Extraction of an Overlapped Second Harmonic Chirp Component using the Fractional Fourier Transform

In ultrasound harmonic imaging with chirp coded excitation, the axial resolution can be improved by increasing the excitation signal bandwidth. However, increasing the bandwidth will cause overlapping between the received nonlinear second harmonic chirp component (SHCC) and the fundamental component. For the spectrally overlapping harmonics, signal decoding using the second harmonic matched filter (SHMF) typically produces higher range sidelobes level (RSLL), which reduces the image contrast. A multi-pulse detection scheme such as pulse inversion can be used to extract the overlapped SHCC; however it is susceptible to motion artifacts and reduced system frame-rate. In this study, the fractional Fourier transform (FrFT) is proposed with chirp coded excitation for the extraction of the overlapped SHCC. The experimental results indicate at least a 13 dB improvement in the RSLL of the FrFT filtered compressed SHCC when compared with the unfiltered compressed SHCC.