ATLANTA—For patients with implanted cardiac devices, MRI has never been a viable way to obtain diagnostic tests due to device contraindications and risks. However, new research has found that MRI testing in this patient population can be performed at little or no risk, Robert H. Helm, MD, from Boston Medical Center said during a presentation at the American College of Cardiology (ACC) scientific sessions on Monday.
“The data for imaging such patients is limited and controversial,” Helm explained. To amend this, Helm studied the safety of MRI in patients with implantable cardioverter-defibrillators (ICDs).
Helm’s evaluations conveyed that certain safety measures utilized during MRI can make diagnostic testing feasible for patients--exposing them to little or no blatant risk. However, Helm said that the procedure must be performed using the right protocols to reduce this risk.
“Pacemakers and ICDs are an absolute contraindication for undergoing MRI imaging,” said Helm, and this, he explained, is based on data assorted with imaging patients in unsafe MRI environments.
According to Helm, 30 million MRI exams are performed in the U.S. annually. In comparison, the use of ICDs and pacemaker devices as therapy for cardiac patients is also on the rise. Helm said that 50 to 70 percent of patients implanted with an ICD will have a need for clinical indication for MRI at some point and approximately 200,000 patients will be denied an MRI due to the device.
Over the years, pacemaker technology has advanced. However, Helm said that “magnets in cardiac devices have a long history together” and can increase the risks of these ICD patients by having the potential to temporarily alter pacing mode.
Helm acknowledged three definitions in regards to safety while performing MRI for patients with implantable devices:
- MRI is safe: patients have a first device that causes no hazard in any MRI environment. This would include non-metallic, non-electrical or non-magnetic devices such as intravenous tubes;
- A device that causes no hazard in a specific MRI environment: one that is specified according to the static magnet field strength, spatial gradient and radiant field, radiofrequency energy and the specific absorption rate; and
- Unsafe devices: these pose a hazard in all MRI environments.
Today, only one pacemaker, which is approved for use in Europe, is conditioned to undergo MRI. All other pacemakers and defibrillators are deemed unfit for MRI.
During the presentation, Helm gave an overview of the mechanisms associated with both MRI and ICD/defibrillators that clash and prevent patients from undergoing diagnostic testing.
According to Helm, one issue is that the static magnetic fields associated with MRIs hold a magnetic force that is fifty times the earth’s magnetic field. “It is the force of this field that exerts attractive forces on magnetic objects—pacemakers/defibrillators” that can cause rotation and interference of the device.
Helm said that the gradient magnetic field of MRI has the ability to turn MRI acquisition on and off, producing a reduction of currents within the device.
To properly evaluate MRI as a safe, practical option for these patients, Helm said that clinicians must “weigh the benefits and make sure they outweigh the risks.” He said that physicians need to analyze whether implantable devices are in fact a strong contraindication for MRI ensuring that there is “no alternative to acceptable imaging modality.”
Additionally, a review of patient history is important to be able to pinpoint previous ICD failure rates, such as abandoned leads.
To prep an ICD patient for the possibility of MRI, Helm said the device should be programmed to be “less vulnerable to the MRI environment.” For patients who are not pacemaker dependent, the device can be powered off, while in dependent patients, the device can be switched into asynchronized mode. He said that the “bells and whistles,” which he referred to as magnet and rate response, should be disabled to alleviate further magnetic field concern.
According to Helm, 300 to 400 patients have already undergone MRI with ICD at various centers. The research has shown that there “have been no significant adverse effects.”
To achieve successful MRI without creating adverse events, Helm said the following protocols should be implemented:
- Lowering the static magnetic fields to 1.5 tesla or lower;
- Minimizing the total specific absorption rate to be less than two watts per kg;