Relaxation Properties of Contrast Media for MRI Based on Iron Oxide Nanoparticles in Different Magnetic Fields.

We studied dependences of T2 relaxation time on magnetic field and concentration of nanoparticles. It was found that nanocontrast media are effective under the influence of the magnetic fields in the range 0.3-7 T. Data of electron paramagnetic resonance confirm the assumption on aggregation of nanoparticles not coated with proteins in high magnetic fields.

Magnetic Vortices as Efficient Nano Heaters in Magnetic Nanoparticle Hyperthermia.

Magnetic vortices existing in soft magnetic nanoparticles with sizes larger than the single-domain diameter can be efficient nano-heaters in biomedical applications. Using micromagnetic numerical simulation we prove that in the optimal range of particle diameters the magnetization reversal of the vortices in spherical iron and magnetite nanoparticles is possible for moderate amplitudes of external alternating magnetic field, H0 < 100 Oe. In contrast to the case of superparamagnetic nanoparticles, for the vortex configuration the hysteresis loop area increases as a function of frequency. Therefore, high values of the specific absorption rate, on the order of 1000 W/g, can be obtained at frequencies f = 0.5-1.0 MHz. Because the diameter D of a non single-domain particle is several times larger than the diameter d of a superparamagnetic particle, the volume of heat generation for the vortex turns out to be (D/d)3 times larger. This shows the advantage of vortex configurations for heat generation in alternating magnetic field in biomedical applications.

Interaction Effects in Assembly of Magnetic Nanoparticles.

A specific absorption rate of a dilute assembly of various random clusters of iron oxide nanoparticles in alternating magnetic field has been calculated using Landau-Lifshitz stochastic equation. This approach simultaneously takes into account both the presence of thermal fluctuations of the nanoparticle magnetic moments and magneto-dipole interaction between the nanoparticles of the clusters. It is shown that for usual 3D clusters, the intensity of the magneto-dipole interaction is determined mainly by the cluster packing density η = N p V/V cl , where N p is the average number of the particles in the cluster, V is the nanoparticle volume, and V cl is the cluster volume. The area of the low frequency hysteresis loop and the assembly-specific absorption rate have been found to be considerably reduced when the packing density of the clusters increases in the range of 0.005 ≤ Î· < 0.4. The dependence of the specific absorption rate on the mean nanoparticle diameter is retained with an increase of η, but becomes less pronounced. For fractal clusters of nanoparticles, which arise in biological media, in addition to a considerable reduction of the absorption rate, the absorption maximum is shifted to smaller particle diameters. It is found also that the specific absorption rate of fractal clusters increases appreciably with an increase of the thickness of nonmagnetic shells at the nanoparticle surfaces.