Imaging with two or more imaging modalities is widely becoming standard practice because of their needed additional information (Esenaliev et al. 1993). Much research is being performed to develop multimodal systems such as MRI/PET and CT/PET and to optimize them to acquire complementary information of disease in the form of both functional and anatomical details. Anatomical imaging visualizes internal structures of the body, and functional imaging visualizes and oen quanties physiological processes such as blood ow, oxygenation, and metabolism (Moseley and Donnan 2004).