| It is possible to safely and effectively perform magnetic resonance imaging for many people with an implanted defibrillator or pacemaker It is possible to safely and effectively perform magnetic resonance imaging for patients with 24 recent-model implanted defibrillators or pacemakers, according to an article in the September 18 issue of Circulation. Henry Halperin, MD, and colleagues at the Johns Hopkins University have developed a combination of methods that reduces the risk of life-threatening meltdowns and other complications. Halperin said, “We have turned a once exceptional procedure into one that is now a routine at Hopkins.” Among other things, the Hopkins group reprograms devices, fixing them to a specific sequence. This makes the implanted devices blind to their external environment, reducing the potential for their electronics to confuse the radiofrequency generated by the imaging equipment with an irregular heartbeat. They also turn off the defibrillators’ shocking function for the duration of the scan. Modifications extend to the imaging: The strength of the electromagnetic field is roughly halved, from as much as 4 watts per kilogram to 2 watts per kilogram per patient. “This lower-energy scan still provided images of sufficient quality to make an accurate diagnosis in more than 90 percent of cases tested,” Halperin said. The journal article reported on 55 of more than 100 patients scanned so far. The report comes just two years after the same journal published the team’s initial, positive findings in animals, stirring fierce debate at several international conferences as to whether or not magnetic resonance imaging could truly be made safe. Since 2004, the Hopkins team says its expanded use of the imaging technology has made more than a dozen potentially life-saving diagnoses, despite the fact that the tiny, battery-driven heart devices have long been considered unsafe and off limits for testing. “The risk to patients of burning heart tissue or misfiring is still there,” Halperin cautioned. “But our results show that with appropriate precautions, magnetic resonance imaging is a safe and effective diagnostic tool to use for those with modern implanted heart devices.” An electro-physiologist and professor of medicine, radiology and biomedical engineering at The Johns Hopkins University School of Medicine and its Heart Institute, Halperin has long led efforts to expand access to magnetic resonance imaging. Except for research purposes, the U.S. Food and Drug Administration has not authorized any implanted cardiac device for magnetic resonance imaging testing. But Halperin said that opening this diagnostic option is important for the estimated 2 million Americans, many of them elderly, who have these implanted devices and who are currently denied the benefits of the quick and accurate MR images. “Once these precautions are better understood and further refined, we hope policy makers will see fit to review current restrictions on scanning anyone with a device,” said lead author Saman Nazarian, M.D., a cardiac electrophysiology, clinical and research fellow at Hopkins. “These images are critical to early diagnosis of certain cancers of the brain, head and neck, and to guide invasive procedures,” he added. Of those scanned in the study, 31 had a pacemaker and 24 had an implantable defibrillator. Only modern devices - pacemakers made after 1996 and defibrillators manufactured after 2000 - were tested. Newer models are made of titanium, a non-magnetic metal, and they are smaller and more lightweight and have better protection from the radiofrequency energy of the scanner. Using a single scanner, a 1.5 Tesla by General Electric, the Hopkins group was able to help plan artery-opening procedures for more than a half dozen patients in the test group, improve measurements of tumor growth in nine others, and detect two strokes, a benign brain mass and a blood clot in the spine that had been missed by alternative imaging with computed tomography. Nazarian cautioned that only physicians specially trained in MRI safety, or with access to specialists familiar with the specific precautions taken in his study, should undertake this approach. “It is also important at this time to restrict MRI use to those with implanted devices specifically tested, and scanners of the same type and magnetic strength as that used in our study,” he added. All study participants were closely monitored during the scans with electrocardiography and pulse oximetry, and staff were on hand to resuscitate patients in the event of an emergency. All subjects were over age 19 and were followed from three months to six months to look for any post-test heart damage or changes in device programming. Patients were disqualified if they had any leads placed on the surface of the heart or leads that were capped with metal, and therefore not connected to the battery and at greater risk of overheating. An analysis of records showed that scans provided definitive answers to physicians’ diagnostic questions 100 percent of the time for conditions affecting areas outside the chest and 93 percent of the time for conditions that affected the heart and upper body. In the latter category, the remaining 7 percent of scans were too distorted by imaging artifacts from the implanted devices to make a clear diagnosis. Device monitoring showed that lead sensing did not fluctuate or change during the scan. Battery measurements showed that scans did not deplete or strengthen the battery’s charge. Indeed, pacemakers and defibrillators performed successfully after the scans without any premature firing or false alarms. LIST OF TESTED IMPLANTED CARDIAC DEVICES --- Satisfactory test results Pacemakers with Satisfactory MRI Testing Manufacturer: St Jude Manufacturer: Guidant Manufacturer: Medtronic Defibrillators with Satisfactory MRI Testing Manufacturer: St Jude Manufacturer: Guidant Manufacturer: Medtronic |