A Weaker, Cheaper MRI

Magnetic fields that would barely budge a compass might find tumors faster

PHOTO: VADIM ZOTEV/LOS ALAMOS NATIONAL LABORATORY

HEAD SHOTS

Four slices of researcher Vadim Zotev’s head are the first medical images made with low-magnetic-field MRI.

Researchers at Los Alamos National Laboratory have made what they say are the first images of a human brain using magnetic fields a hundred-thousandth the strength of conventional magnetic resonance imaging (MRI), paving the way for lower cost medical images that might be better at detecting tumors.

Though the resolution is much lower than that in conventional MRIs, the images ”show we have a potential for pretty good results,” says Vadim Zotev, a researcher in Los Alamos’s applied modern physics group. (That’s his head in the images.)

MRI works by ­subjecting the human body to a strong magnetic field, which causes the ­proton in the nucleus of each hydrogen atom in the body to line up along the magnetic field’s lines of force. An RF pulse briefly knocks the ­protons out of alignment. As they snap back into position, the ­protons emit an RF signal that can be used to construct a three-dimensional image. Most MRI machines have a magnetic field of about 1.5 teslas, strong enough to yank metal objects out of the hands of the unwary.

Zotev’s machine, however, generates a magnetic field of only 46 microteslas, roughly the same strength as the Earth’s magnetic field. Few protons align at this lower strength, so he must first apply a 1-­second prepolarization pulse—at 30 milli­teslas, it’s about as strong as a small bar magnet—which primes the protons to respond to the microtesla field. To detect the weaker signals, he uses an array of seven super­sensitive magnetometers called superconducting quantum interference devices, or SQUIDs. In a SQUID, ­electrons are in an odd ­quantum state that allows individual ­electrons to move in two directions at once and interfere with themselves. The amount of interference depends on the strength of an external magnetic field and translates into a measurable resistance to the flow of current in the SQUID.

A weak magnetic field MRI machine might cost as little as US $100 000 compared with $1 million or more for a standard MRI system

Because it needs fewer costly magnets, a weak­magnetic-field MRI machine might cost as little as US $100 000, compared with $1 million or more for a standard MRI system, says Zotev. But perhaps the most exciting thing about low-field imagers is that they can also perform another imaging technique, magneto­encephalography (MEG), which, conveniently, also relies on SQUIDs. MEG measures the magnetic fields produced by brain activity and is used to study seizures. Putting the two imaging modes together could mean matching images of brain activity from MEG with images of brain structure from MRI, and it might make for more precise brain surgery.

Low-field MRI has other advantages, says John Clarke, a physicist at the University of California, Berkeley, who uses a single-SQUID MRI device to image tissue samples. ”I’m personally quite excited about the idea of imaging tumors” with low-field MRI, he says. The difference between cancerous and noncancerous tissue is subtle, particularly in breast and prostate tumors, and the high-field strengths used in conventional MRI can drown out the signal. But low-field MRI will be able to detect the differences, Clarke predicts. A low-field MRI might also allow for scans during surgical procedures such as biopsies, because the weaker magnetic field would not heat up or pull at the metal biopsy needle.

Groups in Europe and Japan are also developing low-field MRI, both for identifying tumors and for matching with MEG. Zotev is working on improving the image quality, perhaps by increasing the strength of the prepolarization field, and studying what signals might be read in low-field MRI that conventional MRI might miss. He says that, with enough focus on the engineering issues, practical devices might be ready for clinical trials within a ­couple of years.

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