Complex Relationship Between Signal Intensity Properties in Magnetic Particle Imaging (MPI) and Iron Oxide Nanoparticle Degradation

Magnetic particle imaging (MPI) is an exciting new biomedical imaging technology that uses superparamagnetic nanoparticles as an imaging tracer. MPI is touted as a quantitative imaging modality but MPI signal properties have never been characterized for nanoparticles undergoing biodegradation. Here we characterize the nature of the MPI signal properties as a function of degradation of various magnetic particle formulations. We show that MPI signal properties can increase or decrease as a function of nanoparticle formulation and chemical environment and that long-term in vitro experiments only roughly approximate long-term in vivo MPI signal properties. Data are supported by electron microscopy of nanoparticle degradation. Knowledge of MPI signal property changes during nanoparticle degradation will be critical in design and interpretation of all MPI experiments. Further, we demonstrate for the first time, an environmentally sensitive MPI contrast mechanism opening the door to smart contrast paradigms in MPI.