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 Table of Contents  
REVIEW ARTICLE
Year : 2018  |  Volume : 3  |  Issue : 1  |  Page : 16-24

Magnetic resonance imaging-conditional devices: Where have we reached today?


1 Department of Cardiology, Delhi Heart and Lung Institute, New Delhi, India
2 Department of Cardiac Electrophysiology, Mount Sinai School of Medicine and Hospital Centre, New York, USA

Date of Web Publication23-Jul-2018

Correspondence Address:
Prof. Kamal K Sethi
Department of Cardiology, Delhi Heart and Lung Institute, 3 MM II, Panchkuian Road, New Delhi - 110 055
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/IJHR.IJHR_12_16

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  Abstract 

Scientific growth in the field of magnetic resonance imaging (MRI) and cardiac devices has been exponential in recent decades. Cardiac implantable electronic devices due to their ferromagnetic constituents in leads and device body have always been an issue if patients need MRI. MRI is relatively safe. Recent introduction of changes in leads and device body constituents renders them less ferromagnetic, making MRI less frightening to a certain extent. Simultaneously, there is increasing research interest in MRI. Not only anatomy and pathology but also physiology of cardiac and nervous structures can be imaged. It is estimated that 53%–64% of intracardiac defibrillator (ICD) patients will require an MRI determination over a 10-year time horizon, highlighting the importance of MRI-conditional devices for this patient population. In this article, we briefly describe evolution and current status of conditioning of cardiac devices to make them MRI-friendly and briefly discuss where we are in terms of our physician role with respect to MRI-conditional devices.

Keywords: Cardiac implantable electronic device, intracardiac defibrillator, magnetic resonance imaging


How to cite this article:
Sethi KK, Chutani SK. Magnetic resonance imaging-conditional devices: Where have we reached today?. Int J Heart Rhythm 2018;3:16-24

How to cite this URL:
Sethi KK, Chutani SK. Magnetic resonance imaging-conditional devices: Where have we reached today?. Int J Heart Rhythm [serial online] 2018 [cited 2018 Oct 21];3:16-24. Available from: http://www.ijhronline.org/text.asp?2018/3/1/16/237367


  Introduction Top


Preamble

Between 1993 and 2009, the number of pacemaker implantations increased by >50%, and approximately 3 million patients have implanted pacemakers during this period [Figure 1].[1] During the same period, magnetic resonance imaging (MRI) has emerged as the gold standard of imaging for a variety of reasons as shown in [Figure 2]. During 1993–2011, the number of MRI scans increased in the USA from 7 million to 32 million, the largest number in the world. The most common reasons for MRI scan [Figure 3] are stroke, prostate cancer, osteoarthritis, and colorectal cancer. Eighty-five percent of pacemaker recipients are >65 years of age, and MRI is refused in >80% of this cohort due to pacemaker.[2] It has been estimated that 50 to 75% of pacemaker patients shall need an MRI determination over their life time.[2],[3],[4]
Figure 1: Trends in permanent pacemaker implantation in the USA between 1993 and 2009[1]

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Figure 2: Use of magnetic resonance imaging worldwide (IMV Benchmark Report 2012)

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Figure 3: Most common potential reasons for MRI examination in patients with pacemakers. PPM = Permanent pacemaker, MRI = Magnetic resonance imaging

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Increasing use of magnetic resonance imaging

Advances in imaging anatomical structure and physiology with MRI and MR angiography over the last two decades have led to MR becoming an increasingly attractive imaging modality, especially in the brain and heart imaging. Brain anatomy, physiology, spatial resolution, and spatial specificity of neuronal activation as detected by MRI have markedly improved over the past decade. Multimodal MRI techniques including quantitative blood flow, blood volume, and oxygen extraction fraction measurements are becoming routine. These techniques have been applied to study cerebral physiology and neuroenergetics under various experimental conditions in normal brains and diseased states such as stroke. MRI provides excellent spatial resolution and multiplanar three-dimensional analysis, while not exposing patients to ionizing radiation, risks of invasive procedures, or potentially nephrotoxic-iodinated contrast agents. MRI has emerged as a broadly applied diagnostic tool for patients with neurovascular and cardiovascular diseases, and therefore, the number of patients undergoing scanning each year is increasing. In parallel, increasing number of patients is being treated with permanently or temporarily implanted cardiovascular devices.

Cardiac MRI is performed to evaluate the anatomy and function of heart chambers, valves, size, and blood flow through major vessels and surrounding structures, diagnosing a variety of heart and blood vessel disorders such as tumors, infections, and inflammatory conditions. It began with late gadolinium enhancement as a means to assess myocardial infarct size, for which it is now the gold standard. Over time, it was recognized that other nonischemic causes of myocardial fibrosis, such as hypertrophic cardiomyopathy or sarcoidosis, could be visualized with late gadolinium enhancement. MRI helps in evaluation of effects of coronary artery obstruction, resulting in limited blood flow to the heart muscle and scarring within the heart muscle after Myocardial Infarction (MI). MRI also helps in monitoring the progression of certain disorders over time, evaluating the effects of surgical changes, especially in patients with congenital heart disease, and evaluating the anatomy of the heart and blood vessels in children and adults with congenital heart disease.[3],[5]

Atherosclerotic plaque imaging and T1-weighted magnetic resonance imaging

Atherosclerotic plaque composition is known to be associated with its propensity to rupture and cause vascular events. MRI of atherosclerotic plaque using 1.5 T scanners can detect plaque composition. Recently, a new pulse sequence called modified Look-Locker inversion recovery was developed for T1-mapping enabling the quantitative evaluation of myocardial T1 either without contrast, so-called native T1, or postcontrast. Native T1 is especially useful in patients who cannot receive gadolinium-based contrast agents because of Stage 4 or 5 chronic kidney disease. Native T1 is the highest in patients with amyloidosis, many of whom have chronic kidney disease. It is elevated to an intermediate degree in hypertrophic cardiomyopathy and other nonischemic cardiomyopathies. Postcontrast T1-mapping is used to calculate extracellular volume (ECV), which is elevated in the setting of interstitial fibrosis or other causes of increased extracellular space such as edema and/or inflammation. Elevated ECV has been documented in hypertensive heart disease, hypertrophic cardiomyopathy, and heart failure with preserved ejection fraction. Increased ECV has been associated with adverse short-term cardiac prognosis in a diverse patient population.[4]

T1-weighted magnetic resonance imaging and myocardial ischemia: Native T1 at rest and with adenosine stress

The group at Oxford has studied native T1 at rest and with adenosine stress, showing that native T1 is sensitive to changes in myocardial blood volume, which may be a sensitive marker of ischemia. They found that normal myocardium increased native T1 with adenosine by >6%, whereas infarcted myocardium showed essentially no increase. Ischemic myocardium showed an intermediate increase of approximately 4%. A major advance here is the ability to perform stress cardiac MR studies without using gadolinium.[4]


  Magnetic Resonance Imaging and Cardiac Implantable Devices Top


How does magnetic resonance imaging affect cardiac devices?

Risks associated with MRI generally arise from three distinct mechanisms related to MRI: (1) static main magnetic field, (2) radiofrequency (RF) energy, and (3) gradient magnetic fields [Figure 4]. There are several potential risks associated with MR scanning of specific cardiovascular devices that result from these processes [Table 1]. Most of these risks can be understood by consideration of the areas as discussed below [Table 2].
Figure 4: MRI with three powerful fields. MRI = Magnetic resonance imaging

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Table 1: Major magnetic resonance imaging-related potential pacemaker complications

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Table 2: Multiple potential interactions

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Static magnetic fields

Most currently used clinical MR scanners are 1.5–3 T, which corresponds to ~30,000–60,000 times the strength of the earth's magnetic field. The greatest risk from the main magnetic field is attraction of a ferromagnetic object into the scanner. The term “ferromagnetic” is used to denote a substance that experiences an attractive force in the presence of a magnetic field. As a result of ferromagnetic interactions, a device may be moved, rotated, dislodged, or accelerated toward the magnet at dangerously high velocities and dangerously high forces, creating a “projectile effect” that could lead to significant patient injury or damage to the MR system. Device function may also be altered or negated as a result of interactions with the strong static magnetic fields.[6]

Radiofrequency energy

During MRI, RF energy is “pulsed” into the body to generate the MR image. The body will absorb some of the RF power and therefore will heat up (usually <1°C) directly owing to ohmic heating. The dosimeter term used to characterize RF energy is the specific absorption rate (SAR, measured in watts per kilogram). SAR increases with the square of the field strength. Certain metallic devices (such as leads) can act as an “antenna” and concentrate this RF energy, which leads to excessive local heating, especially at the tip of these devices. Fractured leads may pose a particularly high risk of thermal injury. Concentration of RF energy is frequency dependent and therefore changes for a given device in different field strength. RF energies used in the MRI process can also induce electrical currents in wires and leads, which could possibly induce arrhythmias.[7]

Gradient magnetic fields

Time-varying magnetic fields called gradients (dB/dt, measured in Tesla per se cond) are used to encode for various aspects of the image acquisition. Although the gradients are much weaker than the main magnetic field, the gradients are repeatedly and rapidly turned on and off. The rapidly changing magnetic fields from the gradients can induce electrical currents in electrically conductive devices and may directly excite peripheral nerves. Although current-generation scanners operate at levels that will not directly excite cardiomyocytes, the gradients can induce currents within electrically conductive wires and leads that could cause arrhythmias.[8]

Other issues

Magnetic resonance imaging system and scan locations of the device

The body area to be imaged with MRI matters, especially in reference to the location of the device, whether it is inside or outside of the magnet bore. For example, the use of a dedicated extremity scanner may allow the device to remain outside of the magnet bore as well as away from the RF and gradient fields, resulting in no significant magnetic field interactions for both pacemakers and intracardiac defibrillators (ICDs). Hence, peripheral scan locations such as knee and brain scanning do not have significant effects on programmed parameters, pacemaker components, and pacing capture.[9],[10]

Electrocardiographic aberrations

The very flow of electrically conductive blood in the presence of powerful static magnetic fields produces very small voltages that may produce electrocardiographic aberrations, including elevation of the ST-segment, T-wave abnormalities, and even the appearance of arrhythmias. The stronger the static magnetic field, the greater the magnitude of these observed perturbations. This phenomenon may involve monitoring of the heart rhythm during scanning, leading to inappropriate inhibition of pacemaker function, or creating arrhythmia artifacts on event loop recorders.[11]

Food and Drug Administration terminology used for labeling implanted devices

In 1997, the Food and Drug Administration (FDA), Center for Devices and Radiological Health, proposed the definitions for the terms [Figure 5] as shown below.
Figure 5: ASTM Standard F2503* accepted by the Food and Drug Administration defined terms

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Magnetic resonance safe

It refers to an item that poses no known hazards in any MR environment. “MR safe” items include nonconducting, nonmetallic, and nonmagnetic items, such as a plastic  Petri dish More Details. There is no MRI safe cardiac pacing device or defibrillator.

Magnetic resonance conditional (i.e., conditioned to work properly in a magnetic resonance imaging environment)

Conditional means that device has been conditioned or modified such that it poses no known hazards in a specified MRI environment with specified conditions of use. Conditions that define the MR environment include static magnetic field strength, spatial magnetic gradient, dB/dt (time-varying magnetic fields), RF fields, and SAR. Additional conditions, including specific configurations of the device, e.g., the routing of leads, may be required.

Magnetic resonance unsafe

It is an item that is known to pose hazards in all MR environments. “MR unsafe” items include ferromagnetic items in leads and devices.[12],[13]

Magnetic resonance-conditional leads

Pacing leads can potentially act as antennae for electromagnetic energy impulses. The transfer of electromagnetic energy can lead to myocardial electrical stimulation, tissue destruction at the lead tip–endocardial interface, pain, and damage to the pulse generator circuitry or battery. This may produce adverse effects on sensing, pacing thresholds, and lead impedances and can cause inappropriate pacing acceleration or inhibition and battery depletion.

A pacemaker lead is composed of an outer and inner insulation and an outer and inner lead coil. The lead coil is arranged in a configuration to maximize energy-efficient conduction while maintaining flexibility, durability, and minimization of lead diameter. The inner coil is made of filaments wound in a three-dimensional relationship coiled with a certain pitch. This has implications for MRI-related energy conduction based on the resonant frequency of the lead. One of lead design changes in an MR-conditional lead is a modification in the pitch of the inner coil. The inner coil was designed with a decreased number of coiled filars, increasing the number of winding turns and therefore increasing the lead inductance. This geometry limits the RFs that can conduct through the lead filaments. The decrease in the number of filars required an increase in the filar diameter to increase the strength of the lead [Figure 6] and [Figure 7].
Figure 6: Common industry coil designs

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Figure 7: Coil designs

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Other challenge in the development of device leads includes the elimination of their movement, heating, and current induction in MR environment. Some movement is possible in leads that have been in place even for more than several weeks to months. Modification of the materials within the leads could reduce the risk of heating and current induction. The modifications include replacement of stainless steel components with copper or nickel alloys in the inner coil or lead shaft and the use of platinum/iridium electrodes at the lead tip. MR-conditional leads have also been modified to conform to the geometry of the inner coil to reduce the transfer of energy which in turn reduces lead-tip heating.

Magnetic resonance-conditional devices

MR-conditional devices are now smaller with less ferromagnetic material with consequently improved protection against electromagnetic interference. These changes improved their resilience to artifact and heating induced by MR scanning. These conditioned cardiac implantable electronic device (CIED) systems could undergo MR scanning at 1.5 T without additional risk. The device components have been redesigned to minimize the energy induced and discharged. This has been possible by changes including reduction of the ferromagnetic content of the pulse generator and enhanced protection of the circuitry and internal power supply.

Changes in reed switch

The use of a Hall effect sensor in lieu of a reed switch has been introduced that reduces the risk of the device reverting to a “magnet mode” when exposed in an MR imager, which sometimes can lead to asynchronous pacing in pacemakers. The Hall effect sensor's predictable behavior is not influenced by the static magnetic field of the MR environment.

Besides alterations in structural design, software changes have been developed for MR-conditional devices. Most devices use a special MR programming mode in which the CIED will revert to an asynchronous pacing mode at higher pacing outputs to avoid suppression of pacing during MR scanning. For MR-conditional ICDs, therapy for ventricular tachyarrhythmia is temporarily disabled during MRI [Table 3] and [Table 4].[12],[13],[14]
Table 3: Modifications to produce magnetic resonance imaging-conditional pacemakers

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Table 4: Magnetic resonance imaging-conditional devices in the US and Europe markets

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  Current Status of Operative Issues Top


Potential perioperative issues with cardiac devices

The most widely appreciated potential complication with the cardiac device in the operative setting is the inappropriate sensing of electromagnetic interference caused by electrocautery. The consequence of such inappropriate sensing depends on the type of device involved, patient characteristics, and device settings. In general, however, the results may include:

  1. Inappropriate inhibition of pacing
  2. Inappropriate rapid pacing
  3. Inappropriate sensing and triggering of ICD therapy (shock or rapid pacing and antitachycardia pacing).


Potential implications for device function associated with anesthesia and surgery

  1. Reprogramming of device secondary to cautery use or associated with radiotherapy or RF ablation procedures to “power-on reset” mode – a simple backup pacing mode, typically Ventricle paced, ventricle sensed; pacing inhibited if beat sensed (VVI) or Ventricular asynchronous pacing (VOO)
  2. Anatomical complications associated with surgical procedures – pneumothorax with thoracotomy or cardiac surgery – may lead to pacemaker malfunction due to lead dislodgment, increase in impedance, particularly with unipolar pacing systems, or increase in the defibrillation threshold
  3. Damage to leads through direct trauma; device or lead infection associated with perioperative bacteremia
  4. Burn to tissue associated with electromagnetic interference conduction through leads
  5. Complications associated with the failure of device to return to preoperative setting.[15]


Abandoned leads

The presence of abandoned pacing or ICD leads, capped or uncapped, or lead remnants after a partially successful device extraction is considered to be absolute contraindications for MR scanning. Abandoned leads are associated with an increased risk of heating and myocardial damage. Metallic remnants of leads can heat, dislodge, or embolize. In rare circumstances when MR scanning is absolutely required, extraction of abandoned leads might be considered, but this can be associated with a 1%–2% chance of a life-threatening complication. Because of these risks, it is unusual to perform device extraction for patients with leads in situ for many years to allow them to undergo MR scanning.[16]

Market availability of magnetic resonance-conditional cardiac implantable electronic device technology

The first MR-conditional pacemaker system was introduced in Europe by BIOTRONIK in 2010 (Berlin, Germany), closely followed by Health Canada and then with the US FDA approval in January 2011 for Medtronic's MR-conditional pacemaker (Minneapolis, MN).

Since then, almost all manufacturers have MR conditional devices that can be used for full body scan at upto 3 Tesla.[17]

Previous studies regarding magnetic resonance imaging-device interactions

The potentially harmful effects have mainly been identified in older pacemaker and lead technology. During the last decade, a number of small studies have asserted that MRI scans (at 0.5 T and 1.5 T) can be safely performed in patients with implanted pacemakers in carefully selected clinical circumstances when appropriate strategies are used.

Roguin et al.[18] reported that following a specific safety protocol, pacemakers manufactured after 2000 can safely go into MRI. These devices are now smaller, with less magnetic material and improved electromagnetic interference protection. Moreover, the absence of clinically relevant cumulative changes in pacing capture threshold, lead impedance, or battery voltage has been shown by Nazarian [19] in permanent pacemaker patients who underwent two or more MRI examinations.

Nazarian [19] followed 438 patients with implanted cardiac devices (54% with pacemakers and 46% with defibrillators) who had 555 MRI scans. After MRI, the observed changes in the electrical parameters did not require device revision or reprogramming. The researchers concluded that with a protocol based on device selection, programming, and careful patient monitoring, MRI can be performed safely in many patients who have an implanted device. Finally, in a very recent paper, a German group showed that even cardiac MRI may be performed safely when limiting SAR, appropriately monitoring patients, and device reprogramming after MRI, if necessary. Cardiac MRI delivered good image quality and diagnostic value in patients with right-sided devices. Hence, the authors conclude that cardiac MRI in patients with right-sided devices may therefore be performed with an acceptable risk/benefit ratio, whereas the risk/benefit ratio is rather unfavorable in patients with left-sided devices. This finding may simply be due to MRI artifact caused by left-sided devices.

Role of electrophysiology and radiologist versus billing code

It is interesting that the Heart Rhythm Society has not yet published any guideline document regarding MRI compatible devices. Clearly, electrophysiology (EP) physicians are those who implant and follow devices, while radiologists are those who scan the patients. The American college of radiology (ACR) and the Radiological Society of North America warn that medical devices may malfunction or cause problems during an MRI examination. Scanning a patient with MRI-conditional device requires some basic precautions that include effective communication and close collaboration between cardiologists and radiologists. The service burden of scanning device patients raises several logistic problems and requires flexibility and close cooperation between those involved (patient, MRI availability, EP physician, EP nurse, EP trainee, radiologist, radiology trainee, MRI technician, and device representative). All experts in the field agree that competent supervision by an EP physician and team is required before, during, and after MRI to monitor and reprogram the device and to manage any complication [Table 5] and [Table 6].
Table 5: Requirements for scanning with magnetic resonance imaging-conditional permanent pacemakers

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Table 6: Magnetic resonance imaging suite requirements

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Anticipating that an MRI examination takes approximately 45 min, plus the time required for device testing before and after MRI, we can expect 90 min per MRI. Scanning an average of 50 device patients per year, it is possible to see how much this service might impact the workload of all the personnel involved. Nevertheless, such costly support is mandatory as scanning device patients may also have serious medicolegal implications. Time-dependent billing code for supervision and management by EP team is not available.[20]

Protocol and facility for performing magnetic resonance imaging patients with magnetic resonance-conditional cardiac implantable electronic devices

Every imaging facility must have a protocol for MR scanning of patients with CIEDs, developed through a collaborative effort of MR and EP/device specialists. Ideally, it should consist of an onsite CIED clinic to interrogate and program the CIED systems. In some cases, it might be possible for radiology suite to establish close collaboration with an offsite CIED clinic where patients are assessed before and after MR.

However, on the day of MR scan, a member of the CIED team (technician, nurse, or physician) should be readily accessible for device troubleshooting or reprogramming, if required. Yet, it might not be possible for a member from the CIED clinic to be physically present in the MR suite during the entirety of the scan because of logistic reasons. However, as MR-conditional CIED technology evolves and experiences with MR scanning accrues, the need for onsite CIED support might diminish over time.

The way by which the CIED team provides onsite support for patients who undergo MR scanning should be based on a mutual and collaborative agreement between the CIED clinic and the MR Radiology Department. It should be tailored according to the practice standards and resources of the institution. Some might require that a CIED team member be present during the entire MR scan, and others might require that the CIED team provide the same-day assessment before and after MR scanning. The specific personnel requirement will be left to the discretion of the institution. Finally, a cardiologist with expertise in CIED management should be readily available for consultation before, during, and after MR scanning. This physician does not need to be physically present but should be easily accessible to provide advice, if required. The imaging facility should develop a standardized protocol to triage CIED patients for MR scanning.[21]

This protocol will systematically:

  1. Identify patients with CIED systems
  2. Alert the MR team of the presence of a CIED in a given patient
  3. Formalize a referral process to the CIED clinic to obtain information on the CIED and to assess its function
  4. Identify potential relative contraindications that might increase risk during MR scanning
  5. Ensure that the CIED and patient have been properly assessed in preparation for MR scanning
  6. Ensure that the patient's CIED is reinterrogated and reprogrammed after MR scanning
  7. Alert physicians (MR radiologist and CIED cardiologist) of potential CIED malfunction before, during, and after MR scanning.


The facility should also have the capability to monitor the patient's vital status during MR scanning. All of the following modalities should be available: (1) pulse oximetry, (2) electrocardiographic monitoring, and (3) capability for verbal communication between the MR scan operator and patient. The facility should also have emergency resuscitation equipment available including, at minimum, an external defibrillator and ready access to an onsite emergency resuscitation cart and team.

Programming of device to magnetic resonance imaging mode

Most manufacturers have programmable device modification which has to be switched on before MRI, and the device is reverted back to preprogram status post-MRI. This programming has to be done by EP physician/nurse, and postprocedure, all lead parameters and therapy parameters have to be rechecked and reprogrammed as per need of patient.[21]

Monitoring requirements for the cardiac implantable electronic device patient undergoing magnetic resonance with a magnetic resonance-conditional device

To date, there are no formalized recommendations with regard to the optimal method to monitor the vital status of CIED patients undergoing MR scanning.[22]

Currently, three modes of monitoring are often used: electrocardiographic monitoring, (2) pulse oximetry, and (3) intermittent verbal communication.

Monitoring of the CIED patient during MR scanning might be performed by a number of qualified individuals including MR technologist, MR nurse, MR radiologist, cardiologist with expertise in CIED management, or CIED clinic nurse.

Three important factors determine the personnel composition required for monitoring of a given CIED patient during MR scanning: (1) the patient's medical status, (2) the functional status of CIED, and (3) experience of the team members.

Additional monitoring requiring situations

  1. A patient with an active arrhythmia: it is recommended that MRI be deferred until the arrhythmia is controlled
  2. A patient with an MR-conditional CIED with some abnormal function, but none that are absolute contraindications for MRI, e.g., recent, nonsignificant changes in pacing and sensing thresholds
  3. A patient with an MR-conditional ICD that is functioning normally who has had recent therapies (pacing or shocks) for ventricular arrhythmias.


Magnetic resonance imaging-conditional devices: Ethical and legal issues

Regarding the choice of MRI-compatible/conditional device, some important medicolegal questions should be carefully considered:

  1. The patient's right to select his/her device
  2. The responsibility of electrophysiologist/device nurse
  3. The hospital's responsibility
  4. As correct quantification as possible of the patient's potential damage if MRI is needed
  5. Patient clearly informed that the device is MRI conditional and not safe.


The major legal issue concerns the possible liability of the EP physician, and the institution if a patient, who received a conventional device, needs an MRI. It is difficult for the physician to justify the implantation of a conventional system if an MRI-compatible system is available.

With regard to the doctor's responsibility, this issue is greatly influenced by the differences in the legal and health-care systems in different countries. The guiding principles are similar, and the responsibility of explaining risks, benefits, and alternatives and its clear understanding by patient is of paramount importance.[22],[23]

Finally, potential damage to patients caused by not performing an MRI scan, which is certainly debatable, often in the context of a lack of data, with regard to type of illness, availability of other diagnostic techniques, need to guide therapy, etc., has to be considered.


  Conclusion Top


MRI in CIED patients is no more considered an absolute contraindication to imaging. Potentially deleterious effects have been identified, including inhibition of pacing, asynchronous/high-rate pacing, lead-tip heating, loss of capture, and alteration of programming with potential damage to pacemaker circuitry. The patient is subjected to an intense magnetic field, which could impose mechanical forces on ferromagnetic components or inhibit stimulation and cause unpredictable magnetic sensor activation or reversion to an asynchronous program. The reed switches contained in the most current pacemakers are susceptible to the intense magnetic fields generated during MRI. In addition, patients are exposed to a magnetic gradient and RF voltage that could cause a dangerous arrhythmia due to high voltage through the body of the lead, pacemaker reprogramming or power-on reset, and RF interactions with the device such as over- and under-sensing. In addition, a current flow through the lead could result in overheating and thermal damage of cardiac tissue in the area adjacent to the lead electrodes. In fact, a temperature increase at the lead-tip in vitro may result in loss of pacing capture or myocardial perforation.

No CIED system is “MR safe,” but selected CIED systems are “MR conditional.” At least four following conditions have to be met with for a relatively safe MRI, namely (a) device hardware has been modified such that MR influence is negligible, (b) conditions to do MRI safely are met with including preprocedure assessment and programming of device to MRI mode, (c) post-MRI, the device should be programmed back to normal therapy, and (d) MRI suite is conditioned to provide appropriate resuscitative therapy in event of complications.

Patients may undergo MR scanning without additional known risks as long as manufacturer-specified scanning parameters are followed. It is important to note that the recommended scanning parameters vary among CIED manufacturers. This means that the MR scanning protocol will vary in accordance with the patient's CIED system. As such, clear communication between the CIED cardiologist and the MRI specialist must be established to ensure that these standards are conformed to. For example, some CIED systems permit full-body scanning and others specify an exclusion zone, which prohibits imaging in the thoracic region. In general, all CIED manufacturers recommend maximization of the distance between CIED and scanner, if possible. Most manufacturers also recommend a maximum static magnetic field strength of 1.5 T, with a maximum SAR value of 2 W/kg for each sequence and a maximum gradient slew rate of 200 T/m/s. Most studies that evaluated the safety of MR scanning in patients with MR-conditional CIED systems have been conducted with 1.5 T scanners. There are some data on the safety of MR scanning for MR-conditional CIEDs at 3.0 T. In addition, the long-term (e.g., >5 years) product performance of conditional leads and pulse generators is unknown.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
  References Top

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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]



 

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