Study of the Effect of Laser Radiation Parameters on the Efficiency of Lithotripsy

Abstract

In this article, we report on experimental studies of the influence of several laser radiation parameters, such as the duration of the laser pulse, the radiation wavelength, and the pulse energy, on the efficiency of the destruction of urinary calculi. The study used a laser lithotripter based on a fiber Tm laser generating at a wavelength of 1940 nm with pulses with a duration of about 1800 μs and pulse energy of up to 6 J, as well as a femtosecond solid-state Yb laser generating at a wavelength of 1032 nm with a pulse duration of about 250 fs and pulse energy of up to 400 μJ. A comparative analysis was carried out according to such criteria as the productivity of lasers when removing a unit mass of images and the amount of sample displacement resulting from the retropulsion effect. The results obtained in this work demonstrated that the femtosecond laser loses approximately two times its efficiency in terms of sample material removal. However, this shows the absolute advantage of the photoionization mechanism of femtosecond laser ablation in the study of retropulsion and thermal heating, which were completely absent in this case.

1. Introduction

Today, the prevalence of urolithiasis varies from 2 to 15% in different regions of the world, with a general upward trend. Studies in the United States have shown that urolithiasis prevalence increased from 3.6% in 1976–1980 to 10.1% in 2015–2016 [1]. The urolithiasis prevalence in 2023 in Western Europe was about 5–9%, in Canada 12%, and in the United States 7–15%. Kidney stones affect approximately 1 in 11 people in the United States [2]. The prevalence of urolithiasis in the eastern hemisphere is about 1–5% [3,4,5]. However, it is worth noting that approximately 60% of all kidney surgeries are performed for urolithiasis. However, surgical treatment is not isotropic and is fraught with complications of varying severity, and almost one-third of operated patients experience calculi recurrence. The percentage of severe complications increases proportionally with the frequency of surgical interventions in patients with nephrolithiasis. Due to this fact, current trends in operative urolithiasis treatment are aimed at reducing the “invasiveness” of the intervention. Endourological surgical interventions (contact ureteral lithotripsy, retrograde intrarenal surgery, and percutaneous interventions) and extracorporeal remote shockwave lithotripsy are considered to be the main methods of urolithiasis surgical treatment [6,7,8,9,10]. Methods of urinary stone crushing are constantly studied and improved by the global scientific community. It is worth noting that, to date, the most active research in the urology field has been aimed at developing new methods of delivery of concentrated energy flows for the destruction and disintegration of urinary stones. Thus, in recent years, the laser lithotripsy (LL) method has been actively introduced into clinical practice. From the 1990s until the 2000s, neodymium solid-state lasers with a wavelength of 1064 nm, pulse energy from 1 to 10 J, and pulse duration from 1 to 2 µs were used [11,12]. The residual fraction size of renal concrements after treatment with such lasers ranged from 2 to 4 mm [13]. At the same time, the thermal influence zone was of the order of 1 to 2 mm [14]. LL with a neodymium laser is mostly shockwave in nature, which entails the formation of large residual fractions and a significant thermal effect. In this regard, we can talk about the extremely low efficiency of this approach and the long rehabilitation period. In the second stage of laser lithotripsy from 2000 to the present, using solid-state holmium laser lithotripters with a wavelength of 2120 nm, a pulse energy of the order of 1 J, a pulse duration of 300 µs, and an energy density of 200 to 600 J/cm² [15,16]. The thermal influence zone during such treatment is about 300 µm [14]. The residual fraction size ranges from 1 to 2 mm [16,17]. LL by holmium laser has a photothermal [11] character; therefore, it differs in the residual fraction average size and universality of influence on renal concrements of various chemical compositions, which causes the feasibility of its use concerning neodymium lasers. The third stage of laser lithotripsy development is the search for and mastery of new promising laser emitters and treatment methods. Among the promising emitters, we should highlight the erbium laser (Er: YAG) and thulium fiber laser (Tm: fiber) [17,18,19,20,21]. Er: YAG uses laser radiation with a wavelength of 2940 nm, a pulse energy in the order of 100 mJ, a pulse duration of 300 µs, and an energy density of processing 50–700 J/cm². The thermal influence zone during such treatment in various works is from 5 to 50 μm [14]. LL with erbium laser is photothermal and is more effective than neodymium laser (Nd: YAG) and holmium laser (Ho: YAG), but has large unresolved problems of radiation delivery due to the long wavelength [17]. Tm: YAG uses laser radiation with a wavelength of 1940 nm, pulse energy up to 8 J, and a pulse duration of 50 ms. The residual fraction size is 0.25 mm [22]. Thulium lasers are universal in the treatment of kidney stones of various chemical compositions and have power comparable to holmium lasers [11,22].

Another promising direction is to reduce the laser pulse duration, which, due to the physical basis of laser interaction with the substance, proportionally reduces the thermal influence zone, shock wave impact, and residual fraction size [23,24,25,26]. The main accompanying processes of calculus crushing by ultrashort pulse (USP) laser radiation are the formation of a plasma torch and the evaporation of the target material. At high peak powers of laser radiation in the focusing spot region, the radiation is rapidly absorbed, causing the development of local nonlinear processes and instabilities, which is accompanied by shock wave development in the environment [12,13,14,15,27]. At the same time, these shock waves have much lower energy than those generated by lasers with high pulse energies, which are used in LL today. In this regard, a promising development direction may be the application of ultrashort pulse fiber lasers generated in the near-infrared range with high energy and short pulse duration to create laser lithotripters. The main advantage of lasers in this spectral range is the reduced absorption of radiation by water molecules. This feature will minimize the heating of the fluid in the kidney parenchyma and increase the efficiency of renal calculi destruction. At the same time, the relatively low energy of USP laser pulses, combined with their high peak power, will make it possible to destroy kidney calculi without a large shock wave effect on kidney tissue. For example, a femtosecond laser Coherent Hidra 10 with a wavelength of 800 nm, pulse energy of the order of 0.64 mJ, a pulse duration of 140 fs, and processing power density of 107 W/cm² showed in [28] the possibility of ensuring the residual fraction size in the range from 1 to 20 μm. The mechanism of such laser treatment is photoionization and differs from the photothermal one by the essential decrease of shock wave influence from 1.5 bar when using a nanosecond pulse duration to 0.4 bar when using a laser pulse duration of 0.1 ps [28].

In this regard, the purpose of this study is to experimentally establish the influence of the main characteristics of laser radiation, such as the generation wavelength, energy, and pulse duration, on the efficiency of laser lithotripsy of renal calculus samples and their phantoms, among which the main focus was on the performance when removing the material volume in a unit of time, as well as the processed sample displacement due to the retropulsion phenomenon and environmental thermal heating, which should not exceed 40 °C.

2. Materials and Methods

To achieve the project’s key goal, we performed several experiments comparing the performance of urinary nodule phantom destruction when exposed to laser radiation with microsecond and femtosecond pulse durations. In addition, we investigated the retropulsion effect, which plays a significant role in the course of urinary concretion removal surgery. The characteristics of the used equipment are presented in

Table 1. Characteristics of the used lasers.

	Super-Pulsed Tm-Fiber Laser (Microsecond)	Femtosecond Pulse Laser
Active medium	Tm-doped fiber laser	Bulk Yb: YAG
Wavelength, nm	1940	1032
Pulse duration	≥0.2 ms	250 fs
Pulse energy	0.025–6 J	400 μJ
Repetition rate	1–2400 Hz	10 kHz
Average power	~60 W	10 W

.

To ensure a large sample of data and the result’s repeatability, we carried out experimental studies of laser performance on urinary stone phantoms, which are 5 mm × 5 mm × 5 mm cubes, the base of which was dental plaster of the fourth-strength class (BegoStone, Bego GmbH, Bremen, Germany). The experimental sample photo is shown in Figure 1a. It is worth noting that fourth-strength dental plaster is a common material for laser lithotripsy performance studies [29]. The main objective of each experiment series was to evaluate the rate of urinary concrement destruction by laser irradiation. The task was solved by analyzing the dependence of the obtained craters’ parameters on the laser radiation parameters acting on the sample. The main problem in comparing the studied lithotripters was the different physical mechanisms of urinary stone phantom destruction. First, this was due to the difference in the working average output power of the optical radiation of the studied laser sources. In this regard, for an accurate analysis of the processes occurring when exposed to radiation of different durations on the samples under study, we have chosen reference values such as throughput and average efficiency in removing the mass of sample material in 1 s.

In experiments with a super-pulsed Tm-fiber laser, we studied the performance using a specially developed technique, which afterward made it possible to compare the results obtained using different laser radiation sources. A key step in the experimental study was weighing the samples before and after treatment. The stone phantom was immersed in a container with physiological saline to create conditions close to the operating conditions of laser lithotripters. The radiation was delivered to the sample by means of an optical fiber 150 μm in diameter. In this case, the end of the fiber was located at a distance of 3–5 mm from the surface of the calculus. (See Figure 2b) We chose such a distance to protect the fiber end of the radiation delivery system from the impact of fractions separated from the stone. In this case, the laser radiation spot on the sample surface was ~200 μm. Accordingly, the peak power density of radiation was 4.35 GW/cm². In this case, the laser radiation profile corresponded to the Gaussian. The main series of experiments included 36 renal calculus phantoms. Laser exposure was carried out with different energies from 0.2 J to 2.5 J, pulse durations from 0.2 ms to 2.5 ms, and pulse repetition rates ranging from 18 Hz to 5 Hz. The average power of laser processing and the total energy can be considered as derivatives of variable technological parameters. To measure the removed material’s volume, we weighed the parts before and after laser treatment. Weighing was carried out on a non-automatic balance (Analytical Balance ME104/A). To minimize the error in measuring the sample’s mass associated with moisture removal during the weighing process, the authors decided to measure the mass of dehydrated samples. In this regard, before re-weighing, the samples were dried on blotting paper for two hours.

When studying the value of sample retropulsion resulting from cavitation, the object of study was immersed in the container with a NaCl solution, a linear scale applied, and placed near the zero mark on the scale. Next, we brought the fiber working end to the sample, and layered generation began. Since the study was carried out according to a static method, we placed the fiber working part close to the stone phantom and rigidly fixed it. After the laser worked for 10 s, we stopped the experiment and measured the distance the sample was displaced. It should be noted that we have chosen a static technique for studying retropulsion since it is closest to the actual operating conditions of laser lithotripters.

We experimented with the effect of femtosecond pulses on urinary calculi using the commercially available ytterbium laser system TEMA-6 of AVESTA-PROJECT LLC, which makes it possible to generate linearly polarized spectrally limited pulses with a duration of 250 fs at 1032 nm with a pulse energy of up to 400 μJ. Thus, the pulses’ peak power acting on the sample was about 1.6 GW. Due to such high peak power, radiation delivery to the calculi phantom using an optical fiber was a problem. Therefore, we decided to use bulk optics for this task. To deliver radiation to the sample, we used a system of mirrors with a focusing lens at the output (f = 25 mm). In this case, the laser radiation spot on the sample’s surface was ~300 μm. Accordingly, the peak power density of radiation was 241.91 TW/cm². To create experimental conditions close to those of the microsecond laser study, the samples were preliminarily hydrated for a day and then fixed in the holder so that the test object was in the focus of radiation at each iteration of the experiment. The pulse energy during the treatment course varied from 200 to 400 μJ, and the exposure time from 2 to 30 s. In addition, studies were carried out at three pulse repetition rates (10, 15.6, and 25 kHz) to characterize the magnitude of the influence of the peak radiation power on the ample ablation efficiency. The pulse duration did not change throughout the experiment and was 250 fs. It is worth noting that due to the lack of a system for delivering radiation with a femtosecond pulse duration in this research work, it was necessary to use a scanning system to move the affected area over the sample. In this case, we moved the processing zone horizontally with a step of 1 mm/s. After completely passing the specified processing perimeter, the radiation was shifted deep into the sample by 1 mm. It is worth noting that the speed and number of movements were selected empirically and were not optimized, which made a negative contribution to the femtosecond laser efficiency.

In addition, we studied retropulsion in water when the sample was exposed to femtosecond pulses. To measure retropulsion, the test object was placed in a rectangular pure quartz cuvette (see Figure 2a,b), which did not absorb at the laser radiation wavelength, with a linear scale applied to the bottom, and the sample was processed through the cuvette wall.

It is also necessary to emphasize that one of the most significant parameters in the study of the effectiveness of laser lithotripters is the amount of heating in the environment. Since the mechanism of renal calculi destruction is photothermal ablation, during laser lithotripsy, direct absorption of infrared energy by urinary calculi leads to an accumulation of heat and a subsequent increase in temperature, which leads to renal calculus chemical decomposition. Thresholds for photothermal ablation depend on both the chemical composition and the radiological density of the stones. However, it is worth noting that urinary stones very often have a mixed chemical structure, so these thresholds are variable. In this regard, when conducting research, it is necessary to control the temperature near the sample, which should not exceed 40 °C. Exceeding the specified value increases the likelihood of burns to surrounding tissues and can also lead to protein coagulation and impaired functionality of the kidney parenchyma. Following the above, during the experiment, we continuously monitored the ambient temperature near the test sample using a high-precision digital thermometer (Fluke 233, Fluke Corporation, Everett, WA, USA).

3. Results

As mentioned above, the main idea of the study was to compare the performance of laser sources that can be used as medical lithotripters. In this regard, when comparing lasers with microsecond and femtosecond pulse durations, we evaluated the efficiency of photothermal and photoionization ablation mechanisms. As a result of our studies on a large sample (60 samples), we experimentally showed that when operating in the region of the same average output power of about 9 W, the microsecond laser’s average productivity by processing a gypsum phantom is higher than that of a laser with a femtosecond pulse duration. The experimental results of the measured average productivity are shown in Figure 3a. As can be seen from the graph, in the case of a linear approximation, a microsecond laser source has a faster increase in average productivity than a femtosecond one.

Qualitatively, the predominance of the efficiency of a microsecond laser source over a femtosecond one is shown in Figure 3b. It can be seen from the above bar graph that the average material removal efficiency for a microsecond laser applied is two times higher.

Figure 4a shows the dependence of the average productivity on the average and the peak powers for a laser facility generating femtosecond pulses. Of note is that we achieved the best performance at the lowest peak power, which corresponds to the highest average pulse train output power. This fact can also be considered evidence of the advantage of the photothermal destruction mechanism over photoionization during urinary calculi treatment.

It is also necessary to emphasize that in research practice, when treating urinary calculi with a microsecond laser, high pulse energy and a low repetition rate cause vibrations of the working end of the fiber. It leads to the random movement of the area of photothermal impact on the destructible sample. As mentioned above, in the experiment with a femtosecond laser, we used an acoustic-optical scanning system to move radiation over the sample surface, the speed and step of movement of which were chosen empirically and were not optimized. This fact, in turn, is reflected in the performance assessment. Figure 4b shows the dependence of the average productivity on the time of exposure to femtosecond pulses at the maximum average laser power. Judging by the nature of the change in the average values with increasing exposure time, it can be concluded that the key condition for obtaining high-performance indicators from a laser with a femtosecond pulse duration is the choice of the optimal time and scanning step. As you can see from the graph, at an average output power of about 9.6 W, the maximum performance level corresponded to a processing time of about 3.5 s, after which a sharp decline began. It is a consequence of the fact that the laser exposure area is processed faster compared to the scan step duration.

As mentioned above, in addition to the productivity and efficiency of sample processing, the main characteristic of laser emitters used in lithotripsy is the retropulsion of the samples under study. The experimental results of the retropulsion study are presented in Figure 5. Graphical diagrams confirm the complete absence of retropulsion between the sample and the surrounding liquid when samples are exposed to femtosecond pulses.

This result is explained by the superposition of several factors, among which the lower water absorption in the region of 1.03 μm compared to the absorption at 1.94 μm for the Tm laser source, as well as the low average radiation power. However, the main reason is the difference in the physical mechanism of material removal. The ablation process during femtosecond laser lithotripsy, as described above, is due to the mechanism of photoionization ablation, as a result of which it is fundamentally different from the process of microsecond laser lithotripsy due to the photothermal mechanism. In microsecond laser lithotripsy, shock waves are generated by both plasma expansion and bubble collapse with an amplitude of more than 100 bar² [28]. These bubbles appear due to the local evaporation of the liquid near the sample. At the same time, the shock waves generated by microsecond radiation are strong enough to crush stones when they reach their surface. In the case of laser lithotripsy with a femtosecond pulse duration, the calculus undergoes a photoionization process and forms a plasma mixture of ions and electrons. After ionization and plasma generation, the incoming laser energy is absorbed by free electrons, providing acceleration and the production of additional free electrons [30]. The high-speed free electron number is large enough to remove sample material. It is worth noting that the photoionization mechanism of ablation provides the smallest residual fractions. In comparison, nanosecond laser lithotripsy produces large fragments that range in size from a few hundred microns to several millimeters [31].

It is necessary to emphasize that due to the short duration of exposure to radiation and the material being processed, corresponding to a pulse duration of about 250 fs, only a small part of the incident laser energy was transferred to heat the ions and create shock waves. Due to these factors, when we exposed the sample to femtosecond laser pulses, the liquid did not evaporate. It leads to the appearance of vapor bubbles near the sample, which causes retropulsion [30,31]. At the same time, as can be seen from the graph, when using the millisecond stone processing mode, the effect of retropulsion is observed due to cavitation in a liquid medium, amounting to about 9.6 mm per 9 W of averaged power.

4. Conclusions

As a result of the experimental study, we were able to compare the mechanisms of interaction of radiation energy with the material of urinary calculus phantoms using laser radiation of different wavelengths and pulse durations. It is shown that at an average power of about 9 W, the efficiency of removing phantom masses from gypsum by radiation with a wavelength of 1064 nm and a duration of 250 fs is 1.56 g/s. At the same time, a fiber Sp-Tm laser with a wavelength of 1940 nm showed an average result of about 3.19 g/s with a pulse duration of about 1.8 ms, which corresponds to an almost twofold increase in productivity in the femtosecond treatment regime. It is partly due to the discrepancy between the time it takes to remove material and the step of the femtosecond laser scanning head; on the other hand, when analyzing the obtained data on the laser characteristics that affect the processing productivity, we established the fact that the photothermal destruction mechanism is superior to photoionization ablation. At the same time, the magnitude of retropulsion during the treatment of urinary calculi phantoms with a Tm-laser lithotripter averaged 9.6 mm at an average power of 9 W. When using radiation of the same power of femtosecond pulse duration, retropulsion and thermal exposure were not observed. This phenomenon is explained by several factors, among which it is worth highlighting the operating wavelength of the radiation located in the hydroxyl groups’ transparency window. This condition makes it possible to avoid significant heating of the liquid around the urinary calculus phantom and prevent the formation of vapor bubbles, which create shock waves due to the cavitation effect after they have collapsed. We also should note the duration of the interaction of radiation with the processed material, during which the radiation energy does not have time to be transferred to neighboring molecules of the material. However, the limiting factor for the femtosecond lasers implementation in LL is the lack of a delivery channel capable of transmitting high peak power radiation. Perhaps, in the future, this problem will be solved by using photonic crystal fibers and hollow-core fibers. In [32,33], radiation transmission through a hollow-core fiber was demonstrated with a peak power of 0.7 TW in the mid-IR and with a peak power of 100 MW at a wavelength of 1030 nm, respectively. However, the possibility of their use requires practical confirmation. In addition, as far as we know, samples capable of transmitting femtosecond pulses with high-peak-power in a fiber trace with small bending radii (the bending radius of a urethroscope) have not yet been demonstrated. Following the above, we believe that, despite the lower productivity, the absence of retropulsive and thermal effects on the environment should favorably affect both the quality of the operations themselves and the postoperative recovery process of the patient.