Medical imaging is invaluable when it comes to gaining insight into human body. From the beginning, medical images had to deal with artifacts. It is well-known that metallic implants cause dark and bright streaking artifacts, due to the high atomic number of the metal. In this book new procedures for metal artifact reduction in computed tomography (CT) are proposed, which rely on additional information extracted from complementary magnetic resonance (MR) images. These methods exploit the possibilities arising from the use of emergent tri-modality imaging systems.