Atrial fibrillation (AF) is the most common form of cardiac arrhythmia and is associated with high rates of morbidity and mortality.1 AF can be paroxysmal or persistent, and remains asymptomatic in some cases. However, in more severe and persistent cases AF may cause congestive heart failure, palpitations, syncope, and chest pains. Additionally, patients with AF have a significantly higher risk for suffering from stroke.2,3
The management of AF is driven by three principal goals: rate control, prevention of thromboembolism, and restoration of normal heart rhythm. In terms of rhythm control, pharmacotherapy is the recommended first-line treatment, with atrial ablation as a second-line option, especially in patients with symptomatic lone AF.1 Alternative treatment may be necessary for patients with persistent, highly symptomatic AF. Pacemakers prevent slow heartbeats and reduce the likelihood of AF, although once it is inserted patients are dependent on the device. Cardioversion is used to shock the heart back into a normal sinus rhythm; however, in some cases this is not a permanent solution and the arrhythmia often returns. The use of catheter ablation in AF is now becoming an increasingly popular method to treat symptomatic and persistent disease, especially in patients with little or no left atrial enlargement.1
Catheter Ablation
Since the role of the pulmonary veins was identified in AF, several pulmonary-vein- based catheter systems have been developed and catheter ablation is now increasingly being utilized in patients with AF ablation procedures.4 Pulmonary vein isolation is the most common ablation strategy, although the isolation line is moving into the atrium and away from the pulmonary vein ostia. Additional ablation may be performed in some patients with symptomatic paroxysmal AF, and in most patients with persistent AF in the form of linear lesions joining the lesion sets encircling the pulmonary veins, focal ablation at fractionation sites, and isolation of the coronary sinus or left atrial appendage.5
The goal of catheter ablation is to eliminate the trigger and the substrate of the AF. Guidelines for AF have stated that catheter ablation is a reasonable alternative to pharmacological therapy to prevent recurrent AF in symptomatic patients with little or no left atrium enlargement.1 There is a general consensus that the primary indication for the use of catheter ablation is the presence of symptomatic AF or an intolerance to class I–III antiarrhythmogenic medication./p>
Successful ablation should produce myocardial lesions that block the propagation of the AF wave fronts from a rapidly firing triggering source or modify the arrhythmogenic substrate responsible for re-entry of the wave. In catheter ablation, radiofrequency energy is delivered through a transvenous electrode catheter that achieves ablation by conducting an alternating electrical current through targeted myocardial tissue./p>
The highly complex nature of the pulmonary veins and left atrium makes performing AF ablation with standard fluoroscopy alone difficult. Catheter ablation using standard fluoroscopy is successful in approximately 75% of cases and leads to pulmonary vein stenosis, tamponade, or stroke in 5% of patients.6 Therefore, there is need for a more precise method of imaging to increase the efficacy and reduce the risks of AF ablation procedures. The alignment of pre-procedural computed tomography (CT) and magnetic resonance imaging (MRI) images is a new focus in catheter ablation in AF procedures to improve the accuracy of imaging.
Image Integration
CT and MRI can produce detailed images of the atrial and pulmonary vein anatomy.7,8 The 2D CT/MRI slices of individual cardiac structures are rendered into 3D surface reconstructions, which are then used in image registration. This is achieved in a three-step process involving a series of algorithms. Image registration is then performed, which involves aligning the 3D surface reconstructions to realtime electroanatomical maps derived from catheter mapping./p>
Two computerized algorithms are used to superimpose pre-procedural CT/MRI images of the same time-point in the cardiac cycle onto the electroanatomical maps. These are referred to as landmark registration and surface registration. Landmark registration uses at least three landmark matching pairs created by the realtime catheter tip locations and the estimated locations on the 3D CT/MRI reconstructions. Surface registration uses an algorithm that aligns two sets of images by minimizing the average distance from multiple endocardial locations to the surface of 3D CT/MRI image reconstructions.CartoMerge™ (Biosense Webster)
The first device to be introduced for image integration in catheter ablation was the CartoXP™, which has since been updated to the CartoMerge™ device.9 CartoMerge registers the ablation catheter to 3D images of the left atrium and pulmonary veins of the heart, resulting in improved navigation of catheters to target ablation points.10 The 3D image is generated using a pre-procedural MRI or CT scan. The catheter is then localized using magnetic localization technology. The CartoMerge system has been associated with an increased efficacy of ablation and a reduction in procedure time.11/p>
CartoMerge was first examined for clinical efficacy and safety in nine dog models.12 The authors concluded that the study supported the clinical use of the image integration system in catheter ablation procedures either in the atria or in the ventricles. In addition, the CT image registration process presented a significant advantage over the less detailed reconstructed 3D maps and was suggested to be helpful for the ablation of AF./p>
The first human clinical study was performed in six patients with drug-refractory AF.13 A position error of 3mm was reported in this study using a combined registration strategy, where the pre-acquired MRI and CT images were registered to the 3D mapping space. Both landmark registration and surface registration were implemented in the study. The registration error after each landmark registration resulted in 60% pulmonary vein misalignment. Surface registration resulted in a significant improvement in pulmonary vein alignment and reduced the distance between 3D MRI/CT reconstruction and realtime mapping. The study demonstrated that image integration in ablation improves the success of ablation procedures and the combination of two registration techniques increases the accuracy of the image fusion./p>
A recent study compared the clinical outcomes of AF using the previous model (CartoXP) and the new CartoMerge system.14 In the study, 81 patients were randomly assigned to ablation with either the CartoXP or the CartoMerge system. The isolation of pulmonary veins was achieved in 95 and 92% of patients in the CartoMerge and CartoXP groups, respectively. In addition, AF was cured in 79 and 74% of cases treated with the CartoMerge and CartoXP, respectively. CartoMerge also reduced procedure time and hence X-ray exposure. However, CartoMerge did not appear to have any effect on the outcome of the ablation procedure, although this may have been due to all of the procedures being carried out by experienced operators.
Computed Tomography–Fluoroscopy Guidance Systems
CT–fluoroscopy guidance systems have now been introduced that provide an automatically segregated image combining pre-procedural CT images with realtime fluoroscopy mapping. The use of these systems during an ablation procedure may offer advantages over the electroanatomical CartoMerge system. For example, through registration of detailed CT of the left atrium with the realtime fluoroscopy image, tissue delineation is included in the fluoroscopic view and the course and position of the ablation catheter are depicted. This may result in improvements in the manipulation of the catheter, which leads to shorter procedure times. Unlike the tracking catheter used in the electro-physiological catheter, the coronary sinus was used as a landmark, allowing registration to be performed automatically and more quickly. In addition, the availability of realtime background fluoroscopy allows as many validations of the registration process as required, and also allows for patient movement during the procedure. Using this software, registration can also be achieved in different views by automatically adjusting the CT image to the fluoroscopy orientation./p>
A CT–fluoroscopic guidance system was evaluated in a controlled study including 50 patients with symptomatic AF.15 Patients were randomized to undergo catheter ablation with or without the CT–fluoroscopy guidance system. In the CT–fluoroscopy arm there were shorter procedure times and a trend toward a higher success rate for AF ablation. However, this software could not record ablation points on the CT scan, so the CartoMerge system of recording the delivered ablation lesions was used. Work is ongoing to develop a way of marking lesion locations on the fluoroscopy guidance system. Further larger-scale studies are required to assess the impact of this system on the outcome of AF ablation procedures.
EP Navigator (Philips Healthcare)
The EP Navigator is a recently introduced imaging tool to aid the treatment of complex cardiac rhythm disorders. The EP Navigator enables electrophysiologists to segment pre-interventional 3D CT images of a patient’s cardiac anatomy and overlay and register these 3D images with live X-ray fluoroscopy catheter position information (see Figure 1). The resulting single image supports catheter/device navigation during specified procedures. This helps physicians to navigate through the heart during complex procedures.
The EP Navigator instantly indicates the position of any catheter, including the ablation catheter, with respect to complex 3D cardiac anatomy (see Figure 2). The design of the EP Navigator is intended to allow users to reach ablation points and perform complex procedures when there is no access to mapping./p>
Each patient undergoes a pre-procedural multislice CT scan, which is then transferred to the EP Navigator. Data are then selected by the operator for 3D reconstruction and a segmented volume of the desired cardiac structures is created. X-ray images are acquired at multiple angles; the 3D volumes are overlaid onto the X-ray images, and the user manipulates the 3D volumes to optimize registration of both images. In realtime the registered image shows the position of the catheter in relation to the 3D cardiac anatomy. To optimize the visualization of the anatomy, the composite 3D image will automatically rotate to follow the movement of the X-ray system in order to track the catheter. The German Heart Institute in Berlin has been using the EP Navigator system for over a year and has reported extremely pleasing results on its use in catheter ablation procedures.
Limitations to Image Integration
There are limitations to the use of image integration in AF ablation procedures. To achieve high-resolution 3D images of the left cardiac geometry using conventional mapping systems, the catheter has to be sequentially placed at many different sites in the left atrium; this can be very time-consuming and does not always show all anatomical details of the left atrium. CT and MRI images take time to produce, increasing both the administrative burden of scheduling and the financial burden. Radiation exposure is also an important issue during the imaging procedure./p>
In addition, static images of the registered MRI/CT reconstructions are in general taken at least one day before the ablation procedure, and in some cases weeks before; therefore, physiological differences in volume status, heart rhythm and rate, and respirations are inevitable. As a result, fusion of CT/MRI images with fluoroscopy can give a false impression of the real anatomy during the procedure. This may be misleading and in most cases a perfect match is not possible, hence before ablating a site operators should always question the possibility that the anatomical information they are viewing is not entirely accurate.Rotational Angiography
Rotational angiography is an imaging method involving rotating a fluoroscopy system around an anatomy, taking angiographic images at different angles in rapid sequence. This technique can produce high-resolution images of the cardiac anatomy, such as the left atrium and pulmonary veins, which are clearly defined in both left and right atrial orientations in the majority of patients. The German Heart Institute in Berlin has used rotational angiography in over 100 patients and all ancillary pulmonary veins were clearly identified. This technique does not involve separate registration of images. The acquired 3D rotational scan automatically corresponds directly to the anatomy of the patient at the time of the procedure. Rotational angiography is simple to perform and is time-saving since images are acquired in only five to seven minutes. The technique also has lower radiation exposure than conventional CT scans and negligible costs, and no safety issues have arisen to date. Therefore, it has potential for use in the future for AF ablation procedures.
Summary
AF is the most common arrhythmia and is associated with high rates of morbidity and mortality. AF ablation procedures are necessary for patients with highly symptomatic or persistent AF. In ablation procedures 3D electroanatomical mapping systems are useful for guidance. However, chamber reconstruction with these guidance systems is not able to replicate the highly complex anatomy of the heart and accurately represent the variable pulmonary vein and left atrium anatomy. Therefore, the alignment of pre-procedural CT and MRI images is a new focus in catheter ablation in AF procedures and may enhance the efficacy and safety of the procedure. CartoMerge is currently the most widely used image integration system that registers pre-procedural MRI and CT images of the cardiac anatomy with realtime maps. CT–fluoroscopy guidance systems such as the EP Navigator may offer advantages in terms of ease of use during the procedure. Both of these systems have enhanced the efficacy and safety of AF ablation, although they have limitations such as exposure to radiation, expense, and procedure length. Rotational angiography may offer an alternative imaging system to pre-procedural MRI or CT. Direct comparisons between the two devices have not been completed and will be necessary to compare the clinical benefits of each.