Fragmentation of Urinary Calculi In Vitro by Burst Wave Lithotripsy

Abstract

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.

Pressure Pulse Distortion by Needle and Fiber-Optic Hydrophones due to Nonuniform Sensitivity

“…Moreover, for HITU signals, fiber optic hydrophones are generally preferred due to their superior robustness to high pressures ( e.g . > 10 MPa) [17–24], and the cylinder model performs very well for frequencies up to 100 MHz for the fiber optic hydrophone investigated, as may be seen in Fig. 10 (maximum relative magnitude error = 23% and maximum phase error = 13 degrees).…”

“…In addition, some needle [5] and fiber-optic [6–14] hydrophones are more robust for high-intensity therapeutic ultrasound (HITU) measurements than most membrane hydrophones, although robust membrane hydrophone designs also have been proposed [15, 16]. Fiber–optic hydrophones have been used extensively for calibration of HITU [17–23] and lithotripsy [24] systems. Some fiber optic hydrophones have exceptional spatial resolution on the order of 100 μm or less, far superior to most membrane hydrophones.…”

“…Since the fiber-optic hydrophone core typically has far higher characteristic impedance than the surrounding fluid, it may be accurately modeled as a rigid disk [31]. This model applies to many fiber-optic hydrophones, such as those commonly used in HITU [17, 18, 21–23] and lithotripsy [24]. Other fiber-optic hydrophones, such as those based on multi-layer [32], distributed Bragg reflector [33], Fabry-Perot interferometer [34–36] and other [37] designs, have a more complex response than the rigid, semi-infinite rods model predicts.…”

Cavitation-Enhanced Mechanical Effects and Applications

“…While the use of HIFU in lithotripsy is relatively recent (Ikeda et al 2006;Yoshizawa et al 2009;Duryea et al 2011;Harper et al 2014;Maxwell et al 2015), it has some significant advantages over SWL. HIFU can be focused much more precisely than shock waves such that it can enhance the damage at the desired site while reducing collateral tissue damage.…”

“…Moreover, Maxwell et al developed a new method of lithotripsy that uses short, broadly focused bursts of ultrasound rather than shock waves to fragment urinary calculi, and this technique is called burst wave lithotripsy (BWL) (Maxwell et al 2015). Their results demonstrate that stone comminution with BWL using broadly focused ultrasound bursts is feasible as a potential noninvasive treatment method for nephrolithiasis.…”