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Iron-oxide nanoparticles (IONPs) have proven utility as contrast agents in many MRI applications. Previous quantitative IONP mapping has been performed using mainly T2* mapping methods. However, in applications requiring high IONP concentrations, such as magnetic nanoparticles based thermal therapies, conventional pulse sequences are unable to map T2* because the signal decays too rapidly. In this article, sweep imaging with Fourier transformation (SWIFT) sequence is combined with the Look-Locker method to map T1 of IONPs in high concentrations.T1 values of agar containing IONPs in different concentrations were measured with the SWIFT Look-Locker method and with inversion recovery spectroscopy. Precisions of Look-Locker and variable flip angle (VFA) methods were compared in simulations.The measured R1 (=1/T1) has a linear relationship with IONP concentration up to 53.6 mM of Fe. This concentration exceeds concentrations measured in previous work by almost an order of magnitude. Simulations show SWIFT Look-Locker method is also much less sensitive to B1 inhomogeneity than the VFA method.SWIFT Look-Locker can accurately measure T1 of IONP concentrations ≤53.6 mM. By mapping T1 as a function of IONP concentration, IONP distribution maps might be used in the future to plan effective magnetic nanoparticle hyperthermia therapy. Magn Reson Med 71:1982–1988, 2014. © 2014 Wiley Periodicals, Inc.