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Objective Ferumoxytol, an iron oxide nanoparticle that targets phagocytic cells, can be used in magnetic resonance imaging of malignant brain tumors and can be administered as a bolus, allowing dynamic imaging. Our objectives were to determine the optimum time of delayed contrast enhancement of ferumoxytol, and to compare ferumoxytol and gadolinium contrast agents for magnetic resonance angiography and perfusion. Methods Twelve patients with malignant brain tumors underwent serial magnetic resonance imaging multiple times up to 72 hours after ferumoxytol injection at both 1.5 and 3-T. The enhancement time course was determined for ferumoxytol and compared with a baseline gadolinium scan. Perfusion, time-of-flight and dynamic magnetic resonance angiography and T1-weighted scans were compared for the two agents. Results The lesions were detectable at all field strengths, even with an intraoperative 0.15-T magnet. Maximal ferumoxytol enhancement intensity was at 24 to 28 hours after administration, and the enhancing volume subsequently expanded with time into a non-gadolinium-enhancing, high T2-weighted signal region of tumor-infiltrated brain. Dynamic studies were assessed with both agents, indicating early vascular leak with gadolinium but not with ferumoxytol. Conclusion Our most important finding was that gadolinium leaks out of blood vessels early after injection, whereas ferumoxytol stays intravascular in the "early" phase, thereby increasing the accuracy of tumor perfusion assessment. As a magnetic resonance imaging contrast agent, ferumoxytol visualizes brain tumors at all field strengths evaluated, with delayed enhancement peaking at 24 to 28 hours after administration.