"It's by far the most sensitive MRI imaging technique that has been demonstrated," says Raffi Budakian, assistant professor of physics at the University of Illinois at Urbana-Champaign, commenting on combining atomic force microscopy (AFM) with magnetic resonance imaging (MRI)–magnetic resonance force microscopy (MRFM). MRFM enables 3D visualization of tiny specimens. MRI offers unparalleled 3D imaging of living tissue without inflicting damage, but with resolution limited to several cubic microns.
In 2009, Christian Degen, assistant professor of chemistry at the Massachusetts Institute of Technology (MIT), and colleagues at the IBM Almaden Research Center, built the first MRFM device capable of imaging viruses in 3D.1 On April 25, 2010, the paper reporting this ability was awarded a 2009 Cozzarelli Prize by the National Academy of Sciences.
MRFM involves attaching the sample to the end of a tiny silicon cantilever. As a magnetic iron cobalt tip nears the sample, the atoms' nuclear spins become attracted to it and generate a small force on the cantilever. Spins are repeatedly flipped, causing the cantilever to gently sway. Displacement is measured with a laser beam to create a series of 2D images, then combined to generate a 3D image. MRFM resolution is nearly as good (within a factor of 10) as that of electron microscopy. But electron microscopy damages delicate samples.
Degen and two of his students are pursuing another new approach to nanoscale MRI that uses fluorescence instead of magnetism, replacing the magnetic tip with a diamond that has a nitrogen-vacancy defect in its crystal structure. The diamond functions as a sensor because its fluorescence intensity is altered by interactions with magnetic spins.
- C.L. Degen et al., PNAS 106(5), 1313–1317, Feb. 3, 2009.