Maxwell AD et al, 2015: Fragmentation of Urinary Calculi in Vitro by Burst Wave Lithotripsy.
Maxwell AD, Cunitz BW, Kreider W, Sapozhnikov OA, Hsi RS, Harper JD, Bailey MR, Sorensen MD
Center for Industrial and Medical Ultrasound, Applied Physics Laboratory, University of Washington, Seattle, Washington;
Department of Acoustics, Physics Faculty, Moscow State University, Moscow, Russia.
Department of Urology, School of Medicine, University of Washington, Seattle, Washington
PURPOSE: We have developed a new method of lithotripsy that uses short, broadly focused bursts of ultrasound rather than shock waves to fragment stones. This study investigated the characteristics of stone comminution by burst wave lithotripsy in vitro.
MATERIALS AND METHODS: Artificial and natural stones (mean 8.2±3.0 mm, range 5-15 mm) were treated with ultrasound bursts using a focused transducer in a water bath. Stones were exposed to bursts with focal pressure amplitude ≤6.5 MPa at 200 Hz burst repetition rate until completely fragmented. Ultrasound frequencies of 170 kHz, 285 kHz, and 800 kHz were applied using 3 different transducers. The time to achieve 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 ≥2.8 MPa at 170 kHz. Fractures appeared along the stone surface, resulting in fragments separating 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 treatment durations between a mean of 36 seconds for uric acid to 14.7 minutes for cystine stones. At a frequency of 170 kHz, the largest artificial stone fragments were <4 mm. Exposures at 285 kHz produced only fragments <2 mm, and 800 kHz produced only fragments <1 mm.
CONCLUSIONS: Stone comminution with burst wave lithotripsy is feasible as a potential noninvasive treatment method for nephrolithiasis. Adjusting the fundamental ultrasound frequency allows control of stone fragment size.
J Urol. 2014 Aug 8. pii: S0022-5347(14)04179-2. doi: 10.1016/j.juro.2014.08.009. [Epub ahead of print]. FREE ARTICLE
Obviously, ultrasound seems to be a logic alternative instead of shock waves for stone fragmentation. Promising in vitro results are reported. The mechanism of fragmentation differs from shock wave disintegration due to ultrasound bursts of 10 cycles of relatively low pressure amplitudes of 2.8-6.5 MPa vs. single pulses of 40-100 MPa for shock waves. Besides shear waves also resonance and reverberation effects seem to contribute to fragments of different sizes as shown in Figure 6. Although pressure peaks are only in the range of 1:10 compared to shock waves, the integrated power as well as total energy delivered is much higher for BWL (≤15W/cm2, BWL vs. ≤ 1 W/ cm2, SWL). In addition, as we know from SWL, the discrepancy of in vitro vs. in vivo fragmentation of 1:20 or more has to be compensated by an according increase in pulse numbers resp. treatment time. This interesting approach may open an alternative for SWL however, clinical and approval hurdles will be high.