Nanoparticle core size optimization for magnetic particle imaging

In magnetic particle imaging (MPI), the changing magnetization of magnetic nanoparticle (MNP) tracers subjected to an alternating magnetic field is detected. The physical properties of the MNP tracers have a direct effect on the quality of the resulting signal. In order to improve MPI image resolution and sensitivity, optimizing these properties, in particular the MNP core size, is essential. In this work, we investigate the existence of an optimal MNP core size for MPI using stochastic simulations of Langevin equations, supported by magnetic particle spectroscopy (MPS) measurements of highly monodisperse single-core nanoparticles with carefully tailored core sizes. We demonstrate that once the MNP core diameter exceeds around 28 nm (with the exact value depending on applied field properties and non-magnetic nanoparticle coating), relaxation effects will begin to dominate. Furthermore, as nanoparticle size is increased, interparticle interactions make it difficult to stabilize the particles in water and maintain their monodispersity. Taken together, we conclude that 28 nm in core diameter is an optimal size for single-core, monodisperse, magnetite particles used in MPI.