Ultrasmall superparamagnetic iron oxide: characterization of a new class of contrast agents for MR imaging

Radiology. 1990 May;175(2):489-93. doi: 10.1148/radiology.175.2.2326474.


An ultrasmall superparamagnetic iron oxide (USPIO) preparation was developed that is small enough to migrate across the capillary wall, a prerequisite in the design of targetable particulate pharmaceuticals. Seventy percent of particles were smaller than 10 nm; 26%, smaller than 5 nm. The blood half-life of USPIO in rats was 81 minutes, considerably longer than that of larger superparamagnetic iron oxide preparations such as AMI-25 (6 minutes). Electron microscopy demonstrated that USPIO particles transmigrate the capillary wall by means of vesicular transport and through interendothelial junctions. Twenty-four hours after intravenous administration, 3.6% of the injected dose per gram of tissue was found in lymph nodes, 2.9% per gram in bone marrow, 6.3% per gram in liver, and 7.1% per gram in spleen. The major potential applications for USPIO are as (a) an intravenous contrast agent for the lymphatic system, (b) a bone marrow contrast agent, (c) a long-half-life perfusion agent for brain and heart, and (d) the magnetic moiety in organ-targeted superparamagnetic contrast agents for magnetic resonance imaging.

MeSH terms

  • Animals
  • Capillary Permeability
  • Cells, Cultured
  • Contrast Media*
  • Endothelium, Vascular / metabolism
  • Ferrosoferric Oxide
  • Iron* / pharmacokinetics
  • Lymph Nodes / metabolism
  • Magnetic Resonance Imaging*
  • Male
  • Oxides*
  • Particle Size
  • Rats
  • Rats, Inbred Strains
  • Tissue Distribution


  • Contrast Media
  • Oxides
  • Iron
  • Ferrosoferric Oxide