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December 4, 2006

Hot Issue — MRI and Implantable Devices
By Beth W. Orenstein
Radiology Today
Vol. 7 No. 24 P. 8

Researchers are investigating whether some pacemakers and cardioverter defribrillators are safe for use in MRI’s gradient magnetic fields.

More than 3 million Americans have pacemakers to keep their heart beat in regular rhythm or with implantable cardioverter defibrillators (ICDs) to shock their heart should they suffer serious abnormal rhythms.

With the population aging and advances being made in disease detection and prevention, the number of people with such implantable cardiac devices is only going to increase. Experts estimate that at least one half of patients with implantable cardiac devices will eventually have some other medical issues that call for an MRI exam.

“MRI is probably the best imaging modality available today for a lot of different reasons,” says Henry R. Halperin, MD, MA, professor of medicine, radiology, and biomedical engineering at The Johns Hopkins University School of Medicine in Baltimore. “MRI is very useful in diagnosing and managing a large number of diseases, including cancer and stroke.”

The problem is that until recently it was widely accepted that MRI exams and implantable cardiac devices were absolute contraindications. The FDA lists pacemakers and defibrillators as contraindications to MRI procedures and device manufacturers warn against using MRI in patients in whom they have been implanted.

However, physicians at numerous medical centers, including The Johns Hopkins and the Oklahoma Heart Institute, are finding that pacemakers and even ICDs and MRI exams can be acceptable for some patients under certain conditions.

One fear is that the radio frequency (RF) energy emitted by the MRI scanners would heat the wires leading from the pacemaker or ICD to the heart and burn the surrounding tissue. Another concern is that because MRI systems emit electromagnetic fields, they could confuse the electronic implants and cause them to stop working or activate at inappropriate times.

A third concern is that the gradient magnetic fields used in the MRI process may directly stimulate the heart to contract and inadvertently pace the patient’s heart by the MRI process itself.

New Devices
Halperin says it appears that those fears may have been more applicable to earlier pacemakers. In recent years, pacing devices have been made more resistant to electromagnetic energies because of the growing use of wireless phones and other electronic devices. “It turns out that a lot of the same considerations that protect implantable cardiac devices from cell phones also protect them from MRI scans,” he says.

Frank G. Shellock, PhD, FACC, FACSM, director of MRI Studies of Biomimetic MicroElectronic Systems (BMES) Implants at the National Science Foundation, BMES Engineering Research Center, University of Southern California and Institute for Magnetic Resonance, says newer pacemakers have more sophisticated circuitry, making them much less likely to be affected by electromagnetic interference.

As an electrophysiologist, Halperin says he became interested in the dilemma of MRI exams being unsafe for patients with implantable cardiac devices about four or five years ago. He and colleagues at Hopkins began testing devices to see whether they could safely be used in the scanner.

A variety of inpatient and outpatient services at Johns Hopkins Hospital referred patients with pacemakers or ICDs who needed MRI exams and who, for different reasons, had no other imaging alternative.

From February 2003 to September 2005, the researchers, led by Saman Nazarian, MD, scanned 55 patients, 31 who had permanent pacemakers and 24 who had an ICD for at least six weeks. A radiologist with more than 10 years of experience with MRI and a cardiac electrophysiologist were present during all the scans.

During the scans, the pacemakers were reprogrammed and the defibrillator shocking action turned off. As a safety precaution, the researchers also cut the strength of the RF energy in half. The imaging results were useful for all conditions outside the chest and 93% of those involving the heart or upper body, Halperin says.

The researchers found that none of the pacemakers manufactured after 1996 or defibrillators manufactured after 2000 were affected by the scanners.

Since their study was reported in the September 19 issue of Circulation, the researchers have scanned another 50 patients with similar results, Halperin says.

No Blanket Approval
While their research shows some patients with pacemakers and ICDs may be able to undergo MRI scans safely under certain circumstances, Halperin says it should not be seen as a blanket approval for all patients with implantable cardiac devices. He recommends that MRI scans be done only on those patients with the devices that have been tested and proven safe, with physicians who have special training in MRI safety present, and with MRI scanners of the same type and magnetic strength as those used at Johns Hopkins. “Because we haven’t had any problems with the patients we tested so far doesn’t guarantee we won’t have any problems in the future,” Halperin says.

Shellock, who directs the for-profit MRIsafety.com, and colleagues have also been testing pacemakers and ICDs on a device-by-device basis as have other groups. In one study in the American Heart Journal in February, Shellock’s group reported findings from tests on a modern cardiac pacemaker (INSIGNIA 1 PLUS, Model 1298 and FINELINE II, Model 4471, pacing leads; Guidant Corporation, St. Paul, Minn.) in vitro at 1.5 Tesla; they found only minor magnetic field interactions and no safety risks. Another article is pending publication in the Journal of Cardiovascular Magnetic Resonance pertaining to pacemakers and ICDs from St. Jude Medical, which is headquartered in St. Paul.

Tricky Study Conditions
Shellock, like Halperin, says the findings of these studies can be applied only to specific pacemakers and the MRI conditions used in their investigations.

Emanuel Kanal, MD, FACR, FISMRM, director of Magnetic Resonance Services and professor of radiology and neuroradiology at the University of Pittsburgh Medical Center (UPMC) and chair of the American College of Radiology’s (ACR) MRI Safety Committee, says trying to precisely reproduce the exact conditions under which the pacing device and leads had been tested and found safe may be clinically impossible. “That is one of the major clinical limitations of these studies,” he says. That is also one reason why, Kanal says, many do not agree with the attempt to apply these results to broad clinical settings.

The FDA and the ACR still strongly advise against attempting to routinely scan pacemaker patients in MR scanners regardless of which model may be implanted, Kanal says. “Many potential limitations to the Hopkins studies exist, not the least of which is the fact that there is such substantial variation in the physical layout of how pacing devices and their wires or leads are implanted in patients, which may significantly alter their safety outcomes in MR imaging studies.”

Adequate studies to ensure that no thermal injuries occur with MR scanning of humans with pacemakers are not yet available, Kanal says.

Because of the studies’ limitations, Halperin, Shellock, and Kanal agree it is important to develop a device that is approved for use in the MR environment.

The life span of an ICD is approximately five years while a pacemaker’s is roughly eight years, Halperin says. Approximately 500,000 new devices are implanted in patients every year. “If they started coming out with devices that proved to be MRI-safe today, in 10 years, all devices would be MRI safe.”

Several manufacturers are attempting to develop implantable cardiac devices that would be acceptable for MRI procedures. One of them, Medtronic Inc., based in Minneapolis, says it has developed a pacemaker designed to be acceptable for use with MRI equipment and is seeking FDA approval for the first clinical study of the device.

Tracy McNulty, a spokeswoman for Medtronic’s Cardiac Rhythm Disease Management division, says the device that Medtronic is developing to be acceptable for MRI is being designed from the ground up. “MR safety is not simply a software upgrade to a patient’s currently implanted pacemaker,” she explains. “MR safety would require an entire system [device, leads, and internal components] that has been designed and tested for use in the MR environment.”

Also, McNulty says, the device would have to pass rigorous scientific testing to address the many variables that can impact patient safety before receiving approval.

Although studies such as the one at Johns Hopkins and others in peer-reviewed literature show that some devices are acceptable for MRI, Medtronic’s position on their safety has not changed, McNulty notes. “We do not consider MRI scans to be safe until these devices and leads are specifically designed for use with MRI technology and [we won’t] until they receive the necessary approvals from regulatory authorities,” she says.

Medtronic is interested in developing implantable devices acceptable for MRI procedures because it estimates that 50% to 75% of patients with them could benefit from MRI studies over the lifetime of their devices.

Halperin says the manufacturers of pacemakers and defibrillators “see the handwriting on the wall,” and that “the first company that comes out with an MRI-safe device will have a definite advantage.”

Research is also underway at UPMC on an RF-shielding device that could be used to cover the chest of the patient with a pacemaker while scanning other body parts. The FDA has approved the device to shield anatomic regions such as the arms to decrease certain imaging artifacts, but it has not been approved as a safety device or to isolate implants and devices.

Kanal has tested the RF shield using phantoms and found that it kept the pacemakers’ leads from heating. The researchers are about to start human testing.

Shielding Device
The RF shield, made by MTW Materials of Passaic, N.J., is composed of the same material used in stealth bombers to render them less detectable on radar. It works by safely absorbing and dissipating the RF energy of the MR scanner. If a patient had a pacemaker or ICD, the shield could be wrapped around his or her chest and held in place with a buckle.

Obviously, the drawback of the shield is that it would not allow imaging of the chest. However, Kanal does not believe that would be a problem in the majority of cases. At least 90% of the MRIs performed in the United States today are of the brain, the cervical and lumbar spines, and the major joints (knees and shoulders). So, if this shield for devices and implants from RF energy in the MRI environment is approved, it could prove quite useful, he says.

— Beth W. Orenstein is a freelance medical writer and a regular contributor to Radiology Today. She writes from her home in Northampton, Pa.

New Terminology With Regard to MRI and Implants and Devices
MR Safe: an item that poses no known hazards in all MRI environments. Safe items include nonconducting, nonmagnetic items such as a plastic Petri dish. An item may be determined to be MR Safe by providing a scientifically based rationale rather than test data.

MR Conditional: an item that has been demonstrated to pose no known hazards in a specified MRI environment with specified conditions of use. Field conditions that define the specified MRI environment include static magnetic field strength, spatial gradient, dB/dt (time varying magnetic fields), radio frequency (RF) fields, and specific absorption rate. Additional conditions, including specific configurations of the item, may be required.

For MR Conditional items, the item labeling will include results of testing sufficient to characterize the behavior of the item in the MRI environment. In particular, testing for items that may be placed in the MRI environment should address magnetically induced displacement force and torque and RF heating. Other possible safety issues include but are not limited to thermal injury, induced currents/voltages, electromagnetic compatibility, neurostimulation, acoustic noise, interaction among devices, and the safe functioning of the item and the safe operation of the MR system. Any parameter that affects the safety of the item should be listed and any condition that is known to produce an unsafe condition must be described.

MR Unsafe: an item known to pose hazards in all MRI environments. MR Unsafe items include magnetic items such as a pair of ferromagnetic scissors.

— Source: American Society for Testing and Materials (ASTM) International, Designation: F 2503-05. Standard Practice for Marking Medical Devices and Other Items for Safety in the Magnetic Resonance Environment. ASTM International, 100 Barr Harbor Drive, P.O. Box C700, West Conshohocken, PA 19428. 2005.