Two-dimensional ultrasound phased array design for tissue ablation for treatment of benign prostatic hyperplasia

This paper describes the design, construction and evaluation of a two-dimensional ultrasound phased array to be used in the treatment of benign prostatic hyperplasia. With two-dimensional phased arrays, the focal point position can be controlled by changing the electrical power and phase to the individual elements for focusing and electronically steering in a three-dimensional volume. The array was designed with a steering angle of +/-14 degrees in both transverse and longitudinal directions. A piezoelectric ceramic (PZT-8) was used as the material of the transducer, since it can handle the high power needed for tissue ablation and a matching layer was used for maximum acoustic power transmission to tissue. Analysis of the transducer ceramic and cable impedance has been designed for high power transfer with minimal capacitance and diameter. For this initial prototype, the final construction used magnet compatible housing and cabling for future application in a clinical magnetic resonance imaging system for temperature mapping of the focused ultrasound. To verify the capability of the transducer for focusing and steering, exposimetry was performed and the results correlated well with the calculated field. Ex vivo experiments were performed and indicated the capability of the transducer to ablate tissue using short sonications. For sonications with exposure time of 10, 15 and 20 s, the lesion size was roughly 1.8, 3.0 and 4.3 mm in diameter, respectively, which indicates the feasibility of this device.