Magnetic relaxation in tissues can be enhanced using pharmaceuticals called contrast agents. The most commonly used of these agents for orthopedic imaging are the paramagnetic compounds, which have their strongest effect on the T1 in tissues where they have accumulated. These contrast agents can be injected intravenously, increasing T1 signal intensity in tissues where they accumulate. The MR signal collected still results from the water protons, but the presence of these contrast agents enhances the relaxation of water protons in their vicinity. Paramagnetic contrast agents contain magnetic centers that create magnetic fields approximately one thousand times stronger than those corresponding to water protons. These magnetic centers interact with water protons in exactly the same way as the neighboring protons described in Sections 1.3.2 and 1.3.3 above, but with much stronger magnetic fields, and therefore, have a much greater impact on relaxation rates, particularly on T1. In orthopedic imaging, contrast agents are routinely injected intravenously to help identify areas of hypervascularity, as in malignant tumors.