As one of the important tools of NDE, imaging technology provides fast, intuitive, and reliable diagnostic information for various research and industrial NDE applications. While as digital computer and electronic technologies bloom, a large number of advanced imaging systems as well as digital image processing algorithms have been successfully developed in the last two decades. These devices and algorithms have been adapted for NDE applications. A high quality NDE image obtained by an imaging system provides the visible information of the material structure. However, qualities of NDE images are often limited by some factors such as noise and unsharpness, incomplete data and information loss, sensitivities of the sensors and capability of the devices [1, 2, 3]. Many image processing methods can be applied to increase reliability of the images and to analyze the images. These methods includes image transform, reconstruction, enhancement, restoration, and feature extraction. The references of these methods can be readily found in many textbooks and research journals and we try not to do an exhaustive list in here. To have a quick review of different image processing techniques, one may find useful a recent review paper by Demoment [4] and the textbook written by Gonzalez and Wintz [5].