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
New generation of electro-anatomic mapping: full intracardiac ultrasound image integration
Cardiac Translational Electrophysiology Laboratory, Saint Marys Hospital Complex, Mayo Clinic and Foundation, 2-416 Alfred Building, Rochester, MN 55902, USA
* Corresponding author. Tel: +1 507 255 6263; fax: +1 507 255 3292. E-mail address: packer.douglas{at}mayo.edu
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Abstract |
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Surrogate electro-anatomic-derived geometries are used as the three-dimensional (3D) basis for mapping of cardiac arrhythmias. While merged computed tomography (CT) imaging may provide stellar pulmonary vein (PV) and left atrial (LA) anatomy, the applied scans must be obtained prior to ablation, and may not reflect physiologic conditions at the time of intervention. Patient-specific, ultrasound-derived 3D imaging has been developed as an alternative basis for new generation electro-anatomic mapping. An electro-anatomic sensor positioned at the tip of the phased-array intracardiac ultrasound catheter, provides the means to specify both location and orientation of each image as the ‘context’ for creating the 3D volumes for co-registration with electro-anatomic mapping. Specific anatomic details such as the pulmonary veins, membranous fossa, papillary muscles, or valve structures derived from real-time imaging can also be integrated into each segmented volume. This presentation reviews the basis and methods for this novel multi-modality image fusion for the creation of robust, nearly real-time anatomic images for guiding electro-anatomic mapping and ablation without requiring pre-acquired CT image sets, with accompanying limitations.
Key Words: Intracardiac ultrasound, Multi-slice computed tomography, Atrial fibrillation, Cardiac anatomy, Electro-anatomic mapping
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Introduction |
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Over the last 10 years, the close relationship between cardiac electrophysiology and underlying anatomy has been increasingly appreciated. Paroxysmal atrial fibrillation, for example, has been linked to critical triggering events arising in the pulmonary veins, coronary sinus, or the superior vena cava.1
–7 The nomenclature of classical saw-tooth atrial flutter has been changed to cavotricuspid isthmus-dependent, counter-clockwise atrial flutter, as a reflection of the connection between anatomy and physiology.8 Similar links have been forged between ventricular tachycardias and the underlying anatomy of the aortic root9,10 and the right ventricular outflow tract.11–14 In each case, this relationship has led to a more anatomically based approach to ablation than previously possible.
Moreover, both multi-row spiral computed tomography (CT) and magnetic resonance (MR) images have provided a better reflection of the underlying anatomy of the left atrium.15–22 These have been downloaded for displaying side-by-side with acquired electro-anatomic maps. More recently, these stellar depictions of anatomy have been ‘merged’ with electro-anatomic maps to form the basis of a better anatomic understanding.15–19 Nevertheless, their utility has been limited by inherent mismatch of the off-line rendered anatomy, and that actually observed at the time of ablation.21–23 This is, in part, due to differences in volume, inspiratory/expiratory phase,23–25 and cardiac cycle, as well as the occurrence of arrhythmia between the time of image acquisition and the actual ablative intervention.
Alternatively, the use of intracardiac ultrasound for anatomy-guided ablation of AF intervention is also increasing26–30 and with other arrhythmias,31–36 which provides real-time imaging of underlying cardiac structures and the catheters used during ablative intervention.26,37–39 While this ameliorates some of the issues created by changing chamber volumes or arrhythmias, most work with intracardiac ultrasound has been limited to the acquisition and display of two-dimensional (2D), planar phased-array images. Primary three-dimensional (3D) imaging has only recently been available. The value of such 3D imaging on a real-time basis may be considerable, although this has not been previously reported.
This review is therefore presented to (i) describe the feasibility of the recreation of underlying atrial and ventricular 3D anatomy using real-time, phased-array ultrasound imaging and to (ii) demonstrate the integration of this reconstructed, volume rendered 3D anatomy as the basis for electro-anatomic mapping.
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Two-dimensional ultrasound imaging
An imaging approach for this process has been developed in lab.23
The 2D images used in this process are obtained with a 5–10 MHz phased-array system, creating 90° sector images at a frame rate of 30 per second (Siemens Acunav, Mountain View, CA, USA). The four-directional tip delectability, along with catheter rotation, allows detailed imaging of right and left heart cardiac structures from an ultrasound catheter positioned in the RA. Typical images are obtained at 7.5 MHz, which optimizes both tissue resolution and penetration.26 The physiology of blood flow can also be examined using pulsed-wave spectral Doppler, adding a physiologic dimension to the anatomic characterization.38 The catheter is interfaced with a standard electro-anatomic mapping system (Biosense Webster, Diamond Bar, CA) to allow downloading of images in the mapping system's workspace.
Three-dimensional ultrasound imaging
Three-dimensional ultrasound image creation is facilitated by the implantation of a three-coil, electro-anatomic mapping ‘sensor/locater’ within the tip of the intracardiac ultrasound catheter.23 Using a standard electro-anatomic system emitter triangle, the location of the ICE catheter tip is established within 3D space, and displayed using customized CartoSound software (Biosense Webster).23 The location and the accompanying direction of the phased-array image plane, with characteristic pitch, yaw, and roll components of catheter tip motion, are thereby displayed within the x, y, and z co-ordinates of the electro-anatomic mapping system.
For atrial image acquisition, the intracardiac ultrasound catheter is positioned in the RA with the sector directed anteriorly and laterally toward the LA. For 3D image generation, the endocardial surface demonstrated on each planar ultrasound image is traced or circumscribed manually in the workstation's ultrasound viewer window, as shown in Figure 1. Thereafter, the catheter is rotated 1–3° more posteriorly and a second planar image of the LA acquired, enabled with its matching endo-cardial surface perimeter tracing (Figure 1, Panel 2). Sequential images of the LA are obtained with additional catheter rotation, image acquisition, and endocardial surface perimeter marking.23 Because each of these images is acquired within the context of the electro-anatomic system co-ordinates, the exact location of each image plane within three space is referenced to each neighboring plane. Each of the adjoining rings, are thereby collated (Figure 1, Panel 3) into a complete 3D volume image, with the points along and between each ring interpolated to form a rendered volume (Panel 4) which can be segmented from the surrounding non-cardiac components of the planar echocardiography image (Panel 5).
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A similar process is used for acquisition, orientation, collation, and volume rendering of each of the four pulmonary veins. In this case, ultrasound catheter rotation within 1–5°, with 5–7 planar acquisitions is sufficient to completely render a volume object map. These images were likewise segmented from surrounding structures. Following a similar process, the right atrium, superior vena cava, inferior vena cava, and the azygous vein are likewise imaged as seen in Figure 2.
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Ventricular image generation and integration is similarly performed, although the best RV and LV images are produced during imaging from across the tricuspid valve into the RV (Figure 2). Typically all images are gated to the surface QRS at end expiration. A series of images of the RV including the tricuspid valve region and the RV outflow tract can be created by sequential catheter rotation, image acquisition, planar image display, endocardial surface circumscription, and volume rendering as shown in Figure 2. The position of papillary muscles and tricuspid valve leaflets can be marked directly from accompanying real-time ultrasound images. Work is now underway to automate the entire process of establishing the enodcardial surface. The site of echo image transition between chamber and wall can be established manually with this level of intensity shift subsequently used as a fiducial for automatically selecting any similar interface between the chamber and the surrounding wall. Subsequent ‘echo cloud sculpting’ during ultrasound catheter rotation will permit 3D volume rendering of the cardiac chamber and segmentation from the surrounding areas of increased echogenicity.
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Animal testing
The application of these 3D ultrasound imaging techniques was recently validated in an animal model and tested in 15 patients with atrial fibrillation by Okamura et al.23
To examine overall feasibility, 3D geometries were created from a collated family of 2D phased-array sector images of the LA. The images were compared qualitatively and quantitatively with those derived from electro-anatomic mapping and pre-acquired CT images. This examination included comparisons of specific anatomic structures and site-specific clips previously implanted at specific endocardial locations, prior to imaging. Using separate, real-time 2D intracardiac echocardiographic imaging as the gold standard, the point-to-point and surface errors between directly imaged pre-implanted clip or actual underlying atrial and ventricular sites and those of apparent 3D echo or electro-anatomic surrogate map geometry sites were established. The validation distance between this actual underlying anatomy and 3D ultrasound sites, using methods shown in Figure 3, was only 2.1 ± 1.1 mm for atrial and 2.4 ± 1.2 mm for ventricular sites.23 These were significantly less than the variance seen between apparent CartoMerge CT images and real-time ICE images (atria: error was 3.3 ± 1.6 mm; and ventricles 4.8 ± 2.0 mm; P < 0.001).
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Ablative lesions were guided by the 3D ultrasound-derived geometry. Error between 3D ultrasound anatomic sites and blinded real-time 2D phased-array imaging locations was likewise significantly lower using the 3D ultrasound approach (as developed in Figure 4). Resulting lesions created with radiofrequency (RF) energy delivery at specific CartoSound sites were within 1.1 ± 1.1 mm of the actual anatomic or clip site.
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In addition, the images were sufficiently robust to guide PV isolation with very little fluoroscopy.
Patient validation
In addition, similar analyses were undertaken in 15 patients undergoing ablation for atrial fibrillation.23 Fifty-eight pulmonary veins were rendered with the use of 7 ± 3 individual ultrasound contours, while the left atrium in these patients were similarly rendered using 23 ± 5 contours. The volumes created by ultrasound imaging were slightly smaller than those from electro-anatomic mapping (98 ± 24 vs. 109 ± 25 cm3, P < 0.05) as similarly shown in Figure 5. In this process, 3D ultrasound volume creation yielded significantly less error between apparent and actual anatomic sites than possible with electro-anatomic or CT imaging, and complete ablation of the PVs was accomplished from the 3D image guidance, including during robotically guided sheath deliveries (Figure 6).
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Wide area circumferential ablation around both right (13 ± 4 lesions) and left (14 ± 6 lesions) pulmonary vein sets guided largely by 3D ultrasound imaging resulted in establishment of pulmonary vein entrance block, with subsequent lasso-guided, touch-up ablation requirement similar to that with Carto map-guided ablation alone.40
With experience, the time required for the 3D ultrasound mapping of the left atrium and pulmonary veins decreased from 41 ± 13 min in the first seven patients to 23 ± 4.7 min in the next eight patients (P < 0.0001).23
ICE catheter-guided linear ablation has been increasingly used in ablation of persistent or chronic AF in the setting of underlying heart disease.23,41–44 Twelve of the validation study patients also underwent successful linear ablation, with an example of extensive linear ablation shown in Figure 7, and explained in the figure legend.
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Endocardial electro-anatomic mapping and surface registration
The utility of image integration has also been tested in a canine model. Figure 8 shows an electro-anatomic map of the right atrium created during sinus rhythm. The resulting surface electro-anatomic map was registered to the acquired 3D ultrasound volume image. Earliest activation is seen antero-laterally at the SVC/RA junction, with subsequent activation occurring more rapidly in a superior to inferior direction than seen in an anterior or posterior direction. A point of breakthrough in the electro-anatomic map from the underlying RA is as noted at the arrow point. The difference between these surface displays is on the order of 1–2 mm. The accuracy of registration is facilitated by the use of the same 3D co-ordinate system for both ultrasound image rendering and electro-anatomic mapping.
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Ablative intervention
Guidance of linear ablation is also enabled by the 3D ultrasound imaging approach. A representative postero-superior ablative line is shown in Figure 9. Multiple energy deliveries were made between the left superior and the right superior pulmonary veins with complete juxtaposition of the lesions as cataloged on the surface of the ultrasound image. The creation of an intercaval line from the superior to the inferior vena cava, along with an ablative line across the cavotricuspid isthmus was likewise created without fluoroscopy. Figure 4 shows the gross pathology seen on direct tissue inspection after extensive linear ablation. Figure 4B–D shows the accompanying tissue pathology occurring without gaps along this linear lesion. Of note, the length of this line was extensive and line creation was completely guided by the 3D imaging along with fused real-time ultrasound imaging, without requiring any additional fluoroscopy use.
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Figure 10 shows circumferential ablation around the superior pulmonary veins of a similarly created 3D volume rendering of the left atrium and accompanying pulmonary veins. Again, no fluoroscopy was required in this process, with all catheter manipulation guided by the 3D image itself.
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These studies demonstrate an acceptable level of accuracy of the 3D ultrasound imaging and its seamless integration with electro-anatomic mapping.23
This was facilitated by the concomitant use of the same co-ordinate system of the electro-anatomic mapping technology. Resulting images permitted a novel means of (i) guiding catheter positioning, (ii) catheter tip/tissue orientation assessment, (iii) tissue contact monitoring, (iv) lesion formation assessment, and (v) monitoring of micro bubble formation. All point source or position errors were at 
2 mm, which is in the rage of location error inherent in the electro-anatomic mapping system.21–23 While differences in chamber size through the course of the validation studies was apparent, this was significantly less than that expected with the use of 3D CT images acquired 2–7 days prior to the ablation.
This and other recent studies,15–19,21–23 highlight the potential for error with the Carto Merge approach. Whether a surface or landmark pair match approach is used to register electro-anatomic maps to pre-acquired CTs, significant error may be generated.21–23 As such, simply guiding an ablation based on the apparent location of the annotated ablative lesions registered to a CT surface does not guarantee that the apparent location of that anatomy actually correlates with the location of underlying anatomy at the time of energy delivery. The use of intracardiac ultrasound in the process of creating 3D geometry significantly reduces the chance of point-to-point error, which should improve both efficacy and safety.23 This is in turn was likely related to minimization of differences created (i) during the time interval between acquisition and intervention, as well as changes in (ii) volume, (iii) rhythm, (iv) cardiac cycle, or (v) respiratory cycle.24–25
This is further demonstrated by the appropriate localization of ablative lesions, with energy delivery guided by the 3D geometry, whether it is focused or linear ablation around the pulmonary veins, the dome of the left atrium, and the lateral LA isthmus in both animals and humans, including in those guided by sheath robotics. In addition, the actual position of the ablation catheter is obvious on the 2D-fused images, rather than simply providing this information from the icon within a merged CT image.
Real time detection and monitoring of evolving RF lesions by intracardiac ultrasound may also be useful in guiding ablation, although the sensitivity, specificity and positive predictive value for lesion formation and trans-murality have been incompletely studied.45 Up to 70–80% of lesions created in ventricular myocardium are visible on ultrasound inspection,46 while the yield in atrial tissue is less prominent, ranging between 30–60%.47 This has prompted the use of alternative imaging approaches to highlight the lesions such as echo contrast imaging or the use of Doppler tissue imaging. At some point, the full integration of both physiologic and lesion imaging components is likely to be even more useful than that of simple anatomic rendering alone.
Limitations of the method
This represents a test of the image acquisition, display, collation, interpolation, volume rendering, and segmentation to produce 3D ultrasound images. While each component phased-array image is acquired in real-time, there is a mandatory calculation time required to fully render the images as 3D volumes. Recent experience has also demonstrated an improvement in the image acquisition time, with semi-automated selection of the interface between chamber and the myocardial wall. Additional development of 3D ultrasound acquisition systems will require more precise gating to respiratory phase and the cardiac cycle.
While not completely real-time, the fusion of real-time phased-array images and the 3D volumes still demonstrates little change in volume over the course of the intervention. These images are still closer to the prevailing anatomy than possible with a CT obtained one or more days prior to the ablative intervention.21–23
Creation of these images also requires a fundamental understanding of intracardiac ultrasound projections and component surfaces. While this may require a steep learning curve in the absence of prior ultrasound experience, it is relatively straightforward in the setting of prior experience.
Clinical implications
These data suggest both the feasibility and practicality of new generation 3D volume imaging using intracardiac ultrasound as its basis. This is likely to be readily extrapolated to the clinical arena. Of major significance, the ablative interventions undertaken in this study were performed with limited or no fluoroscopy. Similar approaches in patients with cardiac arrhythmias is likely to yield a robust, fluoroscopy-saving means of undertaking complex, yet anatomically-based intervention. The accompanying utility of registration of electro-anatomic mapping provides an additional means of physiologically guiding ablation.
Conflict of interest: D.L.P. has received other research grants from Biosense Webster.
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This study was funded by Biosense Webster.
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