| Phased array offers several advantages over single focused transducer such as simultaneous multi foci, flexible focus patterns and aberration correction. This study will take the optimal schemes to elongate the focal region without grating lobes, to reduce the numbers of the channel drivers, as well as to maintain the flexible multi focus patterns. This paper presents two styles of new array structures, the 128 4-combined elements and the 128 9-combined elements, to evidently elongate the focal region without grating lobes. The combined element is formed by several nearby elements being combined as an element excited by one channel driver. In the array, each center of the combined element is shifted a basic element between two neighbor rows. The simulations of the 128 4-combined elements and the 128 9-combined elements show a same elongated focal region of 18mm×18mm×40mm, 10 times of that of an ordinary no-center-shift array. In addition, the 128 4-combined elements and the 128 9-combined elements can steer the C-shape multi foci very well. In further optimization, this study presents the 256 4-combined elements which can well steer up to 16 foci of fit-to-shape square multi foci, and steer the single focus laterally in a range of 34 mm and axially in a range of 46 mm without grating lobes. |