Abstract
A new digital beam-forming technique is proposed for the reflective-mode ultrasound imaging. This technique is based on the holographic beam-forming using uncorrelated transmitting pulses, and it permits high speed data acquisition with simple hardware.
In the proposed method, a carrier signal is modulated with uncorrelated pulse sequences, and ultrasound pulses emitted from transmitters construct a wavefront of non-directional power propagation. Beam-forming on the transmitting and receiving stages can be accomplished numerically using delay-and-sum operation and correlation.
Point spread function (P.S.F.) of the imaging system is derived as a function of the range and azimuth differences. From the analysis of the P.S.F., it is shown that both the range and azimuth resolution limit is sufficiently small when the waveforms are uncorrelated to each other and the sum of their auto-correlation functions has a single sharp peak, even if the band-width of each waveform is narrow.
We consider the utilization of Walsh functions as the modulating sequences. An experimental system utilizing a 50kHz airborne sensor array is constructed. This system has a linear array which consists of 16 transmitters and a single receiver. The experimental results indicate the feasibility of the method. Computer simulation shows that the additional receivers improve the P.S.F. of the system.
