MRI relies on the fact that atomic nuclei with an unpaired proton or neutron spin like miniature tops - a characteristic that gives them magnetic moments. When exposed to a magnetic field, these tiny magnets attempt to align their axes with it. The alignment is not exact, however, so the nuclei precess in a way that is unique to each type of atom. When a gradient magnetic field is applied, the precession depends also on the physical location of the nuclei, and this spin manipulation of the nuclei with the magnetic field is known as "signal encoding".

In a conventional MRI system, the nuclei are also hit with an RF pulse that makes them absorb and re-emit energy at a frequency dependent on their rate of precession. This energy is picked up by the RF coil, which sends the data to a computer system to generate the image. To generate a signal strong enough for the coil to detect, the magnetic field used to align the nuclei must be exceptionally strong - so large that expensive magnets are required.

The California team's breakthrough was to come up with a more sensitive detection technique, enabling MRI to be carried out with a much weaker magnetic field. Doing away with the large magnets means that the equipment can be relatively compact, and the potential for side-effects from exposure to strong magnetic fields is eliminated. The equipment could also be brought out of the hospital because there is no need for a shielded room.

"Our technique provides a viable alternative for MRI detection with substantially enhanced sensitivity and time resolution for various situations where traditional MRI is not optimal," said team leader Alexander Pines, a professor of chemistry at UC Berkeley who also works at Lawrence Berkeley.

Separation and optimization

The laser-based approach to MRI physically separates the two basic steps of MRI - signal encoding and detection - so that each can be optimized for sensitivity. For example, when the team tested the technique by imaging water flow, the water was first passed through a weak magnetic field where the nuclei were spatially encoded. It was then transported to a separate detection area where the signal was read by a pair of devices called optical atomic magnetometers.

Each magnetometer contains a sample of vaporized atoms, with each atom featuring a single unpaired electron. These lone electrons make the atoms behave like spinning bar magnets. A polarized laser beam is then passed through the vapour; the laser's interaction with the atoms - in the presence of the water - causes the angle of polarization to change. This effect correlates to the magnetization of the encoded water sample and can be measured precisely.

While the technology is unlikely to compete with conventional MRI, being small, cheap and relatively simple to use could mean that it will find its niche outside the hospital. The team hopes the invention might form the basis of a device suitable for use in countries without access to expensive imaging equipment, for example. "Our system is fundamentally simple and does not involve any single expensive component," said UC Berkeley physicist Dmitry Budker. "We anticipate that the whole apparatus will become quite compact and deployable as a battery-powered portable device."

Clearly, it's a big step from imaging water flow to imaging a person, but there is no reason why the technique should not be able to develop in this direction. "We are optimistic about the further development of our technique and applying it for medical applications in the near future," said Xu. "One of the challenges is to improve the detection efficiency since the human body is of substantial size and the magnetic field decreases when the detector is far away from the source."