Optimal transmit phasing for harmonic-background suppression with bipolar square-wave pulser

Optimal transmit phasing has been proposed to increase contrast-to-tissue ratio (CTR) by relatively phasing the tissue and leakage harmonic components to cancel each other out for tissue-background suppression in harmonic imaging. Since most clinical systems are only equipped with a bipolar square-wave pulser, effective procedures for binary conversion of continuous transmit signals become essential in optimal transmit phasing to generate arbitrarily-phased bipolar waveforms. In this study, sigma-delta modulation is combined with code tuning to achieve this goal. Measurements of echoes from a wire phantom were performed to investigate the difference in harmonic suppression between the original transmit waveform and its bipolar counterpart. In-vivo cardiac imaging of a rabbit model was also established in the clinical ultrasound system to test the efficacy of a bipolar waveform in optimal transmit phasing. Our results indicate that although the harmonic magnitude becomes abrupt with the transmit phasing when a bipolar waveform is utilized in optimal transmit phasing, effective harmonic suppression is still achievable in the tissue background. Based on in-vivo images of seven consecutive cardiac cycles, a bipolar transmit waveform with the optimal suppression phase generally improves the CTR by 4.9 dB (p < 10(-8)). It should be noted, however, that the abrupt harmonic magnitude with transmit phasing could pose difficulties in the selection of the optimal suppression phase and thus limit the clinical applicability of optimal transmit phasing.