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Purpose:
To assess if real-time magnetic resonance (MR) imaging– guided radiofrequency (RF) ablation for atrial flutter is feasible in patients.
Materials and Methods:
The study complied with the Declaration of Helsinki and was approved by the local ethics committee. All patients were informed about the investigational nature of the procedures and provided written informed consent. Ten patients (six men; mean age 6 standard deviation, 68 years 6 10) with symptomatic atrial flutter underwent isthmus ablation. In all patients, two MR imaging conditional steerable diagnostic and ablation catheters were inserted into the coronary sinus via femoral sheaths and into the right atrium with fluoroscopic guidance. The patients were then transferred to a 1.5-T whole-body MR imager for an ablation procedure, in which the catheters were manipulated by an electrophysiologist by using a commercially available interactive real-time steady-state free precession MR imaging sequence.
Results:
All catheters were placed in standard positions successfully. Furthermore, simple programmed stimulation maneuvers were performed. In one of 10 patients, a complete conduction block was performed with MR imaging guidance. In nine of 10 patients, creating only a small number of additional touch-up lesions was necessary to complete the isthmus block with conventional fluoroscopy (median, three lesions; interquartile range, two to four lesions).
Conclusion:
Real-time MR imaging–guided placement of multiple catheters is feasible in patients, with subsequent performance of stimulation maneuvers and occasional complete isthmus ablation. RSNA, 2014
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From the Departments of Radiology (M. Grothoff, L.L., C.L., J.H., L.H., M. Gutberlet), Electrophysiology (C.P., C.E., T.G., G.H., P.S.), and Obstetrics (J.H.), University of Leipzig Heart Center, Struempellstr 34, 04289 Leipzig, Germany; Imricor Medical Systems, Burnsville, Minn (S.W., T.L., D.S.); and Philips Healthcare, Hamburg, Germany (B.S.). Received January 7, 2013; revision requested February 12; revision received August 1; accepted August 16; final version accepted November 13. Address correspondence to M. Grothoff (e-mail: matthias.grothoff@herzzentrum-leipzig. de).
Online supplemental material is available for this article.
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Matthias Grothoff, MD Christopher Piorkowski, MD Charlotte Eitel, MD Thomas Gaspar, MD Lukas Lehmkuhl, MD Christian Lücke, MD Janine Hoffmann, MD Lysann Hildebrand, MD Steve Wedan, MS Thomas Lloyd, MS Daniel Sunnarborg, MS Bernhard Schnackenburg, PhD Gerhard Hindricks, MD Philipp Sommer, MD Matthias Gutberlet, MD
Original Research n Cardiac
MR Imaging–guided Electrophysiological Ablation Studies in Humans with Passive Catheter Tracking: Initial Results1
CARDIAC IMAGING: MR Imaging–guided Electrophysiological Ablation with Passive Catheter Tracking
R
adiofrequency catheter ablation has been established as the standard therapy in atrial flutter, atrial fibrillation, and other arrhythmias of both atrial and ventricular origin (1,2). The current imaging modality for catheter guidance is fluoroscopy, which offers real-time visualization of the entire intracardiac body of the inserted ablation devices in a two-dimensional summation image and has proved to be suitable for precise placement of the ablation catheters at the desired target sites. However, there are also some disadvantages inherent to this method. First, fluoroscopy is associated with substantial radiation exposure to both the patient and the investigator (3,4). Second, fluoroscopy is unable to visualize the structural lesions that are generated during the ablation procedure. Finally, fluoroscopy offers very limited capabilities in imaging adjacent structures and allowing recognition of possible complications. Over the past two decades, magnetic resonance (MR) imaging has emerged as a valuable tool in cardiac imaging, and other publications have demonstrated the potential and development of MR imaging–guided endovascular interventions (5–7). MR imaging is free of ionizing radiation and provides high-contrast images with any
Advances in Knowledge nn MR imaging–guided electrophysiological (EP) interventions for ablation of atrial flutter were conducted safely in 10 patients by using passive catheter tracking. nn Besides catheter guidance, MR imaging might offer the possibility of visualizing the postablation edema and necrosis of the cavotricuspid isthmus by using T2-weighted and late gadoliniumbased enhancement sequences. nn Problems with completing the ablation procedure arise especially from the handling of the MR imaging conditional catheter and the MR imaging–guided intubation of the coronary sinus. 696
user-defined angulation. Moreover, it allows for the visualization of edema by using T2-weighted imaging and an accurate detection of necrosis and/or fibrosis by using late gadolinium-based enhancement techniques, even in the thin walls of the atria (8–10). In radiofrequency catheter ablation, these advantages might be helpful in identifying the arrhythmogenic substrate and in visualizing ablation-induced cardiac lesions (11). Performing diagnostic and therapeutic electrophysiological (EP) interventions with MR imaging guidance requires the development of a compatible ablation system, a reliable method to visualize the catheter tip during intervention, and a workflow in patient management that accounts for the high magnetic field environment. MR imaging–guided invasive diagnostic EP studies have been reported in animal studies and in a small series of five patients in which active and passive catheter-tracking techniques were used (12–16). We hypothesize that realtime MR imaging–guided radiofrequency ablation of atrial flutter is feasible in patients.
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Competent Authority (Bundesinstitut für Arzneimittel und Medizinprodukte). All patients were fully informed about the investigational nature of the procedures and provided written informed consent.
Patient Population The study cohort consisted of 10 patients scheduled for MR imaging– guided ablation to treat right atrial flutter. In isthmus-dependent atrial flutter, the tachycardia takes a circular pathway in the right atrium. Ablation of the inferior isthmus as the therapeutic method of choice means to create a gapless ablation line, connecting to nonconducting structures to block the reentry circuit. The easiest way is to connect the tricuspid annulus with the inferior vena cava. The completeness of this block line needs to be proved at the end of the procedure by measuring the conduction delay at both sides of the ablation line (Fig 1). Inclusion criteria for the study were age of at least 18 years, a willingness and ability to sign the study-specific informed consent, a left atrial diameter less than or equal to 55 mm, an ejection fraction greater than or equal to
Materials and Methods The study was supported by the provision of MR imaging conditional catheters from Imricor Medical Systems (Burnsville, Minn). S.W., T.L., and D.S. are employed by Imricor. B.S. is employed by Philips. M. Grothoff, who is not employed by Imricor or Philips, had control of the data. The study complied with the Declaration of Helsinki and was approved by the local ethics committee, as well as the German
Implication for Patient Care nn In this early phase, several limitations have to be overcome before MR imaging–guided EP ablation studies can be used in clinical routine; however, this method might have clinical potential because of the elimination of ionizing radiation and the additive benefit of tissue characterization.
Published online before print 10.1148/radiol.13122671 Content codes: Radiology 2014; 271:695–702 Abbreviations: EP = electrophysiological icECG = intracardiac electrocardiography LAO = left anterior oblique RAO = right anterior oblique sECG = surface electrocardiography Author contributions: Guarantors of integrity of entire study, M. Grothoff, M. Gutberlet; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; approval of final version of submitted manuscript, all authors; literature research, M. Grothoff, C.P., C.E., T.G., L.L., J.H., S.W., B.S., G.H., M. Gutberlet; clinical studies, M. Grothoff, C.P., C.E., T.G., L.L., L.H., S.W., T.L., D.S., G.H., P.S., M. Gutberlet; experimental studies, C.P., T.G., S.W., T.L., D.S., B.S., G.H., M. Gutberlet; statistical analysis, M. Grothoff, L.L., J.H., L.H.; and manuscript editing, M. Grothoff, C.P., C.E., T.G., L.L., C.L., J.H., L.H., S.W., T.L., B.S., G.H., P.S., M. Gutberlet Conflicts of interest are listed at the end of this article.
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Figure 1 Patient Characteristics Characteristic Patient sex No. of women No. of men Patient age (y) Age of women Age of men Height (m) Weight (kg) Body mass index (kg/m2) Left ventricular ejection fraction (%) No. of patients with syncope No. of patients with typical atrial flutter No. of patients with atypical atrial flutter
Figure 1: Photograph shows anatomic background of typical atrial flutter. The tachycardia usually runs counterclockwise in the right atrium (RA) (arrows); the ablation line (red points) is drawn to create an electrical block between the tricuspidal annulus (TA) and the inferior vena cava (VCI). CSO = ostium of the coronary sinus, FO = fossa ovalis. (The human heart was courtesy of the Institute of Anatomy, University of Leipzig, Leipzig, Germany.)
45%, and an ability to undergo anticoagulation therapy to achieve adequate anticoagulation. Exclusion criteria were usual MR contraindications, such as claustrophobia (four exclusions) and implanted defibrillators, pacemakers, or ferromagnetic intracranial metallic implants (one exclusion). Further exclusion criteria were an ejection fraction less than 45% (two exclusions) and a severely enlarged left atrium due to limitations of the maximum reach of the MR imaging conditional catheter, without the use of additional sheaths. Patient characteristics are given in the Table. Age differences between male and female patients were analyzed by using the Mann-Whitney test.
MR Imaging Conditional Catheters and EP Recording System The EP tools used in the study included an MR imaging conditional ablation and diagnostic catheter and an MR EP recording system, both designed for use in 1.5-T closed-bore imagers. They provide basic EP functionality, such as delivering programmed stimulation, sensing intracardiac electrocardiography
Value 4 6 70.0 (61.2, 76.3) 70.0 (65.5, 74.5)* 72.0 (51.5, 78.0) 1.72 (1.63, 1.81) 78.0 (73.2, 98.3) 26.5 (25.8, 29.3) 62.0 (54.5, 65.0) 3 8 2
Note.—Continuous data are presented as medians, with interquartile ranges in parentheses. * P = .91.
(icECG) and surface electrocardiography (sECG) signals, monitoring ablation tip temperature, and delivering transmural lesions. The catheter is an 8.5-F, bipolar, passively tracked, steerable catheter with open irrigation (Vision; Imricor Medical Systems). It allows for all clinical imaging protocols. The stainless steel structural braid and pull wire are replaced with Kevlar. An optical temperature sensor is integrated into the tip for real-time temperature monitoring. Filters located in the catheter allow the transmission of ablation and icECG signals while attenuating current induced by MR imaging. Passive markers placed near the tip of the catheter facilitate MR imaging guidance. Gold electrodes are used to minimize susceptibility artifacts. The MR EP recording system consists of two components—a patient interface module and a host computer or “bridge workstation” (Fig 2). The patient interface module receives, filters, and digitizes icECG and sECG signals, which are then sent to the host computer via a fiber-optic cable. The
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patient interface module also receives commands from the host computer and acts as a programmable stimulator. Furthermore, it passes tip temperature data and delivers ablation energy from a conventional ablation generator. The MR EP recording system host computer is the interface between the electrophysiologist and the MR EP recording system. The MR EP recording system is used to display and record sECG and tip temperature data, program stimulation parameters, and record relevant EP end points. To deliver therapy, the MR EP recording system and ablation catheters connect to a conventional ablation generator and irrigation pump.
MR Imaging The technique of MR imaging passive tracking has been described previously (15). In short, fully balanced steadystate free precession sequences with a frame rate of eight per second were used to visualize the inserted and manipulated MR imaging conditional catheters in orientations equivalent to right anterior oblique (RAO) and left anterior oblique (LAO) views at fluoroscopy (Fig 3). With passive tracking, planes had to be manipulated by advancing stepwise in RAO and LAO equivalent orientation for continuous visualization of the catheters. This was performed by radiologists with 13 years (M. Grothoff) and 18 years (M. Gutberlet) of experience in cardiac MR imaging. The tip of the catheters was identified by passive tracking markers. Communication between the investigator in the imaging room and the radiologist at the MR imaging console was provided by a wireless communication system. In this feasibility study, the maximum time in the MR imaging room was set to 90 minutes to ensure that the stress for the patient was limited. After that time, we switched to the established fluoroscopic guidance, in case no complete ablation could be achieved. Owing to time constraints, postablation imaging was performed 24 hours after the intervention in three patients 697
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T1-weighted phase-sensitive inversionrecovery turbo gradient-echo sequence with the following parameters: standard inversion time, 270 msec; repetition time msec/echo time msec, 4.7/2.3; section thickness of 10 mm; and flip angle, 15°. Typical in-plane resolution was 1.8 3 2.4 mm.
Figure 2
Figure 2: Computer-generated image shows the setup of the MR imaging electrophysiology laboratory. RF = radiofrequency.
Figure 3
EP Studies and Ablations In all patients, two MR imaging conditional catheters were placed in the right atrium and the coronary sinus with fluoroscopic guidance by using long, nonsteerable 9-F femoral sheaths, as we knew that coronary sinus intubation with passive MR imaging visualization is difficult (15). After transferring the patients to the MR imaging laboratory, ablation of the cavotricuspid isthmus was performed with conventional technique: The MR imaging–guided ablation started close to the tricuspid valve, and the catheter was slowly drawn back to the inferior caval vein. After ablation, the completeness of the isthmus block was tested with pacing maneuvers. The examinations were performed by G.H. (with 25 years of experience in EP interventions), C.P. (with 10 years of experience), and P.S. (with 10 years of experience). Patients were followed up 24 hours after the MR imaging procedure and 30 days after the procedure, in the outpatient clinic. Results
Figure 3: MR images demonstrate passive catheter tracking in (a) RAO and (b) LOA orientations by using a steady-state free precession sequence. In a, the catheter tip is located above the cavotricuspid isthmus. In b, the catheter is placed on the lateral side of the isthmus. AA = ascending aorta, IVC = inferior vena cava, LA = left atrium, LV = left ventricle, RA = right atrium, RV = right ventricle. ∗ = passive marker near the catheter tip.
only. For visualization of the myocardial edema, a black-blood breath-hold T2-weighted short inversion time turbo spin-echo sequence in RAO orientation was used (Fig 4). The imaging parameters were repetition time, two R-R intervals; echo time, 80 msec; section thickness, 8 mm; and no gap. Typical in-plane resolution was 1.4 3 1.8 mm. 698
Furthermore, RAO-oriented images with late gadolinium-based enhancement (Fig 4) in these patients were acquired for visualization of necrosis 10–15 minutes after application of 0.2 mmol/ kg of body weight of gadobutrol (Gadovist; Bayer Healthcare Pharmaceuticals, Leverkusen, Germany) by using a multiple–breath-hold two-dimensional
Passive Catheter Tracking Visualization and MR imaging–guided passive tracking of the inserted catheters by using real-time steady-state free precession imaging was feasible in all patients (Movie 1 [online]). No deviceor procedure-related adverse events occurred during or after MR imaging. With MR imaging guidance, the ablation catheter could be placed on the cavotricuspid isthmus by using LAO and RAO equivalent orientations in all patients. Furthermore, the ablation catheter was placed on the lateral side of the isthmus to test for isthmus block. Contact between the catheter tip and the apical myocardium was confirmed with
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Figure 4
Figure 4: MR images demonstrate postablation imaging of the cavotricuspid isthmus. (a) On the T2weighted image, the edema of the isthmus is clearly visible (arrows). (b) On images with late gadoliniumbased enhancement, hyperintense areas represent postablation necrosis (arrowheads). AA = ascending aorta, RA = right atrium, RV = right ventricle, VCI = vena cava inferior.
local icECG. The mean time 6 standard deviation in the MR imager was 65 minutes 6 15. In case a catheter dropped out of the adjusted plane, it was either redirected by the investigator or the selected plane had to be adapted to the new position of the catheter tip. This was usually achieved within one to three attempts. In six cases, however, it took up to 15 attempts, with up to 40 seconds until the catheter tip was relocated. MR imaging–guided attempts to intubate the coronary sinus with realtime steady-state free precession guidance were only successful in three of 10 patients. Imaging of the catheter tip was also feasible during the ablation process. However, image quality was slightly impaired owing to lamellar radiofrequency-induced artifacts during energy delivery (Movie 2 [online]). During the interventional procedures with passive tracking, the specific absorption rate did not exceed 3 W/kg.
EP Ablation Studies Ablations of the right atrial isthmus were attempted in 10 patients; a complete block of the isthmus line could only be demonstrated in the last patient. The remaining patients had to be retransferred to the conventional EP laboratory, where a complete isthmus
block was achieved in all patients by using standard ablation catheters. The median number of additional touch-up lesions created was three, with an interquartile range of two to four per patient. No adverse events occurred during the procedures or within the first 30 days of follow-up.
MR Imaging of Atrial Lesions In all three patients who underwent postablation imaging 24 hours after the intervention, circumscribed hyperintense signal intensity alterations could be visualized at the ablation site with both the T2-weighted sequence and the late gadolinium-based enhancement sequence (Fig 4). The extent of edema at T2-weighted imaging clearly exceeded the extent of the scar tissue in all patients. The scar tissue was visible as bright, well-defined spots. Although the spatial resolution of the late gadolinium-based enhancement sequence was increased compared with our standard cardiac T2-weighted sequence, owing to the thin atrial structures, we were not able to visualize the continuity of the ablation lines. Discussion There are substantial challenges that have to be overcome when performing
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EP studies in an MR imaging environment: visualization and tracking of the catheters, recording of icECG and sECG in a high magnetic field, and safe delivery of energy during ablation. We showed that EP ablation procedures of atrial flutter with intracardiac MR imaging guidance of the inserted catheters are feasible, albeit subject to major limitations in this early stage of evaluation and far from routine clinical use. We did not use any experimental sequences or imaging hardware; we used a standard 1.5-T MR imager with a standard real-time steady-state free precession sequence, which implies that almost any MR imaging unit is suitable for such interventions. The use of MR imaging–guided EP studies could be advantageous in the diagnosis and therapy of more complex arrhythmias, like atrial fibrillation and ventricular tachycardias, in the future. A common characteristic of these arrhythmias is that they are ablated with an “anatomic,” substrate-based approach. This means that the goal in atrial fibrillation ablation is to completely isolate the pulmonary vein ostia from the left atrium and additionally to address areas of fibrosis and electrical disturbances within the left atrium, if present (11). In ventricular tachycardia ablation, mostly a circumscribed area of scar or abnormal myocardium is present, which needs to be treated by ablating zones of slow electrical conduction to avoid occurrence of reentry circuits around this scar area. Visualization of the underlying substrate would therefore be of great interest in most ablation procedures today. In this initial phase of MR imaging– enabled ablation procedures, there was a steep learning curve with handling of the ablation catheters and precise navigation in the heart chambers. The mechanical properties of these firstgeneration EP catheters differ from conventional diagnostic or ablation catheters. Diagnostic catheters are usually softer, smaller in diameter (6 F instead of 8.5 F), and easier to handle in terms of catheter deflection. Conventional irrigated-tip catheters are comparable in stiffness and diameter (8 F vs 8.5 F); the 699
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deflection radius (the “reach”) of the MR imaging–enabled catheter is larger and therefore will be modified in the next version. Since we set a time limit for the isthmus ablation in the MR imaging laboratory, we were not able to complete the entire procedure with MR imaging support in most patients but had to create a few new lesions in the conventional fluoroscopy setting. In these patients only, a limited approach was needed with conventional fluoroscopy to complete the procedure. This might be beneficial in complex procedures to reduce radiation exposure; however, the ultimate goal remains full MR imaging guidance without the need for additional fluoroscopy. The last patient finally underwent successful ablation in the MR laboratory. All catheters could be visualized sufficiently and the target area addressed. One of the main findings of our study was that visualization and complex three-dimensional guidance of the inserted catheters was possible with passive catheter tracking only. Passive catheters are recognized by local susceptibility artifacts (17). For real-time imaging, we used a commercially available standard cine steadystate free precession sequence that is typically applied for the adjustment of standard imaging planes in cardiac MR imaging. Selection and adjustment of planes were performed manually during the whole procedure to follow the susceptibility artifact when the catheter was manipulated. This required continuous coordination between the investigator who manipulated the catheters and the radiologist at the MR imaging console, which could be achieved with a wireless communication system. In such a setting, it is beneficial to use standardized planes for imaging. In our approach, we used 30° RAO and 60° LAO equivalent orientations, which are also used in conventional fluoroscopic EP interventions. There were difficulties with intubating the coronary sinus with real-time steady-state free precession guidance in most patients. This was mainly due to the different handling of the MR imaging conditional EP catheter, as compared with conventional EP catheters, and the 700
absence of a guidewire. Therefore, the catheters, and in particular the coronary sinus catheter, were initially inserted with fluoroscopic guidance to be able to spend more time on the ablation procedure. A previous study, however, demonstrated that MR imaging–guided intubation of the coronary sinus is reproducibly feasible in swine (18). The authors visualized the anatomy of the coronary sinus by using a three-dimensional steady-state free precession whole-heart sequence prior to real-time steady-state free precession guidance. They furthermore used a modified noncommercial guidewire and a modified cobra catheter to intubate the sinus, since the handling was not comparable with our EP catheter. Nevertheless, this approach might be helpful when transferred to humans to achieve a fully MR imaging–guided ablation process with passive catheter tracking. A further study, in which isthmus ablation in dogs was reported, involved passive catheter tracking by using gradient-echo sequences to lower specific absorption rate as compared with steadystate free precession sequences (14). We could show that MR imaging guidance is also safe with steady-state free precession sequences with a flip angle of 25° and a frame rate of eight images per second, which provide better contrast compared with standard gradient-echo sequences. No specific absorption rate conflicts occurred. Our findings agree with the findings of Hoffmann et al, who also used steady-state free precession sequences for real-time imaging (13). The exact position of a catheter in atrial flutter ablation procedures is defined by the correct anatomic position (visualization in RAO and LAO orientations, demonstrating the position of the catheter tip at the lateral cavotricuspid isthmus) and the information provided by the icECG (tricuspidal annulus with small atrial and big ventricular ECG signals, clear potentials, and no ST elevation in unipolar icECG). This information was provided by MR imaging in the corresponding planes to visualize the catheter tip and the EP recording system. Visualization of the exact position of the catheter tip was also possible
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during the ablation process. This is an important finding to control the exact position of the catheter tip during energy delivery and to avoid complications caused by an accidental dislocation of the catheter tip during ablation. Visualization of edema by using T2weighted imaging and visualization of necrosis by using late gadolinium-based enhancement sequences after radiofrequency ablation have been described in several studies (19–22) and have even been reported with real-time imaging of the developing lesions (23). Although feasible, imaging of edema and necrosis in the atria still remains challenging. The atrial myocardium is thin, and the spatial resolution of a 1.5-T MR imaging unit is limited. Moreover, motion artifacts caused by cardiac contraction and respiration may further impair image quality (24). T2-weighted short tau inversion-recovery sequences have a low signal-to-noise-ratio and relatively small differences in contrast-to-noise ratios between injured and normal myocardium (25), which is particularly disadvantageous, considering the low atrial myocardial mass. In the present study, however, we could demonstrate that postablational edema and necrosis of the cavotricuspid isthmus can be visualized by using T2-weighted and late gadolinium-based enhancement sequences. The benefit of this information has to be further evaluated. The preablational detection of scar tissue that causes arrhythmia helps the electrophysiologist to identify arrhythmia substrates. The postablational visualization of radiofrequency-induced necrosis might help to detect ablation gaps and to decide whether an ablation procedure is complete. Whether or not such tissue characterization strategies have influence on therapeutic decisions and procedural success remains to be seen. There are a number of limitations inherent to this study: In only one of 10 patients, a complete block of the cavotricuspid isthmus could be achieved in the MR imaging– guided ablation procedure. In the other nine patients, conventional fluoroscopy
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time to complete the ablation was reduced, but nevertheless remained necessary. This problem was primarily caused by handling problems of the ablation catheter and can hopefully be overcome with the next generation of catheters to achieve a higher number of complete MR imaging–guided ablations. We did not compare our passive tracking approach with active tracking. Intubation of the coronary sinus was difficult with passive catheter tracking. The implementation of an active tracking system might be helpful to better visualize the catheters in relatively small tubular structures. According to our study protocol, procedure time in the MR imaging room was limited to 90 minutes for safety reasons. Therefore, we did not perform MR imaging of edema and necrosis during or directly after ablation. Only three patients were available for postablation imaging after 24 hours. Although we used an adapted late gadolinium-based enhancement sequence with increased spatial resolution, we were not able to visualize the continuity of the ablation lines. One of the major problems in MR imaging–guided EP studies is how to perform an immediate defibrillation in a high-field-strength magnetic environment. This question has to be answered before this technique can be applied to more complex ablations with a higher risk of adverse events. In conclusion, we showed that, in combination with fluoroscopy, MR imaging–guided EP diagnostic and therapeutic interventions are feasible with passive catheter tracking. Besides the radiation-free environment, other advantages of this technique might be the visualization of the catheters in relationship to the true cardiac anatomy and the visualization of tissue damage during the ablation procedure. As a next step, the handling of the catheters has to be modified to enable fully MR imaging–guided ablation procedures. However, there remain several major limitations to be overcome before this technique can be considered as an alternative to standard fluoroscopy-guided procedures.
Disclosures of Conflicts of Interest: M. Grothoff Financial activities related to the present article: institution received the MR imaging conditional catheters (nonfinancial support) from Imricor Medical Systems. Financial activities not related to the present article: author received lecturing fees from Siemens. Other relationships: none to disclose. C.P. Financial activities related to the present article: none to disclose. Financial activities not related to the present article: author received lecturing fees from St Jude Medical and Biotronic and is a member of the St Jude medical advisory board. Other relationships: none to disclose. C.E. No relevant conflicts of interest to disclose. T.G. Financial activities related to the present article: none to disclose. Financial activities not related to the present article: author received lecturing fees and congress sponsoring from St Jude Medical, Biosense, and Biotronic. Other relationships: none to disclose. L.L. Financial activities related to the present article: none to disclose. Financial activities not related to the present article: author received lecturing fees from Siemens. Other relationships: none to disclose. C.L. No relevant conflicts of interest to disclose. J.H. No relevant conflicts of interest to disclose. L.H. No relevant conflicts of interest to disclose. S.W. Financial activities related to the present article: author is an employee of Imricor Medical Systems. Financial activities not related to the present article: author has patents pending. Other relationships: none to disclose. T.L. Financial activities related to the present article: author is a full-time employee of Imricor Medical Systems. Financial activities not related to the present article: none to disclose. Other relationships: author has a patent and a null patent pending. D.S. Financial activities related to the present article: author received nonfinancial support from Imricor Medical Systems. Financial activities not related to the present article: author received personal fees from St Jude Medical. Other relationships: none to disclose. B.S. No relevant conflicts of interest to disclose. G.H. Financial activities related to the present article: none to disclose. Financial activities not related to the present article: author received lecturing fees from St Jude Medical, Biosense, and Biotronic and is a member of the St Jude medical advisory board and Biosense advisory board. Other relationships: none to disclose. P.S. Financial activities related to the present article: none to disclose. Financial activities not related to the present article: author received lecturing fees and congress sponsoring from St Jude Medical, Biosense, and Biotronic. Other relationships: none to disclose. M. Gutberlet Financial activities related to the present article: none to disclose. Financial activities not related to the present article: author received lecturing fees from Siemens and Philips. Other relationships: none to disclose.
References 1. Calkins H, Canby R, Weiss R, et al. Results of catheter ablation of typical atrial flutter. Am J Cardiol 2004;94(4):437–442. 2. Spector P, Reynolds MR, Calkins H, et al. Meta-analysis of ablation of atrial flutter and
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supraventricular tachycardia. Am J Cardiol 2009;104(5):671–677. 3. Lickfett L, Mahesh M, Vasamreddy C, et al. Radiation exposure during catheter ablation of atrial fibrillation. Circulation 2004;110(19):3003–3010. 4. Perisinakis K, Damilakis J, Theocharopoulos N, Manios E, Vardas P, Gourtsoyiannis N. Accurate assessment of patient effective radiation dose and associated detriment risk from radiofrequency catheter ablation procedures. Circulation 2001;104(1):58–62. 5. Muller L, Saeed M, Wilson MW, Hetts SW. Remote control catheter navigation: options for guidance under MRI. J Cardiovasc Magn Reson 2012;14:33. 6. Saeed M, Hetts SW, English J, Wilson M. MR fluoroscopy in vascular and cardiac interventions (review). Int J Cardiovasc Imaging 2012;28(1):117–137. 7. Ozturk C, Guttman M, McVeigh ER, Lederman RJ. Magnetic resonance imaging– guided vascular interventions. Top Magn Reson Imaging 2005;16(5):369–381. 8. Nordbeck P, Hiller KH, Fidler F, et al. Feasibility of contrast-enhanced and nonenhanced MRI for intraprocedural and postprocedural lesion visualization in interventional electrophysiology: animal studies and early delineation of isthmus ablation lesions in patients with typical atrial flutter. Circ Cardiovasc Imaging 2011;4(3):282– 294. 9. Peters DC, Wylie JV, Hauser TH, et al. Detection of pulmonary vein and left atrial scar after catheter ablation with three-dimensional navigator-gated delayed enhancement MR imaging: initial experience. Radiology 2007;243(3):690–695. 10. Yokokawa M, Tada H, Koyama K, et al. The change in the tissue characterization detected by magnetic resonance imaging after radiofrequency ablation of isthmus-dependent atrial flutter. Int J Cardiol 2011;148(1):30– 35. 11. Mahnkopf C, Badger TJ, Burgon NS, et al. Evaluation of the left atrial substrate in patients with lone atrial fibrillation using delayed-enhanced MRI: implications for disease progression and response to catheter ablation. Heart Rhythm 2010;7(10):1475–1481. 12. Eitel C, Piorkowski C, Hindricks G, Gut berlet M. Electrophysiology study guided by real-time magnetic resonance imaging. Eur Heart J 2012;33(15):1975. 13. Hoffmann BA, Koops A, Rostock T, et al. Interactive real-time mapping and catheter ablation of the cavotricuspid isthmus guided
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18. Neizel M, Krämer N, Schütte A, et al. Magnetic resonance imaging of the cardiac venous system and magnetic resonanceguided intubation of the coronary sinus in swine: a feasibility study. Invest Radiol 2010;45(8):502–506.
cardiac MRI reconstructions to guide ventricular tachycardia ablations and assess ablation lesions. Circ Arrhythm Electrophysiol 2012;5(2):e31–e35.
by magnetic resonance imaging in a porcine model. Eur Heart J 2010;31(4):450–456. 14. Nazarian S, Kolandaivelu A, Zviman MM, et al. Feasibility of real-time magnetic resonance imaging for catheter guidance in electrophysiology studies. Circulation 2008;118(3):223–229. 15. Sommer P, Grothoff M, Eitel C, et al. Feasibility of real-time magnetic resonance imaging-guided electrophysiology studies in humans. Europace 2013;15(1):101–108. 16. Ganesan AN, Selvanayagam JB, Mahajan R, et al. Mapping and ablation of the pulmonary veins and cavo-tricuspid isthmus with a magnetic resonance imaging–compatible externally irrigated ablation catheter and integrated electrophysiology system. Circ Arrhythm Electrophysiol 2012;5(6):1136–1142. 17. Peeters JM, Seppenwoolde JH, Bartels LW, Bakker CJ. Development and testing of passive tracking markers for different field strengths and tracking speeds. Phys Med Biol 2006;51(6):N127–N137.
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19. Kolandaivelu A, Lardo AC, Halperin HR. Cardiovascular magnetic resonance guided electrophysiology studies. J Cardiovasc Magn Reson 2009;11:21. 20. McGann C, Kholmovski E, Blauer J, et al. Dark regions of no-reflow on late gadolinium enhancement magnetic resonance imaging result in scar formation after atrial fibrillation ablation. J Am Coll Cardiol 2011;58(2):177–185. 21. Schmidt EJ, Mallozzi RP, Thiagalingam A, et al. Electroanatomic mapping and radiofrequency ablation of porcine left atria and atrioventricular nodes using magnetic resonance catheter tracking. Circ Arrhythm Electrophysiol 2009;2(6):695–704.
23. Vergara GR, Vijayakumar S, Kholmovski EG, et al. Real-time magnetic resonance imaging–guided radiofrequency atrial ablation and visualization of lesion formation at 3 Tesla. Heart Rhythm 2011;8(2):295–303. 24. Klemm HU, Steven D, Johnsen C, et al. Catheter motion during atrial ablation due to the beating heart and respiration: impact on accuracy and spatial referencing in three-dimensional mapping. Heart Rhythm 2007;4(5):587–592. 25. Abdel-Aty H, Simonetti O, Friedrich MG. T2-weighted cardiovascular magnetic resonance imaging. J Magn Reson Imaging 2007;26(3):452–459.
22. Tian J, Ahmad G, Mesubi O, Jeudy J, Dickfeld T. Three-dimensional delayed-enhanced
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Purpose:
To assess if real-time magnetic resonance (MR) imaging– guided radiofrequency (RF) ablation for atrial flutter is feasible in patients.
Materials and Methods:
The study complied with the Declaration of Helsinki and was approved by the local ethics committee. All patients were informed about the investigational nature of the procedures and provided written informed consent. Ten patients (six men; mean age 6 standard deviation, 68 years 6 10) with symptomatic atrial flutter underwent isthmus ablation. In all patients, two MR imaging conditional steerable diagnostic and ablation catheters were inserted into the coronary sinus via femoral sheaths and into the right atrium with fluoroscopic guidance. The patients were then transferred to a 1.5-T whole-body MR imager for an ablation procedure, in which the catheters were manipulated by an electrophysiologist by using a commercially available interactive real-time steady-state free precession MR imaging sequence.
Results:
All catheters were placed in standard positions successfully. Furthermore, simple programmed stimulation maneuvers were performed. In one of 10 patients, a complete conduction block was performed with MR imaging guidance. In nine of 10 patients, creating only a small number of additional touch-up lesions was necessary to complete the isthmus block with conventional fluoroscopy (median, three lesions; interquartile range, two to four lesions).
Conclusion:
Real-time MR imaging–guided placement of multiple catheters is feasible in patients, with subsequent performance of stimulation maneuvers and occasional complete isthmus ablation. RSNA, 2014
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From the Departments of Radiology (M. Grothoff, L.L., C.L., J.H., L.H., M. Gutberlet), Electrophysiology (C.P., C.E., T.G., G.H., P.S.), and Obstetrics (J.H.), University of Leipzig Heart Center, Struempellstr 34, 04289 Leipzig, Germany; Imricor Medical Systems, Burnsville, Minn (S.W., T.L., D.S.); and Philips Healthcare, Hamburg, Germany (B.S.). Received January 7, 2013; revision requested February 12; revision received August 1; accepted August 16; final version accepted November 13. Address correspondence to M. Grothoff (e-mail: matthias.grothoff@herzzentrum-leipzig. de).
Online supplemental material is available for this article.
RSNA, 2014
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Matthias Grothoff, MD Christopher Piorkowski, MD Charlotte Eitel, MD Thomas Gaspar, MD Lukas Lehmkuhl, MD Christian Lücke, MD Janine Hoffmann, MD Lysann Hildebrand, MD Steve Wedan, MS Thomas Lloyd, MS Daniel Sunnarborg, MS Bernhard Schnackenburg, PhD Gerhard Hindricks, MD Philipp Sommer, MD Matthias Gutberlet, MD
Original Research n Cardiac
MR Imaging–guided Electrophysiological Ablation Studies in Humans with Passive Catheter Tracking: Initial Results1
CARDIAC IMAGING: MR Imaging–guided Electrophysiological Ablation with Passive Catheter Tracking
R
adiofrequency catheter ablation has been established as the standard therapy in atrial flutter, atrial fibrillation, and other arrhythmias of both atrial and ventricular origin (1,2). The current imaging modality for catheter guidance is fluoroscopy, which offers real-time visualization of the entire intracardiac body of the inserted ablation devices in a two-dimensional summation image and has proved to be suitable for precise placement of the ablation catheters at the desired target sites. However, there are also some disadvantages inherent to this method. First, fluoroscopy is associated with substantial radiation exposure to both the patient and the investigator (3,4). Second, fluoroscopy is unable to visualize the structural lesions that are generated during the ablation procedure. Finally, fluoroscopy offers very limited capabilities in imaging adjacent structures and allowing recognition of possible complications. Over the past two decades, magnetic resonance (MR) imaging has emerged as a valuable tool in cardiac imaging, and other publications have demonstrated the potential and development of MR imaging–guided endovascular interventions (5–7). MR imaging is free of ionizing radiation and provides high-contrast images with any
Advances in Knowledge nn MR imaging–guided electrophysiological (EP) interventions for ablation of atrial flutter were conducted safely in 10 patients by using passive catheter tracking. nn Besides catheter guidance, MR imaging might offer the possibility of visualizing the postablation edema and necrosis of the cavotricuspid isthmus by using T2-weighted and late gadoliniumbased enhancement sequences. nn Problems with completing the ablation procedure arise especially from the handling of the MR imaging conditional catheter and the MR imaging–guided intubation of the coronary sinus. 696
user-defined angulation. Moreover, it allows for the visualization of edema by using T2-weighted imaging and an accurate detection of necrosis and/or fibrosis by using late gadolinium-based enhancement techniques, even in the thin walls of the atria (8–10). In radiofrequency catheter ablation, these advantages might be helpful in identifying the arrhythmogenic substrate and in visualizing ablation-induced cardiac lesions (11). Performing diagnostic and therapeutic electrophysiological (EP) interventions with MR imaging guidance requires the development of a compatible ablation system, a reliable method to visualize the catheter tip during intervention, and a workflow in patient management that accounts for the high magnetic field environment. MR imaging–guided invasive diagnostic EP studies have been reported in animal studies and in a small series of five patients in which active and passive catheter-tracking techniques were used (12–16). We hypothesize that realtime MR imaging–guided radiofrequency ablation of atrial flutter is feasible in patients.
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Competent Authority (Bundesinstitut für Arzneimittel und Medizinprodukte). All patients were fully informed about the investigational nature of the procedures and provided written informed consent.
Patient Population The study cohort consisted of 10 patients scheduled for MR imaging– guided ablation to treat right atrial flutter. In isthmus-dependent atrial flutter, the tachycardia takes a circular pathway in the right atrium. Ablation of the inferior isthmus as the therapeutic method of choice means to create a gapless ablation line, connecting to nonconducting structures to block the reentry circuit. The easiest way is to connect the tricuspid annulus with the inferior vena cava. The completeness of this block line needs to be proved at the end of the procedure by measuring the conduction delay at both sides of the ablation line (Fig 1). Inclusion criteria for the study were age of at least 18 years, a willingness and ability to sign the study-specific informed consent, a left atrial diameter less than or equal to 55 mm, an ejection fraction greater than or equal to
Materials and Methods The study was supported by the provision of MR imaging conditional catheters from Imricor Medical Systems (Burnsville, Minn). S.W., T.L., and D.S. are employed by Imricor. B.S. is employed by Philips. M. Grothoff, who is not employed by Imricor or Philips, had control of the data. The study complied with the Declaration of Helsinki and was approved by the local ethics committee, as well as the German
Implication for Patient Care nn In this early phase, several limitations have to be overcome before MR imaging–guided EP ablation studies can be used in clinical routine; however, this method might have clinical potential because of the elimination of ionizing radiation and the additive benefit of tissue characterization.
Published online before print 10.1148/radiol.13122671 Content codes: Radiology 2014; 271:695–702 Abbreviations: EP = electrophysiological icECG = intracardiac electrocardiography LAO = left anterior oblique RAO = right anterior oblique sECG = surface electrocardiography Author contributions: Guarantors of integrity of entire study, M. Grothoff, M. Gutberlet; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; approval of final version of submitted manuscript, all authors; literature research, M. Grothoff, C.P., C.E., T.G., L.L., J.H., S.W., B.S., G.H., M. Gutberlet; clinical studies, M. Grothoff, C.P., C.E., T.G., L.L., L.H., S.W., T.L., D.S., G.H., P.S., M. Gutberlet; experimental studies, C.P., T.G., S.W., T.L., D.S., B.S., G.H., M. Gutberlet; statistical analysis, M. Grothoff, L.L., J.H., L.H.; and manuscript editing, M. Grothoff, C.P., C.E., T.G., L.L., C.L., J.H., L.H., S.W., T.L., B.S., G.H., P.S., M. Gutberlet Conflicts of interest are listed at the end of this article.
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Figure 1 Patient Characteristics Characteristic Patient sex No. of women No. of men Patient age (y) Age of women Age of men Height (m) Weight (kg) Body mass index (kg/m2) Left ventricular ejection fraction (%) No. of patients with syncope No. of patients with typical atrial flutter No. of patients with atypical atrial flutter
Figure 1: Photograph shows anatomic background of typical atrial flutter. The tachycardia usually runs counterclockwise in the right atrium (RA) (arrows); the ablation line (red points) is drawn to create an electrical block between the tricuspidal annulus (TA) and the inferior vena cava (VCI). CSO = ostium of the coronary sinus, FO = fossa ovalis. (The human heart was courtesy of the Institute of Anatomy, University of Leipzig, Leipzig, Germany.)
45%, and an ability to undergo anticoagulation therapy to achieve adequate anticoagulation. Exclusion criteria were usual MR contraindications, such as claustrophobia (four exclusions) and implanted defibrillators, pacemakers, or ferromagnetic intracranial metallic implants (one exclusion). Further exclusion criteria were an ejection fraction less than 45% (two exclusions) and a severely enlarged left atrium due to limitations of the maximum reach of the MR imaging conditional catheter, without the use of additional sheaths. Patient characteristics are given in the Table. Age differences between male and female patients were analyzed by using the Mann-Whitney test.
MR Imaging Conditional Catheters and EP Recording System The EP tools used in the study included an MR imaging conditional ablation and diagnostic catheter and an MR EP recording system, both designed for use in 1.5-T closed-bore imagers. They provide basic EP functionality, such as delivering programmed stimulation, sensing intracardiac electrocardiography
Value 4 6 70.0 (61.2, 76.3) 70.0 (65.5, 74.5)* 72.0 (51.5, 78.0) 1.72 (1.63, 1.81) 78.0 (73.2, 98.3) 26.5 (25.8, 29.3) 62.0 (54.5, 65.0) 3 8 2
Note.—Continuous data are presented as medians, with interquartile ranges in parentheses. * P = .91.
(icECG) and surface electrocardiography (sECG) signals, monitoring ablation tip temperature, and delivering transmural lesions. The catheter is an 8.5-F, bipolar, passively tracked, steerable catheter with open irrigation (Vision; Imricor Medical Systems). It allows for all clinical imaging protocols. The stainless steel structural braid and pull wire are replaced with Kevlar. An optical temperature sensor is integrated into the tip for real-time temperature monitoring. Filters located in the catheter allow the transmission of ablation and icECG signals while attenuating current induced by MR imaging. Passive markers placed near the tip of the catheter facilitate MR imaging guidance. Gold electrodes are used to minimize susceptibility artifacts. The MR EP recording system consists of two components—a patient interface module and a host computer or “bridge workstation” (Fig 2). The patient interface module receives, filters, and digitizes icECG and sECG signals, which are then sent to the host computer via a fiber-optic cable. The
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patient interface module also receives commands from the host computer and acts as a programmable stimulator. Furthermore, it passes tip temperature data and delivers ablation energy from a conventional ablation generator. The MR EP recording system host computer is the interface between the electrophysiologist and the MR EP recording system. The MR EP recording system is used to display and record sECG and tip temperature data, program stimulation parameters, and record relevant EP end points. To deliver therapy, the MR EP recording system and ablation catheters connect to a conventional ablation generator and irrigation pump.
MR Imaging The technique of MR imaging passive tracking has been described previously (15). In short, fully balanced steadystate free precession sequences with a frame rate of eight per second were used to visualize the inserted and manipulated MR imaging conditional catheters in orientations equivalent to right anterior oblique (RAO) and left anterior oblique (LAO) views at fluoroscopy (Fig 3). With passive tracking, planes had to be manipulated by advancing stepwise in RAO and LAO equivalent orientation for continuous visualization of the catheters. This was performed by radiologists with 13 years (M. Grothoff) and 18 years (M. Gutberlet) of experience in cardiac MR imaging. The tip of the catheters was identified by passive tracking markers. Communication between the investigator in the imaging room and the radiologist at the MR imaging console was provided by a wireless communication system. In this feasibility study, the maximum time in the MR imaging room was set to 90 minutes to ensure that the stress for the patient was limited. After that time, we switched to the established fluoroscopic guidance, in case no complete ablation could be achieved. Owing to time constraints, postablation imaging was performed 24 hours after the intervention in three patients 697
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T1-weighted phase-sensitive inversionrecovery turbo gradient-echo sequence with the following parameters: standard inversion time, 270 msec; repetition time msec/echo time msec, 4.7/2.3; section thickness of 10 mm; and flip angle, 15°. Typical in-plane resolution was 1.8 3 2.4 mm.
Figure 2
Figure 2: Computer-generated image shows the setup of the MR imaging electrophysiology laboratory. RF = radiofrequency.
Figure 3
EP Studies and Ablations In all patients, two MR imaging conditional catheters were placed in the right atrium and the coronary sinus with fluoroscopic guidance by using long, nonsteerable 9-F femoral sheaths, as we knew that coronary sinus intubation with passive MR imaging visualization is difficult (15). After transferring the patients to the MR imaging laboratory, ablation of the cavotricuspid isthmus was performed with conventional technique: The MR imaging–guided ablation started close to the tricuspid valve, and the catheter was slowly drawn back to the inferior caval vein. After ablation, the completeness of the isthmus block was tested with pacing maneuvers. The examinations were performed by G.H. (with 25 years of experience in EP interventions), C.P. (with 10 years of experience), and P.S. (with 10 years of experience). Patients were followed up 24 hours after the MR imaging procedure and 30 days after the procedure, in the outpatient clinic. Results
Figure 3: MR images demonstrate passive catheter tracking in (a) RAO and (b) LOA orientations by using a steady-state free precession sequence. In a, the catheter tip is located above the cavotricuspid isthmus. In b, the catheter is placed on the lateral side of the isthmus. AA = ascending aorta, IVC = inferior vena cava, LA = left atrium, LV = left ventricle, RA = right atrium, RV = right ventricle. ∗ = passive marker near the catheter tip.
only. For visualization of the myocardial edema, a black-blood breath-hold T2-weighted short inversion time turbo spin-echo sequence in RAO orientation was used (Fig 4). The imaging parameters were repetition time, two R-R intervals; echo time, 80 msec; section thickness, 8 mm; and no gap. Typical in-plane resolution was 1.4 3 1.8 mm. 698
Furthermore, RAO-oriented images with late gadolinium-based enhancement (Fig 4) in these patients were acquired for visualization of necrosis 10–15 minutes after application of 0.2 mmol/ kg of body weight of gadobutrol (Gadovist; Bayer Healthcare Pharmaceuticals, Leverkusen, Germany) by using a multiple–breath-hold two-dimensional
Passive Catheter Tracking Visualization and MR imaging–guided passive tracking of the inserted catheters by using real-time steady-state free precession imaging was feasible in all patients (Movie 1 [online]). No deviceor procedure-related adverse events occurred during or after MR imaging. With MR imaging guidance, the ablation catheter could be placed on the cavotricuspid isthmus by using LAO and RAO equivalent orientations in all patients. Furthermore, the ablation catheter was placed on the lateral side of the isthmus to test for isthmus block. Contact between the catheter tip and the apical myocardium was confirmed with
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Figure 4
Figure 4: MR images demonstrate postablation imaging of the cavotricuspid isthmus. (a) On the T2weighted image, the edema of the isthmus is clearly visible (arrows). (b) On images with late gadoliniumbased enhancement, hyperintense areas represent postablation necrosis (arrowheads). AA = ascending aorta, RA = right atrium, RV = right ventricle, VCI = vena cava inferior.
local icECG. The mean time 6 standard deviation in the MR imager was 65 minutes 6 15. In case a catheter dropped out of the adjusted plane, it was either redirected by the investigator or the selected plane had to be adapted to the new position of the catheter tip. This was usually achieved within one to three attempts. In six cases, however, it took up to 15 attempts, with up to 40 seconds until the catheter tip was relocated. MR imaging–guided attempts to intubate the coronary sinus with realtime steady-state free precession guidance were only successful in three of 10 patients. Imaging of the catheter tip was also feasible during the ablation process. However, image quality was slightly impaired owing to lamellar radiofrequency-induced artifacts during energy delivery (Movie 2 [online]). During the interventional procedures with passive tracking, the specific absorption rate did not exceed 3 W/kg.
EP Ablation Studies Ablations of the right atrial isthmus were attempted in 10 patients; a complete block of the isthmus line could only be demonstrated in the last patient. The remaining patients had to be retransferred to the conventional EP laboratory, where a complete isthmus
block was achieved in all patients by using standard ablation catheters. The median number of additional touch-up lesions created was three, with an interquartile range of two to four per patient. No adverse events occurred during the procedures or within the first 30 days of follow-up.
MR Imaging of Atrial Lesions In all three patients who underwent postablation imaging 24 hours after the intervention, circumscribed hyperintense signal intensity alterations could be visualized at the ablation site with both the T2-weighted sequence and the late gadolinium-based enhancement sequence (Fig 4). The extent of edema at T2-weighted imaging clearly exceeded the extent of the scar tissue in all patients. The scar tissue was visible as bright, well-defined spots. Although the spatial resolution of the late gadolinium-based enhancement sequence was increased compared with our standard cardiac T2-weighted sequence, owing to the thin atrial structures, we were not able to visualize the continuity of the ablation lines. Discussion There are substantial challenges that have to be overcome when performing
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EP studies in an MR imaging environment: visualization and tracking of the catheters, recording of icECG and sECG in a high magnetic field, and safe delivery of energy during ablation. We showed that EP ablation procedures of atrial flutter with intracardiac MR imaging guidance of the inserted catheters are feasible, albeit subject to major limitations in this early stage of evaluation and far from routine clinical use. We did not use any experimental sequences or imaging hardware; we used a standard 1.5-T MR imager with a standard real-time steady-state free precession sequence, which implies that almost any MR imaging unit is suitable for such interventions. The use of MR imaging–guided EP studies could be advantageous in the diagnosis and therapy of more complex arrhythmias, like atrial fibrillation and ventricular tachycardias, in the future. A common characteristic of these arrhythmias is that they are ablated with an “anatomic,” substrate-based approach. This means that the goal in atrial fibrillation ablation is to completely isolate the pulmonary vein ostia from the left atrium and additionally to address areas of fibrosis and electrical disturbances within the left atrium, if present (11). In ventricular tachycardia ablation, mostly a circumscribed area of scar or abnormal myocardium is present, which needs to be treated by ablating zones of slow electrical conduction to avoid occurrence of reentry circuits around this scar area. Visualization of the underlying substrate would therefore be of great interest in most ablation procedures today. In this initial phase of MR imaging– enabled ablation procedures, there was a steep learning curve with handling of the ablation catheters and precise navigation in the heart chambers. The mechanical properties of these firstgeneration EP catheters differ from conventional diagnostic or ablation catheters. Diagnostic catheters are usually softer, smaller in diameter (6 F instead of 8.5 F), and easier to handle in terms of catheter deflection. Conventional irrigated-tip catheters are comparable in stiffness and diameter (8 F vs 8.5 F); the 699
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deflection radius (the “reach”) of the MR imaging–enabled catheter is larger and therefore will be modified in the next version. Since we set a time limit for the isthmus ablation in the MR imaging laboratory, we were not able to complete the entire procedure with MR imaging support in most patients but had to create a few new lesions in the conventional fluoroscopy setting. In these patients only, a limited approach was needed with conventional fluoroscopy to complete the procedure. This might be beneficial in complex procedures to reduce radiation exposure; however, the ultimate goal remains full MR imaging guidance without the need for additional fluoroscopy. The last patient finally underwent successful ablation in the MR laboratory. All catheters could be visualized sufficiently and the target area addressed. One of the main findings of our study was that visualization and complex three-dimensional guidance of the inserted catheters was possible with passive catheter tracking only. Passive catheters are recognized by local susceptibility artifacts (17). For real-time imaging, we used a commercially available standard cine steadystate free precession sequence that is typically applied for the adjustment of standard imaging planes in cardiac MR imaging. Selection and adjustment of planes were performed manually during the whole procedure to follow the susceptibility artifact when the catheter was manipulated. This required continuous coordination between the investigator who manipulated the catheters and the radiologist at the MR imaging console, which could be achieved with a wireless communication system. In such a setting, it is beneficial to use standardized planes for imaging. In our approach, we used 30° RAO and 60° LAO equivalent orientations, which are also used in conventional fluoroscopic EP interventions. There were difficulties with intubating the coronary sinus with real-time steady-state free precession guidance in most patients. This was mainly due to the different handling of the MR imaging conditional EP catheter, as compared with conventional EP catheters, and the 700
absence of a guidewire. Therefore, the catheters, and in particular the coronary sinus catheter, were initially inserted with fluoroscopic guidance to be able to spend more time on the ablation procedure. A previous study, however, demonstrated that MR imaging–guided intubation of the coronary sinus is reproducibly feasible in swine (18). The authors visualized the anatomy of the coronary sinus by using a three-dimensional steady-state free precession whole-heart sequence prior to real-time steady-state free precession guidance. They furthermore used a modified noncommercial guidewire and a modified cobra catheter to intubate the sinus, since the handling was not comparable with our EP catheter. Nevertheless, this approach might be helpful when transferred to humans to achieve a fully MR imaging–guided ablation process with passive catheter tracking. A further study, in which isthmus ablation in dogs was reported, involved passive catheter tracking by using gradient-echo sequences to lower specific absorption rate as compared with steadystate free precession sequences (14). We could show that MR imaging guidance is also safe with steady-state free precession sequences with a flip angle of 25° and a frame rate of eight images per second, which provide better contrast compared with standard gradient-echo sequences. No specific absorption rate conflicts occurred. Our findings agree with the findings of Hoffmann et al, who also used steady-state free precession sequences for real-time imaging (13). The exact position of a catheter in atrial flutter ablation procedures is defined by the correct anatomic position (visualization in RAO and LAO orientations, demonstrating the position of the catheter tip at the lateral cavotricuspid isthmus) and the information provided by the icECG (tricuspidal annulus with small atrial and big ventricular ECG signals, clear potentials, and no ST elevation in unipolar icECG). This information was provided by MR imaging in the corresponding planes to visualize the catheter tip and the EP recording system. Visualization of the exact position of the catheter tip was also possible
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during the ablation process. This is an important finding to control the exact position of the catheter tip during energy delivery and to avoid complications caused by an accidental dislocation of the catheter tip during ablation. Visualization of edema by using T2weighted imaging and visualization of necrosis by using late gadolinium-based enhancement sequences after radiofrequency ablation have been described in several studies (19–22) and have even been reported with real-time imaging of the developing lesions (23). Although feasible, imaging of edema and necrosis in the atria still remains challenging. The atrial myocardium is thin, and the spatial resolution of a 1.5-T MR imaging unit is limited. Moreover, motion artifacts caused by cardiac contraction and respiration may further impair image quality (24). T2-weighted short tau inversion-recovery sequences have a low signal-to-noise-ratio and relatively small differences in contrast-to-noise ratios between injured and normal myocardium (25), which is particularly disadvantageous, considering the low atrial myocardial mass. In the present study, however, we could demonstrate that postablational edema and necrosis of the cavotricuspid isthmus can be visualized by using T2-weighted and late gadolinium-based enhancement sequences. The benefit of this information has to be further evaluated. The preablational detection of scar tissue that causes arrhythmia helps the electrophysiologist to identify arrhythmia substrates. The postablational visualization of radiofrequency-induced necrosis might help to detect ablation gaps and to decide whether an ablation procedure is complete. Whether or not such tissue characterization strategies have influence on therapeutic decisions and procedural success remains to be seen. There are a number of limitations inherent to this study: In only one of 10 patients, a complete block of the cavotricuspid isthmus could be achieved in the MR imaging– guided ablation procedure. In the other nine patients, conventional fluoroscopy
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time to complete the ablation was reduced, but nevertheless remained necessary. This problem was primarily caused by handling problems of the ablation catheter and can hopefully be overcome with the next generation of catheters to achieve a higher number of complete MR imaging–guided ablations. We did not compare our passive tracking approach with active tracking. Intubation of the coronary sinus was difficult with passive catheter tracking. The implementation of an active tracking system might be helpful to better visualize the catheters in relatively small tubular structures. According to our study protocol, procedure time in the MR imaging room was limited to 90 minutes for safety reasons. Therefore, we did not perform MR imaging of edema and necrosis during or directly after ablation. Only three patients were available for postablation imaging after 24 hours. Although we used an adapted late gadolinium-based enhancement sequence with increased spatial resolution, we were not able to visualize the continuity of the ablation lines. One of the major problems in MR imaging–guided EP studies is how to perform an immediate defibrillation in a high-field-strength magnetic environment. This question has to be answered before this technique can be applied to more complex ablations with a higher risk of adverse events. In conclusion, we showed that, in combination with fluoroscopy, MR imaging–guided EP diagnostic and therapeutic interventions are feasible with passive catheter tracking. Besides the radiation-free environment, other advantages of this technique might be the visualization of the catheters in relationship to the true cardiac anatomy and the visualization of tissue damage during the ablation procedure. As a next step, the handling of the catheters has to be modified to enable fully MR imaging–guided ablation procedures. However, there remain several major limitations to be overcome before this technique can be considered as an alternative to standard fluoroscopy-guided procedures.
Disclosures of Conflicts of Interest: M. Grothoff Financial activities related to the present article: institution received the MR imaging conditional catheters (nonfinancial support) from Imricor Medical Systems. Financial activities not related to the present article: author received lecturing fees from Siemens. Other relationships: none to disclose. C.P. Financial activities related to the present article: none to disclose. Financial activities not related to the present article: author received lecturing fees from St Jude Medical and Biotronic and is a member of the St Jude medical advisory board. Other relationships: none to disclose. C.E. No relevant conflicts of interest to disclose. T.G. Financial activities related to the present article: none to disclose. Financial activities not related to the present article: author received lecturing fees and congress sponsoring from St Jude Medical, Biosense, and Biotronic. Other relationships: none to disclose. L.L. Financial activities related to the present article: none to disclose. Financial activities not related to the present article: author received lecturing fees from Siemens. Other relationships: none to disclose. C.L. No relevant conflicts of interest to disclose. J.H. No relevant conflicts of interest to disclose. L.H. No relevant conflicts of interest to disclose. S.W. Financial activities related to the present article: author is an employee of Imricor Medical Systems. Financial activities not related to the present article: author has patents pending. Other relationships: none to disclose. T.L. Financial activities related to the present article: author is a full-time employee of Imricor Medical Systems. Financial activities not related to the present article: none to disclose. Other relationships: author has a patent and a null patent pending. D.S. Financial activities related to the present article: author received nonfinancial support from Imricor Medical Systems. Financial activities not related to the present article: author received personal fees from St Jude Medical. Other relationships: none to disclose. B.S. No relevant conflicts of interest to disclose. G.H. Financial activities related to the present article: none to disclose. Financial activities not related to the present article: author received lecturing fees from St Jude Medical, Biosense, and Biotronic and is a member of the St Jude medical advisory board and Biosense advisory board. Other relationships: none to disclose. P.S. Financial activities related to the present article: none to disclose. Financial activities not related to the present article: author received lecturing fees and congress sponsoring from St Jude Medical, Biosense, and Biotronic. Other relationships: none to disclose. M. Gutberlet Financial activities related to the present article: none to disclose. Financial activities not related to the present article: author received lecturing fees from Siemens and Philips. Other relationships: none to disclose.
References 1. Calkins H, Canby R, Weiss R, et al. Results of catheter ablation of typical atrial flutter. Am J Cardiol 2004;94(4):437–442. 2. Spector P, Reynolds MR, Calkins H, et al. Meta-analysis of ablation of atrial flutter and
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supraventricular tachycardia. Am J Cardiol 2009;104(5):671–677. 3. Lickfett L, Mahesh M, Vasamreddy C, et al. Radiation exposure during catheter ablation of atrial fibrillation. Circulation 2004;110(19):3003–3010. 4. Perisinakis K, Damilakis J, Theocharopoulos N, Manios E, Vardas P, Gourtsoyiannis N. Accurate assessment of patient effective radiation dose and associated detriment risk from radiofrequency catheter ablation procedures. Circulation 2001;104(1):58–62. 5. Muller L, Saeed M, Wilson MW, Hetts SW. Remote control catheter navigation: options for guidance under MRI. J Cardiovasc Magn Reson 2012;14:33. 6. Saeed M, Hetts SW, English J, Wilson M. MR fluoroscopy in vascular and cardiac interventions (review). Int J Cardiovasc Imaging 2012;28(1):117–137. 7. Ozturk C, Guttman M, McVeigh ER, Lederman RJ. Magnetic resonance imaging– guided vascular interventions. Top Magn Reson Imaging 2005;16(5):369–381. 8. Nordbeck P, Hiller KH, Fidler F, et al. Feasibility of contrast-enhanced and nonenhanced MRI for intraprocedural and postprocedural lesion visualization in interventional electrophysiology: animal studies and early delineation of isthmus ablation lesions in patients with typical atrial flutter. Circ Cardiovasc Imaging 2011;4(3):282– 294. 9. Peters DC, Wylie JV, Hauser TH, et al. Detection of pulmonary vein and left atrial scar after catheter ablation with three-dimensional navigator-gated delayed enhancement MR imaging: initial experience. Radiology 2007;243(3):690–695. 10. Yokokawa M, Tada H, Koyama K, et al. The change in the tissue characterization detected by magnetic resonance imaging after radiofrequency ablation of isthmus-dependent atrial flutter. Int J Cardiol 2011;148(1):30– 35. 11. Mahnkopf C, Badger TJ, Burgon NS, et al. Evaluation of the left atrial substrate in patients with lone atrial fibrillation using delayed-enhanced MRI: implications for disease progression and response to catheter ablation. Heart Rhythm 2010;7(10):1475–1481. 12. Eitel C, Piorkowski C, Hindricks G, Gut berlet M. Electrophysiology study guided by real-time magnetic resonance imaging. Eur Heart J 2012;33(15):1975. 13. Hoffmann BA, Koops A, Rostock T, et al. Interactive real-time mapping and catheter ablation of the cavotricuspid isthmus guided
701
CARDIAC IMAGING: MR Imaging–guided Electrophysiological Ablation with Passive Catheter Tracking
Grothoff et al
18. Neizel M, Krämer N, Schütte A, et al. Magnetic resonance imaging of the cardiac venous system and magnetic resonanceguided intubation of the coronary sinus in swine: a feasibility study. Invest Radiol 2010;45(8):502–506.
cardiac MRI reconstructions to guide ventricular tachycardia ablations and assess ablation lesions. Circ Arrhythm Electrophysiol 2012;5(2):e31–e35.
by magnetic resonance imaging in a porcine model. Eur Heart J 2010;31(4):450–456. 14. Nazarian S, Kolandaivelu A, Zviman MM, et al. Feasibility of real-time magnetic resonance imaging for catheter guidance in electrophysiology studies. Circulation 2008;118(3):223–229. 15. Sommer P, Grothoff M, Eitel C, et al. Feasibility of real-time magnetic resonance imaging-guided electrophysiology studies in humans. Europace 2013;15(1):101–108. 16. Ganesan AN, Selvanayagam JB, Mahajan R, et al. Mapping and ablation of the pulmonary veins and cavo-tricuspid isthmus with a magnetic resonance imaging–compatible externally irrigated ablation catheter and integrated electrophysiology system. Circ Arrhythm Electrophysiol 2012;5(6):1136–1142. 17. Peeters JM, Seppenwoolde JH, Bartels LW, Bakker CJ. Development and testing of passive tracking markers for different field strengths and tracking speeds. Phys Med Biol 2006;51(6):N127–N137.
702
19. Kolandaivelu A, Lardo AC, Halperin HR. Cardiovascular magnetic resonance guided electrophysiology studies. J Cardiovasc Magn Reson 2009;11:21. 20. McGann C, Kholmovski E, Blauer J, et al. Dark regions of no-reflow on late gadolinium enhancement magnetic resonance imaging result in scar formation after atrial fibrillation ablation. J Am Coll Cardiol 2011;58(2):177–185. 21. Schmidt EJ, Mallozzi RP, Thiagalingam A, et al. Electroanatomic mapping and radiofrequency ablation of porcine left atria and atrioventricular nodes using magnetic resonance catheter tracking. Circ Arrhythm Electrophysiol 2009;2(6):695–704.
23. Vergara GR, Vijayakumar S, Kholmovski EG, et al. Real-time magnetic resonance imaging–guided radiofrequency atrial ablation and visualization of lesion formation at 3 Tesla. Heart Rhythm 2011;8(2):295–303. 24. Klemm HU, Steven D, Johnsen C, et al. Catheter motion during atrial ablation due to the beating heart and respiration: impact on accuracy and spatial referencing in three-dimensional mapping. Heart Rhythm 2007;4(5):587–592. 25. Abdel-Aty H, Simonetti O, Friedrich MG. T2-weighted cardiovascular magnetic resonance imaging. J Magn Reson Imaging 2007;26(3):452–459.
22. Tian J, Ahmad G, Mesubi O, Jeudy J, Dickfeld T. Three-dimensional delayed-enhanced
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