Diverging Polymer Acoustic Lens Design for High-Resolution Row-Column Array Ultrasound Transducers

Spherical diverging acoustic lenses mounted on flat 2D Row-Column Addressed (RCA) ultrasound transducers have shown the potential to extend the Field of View (FOV) from a rectilinear to a curvilinear volume region and thereby enable 3D imaging of large organs. Such lenses are usually designed for small aperture low-frequency transducers, which have limited resolution. Moreover, they are made of off-the-shelf pieces of materials, which leaves no room for optimization. We hypothesize that acoustic lenses can be designed to fit high-resolution transducers and they can be fabricated in a fast, cost-effective, and flexible manner using a combination of 3D printing and casting or CNC machining techniques. These lenses should increase the FOV of the array while preserving the image quality. In this work, such lenses are made in concave, convex, and compound spherical shapes, and from thermoplastics and thermosetting polymers. Polymethylpentene (TPX), Polystyrene (PS), Polypropylene (PP), Polymethyl methacrylate (PMMA), Polydimethylsiloxane (PDMS) and Room-temperature-vulcanizing silicone (RTV) diverging lenses have been fabricated and mounted on a 128+128 6 MHz RCA transducer. The performances of the lenses have been assessed and compared in terms of FOV, Signal to Noise Ratio (SNR), bandwidth, and potential artefacts. The largest FOV (24.0°) is obtained with a 42.64 mm radius PMMA-RTV compound lens, which maintains a decent fractional bandwidth (53%) and SNR at 6 MHz (-9.1 dB amplitude drop compared to the unlensed transducer). The simple PMMA TPX, PS, PP, PDMS and RTV lenses provide a FOV of 12.2°, 6.3°, 8.1°, 11.7°, 0.6° and 10.4°, a fractional bandwidth of 97%, 46%, 103%, 46%, 97%, 53% and 49%, and an amplitude drop of -5.2 dB, -4.4 dB, -2.8 dB, -15.4 dB, -6.0 dB and -1.8 dB respectively. This work demonstrates that thermoplastics are suitable materials for fabricating low-attenuation convex diverging lenses for large-aperture, high-frequency 2D transducers. This is highly desired to achieve high-resolution volumetric imaging of large organs.