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October 2008

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Applications of cardiac CT in electrophysiology interventions

Laurens F. Tops, MD, Joanne D. Schuijf, MSc, PhD, and Jeroen J. Bax, MD, PhD, FACC, FESC

In the past decade, important new techniques have entered the clinical arena of both cardiac imaging and cardiac electrophysiology. Recent technical improvements have expanded the role of multislice computed tomography (MSCT) in cardiac imaging.1 Due to its high temporal and spatial resolution, MSCT can provide detailed information on cardiac function and morphology, including coronary artery disease.

At the same time, new electrophysiological procedures have dramatically changed the treatment of patients with atrial fibrillation (AF) and patients with chronic heart failure. Catheter ablation procedures offer a potential curative treatment in patients with highly symptomatic drug-refractory AF.2 Cardiac resynchronization therapy (CRT) is now an established therapeutic option for patients with end-stage heart failure.3 For these procedures, detailed knowledge on cardiac anatomy is crucial.

Both the evaluation of pulmonary vein (PV) anatomy prior to invasive radiofrequency ablation for AF and noninvasive coronary vein mapping prior to placement of a CRT device were recently determined to be appropriate indications for cardiac CT.4 This article will focus on the assessment of PV anatomy for catheter ablation procedures for AF and the assessment of coronary venous anatomy for left ventricular (LV) lead positioning in CRT. In addition, new techniques that allow the integration of MSCT images with other imaging modalities will be discussed.

Pulmonary vein anatomy for AF ablation procedures

The PVs are the main source of triggers that initiate paroxysmal AF.5 Importantly, with the use of radiofrequency energy during catheter ablation procedures, these triggers can be eliminated.5 At present, catheter ablation is considered a reasonable option in patients with paroxysmal AF, when at least 1 antiarrhythmic drug has failed.2 Several ablation strategies that target electrical isolation of the PVs have been proposed. Focal ablation of PV triggers,6 segmental ablation of the PV ostium,7 and anatomical-based circumferential ablation8 have been proven to be effective in the treatment of AF.

Regardless of the ablation strategy that is applied, detailed knowledge of the anatomy of the left atrium and PVs is mandatory for catheter ablation procedures.9 Multislice CT allows visualization of these and other critical anatomic structures and, therefore, can provide a detailed “road map” for the ablation procedure.

Technical considerations

For imaging of the PVs, a contrast-enhanced helical scan is typically performed. Retrospective electrocardiographic (ECG)-gating can be used; however, a large proportion of patients referred for PV imaging may have AF during MSCT scanning, which limits proper image acquisition. In some studies no ECG-gating during scanning is used, which also lowers radiation exposure.10 In addition, adjustments in pitch can be made to minimize radiation exposure.11 Correct contrast timing is important to ensure good enhancement of the left atrium and PVs. With the use of a dual-barrel CT injection system, the contrast material and saline chaser bolus can be administered at high flow rates (typically 4 to 5 mL/sec). Automated peak enhancement detection in the descending aorta can be used to time the initiation of the data acquisition. During postprocessing, PV anatomy is assessed using both cross-sectional reconstructions (coronal, sagittal, and transversal views) and 3-dimensional (3D) volume-rendered reconstructions.

Assessment of PV anatomy

Normal PV anatomy is characterized by the presence of 4 PV ostia: 2 separate ostia of the right-sided PVs and 2 separate ostia of the left-sided PVs.12 Variations in PV anatomy include the presence of a “common ostium” (single insertion) of the PVs or “additional” PVs (Figure 1). In 200 patients referred for MSCT scanning prior to catheter ablation for AF, Cronin et al10 observed the presence of normal PV anatomy in 164 patients (82%). A common ostium of the PVs most frequently involved the left-sided PVs (n = 13; 6.5%), whereas an additional PV was most frequently present on the right side (n = 6; 3%). In addition to anatomic variations, the diameter of the PV ostia can be assessed with MSCT. It has been shown that, in particular, the left-sided PVs are oval-shaped with a larger superior-inferior than anterior-posterior diameter.13 Importantly, the preprocedural PV diameters can be compared with PV diameters during follow- up to detect PV stenosis after catheter ablation.14 Although the prevalence of PV stenosis after catheter ablation is relatively low (approximately 5%), it is a severe complication and difficult to treat.15,16

Furthermore, the anatomy of the left atrium (including the left atrial roof, left atrial appendage, and the ridge between the left atrial appendage and the left-sided PVs) can be assessed with MSCT. It has been shown that there is large variability in left atrial anatomy in patients referred for catheter ablation of AF.17 In addition, the location and the relationship of the structures that surround the PVs can be assessed with MSCT (Figure 2). The location of the esophagus,18 the phrenic nerves,19 and the coronary sinus and circumflex artery20 can be evaluated. This has important implications for the catheter ablation procedure, since injuries of the esophagus, phrenic nerves, and circumflex arteries during catheter ablation procedures have been reported.21-23 Preprocedural imaging of these structures with the use of MSCT may help to avoid these severe complications.

Image integration

Ideally, the anatomy of the PVs, the left atrium, and the adjacent structures as provided by MSCT, is available during the actual catheterablation procedure. Conventional electroanatomical maps provide detailed electrophysiological information during these procedures.24 However, these maps are limited by the use of reconstructed anatomy.25 With the use of new image integration systems (ie, CartoMerge, Biosense Webster, DiamondBar, CA), the integration of preprocedural acquired MSCT images and online electroanatomical maps has become available.26

Image integration consists of 2 processes: segmentation and registration.27 The segmentation process is the extraction of the anatomy of the left atrium and the PVs from the raw MSCT data before the catheter ablation procedure (Figure 3). This process is based on setting of a certain intensity threshold (in HUs). Therefore, correct contrast timing during the MSCT data acquisition is crucial, since it allows for easy differentiation between the endocardium (low intensity level) and the blood pool (high intensity level). Automatic competitive region-growing algorithms subsequently create the volume-rendered reconstruction of the left atrium and PVs.

The registration process is the actual alignment of the MSCT image and the reconstructed electroanatomical map during the catheter ablation procedure (Figure 4). With the use of markers placed on distinct anatomic locations (eg, left atrial–PV junction), the MSCT image and the electroanatomical map are superimposed. Finally, with the use of dedicated algorithms that minimize the distance between the MSCT image and the electroanatomical map, the 2 structures are fused.27

The feasibility and accuracy of the integration of MSCT and electroanatomical maps has been reported in several studies.27-29 Importantly,it has been noted that the integration of MSCT and electroanatomical maps can result in a reduction in fluoroscopy time during the procedure, and an improved outcome.30 However, a few limitations of the technique need to be addressed. First, in general, the registration process results in a distance of roughly 2 mm between the MSCT image and the electroanatomical map.31 This distance may not be clinically relevant, however, since ablation catheters usually have a diameter of 4 to 8 mm. Second, the image integration systems do not account for cardiac motion, breathing variations, or differences in heart rate and rhythm during scanning and the ablation procedure. Nonetheless, the use of MSCT images during the catheter ablation procedure has been shown to be very valuable, and further improvements in the technique can be expected.25

Anatomy of the coronary venous system for CRT

In recent years, CRT has become an established treatment option in selected patients with symptomatic heart failure, despite optimal medical treatment, a broad QRS complex on the surface ECG, and poor LV systolic function.3,32 In selected patients, CRT reduces symptoms, improves exercise capacity, and may even reduce morbidity and mortality compared with optimized medical treatment.33

Implantation of a CRT device involves placement of an LV pacing lead. For this purpose, a suitable coronary vein, preferably located at the site of latest activation (in general the posterolateral wall of the LV) is necessary. In large clinical trials, the success rate for placement of a transvenous cardiac resynchronization system has ranged from approximately 88% to 92%.34 Reasons for the failure of transvenous LV lead placement include the lack of suitable side branches, a narrowing of the ostium of the coronary sinus, and the inability to advance the lead through the coronary venous system.35,36 In these cases, a minimal invasive surgical approach may be performed to place an epicardial LV pacing lead.

Ideally, coronary venous anatomy is assessed noninvasively, before the LV lead implantation procedure, to determine whether a transvenous approach is feasible.37 Because of its high spatial and temporal resolution, MSCT has the ability to noninvasively visualize the coronary venous system.

Technical considerations

A contrast-enhanced helical scan is typically performed for imaging of the coronary venous system. In general, a protocol similar to coronary artery scanning can be used with slight modifications for contrast timing.38 The amount of contrast material may vary (approximately 80 to 110mL) depending on scan duration, and it is typically administered with at an infusion rate of 4 to 5 mL/sec. Scan initiation and the saline chaser bolus may be delayed to optimize opacification of the coronary veins. Retrospective ECG gating is required for accurate assessment of the small coronary veins. The temporal resolution using retrospective gating can range from 80 to 250 msec.11 Data acquisition and reconstruction protocols may vary among the different 64-slice MSCT scanners. Data acquisition is typically performed using a 0.5 to 0.6-mm section thickness,and images are subsequently reconstructed with 0.5 to 0.75-mm section thickness and 0.3 to 0.4-mm reconstruction interval. With the use of 3 dimensional volume-rendered reconstructions, an overview of the coronary venous anatomy can be acquired. In addition, cross-sectional andmultiplanar reconstructions can be used to assess diameters and patency of the veins.39

Assessment of coronary venous anatomy

The major components of the cardiac venous system relevant for LV lead implantation include the coronary sinus, the great cardiac vein, the anterior interventricular vein, posterior interventricular vein, posterior vein of the LV, and the left marginal vein (Figure 5). Anatomic studies have described large interindividual variations in coronary venous anatomy.40 The posterior vein of the LV and the left marginal vein drain the posterolateral wall of the LV and are the target veins during LV lead implantation procedures.

Several studies have shown the feasibility of MSCT for the noninvasive evaluation of coronary venous anatomy.39,41,42 In a direct comparison between MSCT and invasive venography (the gold standard), it has been shown that MSCT can accurately depict coronary venous anatomy, and may even be superior to invasive venography for the visualization of small veins.41

Importantly, a possible association between the anatomic variations and the history of a myocardial infarction has been reported. In a series of 100 patients referred for coronary CT angiography, patients with a history of myocardial infarction were less likely to have a left marginal vein.42 The absence of a left marginal vein may hamper transvenous LV lead implantation for CRT, requiring surgical epicardial LV lead placement. Preoperative information derived from MSCT may help in selecting the optimal strategy.

In addition to information on anatomic variations, MSCT can provide information on the diameters of the coronary veins and distance to the various tributaries. It has been noted that the proximal coronary sinus is oval-shaped with a larger superior-inferior diameter.39 However, MSCT may slightly overestimate vessel diameter, as compared with invasive venography.41 The distance from the coronary sinus and the various tributaries, as provided by MSCT, may be helpful in planning the LV lead implantation procedure.42 Furthermore, MSCT can accurately visualize the relation between the target vein and the phrenic nerves.19 Thereby, MSCT may help in avoiding phrenic nerve stimulation by the LV pacing lead.

Finally, MSCT may be of value in the follow-up of patients after CRT (Figure 6). When repositioning of the LV lead is required because of loss of capture, excessive increase in lead threshold, or phrenic nerve stimulation, MSCT can visualize the actual lead position in relation to other cardiac veins and adjacent structures (such as the phrenic nerve) for potential lead positioning.43

Image integration

Knowledge of venous anatomy is crucial for LV lead implantation. However, integrated information on the site of latest activation (maximum LV dyssynchrony) and venous anatomy may even be more important. It has been shown that concordance of the location of the LV lead and thesite of latest activation (assessed with echocardiography) results in improved CRT outcome.44 Accordingly, it would be desirable to integrate the echocardiographic images with the MSCT images, in order to coregister electromechanical delays with venous anatomy.45

Recently, the feasibility of the integration of anatomic information derived from MSCT and functional information derived from echocardiography was demonstrated. This new technique enables the identification of a suitable coronary vein closest to the site of latest activation before the LV lead implantation procedure.46 Although these results are preliminary, the concept of image registration to guide LV lead placement has the potential to improve the response to CRT.

Conclusion

Because of its high spatial and temporal resolution, MSCT can provide detailed information on PV and coronary venous anatomy. Anatomic variations, diameters of the veins, and the relation with adjacent structures can clearly be visualized with MSCT. Thereby, it can provide a detailed road map for catheter ablation procedures and LV lead implantations. New techniques that enable the integration of MSCT images with electroanatomical maps or echocardiography may facilitate these procedures and potentially improve the outcome.

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