Complex pulsing schemes for high frame rate imaging

High frame rate ultrasound imaging can be achieved by simultaneous transmission of multiple focused beams along different directions. However, image quality degrades by the interference among beams. An alternative approach is to transmit spherical waves of a basic short pulse with frequency coding and a constant transmit delay from channel to channel. In this way, transmit diversity is provided on a time and channel basis rather than on a beam direction basis. The non-focused transmitted acoustic waves carry spatial information from the entire imaging region. At a given imaging point, all pulses will add up to a pulse train. The acoustically generated high time-bandwidth (TB) product waveforms can be compressed by using a filter bank of matched filters one for every beam direction. Matched filtering compresses the pulse train to a single pulse at the scatterer position plus a number of spike axial sidelobes. Frequency and phase modulation of the transmitting pulses allows control and elimination of the ambiguous spikes. QLFM pulse trains are found to give the best performance. Simulation results and images are presented showing the feasibility of the method. The excitation consists of 32 pulses with linear frequency modulation along the transducer elements, that cover the 70% fractional bandwidth of the 7 MHz transducer. The resulted images (after beamforming and matched filtering) show an axial resolution at the same order as in conventional pulse excitation and axial sidelobes down to -45 dB. With the proposed imaging strategy of pulse train excitation, a whole image can be formed with only two emissions, making it possible to obtain high quality images at a frame rate of 20 to 25 times higher than that of conventional phased array imaging