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Two magnets are cheaper than one: Stanford engineers construct an inexpensive MRI scanner

There's a bargain in the basement of Stanford's Packard Electrical Engineering building: a low-cost magnetic resonance imaging (MRI) scanner. MRI scanners take sharp inner pictures of the body including the brain, spine and joints. MRI images provide better contrast in soft tissue like the brain compared with other imaging techniques such as X-ray, CT or ultrasound. But MRI scanners don't come cheap. One whole-body scanner costs $1 million to $3 million, and scan charges can exceed $1,000.

"Personally, I never liked the cost of MRI. I'm very frugal," says Steven Conolly, a senior research associate on the engineering team creating the new MRI scanner. Professor emeritus of engineering and radiology Albert Macovski inspired the project, which is led by Conolly and engineering research associate Greig Scott. One of the team's goals is to create a high-quality scanner that would sell for about $150,000.

The scanner might become useful in the developing world, says Conolly, or as a tool for basic science research.

The team's low-cost approach to building MRI scanners is practical and challenging. And after five years of work, the researchers now have their first human images in hand.

The trick, Conolly says, is to use two inexpensive resistive magnets instead of an expensive superconducting magnet.

MRI works in two steps. First, it exposes the human body to a strong magnetic field. Some elements, including hydrogen atoms inside water and body fat, respond to a strong magnetic field by lining up with it much as iron filings align with the field of a desktop magnet. Here the magnetic field needs to be very strong because hydrogen atoms don't respond as easily to a magnetic field as iron filings do. Today's MRI scanners use magnets as strong as those used to pick up cars in a junkyard.

Once the hydrogen atoms have lined up, they create their own magnetic signal. Because hydrogen atoms in different tissues have slightly different signals, the MRI scanner measures those differences, detecting contrast in an image. For this second step -- measuring the difference between, for example, a hydrogen atom inside a tumor and one inside muscle -- the magnetic field has to be extremely precise, Conolly says. The field cannot vary by more than one ten-thousandth of a percent, which means if the Earth was as flat as an MRI magnetic field, the world's highest hill would be only 20 feet high.

The only magnets available today that are both very strong and homogeneous are superconducting magnets. They are the biggest single cost in an MRI scanner. But it turns out, says Conolly, that the magnet inside an MRI scanner doesn't need to be simultaneously strong and consistent. So the team built an entire MRI scanner from scratch, using two magnets to replace the conventional superconducting magnet. The first magnet is very strong and capable of lining up the hydrogen atoms. It needn't be very precise, though, and has about 40 percent variation. "It's like using a lamp to illuminate a book," says Conolly. "The light intensity may vary over the surface of the page by 40 percent, but as long as it's bright enough, you can still read the page." The second magnet creates a homogeneous magnetic field, but it needn't be strong. In fact, it's weak, requiring the power of about two hairdryers. The MRI team turns on one magnet to line up the hydrogen atoms and turns on the other to record the body's signal.

Both magnets are simple copper resistive magnets -- made of stuff anyone might find at a hardware store. As soon as the Stanford MRI team created a working scanner, they started to take pictures. One of the team scientists, Blaine Chronik, went to the grocery store when they first started getting data "and just looked for interesting things to image," says Conolly. "We tried tomatoes," said Sharon Ungersma, a graduate student on the project, "and grapes and other foods." Bacon showed the most interesting contrast. The fat and the muscle stripes on the bacon showed up in stark contrast to each other. Soon the team, including graduate students Hao Xu and Nate Matter, started imaging human hands and wrists. Image slices show the carpal bones, the tendons and the soft tissue. "The hand images are definitely not yet at the same quality as conventional MRI scanners," says Conolly, "but we can actually talk about anatomy now. And we can measure improvements."

By this summer, Conolly anticipates the images will be far better. A new-and-improved homogeneous magnet is almost completed. This one is bigger and can work at higher field strength. It'll fit a knee, not just a wrist. It's more energy-efficient and, fittingly, even cheaper. The team is still working to attain the homogeneity that they need in the low-strength magnet. Because they create the magnet by coiling copper tape, each turn needs to be placed precisely with respect to the other coils. "There are more than a hundred turns of copper tape in the coils, so every little kink and bind accumulates," says Conolly. "The outside diameter can be off by about 50-thousandths of an inch from ideal -- which is pretty good, but it's still a problem." Ungersma is now creating a new set of coils to fix this problem.

The entire team is busy trying to improve image quality. The team is also excited about the prospect of basic scientific research, which opens the door for exploring many new contrast mechanisms.

The team has received grants to create scanners for imaging the knee, brain and breast. One of the benefits of the low-cost MRI scanner is that hospitals could use smaller scanners specific to certain parts of the body, instead of buying a second whole-body scanner. With the Stanford technology, MRI breast imaging might become cost effective for breast cancer screening. In an age of rising medical costs, the technology may make MRI available to a larger clientele.


By Louisa Dalton

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