ON THE DEVELOPMENT OF SMALL FORM FACTOR HISTOTRIPSY DEVICES FOR NEUROSURGICAL APPLICATIONS AND SMALL ANIMAL EXPERIMENTS
The development of surgical instruments is a growing area where histotripsy is relatively under represented. This work focuses on the miniaturization of histotripsy devices for introduction into the neurosurgery, cardiac, and tumor resection fields. Three devices were developed where the key to miniaturizing was a novel transducer stack using an elliptical aluminum lens. A 10mm aperture aluminum lens histotripsy transducer with an f-number of 0.7 was fabricated using an air-backed 5.0MHz, PTZ-5A, 1-3 dice-and-fill piezoelectric composite. A KLM model of the device showed maximum output pressure at 6.8MHz. Cavitation was observed in water by driving the composite with single-cycle, 6.8MHz pulse at a 50Hz PRF and a bubble cloud 264µm long by 124 µm wide was measured, demonstrating the highest frequency histotripsy bubble cloud to date. Co-registered imaging through the lens center was added by inserting a 30MHz phased array endoscope. Ex-vivo sub-surface tissue ablation was also demonstrated. Two 5mm square aluminum lens histotripsy devices were built with two materials: a 40% volume fraction 1-3 PZT-5A composite and Pz-39, a porous ceramic. The composite-based could not cavitate in water up to a 600V drive level, whereas the Pz39 based device was able to cavitate in water at a drive level of 220V. In vivo ablation of rat brain tissue was demonstrated through a skull opening while monitoring using an endoscopic 30MHz ultrasound phased-array with B-mode imaging and power Doppler overlay. Power Doppler showed the ablation zone grow steadily over 12s. Immediately after treatment the ablated area appeared anechoic, slowly becoming specular. The final project focused on developing a 15mm diameter Fresnel-lens device, filling the lens with epoxy to create a flat aperture to simplify coupling. The Fresnel lens has a reduced curvature compared to a non-Fresnel lens, allowing integration of an imaging endoscope without excessive signal loss in the epoxy. The device was designed for 6MHz and demonstrated cavitation free-field in water. Hydrophone measurements showed the optimal drive frequency was 6.0MHz, as designed. The center element was removed to demonstrate future endoscope integration, and although approximately 40% pressure loss was observed the device was still capable of free-field cavitation.