New Pacemakers Prove MRI-Proof
With some precautions, modern models are immune to scanner's deadly influence
According to both medical wisdom and regulatory decree, magnetic resonance imaging (MRI) scans and implanted heart devices such as pacemakers do not mix. Henry Halperin, an associate professor of medicine, radiology, and biomedical engineering at Johns Hopkins University School of Medicine in Baltimore, bluntly sums up the problem: "It is feared that the electromagnetic fields of the MRI may heat up metal components or pull on and dislodge the device, causing tissue damage, device malfunction, or possibly death."
Still, forgetful or comatose patients have inadvertently gotten scans, with more than two dozen deaths thought to be associated with the procedure. How many deaths were really due to the scan is anybody's guess; then again, nobody knows how many lives might be saved if patients with implants could get diagnostic MRI scans, which are regarded as the best imaging technology for the diagnosis of many cancers; diseases of the brain, head, and neck; and many cardiovascular conditions. It is estimated that half of patients with pacemakers become candidates for scanning at some point in their lives [see photo, " Safe Inside"].
For those who need it most, an option is now at hand. As part of an industry-supported study published in the 3 August issue of Circulation, Halperin and his colleagues reported that pacemakers made after 2000 can go through MRI machines safely if a team of specialists, including a cardiologist, supervises, following a specific safety protocol.
In a typical MRI, a 1.5-tesla magnet strings the body's protons tautly along the magnet's lines of force. Then radio frequency waves twang the protons out of alignment; when they snap back, they produce an RF signal from which an image of the body's organs is constructed. With all those electromagnetic fields, an electronic device that has an antenna-like electrode lead running into a human heart would seem, on the face of it, a dangerous thing.
The evolution of pacemaker technology and the way that new implants interact with an MRI is what makes scanning possible now, according to the researchers. At about 40 grams, modern pacemakers are, on average, just a fifth the weight of their ancestors, and the pull they're subjected to from a 1.5-T machine is only about the weight of two golf balls--hardly enough to dislodge the device.
But more striking was the discovery, in both animal and human subjects, that when the electrodes absorbed energy from the scanner's RF field, their temperature rose just 5 degrees C--hardly the flesh-cooking inferno doctors feared. According to the Johns Hopkins study, the lead, which runs from the pacemaker to the heart, is too short to couple well with the RF field, and capacitors in the device filter out a good deal of the energy that would cause heating.
All this good news isn't exactly welcome at Biophan Technologies Inc., a West Henrietta, N.Y., company founded in the late 1990s with the sole purpose of developing technologies that can make implants MRI-friendly. Michael Weiner, the company's chief executive officer, finds fault with the Johns Hopkins study, saying the heating result depends on where and when you measure the temperature. The researchers took the temperature at the pacemaker's electrode tip, which is metal and "works like a heat sink," Weiner says. "It's the tissue a few millimeters away that heats up." Because the study's subjects had gotten their implants just four weeks previously, he adds, not enough fibrous scar tissue had formed around the tip to furnish a potential hot spot.
Besides heat and the pull of the magnet, there are at least two other potential problems with the scanning of pacemakers. First, the RF wave front that sweeps over the body 200 to 300 times per second can set up a heart-quickening voltage gradient along the lead to the heart. Second, the current the RF field induces in the device could also, in principle, reset the pacemaker's rate. The Hopkins researchers found no evidence of either problem but acknowledged that they examined pacemakers in only 24 volunteer patients.
The safety protocol outlined for the Johns Hopkins study begins with the conventional remote testing of the implanted device--a pacemaker or a defibrillator. Next, doctors explain the potential risks to the patient and then turn off the device or put it in safe mode, using the device's built-in wireless link. They then scan the patient with an MRI machine in the presence of a cardiologist. Finally, the cardiologist uses the wireless link to check the implant, to make sure it still functions properly.
Johns Hopkins's Halperin says his group's study doesn't threaten Biophan's business. "We tested existing models, but the problem isn't solved, because each new device that comes out would have to be tested separately," he notes. "Besides, the monitoring protocol we use is fairly involved, and with what Biophan is talking about, you wouldn't have to do any of it. Everybody who needed an MRI could get one anywhere, instead of having to go to a special center that can take precautions."
Biophan's safety technologies include an in-lead RF filter--developed, by the way, at Johns Hopkins and licensed exclusively to Biophan--that reduces heating of the electrode in the heart by more than 95 percent. It also has what the company calls an anti-antenna. Normally, the pacemaker's lead makes a circuit, called the primary loop, by conduction from the electrode in the heart, through the patient, and back to the pacemaker. The anti-antenna, a structure in the lead, provides a reverse loop to cancel out any voltage gradient that might build up along the primary loop.
Other companies are working on similar technologies. Minneapolis-based Medtronic Inc., the leading pacemaker company and a backer of the Johns Hopkins research, says its pacemakers and defibrillators will be fully safe for MRI scanning by next year.