醫用超音波系統用於流速影像上之都卜勒向量(Vector Doppler)估計會受到血管角度的影響及最大流速的限制，因此本文使用較高幀率的平面波成像系統，並嘗試將Crossed-Beam Vector Doppler (VD)與斑點追蹤技術(Speckle Tracking, ST)做結合，來改善血管角度對流速估計的影響。VD為將不同發射旋轉角收到的回音信號，運用自相關(Autocorrelation)與最小方差法(Least-Squares)計算出流速向量。並利用VD所計算出的軸向流速(Vz-VD)與一維斑點追蹤技術(1-D ST)做結合，計算出更準確的橫向流速(Vx-ST)。由於不做聚焦的單一平面波成像(Single Plane Wave, SPW)能提高幀率，但影像品質與訊雜比均較差，因此藉由不同角度的平面波進行同調性平面波複合成像(Coherent Plane Wave Compound, CPWC)以改善上述缺點，達到準確的斑點追蹤效果。
本研究模擬及血流仿體實驗結果顯示，本文所使用的方法(VD+1-D ST)與VD相比，能夠在血管角度45度以下，對流速估計的標準差STD及誤差BIAS皆能有所改善，能夠得到較為穩定以及準確的流速估計，且在血管角度0度的橫向流速估計獲得大幅度改善。

Use vector Doppler to estimate flow velocity is limited by vessel angle and maximum flow velocity in medical ultrasound system. In this study, we tried to overcome the influence of vessel angle by combining crossed-beam vector Doppler (VD) with speckle tracking (ST) in high frame rate plane wave imaging. In VD, we used radio frequency signals from different steered transmission angles to estimate autocorrelation separately, and used least-squares to obtain flow vector. Then we combined the axial velocity from VD (Vz-VD) with one-dimension speckle tracking (1-D ST) to get more accurate lateral velocity (Vx-ST). Although single plane wave excitation (SPW) could get high frame rate, its image quality and signal-to-noise ratio became worse. In order to improve estimation, we used coherent plane wave compound imaging instead of single plane wave imaging to do speckle tracking.
The results of both simulation and experiment indicate that STD and BIAS performance of VD+ST is much better than VD below 45-degree vessel. Therefore, VD+ST has more stable and accurate flow velocity estimation, especially in lateral vessel.

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