Free electron laser lithotripsy: threshold radiant exposures

Warm irrigation fluid effect on Thulium fiber laser (TFL) ablation of uroliths

Prior laser studies have demonstrated that as the temperature of a medium increases, the amount of energy delivered to the target increases. We sought to investigate the role of irrigation fluid temperature on Thulium fiber laser (TFL) urolith ablation. 360 calculi were divided in vitro according to chemical composition: calcium oxalate monohydrate (COM), cystine (CYS), struvite (STR), calcium phosphate (CAP), uric acid (UA), and calcium oxalate dihydrate (COD). A 200 μm TFL was placed directly on each stone, while immersed in 0.9% NaCl at four different temperatures (25 C, 37 C, 44 C, 60 C) and a single laser pulse administered at distinct energy settings (0.1 J, 0.5 J, 1.5 J). Optical coherence tomography assessed the resulting ablation cone volume. Mean stone volume and porosity were evaluated through ANOVA and Tukey post-hoc analysis. A multivariate generalized model for each composition accounted for the impact of fluid temperature and laser energy on stone ablation. Warmer fluid temperatures yielded greater ablation cone volumes for most energy settings, excluding UA stones. When accounting for chemical composition, higher tensile strength stones (COM, CYS) benefited most from warmer fluid in comparison to frangible stones (CAP, STR). The effects of increasing fluid temperature are modest relative to laser pulse energy as a large temperature increase (i.e. 7ºC) is equivalent to a minor energy increase (i.e. 0.1 J). For non-UA stones, TFL ablation efficiency increases with warmer irrigation fluid. The effect, albeit modest compared to laser pulse energy, was most notable for COM and CYS stones.

Laser Heating of Fluid With and Without Stone Ablation: In Vitro Assessment

Introduction: Laser lithotripsy can cause excessive heating of fluid within the collecting system and lead to tissue damage. To better understand this effect, it is important to determine the percentage of applied laser energy that is converted to heat and the percentage used for stone ablation. Our objective was to calculate the percentage of laser energy used for stone ablation based on the difference in fluid temperature measured in an in vitro model when the laser was activated without and with stone ablation. Methods: Flat BegoStone disks (15:5) were submerged in 10 mL of deionized water at the bottom of a vacuum evacuated double-walled glass Dewar. A Moses 200 D/F/L laser fiber was positioned above the surface of the stone at a distance of 3.5 mm for control (no stone ablation) or 0.5 mm for experimental (ablation) trials. The laser was activated and scanned at 3 mm/second across the stone in a preprogrammed pattern for 30 seconds at 2.5 W (0.5 J × 5 Hz) for both short-pulse (SP) and Moses distance (MD) modes. Temperature of the fluid was recorded using two thermocouples once per second. Results: Control trials produced no stone ablation, while experimental trials produced a staccato groove in the stone surface, simulating efficient lithotripsy. The mean temperature increase for SP was 1.08°C ± 0.04°C for control trials and 0.98°C ± 0.03°C for experimental trials, yielding a mean temperature difference of 0.10°C ± 0.06°C (p = 0.0005). With MD, the mean temperature increase for control trials was 1.03°C ± 0.01°C and for experimental trials 0.99°C ± 0.06°C, yielding a smaller mean temperature difference of 0.04°C ± 0.06°C (p = 0.09). Conclusions: Even under conditions of energy-efficient stone ablation, the majority of applied laser energy (91%-96%) was converted to heat.

The Efficiency of Moses Technology Holmium Laser for Treating Renal Stones During Flexible Ureteroscopy: Relationship Between Stone Volume, Time, and Energy

Introduction: There are limited data on how stone factors relate to flexible ureteroscopy (f-URS) laser lithotripsy efficiency and outcomes when using the Moses Technology Ho:YAG system. We examined the relationship of stone volume and density to lithotripsy, lasing times, and energy used to treat a single renal stone. We also assessed short-term clinical outcomes. Methods: We analyzed patients undergoing f-URS by a single surgeon using high-power Moses Technology Ho:YAG system (Lumenis). We only included cases with a CT confirming a solitary renal stone. Ureteral stones, staged and bilateral procedures were excluded. Stone dimensions and HU were obtained. Volume (mm³) was calculated using European Association of Urology criteria. Laser energy (J), lithotripsy, and lasing times were obtained. Laser activity was calculated by dividing lasing time by lithotripsy time. Relationships between time, stone density, volume, and energy were assessed using Spearman correlation. Complications were assessed using Clavien-Dindo grade. Residual fragments (RF) were determined on imaging within 3 months. Results: Twenty-nine patients met the inclusion criteria. Mean (range) stone volume and density were 290 mm³ (42-1700) and 814 HU (170-1675), respectively. Mean lithotripsy and lasing times were 11.9 (3.6-26.0) and 6.0 (0.6-19.6) minutes, respectively. Mean laser activity was 47%. Mean fragmentation speed was 0.9 mm³/s. Mean energy used per unit stone volume was 38.2 J/mm³. Time taken to perform fragmentation had a stronger association with the stone volume vs stone density. Three (10.3%) and 2 (6.9%) patients had a Clavien Grade 1 and 2 complications, respectively. At follow-up the zero-fragment rate was 79.3%. Conclusions: When using the Moses Technology laser to ablate a single renal stone with f-URS, the fragmentation speed was ∼1 mm³/s. Stone volume, not density was correlated to lasing time. We propose mm³/s be considered a measure that has implications for technique efficiency and comparing laser platforms.

Mechanisms of Pulse Modulated Holmium:YAG Lithotripsy

Introduction: This study aimed at answering three research questions: (1) Under the experimental conditions studied, what is the dominant mechanism of Holmium:YAG lithotripsy with or without pulse modulation? (2) Under what circumstances can laser pulse modulation increase crater volume of stone ablation per joule of emitted radiant energy? (3) Are BegoStone phantoms a suitable model for laser lithotripsy studies? Materials and Methods: The research questions were addressed by ablation experiments with BegoStone phantoms and native stones. Experiments were performed under three stone conditions: dry stones in air, hydrated stones in air, and hydrated stones in water. Single pulses with and without pulse modulation were applied. For each pulse mode, temporal profile, transmission through 1 mm water, and cavitation bubble collapse pressures were measured and compared. For each stone condition and pulse mode, stones were ablated with a fiber separation distance of 1 mm and crater volumes were measured using optical coherence tomography. Results: Pulses with and without pulse modulation had high (>80%) transmission through 1 mm of water. Pulses without pulse modulation generated much higher peak pressures than those with pulse modulation (62.3 vs 11.4 bar). Pulse modulation resulted in similar or larger craters than without pulse modulation. Trends in BegoStone crater volumes differed from trends in native stones. Conclusions: This results of this study suggest that the dominant mechanism is photothermal with possible photoacoustic contributions for some stone compositions. Pulse modulation can increase ablation volume per joule of emitted radiant energy, but the effect may be composition specific. BegoStones showed unique infrared ablation characteristics compared with native stones and are not a suitable model for laser lithotripsy studies.

Stone ablation rates using innovative pulse modulation technology: Vapor tunnel, virtual basket, and bubble blast. An in vitro experimental study

INTRODUCTION AND OBJECTIVES

Virtual Basket^TM , Bubble Blast^TM , and Vapor Tunnel^TM are three laser pulse modulation technologies that modify the holmium: yttrium-aluminum-garnet (Ho:YAG) laser pulse transmission through the creation of bubbles emerging from the fiber tip with different effects on the target stone. The primary outcome of the current study was to test the stone ablation rates for the different pulse modulation modes, Virtual Basket, Bubble Blast, and Vapor Tunnel, using different power, energy, and frequency settings.

MATERIALS AND METHODS

Quanta Cyber: Ho 150 W^TM , a 365 µm Precision^TM fiber, and hard and soft phantom BegoStones^TM were used in an in vitro experimental configuration in a saline bath. In the Virtual Basket mode, the combinations of power, energy and frequency were tested; 10 W = 0.5 J × 20 Hz, 10 W = 0.5 J × 20 Hz, 60 W = 1 J × 60 Hz and 60 W = 2 J × 30 Hz. In the Bubble Blast mode, the combinations, 12 W = 1.2J × 10 Hz, 60 W = 1.2J × 50 Hz and 60 W = 2 J × 30 Hz, were tested. Similarly, the combination of 10 W = 0.5 J × 20 Hz was tested with Vapor Tunnel mode. High-speed camera captures of the bubble formation and regular photographs of the fragmentation pattern were also taken for each mode.

RESULTS

High power lithotripsy was faster and related to higher ablation rates. The Virtual Basket, Bubble Blast, and Vapor Tunnel modalities showed different ablation rates for the same energy and frequency settings. For hard stones, there was an improvement in the ablation rate using 60 W = 2 J × 30 Hz compared with 60 W = 1 J × 60 Hz and 60 W = 1.2 J × 50 Hz. The highest ablation rates were recorded using the Virtual Basket mode with the high-power settings of 2 J of energy and 30 Hz of frequency.

CONCLUSIONS

The Virtual Basket^TM pulse-modulation technology was related to the highest ablation rates for both hard and soft stones, compared to the Bubble Blast^TM and the Vapor Tunnel^TM technologies in high-power and low-power lithotripsy respectively. For the same high power settings, higher energy seems to provide higher ablation rates.

Gas Bubble Anatomy During Laser Lithotripsy: An Experimental In Vitro Study of a Pulsed Solid-State Tm:YAG and Ho:YAG Device

Purpose: To examine gas bubbles generated by two laser lithotripsy devices, a pulsed thulium solid-state laser and a holmium:yttrium-aluminum-garnet (Ho:YAG) device, and their possible effects in lithotripsy. Materials and Methods: We investigated two Dornier laser devices, a Medilas H Solvo 35 and a pulsed solid-state thulium laser evaluation model (Dornier MedTech Laser GmbH, Wessling, Germany). Our setup consisted of a water-filled glass tank heated to 37/60/70°C. Different laser power/frequency settings and short/long pulses were examined for both laser devices. We analyzed the impact of degraded, cut, and broken fibers on gas bubble anatomy. Furthermore, high-speed recordings of BegoStone ablation were analyzed. For all recordings, we used a Photron Nova S12 camera. Results: These two devices produced differently shaped gas bubbles under different fiber conditions, temperatures, power settings, and short and long pulse settings, which explain the differing repulsive force and pressure values. Inside the gas bubble, a cone was visible whose angle correlates with the protruding jet. We observed turbulences and swirls moving back and forth the fiber tip. During fragmentation, sparks are generated that demonstrate the photothermal effect, and we recorded stone fragments being pulled toward the fiber. Both devices showed comparable results with differences mainly due to pulse lengths. Conclusion: The shapes of the vapor bubbles formed during laser lithotripsy depend on several factors. Excessive transoperative fiber cleavage seems to be unnecessary. Due to the large gas bubbles observed and because of the amount of potential pressure generated, only low energies should be applied in the ureter.

Assessing the Role of Light Absorption in Laser Lithotripsy by Isotopic Substitution of Kidney Stone Materials

Understanding the chemical characteristics of kidney stones and how the stone composition affects their fragmentation is key to improving clinical laser lithotripsy. During laser lithotripsy, two mechanisms may be responsible for stone fragmentation: a photothermal mechanism and/or microexplosion mechanism. Herein, we carry out an isotopic substitution of crystal H₂O with D₂O in calcium oxalate monohydrate and struvite stones to alter their optical properties to study the relationship between the absorption of the stones, at the wavelength of the Ho:YAG (2.12 μm) laser, and the fragmentation behavior. Changing the absorption of the stones at 2.12 μm changes the extent of fragmentation, whereas changing the absorption of the bulk medium has a negligible effect on fragmentation, leading to the conclusion that kidney stone ablation is dominated by a photothermal mechanism.

How Lasers Ablate Stones: In Vitro Study of Laser Lithotripsy (Ho:YAG and Tm-Fiber Lasers) in Different Environments

Introduction: There are two main mechanisms of stone ablation with long-pulsed infrared lasers: photothermal and photomechanical. Which of them is primary in stone destruction is still a matter of discussion. Water holds importance in both mechanisms but plays a major role in the latter. We sought to identify the prevailing mechanism of stone ablation by evaluating the stone mass loss after lithotripsy in different media. Materials and Methods: We tested a holmium:yttrium-aluminum-garnet (Ho:YAG) laser (100 W; Lumenis), a thulium-fiber laser U1 (TFL U1) (120 W; NTO IRE-Polus, Russia), and a SuperPulse thulium-fiber laser U2 (TFL U2) (500 W; NTO IRE-Polus). A single set of laser parameters (15 W = 0.5 J × 30 Hz) was used. Contact lithotripsy was performed in phantoms (BegoStones) in different settings: (a) hydrated phantoms in water, (b) hydrated phantoms in air, (c) dehydrated phantoms in water, and (d) dehydrated phantoms in air. Laser ablation was performed with total energy of 0.3 kJ. Phantom mass loss was defined as the difference between the initial phantom mass and the final phantom mass of the ablated phantoms. Results: All lasers demonstrated effective ablation in hydrated phantoms ablated in water; no visual differences between the lasers were detected. The ablation of dehydrated phantoms in air was also effective with visible vapor during ablation and condensation on the cuvette wall. Dehydrated phantoms in water and in air show minimal to no ablation accompanied with formation of white crust on phantom surface. Among laser types, TFL U2 had the highest phantom mass loss in all groups except for dehydrated phantoms ablated in air. Conclusions: Our results suggest that both photothermal and thermomechanical ablation mechanisms (explosive vaporization) occur in parallel during laser lithotripsy. In Ho:YAG and TFL U2 stone ablation explosive vaporization prevails, whereas in TFL U1 ablation photothermal mechanism appears to predominate.

Polymer-Mineral Composites Mimic Human Kidney Stones in Laser Lithotripsy Experiments

Despite the widespread use of laser lithotripsy to fragment kidney stones in vivo, there is a lack of robust artificial stone models to replicate the behavior of human stones during lithotripsy procedures. This need for accurate stone models is particularly important as novel laser technologies are introduced in the field of lithotripsy. In this work, we present a method to prepare composite materials that replicate the properties of human kidney stones during laser lithotripsy. Their behavior is understood through the lens of near-IR spectroscopy and helps to elucidate the mechanism of laser lithotripsy in kidney stone materials.

High power holmium:YAG versus thulium fiber laser treatment of kidney stones in dusting mode: ablation rate and fragment size studies

OBJECTIVES

The experimental Thulium fiber laser (TFL) is currently being studied as a potential alternative to the gold standard Holmium:YAG laser for lithotripsy. Recent advances in both Holmium and TFL technology allow operation at similar laser parameters for direct comparison. The use of a "dusting" mode with low pulse energy (0.2-0.4 J) and high pulse rate (50-80 Hz) settings, is gaining popularity in lithotripsy due to the desire to produce smaller residual stone fragments during ablation, capable of being spontaneously passed through the urinary tract.

METHODS

In this study, Holmium and TFL were directly compared for 'dusting' using three laser groups, G1: 0.2 J/50 Hz/10 W; G2: 0.2 J/80 Hz/16 W; and G3: 0.4 J/80 Hz/32 W. Holmium laser pulse durations ranged from 200 to 350 μs, while TFL pulse durations ranged from 500 to 1,000 μs, due to technical limitations for both laser systems. An experimental setup consisting of 1 × 1 cm cuvette with 1 mm sieve was used with continuous laser operation time limited to ≤5 minutes. Calcium oxalate monohydrate stone samples with a sample size of n = 5 were used for each group, with average initial stone mass ranging from 216 to 297 mg among groups.

RESULTS

Holmium laser ablation rates were lower than for TFL at all three settings (G1: 0.3 ± 0.2 vs. 0.8 ± 0.2; G2: 0.6 ± 0.1 vs. 1.0 ± 0.4; G3: 0.7 ± 0.2 vs. 1.3 ± 0.9 mg/s). The TFL also produced a greater percentage by mass of stone dust (fragments <0.5 mm) than Holmium laser. For all three settings combined, one out of 15 (7%) stones treated with Holmium laser were completely fragmented in ≤5 minutes compared to nine out of 15 (60%) stones treated with TFL.

CONCLUSIONS

These preliminary studies demonstrate that the TFL is a promising alternative laser for lithotripsy when operated in dusting mode, producing higher stone ablation rates and smaller stone fragments than the Holmium laser. Clinical studies are warranted. Lasers Surg. Med. 51:522-530, 2019. © 2019 Wiley Periodicals, Inc.

Recent advances in infrared laser lithotripsy [Invited]

The flashlamp-pumped, solid-state, pulsed, mid-infrared, holmium:YAG laser (λ = 2120 nm) has been the clinical gold standard laser for lithotripsy for over the past two decades. However, while the holmium laser is the dominant laser technology in ureteroscopy because it efficiently ablates all urinary stone types, this mature laser technology has several fundamental limitations. Alternative, mid-IR laser technologies, including a thulium fiber laser (λ = 1908 and 1940 nm), a thulium:YAG laser (λ = 2010 nm), and an erbium:YAG laser (λ = 2940 nm) have also been explored for lithotripsy. The capabilities and limitations of these mid-IR lasers are reviewed in the context of the quest for an ideal laser lithotripsy system capable of providing both rapid and safe ablation of urinary stones.

Intracorporeal lithotripsy

Since the introduction of ESWL, PNL and URS during the early 1980s the application rate of ESWL has declined while those of PNL and URS have increased. This is mainly due to the facts that instruments and techniques for Intracorporeal Lithotripsy (IL) have made a continuous progress. This review shows that today an array of options for IL within the entire urinary tract is available to treat stones in a perfect minimal invasive way. At the same time further improvements of IL are already visible.

Stone/tissue differentiation for holmium laser lithotripsy using autofluorescence

BACKGROUND AND OBJECTIVES

Holmium laser lithotripsy is a safe and effective method to disintegrate urinary stones of all compositions in an endoscopic procedure. However, handling and safety could be improved by a real-time feedback system permanently monitoring the position of the treatment fiber. The laser is fired only when the fiber is identified as being placed in front of stone. This work evaluates the potential of fluorescence detection with an excitation wavelength of 532 nm for this purpose.

MATERIALS AND METHODS

A fiber-based fluorescence measurement was set-up to acquire autofluorescence signals from several human renal calculi, artificial stones, and porcine tissue samples (renal calix and ureter). Three different approaches were investigated. First, experiments were performed with a pulsed laser source with a wavelength of 532 nm, pulse energy 36.5 ± 1 μJ, pulse duration 1.2 ± 0.5 nanoseconds, and a repetition rate of 1 kHz with 15 urinary concretions. In the second step, a series of measurements on 42 human urinary calculi samples was carried out using low power continuous wave excitation of 0.4 ± 0.1 mW. Fluorescence was also measured simultaneously to stone fragmentation by holmium laser pulses (pulse energy 240 ± 50 mJ, repetition rate 10 Hz). Finally, a modulated excitation/detection scheme (lock-in technique) was implemented to render fluorescence detection insensitive to white background light.

RESULTS

Unlike porcine renal calix, ureter, and artificial stone human urinary calculi show a strong fluorescence signal when excited with 532 nm. With pulsed excitation on urinary stone (20,000 ± 11,000) counts were registered at 587 nm with the CCD-array of a grating spectrometer in an integration time of 50 milliseconds. Tissue gave lower count rates of ≤(5,500 ± 1,100) even with longer integration times (500 milliseconds/1 second). With a cw excitation power of 0.4 mW (13,000 ± 11,000) counts were registered in an integration time of 200 milliseconds at 587 nm (porcine renal calix: (770 ± 340)). Modulated excitation (66 Hz) with an average power of 0.3 mW and detection with a photodiode resulted in a lock-in amplifier signal of 1.5-4.3V on stone (background and skin: <0.5V).

CONCLUSION

With the lock-in technique, autofluorescence from stones can be detected with only the average excitation power of a green aiming beam overlaid to the Ho:YAG-laser beam (power ≤ 1 mW). Since tissue shows very little autofluorescence when excited with 532 nm, this fluorescence signal enables monitoring of the correct position of the treatment fiber during ureteroscopic procedures.

Therapeutic applications of lasers in urology: an update

There has been renewed interest in the use of lasers for minimally invasive treatment of urologic diseases in recent years. The introduction of more compact, higher power, less expensive and more user-friendly solid-state lasers, such as the holmium:yttrium-aluminum-garnet (YAG), frequency-doubled neodymium:YAG and diode lasers has made the technology more attractive for clinical use. The availability of small, flexible, biocompatible, inexpensive and disposable silica optical fiber delivery systems for use in flexible endoscopes has also promoted the development of new laser procedures. The holmium:YAG laser is currently the workhorse laser in urology since it can be used for multiple soft- and hard-tissue applications, including laser lithotripsy, benign prostate hyperplasia, bladder tumors and strictures. More recently, higher power potassium-titanyl-phosphate lasers have been introduced and show promise for the treatment of benign prostatic hyperplasia. On the horizon, newer and more effective photosensitizing drugs are being tested for potential use in photodynamic therapy of bladder and prostate cancer. Additionally, new experimental lasers such as the erbium:YAG, Thulium and Thulium fiber lasers, may provide more precise incision of soft tissues, more efficient laser lithotripsy and more rapid prostate ablation. This review provides an update on the most important new clinical and experimental therapeutic applications of lasers in urology over the past 5 years.

Preclinical in vivo evaluation of NPe6-mediated photodynamic therapy on normal vasculature

BACKGROUND AND OBJECTIVE

Current treatments of port-wine stain birthmarks typically involve use of a pulsed dye laser (PDL) combined with cooling of the skin. Currently, PDL therapy protocols result in varied success, as some patients experience complete blanching, while others do not. Over the past decade, we have studied the use of photodynamic therapy (PDT) as either a replacement or adjuvant treatment option to photocoagulate both small and large vasculature. The objective of the current study was to evaluate a PDT protocol that involves use of an alternate intravascular photosensitizer mono-L-aspartylchlorin-e6 (NPe6) activated by an array of low-cost light emitting diodes.

STUDY DESIGN/MATERIALS AND METHODS

To monitor the microvasculature, a dorsal window chamber model was installed on 22 adult male mice. The light source consisted of a custom-built LED array that emitted 10 W at a center wavelength of 664 nm (FWHM = 20 nm). The light source was positioned at a fixed distance from the window chamber to achieve a fixed irradiance of 127 mW/cm(2). A retroorbital injection of NPe6 (5 mg/kg) was performed to deliver the drug into the bloodstream. Laser irradiation was initiated immediately after injection. To monitor blood-flow dynamics in response to PDT, we used laser speckle imaging. We employed a dose-response experimental design to evaluate the efficacy of NPe6-mediated PDT.

RESULTS

We observed three general hemodynamic responses to PDT: (1) At low radiant exposures, we did not observe any persistent vascular shutdown; (2) at intermediate radiant exposures, we observed an acute decrease in blood flow followed by gradual restoration of blood flow over the 7-day monitoring period; and (3) at high radiant exposures, we observed acute vascular shutdown that persisted during the entire 7-day monitoring period. Dose-response analysis enabled identification of 85 J/cm(2) as a characteristic radiant exposure required to achieve persistent vascular shutdown at Day 7 following PDT.

CONCLUSION

The experimental data suggest that NPe6-mediated PDT can achieve persistent vascular shutdown of normal microvasculature.

Optimal power settings for Holmium:YAG lithotripsy

PURPOSE

We determined the optimal Ho:YAG lithotripsy power settings to achieve maximal fragmentation, minimal fragment size and minimal retropulsion.

MATERIALS AND METHODS

Stone phantoms were irradiated in water with a Ho:YAG laser using a 365 μm optical fiber. Six distinct power settings were tested, including 0.2 to 2.0 J and 10 to 40 Hz. For all cohorts 500 J total radiant energy were delivered. A seventh cohort (0.2 J 40 Hz) was tested post hoc to a total energy of 1,250 J. Two experimental conditions were tested, including with and without phantom stabilization. Total fragmentation, fragment size and retropulsion were characterized. In mechanism experiments using human calculi we measured crater volume by optical coherence tomography and pressure transients by needle hydrophone across similar power settings.

RESULTS

Without stabilization increased pulse energy settings produced increased total fragmentation and increased retropulsion (each p <0.0001). Fragment size was smallest for the 0.2 J cohorts (p <0.02). With stabilization increased pulse energy settings produced increased total fragmentation and increased retropulsion but also increased fragment size (each p <0.0001). Craters remained symmetrical and volume increased as pulse energy increased. Pressure transients remained modest at less than 30 bars even at 2.0 J pulse energy.

CONCLUSIONS

Holmium:YAG lithotripsy varies as pulse energy settings vary. At low pulse energy (0.2 J) less fragmentation and retropulsion occur and small fragments are produced. At high pulse energy (2.0 J) more fragmentation and retropulsion occur with larger fragments. Anti-retropulsion devices produce more efficient lithotripsy, particularly at high pulse energy. Optimal lithotripsy laser dosimetry depends on the desired outcome.

Comparison of holmium:YAG and thulium fiber laser lithotripsy: ablation thresholds, ablation rates, and retropulsion effects

The holmium:YAG (Ho:YAG) laser lithotriptor is capable of operating at high pulse energies, but efficient operation is limited to low pulse rates (∼10 Hz) during lithotripsy. On the contrary, the thulium fiber laser (TFL) is limited to low pulse energies, but can operate efficiently at high pulse rates (up to 1000 Hz). This study compares stone ablation threshold, ablation rate, and retropulsion for the two different Ho:YAG and TFL operation modes. The TFL (λ = 1908 nm) was operated with pulse energies of 5 to 35 mJ, 500-μs pulse duration, and pulse rates of 10 to 400 Hz. The Ho:YAG laser (λ = 2120 nm) was operated with pulse energies of 30 to 550 mJ, 350-μs pulse duration, and a pulse rate of 10 Hz. Laser energy was delivered through 200- and 270-μm-core optical fibers in contact mode with human calcium oxalate monohydrate (COM) stones for ablation studies and plaster-of-Paris stone phantoms for retropulsion studies. The COM stone ablation threshold for Ho:YAG and TFL measured 82.6 and 20.8 J∕cm(2), respectively. Stone retropulsion with the Ho:YAG laser linearly increased with pulse energy. Retropulsion with TFL was minimal at pulse rates less than 150 Hz, then rapidly increased at higher pulse rates. For minimal stone retropulsion, Ho:YAG operation at pulse energies less than 175 mJ at 10 Hz and TFL operation at 35 mJ at 100 Hz is recommended, with both lasers producing comparable ablation rates. Further development of a TFL operating with both high pulse energies of 100 to 200 mJ and high pulse rates of 100 to 150 Hz may also provide an alternative to the Ho:YAG laser for higher ablation rates, when retropulsion is not a primary concern.

Holmium:YAG (lambda = 2,120 nm) versus thulium fiber (lambda = 1,908 nm) laser lithotripsy

INTRODUCTION

The holmium:YAG laser is currently the most common laser lithotripter. However, recent experimental studies have demonstrated that the thulium fiber laser is also capable of vaporizing urinary stones. The high-temperature water absorption coefficient for the thulium wavelength (mu(a) = 160 cm(-1) at lambda = 1,908 nm) is significantly higher than for the holmium wavelength (mu(a) = 28 cm(-1) at lambda = 2,120 nm). We hypothesize that this should translate into more efficient laser lithotripsy using the thulium fiber laser. This study directly compares stone vaporization rates for holmium and thulium fiber lasers.

METHODS

Holmium laser radiation pulsed at 3 Hz with 70 mJ pulse energy and 220 microseconds pulse duration was delivered through a 100-microm-core silica fiber to human uric acid (UA) and calcium oxalate monohydrate (COM) stones, ex vivo (n = 10 each). Thulium fiber laser radiation pulsed at 10 Hz with 70 mJ pulse energy and 1-millisecond pulse duration was also delivered through a 100-microm fiber for the same sets of 10 stones each.

RESULTS

For the same number of pulses and total energy (126 J) delivered to each stone, the mass loss averaged 2.4+/-0.6 mg (UA) and 0.7+/-0.2 mg (COM) for the holmium laser and 12.6+/-2.5 mg (UA) and 6.8+/-1.7 (COM) for the thulium fiber laser.

CONCLUSIONS

UA and COM stone vaporization rates for the thulium fiber laser averaged 5-10 times higher than for the holmium laser at 70 mJ pulse energies. With further development, the thulium fiber laser may represent an alternative to the conventional holmium laser for more efficient laser lithotripsy.

Lasers in clinical urology: state of the art and new horizons

We present an overview of current and emerging lasers for Urology. We begin with an overview of the Holmium:YAG laser. The Ho:YAG laser is the gold standard lithotripsy modality for endoscopic lithotripsy, and compares favorably to standard electrocautery transurethral resection of the prostate for benign prostatic hyperplasia (BPH). Available laser technologies currently being studied include the frequency doubled double-pulse Nd:Yag (FREDDY) and high-powered potassium-titanyl-phosphate (KTP) lasers. The FREDDY laser presents an affordable and safe option for intracorporeal lithotripsy, but it does not fragment all stone compositions, and does not have soft tissue applications. The high power KTP laser shows promise in the ablative treatment of BPH. Initial experiments with the Erbium:YAG laser show it has improved efficiency of lithotripsy and more precise ablative and incisional properties compared to Ho:YAG, but the lack of adequate optical fibers limits its use in Urology. Thulium:YAG fiber lasers have also demonstrated tissue ablative and incision properties comparable to Ho:YAG. Lastly, compact size, portability, and low maintenance schedules of fiber lasers may allow them to shape the way lasers are used by urologists in the future.

Erbium: YAG laser lithotripsy mechanism

PURPOSE

We tested the hypothesis that the mechanism of long pulse erbium:YAG laser lithotripsy is photothermal.

MATERIALS AND METHODS

Human urinary calculi were placed in deionized water and irradiated with erbium:YAG laser energy delivered through a sapphire optical fiber. Erbium:YAG bubble dynamics were visualized with Schlieren flash photography and correlated to acoustic emissions measured by a polyvinylidene fluoride needle hydrophone. The sapphire fiber was placed either parallel or perpendicular to the calculus surface to assess the contribution of acoustic transients to fragmentation. Stones were irradiated using desiccated stone irradiated in air, hydrated stone irradiated in air and hydrated stone irradiated in water. Ablation crater sizes were compared. Uric acid stones were irradiated in water and the water was assayed for cyanide.

RESULTS

During the early phase of vapor bubble expansion, acoustic transients had minimal effects on calculus fragmentation. Fragmentation occurred due to direct absorption of laser energy transmitted to the calculus through the vapor channel between the sapphire fiber tip and calculus. The forward axial expansion of the bubble occurred more rapidly than the radial expansion. A parallel oriented fiber on the calculus surface produced no fragmentation but generated larger amplitude acoustic transients compared to perpendicular orientation. In perpendicular orientation the erbium:YAG laser did not generate any collapse acoustic waves but fragmentation occurred. Crater width was greatest for desiccated stones irradiated in air (p <0.03). Cyanide production increased as erbium:YAG irradiation of uric acid calculi increased, (r2 = 0.98).

CONCLUSIONS

The erbium:YAG laser fragments stones through a photothermal mechanism.

Laser lithotripsy

PURPOSE OF REVIEW

All literature related to laser lithotripsy published within the past year was reviewed. Salient articles have been reviewed and grouped according to safety issues, efficacy, comparison studies, biliary applications or future directions.

RECENT FINDINGS

There is no evidence of renal deterioration after holmium:yttrium-aluminium-garnet lithotripsy. Laser-related complications occur in less than 1%. Stone-free rates from holmium:yttrium-aluminium-garnet lithotripsy are greater than 90% for ureteral calculi, and 67-84% for renal calculi. This method of lithotripsy is effective for ureteral and renal calculi in morbidly obese patients who are not suitable candidates for shock-wave lithotripsy or percutaneous nephrolithotomy. Holmium:yttrium-aluminium-garnet lithotripsy is more effective than pneumatic lithotripsy for ureteral calculi, but no more effective than shock-wave lithotripsy (Dornier HM-3) for distal ureteral calculi. Holmium:yttrium-aluminium-garnet lithotripsy of biliary calculi is uniformly effective. Preliminary data showed the erbium:yttrium-aluminium-garnet laser to be more efficient than holmium:yttrium-aluminium-garnet energy, but current erbium:yttrium-aluminium-garnet fibers are impractical.

SUMMARY

The holmium:yttrium-aluminium-garnet laser is safe and effective. It is the lithotrite of choice for endoscopic ureteral and ureterorenoscopic lithotripsy.

A perspective on laser lithotripsy: the fragmentation processes

This paper describes in simple terms the physics of laser-calculus interactions and introduces a method with which physicians can understand or evaluate the application of any new laser technique for use in lithotripsy or other medical fields. Tissue optical properties and laser parameters govern the mechanism(s) of fragmentation of urinary or biliary calculi. Laser pulse energies for clinical lithotripsy range from Q0 = 20 mJ to 2 J for short-pulsed lasers to long-pulsed lasers, respectively. Lasers with short pulse durations (i.e., less than a few microseconds) fragment calculi by means of shockwaves following optical breakdown and plasma expansion of ionized water or calculus compositions or by cavitation collapse, thus manifesting a photoacoustical effect. Laser-tissue interactions involving dominant photomechanical or photoacoustical effects are usually stress confined. Long-pulsed lasers (i.e., >100 microsec), on the other hand, generate minimal acoustic waves, and calculi are fragmented by temperatures beyond the thresholds for vaporization of calculus constituents, melting, or chemical decomposition.