser lithotripsy has clinically been implemented to treat urinary stone disease by using a Ho:YAG laser system. Bubble dynamics plays an important role in determining stone ablation efficiency. The current study developed a two-phase opto-thermal model to numerically assess the bubble dynamics during laser irradiation. The simulation involved light propagation, light and water interaction, and multi-phase heat and mass transfer. The simulation was verified by the experimental setup including Ho:YAG lasers, fiber in water, and high-speed camera. Both the numerical simulations and the experimental results showed a good agreement in predicting the effects of laser pulses on the bubble dynamics.
Holmium:YAG laser is commonly used as an efficient technology for lithotripsy, breaking urinary stones into small particles (dust) and larger residual fragments (RF). One of the ultimate goals is to create fine dust for real-time aspiration, eliminating the need for mechanical retrieval of RFs. A recent study of stone dust definition suggests a maximum particle size of 250-µm to allow complete aspiration through the working channel of a flexible ureteroscope.
We have evaluated the particle size generation of a concept Holmium:YAG laser utilizing a pulse width modulation technique. This technology delivers numerous low-energy micro-pulses per pulse with long temporal pulse duration to potentially enable finer dust particles, better ablation rate, and reduced retropulsion. Overall, the concept device generates a high percentage of fine dust compared with prior results found in literature.
(Disclaimers: Bench Test results may not necessarily be indicative of clinical performance. The testing was performed by or on behalf of BSC. Data on file. Concept device or technology. Not available for sale. This device is not yet available for sale in the United States).
Numerical Simulations of Bubble Dynamics in Laser Lithotripsy
Steven Peng1; Thomas Hasenberg1; Timothy Harrah2; James Zhang1; Dongyul Chai1;
Hyun Wook Kang3
1Boston Scientific Corporation, 150 Baytech Dr., San Jose, CA, USA 95134
2Boston Scientific Corporation, 100 Boston Scientific Way, Marlborough, MA 01752
3Department of Biomedical Engineering and Center for Marine-integrated Technology (BK21Plus), Pukyong National University, Busan 48513, Korea
Abstract: Laser lithotripsy has been clinically implemented to treat kidney stone disease by using the Holmium:YAG (λ = 2.12 μm) for two decades. The medical procedure involves bubble formation during delivery of infrared laser energy in water (saline). Bubble dynamics plays an important role of optimization of laser power delivery for best stone fragmentation and dusting effects. A high speed camera image system has been used to observe vapor bubble formation in a water tank, but this method faces more challenge to observe bubble dyna
Objective: To design a durable holmium laser fiber with high power, good transmission efficiency, flexible, and tight bend for holmium YAG laser lithotripsy. The new design has improved the common issues for existing Ho: YAG laser fibers including fiber fracture at bend, fiber burnt, and connector over heat.
Methods and Materials: We developed fiber modeling including ray-tracing optical simulation and CFD thermal modeling. Utilizing optical simulation, we find the over-fill and offset laser light leaking into the glass capillary was effectively absorbed by the fiber and SMA connector. The connector and fiber temperature increases were calculated using CFD thermal modeling based on the optical absorption from the ray-tracing. We introduced dimpled surfaces on the glass capillary tube to strip off the un-wanted rays to prevent the fiber from burning and the connector from overheating. We built and tested several small core (242 um) Super fiber samples for testing on different holmium laser systems (Lumenis P120H and VersaPulse® 100W). We evaluated the fiber durability under high power and tight bend conditions. The % transmission, fiber flexibility, and connector temperature were all measured under these conditions.
Results: We observed good agreement between our fiber test results and our optical and thermal modeling. These results help to illuminate the root cause of the holmium fiber failure modes. Indeed, the test results show the SuperFiber is very durable. For example, it delivered more than 1,000,000 joules under several stringent operating conditions including high power (50- 60W) in air, high frequency (25- 80 Hz), and 180° bend diameters from 16 mm down to 6 mm. One SuperFiber still survived after being tested at 60W (60Hz*1J) and 48W (80Hz*0.6J) while being bent to an 8 mm diameter for 30 minutes in air. This fiber has also showed a low transmission change after bending from 16mm down to 6 mm diameter (low bending induced loss).
Optical fibers for lithotripsy are designed to deliver the maximum energy precisely to the treatment site without a decrease in performance and without increasing the risks to patients and users. One of the obstacles to constant energy delivery is burnback of the optical fiber tip. So far, researchers identified mechanical, thermal, and optical factors as mechanisms in burnback phenomena. Among mechanical factors, the force applied by urologists against a stone is expected to play a dominant role in burnback. In this study, we introduce a novel technique to measure accurately the stone depth and volume ablation under varying force. Our results show varying burnback lengths on the optical fibers and varying stone depth and volume ablation depending on the optical fiber core size. For instance, the slope of the burnback as a function of the applied force for 273 μm fibers was more than two times higher than for the 550 μm fibers. The slope of the total volume of stone ablated as function of force for 550 μm fibers was almost twice as much as for the 273 μm fibers. The data suggest urologists can maximize the stone ablation rate and minimize fiber tip burnback by controlling the applied force on the optical fiber during a lithotripsy procedure.
A previous in vitro study demonstrated that 180W was the optimal power to reduce photoselective vaporization of the
prostate (PVP) time for larger prostate glands. In this study, we investigated anatomic and histologic outcomes and
ablation parameters of 180W laser performed with a new 750-μm side-firing fiber in a survival study of living canines.
Eight male canines underwent anterograde PVP with the 180W 532-nm laser. Four each animals were euthanized 3
hours or 8 weeks postoperatively. Prostates were measured and histologically analyzed after hematoxylin and eosin
(H&E), triphenyltetrazolium chloride (TTC), or Gomori trichrome (GT) staining. Compared to the previous 120W laser,
PVP with the 180W laser bloodlessly created a 76% larger cavity (mean 11.8 vs. 6.7 cm3; p=0.014) and ablated tissue at
a 77% higher rate (mean 2.3 vs. 1.3 cm3/min; p=0.03) while H&E- and TTC-staining demonstrated its 33% thicker mean
coagulation zone (2.0±0.4 vs. 1.5±0.3 mm). H&E-stained cross-sectional prostatic tissue specimens from the 3-hour
(acute) group showed histologic evolution of concentric non-viable coagulation zone, partially viable hyperemic
transition zone of repair, and viable non-treated zone. H&E- and GT-stained specimens from the 8-week (chronic) group
revealed healed circumferentially epithelialized, non-edematous, prostatic urethral channels with no increase in collagen
in the subjacent prostatic tissue vis-á-vis the normal control. Our canine study demonstrates that 180W 532-nm laser
PVP with its new fiber has a significantly higher ablation rate with a more hemostatic coagulation zone, but equally
favorable tissue interaction and healing, compared with our previous 120W canine study.
Photoselective vaporization of the prostate (PVP) has been developed for effective treatment of obstructive benign
prostatic hyperplasia. To maximize tissue ablation for large prostate gland, identifying the optimal power level for PVP
is still necessary. We investigated the effect of various power levels on in vitro bovine prostate ablation with a 532-nm
laser system. A custom-made 532-nm laser was employed to provide various power levels, delivered through a newly
designed 750-μm side-firing fiber. Tissue ablation efficiency was evaluated in terms of power (P; 120~200W), treatment
speed of fiber (TS; 2~8 mm/s), and working distance between fiber and tissue surface (WD; 1~5 mm). Coagulation
depth was also estimated macroscopically and histologically (H&E) at various Ps. Both 180 and 200W yielded
comparable ablated volume (104.3±24.7 vs. 104.1±23.9 mm3 at TS=4 mm/s and WD=2 mm; p=0.99); thus, 180W was
identified as the optimal power to maximize tissue ablation, by removing tissue up to 80% faster than 120W (41.7±9.9
vs. 23.2±3.4 mm3/s at TS=4 mm/s and WD=2 mm; p<0.005). Tissue ablation was maximized at TS=4 mm/s and ablated
equally efficiently at up to 3 mm WD (104.5±16.7 mm3 for WD=1 mm vs. 93.4±7.4 mm3 for WD=3 mm at 180W;
p=0.33). The mean thickness of coagulation zone for 180W was 20% thicker than that for 120W (1.31±0.17 vs.
1.09±0.16 mm; p<0.005). The current in vitro study demonstrated that 180W was the optimal power to maximize tissue
ablation efficiency with enhanced coagulation characteristics.
A novel MoXyTM fiber delivery device with Active Cooling Cap (ACCTM) is designed to
transmit up to 180W of 532 nm laser light to treat benign prostatic hyperplasia (BPH). Under
such high power tissue ablation, effective cooling is key to maintaining fiber power transmission
and ensuring the reliability of the fiber delivery device To handle high power and reduce fiber
degradation, the MoXy fiber features a larger core size (750 micrometer) and an internal fluid
channel to ensure better cooling of the fiber tip to prevent the cap from burning, detaching, or
shattering during the BPH treatment. The internal cooling channel was created with a metal cap
and tubing that surrounds the optical fiber. In this study MoXy fibers were used to investigate
the effect of power levels of 120 and 200 W on in-vitro bovine prostate ablation using a 532 nm
XPSTM laser system. For procedures requiring more than 100 kJ, the MoXy fiber at 200W
removed tissue at twice the rate of the current HPS fiber at 120W. The fiber maintained a
constant tissue vaporization rate during the entire tissue ablation process. The coagulation at
200W was about 20% thicker than at 120W. In conclusion, the new fibers at 200W doubled the
tissue removal rate, maintained vaporization efficiency throughout delivery of 400kJ energy, and
induced similar coagulation to the existing HPS fiber at 120W.
Laser prostatectomy with various lasers has been shown to be effective in the treatment of benign prostate hyperplasia. However, the impact of laser parameters on tissue ablation is still in question. The aim of this study is to experimentally characterize laser-tissue interactions in terms of wavelength by comparing visible (λ= 532 nm) and infrared (λ= 2.01 μm) spectra. Porcine kidney tissue was used as it has thermal properties and glandular structure similar to human prostatic tissue. Q-switched 532 nm (GreenLightTM HPS) and continuous-wave (CW) 2.01 μm (custom-made Tm:YAG) lasers were employed to remove soft tissue under various settings (power, working distance, and treatment speed). For both laser systems, ablation rate increased with power and was maximized at 4 mm/s. The 532 nm laser generated approximately 30% (p<0.005) higher ablation efficiency than the
IR laser. A comparable ablation depth was found between the two wavelengths, but the 532nm laser generated relatively wider (up to 30%; p<0.005) craters. Owing to constant heating due the CW mode, the IR laser induced 20% thicker coagulation depth than the 532 nm (0.94 vs. 0.8 mm at 100 W; p<0.005). Histology also confirmed
coagulation depth in response to each wavelength. Due to light absorption in aqueous environment, the IR laser exhibited a dramatic decrease in power transmission and ablation volume with increasing working distance whereas the 532 nm laser maintained relatively constant features. In conclusion, the characteristics of tissue ablation were contingent upon the applied wavelengths due to optical properties and laser parameters.
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