MRI with 80-nm resolution, far better than for the best medical scans,
has been achieved with a device that combines atomic force microscope
(AFM) and nuclear magnetic resonance (NMR; also known as magnetic resonance
imaging, or MRI) technology.
In the hybrid methodology called magnetic resonance force microscopy
(MRFM), a tiny magnetized particle is attached to a cantilever which
is then brought near a sample which surrounded by a coil that emits
radio waves. When a tiny magnetic domain in the sample feels just the
right amount of magnetic field from the nearby coil and magnetic particle
it will vigorously interact with them resonantly. (The tiny volume being
probed is referred to as a voxel, and the sample-coil-particle combination
is equivalent to the setup in a standard MRI machine for imaging, say,
The sample-particle resonant interaction causes the cantilever to oscillate
(the particle on the cantilever is like a man bouncing resonantly, higher
and higher, on a diving board). The oscillating cantilever, monitored
with a laser beam, is then scanned from place to place, filling out
a two-dimensional and then a three-dimensional map of the resonant interaction.
(The scanned, oscillating cantilever plus laser readout is the AFM part
of the setup.)
The goal is not to help surgeons (the best medical MRI has a spatial
resolution of about a tenth of a millimeter) but to be able to scan
and image small objects---especially particles of biological importance,
such as viruses and proteins---with atomic-scale resolution. In other
words, you would like to increase the sensitivity so as to map the presence
of single spins. The voxel in this case would be shrunk to less that
than 1 nm.
A new experiment at the University of Washington is far from reaching
this goal, but researchers have improved sensitivity by a factor of
almost 10,000 from the time of the earliest MRFM imaging papers in 1996.
(For a report from 1997, see Update
313). The higher sensitivity in general comes by shrink the apparatus
and cooling things (currently, to 80 K) as much as possible, the better
to read out the oscillations and position the sample with greater accuracy.
The Washington voxel of 80 nm---how big is it? One of the team members,
John Sidles (206-543-3690, firstname.lastname@example.org) says that about
a million of these voxels could fit inside a typical blood cell. (Chao,
Dougherty, Garbini, Sidles, Review of Scientific Instruments, May
2004; also see website.)
Other groups are working in this area and are attempting to marshal
the requisite equipment needed for single-spin imaging. According to
Joseph Shih-hui Chao, one of the authors, this would include millikelvin
temperatures, 30-nm-sized magnetic particles, sub-nm positioning accuracy,
and yet softer cantilevers.