Adaptive quadrature demodulation for ultrasound tissue harmonic imaging

Frequency dependent attenuation of ultrasound causes the center frequency of received echoes to be downshifted along depth. This is particularly severe on a harmonic component rather than a fundamental one because higher ultrasound frequency experiences higher attenuation. The different degrees of downshifting along with ultrasound frequency should be considered when quadrature demodulation (QDM) is carried out to secure the best signal-to-noise ratio (SNR). In the conventional second harmonic imaging, however, a constant attenuation coefficient is typically assumed for estimating the center frequency of the harmonic components in QDM. This assumption may not be valid because there is the attenuation variation in an imaging area. To obtain the best SNR, therefore, this paper proposes an automatic center frequency estimator based on autoregressive (AR) modeling for the ultrasound second harmonic tissue imaging. In the proposed method, the estimated center frequency of a second-harmonic component is directly used for dynamic QDM and dynamic low-pass filtering (LPF). The performance of the proposed method was evaluated by Field II simulation and in-vivo experiments. The experimental results demonstrated that the proposed method is capable of providing higher contrast resolution while reducing high-frequency noise under visual examination due to the improvement of SNR.