Superparamagnetic iron oxide

Last revised by Joshua Yap on 11 May 2023

Superparamagnetic iron oxides (SPIOs) are a class of MR contrast agents composed of nanoparticles of iron oxide crystals coated in carbohydrates. They are used to image vasculature and the liver via shortening T1 and T2/T2* relaxation times.

The main agent currently used is ferumoxytol (Feraheme), an ultrasmall SPIO, which is otherwise used to treat iron-deficiency anemia in patients with chronic kidney disease. Knowledge of this other use is important as recent administration of ferumoxytol (within 3 months) can complicate MRI interpretation, manifesting as susceptibility artifact, pre-gadolinium contrast T1 hyperintensity, and T2 hypointensity in the vasculature 1.

Previously available SPIO agents for MRI include

  • ferumoxides (Feridex, Endorem)

  • ferucarbotran (Resovist, Cliavist)


Superparamagnetic iron oxide particles do not leak into the interstitium and therefore act as intravascular (blood pool) agents. Their elimination from the blood is by uptake into the reticuloendothelial system cells in the liver, spleen, bone marrow and lymph nodes and are phagocytosed by macrophages throughout the body.

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