Burst wave lithotripsy (BWL) is a noninvasive technology to fragment urinary stones using short pulses of focused ultrasound. The aim of this study was to assess how different combinations of pulse length and pulse repetition frequency (PRF) impact stone fragmentation rate and cavitation activity in an in vitro-model. Human calcium oxalate stones between 4–7mm were exposed in a tissue phantom with a 350-kHz BWL transducer with a beamwidth of 6 mm. Treatments were delivered in a 45–50% degassed water bath. Ultrasound imaging was used to target the stone and monitor cavitation activity around the stone. Stones were exposed to ten different parameter sets at peak negative pressure = 6.2 MPa in 5-min intervals up to 30 min total. After each interval, the remaining mass of fragments >2mm was measured to determine the rate of comminution. Longer pulse duration generally produced improved fragmentation. Increasing the pulse repetition rate with set pulse duration did not always produce faster comminution. The results suggest that more effective fragmentation may be achieved in BWL by long pulse durations and low PRF. [Work funded by NIDDK, Grants P01 DK043881 and K01 DK104854. Maxwell, Cunitz, and Bailey have consulting agreements with and equity in SonoMotion, Inc.]

Fragmentation of kidney stones by burst-wave lithotripsy (BWL) has shown promising results in preclinical and clinical trials. Based on initial findings, the system was upgraded to enhance imaging, target stones at deeper depths, fragment small stones, and potentially conduct dusting of stones. A 64-element, single-crystal, phased-array imaging probe is coaxially aligned with the therapy probe for image guidance and therapy feedback. Imaging is controlled through a Verasonics Vantage research ultrasound engine capable of harmonic imaging to enhance stone resolution and contrast. These features improve targeting and endpoint detection, particularly for small stones and fragments. New therapy probes were added to effectively target stones with greater skin-to-stone distance, including a higher (800 kHz) frequency transducer to effectively break <4mm stones into sub-millimeter fragments to facilitate passage. The amplifier was upgraded to a custom class D/E design with increased power required by the therapy transducers and is capable of ultrasonic propulsion and real time monitoring of electrical power. This system will provide capabilities to treat a larger patient population as we begin trials breaking and expelling stones in the clinic setting. [Work supported by NIH-P01-DK043881.]

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