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ShareNovember 1, 2007 — Biomedical Engineers found a way to prevent pacemakers from confusing the signals sent out by MRI machines with an actual heart emergency. Decreasing the strength of the electromagnetic field helps, but temporarily re-programming the device to pace the heart at a fixed rate is key to making MRIs safe for patients with pacemakers.
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More than two million Americans depend on pacemakers and defibrillators to keep their hearts beating right. But the devices are not allowed in MRI machines -- until now.
Today, Heather McPherson is getting ready for her very first MRI scan.
“I’m a little nervous about getting the scan,” McPherson told Ivanhoe.
Nervous because McPherson has a heart condition, and a defibrillator implanted in her chest. It’s a device similar to a pacemaker that helps keep her heart beating right.
But her life saving implant also comes with a price. Traditionally, metal devices are unsafe for patients in MRI machines. Now, cardiologists at Johns Hopkins hospital have a new way to safely perform the scans.
“We do put the device in what we consider to be kind of an MRI safe mode,” Henry Halperin, M.D., cardiologist at Johns Hopkins University School of Medicine, told Ivanhoe. During a scan, the devices can heat up, move, or turn off. But now, smaller devices made with fewer metal parts, make what used to be impossible, possible.
"They get less problem from electromagnetic noise that might come from an MRI scanner," Saman Nazarian, M.D., cardiac electro-physiologist at Johns Hopkins University School of Medicine, told Ivanhoe.
Before patients enter the scanner, cardiac electro-physiologists use computers to temporarily re-program the device to pace the heart at a fixed rate. The device then ignores electro-magnetic signals from the MRI machine -- safely producing an image.
“We get a lot of referrals from patients who would need to have these tests done in order to make a diagnosis or make a plan for treatment,” Dr. Nazarian said.
Heather is hoping her scan will help setup a treatment plan of her own for a painful spinal cord condition, so she can get back to a normal life. “I am thrilled to get it done and over with,” McPherson said.
The American Association of Physicists in Medicine contributed to the information contained in the TV portion of this report.
BACKGROUND: Researchers at Johns Hopkins University have figured out how to safely perform magnetic resonance imaging (MRI) scans on people who have any one of 24 modern types of implanted defibrillators and pacemakers. They use a combination of methods to reduce the risk of life-threatening meltdowns and complications posed by MRI machines designed to charge and manipulate the electrical properties of cells in order to produce real-time images from inside the body.
THE PROBLEM: Pacemakers and defibrillators are implantable devices used to treat people with an abnormal heartbeat, a condition known as arrhythmia. More than 2.2 million Americans are living with arrhythmia, which can lead to heart disease, stroke, or sudden cardiac death. But these tiny, battery-driven devices have long been considered unsafe or off-limits for MRI scans, which can be critical to early diagnosis of certain cancers of the brain, head and neck, and are often used to guide invasive surgical procedures. For example, there is a risk of burning heart tissue, or misfiring, and some older models are made of magnetic metals (newer models use titanium or other nonmagnetic materials). Some devices use leads, electrical components capped with metal that connect the device to the heart muscle, although if the leads are attached to the battery or embedded in the blood vessels, they are less likely to be overheated by the MRI electromagnetic field.
SAFETY TWEAKS: 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, thus preventing misfires. The researchers also turn off the defibrillators’ shocking function for the brief duration of the MRI scan: about 30 to 60 minutes. In addition, they changed the amount of electrical energy used at peak scanning in MRI: they reduced the strength of the electromagnetic field by half from as much as 4 watts per kilogram per patient. The lower-energy scan still provides images of sufficient quality to make an accurate diagnosis in more than 90% of the cases tested.
HOW MRI WORKS: Magnetic resonance imaging uses radiofrequency waves and a strong magnetic field instead of X-rays to provide clear and detailed pictures of internal organs and tissues. These radio waves are directed at protons in hydrogen atoms -- one of the most abundant atoms in the human body, because of the body's high water content. The waves "excite" the protons, and when they "relax," they emit strong radio signals. A computer can turn those signals into a high-contrast image showing differences in the water content and distribution in various bodily tissues.
Editor's Note: This article is not intended to provide medical advice, diagnosis or treatment.
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