In-vivo and in-vitro verification of optimal transmit phasing for harmonic background suppression with bipolar square wave pulser

Ultrasonic harmonic imaging has been routinely used to improve the detection of contrast microbubbles, but the contrast-to-tissue ratio (CTR) is generally limited by tissue harmonics and leakage harmonics in the tissue background. We have previously proposed the method of optimal transmit phasing to increase the CTR by relatively phasing these two harmonic components to cancel out each other for tissue background suppression. Nevertheless, since most clinical systems are only equipped with bipolar square wave pulser, effective procedures for binary conversion of continuous transmit signal become essential to generate arbitrarily phased bipolar waveforms. In this study, the sigma-delta modulation is combined with code tuning to achieve this goal. Our results indicate that, though the harmonic magnitude becomes abrupt with the transmit phasing when the bipolar waveform is utilized in optimal transmit phasing, effective harmonic suppression is still achievable in the tissue background. For in-vivo imaging, the bipolar transmit waveform with the optimal suppression phase improves the CTR by about 5 dB. It should be noted that, however, 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.