Identification of pulse echo impulse responses for multisource transmission

In this paper a method for acquiring data from several simultaneously transmitting elements in synthetic transmit aperture (STA) ultrasound imaging is proposed. Several transmitters are excited simultaneously using pseudo-random sequences. The received signal at a given time point and receiver is a mixture of the information corresponding to several transmitters. There is, thus, no direct way of determining which information corresponds to which transmitter, preventing proper focusing. In this paper we decode the received signal by estimating the pulse echo impulse responses between every transmitter and receiver pair, using a least squares estimator. The decoding is done instantaneously, making information from several transmitters available after only one transmission. This limits the influence of motion artifacts both in the decoding step and when the STA focusing scheme is applied. The method is evaluated using the simulation tool Field II. Three point spread functions are simulated where axial movement of 1 m/s is present. The axial resolution for the moving scatterer is 0.249 mm (-3dB) and 0.291 mm (-6dB), which is compared to a standard STA transmission scheme with sequential excitation of the transmitters using a chirp excitation. The axial resolution was in this case 0.260 mm (-3dB) and 0.611 mm (-6dB). Also a blood vessel is simulated with an angle of 45° to the acoustic axis with a peak flow of 1 m/s. The velocity is estimated with a mean bias of 2.57% and a mean standard deviation of 0.505% relative to the peak velocity of the flow.