Magnetic nanoparticle for thermal and magnetic particle imaging

Abstract

The prospect of thermal imaging and control in magnetic particle imaging (MPI) is an important advancement for non-invasive medical diagnostics and therapeutics. Current MPI systems focus on imaging only, from measurement of magnetic nanoparticle concentrations, while leaving out the important in vivo diagnostic of temperature. Here, we report on our progress toward accurate thermal imaging with high resolution MPI and discuss our achievement of 410 mK total temperature uncertainty at 100 ms integration time near room temperature using a high-precision magnetic field-stabilized magnetic particle spectrometer. The magnetic field is stabilized to ~ 14 nTRMS uncertainty, which allows for precise and accurate determination of magnetic nanoparticle temperature by measurement of magnetization under AC driving fields. Figure 1 shows the obtained temperature Allan deviation plot extracted from the measurement of magnetization in 10 nm ferrite magnetic nanoparticles driven by a 2.5 kHz magnetic field [1].It has been reported that the imaging resolution of MPI is critically impacted by nanoparticle relaxation dynamics [2]; specifically, spin relaxation under AC drive fields causes a lagged response that results in image blurring. The dynamic response of magnetic nanoparticles in magnetic fields is intricate, depending strongly on the particles’ inherent magnetic and structural properties, inter-particle interactions, as well as the local environment in which they are suspended. Thus, comprehensive understanding of these effects requires high sensitivity over a broad parameter space. We have recently developed an arbitrary-wave magnetic particle spectrometer for characterizing relaxation dynamics for particles ranging in sizes from 10 – 70 nm. The combined advantage of arbitrary drive waveforms (sine, pulse, chirp, composite, etc.) and high drive amplitudes in our system allows for facile characterization over a large parameter space, using magnetic field frequencies from DC to 50 MHz at amplitudes 0-10 mTRMS. This capability to drive and observe magnetization dynamics occurring on the order 20 ns is required for isolating independent Brownian and Néel relaxation processes, as well as their dependence on particle environment and magnetic field amplitude. We have characterized the dynamic AC susceptibility of magnetic nanoparticles ranging in diameter from 10 nm to 70 nm and in composition (ferrite, cobalt-doped ferrite, zinc-doped ferrite, core-shell, etc.), and we have observed peaks in the imaginary part of the AC susceptibility (χ’’) at frequencies from as low as 50 Hz to > 50 MHz, depending on particle size and composition. As an example, Figure 2 show the χ’’ peak frequency for 15 nm (blue arrow) and 20 nm (green arrow) Fe3O4 nanoparticles suspended in water. This peak is an indication of the effective relaxation timescale (τ ~ 16 ms for 20 nm, and τ ~ 160 ns for 15 nm), which will impact the spatial resolution of MPI, depending on the excitation frequency used. This knowledge will eventually inform strategies for design and synthesis of magnetic nanoparticles with properties targeted for accurate and sensitive thermal imaging using MPI. **