Improving depth of focus for single element annular transducers

Single element annular transducer (SEAT) is a new type of high-frequency single element transducer. Compared with conventional single element uniform transducers (SEUT), in our design SEAT consists of six different thicknesses of concentric rings, and thus each has a different frequency response ranging from 33MHz to 61MHz. In this study, using SEAT, we exploit the dependence of spectral characteristics on position relative to the transducer center and test its ability to move transmit focal depth by using different chirp waveforms. This may potentially overcome the limitation of fixed geometric focus and limited depth of focus (DOF) of SEUT. Up-frequency and down-frequency Gaussian chirp signals were used in SEAT, and single cycle sinusoidal excitations were used in SEUT. Experimental results show that the effective transmit delay profiles can be changed using chirp excitations. In other words, the transmit focal depth is no longer fixed. Pulse compression is also effective in maintaining axial resolution. However, lateral sidelobes are usually higher although the −6 dB mainlobe widths are consistently lower.