| Summary: | 碩士 === 國立臺灣科技大學 === 電機工程系 === 107 === Compared to conventional Delay-and-Sum (DAS) beamforming, Delay-Multiply-and-Sum (DMAS) imaging uses multiplicative coupling of channel pairs for spatial coherence of receiving aperture to improve image resolution and contrast. However, present DMAS imaging is based on the radio-frequency (RF) channel signals (RF-DMAS) and thus requires large oversampling to avoid aliasing and switching of band-pass filtering to isolate the corresponding spectral components for imaging.
Baseband DMAS (BB-DMAS) beamforming in this study is based on the demodulated channel signals to provide similar results but with simplified signal processing. The multiplicative coupling in BB-DMAS always renders baseband signal and thus the need for oversampling is eliminated. The BB-DMAS beamforming scales the magnitude of time-delayed channel signal by p-th root while maintaining the phase. After channel sum, the output dimensionality is restored by p-th power. The BB-DMAS can use any rational p value to provide flexible image quality. Besides, BB-DMAS is applicable to both fixed transmit focused imaging and synthetic focused imaging such as plane wave compounding.
Our results show that the image characteristics between BB-DMAS and RF-DMAS are similar. BB-DMAS beamforming can be seen as the adaptively weighted DAS beamforming with the phase coherence factor (PCF) among receive channels. The suppression of lateral side lobe level, grating lobe level and uncorrelated random noises gradually increases with the p value in BB-DMAS beamforming. The image contrast decreases from -24.8 dB in DAS to -34.3 dB, -43.0 dB and -51.4 dB in BB-DMAS, respectively with p value of 1.5, 2.0 and 2.5.
The DMAS beamforming would exhibit bright region at the focal depth due to higher signal coherence in the transmit focal zone and thus compromise the image uniformity. Therefore, this study attempts to overcome this problem by using unfocused plane wave imaging. For plane-wave (PW) imaging, multi-angle coherent compounding relies on two-dimensional (2D) summation of echo matrix in both dimensions of transmit angle and receive channel to produce the image output. Previously, Delay-Multiply-and-Sum (DMAS) beamforming has been combined with multi-angle PW imaging but only in either one dimension. In this study, a novel 2D-DMAS operation is proposed for multi-angle PW imaging to extract the 2D spatial coherence of echo matrix for further improvement of image quality. Our results show that the lateral width (LW) decreases from 0.53 mm in DAS to 0.42 mm, 0.36 mm, 0.31 mm and 0.28 mm in 2D-DMAS, respectively with p value of 1.5, 2.0, 2.5 and 3.0. Moreover, 2D-DMAS consistently provides the lowest LW when compared with the aforementioned reference methods.
However, DMAS beamforming exhibits a lower CNR caused by obvious granular speckle pattern and thus noticeably elevate the speckle variation. Future work may focus on the combination of DMAS beamforming with the speckle suppression techniques to find the best trade-off between image CR and CNR.
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