MRIs made safe for people with defibrillators and pacemakers
8 Oct 2006
Researchers at Johns Hopkins University claim to have overcome the
problems of taking magnetic resonance images of people with implanted
defibrillators and pacemakers. They have painstakingly figured out how to
safely perform MRI scans on men and women who have any one of 24 modern
types of implanted defibrillators and pacemakers (makes and models listed
Henry Halperin, M.D., and his team have developed a combination of
methods that reduce the risk of life-threatening meltdowns and other
complications posed by MRI’s ability to charge and manipulate the electrical
properties of cells to produce real-time images from inside the body. “We
have turned a once exceptional procedure into one that is now a routine at
Hopkins,” Halperin says.
Among other things, 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
and preventing misfires. They also turn off the defibrillators’ shocking
function for the brief duration of the MRI scan, about 30 to 60 minutes.
Also changed is the amount of electrical energy used at peak scanning in
MRI. The Hopkins team reduced the strength of the electromagnetic field by
half, from as much as 4 watts per kilogram to 2 watts per kilogram per
“This lower-energy scan still provided images of sufficient quality to
make an accurate diagnosis in more than 90 percent of cases tested,”
Halperin said in an article published in the Sept. 18 issue of the journal
Circulation, reporting on 55 of more than 100 patients scanned at
Hopkins so far.
Their report comes just two years after the same journal published the
team’s initial, positive findings in animals, stirring fierce debate at
several international conferences as to whether or not MRI could truly be
Since 2004, the Hopkins team says its expanded use of MRI has made more
than a dozen potentially life-saving diagnoses, despite the fact that the
tiny, battery-driven heart devices, which help the body’s main pump maintain
a beat, have long been considered unsafe and off limits for testing.
“The risk to patients of burning heart tissue or misfiring is still
there,” Halperin cautions. “But our results show that with appropriate
precautions, MRI is a safe and effective diagnostic tool to use for those
with modern implanted heart devices.” An electro-physiologist and professor
of medicine, radiology and biomedical engineering at The Johns Hopkins
University School of Medicine and its Heart Institute, Halperin has long led
efforts to expand access to MRI.
Except for research purposes, the U.S. Food and Drug Administration has
not authorized any implanted cardiac device for MRI testing. But Halperin
says opening up this diagnostic option is important for the estimated 2
million Americans, many of them elderly, who have these implanted devices
but who are also denied the benefits of the quick and accurate images that
“Once these precautions are better understood and further refined, we
hope policy makers will see fit to review current restrictions on scanning
anyone with a device,” says lead author Saman Nazarian, M.D., a cardiac
electrophysiology, clinical and research fellow at Hopkins.
“These images are critical to early diagnosis of certain cancers of the
brain, head and neck, and to guide invasive procedures,” he notes.
Of those scanned in the study, 31 had a pacemaker and 24 had an
implantable defibrillator. Only modern devices — pacemakers made after 1996
and defibrillators manufactured after 2000 — were tested, Nazarian says,
because the latest models were deemed to be safer than older versions. Newer
models are made of titanium, a non-magnetic metal, he points out, and they
are smaller and more lightweight and have better protection from the
radiofrequency energy of the MRI scanner.
Using a single scanner, a 1.5 Tesla by General Electric, the Hopkins
group was able to help plan artery-opening procedures for more than a
half-dozen in the test group; improve measurements of tumour growth in nine
others; and detect two strokes, a benign brain mass and a blood clot in the
spine that had been missed by alternative imaging from CT scanning.
Nazarian cautions that only physicians specially trained in MRI safety,
or with access to specialists familiar with the specific precautions taken
in his study, should undertake this approach. “It is also important at this
time to restrict MRI use to those with implanted devices specifically
tested, and scanners of the same type and magnetic strength as that used in
our study,” he adds.
All study participants were closely monitored during the scans with
electrocardiography and pulse oximetry, and staff were on hand to
resuscitate patients in the event of an emergency.
All subjects were over age 19 and were followed from three months to six
months to look for any post-test heart damage or changes in the devices’
Patients were disqualified from testing if they had any leads placed on
the surface of the heart or leads that were capped with metal, and therefore
not connected to the battery and at greater risk of overheating. Leads are
the electrical components connecting the device to the heart muscle, and
when the leads are attached to the battery or embedded in the blood vessels,
they are less likely to be overheated by the MRI field.
An analysis of records showed that scans provided definitive answers to
physicians’ diagnostic questions 100 percent of the time for conditions
affecting areas outside the chest, and 93 percent of the time for conditions
that affected the heart and the upper body. In the latter category, the
remaining 7 percent of the scans were too distorted by imaging artifacts
from the implanted devices to make a clear diagnosis.
Device monitoring showed that lead sensing did not fluctuate or change
during the scan. Battery measurements showed that scans did not deplete or
strengthen the battery’s charge. Indeed, pacemakers and defibrillators
performed successfully after the scans without any premature firing or false
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. It can occur in a
healthy heart and be of minimal consequence, or it can lead to more serious
heart disease, stroke or sudden cardiac death.
A complete listing of heart devices made safe for MRI scanning is
Tested implanted cardiac devices
Pacemakers with Satisfactory MRI Testing
Manufacturer: St Jude
1. Pacesetter AFP (262)
2. Trilogy (2360)
3. Entity (5326)
4. Affinity (5130, 5330)
5. Integrity (5142, 5342, 5346)
6. Identity (5172, 5370, 5376, 5380, 5386)
7. Vigor (1232)
8. Discovery (1272)
9. Insignia (1194, 1290)
10. EnPulse (AT-500, E2SRO1, E2DRO1)
11. Kappa (701, 901)
12. Prodigy (7860)
13. In Sync BiV (8040, 8042)
Defibrillators with Satisfactory MRI Testing
Manufacturer: St Jude
14. Photon (V-194, V-230, V-232)
15. Atlas (V-240)
16. Epic (V-197, V-235, V-239)
17. Prizm (1850, 1851, 1852, 1860, 1861)
18. Contak (1823, H119, H170, H175)
19. Vitality (T125, T135)
20. Maximo (7232)
21. Gem-II (7273)
22. Gem-III (7275)
23. Marquis (7274)
24. InSync (7272)
Funding for this study was provided by the National
Institutes of Health, and the Donald W. Reynolds Foundation.
Declarations: Dr Halperin is a paid consultant to defibrillator manufacturer
Medtronic, and co-investigators Ronald Berger, M.D., Ph.D., and Albert
Lardo, Ph.D., are paid consultants to Guidant Corp., another device
manufacturer. Co-author David Bluemke, M.D., Ph.D., has also received
honoraria from General Electric Health Care for presentations. None of these
companies provided funding for the study, and the terms of the physicians’
arrangements are managed by The Johns Hopkins University in accordance with
its conflict of interest policies.
Other investigators in this research, conducted solely at
Johns Hopkins, were Ariel Roguin, M.D., Ph.D.; Menekhem Zviman, Ph.D.; Timm
Dickfeld, M.D., Ph.D.; Robert G. Weiss, M.D.; and Hugh Calkins, M.D.