Magnetic nanoparticles (MNPs) are used as T₁-T₂ dual-mode MRI contrast agents (DMCAs) to provide both T₁ and T₂ contrast in the same MRI scanner by changing the operational mode between T₁-weighted and T₂-weighted pulse sequences. These agents are designed to induce positive or negative contrast depending on their r₂/r₁ relaxivity ratio, which should be in the range of 2–10. MNPs containing Gd, Mn, Fe, and other lanthanide ions are used for this purpose. The review discusses the rational design of DMCAs, focusing on strategies for optimizing their properties to achieve dual-mode contrast. Examples include NPs with a single contrast material or those incorporating both T₁ and T₂ contrast materials in the same nanoplatform. The review also covers the basic principles of paramagnetic relaxation in MNPs, the factors influencing their relaxivity, and the in vivo applications of these agents. The study highlights the importance of controlling the r₂/r₁ ratio to achieve efficient dual-mode contrast agents, with examples demonstrating their potential in clinical MRI applications.Magnetic nanoparticles (MNPs) are used as T₁-T₂ dual-mode MRI contrast agents (DMCAs) to provide both T₁ and T₂ contrast in the same MRI scanner by changing the operational mode between T₁-weighted and T₂-weighted pulse sequences. These agents are designed to induce positive or negative contrast depending on their r₂/r₁ relaxivity ratio, which should be in the range of 2–10. MNPs containing Gd, Mn, Fe, and other lanthanide ions are used for this purpose. The review discusses the rational design of DMCAs, focusing on strategies for optimizing their properties to achieve dual-mode contrast. Examples include NPs with a single contrast material or those incorporating both T₁ and T₂ contrast materials in the same nanoplatform. The review also covers the basic principles of paramagnetic relaxation in MNPs, the factors influencing their relaxivity, and the in vivo applications of these agents. The study highlights the importance of controlling the r₂/r₁ ratio to achieve efficient dual-mode contrast agents, with examples demonstrating their potential in clinical MRI applications.