Design and characterisation of a micro-US linear array based on randomised piezocomposite

Microultrasound (µUS) arrays with operating frequencies beyond 25 MHz are suitable for subsurface diagnosis, e.g., dermatology, ophthalmology, intravascular US, because they offer increased image resolution compared to conventional, lower frequency devices. Presently, arrays made with 1–3 piezocomposite active materials are becoming increasingly attractive over bulk piezoelectric materials due to their excellent electrical and acoustic properties. However, because of their repetitive pattern, diced 1–3 piezocomposites are prone to mechanical coupling between spurious modes and the desired thickness resonance, leading to performance degradation. These modes can be successfully separated from the operating frequency by a composite structure comprising either sufficiently fine pillars, with high thickness-to-width aspect ratio (AR), or with a random distribution of pillars and kerfs, namely with a randomised pattern. The promising method presented in this paper describes the use of a gel-casting method combined with micromoulding for fabrication of 1–3 randomised piezocomposites with irregular pillar sections in the range of 2 - 50 µm, The proposed composite is then used as the active material of a 27 MHz, 20-element µUS array. The material's potential in suppressing spurious modes is evaluated through impedance measurements of each channel of the array and through pulse-echo response of the prototype transducer, compared to arrays comprising diced composites.