*Also, see sections on Cardiac Monitors, such as the Reveal and Confirm Products

Cardiac Pacemakers and Implantable Cardioverter Defibrillators
Cardiac pacemakers and implantable cardioverter defibrillators (ICDs) are crucial implanted devices for patients with heart conditions and serve to maintain quality of life and substantially reduce morbidity. Expanded indications for cardiac pacemakers and ICDs (e.g., heart failure, obstructive sleep apnea, and other conditions) emphasize that an increasing number of patients will be treated with these devices. 

Currently, these cardiovascular implants are considered a relative contraindication for patients referred for MR procedures. Additionally, individuals with cardiac pacemakers and ICDs should be prevented from entering the MR environment because of potential risks. 

MR conditional cardiac pacing systems are now available for patients referred for MR examinations and a comprehensive review of this topic has recently been presented in the Journal of Cardiovascular Magnetic Resonance (JCMR) by Shinbane, Colletti, and Shellock (Magnetic resonance imaging in patients with cardiac pacemakers: Era of MR conditional design. Journal of Cardiovascular Magnetic Resonance 2011;197:W457-9).

For additional information on the topic of cardiac pacing systems and MRI, please refer to the following sections, including the sections that refer to “MR Conditional” cardiac pacing systems: Cardiac Pacemaker: Accent MRI Pacemaker System, Cardiac Pacemaker: Advisa DR MRI SureScan Pacing System, Cardiac Pacemaker: Ensura DR MRI SureScan Pacing System, Cardiac Pacemaker: EnRhythm MRI SureScan Pacing System, Cardiac Pacemaker: Revo MRI SureScan Pacing System, Cardiac Pacemakers: ProMRI Pacing Systems, and Extremity MR System.

Cardiac pacemakers and ICDs have been suggested to present potential problems to patients undergoing MR procedures from various mechanisms, including:
1) Movement and/or vibration of the pulse generator or lead(s)
2) Temporary or permanent modification of the function (i.e., damage) of the device
3) Inappropriate sensing, triggering, or activation of the device
4) Excessive heating of the leads
5) Induced currents in the leads
6) Electromagnetic interference 

The effects of the MR environment and MR procedures on the functional and operational aspects of cardiac pacemakers and ICDs vary, depending on several factors including the type of device, how the device is programmed, the static magnetic field strength of the MR system, and the imaging conditions used for the procedure (i.e., the anatomic region imaged, type of transmit RF coil used, the pulse sequence, amount of radiofrequency energy used, etc.). Notably, most of the data concerning the deleterious effects of MR imaging on cardiac pacemakers involved the use of older versions (i.e., pre-1996) of these devices.

Recently, many compelling reports have been published concerning “modern-day” pacemakers (i.e. devices with decreased ferromagnetic components and more sophisticated circuitry) that indicate that certain patients may undergo MR examinations without harmful effects by following specific guidelines to minimize or prevent risks. Thus, there is growing evidence that it may be possible to perform MR procedures safely in patients with cardiovascular devices under highly controlled conditions (see below). Alternatively, as previously mentioned, MR conditional cardiac pacing systems now exist that were specifically designed to provide a high margin of safety for patients referred for MR procedures. 

Implantable cardioverter defibrillators (ICDs) are medical devices designed to automatically detect and treat episodes of ventricular fibrillation, ventricular tachycardias, bradycardia, and other conditions. When a problem is identified, the device can deliver defibrillation, cardioversion, antitachycardia pacing, bradycardia pacing, or other therapy. 

In general, exposure to an MR system or to an MR procedure has similar effects on an ICD as those previously described for a cardiac pacemaker, since some of the basic components are comparable. However, there are several unique aspects of ICDs that impact the possible safe performance of MR procedures in patients with these devices. Therefore, patients and individuals with these devices are generally not allowed to enter the MR environment. In addition, since ICDs also have electrodes placed in the myocardium, patients are typically not permitted to undergo MR examinations because of the inherent risks related to the presence of these conductive materials. 

Similar to cardiac pacemakers, it is anticipated that, in lieu of developing a truly acceptable ICD for the MR environment, safety criteria may be determined for “modern-day” ICDs that entail using specialized programming, monitoring procedures, and MR conditions to allow patients to undergo MR examinations safely. Several recent studies have described patients with ICDs examined by MR imaging without serious problems with the caveat that ICDs, in fact, tend to be more problematic than pacemakers relative to the use of MR examinations. Notably, potential problems remain even for “modern” cardiac pacemakers and ICDs as indicated by reports by Gimbel (2009) and Mollerus et al. (2009), such that extreme caution must be exercised when scanning patients with these devices.

Growing Evidence for Performing MR Procedures in Nonpacemaker Dependent Patients
Harmful effects related to performing MRI procedures in patients with cardiac pacemakers have been documented. Notably, in virtually every case involving a fatality, the patient apparently entered the MR environment without the staff knowing a cardiac pacemaker was present. Importantly, these deaths were poorly characterized, no electrocardiographic data were available for review, it was unknown whether these patients were pacemaker dependent, and the actual cause or mechanism of death were not confirmed. By comparison, no irreversible harm has been reported when patients with cardiac pacemakers were carefully monitored during MR procedures and/or the devices underwent reprogramming prior to the scans.

Regardless of the known hazards of subjecting a patient with a cardiac pacemaker to the MR environment, numerous patients (1,000+) have now undergone MR imaging during purposeful, monitored procedures performed in order to conduct necessary diagnostic examinations. These patients were safely and successfully imaged using MR systems operating at static magnetic fields ranging from 0.2-Tesla to 3-Tesla without serious adverse events.

In consideration of the above, there is increasing evidence from in vitro, laboratory, and clinical studies that strict restrictions prohibiting MR procedures in patients with “modern” cardiac pacemakers and ICDs may be modified. Similar to performing MR procedures in patients with other electronically-activated devices (e.g., bone fusion stimulators, cochlear implants, neurostimulation systems, programmable injusion pumps, etc.), scanning patients with cardiac pacemakers and ICDs involves following highly specific procedures to ensure patient safety. 

Interestingly, a recent study by Gimbel (2008) reported scanning patients with cardiac pacemakers at 3-Tesla with no restrictions placed on pacemaker dependency, region scanned, device type, or manufacturer. This somewhat limited experience suggested that patients may undergo carefully tailored 3-Tesla MRI scans when pre-MRI reprogramming of the device occurs in conjunction with extensive monitoring, supervision, and follow-up.

Proposed Guidelines for Performing MRI in Nonpacemaker Dependent Patients
Guidelines have been presented by Martin et al. (2004), Roquin et al. (2004), Loewy et al. (2004), the American College of Radiology (2007) and the American College of Cardiology/American Heart Association (2007) with regard to performing MR procedures in nonpacemaker dependent patients. These guidelines include, the following: 
-Establish a risk-benefit ratio for the patient 
-Obtain written and verbal informed consent 
-Pretest pacemaker functions using appropriate equipment outside of the MR environment
-A cardiologist/electrophysiologist should decide whether it is necessary to program the pacemaker prior to the MR examination
-A cardiologist/electrophysiologist with Advanced Cardiac Life Support (ACLS) training must be in attendance for the entire MRI examination 
-The patient should be monitored continuously during the MR procedure (e.g., blood pressure, pulse rate, oxygen saturation, and ECG)
-Appropriate personnel, a crash cart, and defibrillator must be available throughout the procedure to address an adverse event 
-Maintain visual and voice contact throughout the procedure with the patient
-Instruct the patient to alert the MR system operator of any unusual sensations or problems so that, if necessary, the MR system operator can immediately terminate the procedure
-After the MRI examination, a cardiologist/electrophysiologist should interrogate the pacemaker to confirm that the function is consistent with the pre-examination state

MRI at 1.5 and 3-Tesla and Cardiac Pacemakers and ICDs: Magnetic Field Interactions
As previously discussed, one important safety aspect of the MR environment on cardiac pacemakers and ICDs is related to magnetic field interactions. Component parts of pacemakers and ICDs, such as batteries, reed-switches, or transformer core materials may contain ferromagnetic materials. Therefore, substantial magnetic field interactions may exist, causing these implants to move or be uncomfortable for patients or individuals. Therefore, as an important part of evaluating pacemakers and ICDs, tests for magnetic field interactions have been conducted using MR systems operating at static magnetic field strengths ranging from 0.2-Tesla (i.e., the dedicated-extremity MR system) to 3-Tesla. 

Luechinger et al. investigated magnetic field interactions for thirty-one cardiac pacemakers and thirteen ICDs in association with exposure to a 1.5-Tesla MR system (Gyroscan ACS NT, Philips Medical Systems, Best, The Netherlands). The investigators reported that “newer” cardiac pacemakers had relatively low magnetic force values compared to older devices. With regard to ICDs, with the exception of one newer model (GEM II, 7273 ICD, Medtronic, Inc., Minneapolis, MN), all ICDs showed relatively high magnetic field interactions. Luechinger et al. concluded that modern-day pacemakers present no safety risk with respect to magnetic field interactions at 1.5-Tesla, while ICDs could pose problems due to strong magnet-related mechanical forces.

The clinical use of 3-Tesla MR systems for brain, musculoskeletal, body and cardiovascular applications is increasing. Because previous investigations performed to determine MR safety for pacemakers and ICDs used MR systems with static magnetic fields of 1.5-Tesla or less, it is crucial to perform ex vivo testing at 3-Tesla to characterize magnetic field-related safety for these implants, with full appreciation that additional MR safety issues exist for these devices, as described-above. 

An important aspect of determining magnetic field interactions for metallic implants involves the measurement of translational attraction. Translational attraction is assessed for metallic implants using the standardized deflection angle test recommended by the American Society for Testing and Materials (ASTM) International. According to ASTM International guidelines, the deflection angle for an implant or device is generally measured at the point of the “highest spatial gradient” for the specific MR system used for testing. Notably, the deflection angle test is commonly performed as an integral part of safety testing for metallic implants and devices. 

Various types of magnets exist for commercially available 1.5- and 3-Tesla MR systems, including magnet configurations that are used for conventional “long-bore” scanners and newer “short-bore” systems. Because of physical differences in the position and magnitude of the highest spatial gradient for different magnets, measurements of deflection angles for implants using long-bore vs. short-bore MR systems can produce substantially different results for magnetic field-related translational attraction, as reported by Shellock et al. 

The implications are primarily for magnetic field-related translational attraction with regard to long-bore vs. short-bore 3-Tesla MR systems (with short-bore scanners producing greater translational attraction for a given implant). Therefore, a study was conducted on fourteen different cardiac pacemakers and four ICDs to evaluate translational attraction for these devices in association with long-bore and short-bore 1.5- and 3-Tesla MR systems. Deflection angles were measured based on guidelines from the ASTM International.

In general, deflection angles for the cardiovascular implants that underwent evaluation were significantly (p<0.01) higher on 1.5- and 3-Tesla short-bore scanners compared to long-bore MR systems. For the 1.5-Tesla MR systems, three cardiac pacemakers (Cosmos, Model 283-01 Pacemaker, Intermedics, Inc., Freeport, TX; Nova Model 281-01 Pacemaker, Intermedics, Inc., Freeport, TX; Res-Q ACE, Model 101-01 Pacemaker; Intermedics, Inc., Freeport, TX) exhibited deflection angles greater than 45 degrees (i.e., exceeding the recommended ASTM International criteria) on both long-bore and short-bore 1.5-Tesla MR systems. The findings indicated that these devices are potentially problematic for patients from a magnetic field interaction consideration. 

With regard to the 3-Tesla MR systems, seven implants exhibited deflection angles greater than 45 degrees on the long-bore 3-Tesla scanner, while 13 exhibited deflection angles greater than 45 degrees on the short-bore 3-T MR system (refer to The List for information on the cardiac pacemakers and ICDs that underwent testing). Importantly, the findings for magnetic field-related translational attraction were substantially different comparing the long-bore (i.e., lower values) and short-bore (i.e., higher values) MR systems.

As stated, other factors exist that may impact MRI issues for these cardiac pacemakers and ICDs. Therefore, regardless of the fact that magnetic field interactions may not present a risk for some of the cardiovascular implants that have been tested, these potentially hazardous mechanisms must be considered carefully for these devices.
 

MR Conditional Cardiac Devices

As a result of considerable design changes, several MR Conditional cardiac pacing systems and implantable cardioverter defibrillators (ICDs, as well as cardiac monitors, are now available for patients referred for MR examinations including those manufacturered by Medtronic, Inc., Sorin, Boston Scientific, Biotronik, and St. Jude Medical. For additional information pertaining to these cardiac devices and MRI, please refer to the pertinent sections in this textbook and visit the websites of the aforementioned manufacturers.

Important Note to MR Healthcare Professionals: Different MRI guidelines may exist for different countries. Therefore, use the appropriate information for your country.

MR healthcare professionals are advised to contact the respective manufacturer in order to obtain the latest safety information to ensure patient safety relative to the use of an MR procedure.

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