Magnetic Tracers for Magnetic Particle Imaging: Insight on the Roles of Frequency-Sustained Hysteresis and Interactions in Quantitative Imaging

Magnetic hysteresis present in tracer materials for magnetic particle imaging (MPI) either by effect of the driving-field frequency or because of interactions among nanoparticles is shown to play a crucial role when such a technique is exploited to get quantitative information about the tissue section being exam-ined. By means of simulations based on a rate-equation model, the nanoparticles are proven to display magnetic hysteresis at the operating frequencies typical of MPI even if they are superparamagnetic in quasistatic conditions. As a consequence, the system function, a quantity crucial for image reconstruction in MPI, becomes markedly different in nature and shape from the curve produced by superparamagnetic nanoparticles. Not taking into account such an effect has detrimental consequences on quantitative MPI, as evidenced by specific examples of image reconstruction involving both monodisperse and polydisperse nanoparticles. The role played in image reconstruction by magnetic dipolar interaction, no longer negligi-ble when nanoparticles accumulate in a small volume of the tissue, is discussed within the rate-equation model.