Coded Spectral Doppler Imaging: From Simulation to Real-Time Processing

Transmission of coded pulses and matched receive filtering can improve the ultrasound imaging penetration depth while preserving the axial resolution. This paper shows that the pulse compression technique may be integrated in a low-cost scanner to be profitably used also in spectral Doppler investigations. By operating on beamformed, demodulated, and down-sampled data in the frequency domain, a single digital signal processor is proved sufficient to perform both pulse compression and multigate spectral Doppler algorithms in real time. Simulations, phantom, and in vivo experiments demonstrate that the transmission of (2.5 or 5 μs long) linear frequency-modulated chirps with bandwidths over the range 1.6-5.4 MHz, rather than of corresponding sine-burst pulses, provides signal-to-noise ratio (SNR) improvements very close to theory. Even in the presence of selective tissue attenuation, SNR gains up to 11 and 13.3 dB have been obtained for the short and the longer chirp, respectively. This may be important in clinical Doppler applications where the needed penetration depth is not achieved with sufficient SNR unless very long bursts are transmitted.