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Kidney Stones and Ultrasound
 One exciting technology that may solve this problem is acoustic bubble coalescence (Duryea et al., 2014a,b). With a relatively weak pulse of ultrasound, bubbles on the stone can be made to linearly oscillate rather than violently ex- pand and collapse. When two bubbles oscillate near each other, there can be a significant force on each bubble due to their radiated fields, known as the secondary Bjerknes forces (the primary Bjerknes force describes the radiation force on a bubble from a standing wave; Bjerknes, 1906). Under the condition that both bubbles are oscillating in phase, the secondary Bjerknes force causes them to be at- tracted. In a cloud, this leads to bubbles translating toward each other and coalescing when a weak acoustic field is ap- plied. In experiments, Duryea et al. (2014b) demonstrated this effect with SWL-generated bubble clouds, reducing a shock-induced bubble cluster to a single bubble (Figure 5) and substantially improving stone fragmentation efficiency.
Additionally, in SWL, breathing and patient motion cause a high number of shocks to miss the stone, reducing the effi- ciency of stone fragmentation while increasing tissue injury. Timing the shocks based on the patient’s respiratory cycle has been shown to improve stone fragmentation but provides no information as to the actual location of the stone. Ultrasound- based techniques that use multiple anatomical features (such as organ contour with vectors relating to the stone position; Koizumi et al., 2011) often outperform algorithms that sim- ply track stone position as the imaging plane changes with respiratory motion. Programming the lithotripter to only fire when the stone is at the focus or motorizing the lithotripter to move with the stone has not been adopted clinically but has the potential to improve stone hit rate by more than 50% (Cleveland et al., 2004; Sorensen et al., 2012).
Another experimental method is burst wave lithotripsy (BWL) that uses sinusoidal focused ultrasound bursts rather than shocks to fragment stones (Maxwell et al., 2015). Oper- ating in the range of 100 kHz to 1 MHz, focused ultrasound bursts are delivered to a stone to produce stresses and cause fractures. Similar to SWL, the dominant mechanisms of frag- mentation are thought to be elastic waves in the stone and acoustic cavitation; however, BWL has some notable unique characteristics that differ from fragmentation in SWL. In particular, elastic wave models for BWL show evidence that surface waves play a particularly important role in produc- ing initial stone fracture. Due to the sinusoidal nature of the pulse, the surface waves form periodic stresses along the stone surface. The spatial period is related to the frequency and the speed of sound for the surface waves that, for a medium such
56 | Acoustics Today | Winter 2017
Figure 5. Bubble cloud development with and without coales- cence caused by low-intensity ultrasound. A bubble cluster is formed by an intense ultrasound pulse similar to a lithotripsy shock wave. Without the bubble removal ultrasound pulse (right), the majority of bubbles visible at t = 0 ms remain at t − 1 ms, and subsequent lithotripter pulses (bottom) grow these remaining bubbles, shielding the stone from the effects of the shock wave. With the bubble removal ultrasound pulse (left), a weak (400-kPa) ultrasound pulse arrives at the bubble cluster at t = 0 ms and lasts for 1 ms. The ultrasound pulse causes bubbles in the cluster to merge together or coalesce, such that at 1 ms only a single bubble remains. Reprinted with permission from Duryea et al. (2014b) ©IEEE.
as a kidney stone, is close to the transverse sound speed of the material. The stresses from surface waves diminish as they extend into the stone and are limited to about one wavelength from the surface. The periodicity and limited depth of stresses created by sinusoidal pulsing in BWL leads to a distinctive fracture pattern and the generation of relatively uniform frag- ment sizes (Figure 6). Furthermore, the fragment sizes can be controlled by changing the wavelength of the ultrasound. The fragments can thus be made uniformly small so that they will pass spontaneously and asymptomatically from the urinary tract. Such a feature could lead to a significant improvement in lithotripsy success.
Repositioning Stones with Ultrasonic Propulsion
Fragments must pass once broken by SWL, BWL, or ureteros- copy. Because of gravity and kidney anatomy, fragments tend to collect and dwell in the lower half of the kidney after a pro- cedure. To solve this problem, ultrasonic propulsion is being

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