Research On Coded Excitation Method In Ultrasonic Imaging

Resolution and penetration are primary criteria for ultrasonic image quality. Conventionally, penetration was often achieved by lowering the image resolution and resolution was achieved by reducing penetration depth, which results in a tradeoff between resolution and penetration. In recent years, the coded excitation technology is applied in ultrasonic imaging system. Coded excitation extends the bounds of this tradeoff by increasing signal-to-noise ratio (SNR) or improving ultrasound penetration corresponding and retaining the resolution simultaneously. Coded excitation is not applied in the ultrasonic imaging system extensive because of the small time bandwidth product (TB) of ultrasonic imaging,more complex system than traditional pulse-echo system and other reasons. Choosing good coding mode and pulse compression method to further improve the quality of the ultrasonic image is still a hot spot of ultrasonic imaging system research.In this paper, the coded excitation system to improve the quality of ultrasonic images is researched from theory. Linear FM Chirp, Baker and complementary Golay signal are used as transmitting signal, and match filter is applied to compress pulse in the receiver. Compared with ultrasonic pluse-echo imaging system, coded excitation system has the potential of higher SNR and deeper penetration depth through the simulation.First, the basic principle of coded excitation is introduced, and the improvement of SNR of ultrasonic image with coded excitation is analyzed, then coded excitation system evaluation index is given. Next, linear FM Chirp signal, Barker and Golay code and there characteristics are described in details. Then, Chirp signal, Barker and Golay coded excitation system are simulated respectively. And compare the echo performance of three code excitation, the results showed that compared with the traditional pulse excitation, the three coding excitation all can improve the SNR with less loss of main lobe, but three coded excitation methods all exist defects. In addition, the influence of the ultrasound transducer bandwidth on selection of the different two-phase code period is analyzed in this paper. The simulation results indicate that two cycles bit length excitation is more suitable for bandwidth limited imaging transducers.At last, Golay-Chirp coded excitation method is presented in this paper, which use Chirp signal to modulate Golay code. This signal both have the good antinoise ability of Chirp signal and excellent autocorrelation, long code of Golay code. Comparison of this method with Golay excitation.The simulation results show that Golay-Chirp code excitation has good performances with higher SNR to reduce the noise influence on the result of the ultrasonic echo.