PNNL

Abstract

Purpose: Magnetic particle imaging (MPI), using magnetite nanoparticles (MNPs) as tracer material, shows great promise as a platform for fast tomographic imaging. To date, the magnetic properties of MNPs used in imaging have not been optimized. As nanoparticle magnetism shows strong size dependence, we explore how varying MNP size impacts imaging performance in order to determine optimal MNP characteristics for MPI at any driving field frequency, ?. Methods: Monodisperse MNPs of varying size were synthesized and their magnetic properties characterized. Their MPI response was measured experimentally, at an arbitrarily chosen ? = 250 kHz, using a custom-built MPI transceiver designed to detect the third harmonic of MNP magnetization. Results were interpreted using a model of dynamic MNP magnetization that is based on the Langevin theory of superparamagnetism and accounts for sample size distribution, and size-dependent magnetic relaxation. Results: Our experimental results show clear variation in the MPI signal intensity as a function of MNP size that is in good agreement with modeled results. A maxima in the plot of MPI signal vs. MNP size indicates there is a particular size that is optimal for the chosen frequency of 250 kHz. Conclusions: For MPI at any chosen frequency, there will exist a characteristic particle size that generates maximum signal amplitude. We illustrate this at 250 kHz with particles of 15 nm core diameter.