Cardiac Image Registration

Long procedure time and somewhat suboptimal results hinder the widespread use of catheter ablation of complex arrhythmias such as atrial fibrillation (AF). Due to lack of contrast differentiation between the area of interest and surrounding structures in a moving organ like heart, there is a lack of proper intraprocedural guidance using current imaging techniques for ablation. Cardiac image registration is currently under investigation and is in clinical use for AF ablation. Cardiac image registration, which involves integration of two images in the context of the left atrium (LA), is intermodal, with the acquired image and the real-time reference image residing in different image spaces, and involves optimization, where one image space is transformed into the other. Unlike rigid body registration, cardiac image registration is unique and challenging due to cardiac motion during the cardiac cycle and due to respiration. This review addresses the basic principles of the emerging technique of registration and the inherent limitations as they relate to cardiac imaging and registration.

Cardiac image integration implications for atrial fibrillation ablation

Cardiac image registration using computed tomography (CT), magnetic resonance imaging (MRI), ultrasound, and fluoroscopy is currently being investigated and clinically used for atrial fibrillation (AF) ablation. Cardiac image registration, in the context of left atrium, is intermodal, with the acquired image and the real-time reference image residing in different image spaces, and involves optimization, where one image space is transformed into the other. Geometry-based methods, which include using fiducial points and/or surface-based techniques, are usually used for cardiac image registration. During fiducial point registration, fiducial points are either external skin markers or manually determined by marking anatomical landmarks, using mapping catheters. Usually, a minimum of three non collinear points are needed for optimal registration. Recently, a catheter placed inside the coronary sinus has also been used as a fiducial marker for the purpose of registration. During surface registration, the process involves characterizing surfaces in each of the images and deriving the best transformation between them. Unlike rigid body registration, such as has been extensively used in imaging the brain, cardiac image registration is unique and challenging. In addition to the errors inherent in intermodal registration, such as errors in pixel and voxel dimension and errors due to fiducial point selection, there are errors specific to cardiac image registration, i.e., errors due to cardiac motion during the cardiac cycle and due to respiration. This review addresses the basic principles of registration and the inherent registration errors as they relate to cardiac imaging and registration.

Biventricular Pacing and QT Interval Prolongation

INTRODUCTION

The purpose of this study was to examine BiV pacing-dependent changes in QT interval and the related potential for proarrhythmia. Biventricular (BiV) pacing has emerged as a promising therapy for patients with advanced congestive heart failure (CHF) and bundle branch block (BBB).

METHODS AND RESULTS

One hundred and seventy-six consecutive patients (123 men and 53 women; mean age 67 +/- 16 years) with ischemic (n = 128) or nonischemic (n = 48) cardiomyopathy in New York Heart Association Class II (8%) or III (92%) CHF (ejection fraction 24 +/- 9%) underwent atrial synchronous BiV pacing. The QRS, QT, and JT intervals were measured at 30 minutes after initiation of BiV pacing, at 24 hours, and at 1 month postimplant. QT interval was defined as the time interval between the initial deflection of the QRS complex and the point at which the T wave crossed the isoelectric line. At baseline, the average QRS duration was 178 +/- 10 ms, attributable to left BBB (n = 158) or intraventricular conduction delay (n = 18). BiV pacing resulted in a small but statistically significant reduction in QRS duration (148 +/- 9 ms during BiV pacing vs 178 +/- 10 ms at baseline [P < 0.0001]), yet the QT increased to 470 +/- 34 ms with BiV pacing versus 445 +/- 32 ms at baseline [P < 0.0001]). The JTc interval during BiV pacing was significantly shorter than during LV pacing (290 +/- 9 ms vs 320 +/- 20 ms, P < 0.0001). During a mean follow-up of 24 +/- 6 months, one patient developed recurrent torsade de pointes. That was eliminated once left ventricular pacing was discontinued.

CONCLUSION

Biventricular pacing prolongs QT interval. However, the occurrence of torsade de pointes is uncommon.

Computed Tomography-Fluoroscopy Image Integration-Guided Catheter Ablation of Atrial Fibrillation

INTRODUCTION

This study examines the feasibility of atrial fibrillation (AF) ablation using registered three-dimensional computed tomography (CT) images of the left atrium with fluoroscopy.

METHODS AND RESULTS

A total of 50 consecutive patients with symptomatic AF refractory to medical therapy (32 paroxysmal, 18 persistent, age 55 +/- 10 years) were randomized to undergo a catheter-based AF ablation procedure with or without the CT-fluoroscopy guidance system. All patients underwent preprocedural contrast-enhanced CT imaging and segmentation of the left atrium. For the CT-fluoroscopy group, circumferential lesions encompassing the pulmonary vein (PV) antrum and linear lesions along the roof of the left atrium between the superior PVs and the mitral isthmus were created on the CT image, which was registered with real-time fluoroscopy. The registered images were then used to navigate the ablation catheters to the sites of planned ablation. After the ablation sites were completed, any remaining PV potentials were isolated with electrophysiological guidance. In the control patients, the same technique was performed without using the CT-fluoro guidance system. CT scans were accurately registered to fluoroscopic images with minimal manual correction. Operators could navigate catheters on the registered images to preplanned, extraostial sites for ablation. CT-fluoroscopy guidance decreased procedure duration and fluoro times (P < 0.05). At a mean follow-up of 9 +/- 2 months, 21 patients (84%) in the CT-fluoro guidance group and 16 patients (64%) in the control group have had no recurrence of AF.

CONCLUSION

CT-fluoroscopic-guided left atrial ablation is feasible and allows appropriate catheter manipulation in the left atrium.

Pulmonary vein isolation and linear lesions in atrial fibrillation ablation

BACKGROUND

Various strategies have been used for atrial fibrillation (AF) ablation. It is unclear whether adding linear lesions to pulmonary vein (PV) isolation has significant advantages.

OBJECTIVES

We assessed the clinical benefit of adding linear lesions in patients undergoing PV isolation for AF.

METHODS

One hundred patients (63 male and 37 female; mean age of 59 +/- 11 years) with documented paroxysmal AF were included in the study. Patients were randomized into two groups. The first group underwent PV isolation alone. The second group underwent PV isolation and had two linear lesions created; one line between the superior PVs, and a second line from the left inferior PV to the mitral valve annulus. Patients' clinical progress after the ablation was evaluated and compared at 1, 3, and 9 months after their respective ablation procedures.

RESULTS

The linear lesions group maintained sinus rhythm and had fewer symptoms than the lone PV isolation group (86 vs. 58%, respectively) (p < 0.05) at 1 month. At 9 months, when patients who reverted to AF underwent additional management to regain sinus rhythm (90 vs. 82%, respectively) (p = NS), there was no statistical difference between the groups regarding the use of antiarrhythmics, the need for electrical cardioversion, and subjective improvement.

CONCLUSION

The addition of linear lesions to PV isolation more effectively achieved sinus rhythm initially and fewer patients required additional management to maintain their rhythm when compared to patients who underwent lone PV isolation. However, at 9 months, the overall results were similar in both groups.

Registration of 3D computed tomographic images with interventional systems: Implications for catheter ablation of atrial fibrillation

Despite the great promise catheter ablation offers in the treatment of complex arrhythmias such as atrial fibrillation (AF), long procedure times and somewhat suboptimal results hinder the widespread use of this technique. As fluoroscopy does not provide contrast differentiation between the area of interest and the surrounding structures, there is a lack of proper intra procedure image guidance. Segmentation of anatomical structures such as the left atrium (LA) can be performed using images obtained with modalities such as computed tomography (CT). However, unlike the cardiac mapping systems, these imaging systems do not track catheters in real time. This review addresses the evolving concept of image registration to deliver therapy in cardiac arrhythmias.

Posterior left atrial–esophageal relationship throughout the cardiac cycle

BACKGROUND

Radiofrequency energy delivered throughout the cardiac cycle has the potential to cause thermal injury to the esophagus if the anatomical relationship between the posterior left atrium and the esophagus changes during cardiac motion.

OBJECTIVE

To assess the posterior left atrial-esophageal relationship throughout the cardiac cycle.

METHODS

In this study, the anatomical relationship between the posterior left atrium and the esophagus was assessed throughout the cardiac cycle in 10 consecutive patients. All patients underwent contrast-enhanced, ECG-gated CT scanning. Left atrial volumes and the esophageal structure were generated from the reconstructed data at 10 phases of the cardiac cycle from 5% to 95% of the R-R interval. The posterior left atrial-esophageal anatomical relationship was measured at four levels, the superior pulmonary vein ostial site, and the upper, mid and lower left atrium.

RESULTS

There were significant variations in the left atrial-esophageal relationship in the 10 patients. The relative movement between the esophagus and the posterior left atrium throughout the cardiac cycle in the anteroposterior and right-to-left orientations was 0.55 +/- 0.99 mm and 0.60 +/- 1.02 mm (95% confidence interval, 2.03 and 1.98 respectively).

CONCLUSIONS

Under normal conditions, there is little change in the anatomical relationship between the posterior left atrium and the esophagus during the entire cardiac cycle. However, due to the interpatient variability at the esophageal location, identification of esophageal location may help prevent complications during catheter ablation procedures involving the left atrium.

Combined use of 1C and III agents for highly symptomatic, refractory atrial fibrillation

BACKGROUND

Despite advances in non-pharmacologic therapy for atrial fibrillation (AF), some patients remain highly refractory.

OBJECTIVE

We report our experience with the unique combined use of 1C and III agents in patients with highly refractory paroxysmal atrial fibrillation.

MATERIALS AND METHODS

Six patients with symptomatic AF (three persistent) were selected after failing multiple antiarrhythmic medications and radiofrequency ablation. They were started on flecainide or propafenone and sotalol or dofetilide during three days of inpatient monitoring. No patient had coronary artery disease. All patients had loop recorder follow-up and ECG recordings during clinic visits for a mean follow-up of 9 +/- 11 months.

RESULTS

After therapy, all patients had complete, sustained control of their symptoms with no evidence of AF or proarrhythmia on monitoring. One patient had recurrence of AF after stopping sotalol and was started back on the drug with complete control.

CONCLUSIONS

Combined therapy with a 1C and III agent may be an effective alternative for the treatment of selective, highly refractory AF. Careful patient selection and hospitalization for initiation is necessary to minimize potential proarrhythmic effects. As this is a short-term therapy, further study is needed to assess the extent of efficacy in a larger number of patients.

Registration of three-dimensional left atrial computed tomographic images with projection images obtained using fluoroscopy

BACKGROUND

Anatomic structures such as the left atrium and the pulmonary veins (PVs) are not delineated by fluoroscopy because there is no contrast differentiation between them and the surrounding anatomy. Representation of an anatomic structure via a 3D model obtained from computed tomography (CT) imaging and subsequent projection of these images over the fluoroscopy system may help in navigation of the mapping and ablation catheter to the appropriate sites during electrophysiology procedures.

METHODS AND RESULTS

In this feasibility study, in vitro experiments were performed with a plastic heart model (phantom) with 2 catheters or radiopaque platinum beads placed in the phantom at the time of CT imaging and fluoroscopy. Subsequently, 20 consecutive patients underwent contrast-enhanced, ECG-gated CT scanning. Left atrial volumes were generated from the reconstructed data at &75% of the R-R interval during the cardiac cycle. Similarly, the superior vena cava and the coronary sinus were also reconstructed from these images. During the electrophysiology procedure, digital records (cine sequences) were obtained. Using predetermined algorithms, both the phantom model and the patients' 3D left atrial models derived from the CT were registered with projection images of fluoroscopy. Registration was performed with a transformation that linked the superior vena cava and the coronary sinus from the CT model with a catheter placed inside the coronary sinus via the superior vena cava. Registration was successfully accomplished with the plastic phantom and in all 20 patients. Registration accuracy was assessed in the phantom by assessing the overlapping beads seen both in the CT and the fluoroscopy images. The mean registration error was 1.4 mm (range 0.9 to 2.3 mm). Accuracy of the registered images was assessed in patients with recordings from a basket catheter placed sequentially in the superior PVs and by injecting contrast into the PVs to assess overlapping of contrast-filled PVs with the corresponding vessels on the registered images. The images could be calibrated quite accurately. Any rotational error, which was usually minor, could be corrected by rotating the images as needed.

CONCLUSIONS

Registration of 3D models of the left atrium and PVs with fluoroscopic images of the same is feasible and could enable appropriate navigation and localization of the mapping and ablation catheter during procedures such as atrial fibrillation ablation.

Atrial Tachycardia Arising From the Coronary Sinus Musculature

OBJECTIVES

We sought to describe the electrophysiological features and long-term outcome after radiofrequency catheter ablation (RFCA) of atrial tachycardia (AT) arising from the coronary sinus (CS) musculature.

BACKGROUND

Atrial tachycardia requiring RFCA deep within the CS has been described in isolated case reports. However, the mechanism and exact site of origin of this tachycardia have not been well elucidated.

METHODS

The study included 8 patients (5 men) of a consecutive series of 283 patients undergoing RFCA for focal AT.

RESULTS

In sinus rhythm, a discrete potential (P) was noted after the CS atrial electrogram and during tachycardia, the CS (P) preceded the surface P-wave by 30 to 50 ms. The CS (P) always preceded the earliest electrogram in the left atrium (LA). Three-dimensional electroanatomical mapping was available in four patients, and in one case it showed earliest activation in the CS with rapid spread to the proximal CS and then to the LA. Ablation of the AT initially attempted from the earliest site in the LA in three patients was unsuccessful. In all patients the tachycardia was safely and successfully ablated at a site 3.6 cm within the CS. There has been no recurrence over a follow-up of 37 +/- 13 months.

CONCLUSIONS

Focal AT emanating deep within the CS musculature can be recognized by a discrete potential associated with the CS atrial signal both during sinus rhythm and tachycardia. Long-term success without complications can be accomplished by ablating within the CS in close proximity to the CS (P).

Feasibility and validation of registration of three-dimensional left atrial models derived from computed tomography with a noncontact cardiac mapping system

OBJECTIVES

The purpose of this study was to determine the feasibility and assess the validity of registering three-dimensional (3D) models from computed tomographic (CT) images using a cardiac mapping system.

BACKGROUND

Registration of 3D anatomic models with an interventional system could help identify and navigate mapping and ablation catheters over a complex structure such as the left atrium (LA).

METHODS

ECG-gated, contrast-enhanced cardiac CT imaging was performed in 14 patients with atrial fibrillation. Segmentation was used to create 3D models of the LA. The 3D models were registered with the mapping system using a series of fiducial points. Registration was accomplished retrospectively in the first 10 patients, and catheter navigation was visualized from recorded data. In the final four patients, registration was accomplished in real time during electrophysiologic study. The mapping catheter position, as it was navigated inside the LA, was applied to the registered model in real time. For the validation study, temporary pacing leads were implanted in the LA of 10 dogs. Following this, CT scanning, segmentation, LA model importation, and registration was described previously. After registration, a mapping catheter was positioned at the site of each buried lead according to the registered model with no fluoroscopic guidance. A radiofrequency lesion was created at this location, and the dog was sacrificed, the heart removed and stained, and the distance between the buried lead and the lesion measured.

RESULTS

During the feasibility study, the location of the catheter in the registered model correlated with fluoroscopy, angiography, and intracardiac electrograms. LA endocardial potentials during sinus rhythm and any premature atrial contractions also were successfully delineated over the registered models. In the validation study, the mean target registration error was 2.0 +/- 3.6 mm.

CONCLUSIONS

Registration of CT-derived 3D models of the LA using a cardiac mapping system is feasible and accurate.

Catheter location, tracking, cardiac chamber geometry creation, and ablation using cutaneous patches

OBJECTIVE

The ability to construct a three-dimensional (3-D) surface model of the endocardium and track the location of catheters within a cardiac chamber, using only cutaneous patches, would be a useful advancement in treating arrhythmias. We tested the feasibility of such a system, Ensite NavX (Endocardial Solutions, Inc., St. Paul, MN, USA), in patients undergoing catheter ablation for SVTs.

METHODS

Sixteen patients with 20 arrhythmias undergoing ablation were selected. Skin electrode patches were placed on the chest to create a 3-D coordinate system. A low-amplitude, 5.7 kHz signal emitted from the patches was received by conventional catheters positioned in the heart. Catheter location was determined by measuring the field strength received by the catheters. Location points were successively acquired while catheters were moved throughout the chamber. This information was collected and processed by a workstation to create a detailed 3-D model of the endocardial surface. Anatomic landmarks were labeled on the model as the mapping catheter was navigated. 3-D cardiac chamber geometry reconstruction, landmark labeling, and real time catheter tracking were performed successfully in all patients. Up to six catheters, with a total of up to 26 intracardiac electrodes, were tracked simultaneously.

RESULTS

Constructed geometries, including major vessels and valves, correlated closely with traditional anatomic models as well as intracardiac recordings and fluoroscopic images.

CONCLUSIONS

Real-time catheter tracking and 3-D cardiac chamber model construction is feasible using cutaneous patches and conventional catheters. This approach may be useful in the treatment of patients with cardiac arrhythmias where ablation therapy is primarily anatomically based.