Magnetic nanoparticle contrast agents for MRI: Structure-property relationships, in vivo applications, and future theranostic directions

Magnetic resonance imaging (MRI) is a non-invasive and non-ionizing imaging modality that provides high-resolution images of internal organs such as the breast, brain, and cardiovascular system, enabling three-dimensional visualization of soft tissues. While MRI offers excellent soft tissue contrast, its sensitivity can be further enhanced using contrast agents, and many clinical applications rely on exogenous agents to improve detection and diagnostic accuracy. Two primary classes are used clinically: paramagnetic substances, exemplified by gadolinium (Gd), which predominantly shorten longitudinal (T1) relaxation, and superparamagnetic iron oxide nanoparticles (SPIONs), which exert strong effects on transverse (T2) relaxation. The performance and safety of these agents are strongly influenced by their pharmacokinetics and biodistribution, including rapid recognition and clearance by the reticuloendothelial system (RES), which can both enable liver-spleen imaging and limit target-specific contrast in other organs. In this review, we first summarize the fundamental principles of MRI contrast generation, with an emphasis on relaxation mechanisms relevant to magnetic nanoparticles (MNPs). We then discuss the use of MNPs as contrast agents in representative biomedical applications, focusing on cardiac, breast, and brain MRI and illustrating how organ-specific physiology constrains nanoparticle design and performance. Finally, we examine biocompatibility and safety considerations for both Gd-based agents and SPIONs, highlighting current regulatory concerns, open questions regarding long-term toxicity, and key challenges that must be addressed to translate next-generation nanoparticle-based MRI contrast agents into routine clinical practice.