Ultrasound harmonic imaging using nonlinear chirp for cardiac imaging

Coded excitation techniques have been used to improve a signal-to-noise ratio (SNR) in tissue harmonic imaging. However, a poor separation between fundamental and target harmonic components causes the second harmonic signals to have a high level of range sidelobes after compression. In terms of the separation performance, pulse inversion (PI) is the best method and thus provides the lowest level of range sidelobes compared to bandpass filtering (BPF) and power modulation. However, the PI requires two transmit steps, thus inevitably decreasing frame rate. This paper proposes a new coded excitation technique using a nonlinear chirp for tissue harmonic imaging, particularly cardiac imaging. Since the spectral overlap between a transmitted nonlinear chirp and its second harmonic counterpart is less than the case of linear chirp, a simple BPF can be used for the separation so as to provide the range sidelobe levels comparable to those produced by PI without a loss of frame rate. In the experiments, the highest level of the sidelobes of the nonlinear chirp was -40.59 dB, which was comparable to the value from PI, i.e., -39.84 dB.