Magnetic resonance imaging, or MRI, is a mainstay of medicine and neuroscience research. It can noninvasively probe deep inside tissues and gives information on the presence of specific chemicals. But because the magnetic forces that it detects are so tiny, MRI isn’t very sensitive: it typically reveals structures on the millimeter to submillimeter scale.
Now researchers at IBM Almaden Research Center, in California, have developed an MRI scanner with resolution 100 million times better than that–good enough to image individual viral particles. With further refinements, the technique could one day be used to generate 3-D images of individual molecules.
“The dream of imaging a single molecule is something that keeps chemists up at night,” says John Marohn, an associate professor of chemistry and chemical biology at Cornell University. “If you had this tool, there’s no end of things that you could do with it, and there’s no end to the good that would come of it.”
MRI makes use of the fact that the nuclei of some elements, such as hydrogen, act like tiny magnets. When an external magnetic field is applied, these nuclei rotate around the direction of the field at characteristic frequencies, generating tiny magnetic fluctuations. In a typical MRI scanner, an electrical coil detects these fluctuations and uses them to map the spatial distribution of hydrogen nuclei, generating an image of the scanned tissue.
Because MRI is so good at creating 3-D images of internal structures, scientists would like to harness it for imaging much smaller biological samples, such as individual proteins. But the detection coil doesn’t scale down very well–the smaller the coil, the lower the sensitivity–leaving smaller samples and finer resolution outside the scope of conventional MRI.
The new scanner developed by IBM harnesses an emerging technology called magnetic resonance force microscopy (MRFM). MRFM circumvents the limitations of MRI by using a physical, rather than electrical, detector to pick up on the minuscule magnetic forces generated by rotating nuclei.
“It’s a much more sensitive way of detecting the magnetism from the nuclei,” says Dan Rugar, manager of nanoscale studies at IBM Almaden Research Center and leader of the team that developed the new device.
Rugar and his colleagues place the sample to be imaged on the tip of a tiny, exquisitely sensitive silicon cantilever. Near the tip is a very small magnet. Using a microscopic wire, the researchers generate an oscillating magnetic field that causes the hydrogen nuclei within the sample to flip back and forth between attracting and repelling the magnet. The resulting physical vibrations in the cantilever are detected by a laser and used to construct an image.