Experimental study on the truncated long code sequence for ultrasound Doppler detection

When applying the coded transmission technique into the pulsed-echo ultrasound system one of the big problems is the limited time frame for the coding sequence to be feed into the pulse-echo duration. Generally, only very short code sequence could be modulated in each transmitted pulse especially in the medical ultrasound system. Due to the poor correlation property of such kind of short code and the nonlinear field attenuation in the ultrasound propagation, the coding result is far from satisfaction. We have presented a truncated long code scheme for the ultrasound Doppler detection [1] [2]. The time duration to be considered for the selection of the length of the code sequence is extended to be as long as the spectrum estimation window allows. Long code sequence modulation can be applied in the pulse-echo system. The long code is first truncated into many short segments and each segment is used for each pulse modulation. Then, the returned signals from the multiple segments are coherently combined for the Doppler detection. The coding effect with a long code gives a good appearance in the process with its good correlation property, which gives solid backup for the field compensation. Some water tank experiments are shown in this paper.