An effective coded excitation scheme based on a predistorted FM signal and an optimized digital filter

This paper presents a coded excitation imaging system based on a predistorted FM excitation and a digital compression filter designed for medical ultrasonic applications, in order to preserve both axial resolution and contrast. In radars, optimal Chebyshev windows efficiently weight a nearly rectangular spectrum. For the small time-bandwidth (TB) products available in ultrasound, the rectangular spectrum approximation is not valid, which reduces the effectiveness of weighting. Additionally, the distant range sidelobes are associated with the ripples of the spectrum amplitude and, thus, cannot be removed by weighting. We show that by using a predistorted chirp with amplitude or phase shaping for amplitude ripple reduction and a correlation filter that accounts for the transducer's natural frequency weighting, output sidelobe levels of -35 to -40 dB are directly obtained. When an optimized filter is applied on receive, contrast or resolution can be traded in for range sidelobe levels down to -86 dB. The digital filter is designed to efficiently use the available bandwidth and at the same time to be insensitive to the transducer's impulse response. For evaluation of the method, simulations were performed with the program Field II.

A commercial scanner (B-K Medical 3535) was modified and interfaced to an arbitrary function generator along with an RF power amplifier (Ritec). Hydrophone measurements in water were done to establish excitation voltage and corresponding intensity levels (I-sptp and I-spta) well below the safety limits. In vivo images of the liver from two healthy volunteers show apparent increased depth of penetration of about 4 cm at 4 MHz, for codes of length 25 mu s. Images taken from a non-attenuating wire phantom show that the -20 dB axial resolution for the coded scheme is as good as with pulse excitation (about 1.5 lambda), depending on the filter design criteria. The axial sidelobes are below -40 dB, which is the noise level of the measuring imaging system. The proposed excitation/compression scheme shows good overall performance and stability to the frequency shift due to attenuation. It increases the penetration depth, and still yields a high resolution and low adjustable sidelobes.