Figure 1. Proton diffusion and dephasing around magnetic nanoparticles.

(a,b) Magnetic nanoparticles (MNPs) with a larger size or magnetization generate a higher degree of magnetic field gradient, leading to higher level of proton dephasing when water molecules diffuse around MNPs. M_xy represents the net nuclei spins on the transverse plane. Dashed arrows indicate the proposed route for water molecule diffusion and dephasing. (c) The magnetic field coupling between adjacent MNPs increases the level of field inhomogeneity, which confers greatly enhanced effect to the loss of phase coherence. (d) Magnetic field inhomogeneity can be further elevated by artificially involving MNPs of different sizes in proximity, owing to the reduced magnetic field symmetry around MNPs.