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 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, published in 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, and diseases of the
brain, head and neck, plus many cardiovascular conditions.
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, electrophysiologist Henry Halperin,
professor of
medicine,
radiology and
biomedical
engineering at the 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 Johns 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
electromagnetic 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; 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, 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 scans 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,"
Halperin said.
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, weighing on
average 40 grams, significantly less than the original
models of decades before, where some weighed well over 200
grams. 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.) Also, tough scar tissue forms
around heart devices after implantation, preventing their
movement without the aid of surgical cutting
instruments.
Device function for the defibrillators was measured
for four weeks after implantation using the 1.5 Tesla MRI
scanner. Selected modern devices, manufactured after 2000,
performed well. Malfunction was observed in the batteries
of some defibrillators manufactured before 2000, and some
noise distortions were misinterpreted as ventricular
defibrillation. The reasons remain unclear for the battery
problems with older defibrillators. Overall, MRI testing
using a variety of existing clinical procedures yielded
good images with negligible distortions or artifacts
(visible abnormalities created by the picture-taking
process).
"Our results show that implantable heart devices can
be made safe for use in MRI," said study lead author Ariel
Roguin. "Our hope is that further research — through
continued device development and human clinical trials
— will produce devices that are prospectively
designed to be MRI safe. Eventually, we will want all
manufacturers to make their implantable heart devices MRI
compatible so that all patients can benefit from the
advantages of both technologies."
Pacemakers and defibrillators are implantable devices
used to treat people with an abnormal heartbeat, a
condition known as arrhythmia. As many as 2.2 million
Americans are living with atrial fibrillation (one type of
rhythm problem). Arrhythmias can occur in a healthy heart
and be of minimal consequence, or they can lead to more
serious heart disease, stroke or sudden cardiac death.
According to statistics from the Heart Rhythm Society,
sudden cardiac death — which is most commonly caused
by arrhythmia — is a leading cause of death in the
United States, claiming more than 400,000 lives each
year.
This study was funded by Medtronic Corp., St. Jude
Medical, the Bogle Foundation and the National Institutes
of Health. Other investigators in this research, conducted
solely at Johns Hopkins, were Menekhem Zviman, Glenn
Meininger, E. Rene Rodriguez, Tim Dickfeld, David Bluemke,
Albert Lardo, Ronald Berger and Hugh Calkins. In
recognition for his work on this study, Roguin was the
recipient of the Melvin Judkins Young Investigators' Award
in cardiovascular radiology at the 2003 annual meeting of
the American Heart Association.