Boosting the benefits of MRI for people with medical implants

A redesign of the wire at the core of the leads that carry signals between implanted medical devices and target structures significantly reduces the heat generated when standard wires are exposed to radio frequency (RF) energy used in MRI.

The system developed at the Martinos Center for Biomedical Imaging at Massachusetts General Hospital (MGH) in Boston, Massachusetts, US is described in a paper published in Scientific Reports.

“Clinical electrical stimulation systems such as pacemakers and deep-brain stimulators are increasingly common therapies for patients with a large range of medical conditions, but a significant limitation of these devices is restricted compatibility with MRI,” said Dr Giorgio Bonmassar of the Martinos Center.

“The tests performed on our prototype deep-brain stimulation lead indicate a threefold reduction in heat generation, compared with a commercially available lead. The use of such leads could significantly expand how many patients may safely access the benefits of MRI.”

For many years the main limitation to using MRI in patients with implanted devices was that those containing ferromagnetic metals might be dislodged. Devices are now available that avoid using such metals.

However, the RF energy used in MRI can increase the electrical current induced in the non-magnetic metallic wires at the centre of device leads, producing heat that can damage tissues at the site where a stimulating signal is delivered.

Even though the US Food and Drug Administration (FDA) has authorised a group of “MR conditional” devices that can be used, these are limited to low-power scanners.

It is estimated about 300,000 patients worldwide a year cannot have MRI exams because of implanted devices.

The wires designed by the team use resistive tapered stripline (RTS) technology that breaks up the RF-induced current increase through an abrupt change in the electrical conductivity of wires made from conductive polymers, a similar “cloaking” technique to that used in some stealth aircraft.

After calculating the features required to produce a lead that would minimise heat generation, the investigators designed and tested a deep-brain stimulation device with such a lead in a standard system used for MRI testing of medical implants, namely a gel model the size of an adult human head and torso.

Compared with a commercially available lead, the RTS lead generated less than half the heat produced by exposure to a powerful MRI-RF field, a result said to be within FDA limits.

The ability to conduct MR examinations on patients with deep-brain stimulation implants would significantly improve the critical process of ensuring the signal is being delivered to the right area, something not possible with CT imaging.

“For epilepsy patients and their providers, brain MRIs could provide much more accurate information about the sites where seizures originate and their relation to other brain structures, maximising the effectiveness and improving the safety of implants that reduce or eliminate seizures,” study co-author Emad Eskandar said in a statement.

The team is now pursuing an FDA Investigational Device Exemption to allow clinical trials of devices with RTS leads.