Cheap, fast, and portable MRI machines could be on the horizon, thanks to a new technique that uses a simple gas-filled chamber to detect the wobbling of atomic nuclei.
Magnetic resonance imaging (MRI) works because atomic nuclei act like small bar magnets. If a nucleus does not line up with an applied magnetic field, it wobbles, or "precesses," much as a spinning top wobbles under the pull of gravity. The wobbling nuclei generate their own oscillating magnetic field, which can be used to create an image. For example, a medical MRI scanner is tuned to twirl hydrogen nuclei, so it can map the abundance of water molecules in living tissues.
Conventional MRI machines sense the oscillating magnetic fields via a coil of wire. But such machines require large, pricey magnets to produce a detectable signal. Alternatively, nuclei precessing in weaker fields can be tracked with a loop of superconductor known as a SQUID, which can detect extremely weak magnetic fields. But SQUIDs must be cooled to near absolute zero, making SQUID-based systems bulky and expensive.
Now, Igor Savukov and Michael Romalis of Princeton University in New Jersey have detected the oscillating field with an exquisitely sensitive sensor called an "atomic magnetometer." Their device consists of a glass chamber filled with potassium vapor. Because of the arrangement of the electrons in potassium, each atom acts like a little magnet, too, and the researchers line them up with a strong "pump" laser. The atoms wobble when exposed to even a tiny magnetic field, and the researchers detect their precession by shining a second "probe" laser through them. Placing a water sample next to the magnetometer, the researchers measured the oscillating field produced by protons exposed to a magnetic field, as they describe in an upcoming issue of Physical Review Letters.
The new scheme requires neither an enormous magnet nor cryogenics, so it can be portable and cheap, the researchers say. It also may open the way for quick, one-shot MRI's instead of tedious scans.
Eventually, the new approach could lead to MRI systems that cost "a few thousand dollars compared to a few million," says Dmitry Budker, a physicist at the University of California, Berkeley, who is also developing atomic magnetometers. Ronald Walsworth of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, says practical challenges must be overcome to transform the idea into a technology. But he adds, "Romalis is to be credited for doing some of that hard engineering work."