Dual-frequency tissue harmonic suppression using phase-coded pulse sequence: Proof of concept using a phantom

Abstract

BackgroundThe presence of tissue harmonic generation during acoustic propagation is one major limitation in nonlinear detection of microbubble contrast agents. However, conventional solutions for tissue harmonic suppression are not applicable in dual-frequency (DF) harmonic imaging. In DF harmonic imaging, the second harmonic signal at second harmonic (2f0) frequency and the inter-modulation harmonic signal at fundamental (f0) frequency are simultaneously generated for imaging and both need to be suppressed to improve contrast-to-tissue ratio (CTR). In this study, a novel phase-coded pulse sequence is developed to accomplish DF tissue harmonic suppression. MethodsPhase-coded pulse sequence utilizes multiple firings with equidistant transmit phase for harmonic cancellation in the sum of respective echoes. For the f0 transmit component, the transmit phase comes from the equidistant set of {−2π/3, 0, 2π/3} to suppress the second harmonic signal at 2f0 frequency. Moreover, in order to provide the inter-modulation harmonic suppression at f0 frequency, the 2f0 transmit phase has to be particularly manipulated for the corresponding f0 transmit phase. ResultsThe proposed three-pulse sequence can remove not only the second-order harmonic signal but also other higher-order counterparts at both f0 and 2f0 frequencies. Measurements were performed at f0 equal to 2.25MHz and using hydrophone in water and contrast agents in tissue phantom. Experimental results indicate that the sequence reduces the tissue harmonic magnitude by about 20dB along the entire axial depths and the corresponding CTR improves at both frequencies. ConclusionIn DF harmonic imaging, the proposed phase-coded sequence can effectively remove the tissue harmonic background at both f0 and 2f0 frequencies for improvement of contrast detection.