Performance evaluation of nonlinear frequency modulated signals in ultrasound harmonic imaging

In ultrasound harmonic imaging with linear frequency modulated (LFM) excitation, the sidelobes level in the compressed harmonic signal can be reduced by applying a windowing function. Windowing on the transmitting signal causes reduced penetration depth, whilst windowing on the receiving side results in reduced signal-to-noise ratio (SNR) gain and axial resolution. To optimize the transmitting signal energy and the SNR gain with reduced sidelobes level in the compressed harmonic signal, the use of nonlinear frequency modulated (NLFM) signals are proposed. The NLFM signal and associated second harmonic matched filter are designed using an analytical approach to minimise correlation errors. In all simulations and experiments, the NLFM signal performance is compared with the reference LFM signal of similar sweeping bandwidth and duration. The results indicate at least a 15 dB reduction in the peak sidelobes level of the NFLM compressed second harmonic signal with comparable axial mainlobe width when compared with the LFM compressed harmonic signal.