{mosimage}News Author: Lisa Nainggolan
CME Author: Désirée Lie, MD, MSEd
The American Heart Association has published a scientific statement on the safety of magnetic resonance imaging (MRI) in patients with cardiovascular devices, which has also been endorsed by the American College of Cardiology, the North American Society for Cardiac Imaging, and the Society for Cardiovascular Magnetic Resonance [1]. The document states that for most devices, if there is a good clinical indication for an MRI scan, then the benefits probably outweigh the risks.
Lead author Dr Glenn N Levine (Baylor College of Medicine, Houston, TX) told heartwire: "This is the first document that specifically addresses these issues." He explained that, over the years, decisions have been made about when it is safe to do an MRI scan with a particular device, but these have not been based for the most part on any good evidence. "When coronary stents first came out, for example, it was decided that you had to wait a minimum of six weeks before doing an MRI scan, but that was not based on any firm data. And there are still people out there who won't do an MRI ever in a patient with a stent, and that is absurd. We wanted to make recommendations that are in line with actual data and expert opinion."
The exceptions to this are pacemakers and implantable cardioverter-defibrillators (ICDs). "Whether one can safely MRI-scan a patient with a pacemaker and/or ICD has been controversial," Levine explained. "However, we didn't want to make a blanket statement that you shouldn't scan, because in certain cases MRI is the best imaging modality to resolve a clinical question. But we also wanted to be careful to stress that MRI should be done in these patients only at very experienced centers, with expertise in MRI physics, MRI safety, and electrophysiology," he noted.
For most devices, MRI is generally safe
Levine said that when his group was trying to put together this guidance, "one of the things I encouraged in our conference calls was that I did not just want to regurgitate what was on the approved labeling but rather state what the experts in the country felt and what they should be telling people."
With regard to the majority of devices, which Levine says are either nonferromagnetic or weakly ferromagnetic, "we decided our overriding principle would be that if there is a good clinical indication to scan anytime after implantation of the device—even if only one day afterward—then one should generally go ahead and perform the scan."
If, however, the MRI is completely elective, "while it may be safe to perform the scan at that time, it is prudent in some cases to wait six weeks after implantation before scanning," he says. Such cases include the following devices: all commonly implanted coronary stents, many peripheral stents, many embolization coils, many inferior vena cava (IVC) filters, many aortic stent grafts, all prosthetic heart valves and annuloplasty rings, certain cardiac closure and occluder devices, and loop recorders.
"Here's the general thing," he says. "For most of these devices, it's probably safe to scan the patient anytime, but let's say someone has had knee pain for five years and needs an MRI but has just had a device implanted two days ago. They've waited five years so they might as well wait another few weeks. But if they are admitted to the hospital after being hit by a car, and now they are having acute knee pain and the surgeons want to do an MRI, then there's a good clinical indication to do the procedure at that time."
Levine stressed, however: "We always caution that for any particular device, one should check the recommendations for that specific device. One of the things emphasized by the physicists on the writing group was that just because one stent is safe to scan that doesn't mean it is safe to scan every stent—in particular, those stents that have not yet been tested for MRI safety. We've given some general advice, but people should still check specific recommendations."
The scientific statement suggests a number of sources of further safety information: what the manufacturer suggests; a number of websites [2,3,4,5], including the particularly popular www.MRIsafety.com [6]; and other published and online documents.
ICDs and pacemakers: Leave it to the experts
With regard to pacemakers and ICDs, Levine says: "We lay out some very specific recommendations. MRI scanning should be done in these patients only at a very experienced center and only when there is a strong indication for MRI." This is because there are legitimate concerns that MRI can cause damage to such devices, reprogram them, or adversely affect their function in other ways, he says.
"When you do an MRI, it's not like a CT scan, its magnitudes more complicated—which scan sequences you use, when and how you adjust scanning parameters, etc. Let's say you have a pacemaker and are going for a brain MRI; just because you are scanning the brain doesn't mean the pacemaker located in the chest is not going to be affected," he explains.
While he says the writing committee felt strongly that they didn't want a blanket ban on MRIs in patients with pacemakers and ICDs, "we also wanted to be very careful that 'the average doctor' didn't look at this and think, 'oh, it's okay for me to scan.' That could be disastrous. We wanted to encourage people to go to the few centers that are really knowledgeable about how to do this safely. This is not something where the average doctor should just try it and see what happens."
The complete list of disclosures is available in the original article.
Sources:
Levine GN, Gomes AS, Arai AE, et al. Safety of magnetic resonance imaging in patients with cardiovascular devices. An American Heart Association Scientific Statement from the Committee on Diagnostic and Interventional Cardiac Catheterization, Council on Clinical Cardiology, and the Council on Cardiovascular Radiology and Intervention. Endorsed by the American College of Cardiology Foundation, the North American Society for Cardiac Imaging, and the Society for Cardiovascular Magnetic Resonance. Circulation 2007 doi: 10.1161/CIRCULATIONAHA.107.187256.
http://www.acr.org
http://www.scmr.org
http://www.nasci.org
http://www.IMRSER.org
http://www.MRIsafety.com
The complete contents of Heartwire, a professional news service of WebMD, can be found at www.theheart.org, a Web site for cardiovascular healthcare professionals.
Clinical Context
MRI has become an increasingly attractive imaging modality that does not expose patients to ionizing radiation, invasive procedures, or iodinated contrast agents, and as more patients are treated with permanently or temporarily implanted cardiovascular devices, guidelines are needed on the safety of MRI in the presence of such devices. MR-related injuries and fatalities occur primarily because of failure to follow safety guidelines or use of outdated information on biomedical devices. These guidelines are endorsed by the American Heart Association and are provided by the Committee on Diagnostic and Interventional Cardiac Catheterization, the Council on Clinical Cardiology, and the Council on Cardiovascular Radiology and Intervention, to address controversy about which patients with cardiovascular devices can safely undergo MRI. Most currently used clinical MR scanners are 1.5 to 3 Tesla, which corresponds to 30,000 to 60,000 times the strength of the earth's magnetic field. Most, but not all, currently implantable devices are nonferromagnetic or weakly ferromagnetic, but the authors cautioned that the manufacturer's product information, dedicated Web sites, or detailed sources on specific devices should be sought for guidance on particular devices.
Study Highlights
Risks associated with MRI arise from 3 distinct mechanisms: static magnetic field, radiofrequency (RF) energy, and gradient magnetic fields.
MR scanners are typically superconducting and thus are always "on."
The greatest risk from the main magnetic field is attraction of a ferromagnetic object into the scanner, and most, but not all, implanted cardiovascular devices are either nonferromagnetic or weakly ferromagnetic.
MRI study should be performed at the lowest field strength available to minimize ferromagnetic risk.
RF energy is pulsed into the body during MRI to generate the MR image, creating heat from ohmic heating.
Certain metallic devices such as leads can act as antennae and concentrate the RF energy, leading to excessive local heating especially at the tips of the devices.
Electrical currents created in wires and leads can lead to arrhythmias.
Gradient magnetic fields result from rapidly changing magnetic fields that can excite peripheral nerves or induce electrical currents in conductive devices.
Flow of electricity in conductive blood can also produce electrographic changes and even arrhythmias.
All patients with a cardiovascular device considered for MRI should undergo screening to accurately identify the implanted device, and inpatients should be examined for temporary devices.
The Food and Drug Administration's definitions of "MR safe" and "MR compatible" have been revised to "MR safe," "MR conditional," and "MR unsafe."
"MR safe" indicates items that are nonmetallic, nonconducting, and nonmagnetic.
"MR conditional" is an item with no known hazards in a specified MR environment with specific conditions.
"MR unsafe" is an item known to pose hazards in all MR environments.
If a device is a nonferromagnetic "passive" implant (such as titanium, titanium alloy, or nitinol), the patient may undergo MRI immediately after implantation.
For weakly ferromagnetic devices, a waiting period of 6 weeks is recommended before MRI.
There is considerable controversy on MR scanning of patients with pacemakers and ICDs.
Presence of a pacemaker or ICD is a strong relative contraindication to MR examination, and scanning should occur only at experienced centers with written informed consent.
Those pacemakers that have been tested have been labeled "MR unsafe."
If scanning occurs with such devices, a clinician with pacemaker and ICD expertise and a crash cart should be present, including a defibrillator.
MR examination should not be performed in patients with known retained fractured intravenous leads because this poses a high risk for thermal injury.
Presence of hemodynamic support devices, including intra-aortic balloon pumps, right ventricular assist devices, and left ventricular assist devices with variable ferromagnetic materials and electrical components are considered absolute contraindications to MR examination.
Most coronary artery and peripheral vascular stents, cardiac closure and occluder devices, and embolization coils are considered "MR safe" with some considered "MR conditional," and MR scanning can be conducted after implantation of nonferromagnetic stents.
Most aortic stent grafts that have been tested are labeled "MR safe," except the Zenith AAA endovascular graft (Cook Vascular, Inc), which is labeled "MR unsafe."
Most prosthetic heart valves and annuloplasty rings that have been tested are labeled "MR safe," with some being "MR conditional."
Patients with loop recorders that are considered "MR conditional" may experience slight movement of the device during MRI and should be warned to avoid undue concern by the patient.
Pearls for Practice
Risks associated with MRI arise from 3 distinct mechanisms: static magnetic field, RF energy, and gradient magnetic fields.
Hemodynamic support devices, including intra-aortic balloon pumps, right ventricular assist devices, and left ventricular assist devices, are considered absolute contraindications to MR examination. ICDs and pacemakers are considered relative contraindications to MRI.
