In animal and laboratory studies, scientists at Johns Hopkins have shown that modern, implanted heart assist devices - such as pacemakers and defibrillators - can be safe for use in magnetic resonance imaging (MRI) machines, a diagnostic and imaging tool long ruled potentially unsafe and off-limits for more than 2 million Americans who currently have a surgically implanted cardiac device.
Their findings, to be published in the latest edition of Circulation online Aug. 3, should eventually make MRI scans more available to people who might benefit from early detection of cancer and other diseases, when treatments are most likely to succeed, and for guiding devices during minimally invasive surgery. Indeed, MRI is now regarded as the imaging modality of choice for diagnosing many cancers, diseases of the brain, head and neck, plus many cardiovascular conditions (in particular, arrhythmogenic right ventricular dysplasia, a weakening of the right ventricle that may result in potentially dangerous cardiac arrhythmias). Precise MRI guidance of surgical instruments would improve laparoscopic, minimally invasive and ablative surgical techniques, but is not currently recommended for people with implanted heart devices.
"Many people, such as the elderly and patients with arrhythmogenic right ventricular dysplasia, who might benefit from an MRI scan are currently denied them because they have an implanted, electrical heart device," said the study's senior author, electro-physiologist Henry Halperin, M.D., professor of medicine, radiology and biomedical engineering at The Johns Hopkins University School of Medicine. "It is feared that the electromagnetic fields of the MRI may heat up metal components, or pull on and dislodge the device, causing tissue damage, device malfunction or possibly death."
Over a six-month period, the Hopkins team tested a broad range of devices from among the hundreds of brands and models currently in use, including nine pacemakers, 18 defibrillators and 40 different lead systems (the electrical component that connects the device to the heart muscle). Each type of device was tested under a variety of electro-magnetic field strengths using one MRI scanner, a 1.5 Tesla by General Electric, the most widely available scanner in North America.
Using models filled with salt water or gel to simulate conditions inside the human body, the researchers evaluated every model for its safety and functionality. They wanted to determine if MRI heated the electrical, metal lead components of the device; to test the electromagnetic field to see if it dislodged or caused a pulling effect on the devices, which are housed in a titanium metal casing; and, fundamentally, to check if MRI caused malfunctions in the devices or produced distortions in the resulting diagnostic image.
Results showed most modern devices are safe and perform well in both standard MRI scans and when scans are performed using electromagnetic fields at maximum strength.
"You can do a high-energy scan for a long period of time without doing any long-term damage to select devices," said Halperin.
Lead components never heated more than 5 degrees Celsius (9 degrees Fahrenheit) when exposed to the electromagnetic fields, and only at maximum strength. It was feared that the radiofrequency waves of the MRI scanner would react with the lead components and, as with any metal antennae, produce intense heat. However, the researchers found that the lead components were too short to provide adequate coupling of the radiofrequency energy to produce sufficient heat to cause tissue damage. Protective capacitors within the devices also filtered out the radiofrequency waves.
The pull of the electromagnetic field on heart devices was negligible, never amounting to more than the equivalent force required to hold two golf balls by hand (less than 100 grams). Modern devices are much smaller, also weighing on average 40 grams, significantly less than the original models of decades before, where some weighed well over 200 grams. Indeed, titanium, the most widely used metal for pacemaker casings, is almost completely nonmagnetic. (The original pacemakers developed in the 1960s did not have a metallic shield and, therefore, were vulnerable to radiation from nearby microwave ovens, so protective metal casings were introduced and made from titanium, a metal inert to body fluids which can shield the device parts from electromagnetic fields.) Tough scar tissue also forms around heart devices after implantation, preventing their movement without the aid of surgical cutting instruments.