Fabrication and characterization of a flat aperture Fresnel lens based histotripsy transducer

Two single element, 8 mm focal depth, 6 MHz PZT-5A 40% volume fraction 1-3 composite Fresnel aluminum lens based therapeutic ultrasound transducers for use in small animal histotripsy applications were built with 15 mm outer diameters — one with a central hole of 5.7 mm diameter for future co-registration and one full-aperture. The device was built with the front face filled with acoustically transparent epoxy to create a flat aperture allowing gel-coupling to tissue, where the Fresnel lens design allowed flattening of the aperture with minimal epoxy fill. Epoxy fill resulted in a 6% loss of focal pressure. The full-aperture device achieved 37 MPa/100 V peak-to-peak focal pressures with the removed center element device achieving 25 MPa/100V - a 32% reduction, which matches COMSOL simulated results. Pulsing between 190 V and 270 V at 17 cycles and 1 kHz PRF, the full-aperture device generated bubble clouds in water ranging from 0.31 mm to 0.51 mm radially, and 0.53 mm to 0.81 mm axially. Cavitation for the removed center element device was observed to begin at 370 V, and was consistent at 400 V.