A Fully Populated Transrectal Array for Boiling Histotripsy Ablation of the Prostate

The endorectal ultrasound (US)-guided thermal ablation of prostate cancer (PCa) using high-intensity focused ultrasound (HIFU) is a widely used focal intervention. While generally safe and effective, it is not without challenges associated with heat diffusion and prefocal heating, which has spurred interest toward nonthermal and mechanical HIFU ablation regimes. Another challenge is the necessity to mechanically translate the HIFU transducer-commonly single-element or annular array-for volumetric treatments, which results in target shifts and transducer position readjustment. The 2-D arrays would address this problem, but their design is challenging in a small form factor. The element pattern must be tightly packed and aperiodic to maximize the active surface area and to suppress grating lobes, respectively. Here, we report on the design, fabrication, and performance evaluation of a 1.5-MHz 128-element transrectal HIFU array driven by Verasonics system capable of mechanical tissue ablation via boiling histotripsy (BH) under real-time coaxial US imaging guidance. A recently developed method for designing randomized, fully populated mosaic arrays was used to create the element pattern. The measured focus steering ranges of the fabricated array were 26 mm axially and 12 mm laterally in the BH regime, with driving voltage compensation by less than 43% and no grating lobe formation. Stress tests with a five-element prototype confirmed safe operating voltage of 850-V peak-to-peak, corresponding to the acoustic intensity of 542 W/cm2 at the array surface. The array integrated with a 128-element US imaging probe driven by the same Verasonics system was successfully used to produce volumetric BH lesions in polyacrylamide (PAA) tissue-mimicking phantoms.