The presence of pacemaker and ICD systems is considered a contraindication to MRI. In the present study we tested several of the main concerns that would prohibit MRI scanning (lead heating, forces and torques acting on the device, device function, inhibition of pacing, arrhythmia induction, image quality, and tissue damage). We found that a number of pacemaker systems and selected ICD systems manufactured after 2000 may be safe for MRI. Although these results and the results of others10–15
are encouraging, clinicians must be aware that manufacturers do not claim that their devices currently are MR safe or MR compatible.
Importance of Obtaining MR Scans in Patients With Implanted Devices
MRI continues to be critical for noninvasive imaging in many areas of the body. Because of superior soft tissue discrimination, imaging of the spine and brain is often critical to assess disorders such as tumors, stroke, and inflammatory and infectious diseases.10
Similar considerations make MRI the noninvasive examination of choice to assess internal derangement of most joints and a wide range of other musculoskeletal disorders.6
More recently, MRI has been applied successfully to assess myocardial wall motion, perfusion, and viability.16,17
In many cases, patients with cardiovascular disease who may have the greatest benefit from MRI are also the individuals increasingly likely to have implanted devices. The first cardiac pacemaker was implanted in 1958, and the first ICD was implanted in 1980. Since then, it is estimated that >2 million patients have had these devices implanted. Cardiac pacemakers are the most common electrically active implants found in patients who may be referred for MR procedures.2,12
Thus, it is critical that there is an understanding of the risks of these devices in the MR environment, with the goal of designing devices that are MR compatible.
Theoretical and Real Safety Concerns
The safety of MRI in patients with implanted pacemaker and ICD systems has been debated for years. Recently, it has been suggested that MRI at 0.5 T12–15
and at 1.5 T11
can be safely performed in patients with implanted pacemakers in carefully selected clinical circumstances when appropriate strategies (programming to an asynchronous mode, adequate monitoring techniques, restricted radiofrequency exposure) are used. The presence of implantable devices during routine MRI, however, is considered a contraindication for MRI,3,4
and there are no cardiac devices that currently have achieved Food and Drug Administration (FDA) clearance for MR compatibility. This is in the context in which modern devices are smaller, have less magnetic material, and have improved EMI safety.
Heating at the Lead Tip
Previous studies testing the potential for production of heat in the body by conventional MRI found temperature increases within acceptable levels.7,18
Even MR procedures performed with a very high averaged SAR can be physiologically tolerated by an individual with normal thermoregulatory function.18,19
In the present study temperature increase of the lead tip in a canine model in vivo was minimal. In vitro, in 0.45% NaCl or placed to a depth of 2 to 3 mm in gel, we found, similar to Shellock,19,20
that the temperature change correlated with the level of SAR. MR protocols within current FDA SAR limitations (1.4 W/kg) resulted in nonsignificant changes in lead temperature. However, when leads were imbedded deeply inside gel (simulating nonperfused tissue), the temperature rise was substantial. These data from leads imbedded deeply within gel indicate the important effect of the conductive medium on heating. However, this does not simulate the real-life situation in patients because leads, even with active fixation, are only 2 to 3 mm deep in the myocardium. The major reason that the leads do not heat up in vivo is likely the cooling effect of blood flow through the heart tissue and around the lead tip–tissue interface.18
Our in vivo chronic studies in animals confirmed no signs of thermal injury. These findings suggest that carefully chosen MRI protocols with low SAR produce insignificant heating of the leads.
In 1 animal, a temporary deterioration in pacing capture threshold occurred immediately after the scan. The capture threshold returned to baseline after 12 hours, and 4 weeks later, the tissue surrounding the lead was normal histologically. We presume that some edema occurred at the lead tip–tissue interface, which subsequently resolved.
Force and Torque on Devices
For all pacemakers, the measured force and torque were small (<14 g and 10 g×cm, respectively). For ICD models manufactured after 2000, the force and torque (16 to 103 g and 23 to 90 g×cm, respectively) were much lower than older-generation devices. Similar findings using different methods have been reported by other investigators.21
For reference, a 100-g weight is near the limit of detection of sensation by the body. We speculate that several weeks after ICD implantation, when there is complete healing of the surgical wounds and fibrous tissue has surrounded the lead tip and generator, there will be no danger of lead dislodgment or tissue rupture. From studies of patients with pacemakers undergoing MRI, there have been no reported complaints of pain or a sensation of pulling.11,13,14
The effects of EMI on pacemaker and ICD systems depends on the intensity of the electromagnetic field, the frequency spectrum of the signal, the distance and positioning (angle) of the device relative to the source, the electrode configuration (unipolar or bipolar), nonprogrammable device characteristics, programmed settings, and patient characteristics. In the present study we demonstrated that ICDs from different manufacturers and manufacturing years differ in susceptibility to MRI.
During the study the devices were in “monitor-on/therapy-off” mode. The devices interpreted the EMI produced by the scanner as arrhythmia. No therapy, however, was delivered. In ICDs, when the reed switch is turned “on” by the static magnetic field of the scanner, the devices continue their pacing function but all monitoring (and therapy) is turned off. This latter effect is a desirable occurrence during an MR scan. Our results suggest that when a patient with an ICD is being prepared for MRI, the device should be programmed to “therapy off” to avoid delivering therapy as a result of interpretation of noise as tachyarrhythmia. Therefore, for an ICD, whether the reed switch is activated is of little importance.
According to our findings, patients with ICDs that could not be interrogated after the scan () should be absolutely prohibited from undergoing an MRI scan because device replacement would be needed after the procedure. Devices that change to “therapy on” on electric reset (placing the patient at risk for a noise-induced shock) and probably those that showed battery deterioration (even temporarily) should probably not undergo MRI. All other newer devices seem to be safe.
Few studies with MRI in patients with ICDs have been published, but the reported data are in agreement with our findings. Gimbel et al22
reported similar findings regarding GEM-II-7273 (no damage), VENTAK-AV-1810, and MINI-IV-1790 (irreversible damage) in patients who underwent inadvertent MRI scanning when no special precautions were taken. Anfinsen et al23
reported similar findings (ie, battery problems and false arrhythmia detection) with a Prizm-VR-1850 in a patient in whom MRI of the brain was performed without realization that an ICD had been implanted 8 days previously, with false detection of electromagnetic noise during the MRI as ventricular fibrillation.
Pacemakers appear to be MRI safe. The scanner exerts minimal force or torque on modern pacemakers. There was no modification of the function of the pacemaker temporarily or permanently by the MR fields, and no significant heating was induced in the lead tips. We recommend setting the device to VOO for pacer-dependent patients and to ODO for nondependent patients. The reed switch may be closed by the static magnetic field, because fields as low as 15 G can close the switch. Closing of the reed switch is not hazardous, however, because it results only in placing the pacemaker into an asynchronous mode, which is done routinely during some types of pacemaker interrogation.11
There are practical concerns in imaging patients with pacemakers. First, routine imaging of these patients by MR centers would not be appropriate because of lack of personnel at these centers to assess pacemaker type and pacemaker dependency of the patient. Second, in some circumstances, such as lead failure, leads are left in place, and replacement leads are positioned. We did not assess the impact of these nonfunctional remaining leads. Certain lead configurations with wire loops in the chest wall are observed occasionally; these loops could result in significant heating and burns.
MR physicians should be present during the examination to use pulse sequences with low SAR levels and to reduce examination time to as brief as is necessary to obtain diagnostic information. Physiological MR-compatible monitoring equipment should be used during the examination, and resuscitation equipment should be available. Finally, pacemaker assessment and programming immediately before and after the MR scan should be performed.
Image distortion was dependent on the imaging plane and protocol used. Most image distortion was in the area adjacent to the generator. Therefore, organ visualization beyond this distance, such as knees, lower spine, liver, or brain, will not be affected by the presence of the pacemaker or ICD and may even change the device classification to MRI compatible.
Our findings and conclusions can be applied only to those devices and clinical situations tested. Technological advances in pacemaker and ICD development mean that new models appear each year. Until those aspects of modern devices that render them immune to MR scanner fields are defined fully, even newer devices using different technologies will have to be tested individually because newer pacing and ICD technologies may not a priori have improved MRI immunity. Further efforts at understanding the characteristics of these devices that render them MR insensitive are needed before broad revision of practice patterns at MR centers is undertaken. Experimental results obtained in vitro or in healthy laboratory animals exposed to radiofrequency-induced heating during MR procedures cannot be automatically extrapolated to predict thermal or other physiological changes in human subjects, particularly those who are pacemaker dependent or have heart disease.3,4
Recommendations for MRI in Patients With ICDs
Patients with certain ICD devices could be eligible for MR scans under appropriate medically necessary circumstances. For example, MRI would only be considered when other noninvasive imaging tests (eg, CT, ultrasound) were unable to provide diagnostic test results. A potential approach for patients in these circumstances would be informed consent after extensive explanations about potential hazards, including death. The device would be programmed to “therapy off.” MR scanning would consist of the minimum scanning necessary to answer the appropriate clinical question. Noninvasive monitoring of blood pressure, pulse oximetry, and ECG during the MR scan would be available, as well as resuscitation equipment in the MR suite. A physician knowledgeable in pacemaker and/or ICD function should be present during the scan. The MR facility must have an emergency resuscitation plan, with the patient moved rapidly out of the scanner to the control room for immediate resuscitation as needed, including defibrillation. Finally, device interrogation and reprogramming immediately after completion of the MR scan would be necessary.
In conclusion, the data presented in this study suggest that certain pacemaker and ICD systems may be compatible with MR scanning at 1.5 T. Further in vivo studies and carefully monitored patient studies are needed before firm recommendations can be made. If these systems can be shown to be fully MRI compatible (to function appropriately and not distort the image) or at least MRI safe (to function appropriately but distort the image), many patients could benefit from the advantages of MRI and obtain information not readily obtained from other imaging modalities.
Our findings may help to change the clinical imaging indications and contraindications of a large number of patients and to revise current imaging practices worldwide.