Solid Contrast Agent Enables Long-Term MRI

An injectable contrast agent for magnetic resonance imaging that coalesces into a solid now enables long-term monitoring of oxygen levels in tissues without the need for an invasive procedure, which is required to implant standard oxygen electrodes or a solid MRI contrast agent (Proc. Natl. Acad. Sci. USA 2014, DOI: 10.1073/pnas.1400015111). The new material maintains the concentration of the contrast agent in the tissue of interest for extended periods of time, permitting repeated quantitative measurements. Michael J. Cima and coworkers at Massachusetts Institute of Technology make the sensors from a saline suspension of oxygen-responsive dodecamethylpentasiloxane embedded in a support matrix of polydimethylsiloxane (PDMS). In rats, the injected PDMS particles coalesced into a solid sensor “depot” in muscle tissue, and there were no signs of an adverse tissue reaction. By using the right combination of MRI pulse sequences, the signal from PDMS is eliminated and the oxygen-responsive signal is preserved. The researchers used the sensors to measure partial pressure of oxygen in the tissue of the rats as they breathed gas streams containing different amounts of oxygen or as they experienced restricted blood flow to a limb. The sensors remained functional for at least a month after injection. By using different contrast agents and support materials, the same strategy could be used to make sensors optimized for other analytes, the researchers suggest.