When it comes to improved magnetic resonance contrast agents, what’s inside really counts. Those are the findings of researchers at the University of California, Berkeley, the Universitá del Piemonte Orientale in Alessandria, Italy, and Lawrence Berkeley National Laboratory in California. The group used a virus shell, or capsid, as a scaffold on which to hang the magnetic resonance-enhancing agent gadolinium. They showed that it’s better to attach the gadolinium to the inside of the shell than to the outside.By doing so, they created a contrast agent that’s hidden and so should have minimal impact on the biodistribution of the capsid. They also left the outside of the shell free to be functionalized to target specific tissue.When using a viral capsid to transport a contrast agent, researchers face a choice. They can bind the agent to the outside of the shell — which might interfere with the capsid distribution in the tissue being imaged — or they can attach the agent to the inside oAttaching an agent inside the shell could reduce the MRI signal because contrast agents have a large number of unpaired electrons that cause nearby water molecules to relax quickly, enhancing the MRI signal. For the effect to continue, though, water must enter and exit the neighborhood of the agent quickly, and being inside a shell might interfere with that process.To investigate the best surface for the agent, the researchers selected the bacteriophage MS2, largely because it has 32 1.8-nm-wide pores spaced around it. They knew that these holes would allow large organic molecules, such as gadolinium-chelating ligands and water, to pass into and out of the shell.They hollowed out the capsid to create an empty shell, and then they attached the contrast agent exclusively to the inside or outside via targeted biochemistries. After evaluating the performance of the two cases, they found that interior binding provided more than 50 percent higher relaxivities on a per particle basis and, therefore, a greater signal than that provided by exterior binding. The inner attachment also was more water-soluble and had greater capsid stability than its outer counterpart. The work is described in the August issue of Nano Letters.The researchers are using an optimized version of this masked payload strategy to produce tunable MRI contrast agents that are aimed at specific targets. These will be used in a number of biomedical imaging applications.