Ultrasound harmonic imaging using nonlinear chirp for cardiac imaging

Coded excitation techniques have been used to improve a signal-to-noiseratio (SNR) in tissue harmonic imaging. However, a poor separation betweenfundamental and target harmonic components causes the second harmonic signals tohave a high level of range sidelobes after compression. In terms of theseparation performance, pulse inversion (PI) is the best method and thusprovides the lowest level of range sidelobes compared to bandpass filtering(BPF) and power modulation. However, the PI requires two transmit steps, thusinevitably decreasing frame rate. This paper proposes a new coded excitationtechnique using a nonlinear chirp for tissue harmonic imaging, particularlycardiac imaging. Since the spectral overlap between a transmitted nonlinearchirp 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 sidelobelevels comparable to those produced by PI without a loss of frame rate. In theexperiments, the highest level of the sidelobes of the nonlinear chirp was 40.59dB, which was comparable to the value from PI, i.e., 39.84 dB.