Ferumoxytol is an iron-containing parenteral treatment for iron deficiency anemia that was recently approved by the Food and Drug Administration. The iron is in the form of a superparamagnetic iron oxide that causes T1, T2, and T2* shortening on magnetic resonance imaging (MRI). Furthermore, the drug has a long intravascular half-life of 14-15 hours; a standard dose can affect MRI for days to months. We describe a case in which a patient underwent contrast-enhanced MRI of the liver 2 days after receiving a dose of ferumoxytol, which was unknown to the radiology team. The blood pool and soft tissues were hyperintense on T1-weighted images, concealing enhancement from the gadolinium-based contrast agent that was administered during the exam and rendering the exam nondiagnostic. Radiologists must be aware of this potential effect in screening patients for MRI and interpreting exams.
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