Harmonic quadrature demodulation for extracting the envelope of the 2nd harmonic component

摘要

An efficient method for separating the harmonic component (2f0) from the fundamental component (f0) using harmonic quadrature demodulation is presented. In the proposed method, the focused ultrasound signal is mixed with cosine and sine signal waveforms of harmonic frequency 2f0 to produce the inphase and quadrature components, respectively. The quadrature component is Hilbert-transformed and then added to the inphase component. This process cancels out both the high and low frequency components of the mixed fundamental signal and the high frequency component of the mixed harmonic signal, leaving only the envelope of the harmonic signal at the base band. This signal is then fed to a low-pass filter to remove out of band noise. In summary, this method can extract the harmonic signal after a single transmit-receive event even when there exists frequency overlap between the f0 and 2f0 components. Hence, the proposed method is superior to the pulse inversion method which requires twice as many transmit-receive cycles as well as the conventional filtering method which has a bandwidth limitation. Therefore, one can find the proposed method useful not only for tissue harmonic imaging but also for contrast agent imaging in applications where high frame rate or low motion artifact is important. The proposed method is verified by both the analytic and computer simulation studies. For a stationary target, the difference between the estimated harmonic signals by the proposed and the pulse inversion methods is within 0.1%.