Quantitative effect of magnetic field strength on PEGylated superparamagnetic iron oxide nanoparticles

With promising applications of superparamagnetic iron oxide nanoparticles (SPIO) in magnetic resonance imaging (MRI) and targeted monitoring of molecular and cellular processes, many different samples of these nanoparticles (NPs) with different compositions synthesized each year. The main challenge in this way is to generate enough contrast that could be traceable on images. In order to compensate for the low quantity of contrast agents in desired sites, surface engineering has to be done to enhance relaxation rates. As many factors such as magnetic field strength can affect relaxation rates of NPs, knowledge of the relation between field strength and relaxation rates is essential to compare results of different fields and choosing an optimum agent for a specific field. In this study, we evaluate the effects of magnetic field strengths of 0.35, 1.5, and 3 T on relaxation rates of PEGylated SPIOs. Longitudinal and transverse relaxation rates of all samples with various concentrations were analyzed quantitatively on appropriate spin-echo sequences. Our results suggest that the increasing of the field strength leads to a marked decrease of longitudinal relaxivity. In the case of transverse relaxivity, all NPs showed an increase between 0.35 and 1.5 T. Upon further increasing the field strength, relaxation rates only slightly increased except for two samples that showed saturation.