Linear hydrophone arrays for measurement of shock wave lithotripter acoustic fields

Abstract

Lithotripter acoustic field characterization is based on single-element hydrophone measurements even though many clinical lithotripters do not have highly repeatable sound fields. Here, linear hydrophone arrays composed of 20 elements, each measuring 0.5 by 0.5 mm and spaced 1.25 mm center to center, are described and characterized. The arrays were fabricated by bonding a 9 µm piezopolymer film to a flex circuit on which an array pattern had been formed using standard printed circuit board etching techniques. After bonding, the devices were backed with an epoxy plug in order to provide structural support. The sensitivity of each hydrophone element was measured at 41 mm axial distance with a pressure of 4.5 kPa using a 5.25 MHz, 14-cycle tone burst. The resulting sensitivities were normalized across each array. The normalized RMS voltages were quite uniform across each array, and between arrays, with an ≈ 6% variation in voltage. The arrays were also placed in a piezoelectric lithotripter in order to determine the shot-to-shot repeatability for peak positive pressures of 60 MPa. The array elements withstood about 500 shock waves before slowly losing sensitivity.