Magnetic particle imaging (MPI), using superparamagnetic nanoparticles as an imaging tracer, is touted as a quantitative biomedical imaging technology, but MPI signal properties have never been characterized for magnetic nanoparticles undergoing biodegradation. We show that MPI signal properties can increase or decrease as iron oxide nanoparticles degrade, depending on the nanoparticle formulation and nanocrystal size and the degradation rate and mechanism. Further, we show that long-term in vitro MPI experiments only roughly approximate long-term in vivo MPI signal properties. Further, we demonstrate, for the first time, an environmentally sensitive MPI contrast mechanism, opening the door to smart contrast paradigms in MPI.

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.<br>

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.<br>

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.<br>