Reference radius in Fresnel Zone Plates to control ultrasound beamforming

Abstract

Fresnel Zone Plate lenses present a high capacity to generate focus spot. In acoustics, these ultrasound lenses can be used in both HIFU and imaging systems. One of the limitations of classical Fresnel Zone Plates is the difficulty to control either the lateral lobes or the length of the focus. These focal geometric parameters are directly related to lateral resolution and depth of focus, respectively. In this work, we study the introduction of a new free parameter in the design of Fresnel lenses called reference radius/reference phase [1-2], which in some cases is equivalent to the introduction of a pupil mask in the lens. The ultrasound beamforming can be manipulated by means of this free parameter. The election of the appropriate value for the reference radius allows the lateral resolution improvement by reducing the lateral lobes or the expansion of the focus. This last effect is achieved through the generation of quasi-Bessel beams due to transformation of spherical waves into quasi-conic waves by means of apodization of the central part of the lens [3-5]. Therefore, the characteristics of the focus can be modified without changing the topology of the lens by using a pupil mask.