Effect of Core Size Distribution of Immobilized Magnetic Nanoparticles on Harmonic Magnetization

Abstract

In magnetic particle imaging (MPI), harmonic magnetization signals detected from magnetic nanoparticles (MNPs) are used to image the spatial distribution of the MNPs. The strength of the harmonic signals is directly related to the sensitivity of the MPI system. In this study, we used numerical simulations based on the Fokker-Planck equation to explore the effect of the core size distribution of an immobilized MNP sample on the harmonic signals. We assumed an anisotropy value of 5 kJ/m^3 and a uniform volume-weighted core size distribution of MNPs ranging from 17.4 to 37.6 nm to simulate a typical MNPs sample. First, we show that the strength of the harmonic signals of the MNP sample were much lower than calculated from the scalar summation of the harmonic signals generated from each MNP in the sample. For example, the strength of the 9th harmonic signal decreased to one-third. This indicates that about 67% of the 9th harmonic signals generated from each MNP are mutually cancelled. We then show that the phase lag of the magnetization due to a finite Néel relaxation time caused lower harmonic magnetization signals of the MNP sample when the core size was distributed. These results indicate that an MNP sample with a narrow size distribution and small anisotropy energy would effectively improve the sensitivity of the MPI system.