使用相位保護去除雜訊來提升超音波圖形輪廓偵測

涂欽鴻
According to the growth of national income, people nowadays pay much more attention to the health issues; Corresponsively the medical care gets improved as well. Today, we have advanced medical devices such as ultrasonic instrument, computed tomography, position emission tomography and magnetic resonance imaging, in which the ultrasonic instruments are now in widespread used because of low-cost, non-side effect, mobile and non-aggressive superiorities.

Ultrasonic instruments can be used as the first step medical inspection, and have become one of the popular medical instruments. Therefore,the thesis focuses on the ultrasonic images. Doctor can diagnose the patients using the organ contour by detected snake algorithm. However, the image is easily corrupy by noise when we capture the ultrasoundic signal. The thesis proposes a pre-processing system of contour detection combining with Log-Gabor filter、contrast enhancement 、histogram equalization and Canny edge algorithm to improve the quality of ultrasonic image. First, the parameters of Log-Gabor filter ,such as the minimum wavelength and the bandwith central frequency are defined to reduce noise. Then, contrast enhancement and histogram equalization are used to enhance image contrast, which makes the object boundary clearer. We apply Canny edge algorithm to compute the edge map. Finally the system can get the contour of ROI by using GVF-based snake method. The thesis demonstrates the pre-processing system increase the accuracy of the results from GVF-based snake when used in contour detection. The produced contour images important references for are the medical inspection.

現今國民所得提高，人們越來越重視健康問題，相對的也提升了醫療品質，在醫療儀器中不乏有超音波、電腦斷層掃描（Computed Tomography,CT）、正子斷層掃描（Position Emission Tomography, PET）及核磁共振（Magnetic Resonance Imaging,MRI）等等，其中又以超音波儀器最為普遍醫療儀器，因為其價格低、無副作用且行動力強，屬於非侵入性檢查，可以作為醫療行為的初步檢查，為醫療從業人員樂於使用的儀器之ㄧ，因此本論文以超音波影像作為研究的目標。使用snake algorithm來找尋身體組織的輪廓，其結果可以協助醫生做醫療上的判斷，但由於超音波影像易受到雜訊干擾，在使用snake描繪超音波圖形中物體輪廓時，容易造成不正確的結果。本論文提出輪廓偵測的影像前處理系統，其中結合Log-Gabor filter、contrast enhancement 、histogram equalization及Canny edge等方法，來改善超音波影像品質；首先本系統設定Log-Gabor filter中的參數來濾除超音波圖形的雜訊，藉此提高影像的清晰度；其參數包括最小波長與中心頻率之頻寬，再加入contrast enhancement和histogram equalization的影像強化功能，以提升超音波影像的對比，使物體的邊緣更加明顯，接下來使用Canny edge演算法來描繪物體邊界，最後藉助於GVF-based snake將物體輪廓描繪出來。實驗結果證明本論文所提出的前處理系統可有效提高GVF-based snake在輪廓偵測上的精確度，使結果用於醫療檢驗中更具有參考的價值。