Resolution and penetration are primary criteria for clinical image quality. Conventionally, high bandwidth for resolution was achieved with a short pulse, which results in a tradeoff between resolution and penetration. Coded excitation extends the bounds of this tradeoff by increasing signal-to-noise ratio (SNR) through appropriate coding on transmit and decoding on receive. Although used for about 50 years in radar, coded excitation was successfully introduced into commercial ultrasound scanners only within the last 5 years. This delay is at least partly due to practical implementation issues particular to diagnostic ultrasound, which are the focus of this paper. After reviewing the basics of biphase and chirp coding, we present simulation results to quantify tradeoffs between penetration and resolution under frequency-dependent attenuation, dynamic focusing, and nonlinear propagation. Next we compare chirp and Golay code performance with respect to image quality and system requirements, then we show clinical images that illustrate the current applications of coded excitation in B-mode, harmonic, and flow imaging.0885-3010/$20.00 c 2005 IEEE chiao and hao: coded excitation and clinical ultrasound systems

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Color flow imaging has suffered from the trade off between resolution and sensitivity ever since it was introduced. The requirement of blood flow sensitivity results in application of narrow bandwidth, long time duration pulses. The problem associated with this type of pulse is the severely degraded spatial resolution, which causes wall overwriting, blobby flow aesthetics, loss of detail flow hemodynamics, etc. Coded Excitation has been applied in B mode imaging to solve the similar tradeoff between SNR and resolution, but has not been so successful in color flow imaging. This is due to the fact that current color flow pulses are already at the regulatory limit and the focus of delivering more power for more sensitivity with coded excitation in color flow imaging. High resolution color flow imaging is an effort to introduce Chirp Coded-Excitation to achieve B-mode-like spatial resolution in color flow without sacrificing sensitivity. Wide bandwidth pulse with time bandwidth product greater than 1 is employed to bring in sensitivity/penetration and resolution. As a result, better spatial definitions, less wall overwriting, and detailed flow hemodynamics are achieved with improved flow sensitivity. Corresponding practical application issues are discussed in this paper.