Reduction of rotational asymmetry in PSF synthesis for a broadband forward-looking ring array

Abstract

Aperture synthesis using a forward-looking ring array is an area of current interest in intracardiac imaging [1]. In general, data from multiple transmits are coherently combined to implement the synthesis, and schemes for the specification of the transmit and receive array weightings of the component beams are required. Often schemes originating in narrowband imaging are applied to broadband, pulse-echo imaging, but the result is usually sidelobes that are very different from those of the narrowband scheme and a rotationally asymmetric two-way response. Beam asymmetry is a potential disadvantage since the orientation of the array with respect to strong sidelobe reflectors may not be controllable. In the present work, we look for pulse-echo aperture synthesis approaches that produce rotationally symmetric PSFs with low sidelobe levels. Such rotational asymmetry can be decreased by minimizing the maximum delay used in a broadband scheme, and we introduce the idea that this minimization can be accomplished by the combination of delays with phase inversions. We also consider the use of more elements, structured as two rings, in order to increase the number of degrees of freedom available in a small number of transmits. The latter approach allows the design of the desired rotationally symmetric PSFs, which also have much lower peak sidelobe levels than alternative schemes. The proposed scheme makes use of two transmissions per look direction of the dual-ring array. Simulations of planar imaging of spherical voids are presented to illustrate the potential contrast improvement of this approach.