Spin-lock MR enhances the detection sensitivity of superparamagnetic iron oxide particles.

PURPOSE To evaluate spin-lock MR for detecting superparamagnetic iron oxides and compare the detection sensitivity of quantitative T1ρ with T2 imaging. METHODS In vitro experiments were performed to investigate the influence of iron oxide particle size and composition on T1ρ . These comprise T1ρ and T2 measurements (B0 = 1.41T) of agar (2%) with concentration ranges of three different iron oxide nanoparticles (IONs) (Sinerem, Resovist, and ION-Micelle) and microparticles of iron oxide (MPIO). T1ρ dispersion was measured for a range of spin-lock amplitudes (γB1 = 6.5-91 kHz). Under relevant in vivo conditions (B0 = 9.4T; γB1 = 100-1500 Hz), T1ρ and T2 mapping of the liver was performed in seven mice pre- and 24 h postinjection of Sinerem. RESULTS Addition of iron oxide nanoparticles decreased T1ρ as well as the native T1ρ dispersion of agar, leading to increased contrast at high spin-lock amplitudes. Changes of T1ρ were highly linear with iron concentration and much larger than T2 changes. MPIO did not show this effect. In vivo, a decrease of T1ρ was observed with no clear influence on T1ρ dispersion. CONCLUSION By suppression of T1ρ dispersion, iron oxide nanoparticles cause enhanced T1ρ contrast compared to T2 . The underlying mechanism appears to be loss of lock. Spin-lock MR is therefore a promising technique for sensitive detection of iron oxide contrast agents.