IntroductionThis 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 availahle in ultrasound, the rectangular spectrum approximation is ditionally, the distant range sidelobes are associated with the not valid, which reduces the effectiveness of weighting. Adripples 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 he insensitive to the transducer's impulse response. For evaluation of the method, simulations were performed with the program Field 11.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 and I,t,,,.) well below the safety limits. In vivo images of the liver from two healthy volunteers show apparent increased depth of penetration of ahout 4 cm at 4 MHz, for codes of length 25 p . 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 l.%), depending on the filter design criteria. The axial sidelobes are below -40 dB, which is the noise tationkompression scheme shows good overall performance level of the measuring imaging system. The proposed exciand stability to the frequency shift due to attenuation. It increases the penetration depth, and still yields a high resolution and low adjustable sidelobes.Coded excitation in medical ultrasound can be used for improving the signal-to-noise ratio (SNR) andlor the penetration depth, as long as both sidelobe level and energy are kept acceptably low for ultrasound imaging. An improvement in SNR of IO dB or more is expected [I], [2], depending on the code parameters, and the available bandwidth. As we reported previously [2], clinical images of the liver from two healthy volunteers demonstrated an improvement in penetration depth of at least 4 to 5 cm at 4 MHz with temporal sidelobe levels lower than -35 dB. In a high contrast imaging system this is not sufficient and this paper primarily focuses on an optimized design of pulse compression schemes applicable to ultrasound.Linear FM waveforms have been used as coded excitation because of their important symmetry properties and thei...
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