Fragmentation of urinary calculi in vitro by burst wave lithotripsy

Abstract

Purpose: We developed a new method of lithotripsy that uses short, broadly focused bursts of ultrasound rather than shock waves to fragment stones. We investigated the characteristics of stone comminution by burst wave lithotripsy in vitro.

Materials and methods: Artificial and natural stones (mean ± SD size 8.2 ± 3.0 mm, range 5 to 15) were treated with ultrasound bursts using a focused transducer in a water bath. Stones were exposed to bursts with focal pressure amplitude of 6.5 MPa or less at a 200 Hz burst repetition rate until completely fragmented. Ultrasound frequencies of 170, 285 and 800 kHz were applied using 3 transducers, respectively. Time to fragmentation for each stone type was recorded and fragment size distribution was measured by sieving.

Results: Stones exposed to ultrasound bursts were fragmented at focal pressure amplitudes of 2.8 MPa or greater at 170 kHz. Fractures appeared along the stone surface, resulting in fragments that separated at the surface nearest to the transducer until the stone was disintegrated. All natural and artificial stones were fragmented at the highest focal pressure of 6.5 MPa with a mean treatment duration of 36 seconds for uric acid stones to 14.7 minutes for cystine stones. At a frequency of 170 kHz the largest artificial stone fragments were less than 4 mm. Exposure at 285 and 800 kHz produced only fragments less than 2 mm and less than 1 mm, respectively.

Conclusions: Stone comminution with burst wave lithotripsy is feasible as a potential noninvasive treatment method for nephrolithiasis. Adjusting the fundamental ultrasound frequency allows for stone fragment size to be controlled.

Keywords: kidney; lithotripsy; nephrolithiasis; sound; ultrasonic therapy.

Figure 1

Modeled focal pressure waveforms for a lithotripsy shock wave (a) and ultrasound burst wave (b). The waveform in (a) approximates the shock from a Dornier HM3 lithotripter, while the burst wave in (b) corresponds with the highest pressure amplitude (p_a = 6.5 MPa) applied in this study. The experimental setup for exposure of stones to burst waves is shown in (c). A focused ultrasound transducer was placed in water tank, and the stone was aligned with the focus using a motorized 3-axis positioning system. The transducer was driven by an amplifier to expose the stone to ultrasound bursts. Fragments were collected in a small container positioned below the stone.

Figure 1

Modeled focal pressure waveforms for a lithotripsy shock wave (a) and ultrasound burst wave (b). The waveform in (a) approximates the shock from a Dornier HM3 lithotripter, while the burst wave in (b) corresponds with the highest pressure amplitude (p_a = 6.5 MPa) applied in this study. The experimental setup for exposure of stones to burst waves is shown in (c). A focused ultrasound transducer was placed in water tank, and the stone was aligned with the focus using a motorized 3-axis positioning system. The transducer was driven by an amplifier to expose the stone to ultrasound bursts. Fragments were collected in a small container positioned below the stone.

Figure 2

Proportion of artificial stones containing fractures after exposure to 60,000 bursts as a function of focal pressure amplitude. At each pressure level, n = 3 stones were tested.

Figure 3

Photographic sequence of an artificial stone during exposure to 170 kHz bursts with p_f = 6.5 MPa over 8 minutes. Ultrasound (US) burst waves are incident on the stone from the left. The photograph to the right shows the fragments generated after 8 minutes of exposure. The scale bar is 1 cm.

Figure 4

Images of uric acid (a), struvite (b), COM (c), and cystine (d) stones. The top images show the stone before 170-kHz burst wave exposure, and the bottom shows after treatment. The scale bars in (d) are 1 cm. All images have identical scales.

Figure 5

Size distribution of fragments post-exposure measured by serial sieving of fragments. The left 4 groups show the size distribution of fragments for natural stones treated with 170 kHz bursts, while the right 3 groups show the size distribution of artificial stones treated with 170 kHz, 285 kHz, and 800 kHz bursts. All measurements are mean values for stones treated in each category. COM = calcium oxalate monohydrate.

Figure 6

Photographs of fractures (top) and fragments (bottom) generated for stones treated with 170 kHz (a), 285 kHz (b), and 800 kHz (c) bursts with similar peak pressure amplitude applied to the stone. Increased ultrasound frequency resulted in stone surface fractures closer together and decreased fragment size. Bursts were incident from the left side of the stone in each of the photographs. The scale bars for the top and bottom rows are both 1 cm. SWL = Shock Wave Lithotripsy BWL = Burst Wave Lithotripsy COM = Calcium Oxalate Monohydrate