BILLED as a non-invasive and harmless way to peer into tissue, MRI scanners are everyone鈥檚 idea of what high-tech medicine should be like. But if the strength of the magnetic fields used in them continues to grow, getting scanned could become a distinctly unpleasant experience.
Physicists and physiologists have long reassured people that static magnetic fields are safe at any strength because there is no known way for them to have any significant effect on living tissues. Now a team in Florida has reported intriguing new evidence that ultra-powerful magnetic fields can alter animal behaviour.
Mice placed in very strong fields ran around in circles for several minutes afterwards and showed a marked dislike for food they had learned to associate with the magnet. And the stronger the magnetic field, the stronger the food aversion. Denesa Lockwood, who carried out the study at Florida State University in Tallahassee, told 快猫短视频 that in some cases the aversion lasted up to two weeks. It seems likely, she says, that the fields were making the animals ill.
Advertisement
This is the first time anyone has exposed lab animals to magnetic fields this powerful. With a strength of 14.1 tesla, the Florida magnet is tens of thousands of times stronger than the Earth鈥檚 magnetic field. Most hospital MRI scanners run at less than 2 tesla, and some experts say that this difference means the mouse findings don鈥檛 have much bearing on what happens to people in scanners. But with field strengths on the increase, others are adamant that it鈥檚 time for doctors, manufacturers and regulators to take note.
While it鈥檚 true that few significant adverse effects have ever been reported from the current generation of MRI scanners, the next generation will use much stronger fields. A handful of US universities already have 7-tesla machines and a 9.4-tesla scanner is currently being built by the University of Illinois at Chicago. And because bigger magnets mean clearer images, further hikes in scanner strength are inevitable.
Against that backdrop鈥攁nd contrary to received wisdom鈥攊f static magnetic fields are found to affect living tissues that doesn鈥檛 mean we have to rewrite the laws of physics. Lockwood believes there is a simple reason why her mice got ill: they moved.
A static magnetic field cannot induce electrical currents or other changes in living tissues. That鈥檚 why scanners are presumed to be safe. But as soon as an animal or person moves within a field, the field is technically no longer static. Moving fields can theoretically transfer energy to electrically active tissues, and hence influence nerve cells.
To see if strong fields would affect mice, Lockwood gave them tasty sugar water before putting them in the field. After a 30-minute exposure Lockwood found that the mice avoided the sugary substance, suggesting they associated it with having a bad time in the magnet. The effects were especially marked in animals that were allowed to move inside the field.
These animals also suffered most dramatically from 鈥渃ircling鈥� behaviour. For up to five minutes after exposure, the mice would run around in anticlockwise circles. Lockwood鈥檚 colleague found that flipping the polarity of the magnet made the animals run the other way. No one knows why, but the researchers speculate that when the mice moved in the magnetic field it somehow disrupted their balance systems.
Extrapolating from rodents to people is not straightforward, but there are already signs that people who move their bodies in scanners can experience subtle symptoms. Up to 10 per cent of people scanned at field strengths of 4 tesla, which are fairly common now in research, later report a weird metallic taste in their mouth.
Moving the person into the scanner induces an electrical current in them, and the taste buds are especially sensitive to this, says Ravi Menon, director of the functional MRI lab at the University of Western Ontario in London, Canada. Other people experience dizziness or bright flashes of light. 鈥淲e don鈥檛 believe any of these effects are dangerous,鈥� he says. 鈥淭hey鈥檙e transient and we know what causes them.鈥�
But with field strengths set to get much stronger, others are far less sanguine. 鈥淭he stronger the field is, the less you should have to move to feel something,鈥� says John Moulder, a professor of radiation oncology at the Medical College of Wisconsin in Milwaukee. Moulder points out that it is impossible for someone to be completely motionless.
But Keith Thulborn at the University of Illinois at Chicago, where the 9.4-tesla system should be up and running by the end of next year, says they are considering all the theoretical possibilities and proceeding with caution.