MRI has become the preferred procedure for diagnosing a wide variety of medical conditions including back and joint pain, stroke symptoms, and cancer.1 Currently, 55% of MRI referrals come from neurologists, oncologists, and orthopedists.2 MRI has been shown to deliver superior imaging for soft tissue without use of ionizing radiation.3 Radiation exposure from recurrent CT adds to cumulative radiation exposure and is associated with increased radiation-induced cancer risk.4 As the number of patients with either a pacemaker or ICD increases in the general population, it is to be expected that the number who will eventually need an MRI due to comorbid medical conditions will increase as well. In fact, it is estimated that 50%-75% of pacemaker patients will have a medical need for an MRI over the lifetime of their device,5 and more than one-third of patients with ICDs — 36 percent — are likely to need an MRI within 4 years of implant.6
To create an image of a desired structure in the body using a nuclear magnetic resonance imaging technique, an MRI scanner applies a strong static magnetic field to the body’s tissue to align the randomly oriented hydrogen nuclei (protons). This is necessary to generate a net (longitudinal) magnetization within the body and make the protons rotate at a predefined frequency. A radiofrequency (RF) pulse at that predetermined frequency is then applied for a certain duration to bring the protons in resonance and flip the longitudinal magnetization into the transverse plane. Once the RF pulse is switched off, the fading of the transverse magnetization and pro-gressive increase in the longitudinal magnetization will occur, thereby restoring the original situation (net longitudinal magnetization). Because this process is specific for different types of tissue, images showing contrast between different structures in the body can be generated.
Traditionally, patients receiving implantable permanent pacemakers or cardiac defibrillators could only receive a CT scan and not an MRI. The reasons for this are varied. As a result of the strong static magnetic field in the MRI scanner, implanted devices containing iron (Fe), cobalt (Co), nickel (Ni), and a few alloys that are strongly magnetic can possibly move within the device pocket. The static magnetic field may also inadvertently switch the pacing functions of the device to either asynchronous pacing or cause unwanted inhibition of pacing. Circuits in the device may be damaged resulting in changes in the programming of the device, unwanted “resetting” of the device back to standard device settings, or an inability to communicate between the device and external programmer. Besides altering the programming, there can be physical damage to the heart as well. The RF field of the MRI may induce current (energy) in the device leads, which act as a type of antenna. Because the tissue near the lead tip has limited conductivity, the energy in the lead will be converted to heat causing thermal damage, edema, and scar tissue.7 This can result in increased stimulation thresholds and unpredictable loss of capture. In ICDs, the fast switching magnetic gradients can potentially mimic intrinsic cardiac activity, which can be interpreted as ventricular tachycardia and cause inappropriate device shocks. Because of these issues, the consensus guidelines have always been that use of an MRI in patients with either a pacemaker or ICD is prohibited unless the need for the MRI was compelling enough to warrant the risk of the exam.
The inability to safely perform MRI scans on these patients has been a significant limitation that has driven development of MRI-compatible devices. The first FDA approved, MRI-compatible pacemaker was released in 2011, and on September 23, 2015, Medtronic received FDA approval for an MRI-compatible ICD: the Evera MRI SureScan ICD System. The Evera MRI ICD, along with the Sprint Quattro Secure MRI SureScan DF4 lead, includes hardware and software design changes from previous generation devices that differentiate it from other ICDs and allow it to undergo full-body MRIs without positioning restrictions.
This approval was based on safety and efficacy data from the Evera MRI Clinical Trial,8 a multicenter, prospective, randomized, controlled clinical trial that enrolled 275 patients at 42 centers around the world. The trial allowed for full-body 1.5 Tesla (the field strength of the magnet) MRI scans and included scans of the chest region where the device is in close proximity to the MRI fields as well as enrollment of high-risk, pacing-dependent patients. An additional subset of patients in the MRI group had ventricular fibrillation induced following their MRI in order to characterize arrhythmia sensing, detection, and therapy delivery. The study met the safety endpoint, demonstrating 100 percent freedom from MRI-related complications (P<.0001) in the MRI group.
Baptist Medical Center Jacksonville was the first hospital in northeast Florida to implant the device since its approval. Baptist Medical Center Jacksonville is a 691-bed tertiary referral center that is part of a more than 1,100-bed regional hospital system, and contains the community’s only dedicated Heart Hospital. The Arrhythmia Program consists of 3 electrophysiologists working with 12 nurses and radiation technologists in 2 dedicated EP labs. The physicians are hospital employed and part of Baptist Heart Specialists, a group of 30 cardiologists. The program strives to bring advanced therapies to the region, including having been a designated hospital for the Micra leadless pacemaker trial with the second largest implant volume in the U.S.
The patient was a 60-year-old woman with multiple myeloma, diabetes, hypertension, and a longstanding history of a nonischemic cardiomyopathy with an ejection fraction of 35% since 2013. She had declined an ICD in the past. She presented with left arm weakness and transient aphasia. Head CT showed possible small infarcts of the right parietal and occipital lobes that were questionably old or new. An MRI of the head revealed a small acute left frontal cortical infarct, and the right parietal and bilateral occipital lobe infarcts were determined to be chronic. Given that she was already on antiplatelet agents, she was anticoagulated post stroke. While recuperating from her acute stroke, she developed spontaneous recurrent episodes of ventricular tachycardia requiring defibrillation. It was recommended that she undergo implantation of an ICD for secondary prevention against sudden cardiac death. However, in light of her recent stroke, it was felt that her likelihood of needing an MRI in the future was high. Therefore, the decision was made to place an MRI-compatible ICD.
In this case, the patient met criteria for ICD implant for both primary and secondary prevention against sudden cardiac death. She actually presented with an acute stroke that was missed on CT scan, but diagnosed accurately with an MRI. This patient’s history of strokes along with multiple myeloma make it likely that further MRIs may be necessary, so the ability to undergo an MRI now enables and expands the success of any future diagnostic evaluations.
- IMV MRI 2010 Benchmark Report.
- RSNA research, radiologists. December 2008.
- CT Scan vs. MRI. Diffen. Available online at http://www.diffen.com/difference/CT_Scan_vs_MRI. Accessed November 9, 2015.
- ACR Practice guideline for diagnostic reference levels in medical x-ray imaging - Revised 2008 (Resolution 3). Recommended diagnostic CT reference levels derived from analysis of the data gathered from the first 3 years of the ACR CT Accreditation Program.
- Kalin R, Stanton MS. Current clinical issues for MRI scanning of pacemaker and defibrillator patients. Pacing Clin Electrophysiol. 2005;28(4):326-328.
- Nazarian S, Reynolds M, Ryan M, et al. Estimating the Likelihood of MRI in Patients After ICD Implantation: A 10-Year Prediction Model. J Am Coll Cardiol. 2015;65(10_S).
- Walton C, Gergely S, Economides AP. Platinum pacemaker electrodes: origins and effects of the electrode-tissue interface impedance. Pacing Clin Electrophysiol. 1987;10:87-99.
- Gold MR, Sommer T, Schwitter J, et al. Full-Body MRI in Patients With an Implantable Cardi-overter-Defibrillator. J Am Coll Cardiol. 2015;65(24):2581-2588.