Application of vernier thinning techniques to segmented annular arrays

Due to the aperture periodicity, the inter-element spacing of two-dimensional squared arrays is maintained near lambda/2 in order to avoid grating lobes. This condition gives rise to severe problems derived from the huge number of array elements and from their little size that causes the signal to noise ratio to bring down. Vernier techniques have been proposed to lower the number of active elements, but the drastic reduction of the ultrasonic energy is still a great problem for the image contrast. In this work, vernier techniques for segmented annular (SA) arrays are theoretically studied. SA arrays produce lower grating lobes than squared arrays and, therefore, allow the element size to be increased beyond the lambda/2 constraint. Using larger elements, SA arrays have advantage to squared arrays because they have larger active area and smaller thinning order for the same complexity (number of channels) of the image system. Theoretical results of the vernier techniques applied to SA arrays in both radial and tangential directions are presented and compared with the equivalent squared array.