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Implantable Devices and Magnetic Resonance Imaging
ORIGINAL ARTICLE
Original Article
Implantable Devices and Magnetic Resonance Imaging Chijen Hsu, FRACP, MMed a,c , Geoffrey Parker, FRANZCR b,c and Rajesh Puranik, FRACP, PhD a,c,∗ a
Department of Cardiology, Royal Prince Alfred Hospital, Sydney, Australia Department of Radiology, Royal Prince Alfred Hospital, Sydney, Australia Specialist Magnetic Resonance Imaging, Royal Prince Alfred Hospital Medical Centre, Sydney, Australia b
c
The indications for cardiovascular implantable electronic devices (CIEDs) are ever expanding, seemingly in parallel to the similar widespread increase in the use of magnetic resonance imaging (MRI), where there are clear advantages of imaging with no ionizing radiation and superior tissue contrast. However, CIEDs have traditionally been considered an absolute contraindication to MRI, posing a major limitation to investigating various pathologies after implantation of such devices. In the last decade the traditional paradigm of avoiding MRI in patients with CIEDs has been challenged with studies demonstrating relative safety at 1.5 T under certain circumstances. Now with the recent approval of ‘MR conditional’ devices, it is becoming increasingly apparent that CIEDs should no longer be considered an absolute contraindication to MRI. (Heart, Lung and Circulation 2012;21:358–363) Crown Copyright © 2012 Published by Elsevier Inc. on behalf of Australian and New Zealand Society of Cardiac and Thoracic Surgeons (ANZSCTS) and the Cardiac Society of Australia and New Zealand (CSANZ). All rights reserved. Keywords. Magnetic resonance imaging; Pacemaker; Implantable-cardioverter-defibrillator
Introduction
cardiovascular pathologies. Therefore, it is estimated that there is a 50–75% probability of a patient with a pacehe use of cardiovascular implantable electronic maker requiring MRI over a lifetime [2]. However, due devices (CIEDs), such as pacemakers and to the ferromagnetic nature of CIEDs, as well as laboimplantable-cardioverter-defibrillators (ICD), is growing, ratory and clinical reports of harm, including 10 deaths as new indications for heart failure, arrhythmia, and in the late 1980s [2], multiple professional societies, and other cardiac conditions are established. The number of cardiac rhythm device manufacturers have continued to implanted CIEDs will continue to grow as the ageing advise against MRI in patients with pacemakers and ICDs population continues to increase globally. Additionally, [3–5]. now that there are greater numbers of adult survivors of Laboratory and clinical studies since the mid-1990s repaired congenital heart disease, it can be expected that till today, employing careful protocols on newer devices a greater number of younger patients will also require [6–10], have demonstrated no untoward complications in CIEDs in the future. patients with CIEDs in situ undergoing MRI. This develIn the last decade, the use of magnetic resonance opment, along with the recent regulatory approval of the imaging (MRI) has increased exponentially [1]. Magnetic first pacemaker specifically designed and proven to be safe resonance imaging does not involve ionizing radiation and in a MR environment [11], has assisted in the rethinking hence has no potentially cumulative effects where serial of the traditional and conservative paradigm regarding studies are required, and has unrestricted choice of twoMRI of patients with CIEDs. Many professional societies dimensional or three-dimensional imaging plane. It is also have started to acknowledge that in some cases the bensuperior to X-ray computer tomography (CT) in displayefit of MRI may outweigh the risks [3,4,12,13]. In the ing soft tissue contrast of the various body organs, and can future, experience of MRI of patients with CIEDs in spetake advantage of gadolinium-containing contrast media. cialised centres is anticipated to grow for certain specified Thus MRI has become a very valuable and safe tool in indications. evaluating central nervous system, musculoskeletal and In this review, we will outline the theoretical effects of MRI on implanted CIEDs, and the results of recent major studies examining the issue of safety. We will also outAvailable online 27 April 2012 line a practical check list for the safe performance of MRI ∗ Corresponding author at: Department of Cardiology, Royal in patients with CIEDs based on recent studies, as well Prince Alfred Hospital, Missenden Rd, Camperdown, Sydney, as discuss issues related to the recently approved ‘MRNSW 2050, Australia. Tel.: +61 2 9515 6111; fax: +61 2 9550 6262. conditional’ pacemakers. E-mail address: [email protected] (R. Puranik). Crown Copyright © 2012 Published by Elsevier Inc. on behalf of Australian and New Zealand Society of Cardiac and Thoracic Surgeons (ANZSCTS) and the Cardiac Society of Australia and New Zealand (CSANZ). All rights 1443-9506/04/$36.00 reserved. http://dx.doi.org/10.1016/j.hlc.2012.04.004
T
Effects of MRI on Devices Basic Physics of MRI Water, and therefore hydrogen nuclei (protons) are abundant in the soft tissues of the human body. Magnetic resonance imaging applies a strong static magnetic field, for example 1.5 Tesla (T) (≈15,000 times the earth magnetic filed), such that otherwise randomly orientated protons within the body are aligned along the magnetic field. A radiofrequency (RF) pulse applied to the body will cause spatial changes in the axis of protons in the body and these protons will emit an RF signal detectable with a receiver coil placed on the body when the RF pulse is removed. The relaxation properties of protons after the RF pulse application are specific for different types of tissues and characterised by T1 and T2 constants, therefore allowing for the generation of image contrast. Additional fast switching magnetic gradient fields are applied to allow for slice selection or 3D encoding of the MR signal [12]. These three essential electromagnetic components of a MRI scanner may have significant effects on CIEDs, which are ferromagnetic and electroconductive.
Mechanical Effects CIEDs usually contain small amounts of metals, such as iron, cobalt, or nickel, etc., and they are generally ferromagnetic [1]. Therefore, there is some concern that CIEDs may potentially move inside a MRI scanner. The newer generations of CIEDs are designed to be smaller, and consist of lesser amounts of ferromagnetic materials, therefore much unlikely to move. Even though the new-generation ICDs still attract 10 times higher magnetic force and torque than pacemakers, that force is similar to the gravity of the earth, and thus not considered to be a safety concern any more [6,14]. However, as a precaution, MRI should be delayed for at least six weeks after CIEDs implantation [15] to allow secure incorporation into tissues and hence minimising movement during MRI.
Effects on Sensing and Pacing CIED function may be altered in the presence of strong magnetic fields. Typically effects may be due to (i) RF effects, (ii) magnetic reed switch activity, (iii) power on reset phenomenon, or (iv) induction effects. RF EFFECTS. Oversensing of non-cardiac signals may occur, and result in inhibition of pacing and cause bradycardia–asystole in a pacemaker-dependent patient; oversensing of ventricular tachyarrhythmia by ICDs may result in inappropriate therapies, including shocks [15,16]. MAGNETIC REED SWITCH ACTIVITY. Most pacemakers and ICDs have built-in magnetic reed switches that are designed to turn ON and OFF circuitry in response to magnets. The presence of a strong background magnetic field, and fast switching magnetic gradient field inside a MRI scanner, may theoretically lead to reed switch activity becoming intermittent and unpredictable [15,17]. However, it is now accepted that these problems are mitigated by appropriately programming the CIED’s prior to MRI.
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POWER ON RESET PHENOMENON. The third concern is ‘power on reset’ phenomenon. This is a common design technique to enable a newly powered on electrical device to a known baseline state. In the circumstance of unusual input conditions such as too low or too high line voltages then a reset may be triggered for circuit stability. Cardiovascular implantable electronic devices have a variable susceptibility to resets and the mode that results is variable. Extreme magnetic fields can cause override of any previous programming in a CIED, and revert it back to factory setting, or other settings [15], which would be potentially dangerous for pacemaker-dependent patients or ICD patients. It should be noted that overall the incidence of resets is low and mostly occurs in a few known more susceptible devices. However, although a low risk, this is perhaps the greatest risk to the pacemaker dependent patient whose CIED is set to VOO mode which may reset to VVI mode, and then the CIED may stop pacing due to inhibition from RF interference. Similarly, ICDs may reset into VVI mode with tachy therapy turned on, and start inappropriately responding to RF interference.
INDUCTION EFFECTS. The lead system in a CIED is a wire, and fluctuating magnetic fields during MRI can induce a current in these leads, especially if they are of certain length or in the form of a loop or coil. The myocardial tissue near the lead tip can sustain thermal injury as the electrical energy is converted into heat [18], and cause oedema or even formation of scar tissue within the myocardium. The increase in temperature locally is limited to approximately 6◦ or less. Generally, by using established clinical MRI protocols, the lead tip may only heat up by <3.9 ◦ C in vitro, and 0.2 ◦ C in vivo [6]. Sensing and pacing threshold and impedance of CIEDs may subsequently transiently change according to these effects. In addition, the induced current can also stimulate the heart near the lead tip, so-called ‘unintended cardiac stimulation’ [15]. These potential risks may also occur in abandoned leads [15].
Clinical Studies of MR Effects on Implantable Devices Pacemakers Studies from 1980s to the mid-1990s had shown some adverse effects of MRI on CIEDs, including 10 deaths in the late 1980s [2,15–17,19]. However, the details of those deaths were not well documented, and no ECG data were available. Also, those early studies were using earlier generation pacemakers, which are no longer in use. Many prospective studies from the mid-1990s to today, using carefully designed reprogramming prior to MRI scan [1,9,10], have not found any clinically significant complications. The reported problems from these studies include: brief pacing output inhibition [20,21], temporary device communication failure [22], diminished battery voltage [10,11,23,24], reed switch activation [11,24], sensing or pacing threshold changes [9,10,22–24], and power-on-reset [24,25]; none of which led to clinically significant outcomes.
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The safety of pacemaker-dependent patients undergoing thoracic and non-thoracic MRI was also analysed in two prospective studies [24,26]. Prior to scanning, patients’ pacemakers were reprogramed to asynchronous pacing (VOO or DOO mode). All scans were completed uneventfully, and no significant pacemaker malfunction was reported.
ICD ICDs are larger sized than pacemakers and generally contain more ferromagnetic material, and therefore are likely to encounter greater magnetic field interactions. Additional concerns facing ICDs are their transformers. In the environment of a strong magnetic field, the transformers might be rendered unable to charge the capacitor, potentially leading to an inactive device [8], and the elective replacement interval (ERI) indicator might also be triggered as a result. This situation is retrievable by reprogramming the ICD after the scan is complete. If the device was close to ERI prior to the scan, then the battery usage during failed charging attempts could trip to ERI and then actually require replacement. Inside a strong magnetic field ICDs may also fail to detect ventricular arrhythmia, and ICDs may misinterpret radiofrequency pulse during MRI as ventricular tachyarrhythmia [6,25]. The detrimental consequences of these scenarios include failure to deliver appropriate therapies or delivery of inappropriate therapies, both of which are highly undesirable for the patient whilst in the MR scanner. Several prospective studies, of almost 500 patients in total, have been performed recently to assess the safety of ICDs and MRI [1,8,23,24,26,27]. The studies were performed with continuous ECG monitoring, where all ICDs were programmed to ‘therapy off’ mode to avoid inappropriate shocks, and there were no clinically significant events reported. The reported problems from these studies include: oversensing of RF as ventricular fibrillation [23], altered sensing or pacing thresholds [28], brief pacing output inhibition [11], power-on-reset [27], and diminished battery voltage [23]. Overall, the incidences of these events were very small, mainly due to appropriate prescan assessment and CIED management. To date, only approximately 37 patients have been reported in the literature whereby MRI safety has been assessed in patients with cardiac resynchronisation therapy devices (CRT) [24,29,30]. These cases thus far have not been shown to have more adverse events than other CIEDs.
Permanent and Temporary Pacing Leads In some situations patients have endocardial leads left in place after pulse generators of CIEDs are removed. These abandoned leads can also receive and induce currents, and result in significant heating and local thermal injury during MRI. The safety of these abandoned leads during MRI has not been systematically studied, and MRI should only be considered with caution [12]. Those lead fragments of
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lesser length, especially when without coiling, are likely to have fewer interactions in the MR environment. Temporary epicardial pacing wires, usually made of stainless steel, are sutured to the epicardial surface of the heart after cardiac surgery. These wires can be connected to an external pacemaker if the patients develop bradycardia or atrioventricular block postoperatively [12]. A study of 51 patients with abandoned epicardial wires undergoing non-thoracic MRI did not demonstrate any significant adverse outcomes [12]. Therefore, epicardial pacing wires are deemed low risk for non-thoracic MR imaging.
Non-electrical Implantable Devices Coronary Stents Many coronary stents are made of alloys, and they are weakly ferromagnetic. Several ex vivo studies have demonstrated their safety at 1.5 T in terms of magnetic field interactions [8]. Large clinical studies have also demonstrated their safety, even for MRI of patients within 1–3 days of stent deployment [31,32]. The safety of stents in a 3T MR scanner was also demonstrated in recent ex vivo studies [8,30].
Heart Valve Prostheses Mechanical heart valves contain many different kinds of metals. Nevertheless, an external magnetic field, even at 3T, produces forces smaller than those generated by a beating heart [8,33]. Clinical studies have not demonstrated any risks in MR scanning of mechanical valves, nor have they demonstrated any risks with metal sternal sutures and mediastinal clips [8,34]. Importantly, this finding includes the Starr-Edwards Model Pre-6000 heart valve prosthesis previously suggested to be a potential risk for a patient undergoing an MR examination.
Safety of 3.0 T MR Scanning on CIEDs Apart from a few small studies [8,11] the majority of clinical studies to date regarding the safety of MRI on CIEDs were performed using scanners with static magnetic field of 1.5 T or less. Therefore, more ex vivo and clinical studies using 3-T scanners are needed before conclusions can be drawn on the safety of CIEDs inside a 3 T scanner.
How to Safely Scan CIED Patients Inside a MRI Scanner CIEDs are no longer considered an absolute contraindication to MRI [3,35]. However, the decision to proceed with MRI must be carefully thought through on a case-by-case basis, and specific scanning and monitoring conditions are followed, as low risk does not mean no risk. To help decide whether to perform MRI in patients with CIEDs, clinicians need to be convinced that the clinical benefit and utility is high, and there are no other alternative imaging modalities whereby the clinical question can be answered. Continuous monitoring of any adverse reports on CIEDs undergoing MRI, by checking latest literature and MRI safety websites, or contacting device representatives prior
Table 1. Safety Check List for MRI of Patients With CIEDs. Decide on the scan 1. MR scanner field strength is 1.5 T or less. 2. Radiologists have discussed with patient’s physician the risks and possible alternative investigation modalities for the condition, and it is explicitly documented. 3. Patient has signed the informed consent. 4. Pacemaker is implanted after 1998, or ICD is implanted after 2000. 5. Leads were implanted ≥6 weeks before MRI. 6. There are no epicardial, abandoned, or no-fixation leads present. 7. Patients, who are pacemaker-dependent, and/or having frequent ventricular arrhythmia, should be excluded. Prior to the scan 1. Device technicians interrogate CIED parameters including extraction of recent device histograms. Deactivate all rate modulating parameters. Program lead polarity to bipolar if possible. Consider excluding patients with high pacing threshold. 2. Reprogram pacemaker in patients who are not pacemaker-dependent to VVI/DDI (inhibited). Deactivate monitoring and all tachyarrhythmia therapies in ICD. 3. Resuscitation trolley is nearby, and equipped with external defibrillator-pacemaker (not AED). During the scan 1. Experienced cardiologist in CIEDs is present. 2. Monitor symptoms, blood pressure, ECG, pulse oximetry, and symptoms. Do not accept the MR scanner’s vectorcardiogram (VCG) as acceptable for arrhythmia monitoring. 3. Minimise number and lengths of sequence. After the scan 1. Device technician recheck device parameters and compare with baseline values. Restore original programming. 2. Follow up interrogation in 3–6 months.
to scanning, will help to avoid scanning more dangerous devices. Clinicians must take patients through the benefits and risks of MRI, and an informed consent is signed beforehand. Based on a number of recent clinical studies which did not show any significant adverse outcomes [1,11,24], the criteria for increasing safety for patients with CIEDs undertaking MRI can be formulated (Table 1). Pacemakers implanted after 1998, or ICD implanted after 2000, using standard transvenous lead systems, are considered relatively safe for MRI in patients who are NOT pacemaker dependent and where ventricular arrhythmias are relatively infrequent; also safety might be increased further by delaying MRI until six weeks after device implantation [15]. It should be noted that MR scanning safety recommendations have been established at 1.5 T, but not at 3 T. This is especially relevant at this time as most newer instalments of MR scanners are at higher field strength to take advantage of improved signal to noise ratio, and hence more optimal spatial resolution. Despite relative safety demonstrated in recent literature for MR scanning of CIEDs, MRI of pacemaker-dependent patients and patients with frequent ventricular arrhythmias, is still best avoided. In patients with CIEDs in whom the pacemaker function is not essential, the CIED pacing setting should be reprogrammed to VVI/DDI (inhibited) to avoid inappropriate pacing from tracking of
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electromagnetic interference (EMI). Similarly, all tachyarrhythmia monitoring and therapies in ICDs are deactivated to avoid inappropriate therapies from inappropriate tracking of EMI [24]. Other sensing and pacing functions, such as rate response, noise, and conducted atrial fibrillation response, etc., are also deactivated to avoid sensing of EMI leading to inappropriate pacing. The presence of an experienced cardiologist in CIEDs during MRI, and continuous monitoring of electrocardiogram (ECG), pulse oximetry, blood pressure and symptoms during scanning are essential. Magnetic resonance imaging facilities should have access to a cardiac arrest trolley and a well rehearsed cardiac arrest protocol from within the MR scanner. There are a number of metal safety issues which need to have been thought through and practiced to ensure good outcomes in a cardiac arrest situation whilst undertaking MRI. The number and length of MR scanning sequences should be reduced to the minimum required to answer the clinical question. Interrogation and restoration of CIED original programming should be performed immediately after the scan, and reviewed again within 3–6 months. It is important to ensure ERI indictor is not triggered, and if so, it might be retrievable by re-programming or initiating capacitor charge/discharge after the scan is complete.
‘MR Conditional’ Cardiac Implantable Electronic Device Despite many advancements, current technologies have not been able to design a completely ‘MR safe’ CIED as it would require non-metallic, non-conducting materials and systems with no known hazards in all MR environments [11]. ‘MR conditional’ refers to devices, which pose no known hazards when applied with specific conditions and a specific MR environment, and the approval of such device requires strict definition of these conditions [11]. Therefore, a ‘MR conditional’ CIED system comprises of ‘MR conditional’ pulse generator attached to ‘MR conditional’ leads. ‘MR conditional’ generators for example cannot be simply attached to pre-existing leads, and still be labelled as a ‘MR conditional’ system [11]. The implications here are to be deemed a dedicated ‘MR conditional’ system would require the explant of the entire previous system, which carries its own significant risks [36]. In 2011 United States Food and Drug Administration approved a ‘MR conditional’ pacing system, the RevoTM MRI SureScan® Pacing System (Medtronic, Inc., Minneapolis, MN), following a study of 258 patients without any significant adverse outcomes, during which thoracic and non-thoracic imaging was conducted at 1.5 T [37]. Biotronik (Berlin, Germany), and St. Jude Medical (St. Paul, MN) also have ‘MR conditional’ pacing systems approved in other countries. Each system has its own approved conditions, which must be checked and followed as part of ‘MR conditional’ system requirements. For example, the Biotronik system has a scan exclusion zone covering thorax and abdomen, and scanning time should not exceed 30 min per session. The St. Jude system advises against positioning the coils directly over the pacing system [11].
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Further, most of these systems have specific limitations on specific absorption rate (SAR) and gradient slew rate performance, which may necessitate altering the sequence parameters used whilst scanning. All current ‘MR conditional’ systems are only approved for use in a MR scanner of 1.5 T, and scanning is not approved for higher field strengths [11]. Instructions regarding the reprogramming of the pacemaker before and after the scan, and close monitoring during scanning still apply to these ‘MR conditional’ pacemakers. It should be noted that there are currently no approved ‘MR conditional’ ICDs available. Many manufacturers have now also produced ‘MR conditional’ implantable cardiac monitors, which are also considered safe for MRI.
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[6]
[7]
[8]
[9]
Conclusions CIED should no longer be considered an absolute contraindication to MRI. However, low risk does not mean no risk, and the assessment of risk and benefit should be evaluated on a case by case basis. In particular, clinicians need to be sure that there is no alternative imaging modality which can adequately answer the clinical question for the patient. Using proven safety protocols in patients who are being imaged at 1.5 T or less and who are NOT dependent on the pacemaker function of their CIEDs and in whom there is relatively infrequent arrhythmia, MRI can be performed safely if the CIED is reprogrammed to an inhibited mode. The safety margin of MR scanning in these patients will be improved further as more patients are implanted with ‘MR conditional’ devices in the future.
[10]
[11]
[12]
[13]
Acknowledgement Chijen Hsu is funded as the CMRS Cardiac MRI Fellow (www.cmrs.org.au).
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ORIGINAL ARTICLE
Heart, Lung and Circulation 2012;21:358–363
Original Article
Implantable Devices and Magnetic Resonance Imaging Chijen Hsu, FRACP, MMed a,c , Geoffrey Parker, FRANZCR b,c and Rajesh Puranik, FRACP, PhD a,c,∗ a
Department of Cardiology, Royal Prince Alfred Hospital, Sydney, Australia Department of Radiology, Royal Prince Alfred Hospital, Sydney, Australia Specialist Magnetic Resonance Imaging, Royal Prince Alfred Hospital Medical Centre, Sydney, Australia b
c
The indications for cardiovascular implantable electronic devices (CIEDs) are ever expanding, seemingly in parallel to the similar widespread increase in the use of magnetic resonance imaging (MRI), where there are clear advantages of imaging with no ionizing radiation and superior tissue contrast. However, CIEDs have traditionally been considered an absolute contraindication to MRI, posing a major limitation to investigating various pathologies after implantation of such devices. In the last decade the traditional paradigm of avoiding MRI in patients with CIEDs has been challenged with studies demonstrating relative safety at 1.5 T under certain circumstances. Now with the recent approval of ‘MR conditional’ devices, it is becoming increasingly apparent that CIEDs should no longer be considered an absolute contraindication to MRI. (Heart, Lung and Circulation 2012;21:358–363) Crown Copyright © 2012 Published by Elsevier Inc. on behalf of Australian and New Zealand Society of Cardiac and Thoracic Surgeons (ANZSCTS) and the Cardiac Society of Australia and New Zealand (CSANZ). All rights reserved. Keywords. Magnetic resonance imaging; Pacemaker; Implantable-cardioverter-defibrillator
Introduction
cardiovascular pathologies. Therefore, it is estimated that there is a 50–75% probability of a patient with a pacehe use of cardiovascular implantable electronic maker requiring MRI over a lifetime [2]. However, due devices (CIEDs), such as pacemakers and to the ferromagnetic nature of CIEDs, as well as laboimplantable-cardioverter-defibrillators (ICD), is growing, ratory and clinical reports of harm, including 10 deaths as new indications for heart failure, arrhythmia, and in the late 1980s [2], multiple professional societies, and other cardiac conditions are established. The number of cardiac rhythm device manufacturers have continued to implanted CIEDs will continue to grow as the ageing advise against MRI in patients with pacemakers and ICDs population continues to increase globally. Additionally, [3–5]. now that there are greater numbers of adult survivors of Laboratory and clinical studies since the mid-1990s repaired congenital heart disease, it can be expected that till today, employing careful protocols on newer devices a greater number of younger patients will also require [6–10], have demonstrated no untoward complications in CIEDs in the future. patients with CIEDs in situ undergoing MRI. This develIn the last decade, the use of magnetic resonance opment, along with the recent regulatory approval of the imaging (MRI) has increased exponentially [1]. Magnetic first pacemaker specifically designed and proven to be safe resonance imaging does not involve ionizing radiation and in a MR environment [11], has assisted in the rethinking hence has no potentially cumulative effects where serial of the traditional and conservative paradigm regarding studies are required, and has unrestricted choice of twoMRI of patients with CIEDs. Many professional societies dimensional or three-dimensional imaging plane. It is also have started to acknowledge that in some cases the bensuperior to X-ray computer tomography (CT) in displayefit of MRI may outweigh the risks [3,4,12,13]. In the ing soft tissue contrast of the various body organs, and can future, experience of MRI of patients with CIEDs in spetake advantage of gadolinium-containing contrast media. cialised centres is anticipated to grow for certain specified Thus MRI has become a very valuable and safe tool in indications. evaluating central nervous system, musculoskeletal and In this review, we will outline the theoretical effects of MRI on implanted CIEDs, and the results of recent major studies examining the issue of safety. We will also outAvailable online 27 April 2012 line a practical check list for the safe performance of MRI ∗ Corresponding author at: Department of Cardiology, Royal in patients with CIEDs based on recent studies, as well Prince Alfred Hospital, Missenden Rd, Camperdown, Sydney, as discuss issues related to the recently approved ‘MRNSW 2050, Australia. Tel.: +61 2 9515 6111; fax: +61 2 9550 6262. conditional’ pacemakers. E-mail address: [email protected] (R. Puranik). Crown Copyright © 2012 Published by Elsevier Inc. on behalf of Australian and New Zealand Society of Cardiac and Thoracic Surgeons (ANZSCTS) and the Cardiac Society of Australia and New Zealand (CSANZ). All rights 1443-9506/04/$36.00 reserved. http://dx.doi.org/10.1016/j.hlc.2012.04.004
T
Effects of MRI on Devices Basic Physics of MRI Water, and therefore hydrogen nuclei (protons) are abundant in the soft tissues of the human body. Magnetic resonance imaging applies a strong static magnetic field, for example 1.5 Tesla (T) (≈15,000 times the earth magnetic filed), such that otherwise randomly orientated protons within the body are aligned along the magnetic field. A radiofrequency (RF) pulse applied to the body will cause spatial changes in the axis of protons in the body and these protons will emit an RF signal detectable with a receiver coil placed on the body when the RF pulse is removed. The relaxation properties of protons after the RF pulse application are specific for different types of tissues and characterised by T1 and T2 constants, therefore allowing for the generation of image contrast. Additional fast switching magnetic gradient fields are applied to allow for slice selection or 3D encoding of the MR signal [12]. These three essential electromagnetic components of a MRI scanner may have significant effects on CIEDs, which are ferromagnetic and electroconductive.
Mechanical Effects CIEDs usually contain small amounts of metals, such as iron, cobalt, or nickel, etc., and they are generally ferromagnetic [1]. Therefore, there is some concern that CIEDs may potentially move inside a MRI scanner. The newer generations of CIEDs are designed to be smaller, and consist of lesser amounts of ferromagnetic materials, therefore much unlikely to move. Even though the new-generation ICDs still attract 10 times higher magnetic force and torque than pacemakers, that force is similar to the gravity of the earth, and thus not considered to be a safety concern any more [6,14]. However, as a precaution, MRI should be delayed for at least six weeks after CIEDs implantation [15] to allow secure incorporation into tissues and hence minimising movement during MRI.
Effects on Sensing and Pacing CIED function may be altered in the presence of strong magnetic fields. Typically effects may be due to (i) RF effects, (ii) magnetic reed switch activity, (iii) power on reset phenomenon, or (iv) induction effects. RF EFFECTS. Oversensing of non-cardiac signals may occur, and result in inhibition of pacing and cause bradycardia–asystole in a pacemaker-dependent patient; oversensing of ventricular tachyarrhythmia by ICDs may result in inappropriate therapies, including shocks [15,16]. MAGNETIC REED SWITCH ACTIVITY. Most pacemakers and ICDs have built-in magnetic reed switches that are designed to turn ON and OFF circuitry in response to magnets. The presence of a strong background magnetic field, and fast switching magnetic gradient field inside a MRI scanner, may theoretically lead to reed switch activity becoming intermittent and unpredictable [15,17]. However, it is now accepted that these problems are mitigated by appropriately programming the CIED’s prior to MRI.
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POWER ON RESET PHENOMENON. The third concern is ‘power on reset’ phenomenon. This is a common design technique to enable a newly powered on electrical device to a known baseline state. In the circumstance of unusual input conditions such as too low or too high line voltages then a reset may be triggered for circuit stability. Cardiovascular implantable electronic devices have a variable susceptibility to resets and the mode that results is variable. Extreme magnetic fields can cause override of any previous programming in a CIED, and revert it back to factory setting, or other settings [15], which would be potentially dangerous for pacemaker-dependent patients or ICD patients. It should be noted that overall the incidence of resets is low and mostly occurs in a few known more susceptible devices. However, although a low risk, this is perhaps the greatest risk to the pacemaker dependent patient whose CIED is set to VOO mode which may reset to VVI mode, and then the CIED may stop pacing due to inhibition from RF interference. Similarly, ICDs may reset into VVI mode with tachy therapy turned on, and start inappropriately responding to RF interference.
INDUCTION EFFECTS. The lead system in a CIED is a wire, and fluctuating magnetic fields during MRI can induce a current in these leads, especially if they are of certain length or in the form of a loop or coil. The myocardial tissue near the lead tip can sustain thermal injury as the electrical energy is converted into heat [18], and cause oedema or even formation of scar tissue within the myocardium. The increase in temperature locally is limited to approximately 6◦ or less. Generally, by using established clinical MRI protocols, the lead tip may only heat up by <3.9 ◦ C in vitro, and 0.2 ◦ C in vivo [6]. Sensing and pacing threshold and impedance of CIEDs may subsequently transiently change according to these effects. In addition, the induced current can also stimulate the heart near the lead tip, so-called ‘unintended cardiac stimulation’ [15]. These potential risks may also occur in abandoned leads [15].
Clinical Studies of MR Effects on Implantable Devices Pacemakers Studies from 1980s to the mid-1990s had shown some adverse effects of MRI on CIEDs, including 10 deaths in the late 1980s [2,15–17,19]. However, the details of those deaths were not well documented, and no ECG data were available. Also, those early studies were using earlier generation pacemakers, which are no longer in use. Many prospective studies from the mid-1990s to today, using carefully designed reprogramming prior to MRI scan [1,9,10], have not found any clinically significant complications. The reported problems from these studies include: brief pacing output inhibition [20,21], temporary device communication failure [22], diminished battery voltage [10,11,23,24], reed switch activation [11,24], sensing or pacing threshold changes [9,10,22–24], and power-on-reset [24,25]; none of which led to clinically significant outcomes.
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The safety of pacemaker-dependent patients undergoing thoracic and non-thoracic MRI was also analysed in two prospective studies [24,26]. Prior to scanning, patients’ pacemakers were reprogramed to asynchronous pacing (VOO or DOO mode). All scans were completed uneventfully, and no significant pacemaker malfunction was reported.
ICD ICDs are larger sized than pacemakers and generally contain more ferromagnetic material, and therefore are likely to encounter greater magnetic field interactions. Additional concerns facing ICDs are their transformers. In the environment of a strong magnetic field, the transformers might be rendered unable to charge the capacitor, potentially leading to an inactive device [8], and the elective replacement interval (ERI) indicator might also be triggered as a result. This situation is retrievable by reprogramming the ICD after the scan is complete. If the device was close to ERI prior to the scan, then the battery usage during failed charging attempts could trip to ERI and then actually require replacement. Inside a strong magnetic field ICDs may also fail to detect ventricular arrhythmia, and ICDs may misinterpret radiofrequency pulse during MRI as ventricular tachyarrhythmia [6,25]. The detrimental consequences of these scenarios include failure to deliver appropriate therapies or delivery of inappropriate therapies, both of which are highly undesirable for the patient whilst in the MR scanner. Several prospective studies, of almost 500 patients in total, have been performed recently to assess the safety of ICDs and MRI [1,8,23,24,26,27]. The studies were performed with continuous ECG monitoring, where all ICDs were programmed to ‘therapy off’ mode to avoid inappropriate shocks, and there were no clinically significant events reported. The reported problems from these studies include: oversensing of RF as ventricular fibrillation [23], altered sensing or pacing thresholds [28], brief pacing output inhibition [11], power-on-reset [27], and diminished battery voltage [23]. Overall, the incidences of these events were very small, mainly due to appropriate prescan assessment and CIED management. To date, only approximately 37 patients have been reported in the literature whereby MRI safety has been assessed in patients with cardiac resynchronisation therapy devices (CRT) [24,29,30]. These cases thus far have not been shown to have more adverse events than other CIEDs.
Permanent and Temporary Pacing Leads In some situations patients have endocardial leads left in place after pulse generators of CIEDs are removed. These abandoned leads can also receive and induce currents, and result in significant heating and local thermal injury during MRI. The safety of these abandoned leads during MRI has not been systematically studied, and MRI should only be considered with caution [12]. Those lead fragments of
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lesser length, especially when without coiling, are likely to have fewer interactions in the MR environment. Temporary epicardial pacing wires, usually made of stainless steel, are sutured to the epicardial surface of the heart after cardiac surgery. These wires can be connected to an external pacemaker if the patients develop bradycardia or atrioventricular block postoperatively [12]. A study of 51 patients with abandoned epicardial wires undergoing non-thoracic MRI did not demonstrate any significant adverse outcomes [12]. Therefore, epicardial pacing wires are deemed low risk for non-thoracic MR imaging.
Non-electrical Implantable Devices Coronary Stents Many coronary stents are made of alloys, and they are weakly ferromagnetic. Several ex vivo studies have demonstrated their safety at 1.5 T in terms of magnetic field interactions [8]. Large clinical studies have also demonstrated their safety, even for MRI of patients within 1–3 days of stent deployment [31,32]. The safety of stents in a 3T MR scanner was also demonstrated in recent ex vivo studies [8,30].
Heart Valve Prostheses Mechanical heart valves contain many different kinds of metals. Nevertheless, an external magnetic field, even at 3T, produces forces smaller than those generated by a beating heart [8,33]. Clinical studies have not demonstrated any risks in MR scanning of mechanical valves, nor have they demonstrated any risks with metal sternal sutures and mediastinal clips [8,34]. Importantly, this finding includes the Starr-Edwards Model Pre-6000 heart valve prosthesis previously suggested to be a potential risk for a patient undergoing an MR examination.
Safety of 3.0 T MR Scanning on CIEDs Apart from a few small studies [8,11] the majority of clinical studies to date regarding the safety of MRI on CIEDs were performed using scanners with static magnetic field of 1.5 T or less. Therefore, more ex vivo and clinical studies using 3-T scanners are needed before conclusions can be drawn on the safety of CIEDs inside a 3 T scanner.
How to Safely Scan CIED Patients Inside a MRI Scanner CIEDs are no longer considered an absolute contraindication to MRI [3,35]. However, the decision to proceed with MRI must be carefully thought through on a case-by-case basis, and specific scanning and monitoring conditions are followed, as low risk does not mean no risk. To help decide whether to perform MRI in patients with CIEDs, clinicians need to be convinced that the clinical benefit and utility is high, and there are no other alternative imaging modalities whereby the clinical question can be answered. Continuous monitoring of any adverse reports on CIEDs undergoing MRI, by checking latest literature and MRI safety websites, or contacting device representatives prior
Table 1. Safety Check List for MRI of Patients With CIEDs. Decide on the scan 1. MR scanner field strength is 1.5 T or less. 2. Radiologists have discussed with patient’s physician the risks and possible alternative investigation modalities for the condition, and it is explicitly documented. 3. Patient has signed the informed consent. 4. Pacemaker is implanted after 1998, or ICD is implanted after 2000. 5. Leads were implanted ≥6 weeks before MRI. 6. There are no epicardial, abandoned, or no-fixation leads present. 7. Patients, who are pacemaker-dependent, and/or having frequent ventricular arrhythmia, should be excluded. Prior to the scan 1. Device technicians interrogate CIED parameters including extraction of recent device histograms. Deactivate all rate modulating parameters. Program lead polarity to bipolar if possible. Consider excluding patients with high pacing threshold. 2. Reprogram pacemaker in patients who are not pacemaker-dependent to VVI/DDI (inhibited). Deactivate monitoring and all tachyarrhythmia therapies in ICD. 3. Resuscitation trolley is nearby, and equipped with external defibrillator-pacemaker (not AED). During the scan 1. Experienced cardiologist in CIEDs is present. 2. Monitor symptoms, blood pressure, ECG, pulse oximetry, and symptoms. Do not accept the MR scanner’s vectorcardiogram (VCG) as acceptable for arrhythmia monitoring. 3. Minimise number and lengths of sequence. After the scan 1. Device technician recheck device parameters and compare with baseline values. Restore original programming. 2. Follow up interrogation in 3–6 months.
to scanning, will help to avoid scanning more dangerous devices. Clinicians must take patients through the benefits and risks of MRI, and an informed consent is signed beforehand. Based on a number of recent clinical studies which did not show any significant adverse outcomes [1,11,24], the criteria for increasing safety for patients with CIEDs undertaking MRI can be formulated (Table 1). Pacemakers implanted after 1998, or ICD implanted after 2000, using standard transvenous lead systems, are considered relatively safe for MRI in patients who are NOT pacemaker dependent and where ventricular arrhythmias are relatively infrequent; also safety might be increased further by delaying MRI until six weeks after device implantation [15]. It should be noted that MR scanning safety recommendations have been established at 1.5 T, but not at 3 T. This is especially relevant at this time as most newer instalments of MR scanners are at higher field strength to take advantage of improved signal to noise ratio, and hence more optimal spatial resolution. Despite relative safety demonstrated in recent literature for MR scanning of CIEDs, MRI of pacemaker-dependent patients and patients with frequent ventricular arrhythmias, is still best avoided. In patients with CIEDs in whom the pacemaker function is not essential, the CIED pacing setting should be reprogrammed to VVI/DDI (inhibited) to avoid inappropriate pacing from tracking of
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electromagnetic interference (EMI). Similarly, all tachyarrhythmia monitoring and therapies in ICDs are deactivated to avoid inappropriate therapies from inappropriate tracking of EMI [24]. Other sensing and pacing functions, such as rate response, noise, and conducted atrial fibrillation response, etc., are also deactivated to avoid sensing of EMI leading to inappropriate pacing. The presence of an experienced cardiologist in CIEDs during MRI, and continuous monitoring of electrocardiogram (ECG), pulse oximetry, blood pressure and symptoms during scanning are essential. Magnetic resonance imaging facilities should have access to a cardiac arrest trolley and a well rehearsed cardiac arrest protocol from within the MR scanner. There are a number of metal safety issues which need to have been thought through and practiced to ensure good outcomes in a cardiac arrest situation whilst undertaking MRI. The number and length of MR scanning sequences should be reduced to the minimum required to answer the clinical question. Interrogation and restoration of CIED original programming should be performed immediately after the scan, and reviewed again within 3–6 months. It is important to ensure ERI indictor is not triggered, and if so, it might be retrievable by re-programming or initiating capacitor charge/discharge after the scan is complete.
‘MR Conditional’ Cardiac Implantable Electronic Device Despite many advancements, current technologies have not been able to design a completely ‘MR safe’ CIED as it would require non-metallic, non-conducting materials and systems with no known hazards in all MR environments [11]. ‘MR conditional’ refers to devices, which pose no known hazards when applied with specific conditions and a specific MR environment, and the approval of such device requires strict definition of these conditions [11]. Therefore, a ‘MR conditional’ CIED system comprises of ‘MR conditional’ pulse generator attached to ‘MR conditional’ leads. ‘MR conditional’ generators for example cannot be simply attached to pre-existing leads, and still be labelled as a ‘MR conditional’ system [11]. The implications here are to be deemed a dedicated ‘MR conditional’ system would require the explant of the entire previous system, which carries its own significant risks [36]. In 2011 United States Food and Drug Administration approved a ‘MR conditional’ pacing system, the RevoTM MRI SureScan® Pacing System (Medtronic, Inc., Minneapolis, MN), following a study of 258 patients without any significant adverse outcomes, during which thoracic and non-thoracic imaging was conducted at 1.5 T [37]. Biotronik (Berlin, Germany), and St. Jude Medical (St. Paul, MN) also have ‘MR conditional’ pacing systems approved in other countries. Each system has its own approved conditions, which must be checked and followed as part of ‘MR conditional’ system requirements. For example, the Biotronik system has a scan exclusion zone covering thorax and abdomen, and scanning time should not exceed 30 min per session. The St. Jude system advises against positioning the coils directly over the pacing system [11].
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Further, most of these systems have specific limitations on specific absorption rate (SAR) and gradient slew rate performance, which may necessitate altering the sequence parameters used whilst scanning. All current ‘MR conditional’ systems are only approved for use in a MR scanner of 1.5 T, and scanning is not approved for higher field strengths [11]. Instructions regarding the reprogramming of the pacemaker before and after the scan, and close monitoring during scanning still apply to these ‘MR conditional’ pacemakers. It should be noted that there are currently no approved ‘MR conditional’ ICDs available. Many manufacturers have now also produced ‘MR conditional’ implantable cardiac monitors, which are also considered safe for MRI.
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[6]
[7]
[8]
[9]
Conclusions CIED should no longer be considered an absolute contraindication to MRI. However, low risk does not mean no risk, and the assessment of risk and benefit should be evaluated on a case by case basis. In particular, clinicians need to be sure that there is no alternative imaging modality which can adequately answer the clinical question for the patient. Using proven safety protocols in patients who are being imaged at 1.5 T or less and who are NOT dependent on the pacemaker function of their CIEDs and in whom there is relatively infrequent arrhythmia, MRI can be performed safely if the CIED is reprogrammed to an inhibited mode. The safety margin of MR scanning in these patients will be improved further as more patients are implanted with ‘MR conditional’ devices in the future.
[10]
[11]
[12]
[13]
Acknowledgement Chijen Hsu is funded as the CMRS Cardiac MRI Fellow (www.cmrs.org.au).
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