Adaptive processing on transmit and receive for high frequency composite transducers

High frequency ultrasound imaging systems are used in many fields (eye, skin, preclinical animal imaging). To date, making high frequency composite transducers for such systems is a challenge due to the small sizes of the composite elements. A width-to-height ratio of less than 0.5 is generally required to avoid interferences from lateral resonances in the composites. This is difficult to reach in making thin composites for high frequency applications. In this study, we investigate the use of adaptive driving pulses on transmit and adaptive filters on receive for composite transducers, to cancel out the effects of lateral resonances and generate shorter pulses. The required adaptive driving pulse and adaptive filter can be determined from simulations and experiments. FEM simulations showed that bandwidth and level of secondary pulses, can be significantly improved using adaptive pulses to drive composite transducers with square, triangular and hexagonal PZT pillars. Two composite transducers, with hexagonal PZT pillars and an aspect ratio of 0.72 and 0.48, respectively, were tested. The -6dB bandwidth measured in the far field increased from 71% to 78% for the first transducer and 40% to 115% for the second. The -20dB pulse width decreased by 48% and 67%, respectively. And the level of secondary pulses decreased from -10dB to -23dB and from -6.5dB to -28dB, respectively. Pulse-echo experiments using a 15 μm wire positioned in the far field of the first transducer in water, showed a decrease of the pulse width, from 0.49 μs with a one-cycle driving pulse to 0.21 μs with an adapted driving pulse and to 0.18 μs after adaptive filtering on receive. Pulse echo experiments and phantom images with the second transducer also showed great improvements using this adaptive processing technique.