This article appears in the following Europace issue: Spotlight Issue: Cardiac Imaging in EP and CRT [View the issue table of contents]
IMAGING IN CATHETER ABLATION FOR AF
Image integration in catheter ablation of atrial fibrillation
1 Division of Cardiology, Johns Hopkins Medical Institutions, Baltimore, MD, USA; 2 Department of Cardiology, Leiden University Medical Center, Leiden, The Netherlands
* Corresponding author. E-mail address: l.f.tops{at}lumc.nl
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Abstract |
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 Top Abstract IntroductionImage integrationImage integration processesFactors affecting image...Impact of image integration...Summary and conclusionsFundingReferences |
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Over the past few years, integration of different imaging modalities to guide catheter ablation procedures for atrial fibrillation has become possible. Various strategies are nowadays available that allow integration of the anatomical information provided by fluoroscopy, computed tomography, magnetic resonance imaging, or intracardiac echocardiography with the information provided by electroanatomic mapping. This review discusses the different image integration techniques, and an overview of the clinical experience with these systems will be provided. In addition, factors that may affect the accuracy of the image integration process will be addressed. Finally, the effect of image integration on procedural characteristics and outcome will be reviewed.
Key Words: Atrial fibrillation, Catheter ablation, Mapping, Computed Tomography, Magnetic Resonance Imaging
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Introduction |
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TopAbstractIntroductionImage integrationImage integration processesFactors affecting image...Impact of image integration...Summary and conclusionsFundingReferences |
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At present catheter ablation is considered a reasonable option in the treatment of atrial fibrillation (AF), when anti-arrhythmic drug therapy has failed.1
The cornerstone for most AF ablation procedures is electrical isolation of the pulmonary veins (PVs).2 However, the anatomy of the PVs and the left atrium (LA) varies significantly. Therefore, accurate visualization of these structures during the catheter ablation procedure is of critical importance.3
In addition to conventional fluoroscopy, several dedicated modalities including electroanatomic mapping,4 computed tomography (CT)5 and magnetic resonance imaging (MRI)6 are available for visualization of the LA and PVs. Recently, integration of the various modalities has become possible. In this article, an overview of the various image integration techniques will be provided. In addition, the clinical experience with these systems will be discussed.
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Image integration |
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TopAbstractIntroductionImage integrationImage integration processesFactors affecting image...Impact of image integration...Summary and conclusionsFundingReferences |
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The rationale of image integration is the process of combining various imaging techniques, in order to overcome the limitations of each technique. For example, although conventional electroanatomic mapping provides important real-time electrophysiological information, it uses reconstructed anatomy of the LA and PVs. In contrast, CT can provide highly detailed information on complex anatomical structures, but lacks electrophysiological information. By combining the two imaging modalities, accurate, real-time, anatomical, and electrophysiological information can be used during the catheter ablation procedure. This may greatly facilitate catheter navigation and may increase the safety and outcome of the ablation procedure. An overview of the currently available strategies and systems for image integration is provided in Table 1, and will be discussed in the following paragraphs.
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Electroanatomic mapping and computed tomography/magnetic resonance imaging
Several conventional electroanatomic mapping systems are currently available,7
and some of them allow the integration of CT or MRI images. Among them, the CartoMergeTM (Biosense Webster, Diamond Bar, CA, USA) and the NavX FusionTM (St Jude Medical, St Paul, MN, USA) systems have been used in catheter ablation procedures for AF.
In the first pre-clinical feasibility study of the CartoMergeTM system, Dong et al.8 used CT markers attached to the epicardium of the cardiac chambers in nine mongrel dogs. After CT scanning, the segmented images were aligned with the reconstructed electroanatomic maps. The accuracy of the system was tested by aiming catheter ablation lesions at the CT markers, while using the registered CT images only. Mean position error of the ablation lesions for the LA was 1.8 ± 1.0 mm (range 0–4.0 mm) at autopsy.8 At present, a large number of clinical studies have reported the feasibility and accuracy of this system in patients undergoing catheter ablation of AF.9–20 Among the various image integration systems, most clinical experience has been reported with the CartoMergeTM system. Therefore, this technique will be discussed more extensively in the next section.
In addition to the CartoMergeTM system, the first experience with NavX FusionTM has been reported recently.21 The mapping component of this system uses voltage gradients generated by external electrical fields to spatially orient and localize the catheter tip. With new dedicated software, CT or MRI images can be imported and used during the actual procedure.22 The image integration process consists of several steps, including ‘field scaling’ of the reconstructed geometry, fusion of the structures using fiducial markers (landmarks), and optimization of the integration by adjusting (moulding and bending) the reconstructed geometry. Brooks et al.21 studied 55 patients undergoing catheter ablation for AF using NavX FusionTM. The authors used three fiducial markers for the initial registration process (most frequently PV ostia), and a mean of 44 ± 19 mapping points to ultimately fuse the geometry and the CT image. After optimization of the integration process, the mean distance between the reconstructed atrial geometry and the imported CT scan was 1.9 ± 0.4 mm. Interestingly, the mean fluoroscopy time normalized for procedural duration was significantly lower in the last 15 patients of the cohort, when compared with the first 15 patients (0.34 ± 0.05 vs. 0.43 ± 0.11, P = 0.009).21
Electroanatomic mapping and intracardiac echocardiography
Recently, the feasibility of the integration of electroanatomic mapping and intracardiac echocardiography has been demonstrated.23 The newly developed CartoSoundTM system (Biosense Webster) is equipped with an intracardiac echocardiography probe with a location sensor tracked by the mapping system. By using this dedicated echocardiography probe, endocardial contours of the LA and PVs can be acquired while the catheter is still in the right atrium (Figure 1). Khaykin et al.24 used this new technique in 15 patients undergoing catheter ablation for AF. Without the use of fluoroscopy, the authors created three-dimensional maps of the LA using a mean of 42 contours (range 20–93). It was concluded that this new technique allows creation of a three-dimensional reconstruction of the LA using intracardiac echocardiography, without entering the actual chamber of interest. The advantage of this new technique is that it combines accurate real-time anatomical information with elecotroanatomic data. However, more data is needed on the feasibility and accuracy of this new technique.
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Fluoroscopy and computed tomography/magnetic resonance imaging
The potential advantage of this strategy is the combination of the actual, real-time fluoroscopic images with the highly detailed images from CT or MRI. The feasibility of the integration of bi-plane fluoroscopy with MRI25
and the integration of single-plane fluoroscopy with CT26,27 has been reported.
Ector et al.25 used conventional fluoroscopy with angiographic images of the right atrium and reconstructed MRI images to guide ablation of right-sided arrhythmias. For the MRI images, a three-dimensional volume of the right atrium was rendered using manual contouring. Newly developed custom software was used to integrate the two modalities based on fluoroscopy geometry calibration and image registration (translation+rotation using two projections). In nine patients, the median distance between the angiographic image and the three-dimensional MRI image ranged from 1.9 mm (posterior) to 2.5 mm (anterior).25
Similarly, the integration of single-plane fluoroscopy and CT has been demonstrated by Sra et al.26 In 20 patients undergoing catheter ablation for AF, registration of the fluoroscopic images and volume-rendered CT images of the LA and PVs was performed. Registration was accomplished by superimposing a coronary sinus catheter on the fluoroscopic image and the coronary sinus from the CT scan. Registration accuracy was confirmed by intracardiac recordings from a multi-electrode basket catheter and PV and coronary sinus venography. Registration of both the techniques was performed successfully in all patients and mean registration error was 1.4 mm (range 0.9–2.3 mm).26 In both studies, custom-made software was used to perform image integration. At present, no studies have reported the use of a commercially available system that allows integration of fluoroscopy and CT or MRI.
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Image integration processes |
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TopAbstractIntroductionImage integrationImage integration processesFactors affecting image...Impact of image integration...Summary and conclusionsFundingReferences |
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As mentioned previously, most studies on image integration have reported results on the integration of CT or MRI with electroanatomic mapping using the CartoMergeTM system. Therefore, this system and the processes needed to perform image integration will be discussed in more depth in the subsequent paragraphs.
Image acquisition
Image acquisition using CT or MRI is the first process needed for image integration. In general, both techniques provide good quality images of the LA and the PVs for image integration. Although the exact scanning protocol may vary among institutions and scanner types, several general issues should be considered. The use of a contrast agent is recommended during scanning. When accurately timed, the contrast bolus enhances the LA and the PVs, thereby facilitating the discrimination of the blood pool and the endocardial border during the ‘segmentation’ process. In addition, a saline chaser bolus following the contrast bolus may facilitate the separation of the pulmonary arteries and the superior PVs.5
Furthermore, the use of ECG gating allows the reconstruction of the LA and PVs images at any time-point during the cardiac cycle, and should therefore be used. However, AF at the time of scanning may hamper ECG-gated data acquisition significantly. New techniques and scanners that allow faster data acquisition may overcome this problem.28 In addition, adjustments in the reconstruction phase (e.g. 50% of the cardiac cycle) may still result in adequate images during AF.17
Segmentation
The second process is ‘segmentation’ of the acquired images. This involves dividing the images into different regions to ultimately select the structures of interest (the LA and PV in case of AF ablation). The segmentation process is performed before the actual catheter ablation procedure, and involves multiple steps (Figure 2). First, the raw CT or MRI data are loaded into the electroanatomic mapping system. Typically, a transverse slice at the level of the chamber of interest is selected for accurate setting of the intensity threshold. By manually adjusting this intensity threshold, the borders of the different structures can be delineated. The presence of contrast in the LA allows easy differentiation between the endocardium (low intensity level) and the blood pool (high intensity level). Subsequently, a three-dimensional volume is created by labelling all volume units of the raw CT or MRI data within the set threshold intensity range. To segment this volume into different structures, specific labels or ‘seeds’ are then placed in the middle of the different areas of the volume (‘region identification’). Subsequently, an algorithm that automatically depicts the different structures of interest based on the placement of the ‘seeds’ and the borders is implemented. Finally, the result of the segmentation process can be verified on sagittal slices and the three-dimensional reconstructions. When an adequate segmentation of the CT or MRI images is achieved, the surface images are stored in the CartoMergeTM system and can be used during the actual ablation procedure.
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Registration
The final process in image integration is ‘registration’ or alignment of the electroanatomic map and the CT or MRI image (Figure 3). This process is performed during the mapping and ablation procedure. Although various registration strategies exist, the main processes are ‘landmark registration’ and ‘surface registration’.
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‘Landmark registration’ involves assignment of a ‘landmark’ to a distinct anatomical structure (e.g. the veno-atrial junction or the LA appendage) on both the electroanatomic map and the CT or MRI image. These landmark pairs can then be used to align the two structures. By using a minimum of three landmark pairs, appropriate registration along the X-, Y-, and Z-axes can be acquired. It has been suggested that landmarks on the posterior aspect of the PV–LA junction yield the best registration accuracy.16
‘Surface registration’ involves the alignment of the whole electroanatomic map and the CT or MRI image. For this purpose, a specific algorithm is implemented that composes the best fit between the two structures by minimizing the distance between all mapping points and the CT or MRI image. The image integration software provides the mean distance between the mapping points and the nearest points on the CT or MRI image, and the minimum and maximum distances. By reviewing the electroanatomic map and the distance between all mapping points and the CT or MRI image, the accuracy of the registration process can be assessed (Figure 4).
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Various registration strategies can be applied to complete the image integration process. Similar to conventional mapping alone, this largely depends on the operator's preference. In some studies only landmark registration was used,16
whereas in others the combination of landmark and surface registration was applied.10 In addition, intracardiac echocardiography can be used to guide the image integration process.9 After reassurance of accurate registration, the actual ablation is performed (Figure 5). Theoretically, the registration strategy (among other factors) may influence the accuracy of the image integration process. In the following paragraphs, the issues that may affect the registration accuracy are discussed.
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Factors affecting image integration accuracy |
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TopAbstractIntroductionImage integrationImage integration processesFactors affecting image...Impact of image integration...Summary and conclusionsFundingReferences |
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In addition to the pre-clinical validation study,8
a number of clinical studies have reported the feasibility and accuracy of the CartoMergeTM system to guide catheter ablation for AF. An overview of the various studies is provided in Table 2. In each study, the mean distance between the mapping points of the electroanatomic map and the CT or MRI image as provided by the CartoMergeTM system, was used as a measure of image integration accuracy. In general, the mean registration error is approximately 2 mm. Importantly, it should be noted that the accuracy of the image integration process depends on all three processes involved: image acquisition, segmentation, and registration. The various factors that could potentially affect the accuracy of image integration will be discussed in the following paragraphs.
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Image acquisition
Theoretically, the spatial resolution of CT is better than MRI and may therefore result in improved registration accuracy. However, in a series of 16 patients (CT n = 8; MRI n = 8), Dong et al.11
found no significant differences in surface-to-point distance between patients with CT imaging and those with MRI imaging (2.96 ± 0.39 vs. 3.14 ± 0.45 mm, P = NS). Similarly, other studies have found no differences between the two imaging modalities based on registration accuracy.14
In addition, differences in breathing during image acquisition and the actual catheter ablation procedure may affect the image integration process. In general, scanning is performed during an inspiratory or expiratory breath-hold, whereas patients are breathing normally during the ablation procedure. It has been noted that LA and PV anatomy changes significantly during respiration,29 which may result in registration errors. Malchano et al.30 demonstrated that an end-expiratory breath-hold during CT scanning best corresponded with the state of ‘quiet respiration’ during the catheter ablation procedure. Importantly, an end-expiratory breath-hold during image acquisition yielded the smallest registration error, when compared with an end-inspiratory breath-hold (end-expiratory 4.7 ± 0.9 vs. end-inspiratory 12.3 ± 11.1 mm).30
Furthermore, the time between image acquisition and the actual ablation procedure may influence image integration accuracy. Typically, the CT or MRI scan is performed 1 or 2 days before the catheter ablation procedure.9,20 Although in a series of 61 patients, no association was found between registration error and the days from image acquisition to ablation,14 it is recommended to keep this interval as short as possible to avoid significant changes in LA volume.31 Indeed, it has been demonstrated that a larger LA volume is associated with a larger registration error.15 Therefore, differences in LA volume between scanning and the ablation procedure should be kept to a minimum.
The actual heart rhythm during image acquisition and any changes in heart rhythm between image acquisition and ablation procedure or even during the actual ablation procedure, all could potentially affect registration accuracy. Changes in heart rhythm may result in (transient) changes in LA volume and anatomy, and subsequently may affect the image integration process. Martinek et al.15 studied 40 patients undergoing catheter ablation for AF using CartoMergeTM. All patients underwent 16-slice CT scanning the day before the catheter ablation procedure. In 21 patients sinus rhythm was present at the time of the CT scan, whereas 19 patients had AF. No significant differences in mean registration error were noted between the two patient groups (sinus rhythm 1.65 ± 1.27 vs. AF 1.54 ± 1.15 mm).15 Importantly, in another study it was noted that a change in heart rhythm during the actual catheter ablation procedure (e.g. after termination of AF) does not significantly influence the overall registration error.17
From all the potential factors during image acquisition, it appears that the breathing pattern and changes in LA volume appear to be the most important. It should be noted however, that in the majority of the abovementioned studies, small sub-group analyses were performed. Larger studies are therefore needed to confirm these findings.
Segmentation
The segmentation process involves several steps, and is largely automatically performed by dedicated software. Therefore, at present no studies have reported the potential influence of the segmentation process on overall image integration accuracy.
Registration
As previously discussed, different registration strategies (using landmark registration, surface registration, or a combination) exist. However, only few studies have investigated the effect of the various strategies on the actual image integration accuracy. Fahmy et al.16 studied 124 patients in three different centres using a standardized image integration protocol. First, only landmark registration using specific areas (including the posterior aspect of the PVs, LA appendage, and the LA anterior wall) was performed. Next, surface registration was performed with a minimum of 40 mapping points. The accuracy of the image integration was assessed by reviewing the mean registration error and the distance between the landmark points as provided by the CartoMergeTM software, and by checking the alignment of specific points on the PV ostia using intra-cardiac echocardiography. After landmark registration alone, the mean landmark error was 5.6 ± 3.2 mm and the mean distance between the specific PV ostia points was 5.2 ± 2.7 mm. However, after surface registration, the mean landmark error increased from 5.6 ± 3.2 to 9.1 ± 2.1 mm (P < 0.01). In addition, the distance between the specific PV ostia points increased to 9.6 ± 2.5 mm. However, the mean distance between all the mapping points and the CT image (mean surface registration error) was only 2.17 ± 1.65 mm. It was concluded that surface registration resulted in shifting of the landmark points, which may ultimately change the accurate image integration.16
In contrast to these results, Dong et al.8 reported an increased accuracy while using surface registration. In a smaller study population (16 patients), the authors used both landmark and surface registration strategies. It was noted that after landmark registration alone, a good alignment between the reconstructed PVs and the CT image was achieved in 26 of 60 PVs (43%), whereas a combination of landmark and surface registration significantly improved the percentage of PVs with a good alignment of 95% (57 of 60 PVs, P < 0.001). In addition, the registration error was reduced using a combination of surface registration and landmark registration when compared with landmark registration alone (3.05 ± 0.41 vs. 3.57 ± 0.98 mm, P = 0.08).8
Of note, during the actual ablation procedure, the accuracy of the registration process may be influenced by the acquired ‘ablation points’ on the electroanatomic map. It has been demonstrated that the mean registration error was significantly different before and after completion of the ablation procedure.15 Therefore, while performing the ablation, the accuracy of the registration should be closely monitored.
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Impact of image integration on procedure time and outcome |
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TopAbstractIntroductionImage integrationImage integration processesFactors affecting image...Impact of image integration...Summary and conclusionsFundingReferences |
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The rationale of image integration is to facilitate catheter ablation procedures by displaying detailed anatomical information in combination with electroanatomic data. Intuitively, the use of image integration decreases procedural and fluoroscopy time and improves the outcome of the procedure. However, at present only a few studies are available that tested this hypothesis.
In a non-randomized study, Kistler et al.13 included a total of 94 patients, using conventional mapping alone (n = 47), or with the use of CartoMergeTM (n = 47). All patients underwent wide encirclement of the ipsilateral PV pairs using irrigated radiofrequency ablation with the endpoint of electrical isolation. A significant reduction in fluoroscopy times was noted in the image integration group (49 ± 27 vs. 62 ± 26 min, P < 0.05). Importantly, the number of patients with maintenance of sinus rhythm without anti-arrhythmic medication after a mean of 25 ± 5 weeks was significantly higher in patients treated using image integration, when compared to those treated with conventional electroanatomic mapping alone (83 vs. 60%, P < 0.05).13
In another study, 47 patients treated using CartoMergeTM were compared with 53 patients using the conventional electroanatomic mapping system.20 All patients underwent 16-slice CT; in the CartoMergeTM group, the CT images were used to guide the catheter ablation procedure. In both groups, a circumferential ablation around the PVs was performed and, if necessary, additional lesions were created. After 6 months follow-up, the overall success was significantly improved in the CartoMergeTM group, when compared with the conventional group (Figure 6). However, no significant changes in mean radiation time and procedure time between the two groups were observed.20
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Finally, a recent study randomized 50 patients between conventional mapping and fluoroscopy-CT image integration.27
In all patients, a 64-slice CT scan was performed and analyzed for LA and PV anatomy. In 25 patients, catheter ablation was guided using an integrated fluoroscopic-CT image, whereas in the remaining 25 patients, a conventional electroanatomic map was used. In both groups, linear lesions were created circumferentially around the PVs, at the LA roof and the mitral annulus. Mean procedure time was significantly reduced by using the fluoroscopy-CT image, when compared with the conventional mapping system (2:01 ± 0:29 vs. 2:51 ± 0:40 h, P < 0.05). In addition, mean fluoroscopy time decreased significantly from 78 ± 16 to 59 ± 11 min (P < 0.05) by using image integration. Unfortunately, the study was not powered to detect significant differences in outcome although a larger proportion of patients in the image integration group remained free from AF during follow-up (84% in the image integration group vs. 64% in the conventional group).27 From these studies, it has become apparent that the use of image integration may reduce procedure and fluoroscopy times and may improve the outcome of catheter ablation procedures. However, larger randomized studies are needed to confirm these findings.
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Summary and conclusions |
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TopAbstractIntroductionImage integrationImage integration processesFactors affecting image...Impact of image integration...Summary and conclusionsFundingReferences |
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At present, several strategies are available for image integration to guide catheter ablation for AF. Among these, the most clinical experience has been reported with the integration of electroanatomic mapping and CT or MRI. The accuracy of the image integration process may be influenced by several factors, including breathing pattern, changes in LA volumes, and registration strategy. Importantly, some small observational studies have demonstrated a reduced fluoroscopy and procedure time and an improved outcome with the use of image integration. Larger, randomized studies are needed to fully appreciate the value of image integration in guiding catheter ablation for AF.
Conflict of interest: M.J.S. receives grants from Biotronik, Medtronic and Boston Scientific. J.J.B. receives grants from Medtronic, Boston Scientific, BMS medical imaging, St Jude Medical, GE Healthcare and Edwards Lifesciences. The remaining authors have no conflicts of interest to declare.
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Funding |
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TopAbstractIntroductionImage integrationImage integration processesFactors affecting image...Impact of image integration...Summary and conclusionsFundingReferences |
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The Leids Universiteits Fonds, Nederlandse Hartstichting, Studiefonds Ketel 1, Stichting De Drie Lichten (to L.F.T.).
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References |
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