Injectable contrast agents or dyes are routinely used to highlight different tissues in the body or help distinguish between healthy and diseased tissue. The most common MRI contrast agents are the gadolinium chelates and the superparamagnetic iron oxides (SPIOs). The first group offer the advantage of consistent identical design but provide low contrast. The second group can provide better contrast but have irregular designs and magnetic properties.

Researchers have recently showed interest in using materials that bridge the gap between the constrained chemical and magnetic structure of the gadolinium chelates with the superparamagnetism of the SPIOs. One such class of materials are the single-molecule magnets (SMMs) – nanoscale magnetic structures that can be designed to have consistent properties and high contrast simultaneously. Among these, the superparamagnetic SMM Fe8O2(OH)12(1,4,7-triazacyclononane)6Br8.9H20, or simply Fe8, which is just 2 nm across, is a promising potential candidate and the first to be extensively explored. "However, modifications to standard SMMs will be needed for successful MRI contrast applications," stresses NIST team member Stephen Russek.

Previous studies by several research groups had reached conflicting conclusions about using SMM Fe8 for enhancing contrast in MRI because they did not take the concentration of the iron nanomagnets into account. By performing new magnetic measurements, the NIST-led team has been able to monitor the magnetic properties of the SMM Fe8 as its concentration in solution was varied.

They found that the MRI contrast correlated with a change in magnetic state of the nanomagnets as they decomposed in aqueous solution. Lower concentrations, below 1.5 millimolar, show larger contrast due to decomposition into smaller magnetic clusters. According to the team, the concentration dependence and selective decomposition of molecular nanomagnets may be advantageous and used as a possible method of varying contrast in vivo.

Molecular nanomagnets have several advantages over existing contrast agents, say the researchers. They can be modified to act as "smart" materials whose contrast can be turned on only when bound to a target molecule or cell. Moreover, they are not believed to be toxic because iron is found naturally in the body. Indeed, previous research has shown that these materials are non-toxic at the concentrations used in MRI.

NIST is working with Florida State University to make SMMs less than 5 nm across and with the University of Colorado to make nanocrystals in the 10 to 50 nm range. The collaboration is pioneering ways to manipulate and measure the magnetic properties of these compounds, as well as developing instruments to understand how contrast agents work and how to control contrast properties. The experiments were performed under non-clinical conditions using imagers at The Children's Hospital in Denver, Colorado.

The research will be published in Polyhedron.