Magnetic resonance imaging (MRI) is a technology widely used in medical applications. But so far, all the images of what is inside our bodies produced by MRI are just black and white. Now, a U.S. research team is developing injectable micromagnets which could bring color to MRI pictures. The researchers also point out that these ‘new micromagnets also could act as smart tags identifying particular cells, tissues, or physiological conditions, for medical research or diagnostic purposes.’ As the technique has not yet been validated on animals, several years will be necessary before it can be used on humans. But read more…
You can see above a diagram showing the magnetic structure and field diagrams: “a, Diagram of the field (black arrows) from two parallel disks magnetized to saturation by B0 (red arrow). Non-magnetic spacer elements are omitted for clarity. b, Calculated (negative) field magnitude in the mid-plane through a typical magnetized disk set. c, Calculated particle volume fraction that falls within a bandwidth about the particle’s frequency shift.” (Credit: NIST/NIH) Here is a link to a larger version of these diagrams, including the full caption.
And here are more details about these micromagnets. “Each micromagnet consists of two round, vertically stacked magnetic discs a few micrometers in diameter, separated by a small open gap in between. Researchers create a customized magnetic field for each tag by making it from particular materials and tweaking the geometry, perhaps by widening the gap between the discs or changing the discs’ thickness or diameter. As water in a sample flows between the discs, protons acting like twirling bar magnets within the water’s hydrogen atoms generate predictable RF signals — the stronger the magnetic field, the faster the twirling—and these signals are used to create images.”
This research project has been led by Gary Zabow, who works both for the Electromagnetics Division of the National Institute of Standards and Technology (NIST) and at the Laboratory of Functional and Molecular Imaging of the National Institutes of Health (NIH).
You can see on the left several scanning electron microscope (SEM) images of the microfabricated magnetic structures: “a, b, SEM images of microfabricated double-disk magnetic structures. c, d, SEM images of externally supported double-disk structures. e, Optical micrograph contrasting a particle still attached to the substrate” (Credit: NIST/NIH) Here is a link to a larger version of these diagrams, including the full caption.
Now, let’s discover how the NIST/NIH team plans to bring colors to MRI. “Unlike the chemical solutions now used as image-enhancing contrast agents in MRI, the NIST/NIH micro-magnets rely on a precisely tunable feature — their physical shape — to adjust the radio-frequency (RF) signals used to create images. The RF signals then can be converted into a rainbow of optical colors by computer. Sets of different magnets designed to appear as different colors could, for example, be coated to attach to different cell types, such as cancerous versus normal. The cells then could be identified by tag color.”
These micromagnets could also be used as “microscopic RF identification (RFID) tags, similar to those used for identifying and tracking objects from nationwide box shipments to food in the supermarket. The device concept is flexible and could have other applications such as in enabling RFID-based microscopic fluid devices (microfluidics) for biotechnology and handheld medical diagnostic toolkits. The microtags would need extensive further engineering and testing, including clinical studies, before they could be used in people undergoing MRI exams.”
This research work has been published in a recent issue of Nature under the title “Micro-engineered local field control for high-sensitivity multispectral MRI” (Volume 453, Number 7198, Pages 1058-1063, June 19, 2008). The figures above have been picked from this article.
Here is the beginning of the abstract. “In recent years, biotechnology and biomedical research have benefited from the introduction of a variety of specialized nanoparticles whose well-defined, optically distinguishable signatures enable simultaneous tracking of numerous biological indicators. Unfortunately, equivalent multiplexing capabilities are largely absent in the field of magnetic resonance imaging (MRI). Comparable magnetic-resonance labels have generally been limited to relatively simple chemically synthesized superparamagnetic microparticles that are, to a large extent, indistinguishable from one another.”
And here is the approach chosen by the NIST/NIH team. “Here we show how it is instead possible to use a top-down microfabrication approach to effectively encode distinguishable spectral signatures into the geometry of magnetic microstructures. Although based on different physical principles from those of optically probed nanoparticles, these geometrically defined magnetic microstructures permit a multiplexing functionality in the magnetic resonance radio-frequency spectrum that is in many ways analogous to that permitted by quantum dots in the optical spectrum.”
For even more information, here is a link to a Technology Review article, “Multicolor MRI for Molecular Imaging” (Katherine Bourzac, June 19, 2008). Here is the last paragraph about the possible toxicity of such injectable micromagnets. “The micromagnets, described this week in the journal Nature, have so far been made from nickel, which is toxic. But Zabow says that they could easily be made from iron, which is nontoxic and magnetic. The researchers are exploring the idea of using the particles as ’sensors of physiological conditions inside the body,’ says Zabow, but he cautions that they are only now taking the first step in that direction, planning tests in cells.”
Sources: NIST news release, June 18, 2008; and various websites
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