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Cardiac MRI of patients with implanted electrical cardiac devices Mark Ainslie, Christopher Miller, Benjamin Brown, Matthias Schmitt Cardiology Department, University Hospital of South Manchester, Manchester, UK Correspondence to Dr Mark Ainslie Cardiology Department, University Hospital of South Manchester, Southmoor Road, Manchester M23 9LT, UK; [email protected] Received 20 May 2013 Revised 1 July 2013 Accepted 2 July 2013 Published Online First 19 July 2013 This is a reprint of a paper that first appeared in Heart, 2014, volume 100, pages 363–369.
ABSTRACT Implantable pulse generators and defibrillators have traditionally been considered contraindications to MRI. However, recent data have challenged this paradigm and demonstrated that patients with newer generation devices can safely undergo MRI, including cardiac MRI, provided basic precautions are taken. Indeed, the introduction of MRI conditional systems has led to a conceptual shift in clinical decision making—‘can this patient undergo MRI safely?’ is being superseded by ‘should this patient be implanted with an MRI conditional device?’. This review outlines the risks associated with MRI in patients with implanted cardiac devices, and discusses practical measures to minimise risks and facilitate safe and diagnostic scanning. INTRODUCTION Reflecting its unparalleled soft tissue contrast and absence of ionising radiation, MRI has seen a substantial expansion over the last 15 years, with more than 60 million MRI scans currently performed annually worldwide.1 More recently cardiovascular magnetic resonance (CMR) imaging has emerged as a cost effective cardiac imaging modality that provides important diagnostic and increasingly prognostic information that has a significant impact on patient management.2 While CMR currently represents only a small proportion of all MRI scans, the number of CMR scans being performed is increasingly rapidly.3 An estimated 250 000 patients in the UK have implantable pulse generators (IPGs) and implantable cardioverter defibrillators (ICDs), with 50 000 patients undergoing implantation in 2011.4 The probability that a patient with an IPG will require MRI over the lifetime of their device is estimated at 50–75%; with most hospitals now offering MRI, including some 60 providing CMR, the issue of MRI in patients with IPGs will be encountered with increasing frequency.5 Serious adverse events during MRI of patients with cardiac devices, including asystole and ventricular fibrillation, have been reported, albeit rarely; therefore, an awareness of the safety issues, the different types of IPGs (MR conditional vs non-MR conditional) and measures to facilitate safe scanning in these patients group, is necessary.5–7
MRI SAFETY TERMINOLOGY
To cite: Ainslie M, Miller C, Brown B, et al. Heart 2014;100:363–369.
The 2005 MRI task group of the American Society for Testing and Materials defined the following terminology with regards to implants and devices relative to the MRI environment: MR safe: An item that poses no known hazards in all MRI environments—for example, non-
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conducting, non-magnetic items such as plastic cannula. MR conditional: An item that has been demonstrated to pose no known hazards in a specified MRI environment with specified conditions of use. Conditions include static magnetic field strength, spatial gradient, dB/dt radiofrequency (RF) fields, and specific absorption rate (SAR). Additional conditions may be required, including specific configurations of the item. MR unsafe: An item that is known to pose hazards in all MRI environments. MR unsafe items include magnetic items such as a pair of ferromagnetic scissors or oxygen cylinders.8
SAFETY ISSUES CONCERNING IPGS AND ICDS IN THE MRI ENVIRONMENT While MRI is an inherently safe imaging modality, the MRI environment harbours the potential for even fatal accidents, particularly in patients with cardiac devices. Indeed of the small number of reported MRI related fatalities, the majority relate to patients with IPGs in situ (10 out of 15 deaths).9–11 Risks associated with MRI in patients with IPGs generally arise from the static magnetic field, gradient magnetic fields, and radiofrequency energy, which can act in isolation or in combination to adversely affect IPG function (table 1). Complications may include lead tip and tissue heating, mechanical pull, inappropriate therapies ( pacing and device discharge), and failure to pace.12–17
Tissue heating Pacing leads can act as antennae, concentrating electromagnetic energy at the un-insulated points of the lead cathode, anode or active fixation helix, which can lead to heating of the surrounding tissue where the energy is dissipated and potentially cause localised oedema and/or fibrosis. Theoretically, this could result in increased pacing thresholds, loss of pacing capture, and cardiac perforation.17–21 The potential for heating depends on the resonant frequency of the lead (dependent on lead length and diameter), the trajectory of the pacing leads, the presence of lead loops, lead or insulation fractures, patient size, and position within the scanner.22–26 In vitro studies without lead tip irrigation have demonstrated lead temperature increases in excess of 60°C during MRI scanning, although such extreme temperature variations have not been observed in more sophisticated models with lead tip irrigation. One in vivo study in pigs found an increase of 20.4°C at the electrode tip in the setting of an SAR approaching 3.8 W/kg, considerably higher than that in routine clinical scanning.27 In 715
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Republished review Table 1 Theoretical effects of static magnetic field, gradient magnetic fields and radiofrequency energy Static Case heating Force and torque Vibration Device interactions Lead heating Stimulation
✓ ✓ ✓
Gradient
Radiofrequency
✓
✓
✓ ✓ ✓
Reed switches ✓ ✓ ✓
another study a small rise in serum troponin was seen following four out of 114 scans performed at 1.5 T in patients implanted with a variety of conventional (ie, MR unsafe) IPGs, which the authors hypothesised likely reflected tissue necrosis secondary to lead tip heating.28
Force and torque IPGs contain ferromagnetic materials, which are subject to force and torque induced by the static and gradient magnetic fields.17 18 28 This can lead to movement and vibration of the device and leads. These forces are directly related to mass of ferromagnetic material, the strength of the magnetic field, and the positioning within the static magnetic field.29 While there are anecdotal reports of pull/drag, modern (ie, post 2000) IPGs appear safe with regards to force and torque at 1.5 T beyond the first 6 weeks following implantation, after which time healing around the device and leads is thought to provide sufficient anchorage.29–31
Inappropriate pacing, shocks, inhibition of therapies Radiofrequency energy pulses can lead to asynchronous pacing, programming changes or battery depletion or can be wrongly interpreted as underlying electrical activity or arrhythmias and thus lead to inhibition of demand pacing or delivery of ICD therapy, respectively.28 31–34 In an ex vivo study by Erlebacher on three IPGs no longer in use, radiofrequency interference caused total inhibition of atrial and ventricular output or resulted in atrial pacing at a high rate.14 No ICD induced defibrillation therapies in the MR environment have so far been reported, although this may relate to the inability of the capacitor to sufficiently charge in the static magnetic field. Nevertheless, repeated attempts by a defibrillator to charge itself could lead to battery depletion.35
Electrical reset ‘Electrical reset’, ‘power on reset’, and ‘factory reset’ are interchangeable terms for the emergency/backup mode that a device reverts to when its battery nears depletion. It is conventionally a VVI pacing mode at the lower rate limit with all advanced functions turned off.19 For ICDs, power on reset is a safety mechanism, which prevents inappropriate shocks from a damaged device. It is a VVI back up mode with no therapies. It cannot be reprogrammed and the device has to be changed. It should not be confused with the normal magnet response of ICDs. This is fixed rate pacing with therapies disabled, but functions return on removal of the magnet. Publications have quoted an MRI related incidence of electrical reset as high as 6.1% in conventional (ie, MR unsafe) IPGs.29 The conversion of asynchronous or fixed rate pacing to VVI mode in an IPG dependent patient without an underlying ventricular rhythm, in combination with 716
radiofrequency energy pulses being wrongly interpreted as intrinsic electric activity, is potentially life threatening and several MRI related deaths in IPG patients have been attributed to this mechanism.
Magnetic operated reed switches were originally incorporated into IPGs to allow device interrogation. Magnet application activates the reed switch which inhibits demand functions and most commonly, but not consistently, sets an IPG to an asynchronous mode.6 28 The position of the reed switch has been shown to be inconsistent within the MRI environment, leading to potential malfunction.31 Variable reed switch responses to MRI have been demonstrated across different IPG models including asynchronous pacing, transient loss of pacing or continuous loss of pacing.28 31 36 Loss of pacing in a pacing dependent patient could have significant consequences, while asynchronous pacing in individuals with an underlying rhythm could heighten the theoretical risk of R on T phenomenon and induce ventricular arrhythmias. In ICDs, activation of the reed switch commonly leads to deactivation of therapies while not affecting backup pacing.37
MRI IN CONVENTIONAL (IE, MR UNSAFE) SYSTEMS While the majority of older studies assessing non-thoracic MRI in patients with MR unsafe IPGs have reported no adverse events, isolated incidents of asystole, ventricular fibrillation, and death have been described,35 although such work is limited by small study size and lack of consistency in reporting type of IPG and leads. More recently, Nazarian et al38 performed 555 MRI scans (40% brain, 22% spine, 16% heart, 13% abdominal, 9% extremities) on 438 patients with either an IPG (54%) or ICD (46%) from various manufacturers. Three patients experienced electrical reset during scanning but there was no device dysfunction on long term (up to 5 years) follow-up. Minor changes in ventricular and atrial lead impedances (3 months) R wave amplitudes and battery voltage compared to non-thoracic scans, suggesting that while non-thoracic MRI can be performed safely, thoracic scans may present a higher risk. While on a smaller scale Junttila et al39 performed three MR scans on each of 10 patients over a period of 12 months. Reassuringly no adverse patient events occurred with a variety of non-MR conditional ICDs and no significant changes were reported in lead threshold, lead impedance or battery voltage. This suggests serial scans may pose no greater risk than single scans. The Magnasafe registry is a multicentre prospective study investigating the frequency of major adverse clinical events and device parameter changes for patients with conventional (MR unsafe) IPGs who undergo clinically indicated non-thoracic MRI at 1.5 T.40 So far over 600 scans across 12 sites have been performed without the occurrence of loss of pacing capture, device failure or death. Decreases in battery voltage and R and P wave amplitudes did occur, and the frequency of one or more clinically relevant device changes occurred in 13% of IPGs and 31% of ICDs. This implies that while technically safe, it is essential to perform a full device interrogation pre- and post-scan.41
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Republished review Box 1 Common design features of MR conditional devices Generator design ▸ Ferromagnetic content reduced ▸ Replacement of reed switch with solid state technology—for example, Hall sensor ▸ Bandstop filter (64 MHz) in casing to shield circuitry Lead design ▸ Lead pitch of the inner coil redesigned to alter resonant frequency of the lead ▸ Lead diameter altered ▸ Bandstop filter (64 MHz) at lead tip (St Jude Tendril lead)
MR CONDITIONAL DEVICES In recent years the device industry has invested significant effort into developing MR conditional systems to address the discussed safety issues. Many new design features of both hardware and software have been incorporated into these devices to allow them to perform more reliably in a specified MRI environment (ie, 1.5 T field strength). Hardware modifications have been made to both the generator and leads whereby some of these changes are manufacturer specific (box 1 and table 2). Software changes include the incorporation of a dedicated MRI pacing mode that can be activated for the duration of the scan. In most instances this requires a trained individual, commonly a pacing technician or electrophysiologist, to activate and deactivate this mode. There are novel alternatives, with St Jude providing a hand held activator to perform this task before and after the scan, and Boston Scientific’s Ingenio and Advantio devices having an MRI time-out mode negating the need to deactivate the safe mode manually post-scanning. Nevertheless current recommendations are that a full IPG interrogation is carried out pre- and post-MRI.38 Activation of the ‘MR safe’ mode switches off advanced functions and the IPG paces in an asynchronous fashion (ie, no sensing) to reduce the risk of electromagnetic interference being misinterpreted as an intrinsic rhythm with the potential of suppressed pacing. In some models the pulse amplitude and pulse duration are also both increased (eg, 5 V and 1 mS in the St Jude Accent), which increases the pacing stimulus and minimises the risk of loss of capture.
In the original EnRhythm trial with the Medtronic Surescan pacing system, 258 implanted patients underwent 14 nonclinically indicated brain and lumbar spine sequences 9–12 weeks post-implant in a 1.5 T scanner. Pacing parameters were compared with a control group (206 implants) immediately, 1 week, and 1 month post-MRI. No MRI related complications occurred and changes to pacing parameters were minimal.22 Thoracic scans were excluded in this trial, but in the more recently published Advisa study, a prospective randomised nonblinded multicentre trial in 263 patients, thoracic/cardiac sequences were included. The Advisa MRI IPG and Capsure-fix safety lead system when scanned in a 1.5 T environment with an SAR limit of 2 W/kg resulted in no power on reset, electrical stimulation or pacing threshold changes.42 The removal of scan area restriction is highly desirable as excluding the thoracic region may negate the performance of up to clinically indicated 40% of MRI scans, including thoracic spinal cord imaging and CMR. To facilitate the recognition of MR conditional devices on a plain film radiograph, radiopaque markers are found on both the generator and the lead, although these are neither standardised nor intuitive and their identification can be a challenge in clinical practice (figure 1A–C).
SCANNING AN IPG PATIENT When presented with a patient with an IPG in situ a number of considerations are pertinent (box 2). Established clinical protocols and algorithms have been proposed with physician led scans and pacing support readily available.18 35 It is good clinical practice to perform device interrogation before and after scanning to assess for battery depletion, programming changes or electrical reset. Patient monitoring throughout the scan period should include monitoring of pulse oximetry, ECG, blood pressure, and verbal responsiveness.38 A suggested algorithm is given in figure 2. Six weeks is the recommended interval between implant and MR scan from published studies. Lead dislodgment is more frequent in the first 6 weeks following an implant; thus studies did not want to subject patients to MR scans and the theoretical risk of force and torque on the leads, while tissue encapsulation was not fully established. No studies exist that we are aware of that address scanning at shorter time intervals. In emergencies, particular spinal cord lesions, the benefit of an MR scan even within a week of implant may outweigh potential risks. A team discussion is essential, as is full patient consent and strict cardiac monitoring. For CMR scanning 6 weeks is acceptable.
Table 2 Current MR conditional implantable pulse generators (IPGs) on the market St Jude
Medtronic
Boston Scientific
Biotronik
Sorin
Device
Accent MRI
Ingenio MRI Advantio MRI
Evia MRI Estella MRI
Reply MRI
Leads
Tendril MRI
Fineline II
Safio/solia
Filtrea
Lead fixation Lead diameter Thorax exclusion Scan time limit SAR limit (W/kg) Manual programming required post-MRI
Active 6.6F No No 4 Activator operated
Advisa MRI Enrhythm MRI Ensura MRI CapsureFix MRI Capsure sense Active and passive 5.4F No No 2 Yes
Active and passive 5.1F No No 2 No (automatic timer)
Active and passive 5.6F Yes Yes 2 Yes
Active 6.5F No No 4 No
SAR, specific absorption rate.
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Figure 1 MR conditional implantable pulse generators . Left: St Jude Accent. Middle: Medtronic Advisa. Right: Boston Ingenio. Red boxes indicate corresponding radiopaque markers.
CMR IMAGING The development of MR conditional devices allows this previously excluded cohort to benefit from CMR imaging; however, device related artefacts can impact significantly on image quality. Image distortion appears dependent on the size of the device (larger devices are associated with more artefact), the position of the implant, the imaging plane, and the scanning protocol.43 The major field distortion is often limited to an area of approximately 15 cm around the device. Artefacts are often more pronounced in the ventricular short axis plane compared to long axis planes, and more so in anterior LV segments.43 Steady state free precession (SSFP) cine imaging is associated with more susceptibility artefact than spoiled gradient echo imaging (figure 3A,B). In the study by Sasaki et al susceptibility artefacts were more pronounced on late gadolinium contrast acquisitions than other imaging sequences and were particularly common in the anterior segments. Strategies to optimise imaging sequences and post-processing are currently being researched.44 Despite the presence of artefacts, one study with conventional (MR
Box 2 Considerations pre-scan Is there an alternative image modality that can answer the clinical question posed? Duration of device implant Manufacturer and model of device What is the MR safety label? What are the restrictions associated with the device (zonal, specific absorption rate)? Has device been used in safety studies? Presence of lead loops or lead fractures Proximity of the device to region being scanned (thoracic vs extrathoracic) What is the likelihood of a non-diagnostic scan 2nd to susceptibility artefacts (left vs right sided implant?; device position over thorax?) Presence of additional hardware—for example, abandoned leads, adapters Risk/benefit ratio for specific patient circumstances (eg, is patient pacing dependent vs good underlying rhythm?) Is diagnosis likely to impact significantly on patient management and/or quality of life or outcome? 718
unsafe) devices showed that 100% of SSFP cine images in 15 pacemaker patients and 86% in 56 ICD patients remained diagnostic for left ventricular assessment.43 In the recent Advisa image quality sub-study, good quality SSFP CMR images assessing cardiac anatomy and biventricular function were obtained in over 90% of patients. Quality was assessed for the left and right ventricle on a seven and five point scale, respectively, whereby grade 1 (excellent image quality) required the absence of both lead and IPG artefacts preventing the interpretation of regional wall motion as defined by systolic wall thickening and endocardial inward movement. In our experience MR conditional pacing systems still exhibit significant artefacts around the generator (figure 3C,D), although commonly this does not prevent left and right ventricular wall motion assessment, especially when being able to switch to spoiled gradient cine imaging. In our experience lead related artefacts are minimal and do not impact significantly on wall motion assessment and do not prevent computing of strain and strain derived indices from tagged images or endocardial feature tracking software packages (eg, Diogenes, TomTec) (figure 3F). It is important to recognise that cardiac devices equally affect CT imaging, whereby lead related artefacts can be particularly prominent. It has been suggested that MRI is superior to assessing the myocardial interface, particularly relevant in ruling out myocardial perforation.43
ISSUES CONCERNING PATIENT SELECTION While it has been demonstrated that MRI of non-MR conditional IPGs can be performed safely when certain logistic, safety and monitoring requirements are adhered to, concerns remain not only about the unpredictable nature of malfunctioning of such devices in the MR environment (especially in pacing dependent patients), but possibly more so about what potential effects the lowering of the acceptance threshold for scanning IPGs would have in real world clinical practice in the long term. The greatest concern would possibly be that the minimum required standards for proceeding ‘safely’ with such exceptional scans could subsequently become eroded, exposing patients to avoidable risk. The second less apparent, but in the long term equally important, aspect is that such a lowering in threshold would remove the incentive for industry to bring their entire product range up to MRI compatible standards. With respect to the latter the core question remains: how safe is safe? In the meantime clinicians and implanters have to address the question of which patients are most likely to benefit from such Ainslie M, et al. Postgrad Med J 2014;90:715–721. doi:10.1136/postgradmedj-2013-304324rep
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Figure 2 Suggested approach with an implantable pulse generator (IPG) patient. BP, blood pressure; ICD, implantable cardiac defibrillator; SAR, specific absorption rate. Adapted from Nazarian et al.38 devices, which basically equates into estimating the lifetime likelihood of requiring an MRI scan. Publications have started to address this issue,45 but consensus and/or societal guidance is still lacking. According to our own analysis performed in collaboration with Medtronic’s Department of Health Economics, based on 2006–7 data from the English Hospital Episode Statistics database and the Hospital Activity Statistics, the 1 year and lifetime risk for a 75–79-year-old to require an outpatient MRI was 7.9% and 46%, respectively.46 While one may argue that all IPG implants should be MR conditional because of the relative high lifetime likelihood of benefiting from MRI, this line of argument ignores the fact that clinical scenarios in which only MRI will provide an adequate (as opposed to the best) answer are actually rare. Such scenarios,
encountered by the authors, include IPG patients with a query of thoracic spinal cord compression, patients with multiple sclerosis and rapidly demyelinating disease, or surgical planning in a case of an osteosarcoma. Beyond the additional premium of MR conditional devices, many of the first generation devices have the downside of lacking some of the more sophisticated complex arrhythmia algorithms and thus may not be suitable for all clinical scenarios. While there are patient groups that very apparently selfselect for implantation of MR conditional devices, including those with conduction disease at a very young age, primary cardiomyopathies or those occurring as part of systemic illness, it may indeed be more appropriate to approach the subject of selection by considering first those patients who would not benefit from an MR conditional device. This logically would
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Figure 3 (A) Four chamber steady state free precession (SSFP) image. (B) Four chamber spoiled gradient. (C) Short axis no artefact. (D) Short axis with significant artefact. (E) Tagging of short axis slice. (F) Feature tracking producing deformational data.
include those that have a definite contraindication to MRI, such as claustrophobia, or those unlikely to require an MRI due to extreme age or terminal disease.
Collaborators Neil Davidson, David J Fox. Contributors MA and MS had the original idea for the article. MA wrote each draft which BB and CM helped edit. The final edit was done by MS. Competing interests None.
CONCLUSIONS MRI in patients with conventional (non-MR safe) devices is no longer an absolute contraindication but requires a multidisciplinary approach and detailed assessment of risk–benefit ratios before and monitoring throughout the scan. The recent introduction of MR conditional pacing systems has become a game changer, facilitating safe scanning, even without zonal exclusion, of patients implanted with these devices, as long as basic precautions are taken. Future guidelines in this field need to reflect this changing technological landscape and provide consensus on clinical standards concerning the MRI/CMR of patients with implanted cardiac devices. 720
Acknowledgements Dr Davidson and Dr Fox provided valuable support.
Provenance and peer review Not commissioned; externally peer reviewed.
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Republished: Cardiac MRI of patients with implanted electrical cardiac devices Mark Ainslie, Christopher Miller, Benjamin Brown and Matthias Schmitt Postgrad Med J 2014 90: 715-721
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Republished review
Cardiac MRI of patients with implanted electrical cardiac devices Mark Ainslie, Christopher Miller, Benjamin Brown, Matthias Schmitt Cardiology Department, University Hospital of South Manchester, Manchester, UK Correspondence to Dr Mark Ainslie Cardiology Department, University Hospital of South Manchester, Southmoor Road, Manchester M23 9LT, UK; [email protected] Received 20 May 2013 Revised 1 July 2013 Accepted 2 July 2013 Published Online First 19 July 2013 This is a reprint of a paper that first appeared in Heart, 2014, volume 100, pages 363–369.
ABSTRACT Implantable pulse generators and defibrillators have traditionally been considered contraindications to MRI. However, recent data have challenged this paradigm and demonstrated that patients with newer generation devices can safely undergo MRI, including cardiac MRI, provided basic precautions are taken. Indeed, the introduction of MRI conditional systems has led to a conceptual shift in clinical decision making—‘can this patient undergo MRI safely?’ is being superseded by ‘should this patient be implanted with an MRI conditional device?’. This review outlines the risks associated with MRI in patients with implanted cardiac devices, and discusses practical measures to minimise risks and facilitate safe and diagnostic scanning. INTRODUCTION Reflecting its unparalleled soft tissue contrast and absence of ionising radiation, MRI has seen a substantial expansion over the last 15 years, with more than 60 million MRI scans currently performed annually worldwide.1 More recently cardiovascular magnetic resonance (CMR) imaging has emerged as a cost effective cardiac imaging modality that provides important diagnostic and increasingly prognostic information that has a significant impact on patient management.2 While CMR currently represents only a small proportion of all MRI scans, the number of CMR scans being performed is increasingly rapidly.3 An estimated 250 000 patients in the UK have implantable pulse generators (IPGs) and implantable cardioverter defibrillators (ICDs), with 50 000 patients undergoing implantation in 2011.4 The probability that a patient with an IPG will require MRI over the lifetime of their device is estimated at 50–75%; with most hospitals now offering MRI, including some 60 providing CMR, the issue of MRI in patients with IPGs will be encountered with increasing frequency.5 Serious adverse events during MRI of patients with cardiac devices, including asystole and ventricular fibrillation, have been reported, albeit rarely; therefore, an awareness of the safety issues, the different types of IPGs (MR conditional vs non-MR conditional) and measures to facilitate safe scanning in these patients group, is necessary.5–7
MRI SAFETY TERMINOLOGY
To cite: Ainslie M, Miller C, Brown B, et al. Heart 2014;100:363–369.
The 2005 MRI task group of the American Society for Testing and Materials defined the following terminology with regards to implants and devices relative to the MRI environment: MR safe: An item that poses no known hazards in all MRI environments—for example, non-
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conducting, non-magnetic items such as plastic cannula. MR conditional: An item that has been demonstrated to pose no known hazards in a specified MRI environment with specified conditions of use. Conditions include static magnetic field strength, spatial gradient, dB/dt radiofrequency (RF) fields, and specific absorption rate (SAR). Additional conditions may be required, including specific configurations of the item. MR unsafe: An item that is known to pose hazards in all MRI environments. MR unsafe items include magnetic items such as a pair of ferromagnetic scissors or oxygen cylinders.8
SAFETY ISSUES CONCERNING IPGS AND ICDS IN THE MRI ENVIRONMENT While MRI is an inherently safe imaging modality, the MRI environment harbours the potential for even fatal accidents, particularly in patients with cardiac devices. Indeed of the small number of reported MRI related fatalities, the majority relate to patients with IPGs in situ (10 out of 15 deaths).9–11 Risks associated with MRI in patients with IPGs generally arise from the static magnetic field, gradient magnetic fields, and radiofrequency energy, which can act in isolation or in combination to adversely affect IPG function (table 1). Complications may include lead tip and tissue heating, mechanical pull, inappropriate therapies ( pacing and device discharge), and failure to pace.12–17
Tissue heating Pacing leads can act as antennae, concentrating electromagnetic energy at the un-insulated points of the lead cathode, anode or active fixation helix, which can lead to heating of the surrounding tissue where the energy is dissipated and potentially cause localised oedema and/or fibrosis. Theoretically, this could result in increased pacing thresholds, loss of pacing capture, and cardiac perforation.17–21 The potential for heating depends on the resonant frequency of the lead (dependent on lead length and diameter), the trajectory of the pacing leads, the presence of lead loops, lead or insulation fractures, patient size, and position within the scanner.22–26 In vitro studies without lead tip irrigation have demonstrated lead temperature increases in excess of 60°C during MRI scanning, although such extreme temperature variations have not been observed in more sophisticated models with lead tip irrigation. One in vivo study in pigs found an increase of 20.4°C at the electrode tip in the setting of an SAR approaching 3.8 W/kg, considerably higher than that in routine clinical scanning.27 In 715
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Republished review Table 1 Theoretical effects of static magnetic field, gradient magnetic fields and radiofrequency energy Static Case heating Force and torque Vibration Device interactions Lead heating Stimulation
✓ ✓ ✓
Gradient
Radiofrequency
✓
✓
✓ ✓ ✓
Reed switches ✓ ✓ ✓
another study a small rise in serum troponin was seen following four out of 114 scans performed at 1.5 T in patients implanted with a variety of conventional (ie, MR unsafe) IPGs, which the authors hypothesised likely reflected tissue necrosis secondary to lead tip heating.28
Force and torque IPGs contain ferromagnetic materials, which are subject to force and torque induced by the static and gradient magnetic fields.17 18 28 This can lead to movement and vibration of the device and leads. These forces are directly related to mass of ferromagnetic material, the strength of the magnetic field, and the positioning within the static magnetic field.29 While there are anecdotal reports of pull/drag, modern (ie, post 2000) IPGs appear safe with regards to force and torque at 1.5 T beyond the first 6 weeks following implantation, after which time healing around the device and leads is thought to provide sufficient anchorage.29–31
Inappropriate pacing, shocks, inhibition of therapies Radiofrequency energy pulses can lead to asynchronous pacing, programming changes or battery depletion or can be wrongly interpreted as underlying electrical activity or arrhythmias and thus lead to inhibition of demand pacing or delivery of ICD therapy, respectively.28 31–34 In an ex vivo study by Erlebacher on three IPGs no longer in use, radiofrequency interference caused total inhibition of atrial and ventricular output or resulted in atrial pacing at a high rate.14 No ICD induced defibrillation therapies in the MR environment have so far been reported, although this may relate to the inability of the capacitor to sufficiently charge in the static magnetic field. Nevertheless, repeated attempts by a defibrillator to charge itself could lead to battery depletion.35
Electrical reset ‘Electrical reset’, ‘power on reset’, and ‘factory reset’ are interchangeable terms for the emergency/backup mode that a device reverts to when its battery nears depletion. It is conventionally a VVI pacing mode at the lower rate limit with all advanced functions turned off.19 For ICDs, power on reset is a safety mechanism, which prevents inappropriate shocks from a damaged device. It is a VVI back up mode with no therapies. It cannot be reprogrammed and the device has to be changed. It should not be confused with the normal magnet response of ICDs. This is fixed rate pacing with therapies disabled, but functions return on removal of the magnet. Publications have quoted an MRI related incidence of electrical reset as high as 6.1% in conventional (ie, MR unsafe) IPGs.29 The conversion of asynchronous or fixed rate pacing to VVI mode in an IPG dependent patient without an underlying ventricular rhythm, in combination with 716
radiofrequency energy pulses being wrongly interpreted as intrinsic electric activity, is potentially life threatening and several MRI related deaths in IPG patients have been attributed to this mechanism.
Magnetic operated reed switches were originally incorporated into IPGs to allow device interrogation. Magnet application activates the reed switch which inhibits demand functions and most commonly, but not consistently, sets an IPG to an asynchronous mode.6 28 The position of the reed switch has been shown to be inconsistent within the MRI environment, leading to potential malfunction.31 Variable reed switch responses to MRI have been demonstrated across different IPG models including asynchronous pacing, transient loss of pacing or continuous loss of pacing.28 31 36 Loss of pacing in a pacing dependent patient could have significant consequences, while asynchronous pacing in individuals with an underlying rhythm could heighten the theoretical risk of R on T phenomenon and induce ventricular arrhythmias. In ICDs, activation of the reed switch commonly leads to deactivation of therapies while not affecting backup pacing.37
MRI IN CONVENTIONAL (IE, MR UNSAFE) SYSTEMS While the majority of older studies assessing non-thoracic MRI in patients with MR unsafe IPGs have reported no adverse events, isolated incidents of asystole, ventricular fibrillation, and death have been described,35 although such work is limited by small study size and lack of consistency in reporting type of IPG and leads. More recently, Nazarian et al38 performed 555 MRI scans (40% brain, 22% spine, 16% heart, 13% abdominal, 9% extremities) on 438 patients with either an IPG (54%) or ICD (46%) from various manufacturers. Three patients experienced electrical reset during scanning but there was no device dysfunction on long term (up to 5 years) follow-up. Minor changes in ventricular and atrial lead impedances (3 months) R wave amplitudes and battery voltage compared to non-thoracic scans, suggesting that while non-thoracic MRI can be performed safely, thoracic scans may present a higher risk. While on a smaller scale Junttila et al39 performed three MR scans on each of 10 patients over a period of 12 months. Reassuringly no adverse patient events occurred with a variety of non-MR conditional ICDs and no significant changes were reported in lead threshold, lead impedance or battery voltage. This suggests serial scans may pose no greater risk than single scans. The Magnasafe registry is a multicentre prospective study investigating the frequency of major adverse clinical events and device parameter changes for patients with conventional (MR unsafe) IPGs who undergo clinically indicated non-thoracic MRI at 1.5 T.40 So far over 600 scans across 12 sites have been performed without the occurrence of loss of pacing capture, device failure or death. Decreases in battery voltage and R and P wave amplitudes did occur, and the frequency of one or more clinically relevant device changes occurred in 13% of IPGs and 31% of ICDs. This implies that while technically safe, it is essential to perform a full device interrogation pre- and post-scan.41
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Republished review Box 1 Common design features of MR conditional devices Generator design ▸ Ferromagnetic content reduced ▸ Replacement of reed switch with solid state technology—for example, Hall sensor ▸ Bandstop filter (64 MHz) in casing to shield circuitry Lead design ▸ Lead pitch of the inner coil redesigned to alter resonant frequency of the lead ▸ Lead diameter altered ▸ Bandstop filter (64 MHz) at lead tip (St Jude Tendril lead)
MR CONDITIONAL DEVICES In recent years the device industry has invested significant effort into developing MR conditional systems to address the discussed safety issues. Many new design features of both hardware and software have been incorporated into these devices to allow them to perform more reliably in a specified MRI environment (ie, 1.5 T field strength). Hardware modifications have been made to both the generator and leads whereby some of these changes are manufacturer specific (box 1 and table 2). Software changes include the incorporation of a dedicated MRI pacing mode that can be activated for the duration of the scan. In most instances this requires a trained individual, commonly a pacing technician or electrophysiologist, to activate and deactivate this mode. There are novel alternatives, with St Jude providing a hand held activator to perform this task before and after the scan, and Boston Scientific’s Ingenio and Advantio devices having an MRI time-out mode negating the need to deactivate the safe mode manually post-scanning. Nevertheless current recommendations are that a full IPG interrogation is carried out pre- and post-MRI.38 Activation of the ‘MR safe’ mode switches off advanced functions and the IPG paces in an asynchronous fashion (ie, no sensing) to reduce the risk of electromagnetic interference being misinterpreted as an intrinsic rhythm with the potential of suppressed pacing. In some models the pulse amplitude and pulse duration are also both increased (eg, 5 V and 1 mS in the St Jude Accent), which increases the pacing stimulus and minimises the risk of loss of capture.
In the original EnRhythm trial with the Medtronic Surescan pacing system, 258 implanted patients underwent 14 nonclinically indicated brain and lumbar spine sequences 9–12 weeks post-implant in a 1.5 T scanner. Pacing parameters were compared with a control group (206 implants) immediately, 1 week, and 1 month post-MRI. No MRI related complications occurred and changes to pacing parameters were minimal.22 Thoracic scans were excluded in this trial, but in the more recently published Advisa study, a prospective randomised nonblinded multicentre trial in 263 patients, thoracic/cardiac sequences were included. The Advisa MRI IPG and Capsure-fix safety lead system when scanned in a 1.5 T environment with an SAR limit of 2 W/kg resulted in no power on reset, electrical stimulation or pacing threshold changes.42 The removal of scan area restriction is highly desirable as excluding the thoracic region may negate the performance of up to clinically indicated 40% of MRI scans, including thoracic spinal cord imaging and CMR. To facilitate the recognition of MR conditional devices on a plain film radiograph, radiopaque markers are found on both the generator and the lead, although these are neither standardised nor intuitive and their identification can be a challenge in clinical practice (figure 1A–C).
SCANNING AN IPG PATIENT When presented with a patient with an IPG in situ a number of considerations are pertinent (box 2). Established clinical protocols and algorithms have been proposed with physician led scans and pacing support readily available.18 35 It is good clinical practice to perform device interrogation before and after scanning to assess for battery depletion, programming changes or electrical reset. Patient monitoring throughout the scan period should include monitoring of pulse oximetry, ECG, blood pressure, and verbal responsiveness.38 A suggested algorithm is given in figure 2. Six weeks is the recommended interval between implant and MR scan from published studies. Lead dislodgment is more frequent in the first 6 weeks following an implant; thus studies did not want to subject patients to MR scans and the theoretical risk of force and torque on the leads, while tissue encapsulation was not fully established. No studies exist that we are aware of that address scanning at shorter time intervals. In emergencies, particular spinal cord lesions, the benefit of an MR scan even within a week of implant may outweigh potential risks. A team discussion is essential, as is full patient consent and strict cardiac monitoring. For CMR scanning 6 weeks is acceptable.
Table 2 Current MR conditional implantable pulse generators (IPGs) on the market St Jude
Medtronic
Boston Scientific
Biotronik
Sorin
Device
Accent MRI
Ingenio MRI Advantio MRI
Evia MRI Estella MRI
Reply MRI
Leads
Tendril MRI
Fineline II
Safio/solia
Filtrea
Lead fixation Lead diameter Thorax exclusion Scan time limit SAR limit (W/kg) Manual programming required post-MRI
Active 6.6F No No 4 Activator operated
Advisa MRI Enrhythm MRI Ensura MRI CapsureFix MRI Capsure sense Active and passive 5.4F No No 2 Yes
Active and passive 5.1F No No 2 No (automatic timer)
Active and passive 5.6F Yes Yes 2 Yes
Active 6.5F No No 4 No
SAR, specific absorption rate.
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Figure 1 MR conditional implantable pulse generators . Left: St Jude Accent. Middle: Medtronic Advisa. Right: Boston Ingenio. Red boxes indicate corresponding radiopaque markers.
CMR IMAGING The development of MR conditional devices allows this previously excluded cohort to benefit from CMR imaging; however, device related artefacts can impact significantly on image quality. Image distortion appears dependent on the size of the device (larger devices are associated with more artefact), the position of the implant, the imaging plane, and the scanning protocol.43 The major field distortion is often limited to an area of approximately 15 cm around the device. Artefacts are often more pronounced in the ventricular short axis plane compared to long axis planes, and more so in anterior LV segments.43 Steady state free precession (SSFP) cine imaging is associated with more susceptibility artefact than spoiled gradient echo imaging (figure 3A,B). In the study by Sasaki et al susceptibility artefacts were more pronounced on late gadolinium contrast acquisitions than other imaging sequences and were particularly common in the anterior segments. Strategies to optimise imaging sequences and post-processing are currently being researched.44 Despite the presence of artefacts, one study with conventional (MR
Box 2 Considerations pre-scan Is there an alternative image modality that can answer the clinical question posed? Duration of device implant Manufacturer and model of device What is the MR safety label? What are the restrictions associated with the device (zonal, specific absorption rate)? Has device been used in safety studies? Presence of lead loops or lead fractures Proximity of the device to region being scanned (thoracic vs extrathoracic) What is the likelihood of a non-diagnostic scan 2nd to susceptibility artefacts (left vs right sided implant?; device position over thorax?) Presence of additional hardware—for example, abandoned leads, adapters Risk/benefit ratio for specific patient circumstances (eg, is patient pacing dependent vs good underlying rhythm?) Is diagnosis likely to impact significantly on patient management and/or quality of life or outcome? 718
unsafe) devices showed that 100% of SSFP cine images in 15 pacemaker patients and 86% in 56 ICD patients remained diagnostic for left ventricular assessment.43 In the recent Advisa image quality sub-study, good quality SSFP CMR images assessing cardiac anatomy and biventricular function were obtained in over 90% of patients. Quality was assessed for the left and right ventricle on a seven and five point scale, respectively, whereby grade 1 (excellent image quality) required the absence of both lead and IPG artefacts preventing the interpretation of regional wall motion as defined by systolic wall thickening and endocardial inward movement. In our experience MR conditional pacing systems still exhibit significant artefacts around the generator (figure 3C,D), although commonly this does not prevent left and right ventricular wall motion assessment, especially when being able to switch to spoiled gradient cine imaging. In our experience lead related artefacts are minimal and do not impact significantly on wall motion assessment and do not prevent computing of strain and strain derived indices from tagged images or endocardial feature tracking software packages (eg, Diogenes, TomTec) (figure 3F). It is important to recognise that cardiac devices equally affect CT imaging, whereby lead related artefacts can be particularly prominent. It has been suggested that MRI is superior to assessing the myocardial interface, particularly relevant in ruling out myocardial perforation.43
ISSUES CONCERNING PATIENT SELECTION While it has been demonstrated that MRI of non-MR conditional IPGs can be performed safely when certain logistic, safety and monitoring requirements are adhered to, concerns remain not only about the unpredictable nature of malfunctioning of such devices in the MR environment (especially in pacing dependent patients), but possibly more so about what potential effects the lowering of the acceptance threshold for scanning IPGs would have in real world clinical practice in the long term. The greatest concern would possibly be that the minimum required standards for proceeding ‘safely’ with such exceptional scans could subsequently become eroded, exposing patients to avoidable risk. The second less apparent, but in the long term equally important, aspect is that such a lowering in threshold would remove the incentive for industry to bring their entire product range up to MRI compatible standards. With respect to the latter the core question remains: how safe is safe? In the meantime clinicians and implanters have to address the question of which patients are most likely to benefit from such Ainslie M, et al. Postgrad Med J 2014;90:715–721. doi:10.1136/postgradmedj-2013-304324rep
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Figure 2 Suggested approach with an implantable pulse generator (IPG) patient. BP, blood pressure; ICD, implantable cardiac defibrillator; SAR, specific absorption rate. Adapted from Nazarian et al.38 devices, which basically equates into estimating the lifetime likelihood of requiring an MRI scan. Publications have started to address this issue,45 but consensus and/or societal guidance is still lacking. According to our own analysis performed in collaboration with Medtronic’s Department of Health Economics, based on 2006–7 data from the English Hospital Episode Statistics database and the Hospital Activity Statistics, the 1 year and lifetime risk for a 75–79-year-old to require an outpatient MRI was 7.9% and 46%, respectively.46 While one may argue that all IPG implants should be MR conditional because of the relative high lifetime likelihood of benefiting from MRI, this line of argument ignores the fact that clinical scenarios in which only MRI will provide an adequate (as opposed to the best) answer are actually rare. Such scenarios,
encountered by the authors, include IPG patients with a query of thoracic spinal cord compression, patients with multiple sclerosis and rapidly demyelinating disease, or surgical planning in a case of an osteosarcoma. Beyond the additional premium of MR conditional devices, many of the first generation devices have the downside of lacking some of the more sophisticated complex arrhythmia algorithms and thus may not be suitable for all clinical scenarios. While there are patient groups that very apparently selfselect for implantation of MR conditional devices, including those with conduction disease at a very young age, primary cardiomyopathies or those occurring as part of systemic illness, it may indeed be more appropriate to approach the subject of selection by considering first those patients who would not benefit from an MR conditional device. This logically would
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Figure 3 (A) Four chamber steady state free precession (SSFP) image. (B) Four chamber spoiled gradient. (C) Short axis no artefact. (D) Short axis with significant artefact. (E) Tagging of short axis slice. (F) Feature tracking producing deformational data.
include those that have a definite contraindication to MRI, such as claustrophobia, or those unlikely to require an MRI due to extreme age or terminal disease.
Collaborators Neil Davidson, David J Fox. Contributors MA and MS had the original idea for the article. MA wrote each draft which BB and CM helped edit. The final edit was done by MS. Competing interests None.
CONCLUSIONS MRI in patients with conventional (non-MR safe) devices is no longer an absolute contraindication but requires a multidisciplinary approach and detailed assessment of risk–benefit ratios before and monitoring throughout the scan. The recent introduction of MR conditional pacing systems has become a game changer, facilitating safe scanning, even without zonal exclusion, of patients implanted with these devices, as long as basic precautions are taken. Future guidelines in this field need to reflect this changing technological landscape and provide consensus on clinical standards concerning the MRI/CMR of patients with implanted cardiac devices. 720
Acknowledgements Dr Davidson and Dr Fox provided valuable support.
Provenance and peer review Not commissioned; externally peer reviewed.
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Republished: Cardiac MRI of patients with implanted electrical cardiac devices Mark Ainslie, Christopher Miller, Benjamin Brown and Matthias Schmitt Postgrad Med J 2014 90: 715-721
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