**1.1. Cavitation bubble dynamics**

Kidney stone diseases are crystallized solids, for example, kidney/ureter/bladder/urethra cal‐ culi or uroliths, which crop up in the urinary tract. The patient suffers acute ache and discom‐ fort. Urolithiasis is the third largest disease in urology following urinary tract infection and prostate condition. It affects 10% of the US population at a significant recurrence of ∼50% [1–3]. Shock wave lithotripsy (SWL) and the ureteroscopic laser lithotripsy (URS) are the top two most frequent treatment options in the USA for the treatment of ureteral stones [4, 5]. The review investigation in Ref. [6] concluded better stone‐free rates (SFRs) for renal stones <15 mm for URS compared with SWL. Even though laser lithotripsy has become the most popular treatment choice for kidney stone disease, the mechanism of calculus disintegration by laser pulse remains unclear. This is due to the multiple physical/chemical processes involved in laser pulse‐caused calculus damage and their sub‐microsecond timescales (as listed in **Table 1**) and their timescales are very short (down to sub‐microsecond level).

For laser lithotripsy, the laser pulse‐induced impact by energy flow can be summarized in **Figure 1** as follows: [7]



**Table 1.** Physical and chemical processes during laser pulse‐induced calculus damage [7].

Investigation of Laser Pulse‐induced Calculus Damage Mechanism by a High‐speed Camera http://dx.doi.org/10.5772/intechopen.69981 81

**Figure 1.** The energy flow block diagram of the laser pulse‐induced calculus damage.


**1. Introduction**

**Figure 1** as follows: [7]

**1.1. Cavitation bubble dynamics**

are very short (down to sub‐microsecond level).

will be a Fresnel reflection loss).

degree accompanied by light emission).

wave after traveling a fraction of a millimeter)

**Table 1.** Physical and chemical processes during laser pulse‐induced calculus damage [7].

Kidney stone diseases are crystallized solids, for example, kidney/ureter/bladder/urethra cal‐ culi or uroliths, which crop up in the urinary tract. The patient suffers acute ache and discom‐ fort. Urolithiasis is the third largest disease in urology following urinary tract infection and prostate condition. It affects 10% of the US population at a significant recurrence of ∼50% [1–3]. Shock wave lithotripsy (SWL) and the ureteroscopic laser lithotripsy (URS) are the top two most frequent treatment options in the USA for the treatment of ureteral stones [4, 5]. The review investigation in Ref. [6] concluded better stone‐free rates (SFRs) for renal stones <15 mm for URS compared with SWL. Even though laser lithotripsy has become the most popular treatment choice for kidney stone disease, the mechanism of calculus disintegration by laser pulse remains unclear. This is due to the multiple physical/chemical processes involved in laser pulse‐caused calculus damage and their sub‐microsecond timescales (as listed in **Table 1**) and their timescales

80 Updates and Advances in Nephrolithiasis - Pathophysiology, Genetics, and Treatment Modalities

For laser lithotripsy, the laser pulse‐induced impact by energy flow can be summarized in

• Photon energy in the laser pulse that includes the laser pulse train propagating through the delivery fiber and passing through the fiber tip (the fiber tip is typically uncoated and there

• Photon absorption that generates heat in the water liquid and vapor (resulting in super‐ heated water exceeding 100°C or a plasma effect with temperatures up to thousands of

• Shock wave generation (at the initial injection of the laser pulse into the water. This is a Bow shock effect that results in an initial strong disturbance but dampens to a regular acoustic


Cavitation bubble [9–16] dynamics are the centerpiece of the physical processes that link the whole energy flow chain from laser pulse to calculus damage.

In this study, cavitation bubble dynamics have been investigated by utilizing a high‐speed cam‐ era and a needle hydrophone. We keep the following three questions in mind when performing this investigation: (1) What are the differences in the characteristics of the bubble dynamics between a short pulse and a long pulse for Ho:YAG laser? A reduction of retropulsion and reduced fiber burn‐back has been demonstrated by employing long‐pulse modes in Ho:YAG lasers in contrast to short‐pulse modes [17]. Several Ho:YAG laser vendors offer variable pulse option including the AMS StoneLightTM 30, pulse with a range from 150 to 800 μs. Indeed, it would be interesting to investigate the cavitation bubble dynamics as a function of pulse width; (2) there has been a dispute as to whether or not a cavitation bubble forms during lithotripsy when the fiber tip is in contact with the surface of the calculus. This contact mode is a com‐ mon practice during treatment of urolithiasis and can be studied with a high‐speed camera; (3) although Ho:YAG lithotripter is the benchmark for laser lithotripsy, the cavitation bubble dynamics and transient pressure level of other laser sources including the Q‐switched (QS) Tm:YAG laser [18, 19] were also investigated. The study revealed the cavitation bubble dynam‐ ics (oscillation and center of bubble movement) and transient pressure levels of the Ho:YAG and Tm:YAG laser pulses at different energy levels and pulse widths. A more detailed investi‐ gation of the relationship between cavitation bubble dynamics and calculus damage (fragmen‐ tation/dusting) will be conducted in a future study.

#### **1.2. Fiber‐tip damage mechanism**

The review investigation in Ref. [6] concluded better stone‐free rates for renal stones <15 mm for URS compared with SWL. However, the delivery fiber employed in URS encountered distal‐end burn‐back [20–23].

As shown in **Figure 2** [11], fiber‐tip (distal‐end) degradation/damage/burn‐back is a constant issue during the URS treatment of kidney stone disease. Fiber‐tip damage leads to a decreased transmittance of laser power, which could lead to significant decrease of stone comminu‐ tion. On certain occasion, the fiber‐tip burn‐back is so much that the degraded fiber tip will consume a lot of the laser power, which can lead to such a high temperature that exceeds the melting temperature of the fiber‐cladding layer or polymer jacket. Although, it is a common

**Figure 2.** Samples of degraded fiber‐end (burn‐back).

sense that the bigger the laser energy density at the fiber tip, the faster the tip deterioration, the burn‐back mechanism of the fiber tip remains unclear.

this investigation: (1) What are the differences in the characteristics of the bubble dynamics between a short pulse and a long pulse for Ho:YAG laser? A reduction of retropulsion and reduced fiber burn‐back has been demonstrated by employing long‐pulse modes in Ho:YAG lasers in contrast to short‐pulse modes [17]. Several Ho:YAG laser vendors offer variable pulse option including the AMS StoneLightTM 30, pulse with a range from 150 to 800 μs. Indeed, it would be interesting to investigate the cavitation bubble dynamics as a function of pulse width; (2) there has been a dispute as to whether or not a cavitation bubble forms during lithotripsy when the fiber tip is in contact with the surface of the calculus. This contact mode is a com‐ mon practice during treatment of urolithiasis and can be studied with a high‐speed camera; (3) although Ho:YAG lithotripter is the benchmark for laser lithotripsy, the cavitation bubble dynamics and transient pressure level of other laser sources including the Q‐switched (QS) Tm:YAG laser [18, 19] were also investigated. The study revealed the cavitation bubble dynam‐ ics (oscillation and center of bubble movement) and transient pressure levels of the Ho:YAG and Tm:YAG laser pulses at different energy levels and pulse widths. A more detailed investi‐ gation of the relationship between cavitation bubble dynamics and calculus damage (fragmen‐

82 Updates and Advances in Nephrolithiasis - Pathophysiology, Genetics, and Treatment Modalities

The review investigation in Ref. [6] concluded better stone‐free rates for renal stones <15 mm for URS compared with SWL. However, the delivery fiber employed in URS encountered

As shown in **Figure 2** [11], fiber‐tip (distal‐end) degradation/damage/burn‐back is a constant issue during the URS treatment of kidney stone disease. Fiber‐tip damage leads to a decreased transmittance of laser power, which could lead to significant decrease of stone comminu‐ tion. On certain occasion, the fiber‐tip burn‐back is so much that the degraded fiber tip will consume a lot of the laser power, which can lead to such a high temperature that exceeds the melting temperature of the fiber‐cladding layer or polymer jacket. Although, it is a common

tation/dusting) will be conducted in a future study.

**1.2. Fiber‐tip damage mechanism**

**Figure 2.** Samples of degraded fiber‐end (burn‐back).

distal‐end burn‐back [20–23].

Fiber‐tip damage/degradation/burn‐back mechanism is a complex subject due to its numerous physical phenomena, for example, sonic shock waves, self‐focusing of the laser beam, and tran‐ sient thermal surge, and so on. It covers three areas of degradation mechanisms: (1) Mechanical: shock wave and debris impulsion; (2) Thermal: heating/liquidating of material, transient ther‐ mal surge because of the absorption of self‐focusing laser beam by the microstructure of the fiber‐tip surface material; (3) Optical: photoionization or plasma [24] as shown in **Figure 3**.

In this investigation, the fiber‐tip damage was studied by visualization of the pressure wave, cavitation bubble dynamics, and ejected phantom stone debris using a high‐speed camera and the Schlieren technique. A high‐speed camera is a great device to study the relationship of the laser pulse with the phantom stone, as well as the recoil motion [8]. The principal chro‐ mophore of the 2.01‐μm Holmium laser is water, which is critical for fragmentation of the calculus during laser lithotripsy [19]. The shock wave [9, 10, 12–14] that the laser pulse gener‐ ates is a disturbance wave that travels faster than sound and is one of the mechanical causes of the fiber‐tip damage (**Figure 2**). Because of the transparency of water liquid, pressure wave imaging inside water is not as straightforward. However, the Schlieren method can reveal the acoustic wave inside water, just like an X‐ray which can reveal the invisible pressure varia‐ tion inside a transparent matter. In this technique, a knife edge is placed at a focal spot to reduce the number of rays that do not interact with the acoustic field to reveal those that do interact; in physical optics terms, the Schlieren technique converts the phase information into an intensity image.

**Figure 3.** The degradation mechanism of the fiber tip (the items in red are studied in this investigation).

A commercially available pulsed Ho:YAG laser at 2.13 with a 365‐μm core diameter fiber impinging on calculus phantoms (Plaster of Paris, white gypsum cement, 10‐mm cube) was employed to simulate the URS process. Laser power‐caused pressure wave, cavitation bubble dynamics, and calculus particles scattering were videotaped by a high‐speed camera with 10,000–930,000 frames per second (fps). The pressure wave is captured by the Schlieren method. The contribution of ejected stone particles in fiber‐tip degradation is also investi‐ gated. The study concluded that using a high‐speed camera combining with the Schlieren technique is a powerful tool to study the movement of the pressure wave and its relationship with bubble dynamics and stone damage. More study in pressure wave shaping by the geo‐ metric shape of the fiber tip and the detailed mechanisms of shock waves, cavitation bubble dynamics, and calculus debris ejection will be investigated in a future study.

#### **1.3. Calculus migration/retropulsion**

During the treatment of urolithiasis, the urinary calculus is subjected to retropulsion forces induced by the combined effects of ablated particle ejection, interstitial water vaporization, and bubble dynamics [25–27]. Therefore, because of the retropulsion, the stone has moved away from the fiber tip. This can cause longer procedure time because of additional steps of finding the dislocated stone and repositioning the fiber tip to it. Recoil motion investigations in the past revealed the relation between retropulsion displacement and laser power, fre‐ quency, and fiber core size [28–31]. Recoil motion is proportional to the laser power and the fiber core size. Furthermore, another research claimed that the recoil motion decreased with a longer laser pulse without compromising dusting effectiveness significantly [32].

The amplitude of stone recoil motion (retropulsion) during kidney stone treatment depends mainly on the power source or instrument. The pneumatic or electrohydraulic lithotripters cause a much bigger recoil motion than laser lithotripters [30, 33–34]. Nevertheless, the laser lithotripters can cause noticeable dislocation of the stone during the procedure. A few inves‐ tigations of the URS treatment of upper ureteral calculi have revealed that the main reason for calculus‐free failures can be due to recoil motion and less frequently to inability to track or seek the stones [35–37]. Recoil‐dislocated calculus could lead to longer operation period, the necessity for another process to deal with recoiled parts and as such reduced stone‐ free level. The stone recoil motion results in additional patient morbidity and health‐care expenses [30, 33]. Besides, left‐behind stone debris can act as a seed for calculus growing back, renal colic, and persistent infection.

The previous studies on stone retropulsion often employed a holder, like a test tube or a "V" shape groove. These approaches, however, have shown large uncertainty and low accu‐ racy, most likely due to the friction between the stone phantom and the holder on which the stone phantom stationed. For instance, stone phantoms (Plaster of Paris, 10‐mm cube) were employed to simulate the URS process, with the previous methodology resulted in <0.5‐mm entire recoil movement (either with a "V" grove or a test tube). When scaling down the stone size to 5‐mm cube (1/8 in volume), the recoil movement was very unpredictable. Our ear‐ lier study of recoil movement on a 5‐mmcube resulted in a 59% standard deviation [38], for example, a peak‐to‐peak recoil movement range of ∼3–10 times as much.

Sroka et al. [39] employed a sphere‐shaped lead ball hung with a nylon string to investigate the recoil motion in URS by a standard CCD camera. In this investigation, a calculus phantom in water formed a pendulum, and the phantom recoil motion is studied using this approach to get rid of any friction that could occur if a holder was employed to host the calculus. This method mostly decreased the migration variation of recoil motion in URS. Furthermore, a high‐speed camera was used to study the movement of the calculus which covered zero‐ order (displacement), first‐order (speed), and second‐order (acceleration) dynamics. This study employed a commercially available pulsed Ho:YAG laser at 2.1‐ and 365‐μm core fiber, and calculus phantoms (Plaster of Paris, 10 × 10 × 10 mm<sup>3</sup> cube) to mimic laser lithotripsy procedure.
