Superparamagnetic iron oxide nanoparticle MRI contrast agent (SPIO (or SPION))

Superparamagnetic iron oxide nanoparticle MRI contrast agents (SPIO/SPION) are inorganic nanoparticles coated with biocompatible materials (e.g., dextran) used to enhance the sensitivity and specificity of MRI, particularly for T2- and T2*-weighted imaging. When administered intravenously, SPIONs are phagocytosed by the mononuclear phagocyte system, accumulating mainly in the liver, spleen, and sites of inflammation or malignancy. This alters local magnetic properties, decreasing T2 signal (appearing as hypointense or "dark" regions on MRI), and can improve detection of tumors, metastases, and inflammation. Some SPION agents can be further functionalized for targeting or as “activatable” probes, responding to biological cues like pH or calcium, thereby expanding their diagnostic capabilities. While historically approved agents have seen variable market availability, research continues into novel formulations with improved targeting, sensitivity, and safety.

Other names

Superparamagnetic iron oxide (SPIO)Superparamagnetic iron oxide nanoparticle (SPION)T2 MRI contrast agentIron oxide nanoparticle MRI contrast agentNegative MRI contrast agent

SPIONs are phagocytosed by macrophages or accumulate in tissues with altered permeability. They create magnetic field inhomogeneities, causing shortened T2 (and T2*) relaxation times, leading to signal loss ("negative contrast") in T2-weighted MRI images. They can be used as activatable probes, e.g., by coating with pH-sensitive materials or ligands for specific cell-surface proteins.