Spatial Homogeneity of Superparamagnetic Nanoparticles and the Relationship to Relaxivity for Magnetic Resonance Imaging

The contribution of spatial homogeneity of magnetic nanofluids to the r 2 -relaxivity (1/ T 2 relaxation time) has been widely investigated for the past decade as a crucial scientific approach to enhance the resolution of T 2 -weighted magnetic resonance imaging (MRI). However, the correlation has not been comprehensively understood, and there are still controversies regarding the interpretation of the correlation. Here, the effects of spatial homogeneity, which is systematically controlled by the PDI (polydispersity index) and D h (hydrodynamic diameter), of SPIONP (superparamagnetic iron oxide nanoparticle) nanofluids on the r 2 -relaxivity were experimentally and theoretically studied to provide scientific clues for solving the unsettled controversies on the correlation between the spatial homogeneity and r 2 -relaxivity. According to the analyzed results, the spatial homogeneity of nanofluids critically affects the r 2 -relaxivity and accordingly the T 2 -weighted MR contrast efficiency due to its contribution to the m ( M ) (or H c ≈ H K ) change of the nanofluids. Moreover, it was demonstrated that the magnetic energy competition model and water accessibility model depending on the degree of spatial homogeneity are critical to interpret the effects of spatial homogeneity on the r 2 -relaxivity for T 2 -weighted MR imaging.