超音波影像機灰階逆向工程方法

Abstract

超音波診斷約從70年開始發展，到現在隨著電子業快速發展的狀況，使其技術也跟著迅速的成長。超音波成像原理系統是基於探頭發出超音波，碰到不同的組織或材料產生反射波，再由探頭接收後，轉成電訊號以產生圖像。 超音波以脈衝的方式傳遞至組織或材料中，並偵測到其反射之聲波，再將這些反射的波之強度轉換為灰階並顯示在超音波診斷儀器上。由於超音波普遍之反射波強度範圍都很廣，而且大部分所需之訊號存在於較弱的反射波上，再加上將聲波轉換成電訊號以灰階顯示的過程為非線性狀態。而本實驗想研究此聲波轉換電訊號之非線性的過程，直接從超音波影像導出該過程，利用圖像的數據來計算用以估計相對反射波強度。 本研究的實驗方法，一開始先從實驗設備中擷取欲偵測之材料的超音波影像，其中包含兩種不同衰減度（0.5 dB/cm-MHz及0.7 dB/cm-MHz）與5種相對於背景不同的分貝值（-9 dB、-6 dB、-3 dB、+3 dB及+6 dB）的圓筒，再使用自訂函數及估測雜訊參數共三種方式取得圈選區域的平均灰階數值後，進而求得影像中各圓筒之圓心座標，接著使用另一個自訂函數將每個圓筒之圓形表面中每一pixel的相對強度計算出來後，再將上述數值與使用MATLAB顯示出來的該pixel的灰階數值結合形成一個二維矩陣，最後將十組相對強度及灰階值的矩陣分成三類分析：第一種以不同衰減度但皆為相對於背景有相同dB值的圓筒來做分析、第二種為相同衰減度下，將五種圓筒的資料匯至同一張散佈圖中來做分析、第三種為將十筆資料合併，觀察灰階數值所對應的相對強度之範圍，再加以探討及分析。

Ultrasonic imaging systems produce images based on the pulse-echo principle. Ultrasound is transmitted in pulses into tissue to be imaged, and returning echoes detected. The intensity of the returning echoes is then converted to gray scale and displayed on ultrasound diagnostic equipment. Due to the large range of echo intensities encountered, and the fact that the majority of the information present is in the weaker echoes, the gray scale conversion process is highly nonlinear. A rough experimental method is described that allows one to estimate the nonlinear conversion process through a series of measurements. Being able to derive this process allows one to estimate relative echo strengths from ultrasonic images alone, and to be able to quantitatively estimate such strengths based solely on image data. The experimental method of this study, involves, at the outset, the acquisition of a series of ultrasonic images from the equipment. The images include those with two different attenuations (0.5 dB/cm-MHz and 0.7 dB/cm-MHz) and 5 different echogenicities in cylinders differing from the background (-9, -6, -3, +3, and +6 dB). After determining the average value of each cylinder with a custom function, another custom function was developed that mapped each pixel with a relative echo value taking into account attenuation for up to five cylinders under two attenuation levels. The purpose of this experiment is that to find software mechanism that we can produce a means for quantifying echo intensities from ultrasonic imaging systems.