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  • Nanomagnets could sharpen MR images

Apr 2007
Caren B. Les

A new class of MRI contrast agents could lead to sharper and more accurate medical images. The contrast agent single-molecule magnet [Fe8O2(OH)12(1,4,7-triazacyclononane)6]Br8 • 9H2O, known as Fe8, has the potential to highlight tumors, lesions and the vascular system, among other structures, as long as the concentration is carefully regulated. Made up of eight iron ions that form a tight bond, the Fe8 molecule has a powerful magnetic field, enabling clearer MR images. Studies have shown that it is nontoxic at clinical concentrations. It also is water-soluble, with the design potential to have consistent properties and high contrast.

The more commonly used MRI contrast agents are easily manufactured but offer relatively low contrast and can have adverse effects on patients. The magnetic ion variety alters the nuclear properties of hydrogen in water, offering the advantage of consistent identical design, but it provides low contrast. Another type consists of the particles of thousands of atoms or crystals that alter local magnetic fields. It provides contrast variation in a larger region but has magnetic designs and properties that are difficult to control.

Studies comparing the efficacy of Fe8 and the commercially prepared contrast agent Magnevist have shown conflicting results in the agent’s relaxivity rates, which affect the quality of contrast in an image.

In an experiment, scientists from four institutions demonstrated that Fe8, at concentrations below 1.5 mM, had a rate of relaxivity that is comparable to that of Magnevist, while at concentrations above 1.5 mM, the rate was significantly lower. The study indicated that these iron-containing magnets, dissolved in water, provide suitable contrast in nonclinical images as long as the concentration is below a certain level.

Researchers at the National Institute of Standards and Technology (NIST) in Boulder, Colo., monitored the molecule’s decomposition and magnetic properties as the composition was varied. They worked with Florida State University in Tallahassee to make single-molecule magnets less than 5 nm in diameter and with the University of Colorado at Boulder to make nanocrystals in the 10- to 50-nm range.

Studies at NIST have demonstrated that molecular nanomagnets produce concentration-dependent contrast in MRI. This image shows higher brightness in the spots that are below left and center, where nanomagnets were used to alter the nuclear properties of hydrogen in water, compared with deonized water (above). Courtesy of NIST.

In the experiment, nuclear magnetic resonance relaxivity rates were obtained with a commercial 300-MHz 7-T system at room temperature within three hours of mixing. MR images of the contrast agents were obtained with a commercial imager operating at 64 MHz (1.5 T). To assess the degree of stability of the Fe8 in aqueous solution, a study on its AC susceptibility was conducted using frozen solutions. The measured properties were correlated to the observed MRI response under nonclinical conditions using imaging devices at Children’s Hospital in Denver.

There are still challenges to be overcome before Fe8 can be used as a mainstream contrast agent. Among them, scientists are looking for ways to increase its stability so that it can be more easily stored and transported.

The molecular magnets also may have applications in quantum computing and in computer memories and high-tech storage devices such as compact discs, where magnetic compounds are employed.

These single-molecule magnets represent a new class of MRI contrast agents that could bridge the gap between the commonly used gadolinium chelates and the iron oxide particles, according to Brant Cage, who led the research at NIST. Through chemical synthesis and physical analysis, the group wants to tune the magnetic properties of the magnets to incorporate the strengths of both classes, he added.

Polyhedron (2007), doi:10.1016/j.poly.2006.12.009.

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