Acoustic bubble removal to enhance SWL efficacy at high shock rate: an in vitro study

Abstract

Rate-dependent efficacy has been extensively documented in shock wave lithotripsy (SWL) stone comminution, with shock waves (SWs) delivered at a low rate producing more efficient fragmentation in comparison to those delivered at high rates. Cavitation is postulated to be the primary source underlying this rate phenomenon. Residual bubble nuclei that persist along the axis of SW propagation can drastically attenuate the waveform's negative phase, decreasing the energy which is ultimately delivered to the stone and compromising comminution. The effect is more pronounced at high rates, as residual nuclei have less time to passively dissolve between successive shocks. In this study, we investigate a means of actively removing such nuclei from the field using a low-amplitude acoustic pulse designed to stimulate their aggregation and subsequent coalescence. To test the efficacy of this bubble removal scheme, model kidney stones were treated in vitro using a research electrohydraulic lithotripter. SWL was applied at rates of 120, 60, or 30 SW/min with or without the incorporation of bubble removal pulses. Optical images displaying the extent of cavitation in the vicinity of the stone were also collected for each treatment. Results show that bubble removal pulses drastically enhance the efficacy of stone comminution at the higher rates tested (120 and 60 SW/min), while optical images show a corresponding reduction in bubble excitation along the SW axis when bubble removal pulses are incorporated. At the lower rate of 30 SW/min, no difference in stone comminution or bubble excitation was detected with the addition of bubble removal pulses, suggesting that remnant nuclei had sufficient time for more complete dissolution. These results corroborate previous work regarding the role of cavitation in rate-dependent SWL efficacy, and suggest that the effect can be mitigated via appropriate control of the cavitation environment surrounding the stone.

FIG. 1.

Finger cot assembly used to hold model kidney stones during shock wave lithotripsy (SWL) treatment. The base of the structure contains a concave basket with 2 mm holes onto which stones were placed. A vinyl finger cot was used to enclose the assembly and to ensure that both stone debris and cavitation nuclei remained in the treatment zone. Complete comminution was defined as the point at which all stone fragments had been reduced to <2 mm and passed through the basket.

FIG. 2.

Experimental setup used to assess SWL comminution efficacy. The finger cot assembly used to hold model stones was filled with deionized water degassed to a dissolved oxygen level 80% of saturation to mimic that of urine; it was subsequently placed into a larger treatment tank containing degassed water (dissolved oxygen level <15% of saturation) with a conductivity of 600 μS/cm. Shock waves (SWs) were delivered from a research electrohydraulic lithotripter patterned after the Dornier HM3, while bubble removal pulses were generated by a separate piezoelectric transducer oriented orthogonal to the axis of SW propagation.

FIG. 3.

Pulse timing utilized for SWL treatments incorporating bubble removal pulses. A 500-μs delay was imposed between the SW and bubble removal pulse in order to allow SW-induced cavitation to undergo an uninhibited growth-collapse cycle. Bubble removal pulses had a frequency of 370 kHz, a duration of 100 ms, and an amplitude of 500 kPa.

FIG. 4.

Number of SWs required to achieve complete stone comminution (all debris <2 mm). The “per-shock” efficiency of conventional SWL was observed to increase as the shock rate was decreased. The incorporation of bubble removal pulses resulted in a drastic reduction in the number of SWs required for complete treatment at both 120 and 60 SW/min, while bubble removal produced no detectable difference in comminution efficacy at the lower rate of 30 SW/min.

FIG. 5.

Representative images capturing the degree of SW-induced prefocal bubble excitation for the six treatment schemes tested in this study. Each image corresponds to the 25th SW in a series of 50 SWs applied at the indicated rate. Cavitation along the SW axis was observed to decrease with a decrease in rate in conventional SWL (top row). The introduction of bubble removal (BR) pulses drastically reduced this cavitation at rates of 120 and 60 SW/min, whereas minimal bubble excitation was produced along the SW axis at the lower rate of 30 SW/min with or without bubble removal.