Abstract
Clustering of magnetic nanoparticles (MNPs) is perhaps the most effective, yet intriguing strategy to enhance T2 relaxivity in magnetic resonance imaging (MRI). However, the underlying mechanism is still not fully understood and the attempts to generalize the classic outersphere theory from single particles to clusters have been found to be inadequate. Here we show that clustering of MNPs enhances local field inhomogeneity due to reduced field symmetry, which can be further elevated by artificially involving iron oxide NPs with heterogeneous geometries in terms of size and shape. The r2 values of iron oxide clusters and Landau–Lifshitz–Gilbert simulations confirmed our hypothesis, indicating that solving magnetic field inhomogeneity may become a powerful way to build correlation between magnetization and T2 relaxivity of MNPs, especially magnetic clusters. This study provides a simple yet distinct mechanism to interpret T2 relaxivity of MNPs, which is crucial to the design of high-performance MRI contrast agents.
Highlights
Clustering of magnetic nanoparticles (MNPs) is perhaps the most effective, yet intriguing strategy to enhance T2 relaxivity in magnetic resonance imaging (MRI)
It is assumed that the increased level of field inhomogeneity may largely enhance the perturbation of proton phase coherence as water molecules diffuse around adjacent MNPs (Fig. 1), which may be the direct reason for the enhanced T2 relaxivity of magnetic clusters
In light of the fact that nonspherical MNPs show enhanced T2 relaxivity compared to spherical ones with equivalent solid volume[14,15], it is natural to assume the role of shape anisotropy in inducing local field a c d
Summary
Clustering of magnetic nanoparticles (MNPs) is perhaps the most effective, yet intriguing strategy to enhance T2 relaxivity in magnetic resonance imaging (MRI). MAR theory illustrates that T2 contrast efficiency (r2) is related to the diffusion of surrounding water protons and the physical properties of MNPs, which can be tuned by their size[12,13], shape[14,15,16,17,18], component[19,20,21], crystal structure[22,23] and surface property[24,25] Another intriguing phenomenon is that cluster formation from single MNPs can cause a marked decrease of T2 relaxation times and increase of r2 values, or vice versa[26]. Encouraged by this rationale, MNPs have been developed as T2 contrast agents due to the ability to induce local field inhomogeneity[38]
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