Foam gratings as an alternative to customised acoustic lenses

Tissue inhomogeneity can affect the accuracy with which ultrasound can be focused into the body. In existing clinical systems, phase corrections are typically made using ultrasound arrays, but with the advent of 3D printers there has been increased interest in producing patient-specific acoustic lenses. Acoustic lenses offer a more compact and affordable system than an equivalent array and may be suited for therapeutic applications where there is well defined target location. This study explored the properties of gratings made of foam machined with holes as an alternative to acoustic lenses. The approach was investigated with an analytical model, a full-wave numerical model, and experimental measurements. A grating is demonstrated that mimics a conventional ultrasound lens, modulating the phase of transmitted ultrasound while maintaining uniform amplitude. The performance of a foam grating is compared to lenses made of PDMS or 3D printed resin. Using two gratings, independent control of amplitude and phase is demonstrated, with increased insertion loss. The primary advantages of this technique over conventional lenses are rapid manufacture (<30 min) and the ability to control both the amplitude and phase of the transmitted ultrasound. Potential applications include generation of complex ultrasound fields for patient specific treatments in ultrasound therapy.