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 radio frequency 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 radio frequency 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.
The American Association of Physicists in Medicine contributed to the information contained in the TV portion of this report.