MR Imaging: New Techniques for the Treatment of Atrial Fibrillation

Dr. Marrouche is the Director of the Cardiac Electrophysiology Laboratories and the Atrial Fibrillation Program at the University of Utah School of Medicine, which for more than 20 years has been a leader in developing innovative MRI applications. In this article, Dr. Marrouche describes his innovative ablation technique using MRI.

More than 3.5 million Americans have atrial fibrillation (AF), and its prevalence increases markedly with age in older adults, from less than 1 percent for those younger than age 60 to roughly 1 in every 10 persons aged 80 years or older.1 People with AF are up to seven times more likely to suffer a stroke than the general population. The Centers for Disease Control estimates that AF is a contributing cause for more than 66,000 deaths and is responsible for more than 1.7 million hospitalizations among persons in the Medicare population. The treatment of AF has been challenging, due in part to the irregular, generalized pervasiveness of electrical abnormalities throughout much of the left and right atria. Since the pulmonary veins have been identified as one of the primary sources of AF, a variety of treatment strategies have been developed to minimize their electrical effects. One method that has proven effective is radiofrequency (RF) ablation, which uses an RF-emitting electrode inserted in the heart through a catheter to destroy tissue responsible for the electrical effects. These procedures often utilize a mapping catheter under fluoroscopic guidance to electrically isolate the pulmonary veins from the rest of the left atrium. Despite years of investment and extensive research, however, the overall curative and complication rates for AF treatment has not changed since the turn of the millennium. In 2005, a worldwide survey of the outcomes of 8,745 ablation procedures demonstrated a 52% success rate (ranging from 14.5% to 76.5% at 777 centers), with an additional 23.9% of patients becoming asymptomatic with the addition of an antiarrhythmic medication. In 27.3% of patients, more than one procedure was required to attain these results, and 6% had at least one major complication.2 The causes of such results are multiple and varied, and include differences in technique, follow-up, definitions of success and in the experience and technical proficiency of the electrophysiologist. One promising approach for improving the efficacy and safety of RF ablation on a more consistent basis involves the use of magnetic resonance imaging (MRI), which gives electrophysiologists a more precise tool for understanding the physiology of the cardiac chambers and the effects of ablation. Of all the imaging modalities, MRI offers the most detailed anatomic and physiologic information about normal and damaged myocardial tissue. In comparison to fluoroscopy and CAT scans, MRI has vastly superior soft tissue contrast and does not expose the patient to ionizing radiation. Current practice and new research show that MRI has potential applicability for AF treatment across the spectrum of care, from patient screening and procedure planning to procedure navigation and post-procedural follow-up.

Patient Screening and Planning

The use of MRI in treating AF starts with an examination to determine if a patient is a good candidate for ablation. With special software, an MRI machine can provide a detailed view of the heart’s fibrotic tissue, which indicates the extent of its damage (Figure 1). If AF has damaged less than 50% of a patient's heart, ablation is usually recommended. But if an MRI exam shows that nearly 60% of heart tissue is fibrotic, the patient is not a good candidate for ablation (Figure 2). A critical component in preparing patients for a successful RF procedure is the identification of left atrial thrombi, which in current practice is done with a transesophageal echocardiogram (TEE). Although these diagnostic tests are highly accurate, some patients may be reluctant to have this procedure because they are semi-invasive. In addition, because of the complex morphology of the left atrial appendage, the estimation/localization of a thrombus may be difficult and, as a result, underestimated.3 In contrast, non-invasive cardiac MRI allows for evaluation of cardiac morphology without any assumptions in regard to cardiac geometry. By imaging the thrombus using multiple adjacent slices, it can provide a better estimation of its overall size and volume, which allows for aggressive anticoagulative therapy as needed. Patients also can take this diagnostic test simultaneously with other studies necessary for the planning of the RF ablation procedure. Another factor that makes MRI essential to procedural planning is the fact that RF ablation in the left atrium (LA) has the potential to damage adjacent structures with potentially deadly side effects. In patients with permanent atrial fibrillation, the left atrium often dilates, which may result in changes to the morphology of the structures to its posterior. In one study of 42 patients about to undergo catheter treatment in the LA for AF, the larger LA size resulted in the spine and aorta frequently impressing on the LA. The esophagus was also a persistent feature running directly posterior to the LA and contacting it in all of the patients imaged. These findings are important, as the topology of the posterior wall of the LA may be altered by the presence of the aorta or spine.4 Segmentation and volume rendering of structures and their relationship to the LA anatomy allows the physician performing the procedure to be careful when performing ablation in points of concern.

MRI Peri-Procedure Applications

MRI is also a crucial element in the treatment phase of an ablation procedure because it allows physicians to see the lesions that form on a patient's heart tissue and determine if the ablation is sufficiently deep. If physicians do not destroy all the fibrotic heart tissue, their patients may have to undergo a second procedure. However, if they ablate on a patient for too long, they run the risk of burning too far into the atrial wall and connecting the atrium to the esophagus, a rare but potentially fatal complication. One approach that has successfully addressed this issue is to have the MRI lab next to the EP lab where the ablation is done. After the ablation procedure, the patient is wheeled into the MRI lab for an exam. If the image shows more ablation is necessary, the patient is returned to the EP lab to have this done. An important component of successful pulmonary vein isolation involves delivering an effective amount of energy for an appropriate duration. Both the choice of catheter and the mode of energy titration during RF ablation are likely to influence an operator’s success in accomplishing permanent changes to the electrical signal propagation within the LA. Striking a balance between effective lesion formation in the LA and preventing damage to the adjacent structures can be difficult, and is particularly important in regards to the recognition of left atrial-esophageal fistula as a potentially fatal complication.5 In one recent study comparing catheter types and imaging modalities, esophageal wall changes (including edema/erythema and necrosis) were seen in 35.7% of the patients that were ablated using an open irrigation tip catheter and in 57.1% of patients where power delivery was monitored using ICE.6 The high rate of injury highlights the need to effectively titrate and monitor tissue response to RF energy. Utilizing techniques such as delayed enhancement, MRI scans will play an important role in helping to determine appropriate power delivery settings, which will result in effective lesion formation without causing unnecessary damage to other structures.

Post-Procedure Care and Follow-Up

Delayed enhancement cardiovascular magnetic resonance imaging (DE-CMRI) is an established clinical method for visualizing tissue necrosis in a variety of cardiac disease processes, including after myocardial infarction and injury due to myocarditis. Given concerns about energy delivery and potential injury to other structures, this method can be a powerful tool for investigation into the injury of tissues near the posterior LA wall. It also has applications in determining the health of atrial tissue and the extent of LA scarring following ablation (Figure 3). Although controlled lesion delivery and scar formation within the LA are indicators of procedural success in preventing the recurrence of AF, the assessment of these factors has been limited to invasive methods. Recent research has defined non-invasive MRI methods that allow for the detection and quantification of LA wall scarring related to RF energy delivery after ablation in patients with AF. Moreover, the study suggests the degree of LA wall injury predicts procedural outcome at three months and may be an important step in improving procedural success.7 DE-CMRI scans also may be used to analyze patients with post-procedural complications and determine how these might be related to initial their RF procedure, their inpatient hospital care, or other factors.

Potential for MRI-Guided Procedures

With its excellent soft tissue contrast and adequate spatial resolution, MRI can provide guidance for electrophysiology procedures without exposing patients to ionizing radiation. Research groups have reported success in using low-pass RF filters in guiding the RF ablation catheter to the atrial wall and the right ventricle apex, even during RF energy applications. The first cardiac ablation procedure with simultaneous MRI imaging is expected to be performed in early 2009. Visualization of RF ablation lesions within the LA using MRI has also recently been reported, and imaging modalities are being developed so that these scans may be run on clinical scanners.8,9 Using delayed enhancement pulse sequences and other novel imaging sequences, it may be possible to assess lesion size and change in tissue pathology in real time within a single display. Such advances are likely to greatly improve the success rate in complex patients.

Conclusion

MRI is an extremely important modality for the treatment of atrial fibrillation. It currently has important implications in preparing patients for the initial treatment by helping to localize left atrial thrombus and defining the anatomy of the pulmonary veins, the left atria, and surrounding structures. During the ablation procedure, MR angiograms help to improve anatomical maps, which are used in conjunction with other imaging modalities to determine appropriate sites for ablation. In post-procedural assessment and follow-up, the use of delayed enhancement MRI makes it possible to visualize the scar with clinical scanners, which will likely prove valuable in diagnosing post-procedural complications and determining how they may be related to RF parameters. As interventional MRI scanners become available, MRI will likely play a greater role in the treatment of AF patients, as well as contribute to improving the overall curative rate and decreasing the complication rate. In addition, as advanced techniques for scar imaging become available, MRI will be an indispensable tool within the catheter lab itself.