Randad A. et al., 2020: Design, fabrication, and characterization of broad beam transducers for fragmenting large renal calculi with burst wave lithotripsy
Randad A, Ghanem MA, Bailey MR, Maxwell AD.
Center for Industrial and Medical Ultrasound, Applied Physics Laboratory, University of Washington, 1606 San Juan Road, Seattle, Washington 98195, USA.
Burst wave lithotripsy (BWL) is a technology for comminuting urinary stones. A BWL transducer's requirements of high-pressure output, limited acoustic window, specific focal depth, and frequency to produce fragments of passable size constrain focal beamwidth. However, BWL is most effective with a beam wider than the stone. To produce a broad-beam, an iterative angular spectrum approach was used to calculate a phase screen that was realized with a rapid prototyped lens. The technique did not accurately replicate a target beam profile when an axisymmetric profile was chosen. Adding asymmetric weighting functions to the target profile achieved appropriate beamwidth. Lenses were designed to create a spherically focused narrow-beam (6 mm) and a broad-beam (11 mm) with a 350-kHz transducer and 84-mm focal depth. Both lenses were used to fragment artificial stones (11 mm long) in a water bath, and fragmentation rates were compared. The linearly simulated and measured broad beamwidths that were 12 mm and 11 mm, respectively, with a 2-mm-wide null at center. The broad-beam and the narrow-beam lenses fragmented 44 ± 9% and 16 ± 4% (p = 0.007, N = 3) of a stone by weight, respectively, in the same duration at the same peak negative pressure. The method broadened the focus and improved the BWL rate of fragmentation of large stones.
J Acoust Soc Am. 2020 Jul;148(1):44. doi: 10.1121/10.0001512. PMID: 32752768
This is an article with advanced physical considerations beyond the horizon of the average urologist. The bottom-line of the in-vitro experiments with burst wave lithotripsy (BWL) disclosed that disintegration of an 11 mm large stone was almost 3 times greater with a broad beam transducer (11 mm) than that achieved with a narrow beam transducer (6 mm). Disintegration was 44±9% and 16±4% with these two devices.
In the experiments described in this article the 350 kHz transducer had a focal depth of 84 mm.
Basically in vitro experiments with BWL clearly showed that the effect is best when the beam width exceeds the stone diameter.
I am indeed looking forward to what can be achieved clinically with BWL.