Nonequilibrium nanoparticle dynamics for the development of Magnetic Particle Imaging

Abstract

Magnetic Particle Imaging (MPI) is a promising new medical imaging platform currently in the preclinical stage. MPI uses iron oxide nanoparticles as tracers, and exploits their nonlinear magnetization response to external oscillating magnetic fields. The nanoparticle response is directly measured to create an image in MPI, and so optimizing nanoparticle properties as well as external magnetic field conditions in order to obtain improved image signal is crucial to the development of MPI as a clinical platform. A deep understanding of the physics of nanoparticle relaxation is fundamental to optimization and further development of MPI. This work focuses on modeling the nonequilibrium dynamics of magnetic nanoparticles in order to optimize conditions for MPI and develop new therapeutic and diagnostic functionalities that can be integrated with MPI. A summary of theoretical models of nanoparticle dynamics is presented, and computational nonequilibrium models are outlined, which currently represent the most sophisticated methods for modeling nanoparticle dynamics. These models are verified and supported through experiment. Using these models, nanoparticle relaxation is explored in depth; the effect of applied field amplitude and frequency, as well as nanoparticle size, on the resulting relaxation mechanism and timescale is investigated in detail. These insights are then applied to the optimization of drive field conditions and nanoparticle size for MPI image resolution and sensitivity. A core size of 28 nm is found to be optimal, with additional tuning required according to specific field conditions used. Finally, a procedure for multicolor MPI, in which the signal from nanoparticles of different types or in different environments is separated, is developed. A multi-channel image reconstruction approach is outlined, and discrimination based on nanoparticle core size is demonstrated, resulting in successful generation of a multicolor MPI image of a 2D phantom. A procedure for quantitative temperature estimation with MPI is also proposed and verified experimentally. Overall, this work provides a theoretical foundation for nanoparticle relaxation physics in MPI, enabling further development towards the clinical stage.