Delay-encoded transmission and image reconstruction method in synthetic transmit aperture imaging

Synthetic transmit aperture (STA) imaging systems usually have a lower SNR compared with conventional B-mode ultrasound systems because only one or a small number of elements are selected for each transmission in STA. Here we propose delay-encoded synthetic transmit aperture (DE-STA) imaging to encode all the transmission elements to increase the SNR of the pre-beamformed RF signals. The encoding scheme is similar to the Hadamard encoding. However, in each transmission of DE-STA imaging, selected transmitting elements are delayed by a half period of the ultrasound wave relative to the rest transmitting elements, rather than using a pulse inversion as in the Hadamard encoding sequence. After all the transmission events, a decoding process in the temporal frequency domain is applied to the acquired RF signals to recover the equivalent traditional STA signals with a better SNR. The proposed protocol is tested with simulated data (using Field II) and experimental data acquired with a commercial linear array imaging system (Ultrasonix RP). The results from both the simulations and the experiments demonstrate increased SNR of pre-beamformed RF signals and improved image quality in terms of peak signal-to-noise ratio (PSNR), resolution and contrast-to-noise ratio compared with traditional STA. The lateral resolution (as assessed by a wire target) of DESTA imaging is improved by 28% and the PSNR of the wire is increased by 7 dB, respectively, compared with traditional STA imaging. The proposed image reconstruction framework can also be extended to other transmission protocols.