A temperature model for laser lithotripsy

Optimizing Fragmentation while Minimizing Thermal Injury Risk with the Thulium Fiber Laser in Ureteral Stone Lithotripsy: An In Vitro Study

Objective: To optimize thulium fiber laser (TFL) settings for effective stone fragmentation although minimizing thermal injury in confined ureteral spaces using a three-dimensional ureter model. Materials and Methods: A hydrogel-based ureter model was maintained at 37.2 ± 0.5°C, with a cylindrical BegoStone (10 × 10 mm, 1.00 ± 0.07 gm) occluding the ureter. Ureteroscopy was performed using a 150 µm TFL fiber for 3 minutes with room temperature irrigation and differing rates (0, 20, 40 mL/min) and power settings (6.4 to 20 W). Maximum sustained temperature (MST) and cumulative thermal dose (cumulative equivalent minutes at 43°C) were assessed against a 120-minute safety threshold. We also evaluated the effects of ureter volume and irrigation temperature. Stone mass treated was calculated by subtracting the mass of residual fragments >3 mm from the initial mass. Results: At 6.4 and 10 W, MSTs were below body temperature, and thermal doses were under 1 minute, indicating minimal thermal risk. At 20 W with 20 mL/min irrigation, MST exceeded 43°C within seconds, and thermal doses surpassed 120 minutes. Treatment efficiency was highest at 20 W (1.58 mg/s), followed by 10 W (1.15 mg/s) and 6.4 W (0.78 mg/s). Among 10 W settings, 1.0 J/10 Hz was more efficient than 2.0 J/5 Hz and 3.0 J/3 Hz. Safe settings produced 95.5% fine dust, whereas high-energy pulses 2-3 J produced significantly more fragments (1-3 mm) compared with settings with pulse energy 0.5-1.0 J. Increasing irrigation to 40 mL/min or using 15°C irrigation effectively reduced MST and improved efficiency, particularly at 20 W. Conclusion: Our study demonstrates the risk of thermal injury with 20 W TFL treatment. Conversely, 10 W settings at 2.0 J/5 Hz are safe and effective for fragmentation. Future research will focus on validating these optimal settings for human stone treatment.

Evaluating safe irrigation rates for Tm fiber laser lithotripsy to prevent thermal injury: an in vitro and numerical simulation

PURPOSE

To evaluate temperature changes and thermal injury during thulium fiber laser (TFL) lithotripsy in the urinary tract and identify the safe irrigation rate, providing a reference for optimal parameter settings in clinical practice.

METHODS

An in vitro thermal injury experiment was conducted using a home-made TFL operating at 40 W. The experiment utilized a glass tube simulating the urinary tract. A peristaltic pump circulated 25 °C saline at irrigation rates ranging from 10 to 40 mL/min. Real-time temperature measurements were recorded using thermocouples, and thermal injury was assessed by calculating the thermal dose threshold. Additionally, a validated numerical simulation model was developed to analyze temperature distributions and predict safe irrigation rates at 20 and 30 W.

RESULTS

In the in vitro experiment, at 40 W, severe thermal injury occurred in the urinary tract when the irrigation rate was below 30 mL/min, particularly in the ureter and renal calyx. The numerical simulation model demonstrated a high degree of consistency with the experimental results. According to the simulation, at 30 W, thermal injury occurred in the renal calyx when the irrigation rate was below 20 mL/min. At 20 W, thermal injury was observed in the renal calyx when the irrigation rate was below 10 mL/min.

CONCLUSIONS

The increase in temperature and the extent of thermal injury were strongly dependent on laser power and irrigation rate, with a lesser dependence on anatomical location. The accuracy of the numerical simulation was validated through experiments, demonstrating its capability to reliably predict temperature variations and thermal injury under different laser power settings and irrigation rates.

Simulation of the temperature distribution of kidney stones induced by thulium fiber laser and Ho: YAG laser lithotripsy

Simulation studies on temperature distribution in laser ablation help predict ablation rates, laser settings, and thermal damage. Despite the limited number of reported numerical studies on the temperature distribution of kidney fluid, there is no simulation study for kidney stone temperature distribution. We employ a numerical approach to study the kidney stone temperature distribution and predict ablation rates, which is an important parameter for clinical lithotripsy. The study looked at how the thulium fiber laser and the Ho:YAG laser differ in terms of temperature profile and ablation depth of kidney stones like calcium oxide monohydrate. The ablation depth increased from 152.7 µm to 489.7 µm when the TFL laser (operated at 10 Hz repetition rate and 1 ms pulse width) fluence increased from 764 J/cm2 to 1146 J/cm2. Correspondingly, the depth increased from 21 µm to 68 µm for the Ho: YAG laser operated at 3 Hz and 0.22 ms pulse width. We attribute this to an increase in temperature with laser energy. We further investigated the effect of pulse width on ablation depth by considering three different TFL pulse widths: 0.5 ms, 0.75 ms, and 1 ms. There was a decrease in ablation depths from 402.5 µm to 242.6 µm when the pulse width increased from 0.5 ms to 1 ms. Because of lower water absorption coefficients, the Ho:YAG laser (70 mJ/10 Hz) produced a smaller ablation depth and temperature profile than the thulium fiber laser (70 mJ/10 Hz). Experimental results from the literature validated the simulation. We found that the Ho:YAG laser worked better for ablation when it was set to 0.2 J/100 Hz for the Ho:YAG laser and 0.4 J/50 Hz for the TFL laser, which were clinical laser settings that we found in the literature. This indicates that, in addition to laser absorption by water, the laser parameters also significantly influence temperature distribution and ablation.

Influence of manual hand pump irrigation on intrapelvic temperature during retrograde intrarenal surgery: a thermography-based in vitro study

Introduction

Thermal injury to kidney tissue during holmium laser lithotripsy represents a significant complication. This issue is often unavoidable due to the variability of renal conditions and the absence of techniques for real-time intrarenal temperature monitoring. The objective of this research was to evaluate influence of manual hand pump irrigation on temperature of the fluid within a pelvicalyceal model during holmium laser lithotripsy.

Material and methods

Laser lithotripsy of artificial stones was carried out in a 3D-printed model of the renal pelvicalyceal system. The irrigation system employed a continuous gravity approach (P = 60 cmH2O), augmented by manual pumping as required. A 9.2 Fr ureteroscope was inserted into the model via a ureteral access sheath (UAS), with sizes of either 10/12 Fr or 12/14 Fr.The power settings for the lithotripsy varied between 12 and 25 W. Temperature monitoring during the procedure was conducted using thermographic methods.

Results

For all laser power settings, the temperatures recorded under gravity irrigation alone were significantly higher compared to those achieved when gravity was combined with a manual hand pump, regardless of the ureteral access sheath size. When using the hand pump system and a 12/14Fr UAS, the median temperatures in none of the laser settings exceeded 30°C. However, using a 10/12Fr UAS, the median temperatures did not exceed 35°C in any of the settings and were significantly lower compared to the use of the gravity flow system alone.

Conclusions

The employment of gravity irrigation supplemented by a manually on-demand pump in retrograde intrarenal surgery is a critical component in mitigating the risk of significant temperature elevations, leading to thermal injury to the adjacent kidney tissues. Moreover, the interquartile ranges of temperatures indicating that gravity system enhanced by an on-demand pump irrigation not only reduce the median temperature but also promote a more consistent thermal environment.

A Novel Hydrodynamic Design Greatly Improves the Debris Clearance Rate of Flexible Ureteroscopy

Objectives: To introduce a novel hydrodynamic design for a flexible ureteroscope that can increase stone debris clearance. Methods: Based on hydrodynamics, the new design allowed the ureteroscope to have six water jets. Fluid gushed from the six jets and would ultimately converge into an eddy. The safety and stone debris clearance efficiency were tested in a 3D-printed kidney model. Stone fragments between 0.5 and 1 mm were used to mimic the debris. A ureteroscope already approved for marketing was used as a control. Results: The new design did not change the local renal pressure and did not raise the whole renal pressure under irrigation at 80 or 100 mL/min but slightly raised it under irrigation at 120 mL/min. The pressures in the 2 g stone clearance procedures were 26.0, 33.1, and 37.5 cmH2O for the new design and 25.1, 30.2, and 39.3 cmH2O for the current design; in the 4 g stone clearance procedures, the pressures were 30.1, 37.2, and 40.0 cmH2O for the new design and 26.9, 30.8, and 39.8 cmH2O for the current design, all under conditions of 80, 100, and 120 mL/min irrigation, respectively. The new design significantly improved the stone clearance rate by ∼10-fold. It effectively cleared 2 and 4 g stones within 900 seconds under the three irrigation rates. In contrast, the current design cleared <10% of the stone debris in all tests. Conclusion: The new hydrodynamic design significantly improved the stone debris clearance rate without causing obviously increased renal pressure, and the improvement was maintained under different irrigation pressures and stone burdens.

The heat is on: the impact of excessive temperature increments on complications of laser treatment for ureteral and renal stones

OBJECTIVE

Technological advancements in the field of urology have led to a paradigm shift in the management of urolithiasis towards minimally invasive endourological interventions, namely ureteroscopy and percutaneous nephrolithotomy. However, concerns regarding the potential for thermal injury during laser lithotripsy have arisen, as studies have indicated that the threshold for cellular thermal injury (43 °C) can be exceeded, even with conventional low-power laser settings. This review aims to identify the factors that contribute to temperature increments during laser treatment using current laser systems and evaluate their impact on patient outcomes.

MATERIALS AND METHODS

To select studies for inclusion, a search was performed on online databases including PubMed and Google Scholar. Keywords such as 'temperature' or 'heat' were combined with 'lithotripsy', 'nephrolithotomy', 'ureteroscopy', or 'retrograde intrarenal surgery', both individually and in various combinations.

RESULTS

Various strategies have been proposed to mitigate temperature rise, such as reducing laser energy or frequency, shortening the duration of laser activation, increasing the irrigation fluid flow rate, and using room temperature or chilled water for irrigation. It is important to note that higher irrigation fluid flow rates should be approached cautiously due to potential increases in intrarenal pressure and associated infectious complications. The utilization of a ureteral access sheath (UAS) may offer benefits by facilitating irrigation fluid outflow, thereby reducing intrapelvic pressure and intrarenal fluid temperature.

CONCLUSION

Achieving a balance between laser power, duration of laser activation, and irrigation fluid rate and temperature appears to be crucial for urologists to minimize excessive temperature rise.

Different ureteral access sheaths sizes for retrograde intrarenal surgery

PURPOSE

There is a trend toward miniaturization in endourological stone therapy. Good visibility, intrarenal pressures and temperature control should be ensured by ureteral sheaths. In the context of the present study, 10/12 Charr. sheaths and 12/14 Charr. sheaths for flexible ureterorenoscopy were investigated regarding stone-free rate, complication rate and efficacy for laser lithotripsy.

METHODS

From January 2020 to January 2022, 100 patients each with kidney stone up to 1.5 cm in diameter were included in the study. Use of a 12/14 Charr. vs. 10/12 Charr. ureteral sheath for flexible ureterorenoscopy was compared. Perioperative data, stone size, volume and density, laser energy, laser duration, stone-free rates and complications based on Clavien-Dindo classification were retrospectively analyzed.

RESULTS

For both groups of ureteral access sheaths, there were no differences in median surgery duration (10/12 Charr: 29 min (7-105 min) vs. 12/14 Charr: 34 min (9-95 min); p = 0.33), overall complication rate (p = 0.61) and hospitalization (p = 0.155). There were no differences in stone-free rates (97.9% vs. 92.7%, p = 0.37). Laser lithotripsy duration usingholmium laser was 1.9 min (0.1-10.8 min) vs. 3.8 min (0.2-20.7 min) (p < 0.01) and applied laser energy was 3.1 J (0.15 J-10.29 J) vs. 6.8 J (1.07 J-26.77 J) (p < 0.01) for 12/14 Charr. sheaths and 10/12 Charr. sheaths, respectively.

CONCLUSION

In terms of stone-free rates, there are no differences between the 10/12 and 12/14 Charr. ureteral access sheaths. The laser duration and energy was increased with 10/12 Charr. sheaths without showing increased risk for clinical complications like trauma or inflammation.

Temperature changes of renal calyx during high-power flexible ureteroscopic Moses holmium laser lithotripsy: a case analysis study

PURPOSE

The risk of thermal damage increases with the introduction of high-power lasers during holmium laser lithotripsy. This study aimed to quantitatively evaluate the temperature change of renal calyx in the human body and the 3D printed model during high-power flexible ureteroscopic holmium laser lithotripsy and map out the temperature curve.

METHODS

The temperature was continuously measured by a medical temperature sensor secured to a flexible ureteroscope. Between December 2021 and December 2022, willing patients with kidney stones undergoing flexible ureteroscopic holmium laser lithotripsy were enrolled. High frequency and high-power settings (24 W, 80 Hz/0.3 J and 32 W, 80 Hz/0.4 J) were performed for each patient with room temperature (25 °C) irrigation. In the 3D printed model, we studied more holmium laser settings (24 W, 80 Hz/0.3 J, 32 W, 80 Hz/0.4 J and 40 W, 80 Hz/0.4 J) with warmed (37 °C) and room temperature (25 °C) irrigation.

RESULTS

Twenty-two patients were enrolled in our study. With 30 ml/min or 60 ml/min irrigation, the local temperature of the renal calyx did not reach 43 °C in any patient under 25 °C irrigation after 60 s laser activation. There were similar temperature changes in the 3D printed model with the human body under the irrigation of 25 °C. Under the irrigation of 37 °C, the temperature rise slowed down, but the temperature in the renal calyces was close to or even exceeded the 43 °C at the setting of 32 W, 30 ml/min and 40 W, 30 ml/min after continuing laser activation.

CONCLUSION

In the irrigation of 60 ml/min, the temperature in the renal calyces can still be maintained within a safe range after continuous activation of a holmium laser up to 40 W. However, continuous activation of 32 W or higher power holmium laser in the renal calyces for more than 60 s in the limited irrigation of 30 ml/min can cause excessive local temperature, in such situation room temperature perfusion at 25 ℃ may be a relatively safer option.

Model-based simulations of pulsed laser ablation using an embedded finite element method

A model of thermal ablation with application to multi-pulsed laser lithotripsy is presented. The approach is based on a one-sided Stefan-Signorini model for thermal ablation, and relies on a level-set function to represent the moving interface between the solid phase and a fictitious gas phase (representing the ablated material). The model is discretized with an embedded finite element method, wherein the interface geometry can be arbitrarily located relative to the background mesh. Nitsche's method is adopted to impose the Signorini condition on the moving interface. A bound constraint is also imposed to deal with thermal shocks that can arise during representative simulations of pulsed ablation with high-power lasers. We report simulation results based on experiments for pulsed laser ablation of wet BegoStone samples treated in air, where Begostone has been used as a phantom material for kidney stone. The model is calibrated against experimental measurements by adjusting the percentage of incoming laser energy absorbed at the surface of the stone sample. Simulation results are then validated against experimental observations for the crater area, volume, and geometry as a function of laser pulse energy and duration. Our studies illustrate how the spreading of the laser beam from the laser fiber tip with concomitantly reduced incident laser irradiance on the damaged crater surface explains trends in both the experimental observations and the model-based simulation results.

Assessing critical temperature dose areas in the kidney by magnetic resonance imaging thermometry in an ex vivo Holmium:YAG laser lithotripsy model

PURPOSE

We aimed to assess critical temperature areas in the kidney parenchyma using magnetic resonance thermometry (MRT) in an ex vivo Holmium:YAG laser lithotripsy model.

METHODS

Thermal effects of Ho:YAG laser irradiation of 14 W and 30 W were investigated in the calyx and renal pelvis of an ex vivo kidney with different laser application times (t_L) followed by a delay time (t_D) of t_L/t_D = 5/5 s, 5/10 s, 10/5 s, 10/10 s, and 20/0 s, with irrigation rates of 10, 30, 50, 70, and 100 ml/min. Using MRT, the size of the area was determined in which the thermal dose as measured by the Cumulative Equivalent Minutes (CEM₄₃) method exceeded a value of 120 min.

RESULTS

In the calyx, CEM₄₃ never exceeded 120 min for flow rates ≥ 70 ml/min at 14 W, and longer t_L (10 s vs. 5 s) lead to exponentially lower thermal affection of tissue (3.6 vs. 21.9 mm²). Similarly at 30 W and ≥ 70 ml/min CEM₄₃ was below 120 min. Interestingly, at irrigation rates of 10 ml/min, t_L = 10 s and t_D = 10 s CEM₄₃ were observed > 120 min in an area of 84.4 mm² and 49.1 mm² at t_D = 5 s. Here, t_L = 5 s revealed relevant thermal affection of 29.1 mm² at 10 ml/min.

CONCLUSION

We demonstrate that critical temperature dose areas in the kidney parenchyma were associated with high laser power and application times, a low irrigation rate, and anatomical volume of the targeted calyx.

Characterization of Fluid Dynamics and Temperature Profiles During Ureteroscopy with Laser Activation in a Model Ureter

Introduction: Ureteral thermal injury has been reported in patients following ureteroscopy with laser lithotripsy due to overheating of fluid within the ureter. Proper understanding of this risk necessitates knowing the volume of fluid available to absorb laser energy. This can be approximated as the volume of fluid that mixes during laser activation, since energy transfer through fluid is dominated by convection. Objectives of this study were to determine the volume of fluid that mixes during laser activation at different irrigation rates and to characterize the temporal/spatial temperature distribution in a model ureter. Methods: The model ureter consisted of a plastic tube-160 mm length and 5.3 mm inner diameter. Irrigation was first applied with clear, then dyed, deionized water at rates from 8 to 40 mL/min. The laser was activated at 20 W (0.5 J/40 Hz). The distances the dyed fluid propagated were measured and volumes calculated. Temperatures were recorded from six thermocouples-five embedded within the tube and one affixed to the ureteroscope. Thermal dose was calculated using the Dewey and Sapareto methodology. Results: The volume of total fluid mixing in the model ureter was ≤1.26 ± 0.10 cm³, consistent with a sharp temperature increase after laser activation from -5 to 25 mm from the ureteroscope tip. With irrigation rates ≤12 mL/min, calculated thermal dose within the model ureter exceeded the threshold of tissue injury and extended greater distances along the ureter with lower irrigation rates. Conclusion: The volume of total fluid mixing within the model ureter was found to be small thus conferring a greater risk of ureteral thermal injury. A thermocouple positioned near the tip of the ureteroscope reasonably approximates temperature in front of the ureteroscope. Until temperature sensors are incorporated into ureteroscopic systems, laser power settings should be carefully selected to minimize risk of ureteral thermal injury.

Determination of Irrigation Flowrate During Flexible Ureteroscopy: Methods for Calculation Using Renal Pelvis Pressure

BACKGROUND

Proper control of irrigation flowrate during ureteroscopy is important to manage thermal and pressure risks. This task is challenging because flowrate is not directly measured by commercially available ureteroscopic or fluid management systems. However, flowrate can be calculated using a hydrodynamic relationship based on measurable values during ureteroscopy. Objectives of this in vitro study were to 1) calculate inflow resistance for different working channel conditions and then using these values 2) calculate irrigation flowrate and determine its accuracy across a range of renal pelvis pressures.

MATERIALS AND METHODS

A 16 Liter container was filled with deionized water and connected by irrigation tubing to a 9.6Fr single-use ureteroscope. Inflow resistance was determined by plotting flowrate (mass of fluid collected from ureteroscope tip in 60 seconds) versus irrigation pressure (range 0-200 cmH2O). Next, the tip of the ureteroscope was inserted into the renal pelvis of a silicone kidney-ureter model and renal pelvis pressure was measured. In conjunction with the previously determined inflow resistance and known irrigation pressure values, flowrate was calculated and compared to experimentally measured values. All trials were performed in triplicate for working channel conditions: empty, 200µm laser fiber, 365µm laser fiber, and 1.9Fr stone basket.

RESULTS

Flowrate was linearly dependent on irrigation pressure for each working channel condition. Inflow resistance was determined to be 5.0 cmH2O/(ml/min) with the 200µm laser fiber in the working channel and calculated flowrates were within 1 ml/min of measured flowrates. Similar results were seen with a 365µm laser fiber, and 1.9Fr basket.

CONCLUSIONS

Utilizing renal pelvis pressure measurements, flowrate was accurately calculated across a range of working channel conditions and irrigation pressures. Incorporation of this methodology into future ureteroscopic systems that measure intrarenal pressure, could provide a real-time readout of flowrate for the urologist and thereby enhance safety and efficiency of laser lithotripsy.

In vitro fragmentation performance of a novel, pulsed Thulium solid-state laser compared to a Thulium fibre laser and standard Ho:YAG laser

The aim of this work was to compare the fragmentation efficiency of a novel, pulsed Thulium solid-state laser (p-Tm:YAG) to that of a chopped Thulium fibre laser (TFL) and a pulsed Holmium solid-state laser (Ho:YAG). During the fragmentation process, we used a silicone mould to fixate the hemispherical stone models under water in a jar filled with room-temperature water. Each laser device registered the total energy applied to the stone model to determine fragmentation efficiency. Our study examined laser settings with single pulse energies ranging from 0.6 to 6 J and pulse frequencies ranging from 5 to 15 Hz. Similar laser settings were applied to explicitly compare the fragmentation efficiency of all three devices. We experimented with additional laser settings to see which of the three devices would perform best. The fragmentation performance of the three laser devices differed statistically significantly (p < 0.05). The average total energy required to fragment the stone model was 345.96 J for Ho:YAG, 372.43 J for p-Tm:YAG and 483.90 J for TFL. To fragment the stone models, both Ho:YAG and p-Tm:YAG needed similar total energy (p = 0.97). TFL’s fragmentation efficiency is significantly lower than that of Ho:YAG and p-Tm:YAG. Furthermore, we found the novel p-Tm:YAG’s fragmentation efficiency to closely resemble that of Ho:YAG. The fragmentation efficiency is thought to be influenced by the pulse duration. TFL’s shortest possible pulse duration was considerably longer than that of Ho:YAG and p-Tm:YAG, resulting in Ho:YAG and p-Tm:YAG exhibiting better fragmenting efficiency.

Pelvicalyceal Volume and Fluid Temperature Elevation During Laser Lithotripsy

BACKGROUND

While high-power laser systems facilitate successful ureteroscopic treatment of larger and more complex stones, they can substantially elevate collecting system fluid temperatures with potential thermal injury of adjacent tissue. The volume of fluid in which laser activation occurs is an important factor when assessing temperature elevation. The aim of this study was to measure fluid temperature elevation and calculate thermal dose from laser activation in fluid-filled glass bulbs simulating varying calyx/pelvis volumes.

MATERIALS AND METHODS

Glass bulbs of volumes 0.5, 2.8, 4.0, 7.0, 21.0, and 60.8 ml were submerged in a 16 L tank of 37˚C deionized water. A 230-µm laser fiber extending 5mm from the tip of a ureteroscope was positioned in the center of each glass bulb. Irrigation with 0, 8, 15, and 40 ml/min of room temperature DI water was applied. Once steady state temperature was achieved, a Ho:YAG laser was activated for 60 seconds at 40W (0.5J x 80Hz, SP). Temperature was measured from a thermocouple affixed to the external tip of the ureteroscope. Thermal dose was calculated using the Dewey and Sapareto t43 methodology.

RESULTS

The extent of temperature elevation and thermal dose from laser activation were inversely related to the volume of fluid in each model and the irrigation rate. The time to threshold of thermal injury was only 3 seconds for the smallest model (0.5ml) without irrigation but was not reached in the largest model (60.8ml) regardless of irrigation rate. Irrigation delivered at 40 ml/min maintained safe temperatures below the threshold of tissue injury in all models with 1 minute of continuous laser activation.

CONCLUSIONS

The volume of fluid in which laser activation occurs is an important factor in determining the extent of temperature elevation. Smaller volumes receive greater thermal dose and reach threshold of tissue injury more rapidly than larger volumes.

High-power, High-frequency Ho:YAG Lasers Are Not Essential for Retrograde Intrarenal Surgery

There is currently insufficient in vivo evidence that high-power Ho:YAG lasers improve retrograde intrarenal surgery or the ablation efficacy. While prospective trials are awaited, a low-cost, silent, low-power Ho:YAG laser that requires only a standard electrical outlet is more than sufficient for retrograde intrarenal surgery.