Magnetic Nanoparticle Imaging: Insight on the Effects of Magnetic Interactions and Hysteresis of Tracers

Abstract

The dynamic properties of magnetite nanoparticles are investigated by rate equations with the aim of clarifying the factors affecting their performance as tracers in magnetic particle imaging (MPI). It is shown that size-dependent effects such as magnetic hysteresis and dipole–dipole interaction may have a great impact on the behavior of MPI tracers. Usually, magnetic imaging exploits the higher-order harmonics of the magnetization waveform without considering either intraparticle hysteresis or interparticle interactions. These assumptions may result in an incorrect estimate (either by excess or by defect) of the nanoparticle concentration, which is the ultimate aim of MPI. The mismatch between real and estimated values is apparent for concentrations typical of some therapeutic applications of magnetic nanoparticles or reached by effect of particle accumulation in organs because of slow clearance processes. We show that this difficulty can be removed by measuring not only the magnitude of the third harmonic of the signal but also the phase shift with respect to the driving field. The proposed technique of signal adjustment makes use of the settings of present-day MPI operating devices. The validity of the adjustment procedure is checked by a proof of concept using nonuniform nanoparticle concentrations.

From spectral analysis to hysteresis loops: a breakthrough in the optimization of magnetic nanomaterials for bioapplications

“…When all harmonics of M are either in phase (ϕ = 0) or in opposition (ϕ = π) with the driving field, the loop area is exactly equal to zero, i.e. the M(H) curve displays no hysteresis, as in the very special (and unlikely) case of superparamagnetic particles whose magnetization can still be described by a Langevin function at the operating frequency [38,40].…”

“…The interpretation of the magnetic behaviour of nanomaterials is often hindered by the intrinsic complexity of the magnetization process, which usually displays hysteretic features. Magnetic hysteresis, originated by the intrinsic irreversibility of the magnetization rotation or switch [36], is not to be neglected when a magnetic nanomaterial is used either as a heat generator [28,37] or as a contrast agent [38]. It should be recognized that hysteresis of the magnetization is the rule rather than the exception in nanomaterials for application in biomedicine, which are typically submitted to high-frequency fields so that frequency-sustained hysteresis [39,40] often appears.…”