Comparison of ferumoxytol and gadodiamide enhancement in the rat U87 glioma xenographed model. (A) T1-weighted coronal MR of the rat brain before contrast. (B) At 60 secs after administration of ferumoxytol (6 mg/kg), no enhancement is visible. (C) At 60 secs postinjection of gadodiamide (0.05 mmol/kg, intravenously), the tumor shows increased signal. The small-molecular-weight agent can leak rapidly into the U87 tumor parenchyma causing contrast enhancement, but ferumoxytol, at early time points, is effectively a blood pool agent.