Duryea AP et al, 2013: Acoustic Bubble Removal to Enhance SWL Efficacy at High Shock Rate: An In Vitro Study
Duryea AP, Roberts WW, Cain CA, Tamaddoni HA, Hall TL
Department of Biomedical Engineering, University of Michigan , Ann Arbor, Michigan
Rate dependent efficacy has been extensively documented in shock wave lithotripsy (SWL) stone comminution, with shock waves (SWs) delivered at 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 that 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.
J Endourol. 2013 Oct 4. [Epub ahead of print]
PMID:23957846 [PubMed - as supplied by publisher]. FREE ARTICLE
With the aim of eliminating cavitation bubbles in the water compartment of the shockwave source (pre-focal cavitation), the authors used a piezoelectric transducer at right angle to shockwave path.
A better effect was recorded at frequencies of 2 and 1 Hz when cavitation removal was used. Interestingly at a frequency of 0.5 Hz there was no difference. With piezoelectric pulses the number of shockwaves required for disintegration was around 20% and 40% of that recorded without the piezoelectric source at frequencies of 1Hz and 2 Hz, respectively at a frequency of 0.5 Hz (30 / minute) there is apparently enough time for cavitation bubbles to disappear between successive pulses.
In addition to the enhanced stone disintegration with a cavitation bubble removing deviceat 2 and 1 Hz, the improved disintegration at a frequency of 0.5 Hz is highly interesting. Despite the lower shockwave frequency disintegration without the new device was complete in 33 minutes at 0.5 Hz compared with 40 minutes at 1 Hz. The corresponding disintegration with the piezoelectric device was 33 and 25 minutes, respectively. It thus seems recommendable to consider both cavitation bubble removal and the option to deliver shockwaves at a frequency of 0.5 Hz in the development modern lithotripters.