New demodulation method for efficient phase-rotation-based beamforming

In this paper, we present a new demodulation method to reduce hardware complexity in phase-rotation-based beamforming. Due to its low sensitivity to phase delay errors, quadrature demodulation, which consists of mixing and lowpass filtering, is commonly used in ultrasound machines. However, because it requires two lowpass filters for each channel to remove harmonics after mixing, the direct use of quadrature demodulation is computationally expensive. To alleviate the high computational requirement in quadrature demodulation, we have developed a two-stage demodulation technique in which dynamic receive focusing is performed on the mixed signal instead of the complex baseband signal. Harmonics then are suppressed by using only two lowpass filters. When the number of channels is 32, the proposed two-stage demodulation reduces the necessary number of multiplications and additions for phase-rotation beamforming by 82.7% and 88.2%, respectively, compared to using quadrature demodulation. We have found from simulation and phantom studies that the proposed method does not incur any significant degradation in image quality in terms of axial and lateral resolution. These preliminary results indicate that the proposed two-stage demodulation method could contribute to significantly reducing the hardware complexity in phase-rotation-based beamforming while providing comparable image quality.