Two years ago, a patient in a British hospital died of heart failure shortly after being given an MRI brain scan.
Despite being asked several times, he had failed to tell doctors that a heart pacemaker was implanted in his chest. The scanner's powerful radio waves are thought to have induced inappropriate voltages in the implant - boosting the patient's pulse so much his heart failed.
MRI images are invaluable. Their ability to show up different densities of soft tissue is particularly useful in cancer diagnosis. But the risk of such accidents means they cannot be used on patients who have implants that contain any kind of metal. This is becoming a serious problem as more people are fitted with implants to control various conditions.
But now a US company has come up with a new coating for implants that should protect such patients from dangerous MRI scanner fields. And there could be spin-offs for everyone if this paves the way for the routine use of MRI scanners during surgery - an activity currently prevented by the use of metal surgical tools.
The material, developed by biomedical devices firm Biophan of West Henrietta, New York state, does two jobs. It prevents most of the powerful radio waves reaching the internal components of the implant by reflecting them. But it also has a very high electrical resistance, preventing high currents flowing around the implant's surface and heating nearby body tissues.
But why are these radio waves needed in any case? In an MRI scanner, a strong magnetic field causes the spins of the nuclei of the hydrogen atoms in body tissue to line up in the same direction. Then, powerful radio wave pulses are directed at the target area, causing some of these nuclei to misalign.
Between pulses, the nuclei attempt to realign themselves, and in doing so re-emit some of the energy absorbed from the radio waves. The frequency of these re-emitted waves depends on the nature of the surrounding tissue, and it is this that allows the scanner to generate images, or density maps, of the body.
The problem is that electromagnetic radio waves can induce currents that are large enough to heat metal implants to over 70 °C - more than hot enough to damage surrounding tissue.
Poorly conducting nanoparticles
The pulses can also induce currents in the long, slender conductors of an implant, like those that killed the British patient in 2001. It is not only pacemakers that prevent MRI use: implanted devices such as drug pumps, nerve stimulators for controlling prostheses and stents that hold open narrowed blood vessels also cannot be scanned.
An established way to shield objects from radio frequencies is simply to surround them with a conducting metal sheath. But in living tissue, these methods do not really help, says Biophan's chief executive Michael Weiner, because they too would be heated up by the currents induced in them.
Biophan's new coating is made from a mixture of poorly conducting nanoparticles held in an insulating matrix. At less than three micrometres thick, the coating is designed to act like a mirror. It reflects most of frequencies of MRI radio waves that hit it and with a resistance in the millions of megaohms, any currents induced in it are tiny. Early studies show that the coating cuts the energy induced in an object by 89 per cent.
The company will have to conduct clinical trials before the coating can be approved by the US Food and Drug Administration. "All the materials we use are biocompatible," says Weiner.
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