New technique speeds nanoMRI imaging

Photo: Magnetic resonance force microscopy setup

Multiplexing technique for nanoscale magnetic resonance imaging developed by researchers in Switzerland cuts normal scan time from two weeks to two days; © H. Hostettler

NanoMRI is a scanning technique that produces nondestructive, high-resolution 3-D images of nanoscale objects, and it promises to become a powerful tool for researchers and companies exploring the shape and function of biological materials such as viruses and cells in much the same way as clinical MRI today enables investigation of whole tissues in the human body.

Producing images with near-atomic resolution, however, is immensely difficult and time-consuming. A single nanoMRI scan can require weeks to complete.

Striving to overcome this limitation, researchers from ETH Zurich in Switzerland developed a parallel measurement technique, which they report in a paper appearing this week on the cover of the journal Applied Physics Letters, from AIP Publishing. Information that normally would be measured sequentially - one bit after another - can now be measured at the same time with a single detector.

"As a loose analogy, think of how your eyes register green, red, and blue information at the same time using different receptors - you are measuring different colors in parallel," said Alexander Eichler, a postdoctoral researcher and teaching assistant in Professor C. Degen's group within the Department of Physics at ETH Zurich.

Parallel measurement is also referred to as "multiplexing." After the scan, the researchers need to be able to distinguish where each bit of information belongs in the final picture. For this reason, "different bits of information are encoded in the detector using different phases," he explained. "The term 'phase' refers to a lag in a periodic signal. The phase can be used to differentiate between periodic signals in a way similar to how color is used to differentiate between light signals in the eye."

Magnetic resonance imaging makes use of the fact that certain atoms - such as 1H, 13C, or 19F - have nuclei that act like tiny spinning magnets. When these atoms are brought into a magnetic field, they rotate around the field axis in much the same way a spinning top rotates around its vertical axis when it is not perfectly balanced.

"This rotation is called 'precession,' and it happens at a very precise frequency, known as the 'Larmor frequency,' which depends on the field strength and type of atom," said Eichler.

In a nonhomogeneous field, atoms at different locations have different Larmor frequencies. The atom's location "can be evaluated from the frequency at which it precesses, and an image of the location of all atoms can be composed," he added. "When you look at a clinical MRI picture, you see bright pixels where the density of atoms - typically 1H - is high, and dark pixels where the density is low."

To achieve this, various strategies have been developed. The research team working with Professor Degen demonstrated phase multiplexing with a particular nanoMRI technique called "magnetic resonance force microscopy" (MRFM), in which the atomic nuclei experience a tiny magnetic force that's transferred to a cantilever acting as a mechanical detector. In response to the magnetic force, the cantilever vibrates and then, in turn, an image can be assembled from the measured vibration.

"Our research overcomes one of the major obstacles toward practical high-resolution nanoMRI, namely the forbidding time scales required for sequential measurements," Eichler said. "It brings us closer to the commercial implementation of nanoMRI."
In other words, the team's work greatly accelerates the speed of nanoMRI measurements. By demonstrating parallel measurements of six data points, they have shown that a normal scan of two weeks can now be compressed to within two days.; Source: American Institute of Physics

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