2.1.

Laser Parameters

An experimental thulium fiber laser (TLR 110-1908, IPG Photonics, Oxford, Massachusetts) was operated at a wavelength of 1908 nm and externally modulated with a function generator (DS345, Stanford Research Systems, Sunnyvale, California) to produce a pulse duration of 500 μs, similar to previous TFL lithotripsy studies.^4,5 The TFL was operated at a constant pulse energy of 35 mJ while varying the pulse rate from 10 to 350 Hz. A clinical holmium:YAG laser (TwoPointOne XE, Coherent, Santa Clara, California) was operated at a wavelength of 2120 nm and fixed pulse duration of 300 μs. The holmium laser was limited to operation at a relatively low pulse rate of 20 Hz while varying the pulse energy from 35 to 360 mJ.

2.2.

Stone Suction Experiments

Spherical, 4-mm-diameter, plaster-of-paris stone phantoms with an average mass of $40.4\pm 2.0\mathrm{mg}$ were formed using a mold and sandpaper to smooth rough spots. These stones were used as an idealized stone model to eliminate potential variability due to stone shape and density. Previous investigators have also used PoP stone phantoms to model urinary stones because of their comparable tensile strength.^11,12 Each stone was dried for at least 24 h and weighed using an analytical balance (AB54-S, Mettler-Toledo, Switzerland). The stones were monitored throughout the suction experiments, and any damaged stone was removed and replaced. A minimum of five PoP stone phantoms was used for each set of laser parameters in all of the studies.

Stones were placed in a saline bath on a level, flat surface with ruler markings. Laser energy was delivered through a 270-μm-core optical fiber (Holmium Lightguide 270D, Olympus Gyrus ACMI, Southborough, Massachusetts) to the stone. The experiment was recorded with a camera at a frame rate of 30 Hz (73K3HN-YC, Mintron, Fremont, California). The fiber was positioned parallel to the surface of the saline bath, with energy being delivered slightly off-center, above the stone. The fiber was then pulled away from the stone at the maximum speed allowable to maintain stone movement without detachment. Figure 1 shows the experimental setup used to measure the stone velocity. Figure 2(a) and 2(b) shows representative images of the initial and final stone locations after pulling for 16 s using the holmium laser at a pulse energy of 70 mJ. The velocity for each set of laser parameters was plotted using the recorded distance and time traveled by the stone. For the TFL, velocity versus pulse rate was plotted. For the holmium laser, velocity versus pulse energy was recorded. The fiber was placed slightly off-center and above the stone phantom for optimal manipulation [Fig. 2(c)].

Fig. 1

Experimental setup used to record stone movement during laser fiber-optic manipulation.

Fig. 2

Before (a) and after (b) snapshots of the 4-mm-diameter PoP stone being pulled by the 270-μm-diameter fiber across a ruled surface in a saline bath using the holmium laser, operating at a pulse rate of 20 Hz and a pulse energy of 70 mJ. (c) Side view of fiber-to-stone orientation. Positioning of the fiber tip slightly off-center, above the stone, provided optimal stone manipulation.

2.3.

Particle Image Velocimetry

Polymer microspheres (Duke Standards 4000 Series, Thermo Fisher Scientific, Waltham, Massachusetts) with diameters ranging from 30 to 50 μm, refractive index of 1.59, and density of $1.05\mathrm{g}/{\mathrm{cm}}^3$ were suspended in water. The microsphere density closely matched that of the water to ensure suspension in the water bath. The microspheres were illuminated from the side by a fiber-optic lamp. A 270-μm-core fiber was inserted into the bath with the length of the fiber level with the surface. Laser energy was delivered in the water bath while videos of particle flow were recorded under magnification. The recorded particle flow was used to map the flow of water as a function of the laser parameters. This method examined the macroscopic effects of a train of laser pulses on the water flow.

2.4.

Thermal Imaging

A thermal camera (SC655, FLIR, Billerica, Massachusetts) capable of a 50-Hz frame rate and $640\times 480$ resolution was used to track the flow of heated water as laser energy was delivered. The water bath was maintained near body temperature ($\sim 37°\mathrm{C}$). Similar to the particle image velocimetry (PIV) experiments described above, a 270-μm-core fiber was inserted into the water bath with the length of the fiber level with the surface. Videos of the thermal signatures from heated water were recorded for TFL pulse rates of 10, 50, 100, 200, and 350 Hz. The thermal signature videos were compared to those using the microspheres as an alternative method to visualize the water flow caused by laser energy delivered in the water bath.

3.1.

Stone Suction Experiments

For the TFL, the effect of a net force pulling the stone toward the trunk end of the fiber was noticeable even at a low pulse energy and pulse rate of 35 mJ and 10 Hz, respectively [Fig. 3(a)]. The effect was weak, and no stone retropulsion was observed. As the pulse rate was increased, the effect of the force pulling the stone toward the fiber tip became stronger. The stone pull velocity increased up to 250 Hz (although the increase between 75 and 250 Hz was not statistically significant). Each pulse caused the water to flow axially in two different directions. The water flow in both directions increased as the laser pulses were delivered at higher rates. The compounding effects of forces from each pulse acted on the stone by either pushing or pulling it away from the fiber tip, depending on where the fiber tip was placed with respect to the stone.

Fig. 3

(a) Stone pull velocity as a function of pulse rate for TFL pulse energy of 35 mJ. (b) Stone pull velocity as a function of pulse energy for a holmium laser pulse rate of 20 Hz.

Furthermore, as the pulse rate was increased up to 250 Hz, strong retropulsion forces were observed. This was a problem when trying to keep the stone attracted to the fiber tip. Because the effect of pulling or pushing the stone is dependent on where the fiber tip is placed, when the net retropulsive force begins to dominate, the location of the fiber tip is critical. This may be the cause for the high error bars in the midrange pulse rate data points shown in Fig. 3(a). Both retropulsive and attractive forces were acting on the stone, and it was difficult to control which force dominated in the 200-Hz range.

As the pulse rate was increased to 350 Hz, retropulsion forces dominated. The stone was not pulled as far without eventually being rapidly pushed away. One of the reasons for this effect is that the retropulsion forces were so strong that a tightly wound vortex (to be seen in the PIV experiments) developed in close proximity to the fiber tip. Because the increase in water flow away from the fiber was stronger than the increase toward the fiber, the microspheres caught in the vortex were swept in the jet flowing away from the fiber tip.

The trend was similar for the holmium laser [Fig. 3(b)]. As the pulse energy was increased, the stone velocity also increased up to $\sim 210\mathrm{mJ}$ (although the increase was not statistically significant for pulse energies between 70 and 210 mJ). The stone velocity then decreased as the pulse energy increased beyond 210 mJ. For operation at 35 mJ, the suction effect was observed without any retropulsion effects. As the pulse energy was increased, the stone was pulled more rapidly, but in a discrete, choppy pattern of motion. The process of pulling the stone was not smooth, as experienced with the TFL. Use of higher pulse energies $>70\mathrm{mJ}$ resulted in greater retropulsion, which required the fiber to be held further away from the stone to maintain the suction effect. At pulse energies $>210\mathrm{mJ}$, the stone bounced randomly in all directions and retropulsion was increasingly difficult to avoid. Pulse energies at 300 mJ and higher yielded a much different result than lower energies. The stone was either pushed far ahead of the fiber tip or pushed far behind the fiber tip in a quick, discrete motion. The attractive force at higher pulse energies required the fiber to be placed completely past the stone, with the potential clinical risk of the laser energy delivered by the fiber being absorbed by tissue structures directly in front of the fiber.

Figure 4 shows the stone pull velocity as a function of average power. This plot provides a direct comparison between holmium and TFL. The TFL was able to exploit the suction effect at more than double the power produced by the holmium laser. More power delivery could make it possible for the TFL to potentially exploit the suction effect to maximize stone ablation rates while minimizing stone retropulsion.

Fig. 4

Stone pull velocity for both lasers as a function of average power.

3.2.

Particle Image Velocimetry

Microspheres ranging from 30 to 50 μm in diameter were used to track the water flow during laser irradiation with both the TFL and holmium. The results of our study show two dominating but opposing forces during lithotripsy. One force, a retropulsive force, is away from the fiber tip. The other force, an attractive force, is toward the fiber tip. The attractive force is suspected to be the cause of the suction effect observed during laser irradiation with both the thulium and holmium lasers.

Figure 5 illustrates the change in flow of the microspheres as the TFL pulse rate was increased. At low pulse rates, the microspheres flowed in two directions, toward and away from the fiber tip [Fig. 5(a)]. As pulse rates were increased, a vortex began to form around the sides of the fiber, causing microspheres pulled toward the fiber to return and become trapped in the jet flowing away from the fiber [Fig. 5(b)]. At pulse rates $>250\mathrm{Hz}$, the vortex surrounding the end of the fiber became tightly wound around the fiber tip [Fig. 5(c)]. This caused microspheres flowing toward the fiber tip to get trapped and then pushed back into the flow away from the fiber tip. This chaotic, tightly wound vortex is suspected to be the primary cause of decreasing stone pull velocity at higher pulse rates in the suction effect experiments.

Fig. 5

Flow of microspheres for TFL pulse rates of 20 (a) 200 (b) and 350 (c) Hz at 35 mJ per pulse.

The effects of the forces on the microspheres were more clear and consistent using the holmium laser than those of the TFL operating at 20 Hz with regard to the flow of microspheres. The vortices seen at higher pulse rates using the TFL were not seen using the holmium as pulse energies were increased. This could be due to the attractive force becoming stronger at roughly the same rate as the retropulsive force. The experiments using the holmium showed that the speed at which the spheres moved away from the fiber tip, in both directions, increased as the pulse energy was increased.

Although the frame rate of the camera used to capture the flow of microspheres was limited to 30 Hz, it was able to capture the shockwave-induced luminescence resulting from the collapse of the cavitation bubbles created by both TFL and holmium (Fig. 6). Such a distinct signature of cavitation bubbles has been previously reported as well.^13,14 Figure 6 also shows distinct differences in the shape and formation of cavitation bubbles created using the TFL [Fig. 6(a)] and the holmium laser [Fig. 6(b)].

Fig. 6

Shockwave-induced luminescence resulting from the collapse of cavitation bubbles for TFL pulse rate of 350 Hz at 35 mJ pulse energy (a) and holmium pulse rate of 20 Hz at 350 mJ pulse energy (b).

The modulated TFL operates at a longer pulse length (500 μs) than the holmium (300 μs). Furthermore, the absorption coefficient of water at the TFL wavelength (${\mu }_a=160{\mathrm{cm}}^{-1}$) is five times greater than that of the holmium (${\mu }_a=28{\mathrm{cm}}^{-1}$) (Ref. 1). Previous investigators have shown that for both increased absorption and longer pulse durations, the cavitation bubble becomes elongated and multiple cavitation bubbles may form.^15–17 Indirect evidence of this is seen in both the results for the TFL microsphere experiments and the shockwave-induced luminescence observed emanating from two distinct points in front of the fiber tip [Fig. 6(a)].

The holmium-induced cavitation bubbles have been shown to be less elongated and pear shaped. This is also supported by the results of the microsphere study performed with the holmium laser [Fig. 6(b)]. The area of luminescence appeared oval and resembled the shape of holmium-induced cavitation bubbles reported by other investigators.^{12,15,16,18,19}

3.3.

Thermal Imaging

A thermal camera was used to track the flow of heated water as laser energy was delivered just beneath the surface of the water bath. For the TFL, the camera recorded a flow of heated water away from the fiber tip and toward the trunk end of the fiber. The thermal camera also recorded small thermal eddies near the fiber tip. Figure 7 shows a recorded video frame. As the TFL pulse rate was increased, the area of thermal flow also increased. Thermal images (not shown here) were also acquired while heating water using the holmium laser, but no discernible eddies or flow pattern were observed. This may be due to the longer optical penetration depth of the holmium laser wavelength in water, resulting in a wider spread of water heating. The videos captured with the thermal camera reinforced the validity of trends observed in the suction effect and particle velocimetry studies.

Fig. 7

Frame of a thermal video captured during fiber-optic delivery of the TFL energy at 35 mJ and 200 Hz in a water bath.