Clusters of magnetic nanoparticles as contrast agents for MRI: The effect of aggregation on T2 relaxivity

Magnetic resonance imaging (MRI) reveals anatomical structures with fascinating details, providing efficient diagnostic tool for medicine and powerful non-invasive method for biological studies in vivo. Specific contrast and labelling agents further enhance MRI applications and the most efficient ones might enable even detection of single cells. However, such enormous sensitivity is accomplished only by means of magnetic nanoparticles. These so-called susceptibility agents markedly decrease the transverse relaxation time T₂, which is described by the transverse relaxivity r₂. The traditional studies have been particularly focused on the dependence of r₂ on magnetic properties and the mean size of crystallites, employing almost exclusively iron-based materials. In contrast, the present contribution attempts to analyse the effect of clustering of magnetic cores on T₂ relaxation and it utilizes nanoparticles of ferromagnetic La_0.75Sr_0.25MnO₃ perovskite phase. Contrary to majority of studies dealing with biomedical applications of magnetic particles, the agglomerates are intentionally isolated and subjected to relaxometry. Furthermore, a supplemental analysis is dedicated to the effect of the silica shell thickness on T₂.