(31e) Effect of Particle Diameter and Magnetic Anisotropy on Magnetorelaxometry and Magnetic Particle Imaging Performance of Immobilized Magnetic Nanoparticles

Magnetic nanoparticles (MNPs) are of interest for application in magnetic particle imaging (MPI), an emerging biomedical imaging technology that relies on the non-linear dynamic magnetization response of MNPs. Compared to other molecular imaging modalities, MPI has the advantages of not requiring ionizing radiation and having high image contrast (zero signal from the tissue background), high sensitivity, and zero signal depth attenuation. In MPI, a static bias magnetic field gradient is superimposed with a uniform alternating magnetic field (AMF) to generate a small “field-free region”, where the particles are able to respond to the AMF and generate a response signal. This signal is monitored by a pick-up coil and is then used to generate a quantitative image of the distribution of MNPs in a field of view. The variation of signal intensity with distance in the space around a point source in the imaging volume is commonly described using the so-called point spread function (PSF), which provides information on the signal strength and theoretical resolution obtainable with a given nanoparticle. A review of the recent literature suggests that no prior computational work has studied the effect of nature of magnetocrystalline anisotropy, strength of magnetocrystalline anisotropy energy, and particle diameter on the MPI performance (through the point spread function) of magnetic nanoparticles undergoing Néel-relaxation. On the other hand, prior computational studies have applied simulations of the Landau-Lifshitz-Gilbert (LLG) equation to predict the magnetorelaxometry of MNPs. However, these did not provide insight into the dipole dynamics for nanoparticles that are fixed in a physical matrix, such as would often be the case for nanoparticles that accumulate inside cells. Thus, further work is needed to fully understand how nanoparticle properties influence non-linear magnetization dynamics of nanoparticles, particularly in the context of the fields often used in MPI.

Here we report a computational study of the effect of particle diameter and magnetic anisotropy (considering both symmetry and energy) on the magnetization dynamics and MPI performance of spherical single-domain MNPs that relax by internal dipole rotation, by carrying out simulations that are based on the LLG equation. In the case of an applied static magnetic field, we studied the magnetic relaxation time of the nanoparticles as the field is suddenly applied and suppressed, for particles with uniaxial and cubic magnetic anisotropies and various anisotropy constants and particle diameters. The results, for both anisotropy symmetries show that for both cases where a static magnetic field is suddenly turned on or off, the MNPs may undergo a successive two-step (periods Ï„₁ and Ï„₂ for field turned on, periods Ï„₃ and Ï„₄ for field turned off) or combined one step (period Ï„₁₂ for field turned on, period Ï„₃₄ for field turned off) relaxation. Whether a particle relaxes with one or two periods when the field is turned on is independent of the particle size, but determined by the competition between the energy due to the applied magnetic field and the magnetic anisotropy energy. For the case where the applied magnetic field is suddenly turned off, our results show good agreement with theoretical predictions for the cases of |K|V/k_BT≤1 and |K|V/k_BT>>1, where K represents the magnetocrystalline anisotropy constant, V represents the particle volume, k_B is the Boltzmann constant and T represents the absolute temperature. On the other hand, for the case of a dynamic applied AMF that is typical of MPI applications, the magnetic responses of the magnetite nanoparticles of various diameters were studied in terms of the magnetization dynamics, dynamic hysteresis loops, harmonic spectra, and PSF of the magnetization signal as a function of particle diameter. Results suggest that increasing the particle diameter leads to improved MPI performance (signal and resolution). Comparison of the MPI performance was also made between nanoparticles with uniaxial and cubic anisotropies. In summary, these computational studies provide theoretical insight into the role of particle diameter and magnetic anisotropy on the performance of MNPs for applications in MPI.