Coded excitation for diagnostic ultrasound: a system developer's perspective

Abstract

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

Ultrasound elasticity imaging using Golay code

“…. .,N) of length N is a Golay code if, and only if, it satisfies [9]: (1) which is known as the complementarity condition. Fig.1 shows the decoding process of Golay codes of length 4 using matched filter.…”

“…Given a Golay pair A, B, another Golay pair of twice the length can be generated by Golay codes cannot be transmitted directly, and they need be modulated by a base pulse sequence in order to achieve better spectral control and pulse shaping as well as greater robustness to motion for multitransmission code [9]. The encoding process of the Golay-code-modulated base sequence can be described as a convolution of base pulse sequence with an oversampled Golay code sequence:…”

The use of phase codes in ultrasound imaging: SNR gain and bandwidth requirements

“…Some of the challenges for implementation of coded excitation in ultrasound included addressing the distortion from frequency-dependent attenuation, nonlinear propagation distortion, and dynamic receive focusing. These issues have been studied in detail [3][4][5][6][7].…”

“…1, a regular (uncoded) 4-chip pulse is compared with a 4-chip Barker code, before and after pulse compression using a matched filter [5]. A single-chip, sometimes referred to in the 0003-682X/$ -see front matter Ó 2008 Elsevier Ltd. All rights reserved.…”

“…However the matched-filter output for the coded Barker pulse has sidelobes of À12 dB compared to À2.5 dB for the uncoded pulse. The matched filter is designed to optimize the signal-to-noise ratio of a signal (SNR out,max ) in the presence of white Gaussian noise, irrespective of coding [5,6,9]. The effect of coding is a decrease in the range sidelobe levels in order to restore the axial resolution.…”