Coded Excitation can Provide Simultaneous Improvements in Sensitivity and Axial Resolution in Doppler Ultrasound Systems

We investigated the possibility of using coded excitation and pulse compression in Doppler ultrasound systems to improve resolution and signal-to-noise ratio (SNR) without increasing transmitted energy or peak power. A small target simulator was used, together with a programmable real-time ultrasound system which has the facility to generate arbitrary transmit waveforms and to acquire the received RF signal. The study demonstrated that it was possible to achieve an axial resolution of 0.197 mm with sidelobe level of -42.3 dB when using a linearly frequency modulated chirp with 5 μs duration, 8 MHz bandwidth and 8 MHz centre frequency. This can be compared with the 0.217 mm resolution obtained when using a 3-cycle non coded pulse with 8 MHz centre frequency. On average the SNR was improved by 8.1 dB when using the chirp compared with the non-coded pulse. This improvement was achieved despite the two pulses having approximately the same transmitted energy and the peak amplitude of the coded signal being 47% of the non coded signal. We conclude that simultaneous improvements in axial resolution and SNR can be made in Doppler ultrasound systems using coded excitation and pulse compression, without increasing the transmitted energy and with a reduction in pulse amplitude, when compared with conventional, non-coded processing.