Protein Cage Relaxivity Measurement for Magnetic Resonance Imaging Contrast Agents

Contrast agents are employed to enhance the differentiation of diseased cells or lesions from normal tissues in magnetic resonance imaging (MRI). Protein cages have been explored as templates to synthesize superparamagnetic MRI contrast agents for decades. The biological origin imparts natural precision in forming confined nano-sized reaction vessels. With natural capacity to bind divalent metal ions, ferritin protein cages have been used for the synthesis of nanoparticles containing MRI contrast agents inside their core. Furthermore, ferritin is known to bind transferrin receptor 1 (TfR1) which is overexpressed on specific cancer cell types and could be used for targeted cellular imaging. In addition to iron, other metal ions such as manganese and gadolinium have been encapsulated within the core of ferritin cages. To compare the magnetic properties of ferritin loaded with contrast agents, a protocol for calculating the contrast enhancement power of protein nanocage is required. The contrast enhancement power is demonstrated as relaxivity and can be measured using MRI and solution nuclear magnetic resonance (NMR) methods. In this chapter, we present methods for measuring and calculating the relaxivity of ferritin nanocages loaded with paramagnetic ions in solution (in tube) with NMR and MRI.