Cardiac Magnetic Resonance for Electrophysiology Procedures

DOI: 10.19102/icrm.2011.020902

¹STEFAN ZIMMERMAN, MD and ²SAMAN NAZARIAN, MD

¹Department of Radiology and ²Division of Cardiology/Cardiac Arrhythmia, Johns Hopkins University, Baltimore, MD

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ABSTRACT. Owing to the lack of ionizing radiation and its capability for characterization of myocardial scar, cardiac magnetic resonance is often the preferred imaging modality prior to complex catheter ablation procedures. The following review highlights imaging techniques and safety issues pertaining to pre-electrophysiology procedural cardiac magnetic resonance.

KEYWORDS. Cardiac Magnetic Resonance, Catheter Ablation.

Dr. Nazarian is funded by a career development grant (K23-HL089333) from the National Institutes of Health. Dr. Nazarian has received honoraria for lectures from Biotronic, Boston Scientific, and St. Jude. The Johns Hopkins University Conflict of Interest Committee manages all commercial arrangements.
Manuscript received June 29, 2011, final version received August 2, 2011.

Address correspondence to: Saman Nazarian, MD, FACC, FHRS, Assistant Professor of Medicine, Johns Hopkins Hospital, Carnegie 592C, 600 N. Wolfe Street, Baltimore, MD, 21287. E-mail: snazarian@jhmi.edu

Introduction

Catheter-based techniques for cure of cardiac arrhythmia have significantly advanced over the past 30 years. Modern electrophysiology procedures for ablation of supraventricular and ventricular tachycardia require the electrophysiologist to interpret simultaneous electrogram data from multiple electrode pairs and within the context of each electrode pair's anatomic location. To simplify the interpretation of these data, three-dimensional mapping systems have been developed that summarize the electrical and anatomic information gathered during the study.^1,2 By enabling three-dimensional voltage and activation mapping, such systems visualize scar tissue, slow conduction zones, and critical paths for arrhythmia. The interpretation of electrical data and formulation of strategies for ablation can be significantly affected by anatomic details of the region of interest. Therefore, current electroanatomic mapping systems also allow image fusion for integration of pre-acquired computed tomography and cardiac magnetic resonance (CMR) images with electrogram data. Owing to the lack of ionizing radiation and its capability for characterization of myocardial scar, CMR is often the preferred imaging modality for pre-procedural imaging. The following review focuses on imaging techniques and safety issues pertaining to pre-electrophysiology procedural CMR.

CMR basics

CMR uses high-strength magnetic and electric fields to evaluate cardiac structure and function, myocardial scar and infiltrative processes, valvular motion, and blood flow. In the presence of an external magnetic field, hydrogen nuclei in molecules of the body (predominantly in water and fat) will become aligned with the axis of the static field. This creates a net magnetization vector. At the same time these protons rotate, or precess, around their own axis, at a rate dependent on the strength of the local magnetic field. Weaker gradient magnetic fields are then applied to introduce regional variability in the precession frequency of hydrogen nuclei, allowing for spatial localization. Radiofrequency pulses tuned to the precession frequency of hydrogen nuclei in the desired spatial location are used to tip the net magnetization vectors of select nuclei from the equilibrium state. After the radiofrequency pulse, the nuclei which received energy from the radiofrequency pulse relax to their equilibrium state, and in the process induce a weak current in the radiofrequency receiver coil. In the final step, radiofrequency receiver coil current, or signal, is converted to the final image via a Fourier transformation.

When performing CMR, multiple pulse sequences (software programs that encode specific radiofrequency pulses and magnetic gradients) are available, each of which is optimized for the evaluation of particular attributes of cardiac structure or function. The availability of a multitude of pulse sequences, each with a series of variable parameters, makes CMR a challenging test to perform, requiring significant expertise for appropriate image acquisition. Owing to the complexity of image acquisition, it is extremely important to communicate the imaging needs for specific procedures to the expert radiology team acquiring the CMR images.

CMR sequences of interest for the electrophysiology laboratory

Magnetic resonance image pulse sequences are generally categorized as either spin echo or gradient echo techniques. Gradient echo techniques have shorter acquisition times and are used for the majority of CMR sequences, including bright blood cine imaging. Spin echo imaging is used for high-resolution, black blood evaluation of morphology, but is limited by longer acquisition times and patient breath-holds. When imaging patients with implanted metal hardware (such as mechanical valves, stents, pacemaker or defibrillator systems, and even sternal wires) spin echo techniques are desirable because of reduced artifact size. Pulse sequences of interest for the electrophysiology laboratory have been summarized in Table 1 and described below.

Since chamber volumes, valvular, and contractile function are always of interest, cine CMR imaging is routinely performed. Spoiled gradient echo and balanced steady state free precession sequences are types of gradient echo imaging used for cine magnetic resonance imaging (MRI) that provide excellent temporal and contrast resolution, optimized for evaluation of cardiac and valvular function.³ Steady state free precession offers improved signal and imaging efficiency but is more susceptible to artifact from metal devices within the field.⁴

Myocardial viability imaging for delineation of scar from normal or heterogeneous border zone myocardium may be of interest prior to catheter ablation. Integration of three-dimensional reconstructed computed tomography datasets of chamber morphology and scar/border zone substrate into electroanatomic mapping systems has been reported.⁵ Our group and others^6–8 are focused on the development of methodologies for CMR scar image integration into electroanatomic mapping systems. CMR techniques for imaging scar take advantage of delayed washout of gadolinium contrast from pathologic tissue (typically scarred, infiltrated, or inflamed myocardium), which results in hyperenhancement of scar compared to normal myocardium.⁹ This technique is called late gadolinium enhancement or delayed enhancement imaging, because images are typically acquired 10–15 min after contrast administration to allow for adequate washout of gadolinium contrast from normal myocardium.¹⁰ Inversion recovery gradient echo pulse sequences are used to null the signal of normal myocardium and accentuate the signal of abnormal tissue that retains contrast. Normal myocardium will be dark, whereas abnormal scarred myocardium will be bright (Figure 1). Obtaining high-quality late gadolinium enhancement images requires attention to a parameter called inversion time, which varies from patient to patient and must be optimized to null the signal of normal myocardium. The proper inversion time is selected from the “TI scout” sequence, which acquires serial images with increasing inversion times. Late gadolinium enhancement imaging can be utilized to identify prior areas of ablation (Figure 2). Atrial late gadolinium enhancement imaging for identification of pre-existing scar or scar after catheter ablation has also been performed, but is challenging due to thinness of the left atrial wall (Figure 3). Specialized free-breathing respiratory and electrocardiogram (ECG)-gated pulse sequences have been developed that allow detailed visualization of the thinner atrial muscular tissue.¹¹ These pulse sequences are not routinely performed and should be specifically requested when required. The potential for thrombus evaluation using late gadolinium enhancement imaging with long inversion time has also been explored and may be useful prior to catheter ablation.¹² However, the accuracy of CMR for thrombus detection has not been compared against transesophageal echocardiography in a systematic fashion.

Electrophysiologists are occasionally called upon to review cases of inflammatory myocarditis presenting with arrhythmia. Visualization of active inflammation may change management and can be performed if CMR with T₂-weighted imaging is specifically requested.¹³ T₂ imaging takes advantage of the longer time required for hydrogen nuclei in water molecules to reach equilibrium out of phase with each other after the applied excitation pulse. T₂-weighted imaging is typically performed using spin echo sequences with various inversion recovery pulses optimized for blood and fat suppression, which improve the visualization of edematous myocardium (Figure 4).¹⁴

Perhaps most important for the electrophysiologist, magnetic resonance angiography (MRA) protocols are necessary for anatomic mapping of end diastolic dimensions of cardiac chambers and vascular structures of interest. MRA images are particularly useful for atrial fibrillation ablation, where knowledge of pulmonary vein anatomy can help avoid complications due to inadvertent ablation too far inside the vein or at the ostium of smaller variant branches.¹⁵ Additionally, by providing the location of the left atrial appendage, image integration may reduce the potential of appendage perforation.¹⁶ The images are typically acquired using a breath-hold three-dimensional gradient echo MRA which uses the high T₁ signal of gadolinium contrast to optimize the signal of blood pool within vessels and cardiac chambers. When MRA is requested, the chambers of interest should be specified to ensure that the field of view and scan timing relative to contrast administration are appropriate. Multiphase time-resolved sequences are also available which utilize short acquisition times and dynamic imaging during contrast administration that eliminate errors related to bolus timing, albeit with lower spatial resolution.¹⁷

CMR in the setting of implanted cardiac devices

Owing to underlying structural heart disease and accompanied conduction system disease and/or risk of ventricular arrhythmia, a significant proportion of patients referred for electrophysiology procedures have permanent pacemakers and implantable defibrillators. Pacemaker and defibrillator systems may be subject to force and torque,¹⁸ heating, current induction,¹⁹ inappropriate pacing, shocks, and/or inhibition of therapies in the CMR environment.²⁰ Owing to these potential risks, CMR is not routinely performed in cardiac device recipients.^21–26 However, several studies have reported the safety of MRI in the setting of pacemakers and defibrillators.^4,27–33 Overall safety has been reported, but it is important to realize that acute changes in battery voltage, lead thresholds,³⁰ and programming³³ can be seen. A recent study has also investigated the safety of MRI in the setting of an MRI conditional pacemaker.³⁴

The safety protocol followed at our institution is based upon selection of device generator platforms previously tested under worst-case scenario (prolonged imaging over the region containing the generator, and SAR up to 3.5 W/kg) magnetic resonance conditions.^4,35,36 We do not perform CMR on patients with implanted device systems prone to electromagnetic interference (generally, generators manufactured prior to the year 2000). We recommend conservative measures to exclude patients with leads that are prone to movement or spontaneous dislodgement, such as patients with less than 6 weeks' time since device implant or patients with no fixation (superior vena cava defibrillation coil) leads. We also recommend avoidance of CMR when device leads that are prone to heating such as non-transvenous epicardial and abandoned (capped) leads are present. To reduce the risk of inappropriate inhibition of pacing due to detection of radiofrequency pulses, we program the device to an asynchronous, dedicated pacing mode in pacemaker-dependent patients. Also, given the lack of asynchronous pacing programming capability and transient loss of pacing capture after worst-case scenario (SAR 3.5 W/kg for 3 h) in vivo testing of 1 of 15 animals implanted with a defibrillator,³⁵ we do not perform CMR on pacemaker-dependent patients with defibrillators. To avoid inappropriate activation of pacing due to tracking of radiofrequency pulses, we program devices in patients without pacemaker dependence to a non-tracking ventricular or dual-chamber-inhibited pacing mode. We also recommend deactivation of rate response and tracking features (such as premature ventricular contraction response, ventricular sense response, and conducted atrial fibrillation response) to ensure that sensing of vibrations or radiofrequency pulses does not lead to unwarranted pacing. Additionally, the magnet mode should be deactivated when possible. Tachyarrhythmia monitoring and therapies should be disabled to avoid unwarranted antitachycardia pacing or shocks. Finally, to reduce the risk of thermal injury and changes in lead threshold and impedance, the estimated whole body averaged SAR of CMR sequences should be minimized (less than 2.0 W/kg) when possible. Blood pressure, ECG, pulse oximetry, and symptoms should be monitored for the duration of the examination. We require the presence of a cardiac electrophysiologist, or advanced cardiac life support-trained individual, familiar with device programming and troubleshooting during all scans.^4,36 At the end of the examination all device parameters are checked and programming is restored to pre-CMR settings.

Image artifacts

The presence of ferromagnetic materials such as pacemaker or defibrillator systems, sternal wires, and/or mechanical valve components causes variations in the surrounding magnetic field, resulting in image distortion, signal voids or bright areas, and poor fat suppression. Susceptibility artifacts are most pronounced on balanced steady state free precession gradient echo sequences. Importantly, artifacts on inversion recovery sequences may show high signal intensity and can mimic areas of delayed enhancement, which would otherwise indicate myocardial scar.⁴ Correlation of suspected areas of scar across different pulse sequences and imaging planes can help avoid misidentification of artifact as scar.

Using spin echo and fast spin echo sequences for morphology, shortening the echo time, using spoiled gradient echo rather than balanced steady state free precession, and using imaging planes perpendicular to the plane of the largest dimension of the metallic implant appears to reduce the qualitative extent of artifact.⁴

Conclusion

When used as an adjunct to local electrograms, CMR images regarding chamber and vascular boundaries, and scar and border zone locations can provide powerful information to aid catheter ablation procedures. Offline analysis of images can provide accurate measures of cardiac function, structural variations, and myocardial viability which can alter strategies even before the case starts. Software upgrades to enable integration of three-dimensional scar images into the electroanatomic system are under development and will likely reduce procedural time devoted to voltage mapping. Basic familiarity with the acquisition, interpretation, and unique advantages of various CMR pulse sequences can greatly enhance patient management in the modern electrophysiology laboratory.

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