Conventional techniques for separating completely the second harmonic signal from the fundamental signal generally involve two or more firings. Not only do these methods cause a reduction in frame rate, but they may perform inadequately for contrast harmonic imaging since the fundamental signals from the contrast agents cannot be eliminated sufficiently. In this work, we present a more efficient method for harmonic extraction that requires only a single transmit and receive event along each scan line.
In the proposed method, the focused rf ultrasound signal is multiplied by a cosine signal with a center frequency of 2f0 so that the harmonic signal should be frequency-shifted to baseband and to +/-4f0 while the fundamental signal at f0 (-f0) is shifted to frequency bands centered at 3f0 (+f0) and -f0 (-3f0). The modulated signal is then subtracted by the original rf signal to cancel out the fundamental signal components centered at +/- f0 in both the original and modulated signals. The resulting signal has the harmonic signal at baseband and around 2f0. Hence, by low-pass filtering, the baseband harmonic signal can be obtained without being contaminated by the fundamental signal even when there exists frequency overlap between fundamental and harmonic signals in the original signal.
The proposed method is verified theoretically and experimentally. The computer simulation results based on modified Rayleigh-Plesset equation shows that the error between the harmonic signals by the proposed method and the pulse inversion method is as small as -37.39dB for a bubble target.