An improvement in magnetic resonance imaging has allowed scientists to view a virus that measures just 18 nanometers across, a study in the early version of PNAS reports.

A group led by Dan Rugar of the IBM Almaden Research Center in San Jose, Calif. used magnetic resonance force microscopy to detect changes in the spins of hydrogen nuclei, a resolution 100 million times better than conventional MRI, allowing them to peer at individual tobacco mosaic virus particles. The technique also provides unprecedented views of the virus's three dimensional structure.

"I think it's very spectacular work, it's really pushing the limits of imaging," said Fedor Jelezko, a physicist at the University of Stuttgart, Germany, who was not involved in the study.


An artistic view of the magnetic tip (blue) interacting with the virus particles at the end of the cantilever.
Currently, people use a grab bag of techniques ranging from x-ray crystallography to nuclear magnetic resonance spectroscopy to uncover the structure of large molecules. But "there's no all-purpose way you can go in and look at structure now," Rugar says.

To detect the flipping of the spins, the researchers attached the virus to a small silicon cantilever sensitive to miniscule forces. The cantilever was placed close to a tiny magnetic tip on the surface, while a current created an alternating magnetic field that flips the spins of protons in hydrogen nuclei. Switching the spins was like flipping over a tiny magnet, causing the nuclei in the sample to ever-so-slightly tug or push the cantilever, Rugar says. A laser tracked the cantilever motion, which was then converted into a three-dimensional image of the sample. In order to measure the tiny shifts in magnetic forces without being obscured by random noise from thermal motion of the atoms, the team cooled the virus to a chilly 300 mK.

"The reason I'm so excited about this paper is that it's an honest-to-goodness biological sample, and the imaging resolution is really good. For the first time, you can start to imagine answering some interesting questions with it," said John Marohn, a chemist at Cornell University, who did not participate in the study.

The group had previously measured the spin of a single electron, but that work only generated two dimensional images, and did not look at a biological sample. By increasing the strength of the magnetic field gradient and refining their method for converting cantilever motion into an image, they were able to boost the resolution of the technique dramatically.

Still, there's a long way to go before the technique can view individual atoms. The microscope currently has a resolution of 4 nanometers, but seeing a single atom requires resolution of 1 angstrom, Rugar said.


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Image courtesy of IBM