Magnetic nanoparticle relaxation processes key for medical imaging techniques
DOI: 10.1063/10.0009250
Magnetic nanoparticle relaxation processes key for medical imaging techniques lead image
Magnetic resonance imaging (MRI) exploits water in the body, causing hydrogen atoms to act as tiny bar magnets and align with an applied magnetic field. Magnetic pulses rotate the hydrogen spin to a specific orientation. After a pulse, the atoms relax back to their original spin orientation, and that relaxation is measured.
In contrast, magnetic particle imaging (MPI) involves inserting magnetic nanoparticles into the body. When these particles magnetically relax, their relaxation frequencies are measured and can be used to trace blood flow.
Bui et al. studied the two magnetic relaxation mechanisms of iron oxide nanoparticles, Brownian and Néel relaxation, as they have important applications in MPI and pose fundamental questions in nanoparticle magnetism.
Brownian relaxation physically rotates particles, turning them upside down to switch the internal spin direction, while Néel relaxation flips the spin internally. In solids, only Néel relaxation plays a role, but in liquid environments like the human body, both are present.
“The two rotation mechanisms vary in timescale by many orders of magnitude, from milliseconds to nanoseconds,” said author Thinh Bui. “We made some technological advances to increase detection speed and magnetic field turn-on time.”
These developments allowed the team to resolve each mechanism separately, as well as investigate the magnetic dynamics as a function of temperature.
In the future, the researchers aim to investigate the interactions between nanoparticles as they aggregate, a process which could create larger signals and better imaging resolution but also dramatically change relaxation timescales and dynamics.
Source: “Advanced characterization of magnetization dynamics in iron oxide magnetic nanoparticle tracers,” by Thinh Q. Bui, Adam J. Biacchi, Cindi L. Dennis, Weston L. Tew, Angela R. Hight Walker, and Solomon I. Woods, Applied Physics Letters (2021). The article can be accessed at https://doi.org/10.1063/5.0077016 .
