Characterization of Magnetic Nanoparticle-Seeded Microspheres for Magnetomotive and Multimodal Imaging.

Magnetic iron-oxide nanoparticles have been developed as contrast agents in magnetic resonance imaging (MRI) and as therapeutic agents in magnetic hyperthermia. They have also recently been demonstrated as contrast and elastography agents in magnetomotive optical coherence tomography and elastography (MM-OCT and MM-OCE, respectively). Protein-shell microspheres containing suspensions of these magnetic nanoparticles in lipid cores, and with functionalized outer shells for specific targeting, have also been demonstrated as efficient contrast agents for imaging modalities such as MM-OCT and MRI, and can be easily modified for other modalities such as ultrasound, fluorescence, and luminescence imaging. By leveraging the benefits of these various imaging modalities with the use of only a single agent, a magnetic microsphere, it becomes possible to use a widefield imaging method (such as MRI or small animal fluorescence imaging) to initially locate the agent, and then use MM-OCT to obtain dynamic contrast images with cellular level morphological resolution. In addition to multimodal contrast-enhanced imaging, these microspheres could serve as drug carriers for targeted delivery under image guidance. Although the preparation and surface modifications of protein microspheres containing iron oxide nanoparticles has been previously described and feasibility studies conducted, many questions regarding their production and properties remain. Since the use of multifunctional microspheres could have high clinical relevance, here we report a detailed characterization of their properties and behavior in different environments to highlight their versatility. The work presented here is an effort for the development and optimization of nanoparticle-based microspheres as multi-modal contrast agents that can bridge imaging modalities on different size scales, especially for their use in MM-OCT and MRI imaging.