Non-planar pad-printed thick-film focused high-frequency ultrasonic transducers for imaging and HIFU applications

Most high-frequency ultrasound transducers for imaging applications are based either on piezopolymers or on lapped bulk piezoceramics. The latter are planar, so focusing can be obtained by adding a lens (at the price of lower sensitivity) or by fracturing into a curved geometry. Pad printed thick-film transducers have been shown to be an interesting alternative, namely because the film is deposited with the required thickness, size and geometry, thus avoiding any subsequent machining and allowing geometrical focusing to be achieved. Film electromechanical properties are close to those of bulk ceramics with similar composition despite higher porosity. In this paper, pad-printed high-frequency transducers based on a low-loss piezoceramic composition are designed and fabricated. High-porosity ceramic cylinders with a spherical top surface are used as substrates and serve as backing material. The transducers are first characterized in view of imaging applications using a short pulse excitation, and their imaging capabilities are evaluated. Secondly, the transducers are excited by a one-period sine wave using several power levels to evaluate their capability to produce high-intensity focused ultrasound (HIFU) at frequencies around 20 MHz. The results, obtained via hydrophone measurements, are discussed showing that transducer performance is promising for applications that would require the same device to be used for imaging and for therapy. Nevertheless, the transducer design can be improved, and simulation studies are performed in order to find a better compromise between low-power and high-power performance. The size, geometry and constitutive materials of optimized configurations are proposed and their feasibility is discussed.