A trip through the MR scanner can wreak havoc with implantable cardiac devices such as pacemakers and defibrillators. But this kind of interference may not be limited to MR imaging, according to Cynthia McCollough, Ph.D.
Her research found that x-rays can also interact with these devices, McCollough said at the 2008 Stanford International Symposium on Multidetector-Row CT.
"We know that semiconductor-type devices are sensitive to radiation therapy, but historically no one has believed that there's been any effect at diagnostic levels," said McCollough, director of the CT Clinical Innovation Center at the Mayo Clinic in Rochester, MN.
A report from Japan correlated interference with a pacemaker at the exact time that the patient was having a CT scan (Circ J 2006:70(3):190-197). The pacemaker was reset to its default setting at the time of CT, she said.
"While (the interference) was a transient effect, the possibility of arrythymia cannot be ignored," said McCollough, who is also an associate professor of radiologic physics at the Mayo Clinic College of Medicine. "That's attention-catching. So I began looking into this."
Teaming up with implantable device maker Medtronic of Minneapolis, McCollough conducted a series of experiments that prospectively measured the response of 21 implantable cardiac rhythm management devices to the radiation delivered by CT for both maximum and typical dose levels (Radiology 2007:243(3):766-774). They found oversensing occurred most often.
"Oversensing means (the device) is sensing an event that didn't come from the heart," she said. "The current from the ionizing radiation fools the pacemaker into thinking that the heart had activity."
McCollough's group observed oversensing in 20 of 21 devices at maximum doses and in 17 of 20 devices at typical doses. Oversensing most often manifested as inhibition, although it occasionally manifested as tracking or safety pacing.
They also found that two devices inhibited for more than four seconds in spiral mode at clinical dose levels. Oversensing was transient and ceased as soon as the device stopped moving through the x-ray beam or the beam was turned off, according to McCollough's group.
"What mechanism would do this?" McCollough said. "Basically, you are inducing current across the p-n junction of the semiconductor. This is an interference effect much like a microwave oven interferes with a pacemaker."
She said that permanent damage to the device is not likely at the standard radiation doses given for common CT exams, such as coronary CT angiography, chest CT, or pulmonary embolism studies.
She added that 63 million CT scans were performed in the U.S. last year, but there were no reports of any serious events involving implantable cardiac devices and CT scanners.
McCollough cautioned, however, that radiologists should be aware that some interference occurs, especially during CT studies — dynamic perfusion, and interventional — when the detector may dwell over the device for longer than four seconds.
"This definitely needs more in vitro and in vivo testing to see if there is something further that we should be doing for these patients," she said.
