Shock wave lithotripsy (SWL) is a widely used, non-invasive procedure to treat symptomatic kidney stones, with more than 300,000 patients using SWL each year in the United States. Although SWL is attractive as a non-invasive treatment option, it often requires repeated treatment and/or adjuvant invasive surgery to be treated. In fact, SWL has only been successful in about 50% of cases. In addition to the relatively low success rate, SWL can cause damage to the kidneys and surrounding tissues, which can have serious consequences. Despite this, extracorporeal shock wave lithotripsy (SWL) remains the only clinical option for the treatment of kidney stones in vitro.
Burst Wave Lithotripsy (BWL) is a new technique that can safely and effectively crush litholites. The University of Washington is studying burst wave lithotripsy (BWL) as a potentially more effective and less damaging method of extracorporeal lithotripsy. BWL uses low-amplitude ultrasound waves delivered at relatively high frequencies to cyclically apply pressure to the stones until they break. During BWL, small stone fragments are detached from the larger stone surface rather than breaking into a series of large fragments, as is common in SWL. With the proper selection of the frequency, the stone can be broken into sufficiently small fragments. BWL uses short, broadly focused ultrasound waves instead of shock waves to break up stones.
Similar to SWL, burst wave lithotripsy (BWL) can focus the target by applying pressure pulses in vitro. The BWL pressure amplitude is significantly lower than that of SWL. Preliminary studies using a 170 kHz transducer have shown that BWL breaks down man-made and natural stones (calcium oxalate monohydrate, uric acid, struvite stones, cystine) into small fragments that are more uniform than SWL and faster than clinical SWL. Parallel studies at higher frequencies have shown that fragment size is controlled by ultrasound frequency. A significant advantage of BWL treatment over SWL is that it can control the size of stone fragments. Studies have shown that BWL crushed stone at three different frequencies, with the higher frequency producing denser cracks, resulting in smaller fragments. Bursts between 400-500 kHz produce fragments up to 1 mm, which may be clinically optimal.
In addition to lithotripsy, initial in vivo experiments have shown that the use of BWL parameters causes less damage to the kidneys. In practice, the pressure of BWL is 30~100 MPa, which is at least an order of magnitude lower than the vibration amplitude of SWL. The BWL burst rate (200 Hz) is much greater than the typical SWL pulse repetition rate (1-2 Hz). The result of this rapid delivery of energy to the stones is especially effective for softer stone compositions (uric acid and struvite stones), which in some in vitro experiments can lead to fragmentation of these stones in just a few seconds. Although BWL takes longer to treat COM and cystine stones, all stones can be completely broken into smaller fragments (≤4mm).
High-speed images taken at t = 80 μs and 130 μs during the 3rd pulse. Scale bar indicates 5 mm.
Since ultrasonic-induced heating is minimized, no thermal damage occurs to the BWL. If the stones are in or near the focal point, the size of these stones will decrease. The researchers observed that BWL creates cracks in both man-made and real stones, resulting in the fragmentation of the whole stone. Lithotripsy can control the size of stone fragments by adjusting the ultrasound frequency. These characteristics suggest that BWL may provide a potentially viable non-invasive alternative to SWL.
Simulated focal pressure waveforms for gravel shock waves (a) and ultrasonic burst waves (b). The waveform in (a) approximates the impact of the Dornier HM3 lithotripter, while the burst wave in (b) corresponds to the highest pressure amplitude applied in this study (pa = 6.5 MPa).
The experimental setup for exposing stones to burst waves is shown in (c). A focused ultrasound transducer is placed in the tank and the stones are aligned with the focal point using a motorized 3-axis positioning system. The transducer is driven by an amplifier that exposes the stone to an ultrasonic burst. Collect the fragments in a small container located below the stones.
Photographic sequence of artificial stone during exposure to 170 kHz pulse, pf = 6.5 MPa over 8 min. Ultrasonic (US) burst waves are incident on the stone from the left side. The figure on the right shows the fragments produced after 8 minutes of exposure. Scale bar is 1 cm.
Photographs of cracks (top) and fragments (bottom) produced by stones treated with 170 kHz (a), 285 kHz (b) and 800 kHz (c) have a similar peak pressure amplitude applied to the stones. The increase in ultrasound frequency results in tighter fractures on the surface of the stone and a decrease in fragment size. Each photo shows an explosion from the left side of the stone. Both the top and bottom rows are 1 cm on a scale.