Ultrasonic imaging with contrast agents combined pulse inversion and generalized S-transform

In this paper, a new method is presented for contrast improvement by combining the pulse inversion technique with generalized S-transform (GST). Two types of GST are studied including hyperbolic S-transform and S-transform with complex window, which modulate the shape and phase of the analyzing function, respectively. Time-frequency filters are designed based on generalized s window to separate the signals form ultrasound contrast microbubbles and surrounding tissues. Filter coefficients can be identified by selecting appropriate back-taper parameter, forward-taper parameter, positive curvature parameter and phase modulation parameter. Radio frequency (RF) data were acquired from a modified digital diagnostic ultrasound platform which transmits two identical pulses with opposite polarity. The efficacy of the proposed method was validated by simulations, in vitro and in vivo experiments. Results in vitro and vivo demonstrated that the CTR was improved by up to 17 dB compared to that obtained using pulse-inversion-based second-harmonic imaging (PIHI). The achieved result was compared with the pulse inversion based wavelet transform imaging (PIWI) on the same set of experimental data. The effect of filtering parameters on the CTR improvement is considered.