Credit: © 2009 NAS

Magnetic resonance imaging (MRI) can provide three-dimensional information on the structure and function of biological samples, but has so far been unable to resolve objects much smaller than a few micrometres in size. Now, Daniel Rugar and co-workers1 at IBM Research Division in San Jose, California have achieved MRI resolution as low as four nanometres, by improving the relatively new technique of magnetic resonance force microscopy (MRFM).

Their MRFM apparatus works by using a magnetic tip to excite nuclear magnetic resonance in samples deposited on a sensitive cantilever. The resonance generates tiny oscillations of the cantilever that can be detected using a laser interferometer.

To demonstrate, the researchers produced high-resolution images of tobacco mosaic viruses. The cantilever was oscillated by the continual flipping of proton spins in hydrogen atoms present in the virus proteins. The researchers then used sophisticated image-reconstruction software to convert the magnetic-force measurements into a three-dimensional map of proton density, which shows nanoscale details of the virus.

The images represent a 100-million-fold improvement in volume resolution over conventional MRI. The researchers suggest they could achieve better resolution, perhaps even lower than one nanometre, by applying more accurate force-detection techniques — to the point where MRI could probe the structure of individual molecules.