Simultaneous multizone focusing method with orthogonal chirp signals

Receive dynamic focusing with an array transducer can provide near optimum resolution only in the vicinity of transmit focal depth. A customary method to increase the depth of field is to combine several beams with different transmit focal depths, with an accompanying decrease in the frame rate. In this paper, we present a simultaneous multizone focusing method in which weighted chirp signals focused at different depths are transmitted at the same time. These chirp signals are mutually orthogonal in a sense that the autocorrelation function of each signal has a narrow mainlobe width and low sidelobe levels, and the crosscorrelation function of any pair of the signals has values smaller than the sidelobe levels of each autocorrelation function. Theoretically, any two chirp signals defined over two nonoverlapped frequency bands are mutually orthogonal. In the present work, however, a fractional overlap of adjacent frequency bands, up to 25%, is permitted to design more chirp signals within a given transducer bandwidth. The crosscorrelation values due to the frequency overlap is reduced by alternating the direction of frequency sweep of the adjacent chirp signals. We also observe that the proposed method provides better images when the low frequency chirp is focused at a near point and the high frequency chirp at a far point along the depth; better lateral resolution is obtained at the far field with reasonable SNR due to the SNR gain in pulse compression imaging