Download Safety of Magnetic Resonance Imaging in Patients

REVIEW
Safety of Magnetic Resonance Imaging in Patients
With Implanted Cardiac Prostheses and Metallic
Cardiovascular Electronic Devices
Nikolaos G. Baikoussis, MD, PhD, Efstratios Apostolakis, MD, PhD,
Nikolaos A. Papakonstantinou, MD, Ioannis Sarantitis, MD, and
Dimitrios Dougenis, MD, PhD
Cardiothoracic Surgery Department, University Hospital of Rio, Patras, Greece
REVIEW
Magnetic resonance imaging (MRI) in patients with
implanted cardiac prostheses and metallic cardiovascular
electronic devices is sometimes a risky procedure. Thus
MRI in these patients should be performed when it is the
only examination able to help with the diagnosis. Moreover the diagnostic benefit must outweigh the risks.
Coronary artery stents, prosthetic cardiac valves, metal
sternal sutures, mediastinal vascular clips, and epicardial
pacing wires are not contraindications for MRI, in contrast to pacemakers and implantable cardioverter-defibrillators. Appropriate patient selection and precautions
ensure MRI safety. However it is commonly accepted
that although hundreds of patients with pacemakers or
implantable cardioverter-defibrillators have undergone
safe MRI scanning, it is not a safe procedure. Currently,
heating of the pacemaker lead is the major problem
undermining MRI safety. According to the US Food and
Drug Administration (FDA), there are currently neither
“MRI-safe” nor “MRI-compatible” pacemakers and implantable cardioverter-defibrillators. In this article we
review the international literature in regard to safety
during MRI of patients with implanted cardiac prostheses and metallic cardiovascular electronic devices.
M
with their devices, putting the devices or even their own
safety in danger [12]. At least 200,000 patients with
cardiac devices are estimated to have been denied an
MRI scan in 2004 [13]. According to the American College
of Cardiology Foundation/American Heart Association
“ACCF/AHA 2007 Clinical Competence Statement on
Vascular Imaging With Computed Tomography and
Magnetic Resonance,” metallic implants such as mechanical heart valves, coronary stents, and sternal sutures are compatible with MRI because they are not
ferromagnetic, although there will be local image artifacts. In contrast, pacemakers and implanted cardioverter-defibrillators are considered a contraindication to
MRI [14], although several case series of patients with
pacemakers have shown that these patients can successfully undergone MRI at 1.5 T [15–17]. Patients who
already have either a pacemaker or implantable cardioverter-defibrillator often need an MRI scan. After implantation of the device, each patient is estimated to have
a 50% to 75% possibility of requiring an MRI scan some
time in his or her life [18, 19].
agnetic resonance imaging (MRI) is a priceless
diagnostic tool used for many diseases and conditions [1–3]. MRI is based on the structure and abundance of water in the different human tissues. It represents the absorption and emission of electromagnetic
energy by atomic nuclei in a magnetic field after excitation by a radiofrequency pulse [4 – 6]. It is an advantageous diagnostic procedure in that it is not at all invasive
because there is no exposure to ionizing radiation or
potentially nephrotoxic iodinated contrast agents [1–3, 7].
Three-dimensional visualization of anatomic structures
and its superiority in soft tissue contrast are additional
advantages [8]. Thus MRI is now considered the gold
standard for imaging the brain, spinal cord, musculoskeletal system, head and neck, and complex congenital
heart malformations [6]. It also appears to be appropriate
for estimating myocardial structure, wall motion, perfusion, and viability. As a result, an important increase in
the number of MRI scans performed annually has been
observed [6, 9 –11]. However, the number of MRI scans in
patients with cardiovascular implantable electronic devices—mostly pacemakers and cardioverter defibrillators— has simultaneously increased [7]. Today millions of
patients have implanted cardiac devices. Nevertheless,
for many years MRI was not allowed for these patients
because of the potential interference of MRI machines
Address correspondence to Dr Baikoussis, Cardiothoracic Surgery Department, University of Rio, Kolokotroni 4, Ovria, 26500 Patras, Greece;
e-mail: [email protected].
© 2011 by The Society of Thoracic Surgeons
Published by Elsevier Inc
(Ann Thorac Surg 2011;91:2006 –11)
© 2011 by The Society of Thoracic Surgeons
Possible Interactions With Materials
Three types of electromagnetic fields are used for the
generation of an MRI: a constant static magnetic field, a
rapidly changing magnetic gradient field, and a strong
radiofrequency field [12]. The most commonly used static
magnetic field strength for clinical MRI scanning is 1.5 to
0003-4975/$36.00
doi:10.1016/j.athoracsur.2011.02.068
3 T [20]. Higher static magnetic fields lead to greater
forces on ferromagnetic materials [21]. Gradient magnetic fields constitute spatial variations in magnetic field
strength indicating the localization of the signals in the
body. Electrical currents in electrically conductive devices and excitation of peripheral nerves can be induced
by these changing magnetic fields [20]. The radiofrequency field can cause tissue heating. In addition certain
metallic devices, such as pacemaker leads, can act as an
antenna and may concentrate radiofrequency energy,
resulting in dramatic local heating [20]. The specific
absorption rate, expressed in watts/kilogram, describes
the absorption of radiofrequency energy, which increases
with the square of magnetic field strength [21]. Heating
effect depends on the dimensions of the leads (length and
shape), on their position in the body, and on their
insulation. The heating effect is greater at the tip, where
the greatest alterations in the electrical field take place
[22]. Magnetic fields are able to provoke either traction
forces proportionally to the mass of the device, causing
shifting movements of the ferromagnetic object, or torsion forces, resulting in rotational movements that tend
to align the device in the direction of the magnetic field
[22]. Finally, it should be kept in mind that because these
objects produce their own static magnetic field [7], image
artifacts or distortions are generated in the area of the
implanted device at the expense of the quality and
reliability of the images [22]. This phenomenon seems to
occur in the direct vicinity of the device [8], being more
intense at a 10- to 15-cm distance from it [22]. Nevertheless, the risk becomes negligible, except when the area
under examination is the thoracic cavity [22]. Therefore,
pacemakers and implantable cardioverter-defibrillators
will have no impact on imaging of more distant organs,
such as knees, lower spine, liver, or brain [7].
Coronary Stents
Some stents are made of alloys including stainless steel,
carbon, or gold, whereas others are composed of cobalt
or tantalum [23]. All but two nonferromagnetic models
are weakly ferromagnetic. The forces caused by the
magnetic field on the implanted stent are proportional to
its length and mass, but they seem to be insufficient to
provoke any shifting [22, 23]. As far as the heating effect
produced by the radiofrequency field is concerned, no
temperature alterations in stents have been observed
[24]. Finally, distortion of the images is possible only
close to the prosthesis [22]. According to several ex vivo
studies, coronary artery stents appear to be absolutely
safe in terms of magnetic field interactions and heating
[25, 26]. Although the manufacturers and cardiologists’
associations suggest waiting 4 to 8 weeks after implantation before performing an MRI in such patients [27], a
recent study carried out at the Mayo Clinic on 111
patients with coronary stents proved safety of the MRI,
even only days after the implantation. No differences
were observed in terms of all-cause deaths, myocardial
infarction, and revascularization in the 30 days after MRI
examination between patients who had undergone MRI
soon after stent implantation and those who had waited
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MRI AND CARDIOVASCULAR ELECTRONIC DEVICES
2007
the recommended time [28]. Porto and colleagues [29]
showed the safety of MRI in the immediate period after
implantation (1 to 3 days). Neither acute thrombosis nor
adverse cardiovascular events (target vessel restenosis or
non–target vessel revascularization) at 9-month clinical
follow-up were documented. Other clinical trials also
showed that MRI at 1.5 T can be performed within 1 to 14
days after stent implantation without any adverse clinical
cardiac outcomes [30, 31]. More recent ex vivo studies,
demonstrated that MRI at 3 T is also safe without risk for
stent migration [32, 33]. Therefore MRI at 3 T or less
appears absolutely safe for patients with coronary stents
any time after the implantation of the stent, even in the
immediate period after implantation [32, 34].
Heart Valve Prostheses
Metals, polymers, and carbons are used in prosthetic
cardiac valves. Titanium, alloys of cobalt and chromium, and alloys of nickel, molybdenum, and tungsten, as well as aluminium and vanadium are the
metals used because of both biocompatibility and
high-grade mechanical resistance. Polytetrafluoroethylene (Teflon; DuPont, Wilmington, DE), polyethylene
terephthalate (Dacron; DuPont) and polyacetal resins
such as Delrin (DuPont) constitute the polymer materials used, whereas the carbon materials include pyrolytic
carbon [22]. An external magnetic field at 3 T produce
forces even smaller than those generated by the beating
heart, so there is negligible risk if a patient with a
mechanical heart valve undergoes MRI [35]. Moreover,
no case of movement of the mobile parts of valves caused
by a magnetic field greater than 1.5 T (which might
render them permanently open or closed) [36], has ever
been reported [22]. The heating effect also is not a hazard
[37], as heat from the area involved is taken up by the
continuous blood flow to the heart during the examination [22]. Finally, image interpretation is rarely influenced by the artifacts provoked by mechanical valves
[38]. In Hartnell and colleagues’ study [38], 25 patients
with mechanical cardiac valves who underwent MRI
examinations at 1 or 1.5 T sustained no cardiac adverse
events, such as arrhythmia or worsening cardiac function, related to MRI. There also appears to be no risk to
prosthetic valves, according to Randall and associates
[39]. No significant heating effects in sternal wires or
other postoperative metal material were documented.
Consequently, prosthetic heart valves, as well as metal
sternal sutures and mediastinal clips, should not be
considered contraindications for an MRI at 3 T or less any
time after implantation [34, 38, 40].
Magnetic Resonance Imaging in Patients With
Implantable Devices
Ferromagnetic materials, complex electrical systems, and
pacemaker leads implanted into the myocardial tissue
constitute pacemakers and implantable cardioverterdefibrillators. Electromagnetic forces produced by MRI
can induce several adverse events in patients with implantable devices, including shifting of the device, dam-
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2011;91:2006 –11
2008
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age to the pacemaker circuit, reed switch malfunction,
inappropriate activation of tachyarrhythmia therapies,
asynchronous pacing, rapid atrial or ventricular pacing or
more rarely ventricular fibrillation, and inhibition of
pacing output and pacemaker lead currents that could
stimulate the heart because their electrodes may act as
antennas [4, 5, 7, 19, 34, 41– 46]. Harm to tissue and high
pacing thresholds may also be provoked by thermal
injury at the pacemaker lead tip, threatening function of
the lead [7, 12, 45, 46].
Pacemakers
REVIEW
Reports from the 1980s to the mid-1990s [41, 43, 44, 47– 49]
showed an adverse impact of MRI on pacemakers. However most of them concerned earlier generation pacemakers that are currently not in use. Studies from the
mid-1990s to today [45, 50 –52] have found no functional
complications in most pacemakers after an MRI examination. It has been observed that during MRI, the stimulation frequency may be raised to the maximum programmed rate. Although this increase will be
symptomatic, it is not life-threatening [22, 53, 54]. However, deactivating the device before performing MRI is
advisable to make the procedure safer [22]. With regard
to the heating effect on catheters and electrodes potentially resulting in thermal injury of the surrounding
tissue, a recent study reported temperature increases
of more than 20°C when the catheters were close to the
area of the image, but increases of 4°C to 5°C when
they were more than 30 cm away (MRI of the brain,
abdomen, pelvis, lumbar spine, and lower extremities)
[55]. However, although several reports [50, 51, 55]
proved an in vitro temperature increase ranging from
7°C to 63.1°C, Roguin and coworkers [50] Schmiedel
and associates [52] and Luechinger and colleagues [45]
found no in vivo thermal damage. Additionally, an
electrical reset can occur in pacemakers during an MRI
scan [50]. An electrical reset means an emergent return
of the device to factory default settings, usually an
inhibited pacing mode like VVI. The incidence of this
phenomenon is 6.1% [16]. An electrical reset threatens
the safety of the patient in two ways: (1) Inhibition of
the pacemaker is possible, potentially resulting in
bradycardia/asystole in patients with previously low
heart rates [56]. (2) Inadequate pacemaker function
may be induced [7] as follows:
(i) Inhibition of pacemaker output in pacemakerdependent patients.
(ii) Insufficient cardiac output in children, who normally have high heart rates.
(iii) Ineffective stimulation in patients needing a high
pacemaker output to ensure a good heart rate.
(iv) Mistaking ventricular fibrillation for bradycardia
and pacemaker stimulation, with subsequent inhibition of implantable cardioverter-defibrillator
therapy when ventricular fibrillation occurs in
patients who have both pacemakers and implantable cardioverter-defibrillators. Deactivating the
pacemaker before MRI is advisable in such cases.
Ann Thorac Surg
2011;91:2006 –11
Hence, it is advisable that patients with pacemakers
undergo MRI with static magnetic field strength of no
more than 0.5 T and a specific absorption rate up to 0.6
W/kg to limit the accompanying dangers [22]. However,
MRI at 1.5 T also appears to be safe according to Martin
and associates’ study [15] in 54 patients with cardiac
stimulators.
Cardioverter-Defibrillators
Despite being significantly smaller and lighter than in the
past (forces on older implantable cardioverterdefibrillators were 1 to 5.9 N), new-generation implantable cardioverter-defibrillators are associated with 10
times higher magnetic forces than pacemakers (0.5 to 1.1
N versus 0.05 to 0.12 N, respectively) [7, 50, 52]. However
patients will not feel forces less than 2 N [50]. Deflection
and rotation of the ferromagnetic materials of defibrillators may take place because of the static magnetic field
generated by MRI, as they are still relatively large [57].
Additionally, in the environment of a static magnetic
field, the implantable cardioverter-defibrillator transformer will be rendered unable to charge the capacitor,
potentially leading to a permanently inactive device, as
happens after repeated unsuccessful attempts to charge
the capacitor. Consequently, although there is no evidence to support this theory, there seems to be a risk that
implantable cardioverter-defibrillators will be incapable
of delivering therapy. That is the reason that deactivation
of therapy delivery before MRI is imperative. However,
there is still the risk of an electrical reset, which may
induce its reactivation [7]. The MRI radiofrequency field
may be misinterpreted by implantable cardioverterdefibrillator devices as ventricular tachyarrhythmia, resulting in inappropriate antitachycardia pacing, cardioversion, or defibrillation. Moreover, ventricular
tachycardia or fibrillation may escape detection by implantable cardioverter-defibrillators because of magnetic
fields [22, 45, 50]. Finally, no thermal injury was reported
by Roguin and associates [50] in an animal study of MRI
performed after cardioverter-defibrillator implantation.
Overall, implantable devices tested in vitro and in animals in the MRI environment were associated with no
significant complications in terms of device function,
tissue damage, or pacing threshold alteration [45, 50, 58,
59]. Moreover, relative safety of MRI at 0.5 to 3.0 T was
documented in recent prospective human studies of
almost 500 patients [15–17, 60 – 63]. There have been only
a few cases of minor pacing threshold alterations, the
need for device reprogramming, and battery depletion
[12]. Wollmann and colleagues [64], Roguin and associates [65], Gimbel and colleagues [61] and Nazarian and
coworkers [17] reported that they had their patients with
implantable cardioverter-defibrillators safely scanned
with MRI. Buendía and associates [66] proved the safety
of MRI in 33 patients: 5 patients with implantable cardioverter-defibrillators and 28 with pacemakers, 4 of whom
were pacemaker dependent. There was no irreversible
device malfunction or any clinical events or significant
threshold changes. No significant irreversible complications were found by Halshtok and coworkers [67], who
studied 34 MRI scans performed at 1.5 T in 18 patients
with 11 pacemakers and 7 implantable cardioverterdefibrillators (1 patient underwent 11 scans). All but 1
MRI scan was diagnostic. The nondiagnostic scan was a
cardiac MRI scan performed in an 11-year-old boy with
hypertrophic cardiomyopathy. The same result concerning safety was proved by Roguin and Goldsher [12], who
performed 49 MRI scans in more than 40 patients with
both types of implantable devices. Therefore patients
with cardiac implantable devices can safely have an MRI
scan at 1.5 T. However, MRI in these patients must be
performed under strict conditions and only if there is an
absolute indication [12, 67].
Permanent and Temporary Pacing Leads
Endocardial pacemaker leads left in place after device
removal potentially act as antennas, resulting in significant heating. The thermal effect may be even more
intense than the one provoked by normally connected
leads. However, these patients can undergo MRI with the
appropriate precautions [7]. Temporary epicardial pacing
wires, composed of stainless steel, are usually sutured to
the epicardial surface of the right ventricle and right
atrium during cardiac operations. These wires are connected to an external pacemaker to prevent atrioventricular block or bradycardia [68]. If their removal is not
possible, they are cut and left in place [38]. Hence when
MRI is performed, there is a risk of carrying an induced
current, potentially leading to pacing complications [68].
However according to Hartnell and colleagues’ study
[38], none of 51 patients with temporary epicardial pacing
wires experienced arrhythmia or other cardiac dysfunction during MRI. Consequently the presence of epicardial pacing wires does not constitute a contraindication to
performing MRI [38].
How To Perform Safe Magnetic Resonance
Imaging in Patients With Implantable Devices
The smaller the implantable device and the fewer ferromagnetic materials it includes, the better is the protection
from the external magnetic environment it has. Reconsideration of the contraindication for MRI in patients
with implantable devices is taking place [18, 34]. Several
reports [15, 17, 50, 60, 69] have proved safe MRI at 1.5 T in
patients with pacemakers or implantable cardioverterdefibrillators within a professionally supervised environment [16, 17, 63]. However, a few surveys have examined
MRI at 3 T in such patients [58, 59, 62]. Currently, new
”MRI-conditional” pacemakers are being introduced [70,
71], but most devices are still ”non-MRI safe” [67]. MRI
should only be performed provided that it is absolutely
necessary and no other diagnostic technique can replace
it [7, 22]. The risks and benefits of MRI should be
balanced, and the patient’s written informed consent
must be obtained before the scan [7,12]. Procedure time
and energy should be minimized [12], as should the risk
of radiofrequency-induced thermal injury by reducing
the specific absorption rate [7, 8]. If the patient is pace-
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maker dependent, stimulation regardless of the interaction with magnetic fields is necessary, so AOO (atrial
pacing without sensing), VOO (ventricular pacing without sensing), or BOO (atrial and ventricular pacing without sensing) are advisable asynchronous modes to which
the device should be programmed. Conversely, if the
patient has sufficient rhythm at rest, the pacemaker
should be temporarily switched off (OOO mode) during
MRI. Finally, when a patient with an implantable cardioverter-defibrillator is going to undergo MRI, interrupting
the therapy before the performance of MRI is recommended [7, 22]. Whatever the conditions, maximum
safety can be obtained only by close patient monitoring,
meaning continuous electrocardiography and pulse oximetry during the procedure [7, 8, 22, 67]. Continuous
verbal communication is also important [67]. Moreover,
an expert cardiologist, an electrophysiologist, a physician
with advanced cardiac life support training, full resuscitation facilities, and advanced cardiac life support equipment must be on site [7, 8, 12, 67]. The function of the
device should be estimated before and directly after
performance of MRI . In addition, thorough follow-up at
6 to 12 weeks is mandatory in every patient with an
implantable device [7, 12]. High-risk patients, such as
those with high thresholds and those who are pacemaker dependent, should be excluded from undergoing MRI [8] or special attention should be paid to them
[7]. Maximum safety for patients with implantable
cardiac device who undergo MRI is ensured if these
guidelines are applied [7].
Conclusions
Magnetic resonance imaging in patients with implanted
cardiac prostheses is sometimes a risky procedure [22].
Thus MRI in these patients should be performed only
when it is the only examination able to help with the
diagnosis. Moreover the diagnostic benefit must outweigh the risks [7, 19, 34]. Coronary artery stents, prosthetic cardiac valves, metal sternal sutures, mediastinal
clips, and epicardial pacing wires are not contraindications for MRI, in contrast to pacemakers and implantable
cardioverter-defibrillators [8]. It appears to be feasible
and safe for MRI to be performed at 1.5 T [15–17, 50, 63,
67]. Appropriate patient selection and precautions ensure
MRI safety [8]. However it is commonly accepted that
although hundreds of patients with pacemakers or implantable cardioverter-defibrillators underwent safe
MRI, it is not a safe procedure [7]. Currently, heating of
the pacemaker leads is the major problem undermining
MRI safety [8]. According to the FDA [19], there are
currently neither ’’MRI-safe’’ nor ’’MRI-compatible’’
pacemakers and implantable cardioverter-defibrillators.
Continuous monitoring—including pulse oximetry and
heart rate control as well as verbal communication with
the patient—along with performing MRI in a specialized
center with experts and resuscitation equipment on
hand, ensure maximum MRI patient safety [67].
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