Low-energy 'fix' for machine, other steps vastly reduce risk
Researchers at Johns Hopkins have painstakingly figured out how to safely perform magnetic resonance imaging (MRI) scans on men and women who have any one of 24 modern types of implanted defibrillators and pacemakers.
Henry Halperin, M.D., and his team have developed a combination of methods that reduce the risk of life-threatening meltdowns and other complications posed by MRI's ability to charge and manipulate the electrical properties of cells to produce real-time images from inside the body.
"We have turned a once exceptional procedure into one that is now a routine at Hopkins," Halperin says.
Among other things, the Hopkins group reprograms the 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 MRI with an irregular heartbeat and preventing misfires. They also turn off the defibrillators' shocking function for the brief duration of the MRI scan, about 30 to 60 minutes.
Also changed is the amount of electrical energy used at peak scanning in MRI. The Hopkins team reduced the strength of the electromagnetic field by half, 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 in an article published in the Sept. 18 issue of the journal Circulation, reporting on 55 of more than 100 patients scanned at Hopkins so far.
Their 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 MRI could truly be made safe.
Since 2004, the Hopkins team says its expanded use of MRI has made more than a dozen potentially life-saving diagnoses, despite the fact that the tiny, battery-driven heart devices, which help the body's main pump maintain a beat, have long been considered unsafe and off limits for testing.
"The risk to patients of burning heart tissue or misfiring is still there," Halperin cautions. "But our results show that with appropriate precautions, MRI 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 MRI.
Except for research purposes, the U.S. Food and Drug Administration has not authorized any implanted cardiac device for MRI testing. But Halperin says opening up this diagnostic option is important for the estimated 2 million Americans, many of them elderly, who have these implanted devices but who are also denied the benefits of the quick and accurate images that MRI provides.
"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," says 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 notes.
Of those scanned in the study, 31 had a pacemaker and 24 had an implantable defibrillator. Only modern devices - pacemakers made after 1996 and defrillators manufactured after 2000 - were tested, Nazarian says, because the latest models were deemed to be safer than older versions. Newer models are made of titanium, a non-magnetic metal, he points out, and they are smaller and more lightweight and have better protection from the radiofrequency energy of the MRI 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 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 from CT scanning.
Nazarian cautions 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 adds.
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 the devices' programming.
作者:admin@医学,生命科学 2011-01-02 05:14