Superparamagnetism occurs when the material is composed of very small crystallites (1-10 nm). In this case, even though the temperature is below the Curie or Néel temperature and the thermal energy is not sufficient to overcome the coupling forces between neighboring atoms, the thermal energy is sufficient to change the direction of magnetization of the entire crystallite. The resulting fluctuations in the direction of magnetization cause the magnetic field to average to zero. The material behaves in a manner similar to paramagnetism, except that instead of each individual atom being independently influenced by an external magnetic field, the magnetic moment of the entire crystallite tends to align with the magnetic field.
In MRI superparamagnetic iron oxide is used as a contrast agent.

See also magnetism, ferromagnetism, paramagnetism, and diamagnetism.

Tumor specific MRI contrast agents are in development to provide better delineation and progression information for various tumors. Clinical oncology has a need for contrast agents that can identify tumors and metastases at a size of 100,000 cells rather than 1,000,000,000 cells. This level of sensitivity requires excellent tumor targeting of imaging agents and a high MRI signal.
Tumor specific agents accumulate at pathological tissues by passive or active targeting mechanisms. Passive targeting agents use e.g., the natural defense mechanisms in which phagocytic cells remove foreign particles from the body. Active targeting is based on a ligand-directed, site-specific accumulation of contrast agents. The availability of macromolecular contrast agents such as feruglose and ultrasmall superparamagnetic iron oxide (USPIO), which permit the assessment of tissue permeability, may also improve the detection of tumor grade, tumor type, and response to drugs that target angiogenesis.

See also Monoclonal Antibodies, Metalloporphyrins, Nitroxides and Ferrioxamine.