Phase-dependent dual-frequency contrast imaging

Subharmonic imaging technique has been shown to provide a higher contrast-to-tissue ratio (CTR) at the cost of relatively low signal intensity from ultrasound contrast agents (UCAs). In this study, we propose a method of dual-frequency excitation to enhance the power of subharmonic component. Dual-frequency excitation pulse is an amplitude-modulated wave which consists of two sinusoids with frequencies of f₁ and f₂. The f₂ at twice of UCAS resonance frequency is adopted to generate subharmonic component, and f1 is included to enhance the high-order nonlinearity of UCAs. In our previous study, the resulting envelope component (f₁-f₂) of dual-frequency excitation can serve as a driving force to excite the nonlinear response of UCAs. The second (f₁-f₂) and third-order (f₂-(f₁-f₂)) nonlinear components related to envelope component would coincide with f₂/2 if a proper set of f₁ and f₂ are selected. We further optimize the subharmonic generation by tuning the phase between second and third-order component. To evaluate the performance of proposed method on CTRs improvement, a speckle-generating flow phantom was constructed in in-vitro experiments. The results show that the CTRs of dual-frequency insonation change periodically with the phase, leading to a difference up to 10 dB between the maximal and minimal CTR. All the phase cases of dual-frequency insonation have higher CTRs than those of traditional subharmonic imaging method. The results show that the phase optimization is an important factor in subharmonic imaging with dual-frequency excitation. The echo produced from the envelope component seems to be specific for UCAs, so the proposed method has the potential in improving SNR and CTR of traditional subharmonic imaging.