Curr Cardiol Rep (2014) 16:511 DOI 10.1007/s11886-014-0511-6
INVASIVE ELECTROPHYSIOLOGY AND PACING (EK HEIST, SECTION EDITOR)
Catheter Ablation Guided by Real-Time MRI Charlotte Eitel & Gerhard Hindricks & Matthias Grothoff & Matthias Gutberlet & Philipp Sommer
Published online: 22 June 2014 # Springer Science+Business Media New York 2014
Abstract Real-time magnetic resonance imaging (MRI) combines the advantages of excellent soft-tissue characterization in a true 3D anatomical and functional model with the possibility of lesion and gap visualization without the need of any radiation. Therefore, real-time MRI presents a particularly attractive imaging technology to guide electrophysiology studies and catheter ablation procedures. This article aims to provide an overview on current routine clinical application of MRI in the setting of interventional electrophysiology. Furthermore, development of real-time MRI guided electrophysiology studies and first experiences with MRI guided catheter ablation procedures are depicted. In this context advantages, challenges and limitations of real-time MRI guided catheter ablation as well as future perspectives are discussed. Keywords Real-time MRI . CMR . Catheter ablation . Electrophysiology . Catheter tracking . MRI guided ablation
This article is part of the Topical Collection on Invasive Electrophysiology and Pacing C. Eitel (*) : G. Hindricks : P. Sommer Department of Electrophysiology, University of Leipzig, Heart Center, Struempellstrasse 39, 04289 Leipzig, Germany e-mail: [email protected]
G. Hindricks e-mail: [email protected]
P. Sommer e-mail: [email protected]
M. Grothoff : M. Gutberlet Department of Diagnostic and Interventional Radiology, University of Leipzig, Heart Center, Leipzig, Germany M. Grothoff e-mail: [email protected]
M. Gutberlet e-mail: [email protected]
Abbreviations AF Atrial fibrillation CS Coronary sinus DE Delayed enhancement LAO Left anterior oblique MRI Magnetic resonance imaging RAO Right anterior oblique
Introduction The number of catheter ablation procedures is growing steadily due to increasing knowledge of pathophysiological mechanisms of complex arrhythmias with the possibility of effective treatment . Understanding and visualization of the underlying anatomical substrate is essential for the treatment of arrhythmias with a complex three-dimensional substrate [e.g., atrial fibrillation (AF), ventricular tachycardia (VT)]. Due to the limitations of 2D fluoroscopy to provide the required precision of 3D orientation, 3D mapping systems have been developed to facilitate accurate understanding of the individual anatomical and electrical substrate [2, 3]. Using these 3D mapping systems consequently did contribute to a significant reduction of fluoroscopy burden in complex ablation procedures and even allowed “zero-fluoroscopy” procedures in less complex ablations . Besides these a novel sensor based 3D navigation system has been introduced recently, integrating 3D non-fluoroscopic catheter navigation on pre-recorded cine-loops (MediGuide Technology) . However, all these technologies are limited by inaccuracies in image acquisition and registration and still require usage of fluoroscopy . In this setting, real-time magnetic resonance imaging (MRI) presents a particularly attractive imaging technology to guide electrophysiology studies and catheter ablation
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procedures. Benefits relate to (1) the fluoroscopy-free environment, (2) substrate analysis, (3) combination of 3D anatomical and functional information, as well as (4) real-time visualization of ablation lesions and introduced catheters. This article aims to provide an overview on current routine clinical applications of MRI in the setting of interventional electrophysiology. Furthermore, development of real-time MRI guided electrophysiology studies and first experiences with MRI guided catheter ablation procedures are depicted. In this context advantages, challenges and limitations of realtime MRI guided catheter ablation as well as future perspectives are discussed.
Peri-procedural use of MRI in the Context of Catheter Ablation Catheter Ablation of Ventricular Tachycardia Delayed enhancement (DE) MRI is the gold standard for delineation of scar/fibrosis and facilitates precise detection of amount and extent of scar that serves as an arrhythmogenic substrate in patients with VT [7, 8]. Therefore, pre-procedural MRI in the setting of VT ablation is increasingly performed to gain more insights regarding the underlying substrate. Techniques have been developed not only to integrate a preacquired anatomical surface model, but also to integrate disease causing scar volumes in the setting of VT ablation [9•]. In this study MRI even facilitated identification of nontransmural scars and infarct grey zones not detected by electroanatomical voltage mapping potentially providing important supplementary substrate information in patients with ischemic cardiomyopathy [9•]. Patients with non-ischemic cardiomyopathy usually exhibit a more complicated morphology and distribution pattern of VT substrate compared to ischemic cardiomyopathy . Characterization of the nonischemic VT substrate in these patients by DE-MRI may lead to optimization of electrogram thresholds for identification of midwall and epicardial scar and enable pre-procedural planning for epicardial access [11, 12]. Despite the benefits of pre-procedural MRI for substrate characterization, the majority of patients referred for VT ablation have an implanted device which is considered a contraindication for performing MRI . However, recent studies indicate the safety and feasibility of performing contrastenhanced cardiac MRI in selected patients with ICDs with the aim of integrating detailed 3D scar maps into clinical mapping systems [14, 15]. This is particularly important as ICD indications are expanding and an increasing number of patients undergoing VT ablation presents with recurrent ICD shocks .
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Catheter Ablation of Atrial Fibrillation In AF patients DE-MRI has been shown to serve as a valuable noninvasive imaging method to assess and quantify the extent of left atrial fibrosis as an indicator of structural remodeling . Consequently higher degrees of fibrosis have been introduced as an independent predictor of AF ablation failure [17, 18••]. Results of the multicenter Delayed Enhancement MRI determinant of successful Catheter Ablation of Atrial Fibrillation (DECAAF) - trial (NCT01150214) confirm that stage of atrial fibrosis assessed by MRI prior to ablation serves as an independent predictor of outcome [19••]. In this way pre-ablation knowledge of the arrhythmogenic substrate might result in avoidance of ablation in patients with very low probability of success on the one hand and individualized treatment concepts with targeted ablation of fibrotic tissue on the other hand. However, this approach still needs to be proven in other well designed studies. Besides performance of DE-MRI for quantification of left atrial fibrosis prior to AF ablation, T1 mapping has been introduced as a new methodology to quantify diffuse fibrotic changes in thin-walled myocardial tissues like the left atrium [20•]. Potential advantages of T1-mapping relate to the lacking dependence of the correct inversion time and the largely automated measurement [20•]. The value of this methodology in the context of catheter ablation procedures still needs to be proven. A recent study indicates that quantification of extracellular volume with the use of T1 measurements facilitates prediction of recurrent AF post–pulmonary vein isolation . Catheter ablation of complex atrial arrhythmias requires accurate knowledge of the true cardiac anatomy of the respective patient, especially in the setting of anatomically defined lesion sets such as in pulmonary vein isolation that still presents the cornerstone of AF ablation. Pulmonary vein anatomy has been described to vary in up to 38 % of patients presenting for AF ablation . This is of interest as additional veins might serve as triggers and as inadvertent delivery of ablation lesions inside a pulmonary vein might increase the risk of subsequent stenosis . Therefore pre-procedural assessment of computed tomography or MRI images of the left atrium is routinely performed in most centers for integration with electroanatomic maps as this facilitates better understanding of the underlying individual 3D anatomy. Post-interventionally, MRI is a very useful tool for detection of complications, like pulmonary vein stenosis following AF ablation. Additionally esophageal injury may be detected and monitored with the use of DE-MRI . Lesion Visualization A unique feature of MRI presents the possibility of imaging deployed lesion sets and remaining gaps [25–29]. Initial
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studies demonstrated the possibility of visualizing spatial and temporal extent of ventricular ablation lesions and correlation with histopathological specimens in dogs [27, 29]. The role of DE-MRI in detecting and quantifying left atrial wall injury after pulmonary vein isolation and correlation with ablation outcome at 3 months has been shown recently [25, 26, 30, 31]. This might facilitate prediction of prognosis and planning of repeat procedures with targeted closure of lesion gaps in AF patients [25, 26, 31]. Despite adequate assessment of radiofrequency lesion size, transmural extent, and interlesional gaps throughout the different stages of gadolinium enhancement , the use of this technique for serial lesion assessment during a procedure is limited due to the time interval of about an hour required for renal clearance between repeated dosing of gadolinium and the ceiling on total allowable gadolinium dose [32, 33]. To overcome this limitation the feasibility to assess acute and subacute radiofrequency lesions with the use of noncontrast enhanced MRI has been described in animal studies [28, 34, 35•]. This may allow intraprocedural evaluation of radiofrequency lesions in the human heart.
Catheter Ablation Guided by Real-time MRI Potential Advantages of Catheter Ablation Guided by Real-time MRI Besides a reduction or even elimination of fluoroscopy, significant advantages of performing catheter ablation directly in the MR scanner relate to true anatomical real-time 3D visualization not only of the underlying anatomy, substrate and neighboring structures, but also of the exact catheter position in relation to these. Another unique feature of real-time MRI in the electrophysiology setting constitutes the immediate detection of lesions and even gaps in ablation lines [25–29, 34]. This might facilitate individualized ablation strategies with preablation planning of the interventional approach, specific targeting of the individual underlying arrhythmogenic substrate or of remaining gaps in previously placed ablation lines. Furthermore, MR thermography offers the possibility to noninvasively image tissue heating potentially facilitating accurate noninvasive prediction of tissue destruction during RF ablation procedures as well as prevention of damaging neighboring structures with esophageal fistulas presenting the most fatal form . Challenges of Catheter Ablation Guided by Real-Time MRI Despite all these promises the clinical application of real-time MRI is hampered by several challenges: First of all the electrophysiologist is facing a completely different setup. Communication between the operator, the nurse and the out-side
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team members, which adjust the scanning planes and manage the electrophysiology system has to be established using wireless headsets in order to overcome the noise being made by the MR scanner. Electrocardiogram and oxygen saturation monitoring requires MR compatible equipment, and perfusion lines connected to monitors containing ferromagnetic material have to be introduced from outside the MR suite. Furthermore until now no acute defibrillation or cardioversion can be performed in the MR scanner. Electrophysiology catheters and devices usually contain metallic materials and are not approved for use in the MRI. Therefore special MR conditional devices and catheters consisting of MR-safe non-ferromagnetic components to reduce MR-induced heating, electrical noise and imaging artifacts have been designed. Development of MR compatible open-irrigated ablation catheters was a prerequisite to achieve adequate lesion depth and improve safety profile in complex ablation procedures [37, 38, 39•, 40•]. Additionally, filters have been introduced for noise suppression and enhancement of small intracardiac electrograms [37, 40•]. Quality of QRS morphology and surface ECG are still hampered, but timing of QRS complexes can be identified quite reliably. Besides 3D imaging of complex cardiovascular anatomy, real-time MRI offers the possibility of 2D imaging along arbitrary imaging planes. From an electrophysiology perspective this means, that reorientation is needed with respect to familiar planes, as fluoroscopy provides summation images of the whole catheter and orientation during electrophysiology procedures is routinely performed within right anterior (RAO) and left anterior oblique (LAO) views. In the setting of passive catheter tracking (catheters that are visualized by metallic artifacts)  an approach of sequential imaging of the catheter in two corresponding orthogonal planes - comparable to fluoroscopic RAO and LAO projections proved helpful for guiding catheter ablation [42•]. However, active catheter tracking (MRI signal received by the catheter)  is desirable as it might be easier to localize the respective catheters potentially leading to improved procedure safety, shorter procedure times and higher success rates . Further developments aim at improving guidance of active catheters with the use of automatic tip alignment algorithms . Animal Studies Lardo et al. for the first time demonstrated the feasibility of performing real-time MRI guided electrophysiology studies in 6 dogs . Nonmagnetic catheters could be successfully placed at right atrial and ventricular targets, intracardiac electrograms could be recorded and placed ablation lesions were visualized . Subsequently, Nazarian and colleagues performed comprehensive electrophysiology studies with placement of catheters at the right atrial, His bundle
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and right ventricular targets, recording of intracardiac signals and performance of atrial and ventricular pacing maneuvers . Instead of passive catheter tracking, further studies developed magnetic resonance based catheter tracking that could be successfully used for performing electroanatomic mapping and radiofrequency ablation of the pulmonary veins and AV node in the MRI environment [45, 46]. Advancements have been made with respect to development of MR-conditional catheters and electrophysiology equipment which were successfully employed for real-time guidance of atrial and ventricular radiofrequency ablation procedures including atrioventricular node modulation , isthmus ablation , ablation of gaps in previously applied lesions in the right atrium  and also biatrial ablation including pulmonary vein ablation . Just recently our group could show the feasibility of MRI guided active catheter tracking for real-time intracardiac navigation, biatrial electroanatomic mapping and atrioventricular node ablation [39•]. Active catheter tracking was performed using the magnetic field to localize inductive coils assembled on the electrophysiology catheter and autosegmentation algorithms created auto-registered 3D models of all cardiac chambers [39•]. The coils were then shown as a virtual catheter icon displayed in real-time in the autosegmented/auto-registered 3D model, in the pre-acquired
MRI planes (Fig. 1), and during further scanning [39•]. Furthermore, electroanatomical mapping can be performed (Fig. 2).
Fig. 1 Active catheter tracking in a pig using the Philips iSuite® platform. The green catheter tip is located in the coronary sinus (CS) and the red catheter tip is located in the right ventricular outflow tract. Catheter
location is shown in three different orthogonal planes (a-c) and in a 3Dshell reconstruction that was built from a pre-recorded 3D-whole-heart data set (d)
Human Studies Limited electrophysiology studies in two humans with targeted guidance of one catheter and confirmation via recording of intracardiac electrograms under guidance of real-time MRI have first been performed by Nazarian et al. . Our group showed for the first time the feasibility and safety of performing diagnostic electrophysiology studies with the usage of multiple MR conditional catheters in humans [37, 40•]. Finally, the first successful real-time MRI guided ablation of the cavotricuspid isthmus could be performed [42•]. A total of ten patients underwent ablation of the cavotricuspid isthmus in a standard 1.5T MRI scanner with a standard real-time SSFP sequence using passive catheter tracking . The feasibility and safety of recording intracardiac electrograms and placement of ablation lesions could be shown. However, in nine of ten patients additional fluoroscopy guided placement of ablation lesions had to be performed in the electrophysiology lab due to incomplete isthmus block. The main reason for this might have been difficult handling of the ablation catheter and a predefined limited time of 90 minutes in the MRI scanner
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Fig. 2 Electroanatomical mapping with active catheter tracking in the right ventricle and right atrium of a pig using the Philips iSuite® platform. Orthogonal planes (a, b) showing the catheter tip (red), green dots representing mapping points (LAT map, precocity of electrograms, c) in a 3D map (d). In c, surface ECG (green) and electrograms from 2 MRI
enabled catheters (white tracing=mapping catheter in RA; yellow tracing=diagnostic catheter in CS, serving as reference signal for the map) are shown. In the lower panel (c) the reference marker is adjusted on CS signal for LAT mapping
due to safety reasons. Visualization and MRI guided passive tracking of the inserted catheters using real time SSFP imaging was feasible in all patients .
important feature for the patient, the interventional electrophysiologist and the nurses.
Conclusions Future Perspective Based on current progress in the field of interventional MRI an increasing number of ablation procedures will target the underlying arrhythmogenic substrate leading to individualized ablation concepts. Besides this, preprocedural MRI serves as a risk stratifying tool enabling identification of patients who will benefit most from a particular procedure or in whom risks outweigh potential benefits. Technical developments aim on further improvement of active catheter tracking algorithms as well as automated integration of mapping and scanning information. Besides this integration of data on tissue feedback and thermometry might lead to more efficient lesion creation potentially leading to higher success and lower complication rates. In the light of a growing amount of ablation procedures the fact that real-time MRI guided procedures will be performed without the use of any radiation presents a unique and
Real-time MRI combines the advantages of excellent soft-tissue characterization in a true 3D anatomical and functional model with the possibility of lesion and gap visualization without the need of any radiation. The safety and feasibility of real-time MRI guided electrophysiology studies [37, 40•, 44] and first ablation procedures [39•, 42•, 51] in humans have been described recently. Until now, these experiences are limited to few centers and widespread application is still impaired due to the above mentioned challenges but developments are under way moving this promising technology closer to routine clinical application. Compliance with Ethics Guidelines Conflict of Interest Charlotte Eitel reports received modest lecture honoraria from Philips GmbH, UB Healthcare, personal fees from St. Jude Medical. Gerhard Hindricks received modest lecture honoraria from St. Jude Medical, Biotronik, Medtronic and Biosense and is a member of the St. Jude Medical and Biosense advisory boards.
511, Page 6 of 7 Matthias Grothoff received modest lecture honoraria from Philips and Siemens Healthcare. Matthias Gutberlet received modest lecture honoraria from Philips and Siemens Healthcare, and is a member of the Siemens MR advisory board. Philipp Sommer received modest lecture honoraria by St Jude Medical, and Siemens Healthcare, and is a member of the St. Jude Medical advisory board. Human and Animal Rights and Informed Consent This article does not contain any studies with human or animal subjects performed by any of the authors.
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