In image processing one is often interested Not only in analyzing one image but in comparing or combining the information given by different images. For this reason, image registration is one of the fundamental tasks within image processing. The task of image registration is to find an optimal geometric transformation between corresponding image data. In practice, the concrete type of the geometric transformation as well as the Notions of optimal and corresponding depend on the specific application. Image registration is a problem often encountered in many application areas like, for example, astro- and geophysics, computer vision, and medicine. For an overview, see, e.g., Brown (1992), Maintz & Viergever (1998), Maurer & Fitzpatrick (1993), van den Elsen et al (1993), and references therein.