超音波亮度模式(Bmode)成像中，波束成像通常藉由時間延遲的觀念估算物體的位置，由於時間延遲的方法預測不穩定，造成影像上假影的產生，所以事先估計的時間延遲影像品質並不佳，所以依據接收訊號判斷的波束成像油然而生，此種波束成像稱為可適應波束成像。
其中一種可適應波束成像稱為最小方差法，而在此種方法中至關重要為共變異矩陣的估測，精準的共變異矩陣可以提升影像品質，反之，不精準的共變異矩陣所產生的影像品質趨近於傳統的波束成像，所以許多的研究者提出不同共變異矩陣的估算方式。
在本篇比較前進平均法與前後平均法使用於共變異矩陣中，在共變異矩陣的估算中，前進平均法由於對角線的估測傾斜，此現象造成影像品質在左右側散射子的亮度低於中間散射子的亮度，與預期的結果相反，進而使用前後平均法改善此問題，在本篇模擬與實驗中皆有改善，其原因為前後平均法相較於前進平均法，其共變異矩陣具有對稱性質。

In adaptive ultrasound beamforming, one of well-known beamforming is minimum variance beamformer. The beamformer in simulation show high performance than traditional beamformer. The crucial purpose for this beamformer is to estimate covariance matrix. The more accurate covariance is, the more high quality image can get. However, in actual environment, MV cannot reaches the simulation performance. Lower SNR transmission result in even worst performance. For this issue, many researcher try to solve this problem and provide new idea to estimate covariance matrix.
In this paper, we use MV beamfomer using single plane wave transmission and discover that wire close to image boundary disappear in the image. The reason why it is disappearance is the estimation error in covariance matrix. We will explain estimation error for covariance matrix and use forward backward averaging to improve this proplem.

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