Potential of coded excitation in medical ultrasound imaging

Improvement in signal-to-noise ratio (SNR) and/or penetration depth can be achieved in medical ultrasound by using long coded waveform, in a similar m anner as in radars or sonars. However, the time-bandwidth product (TB) impr ovement, and thereby SNR improvement is considerably lower in medical ultra sound, due to the lower available bandwidth. There is still space for about 20 dB improvement in the SNR, which will yield a penetration depth up to 2 0 cm at 5 MHz [M. O'Donnell, IEEE Trans. Ultrason. Ferroelectr. Freq. Contr ., 39(3) (1992) 341]. The limited TB additionally yields unacceptably high range sidelobes. However, the frequency weighting from the ultrasonic trans ducer's bandwidth, although suboptimal, can be beneficial in sidelobe reduc tion. The purpose of this study is an experimental evaluation of the above considerations in a coded excitation ultrasound system. A coded excitation system based on a modified commercial scanner is presented. A predistorted FM signal is proposed in order to keep the resulting range sidelobes at acc eptably low levels. The effect of the transducer is taken into account in t he design of the compression filter. Intensity levels have been considered and simulations on the expected improvement in SNR are also presented. Imag es of a wire phantom and clinical images have been taken with the coded sys tem. The images show a significant improvement in penetration depth and the y preserve both axial resolution and contrast. (C) 2000 Elsevier Science B. V. All rights reserved.