Hope for portable MRI


CHEAP, hand-held MRI scanners could one day replace the cumbersome and expensive machines now used in hospitals to create three-dimensional maps of body tissue. The breakthrough is promised by the development of an atomic magnetometer, which can pick up extremely weak magnetic signals.

To generate an image, a conventional MRI scanner applies a strong magnetic field that initially aligns the spins of hydrogen nuclei within the body's water molecules. Zapping the patient with short bursts of radio waves knocks these spins out of alignment, and as they get back into line they wobble or "precess" like spinning tops. This generates an oscillating magnetic field that can be picked up by a magnetic sensor and used to map the tissues.

But these oscillating fields are weak, and to ensure that they are at least strong enough to be picked up by the detectors used in conventional MRI machines the applied magnetic field has to be huge. That requires large and expensive superconducting magnets that must be chilled to near absolute zero.

Cheaper, less powerful magnets are enough if a sensitive superconducting sensor called a SQUID is used to pick up the nuclear response. But in this case the sensor has to be chilled close to absolute zero, again requiring a bulky cooling system that is laborious and costly to maintain. Either way, the scanner ends up neither cheap nor portable.

But that could change, say Igor Savukov and Michael Romalis at Princeton University. They have shown that a sensitive atomic magnetometer can detect magnetic signals from water without giant magnets or complicated cooling systems.

Their sensor consists of a glass cylinder 4 centimetres wide filled with hot potassium vapour and suspended in helium gas. Savukov and Romalis set up the device by using laser light to align the electron spins of the potassium atoms. When they then applied a weak magnetic field to a water sample nearby, and set the hydrogen nuclei inside precessing with a radio pulse, the resulting magnetic signals from the water molecules made the potassium's electrons wobble in turn, and this could be detected by a second laser (Physical Review Letters, vol 94, p 123001).

Today's MRI scanners typically cost $1 million. If atomic magnetometers came into use, the cost could one day be slashed to tens of thousands of dollars, the researchers claim.

The new sensors could also make imaging much quicker. In a conventional scanner a patient typically has to lie still for about 20 minutes. An array of atomic magnetometers could image a body in an instant.

There are several hurdles to overcome before this becomes practical, however. The magnetometer and the sample being probed have to sit inside bulky shields to screen out disturbances from the environment. But Savukov says it should eventually be possible to do away with the shielding and build a hand-held MRI machine that images tissues inside the body as easily as a digital camera takes a photo. "It will require a lot of work, but I don't see any reason why not," he says.

Richard Bowtell, an MRI expert at the University of Nottingham, UK, suspects the technique may struggle to achieve good sensitivity. "But there could be some valuable applications even with these limitations," he says.

Source: Eurekalert & others

Last reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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