Multiple-frequency tissue harmonic background suppression using phase-coded pulse sequence

The presence of tissue harmonic generation during acoustic propagation is one major limitation in nonlinear detection of microbubble contrast agents. However, conventional solutions for suppression of tissue harmonic background are not directly applicable when dual-frequency (DF) waveform is utilize for transmit. In DF harmonic imaging, the second harmonic signal at second harmonic frequency (2f₀) and the inter-modulation harmonic signal at fundamental frequency (f₀) are simultaneously produced and both need to be suppressed for imaging of contrast agents. In this study, novel phase-coded pulse sequences are developed to accomplish DF harmonic suppression. The proposed three-pulse sequence can effectively remove the second-order harmonic signal by producing equidistant phases for cancellation in the sum of different firings. Therefore, the tissue background in DF harmonic imaging can be mostly suppressed. Experiments have been performed to validate the efficacy of the proposed sequence. It is shown that the sequence effectively suppresses the tissue harmonic signal at both f₀ and 2f₀ frequencies to improve CTR.