T₁ estimation for aqueous iron oxide nanoparticle suspensions using a variable flip angle SWIFT sequence

Purpose T₁ quantification of contrast agents, such as super-paramagnetic iron oxide nanoparticles, is a challenging but important task inherent to many in vivo applications in magnetic resonance imaging. In this work, a sweep imaging with Fourier transformation using variable flip angles (VFAs-SWIFT) method was proposed to measure T₁ of aqueous super-paramagnetic iron oxide nanoparticle suspensions. Methods T₁ values of various iron concentrations (from 1 to 7 mM) were measured using VFA-SWIFT and three-dimensional spoiled gradient-recalled echo with VFAs (VFA-SPGR) sequences on a 7 T MR scanner. For validation, T₁ values were also measured using a spectroscopic inversion-recovery sequence on a 7 T spectrometer. Results VFA-SWIFT demonstrated its advantage for quantifying T₁ of highly concentrated aqueous super-paramagnetic iron oxide nanoparticle suspensions, but VFA-SPGR failed at the higher end of iron concentrations. Both VFA-SWIFT and VFA-SPGR yielded linear relationships between the relaxation rate and iron concentrations, with relaxivities of 1.006 and 1.051 s^-1 mM^-1 at 7 T, respectively, in excellent agreement with the spectroscopic measurement of 1.019 s^-1 mM ^-1. Conclusion VFA-SWIFT is able to achieve accurate T₁ quantification of aqueous super-paramagnetic iron oxide nanoparticle suspensions up to 7 mM.