Micro-magnetic simulation study on the magnetic particle imaging performance of anisotropic mono-domain particles.

The performance of magnetic mono-domain particles is of crucial importance in magnetic particle imaging (MPI). So far, the behavior of mono-domain particles has been modeled within the framework of Langevin theory. This theory predicts the dependence of the MPI signal on the particle core size, but cannot account for the influence of the shape, i.e. the anisotropy of the particle core. In this study we present the first micro-magnetic ab initio simulation of spectra of anisotropic particles with different core diameters in an oscillating magnetic field at 25 and 100 kHz. We find that the MPI signal strongly depends on the anisotropy of the magnetic core. Thus, a difference of 3 nm between the principal axes of a prolate ellipsoid with the volume of a 30 nm sphere can result in a complete loss of the MPI signal. Smaller anisotropies, however, can increase the MPI performance of the particle. The simulations show that the effect of the anisotropy on the MPI signal depends on the frequency of the oscillating magnetic field. At 100 kHz, the optimal signal is found at smaller anisotropies than at 25 kHz. Furthermore, the simulations show that experimental spectroscopic results for Resovist® can only be explained quantitatively by particles with a magnetic core size of at least 25 nm.