超音波影像常用來作為初步診斷肝臟疾病，也用來輔助肝穿刺取樣以及經皮腫瘤消融術(Percutaneous tumor ablation)。但超音波影像中的雜訊可能會造成診斷者誤判病灶症狀與位置，也可能使穿刺取樣到非病變組織，因此發展融合電腦斷層或核磁共振影像與超音波影像的超音波診斷訓練器，協助醫護人員強化超音波影像的判讀能力是很重要的，而超音波影像空間定位是開發超音波診斷訓練器所需的基本技術。 本研究主要是推導超音波影像座標系對應到空間座標系的轉換關係，也就是超音波影像的方位校正(Ultrasound calibration)。研究使用一方位校正器和一定位裝置來擷取特徵點及其座標，方位校正器是一具有多組N形線的壓克力樣板。先由超音波掃描方位校正器的N形線，並由定位裝置偵測方位校正器及超音波探頭的方位，再由超音波影像及校正器方位座標分別計算出N形線掃描點(特徵點)的影像座標及空間座標。之後以Horn's方法利用此兩組座標計算得到超音波影像座標系轉換到定位裝置座標系的轉換矩陣。研究使用山型樣板作準確度實驗，實驗結果顯示超音波方位校正的準確度為1.10mm、重複性為0.99mm，此一結果可滿足臨床醫療應用。 Ultrasound scanning is usually used as the primary diagnosis of liver diseases and guidance for liver biopsy and percutaneous tumor ablation. However, ultrasound image noises may cause the examiner misdiagnoses the symptoms and locations of lesions and even extracts non-diseased tissue during liver biopsy. Thus, it is recommended to develop a ultrasound training system with the fusion of ultrasound images and CT images, which enables medical care personnel to enhance their diagnosis ability. One of the key techniques to develop the training system is the calibration of ultrasound images. In this study, the transformation matrix between the frames of ultrasound image and the probe is derived. Two devices, one is an optic tracker and the other is calibration box, are used for the calibration. The tracker is used for tracking instruments and devices while the calibration box with a set of N-shaped nylon wires is used for gathering feature points. First, the N-shaped wires are scanned and the positions of the calibration box and ultrasound scanner are measured by the optic tracker. The image and spatial coordinates of the feature points of scanned images are calculated according to the ultrasound images and the positions of the calibration box respectively. Using the Horn’s method, the two set of coordinate data are inputted to determine the transformation matrix between the ultrasound image frame and the tracker frame. In the experiments, a mountain-shaped plate is used for verify the precision and accuracy of image calibration. The experimental results shown that the accuracy was 1.10mm and the repeatability was 0.99mm.
