Transvenous catheter ablation has become a curative treatment for many arrhythmias. Knowing the precise catheter location in relation to true cardiac anatomy will benefit catheter ablation procedures. A novel electroanatomic mapping system (CartoMerge™, Biosense Webster) with the capability of integrating pre-procedural computed tomography (CT) or magnetic resonance (MR) images with electroanatomic maps has been recently approved for patient care. The employment of this system will improve catheter navigation and therefore may facilitate many catheter ablation procedures.
Clinical Need for Imaging-guided Intervention
Catheter navigation is traditionally guided by fluoroscopy, which provides only limited information about catheter location in relation to cardiac anatomy. Over the past decade, novel ablation strategies mostly based on anatomic considerations have been developed for the treatment of complex arrhythmias, such as atrial fibrillation and non-idiopathic ventricular tachycardia. In order to facilitate these procedures, various three-dimensional (3-D) mapping systems have been developed that enable the realtime display of the ablation catheter in relation to the 3-D maps reconstructed from multiple endocardial locations. However, these mathematically reconstructed 3-D maps cannot replicate the true cardiac anatomy as displayed with CT or MR imaging. Therefore, a clinical need exists for a mapping system that is capable of realtime visualization of ablation catheter location in relation to the true cardiac anatomy provided by CT or MR imaging.
Rationale of Image Integration
Image integration refers to the process of aligning the pre-procedural cardiac CT/MR images with the realtime 3-D maps reconstructed from multiple endocardial locations. The new system is able to track and display the realtime catheter tip location and orientation relative to a reference catheter at a given time-point in the cardiac cycle. Thus, the 3-D electroanatomic maps reconstructed from multiple endocardial locations sampled by catheter navigation represent a snapshot of the mapped cardiac chambers at that time.The impact of respiratory motion or patient movement on the catheter tip location in relation to the 3-D electroanatomic maps is minimized by the reference catheter, which is placed on the patient's back.This results in realtime display of the catheter tip in relation to the static 3-D electroanatomic maps. The system uses computerized algorithms to superimpose the pre-procedural cardiac CT/MR images onto the electroanatomic maps at the same point in the cardiac cycle and, therefore, enables the dynamic display of the catheter tip location on the true cardiac anatomy.
Steps of the Image Integration Process
The process of image integration consists of three steps: pre-procedural CT/MR imaging, image segmentation and extraction, and image registration.
CT/MR Imaging
To control the interval change, we recommend that patients undergo CT/MR imaging within 24 hours before the ablation procedure. In order to obtain high resolution images (0.5–1mm slice thickness), contrast-enhanced CT scanning is performed during a single breath hold with the use of a 32-or 64-slice CT scanner. A simultaneous electrocardiogram (ECG) is recorded to retrospectively assign the source images to the respective phases of cardiac cycle. Axial images at the end diastole are reconstructed at 0.5-1mm interval and transferred to the system for image registration.
MR imaging (MRI) is an alternative image modality for the purpose of image integration. MR angiograms are obtained with a 1.5-T MRI system after intravenous gadolinium injection.With currently available MR scanners, non-ECG gated MR images with about 1-2mm spatial resolution can be obtained during a single breath hold. MRI is good for patients with renal dysfunction, which is a contraindication to contrast-enhanced CT imaging.
Image Segmentation
Image segmentation refers to the separation of the 3-D anatomy of individual cardiac structures from a 3-D volume rendered from the 2-D CT/MR axial slices; it is achieved in a three-step process. First, the volume of cardiac structures is extracted from the whole volume dataset using a computerized algorithm that differentiates the boundary between the blood pool (with contrast filling) and the endocardium (without contrast enhancement). Second, the volumes of individual cardiac structures are separated from each other with the use of another algorithm capable of detecting their boundaries. Finally, by using a third algorithm, the segmented volumes for individual cardiac structures are extracted as 3-D surface reconstructions for image registration (see Figure 1).
Image Registration
Image registration is a crucial part of the integration process and refers to superimposing the 3-D CT surface reconstructions onto the realtime electroanatomic maps derived from catheter mapping. Two computerized algorithms, which are referred to as landmark registration and surface registration, are used to accomplish the image registration process.
Landmark registration aligns the 3-D CT/MR image reconstructions with corresponding electroanatomic maps through matching at least three landmark pairs. The landmark pairs are created by realtime catheter tip locations and their estimated locations on the 3-D CT/MR image reconstructions. Surface registration is an algorithm that aligns the two sets of images by minimizing the average distance from multiple endocardial locations to the surface of 3-D CT/MR image reconstructions. Our experience shows that the two registration methods are complementary. Landmark registration approximates the 3-D CT/MR image reconstructions to the realtime mapping space. It serves as the basis on which surface registration improves the registration accuracy.
Registration Accuracy
Accurate registration is the prerequisite for image integration-guided catheter navigation and ablation. Our laboratory assessed the accuracy of the system in a dog model.1 CT markers were attached to the epicardial surface of each cardiac chamber. Detailed 3-D cardiac anatomy was reconstructed from contrast-enhanced CT images and registered to the electroanatomic maps of each cardiac chamber. Targeted ablations were performed at each of the CT markers, guided only by the reconstructed 3-D images. At autopsy, the position error, defined as the distance between the epicardial projection of the ablation lesion center and the center of the targeted CT marker, was 1.9 ± 0.9mm for the right atrium, 2.7 ± 1.2mm for the right ventricle, 1.8 ± 1.0mm for the left atrium, and 2.3 ± 1.1mm for the left ventricle.2 To evaluate the system's guidance for more complex clinical ablation strategies, ablations of the cavotricuspid isthmus, the fossa ovalis, and the pulmonary veins were performed, which resulted in position errors of 1.8 ± 1.5mm, 2.2 ± 1.3mm and 2.1 ± 1.2mm, respectively.2 These results highlight the accuracy of the image integration system in guiding anatomic-based ablations.
There are very limited data on the registration accuracy with this image integration system in humans. In the authors' experience, and the experience of others, pre-procedural 3-D CT/MRI of the left atrium with pulmonary veins can be successfully registered to the realtime mapping space during clinical atrial fibrillation ablation procedures. The registration error, defined as the distance between multiple realtime left atrial endocardial locations and the surface of the registered 3-D left atrium reconstruction, is about 2–3mm.3,4
Role of Image Integration Guidance in Catheter Ablation
The use of registered CT/MR images to guide catheter ablation presents a significant advantage over the less detailed surrogate geometry created by previously available 3-D mapping systems. As it provides detailed anatomic information on the catheter tip location in relation to the true cardiac anatomy, the image integration technique has the potential to affiliate many ablation procedures, especially those anatomic-based ablation strategies such as atrial fibrillation ablation, non-idiopathic ventricular tachycardia ablation, and ablation of intra-atrial re-entrant tachycardias following corrective surgery for congenital heart diseases.
Since its approval for clinical use, the system has been used for catheter ablation of atrial fibrillation.3-5 Initial experience showed that the registered CT/MRI left atrium reconstructions can provide accurate information on the catheter tip location in relation to the important left atrium structures, such as pulmonary vein ostium and left atrial appendage. It enables the deployment of radiofrequency applications to be tailored to the highly variable pulmonary vein and left atrium anatomy (see Figure 2). Further investigations are needed to determine whether the improvement of catheter navigation with the image integration technique will translate into better clinical outcomes and avoid major procedure-related complications such as pulmonary vein stenosis and tamponade.
Catheter ablation of non-idiopathic ventricular tachycardia and intra-atrial re-entrant tachycardias may also benefit from image integrated electroanatomic mapping, which helps to better define the substrates of ventricular tachycardia and the boundaries of re-entrant circuits in surgically corrected atrium (see Figure 3).
Future Directions
Image integration guided catheter ablation is a promising and growing field. It has broad applicability in the treatment of cardiac arrhythmias. However, the current image integration technique has several limitations. As the CT/MRI is performed prior to the ablation procedures, registration error can arise from interval changes in the heart size because of differences in rhythm, rate, contractility, or fluid status. In addition, the static images of the registered CT/MR image reconstructions give little information on true catheter-tissue contact. Investigations are under way to integrate realtime imaging techniques such as MRI and ultrasound with electroanatomic mapping, which provides realtime information on catheter-tissue contact and lesion formation.