Presentation Authors: Adam Maxwell*, Brian MacConaghy, Michael Bailey, Mathew Sorensen, Oleg Sapozhnikov, Seattle, WA
Introduction: Burst wave lithotripsy (BWL) is a preclinical technology that uses focused ultrasound bursts to fragment stones. Unlike shock wave lithotripsy, stones fracture into specific fragment sizes which are controlled by the ultrasound frequency. To understand and better optimize characteristics of BWL, we aim to determine the mechanisms of stone fracture with focused ultrasound. This study used photoelasticity imaging to visualize burst wave propagation in stone models and identify particular elastic wave modes causing stresses in stones.
Methods: Three focused ultrasound transducers with frequencies of 170, 340, and 800 kHz were used to apply BWL pulses to kidney stone models in a water bath. The models were rectangular, cylindrical, or irregular in shape, made from epoxy or glass (transparent materials displaying stress birefringence) and ranged from 6-26 mm in dimension. Each stone model was positioned in a chamber of index matching fluid submerged in the water bath. An LED light was used to generate circularly-polarized light pulses for back-illumination of the stone. A high-speed camera was used with a stroboscopic method to capture video sequences of ultrasound propagation through each stone model with effective frame rates up to 4 million frames per second. Elastic wave modes in the stone were analyzed and compared with numerical calculations of wave modes. These data were also compared to locations of fractures in experiments where artificial stones were exposed to BWL in vitro.
Results: Formation of longitudinal and shear waves in the model was visualized. Results showed development of periodic stresses in the stone that appear as bright regions on the image with the spacing dependent on frequency (Figure). These patterns were identified as guided wave modes, which form standing waves upon reflection from the distal surfaces of the stone model, causing a uniform 'grid' of oscillating stress points. Observed wave patterns correlated with specific numerically calculated modes dependent on frequency and material. Artificial stones exposed to BWL produced cracks at positions calculated by this mechanism.
Conclusions: These results support guided wave production and reflection as a mechanism of stone fracture in BWL. This mechanism can describe the features of crack spacing and its frequency dependence observed in experiments.
Source of Funding: Work supported by NIH NIDDK grants K01 DK104854, P01 DK043881 and resources from the VA Puget Sound Health Care System.