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

The subharmonic emission from insonified contrast microbubbles was used to create a new imaging modality called Subharmonic Imaging. The subharmonic response of contrast microbubbles to ultrasound pulses was first investigated for determining adequate acoustic transmit parameters. Subharmonic A-lines and gray scale images were then obtained using a laboratory pulse-echo system in vitro and a modified ultrasound scanner in vivo. Excellent suppression of all backscattered signals other than from contrast microbubbles was achieved for subharmonic A-lines in vitro while further optimization is required for in vivo gray scale subharmonic images.

creased significantly by reducing the number of transmit The frame rate in medical ultrasound imaging may be infirings per image frame. Cooley et el. [l] and Lockwood et al. [Z] have described synthetic aperture imaging sysa small number of transmit elements fired in succession.tems where each frame is imaged using data obtained from These "synthetic transmit aperture" systems have potential for very high frame rates, but they a h suffer from low SNR. In this paper we present a method for increasing the SNR of such systems by using spatially-encoded transmits. The transmitted power is increased by having multiple active transmitters in each firing. The active transmitters are encoded in a spatial code which allows the received data to he subsequently sorted by each transmitter for synthetic aperture beamforming. We present the spatial coding and decoding theory and show experimental results to demonstrate its application.

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