Nonlinear saturation effects in ultrasound fields of diagnostic-type transducers used for kidney stone propulsion

Abstract

A novel therapeutic application of ultrasound for repositioning kidney stones is being developed. The method uses acoustic radiation force to expel mm-sized stones or to dislodge even larger obstructing stones. A standard diagnostic 2.3 MHz C5-2 array probe has been used to generate pushing acoustic pulses. The probe comprises 128 elements equally spaced at the 55 mm long convex cylindrical surface with 41.2 mm radius of curvature. The efficacy of the treatment can be increased by using higher transducer output to provide stronger pushing force; however, nonlinear acoustic saturation effect can be a limiting factor. In this work, nonlinear propagation effects were analyzed for the C5-2 transducer using a combined measurement and modeling approach. Simulations were based on the 3D Westervelt equation; the boundary condition was set to match low power measurements. Focal waveforms simulated for several output power levels were compared with the fiber-optic hydrophone measurements and were found in good agreement. It was shown that saturation effects do limit the acoustic pressure in the focal region of the transducer. This work has application to standard diagnostic probes and imaging. [Work supported by RSF 14-12-00974, NIH EB007643, DK43881 and DK092197, and NSBRI through NASA NCC 9-58.]