Improved Second Harmonic Imaging of Ultrasound Contrast Agents Based on Total Least-Squares Adaptive Filtering

In ultrasound contrast imaging, the second harmonic (SH) of contrast agents has a higher resolution than the subharmonic and a higher energy than the ultraharmonic, which is helpful for the visualization of blood flow. However, the contrast-agent SH directly separated from the radio frequency signals may contain SH components generated by tissues. In this study, total least-squares (TLS) adaptive filtering is proposed to improve the second harmonic imaging (SHI) of ultrasound contrast agents. First, the pulse inversion based SH signals before and after injecting contrast agents are the reference signal and input of the TLS model, respectively. Then, the optimal model coefficients are determined by minimizing the perturbations in the input and output signals of the model. Finally, the contrast-agent SH is finally obtained by subtracting the optimal output from the input. In the simulation experiments, simulations before and after injecting contrast agents are used to evaluate the performance of the proposed method. Four random perturbations with different amplitude are added to the simulations after injecting contrast agents, respectively. The TLS-based contrast-to-tissue ratio (CTR) is the best when the tissues are static. For the perturbations of 0.5, 1.0 and 1.5 mm, the TLS-based CTRs are 13.1, 12.3 and 10.1 dB higher than the method combining pulse-inversion with wavelet transform imaging, respectively. In conclusion, the proposed TLS method suppresses more tissue components, and thus provides the improved performance in contrast-agent SHI.