JILA researchers have developed a laser-based source of terahertz radiation that is unusually efficient and less prone to damage than similar systems. The technology might be useful in applications such as detecting trace gases or imaging weapons in security screening.

JILA instrument for generating terahertz radiation. Ultrafast pulses of near-infraredlaser light enter through the lens at left, striking a semiconductor wafer studded with electrodes (transparent square barely visible under the white box connected to orange wires) bathed in an oscillating electric field. The light dislodges electrons, which accelerate in the electric field and emit waves of terahertz radiation. At right is a close-up of the electron source.

Customized microscopic magnets that might one day be injected into the body could add color to magnetic resonance imaging (MRI), while also potentially enhancing sensitivity and the amount of information provided by images, researchers at the National Institute of Standards and Technology (NIST) and National Institutes of Health (NIH) report. The new micromagnets also could act as “smart tags” identifying particular cells, tissues, or physiological conditions, for medical research or diagnostic purposes (www.nature.com). NIH has already filed for a patent for the micromagnets. The micromagnets are compatible with standard MRI hardware.

NIST and NIH investigators have demonstrated the proof of principle for a new approach 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 a 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.

“Current MRI technology is primarily black and white. This is like a colored tag for MRI,” says lead author Gary Zabow, who designed and fabricated the microtags at NIST.

Tiny Tracking Tags

The micromagnets also can be thought of 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 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. The magnets used in the NIST/NIH studies were made of nickel, which is toxic, but was relatively easy to work with for the initial prototypes. But Zabow says they could be made of other magnetic materials, such as iron, which is considered non-toxic and is already approved for use in certain medical agents. Only very low concentrations of the magnets would be needed in the body to enhance MRI images.

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.