Magnetic resonance imaging (MRI) has rapidly become the gold standard for the assessment of many cerebral, systemic, and soft tissue diseases throughout the body. In Canada alone, it is estimated that over 1.5 million MRI scans are performed every year. Traditionally, the presence of any metallic foreign body or implanted device would be an absolute contraindication for MRI imaging because of the potential interactions between the metal and the magnetic field of the imaging machine. This has meant that patients with cardiac implantable electronic devices (CIEDs) such as permanent pacemakers or implantable cardiac defibrillators (ICDs) were automatically excluded from being able to undergo MRI scanning. However, a new generation of CIEDs have become available to allow CIED patients to undergo certain types of MRI scans.
MRI-conditional pacemakers have been clinically available for at least one year in many worldwide jurisdictions, but the development and availability of MRI-conditional ICDs have been more limited. In this report, we detail how one patient at Southlake Regional Health Centre had specific clinical circumstances requiring both an ICD as well as an ongoing need for MRI imaging, and how this problem was addressed by the use of an MRI-conditional ICD system.
The patient is a 35-year-old female who presented to Southlake Regional Health Centre just prior to Christmas last year with a history of three bouts of syncope. She also has a strong family history of myotonic dystrophy (type 1) with a younger brother who was not only affected by myotonic dystrophy, but also died of sudden cardiac death. The patient herself had just recently been diagnosed with myotonic dystrophy. As part of her syncope workup, the patient already had ECGs and Holters that showed some sinus bradycardia, but no advanced heart block or ventricular arrhythmias. She also underwent both an electrophysiologic study and coronary angiogram, which were also unremarkable. While there was a high suspicion that her syncope might be secondary to a tachy- or bradyarrhythmia given her background of myotonic dystrophy, this had not yet been documented; furthermore, there were some clinical aspects to the episodes that were suspicious of being vasovagal. Finally, after discussion with her specialist, the patient had an implantable loop recorder placed. Indeed, she had another episode of syncope, which was documented by the device to be associated with a wide complex arrhythmia that was suspicious for ventricular tachycardia. Therefore, the patient was admitted to hospital for consideration of an ICD implant. However, at around the same time, the patient developed weakness in her right arm. After consultation with neurology, they did not feel that this weakness could be explained by myotonic dystrophy, and a cerebral MRI was ordered prior to ICD implant. On that MRI, a few small enhancing lesions were seen that were suspicious for demyelination. Neurology recommended surveillance cerebral MRIs every six months for the next year, and believed that the patient may need ongoing MRIs after that to monitor for possible progression of these lesions. They felt that serial cerebral CT scans would not be helpful for monitoring her neurological findings.
At this point, our team was in a bind given this patient’s need for both an ICD and ongoing cerebral MRI surveillance. At the time, there was no commercially available MRI-conditional ICD device anywhere in North America. The team had heard of the first MRI-conditional system manufactured and released in Europe just a few months prior to this case (BIOTRONIK’s Lumax 740 ICD series with ProMRI®, with Linoxsmart with ProMRI® and Solia with ProMRI® leads). Thankfully, we were able to import one such system into Canada for this patient by making a special access application to Health Canada, which was approved. The patient now lives with her husband and six-year-old child at home, and has not had any recurrent syncopal episodes. She is scheduled for her first cerebral MRI within the next few days.
The Problem of CIEDs and MRI
It is estimated that 50–75% of patients with CIEDs will require an MRI over the lifetime of their device. In patients with a pacemaker, this number may be higher given the higher mean age and larger number of comorbidities in the pacemaker population. Prior reports have suggested that serious and even fatal injury may occur in patients with CIEDs undergoing MRI imaging. However, these events are quite rare, and other reports have demonstrated the feasibility of using MRI scans safely in patients with CIEDs, albeit in very closely monitored settings. In spite of these promising reports, the risks remain and most MRI imaging centers exclude CIED patients from receiving MRI scans.
There are a number of possible interactions between the CIED and MRI magnetic fields that can result in device failure or damage.1 Radiofrequency forces are generated during the scan, which can result in noise detection on the device. Such noise may cause oversensing that may result in inhibition of pacing or inappropriate tachyarrhythmia detection, which can lead to ICD therapy. Induced voltages may also result within the leads causing both oversensing and undersensing but also potential heating at the lead tips, which can damage cardiac tissues and result in altered lead thresholds/impedances. This risk may be particularly high in abandoned CIED leads. Finally, induced voltages and device heating can also lead to direct thermal damage of device electronic components.
MRI-Conditional CIED Technology: Advances and Limitations
As a way of permitting MRI scanning in patients with CIEDs, manufacturers have made several technological advances to permit MRI compatibility of CIEDs: reduction of ferromagnetic components to decrease magnetic attraction and help avoid corruption of circuitry and device function; circuitry and device modification to accommodate for induced energy field gradients created by the MRI; lead component and design changes to minimize lead tip heating; and integration of special “MRI pacing modes” that eliminate inappropriate sensing and allow for normal device function during MRI imaging. Many companies are also in the process of testing currently available or legacy technologies for MRI safety, and it is likely that some existing technologies may become approved in the future as MRI-conditional with some minimal modification.
Although MRI-conditional CIED technology will open many doors for patients who require both technologies, there will still be limitations. Almost all of the testing for MRI-conditional technologies has occurred with 1.5 Tesla strength magnetic fields. As newer, more powerful 3.0 Tesla scanners become available, it is unclear how these technologies will function in the newer generation scanners. MRI-conditional CIED systems will typically have restrictions on the gradient slew rate (or rate of rise in the gradient amplitude of the magnetic field) to less than 200-216 T/m/s and may also restrict use of additional transmitting coils during the scan. Total body specific absorption rates (SARs) may also need to be restricted (<2.0 W/kg total body, <3.2 W/kg for head). Finally, some devices only allow MRI imaging above or below the chest, while others will allow some types of chest MRI to be performed.
There are also CIED-based limitations that must be addressed prior to subjecting a patient to MRI scanning. The implant must have been in place for a minimum of six weeks and must be in the typical pectoral location. Lead thresholds must be less than 2.0 V at 0.4 ms and lead impedances must be within normal ranges (200-1500 ohms) to ensure that leads are intact and are not fractured prior to subjecting them to magnetic fields. The presence of any other abandoned leads, adapters or extenders must also be confirmed.
Finally, all manufacturers still recommend ECG, oximetry, and blood pressure monitoring during the scan as well as the immediate availability of resuscitative equipment and expertise if needed.
Because of the limitations detailed above, it is important that there is an established workflow protocol for centers performing MRI scanning on patients with MRI-conditional CIEDs.2 While there are no specific guidelines as to how such a workflow should be designed, it must involve a partnership between the local department of radiology and the CIED clinic. First, patients with MRI-conditional devices must be properly identified. MRI-conditional CIED technology consists of an entire “system” that includes both the device and the leads. If non-conditional leads are attached to an MRI-conditional CIED, or vice versa, the system and patient will be at risk during MRI scanning. Thus, patient identification cards must clearly outline that both their generator and their leads are MRI-conditional. Furthermore, the local CIED clinic must keep meticulous documentation of which device and leads the patient has implanted and also maintain documentation of any additional abandoned leads or adapters/extenders. The presence of such additional components would negate the MRI-compatibility of the system. Pre-scanning chest radiography may also be helpful in counting the number of leads/devices and ensuring that the device and leads are correctly positioned. Some MRI-conditional technologies may also have radiopaque markers that can assist in their identification, but not all technologies will possess these, and there is unlikely to be standardization amongst manufacturers.
Immediately prior to scanning, the device should be checked in the CIED clinic to ensure proper function of the device and the turning on of any special “MRI modes” that will prevent loss of device function or inappropriate detection during the scan. During the scan itself, appropriate monitoring of the patient must be in place with staff appropriately trained in resuscitation if required. Immediately after the scan, the device should be re-checked in order to ensure that nothing changed during the MRI, to turn off the “MRI mode,” and restore original device functions. This recommended workflow needs optimal communication and coordination between the radiology department and the CIED clinic.
Future Directions at Southlake
At Southlake, we have already been implanting MRI-conditional pacemakers for about one year, and now, with our case study above, we have begun our journey in implanting MRI-conditional CIEDs. As such, we have been in discussion with our radiology colleagues to draft a workflow procedure in which these patients can be seen both before and after their MRI in the CIED clinic for device evaluation, and during the scan, our radiology department can have the necessary monitoring requirements in place. As these technologies become more available and widespread in their use, the time devoted now to creating the necessary work environment to image patients with these CIEDS will ensure timely and safe MRI imaging with conditional devices for years to come.
Disclosure: Dr. Verma has no conflicts of interest to report.
- Lobodzinski SS. Recent innovations in the development of magnetic resonance imaging conditional pacemakers and implantable cardioverter-defibrillators. Cardiol J. 2012;19(1):98-104.
- Radiologist Information, Checklist and System Verification. Medtronic, Inc. Available online at http://www.medtronic.com/surescan/radiologist.html. Accessed April 15, 2013.