Evidence that SPIO Chain Formation is Essential for High-Resolution MPI

Magnetic Particle Imaging (MPI) is a noninvasive imaging modality that exploits the saturation properties of superparamagnetic iron oxide particles (SPIOs). A major thrust of MPI research aims to sharpen the magnetic resolution of biocompatible SPIOs, which will be crucial for affordable and safe clinical translation. We recently reported on a new class of MPI tracers \---|called superferromagnetic iron oxide nanoparticles (SFMIOs) \---| which offer much sharper magnetic saturation curves. SFMIOs experimentally demonstrate 10-fold improvement in both resolution and sensitivity. However, superferromagnetism is a relatively unexplored branch of physics and the nanoscale physics and dynamics of SFMIOs remain a mystery. Here we show experimentally that chaining of SPIOs can explain SFMIO's boost in SNR and resolution. We show how concentration, viscosity, transmit amplitude, and pre-polarization time can all affect SPIO chain formation and SFMIO behavior. These experiments will inform strategies on SFMIO chemical synthesis as well as SFMIO data acquisition pulse sequences. ### Competing Interest Statement Steven M Conolly is a co-founder of an MPI company, Magnetic Insight, and holds stock in this company. The authors declare no other conflict of interest.