C. L. Degen, M. Poggio, H. J. Mamin, C. T. Rettner, D. Rugar Nanoscale magnetic resonance imaging PNAS 2009, doi: 10.1073/pnas.0812068106.
We have combined ultrasensitive magnetic resonance force microscopy (MRFM) with 3D image reconstruction to achieve magnetic resonance imaging (MRI) with resolution <10 nm. The image reconstruction converts measured magnetic force data into a 3D map of nuclear spin density, taking advantage of the unique characteristics of the 'resonant slice' that is projected outward from a nanoscale magnetic tip. The basic principles are demonstrated by imaging the 1H spin density within individual tobacco mosaic virus particles sitting on a nanometer-thick layer of adsorbed hydrocarbons. This result, which represents a 100 million-fold improvement in volume resolution over conventional MRI, demonstrates the potential of MRFM as a tool for 3D, elementally selective imaging on the nanometer scale.
I think it's important to emphasize that this is a nanoscale magnetic imaging technique. The summary implies that they created a conventional MRI that has nanoscale resolution, as if they can now image a person's brain and pick out individual cells and molecules. That is not the case! And that is likely to never be possible (given the frequencies of radiation that MRI uses and the diffraction limit that applies to far-field imaging.
That having been said, this is still a very cool and noteworthy piece of science. Scientists use a variety of nanoscale imaging tools (atomic force microscopes, electron microscopes, etc.), but having the ability to do nanoscale magnetic imaging is amazing. In the article they do a 3D reconstruction of a tobacco mosaic virus. One of the great things about MRI is that is has some amount of chemical selectivity: there are different magnetic imaging modes that can differentiate based on makeup. This nanoscale analog can use similar tricks: instead of just getting images of surface topography or electron density, it could actually determine the chemical makeup within nanostructures. I expect this will become a very powerful technique for nano-imaging over the next decade.