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Magnetic particle imaging (MPI) is a sensitive, high contrast tracer modality that images superparamagnetic iron oxide nanoparticles (SPIOs), enabling radiation-free theranostic imaging. MPI resolution is currently limited by scanner and particle constraints. Recent tracers have experimentally shown 10x resolution and signal improvements, with dramatically sharper M-H curves. Experiments suggest that this results from interparticle interactions, conforming to literature definitions of superferromagnetism. We thus call our tracers superferromagnetic iron oxide nanoparticles (SFMIOs). While SFMIOs provide excellent signal and resolution, they exhibit hysteresis, with non-negligible remanence and coercivity. We provide the first report on MPI scanning with remanence and coercivity, including the first quantitative measurements of SFMIO remanence decay and reformation using a novel multi-echo pulse sequence. We also describe an SNR-optimized pulse sequence for SFMIOs under human electromagnetic safety limitations. The resolution from SFMIOs could enable clinical MPI with 10× reduced scanner selection fields, reducing hardware costs by up to 100×.

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 inbothresolution 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.