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Cardiac Imaging |
1 From the Departments of Cardiology (M.R.M.J., J.J.B., E.E.v.d.W., M.J.S.) and Radiology (M.S.D., K.G., H.J.L., A.d.R.), Leiden University Medical Center, Albinusdreef 2, 2333 ZA Leiden, the Netherlands; and Department of Epidemiology and Statistics, Erasmus Medisch Centrum, Rotterdam, the Netherlands (E.B.). From the 2003 RSNA Scientific Assembly. Received July 3, 2003; revision requested September 25; final revision received May 25, 2004; accepted June 23. Address correspondence to J.J.B. (e-mail: j.j.bax@lumc.nl).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS: For patients, institutional review board (IRB) approval was not required, and consent was obtained for treatment. Control subjects were part of an IRB-approved research protocol at the institution, in which they had consented to participate. Multi–detector row CT was performed in 23 patients (17 men, six women; mean age, 48 years ± 11 [standard deviation]) with atrial fibrillation who were admitted for isolation of pulmonary veins by means of radiofrequency catheter ablation. Pulmonary vein anatomy was evaluated, and diameters of pulmonary vein ostia were measured. To determine the shape of ostia, a venous ostium index was calculated for all veins by dividing anterior-posterior measurements by superior-inferior measurements. Results were compared with those in a control group of 11 patients (eight men, three women; mean age, 56 years ± 11) without atrial fibrillation. Images were evaluated by two observers in consensus.
RESULTS: Pulmonary veins additional to the four main veins were found in seven (30%) of 23 patients. Common ostia of left and right pulmonary veins were detected in 19 (83%) and nine (39%) patients, respectively. Early branching occurred more often with right than with left veins (19 [83%] vs three [13%] cases, P < .05) in both patients and control subjects. Anterior-posterior diameter of ostia was 12.8 mm ± 3.3 for left veins, 16.2 mm ± 3.8 for right veins, and 18.8 mm ± 7.7 and 28.7 mm ± 5.1 for left and right common ostia, respectively. Ostia of right pumonary veins were more round than were ostia of left pulmonary veins (venous ostium index in patients, 0.91 ± 0.21 vs 0.75 ± 0.17, P < .05; in control subjects, 0.93 ± 0.12 vs 0.82 ± 0.17, P < .05). The CT data were used to determine ablation strategy and guide catheters during radiofrequency ablation.
CONCLUSION: Multi–detector row CT provides a valuable road map for pulmonary vein anatomy prior to radiofrequency catheter ablation. Variations in number and insertion of pulmonary veins were observed in a considerable number of patients and control subjects.
© RSNA, 2005
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Arrhythmogenicity of the pulmonary veins has been attributed to sleeves of atrial myocardium extending into the pulmonary veins (6,7). Anomalies in number and insertion of pulmonary veins have been described, and some authors have indicated a possible role of the right middle pulmonary vein in initiating atrial fibrillation (8). It is not possible to adequately detect these anomalies by using fluoroscopy alone.
Magnetic resonance (MR) imaging has been applied to detect anomalous insertion of pulmonary veins and to evaluate pulmonary vein stenosis after radiofrequency catheter ablation (9–11). The advantage of MR imaging is the lack of ionizing radiation exposure, but MR imaging cannot be performed in patients with pacemakers or claustrophobia or in patients who, because of their clinical conditions, cannot tolerate the considerably long imaging times of MR imaging.
Accordingly, the purpose of our study was to evaluate multi–detector row computed tomographic (CT) depiction of the pulmonary veins to provide a road map for subsequent radiofrequency catheter ablation.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Our institutional review board does not require its approval or written informed consent for a retrospective review, as was performed for the 23 patients undergoing standard practice treatment at our hospital. However, all 23 study patients were extensively informed about the treatment, and they all gave informed consent prior to undergoing multi–detector row CT. The 11 control subjects were part of a research protocol for noninvasive evaluation of coronary artery disease that was approved by the institutional review board at our institution. These 11 control subjects gave informed consent to participate in that study and to the use of the data for scientific research. We retrospectively analyzed the data obtained in these 11 control subjects, and our institutional review board did not require its approval or additional informed consent for that retrospective analysis.
Multi–Detector Row CT
Multi–detector row CT was performed (M.S.D., A.d.R., M.R.M.J.) 2 days prior to radiofrequency catheter ablation by using both a four–detector row 0.5-T system and a 16–detector row system (Aquilion; Toshiba Medical Systems, Otawara, Japan).
Nonionic contrast material (Xenetix 300; Guerbet, Aulnay-sous-Bois, France) was injected into the antebrachial vein (160 mL at a flow rate of 4.0 mL/sec).
In the study group, the first 19 patients were examined with a four–detector row CT scanner, and the other four patients were examined with a 16–detector row CT scanner. In the control group, three control subjects were examined with a four–detector row scanner and the other eight were examined with a 16–detector row scanner. Craniocaudal scanning (coverage length, 80–120 mm) was performed at the level of the atria by using simultaneous acquisition of four sections with a collimated section thickness of 2.0 mm (when using the four–detector row system) and 16 sections with a collimated section thickness of 0.5 mm (when using the 16–detector row system). Helical pitch was 1 mm per 0.5 second with the four–detector row scanner and 4 mm per 0.5 second with the 16–detector row scanner, rotation time was 500 msec, and tube voltage was 120 kV at 250 mA.
Scanning was performed during breath holding. A segmental reconstruction algorithm was used to allow for the inclusion of a range of heart rates among patients without the need for preoxygenation or ß-blocker therapy. Retrospective electrocardiographic gating was performed to eliminate cardiac motion artifacts. Data acquisition was completed in 20 seconds.
Data reconstruction was performed on a postprocessing workstation (Vitrea; Vital Images, Plymouth, Minn) with use of two-dimensional viewing modes and three-dimensional reconstructions.
Analysis of Multi–Detector Row CT Images
Evaluation of pulmonary vein anatomy was first performed by examining the anatomy of the pulmonary veins and their insertion into the left atrium on three-dimensional epicardial reconstructions. Thereafter, reconstructions were evaluated in three different orthogonal planes (transverse, sagittal, and coronal) or with angulated multiplanar reformatting. The number of pulmonary veins, the number of ostia, and the branching pattern of the pulmonary veins were assessed. Two experienced observers (M.S.D. and A.d.R., 6 and 25 years of experience in chest CT image interpretation, respectively) assessed the multi–detector row CT images in consensus.
The ostial insertion of the pulmonary veins was defined as either separate insertion or common ostium. To analyze insertion of the pulmonary veins into the left atrium, definitions were constructed (M.S.D., A.d.R., M.R.M.J., J.J.B., M.J.S.). Application of definitions was as follows: To determine the site of the venoatrial junction, extrapolation of the outer left atrial contour was performed in three orthogonal directions. A line following the outer left atrial contour was drawn (Fig 1) that indicated the border between the left atrium and the pulmonary veins. Pulmonary veins that either entered this "virtual" left atrial contour separately or bifurcated within a distance of less than 0.5 cm from the line were defined as having separate ostia. If the distance between the border of the left atrium and the bifurcation of the pulmonary veins was 0.5 cm or greater on images in the transverse and coronal planes, the ostium was defined as a common ostium. Also, the sagittal plane was used to confirm that both veins either entered the left atrial contour separately or united before entering the left atrium in cases of separate or common ostia, respectively. Multiplanar reformatting was used to obtain optimal insight of the ostial insertion in the three orthogonal planes. Additional pulmonary veins entered the left atrium in a separate ostium, which was located less than 0.5 cm from the virtual line that represented the left atrial contour. Early branching was defined as bifurcation of the pulmonary vein in two or more separate branches within 1 cm of their origin of the left atrium.
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Radiofrequency Catheter Ablation
The aim of radiofrequency catheter ablation was to electrically isolate all pulmonary veins from the left atrium by applying radiofrequency current circumferentially around the ostia of all pulmonary veins, according to the method described by Pappone et al (4). Ablation points were to be targeted at approximately 5 mm outside the ostium of the pulmonary veins to prevent pulmonary vein stenosis. Radiofrequency current was applied by means of a generator (Stockert-Cordis, Freiburg, Germany) at each ablation point for 30 seconds, with a maximum temperature setting at 60°C and maximum radiofrequency energy at 50 W. Prior to the procedure, the ablation strategy was planned by using data obtained with multi–detector row CT.
If a separate pulmonary venous insertion was observed on the multi–detector row CT scan, radiofrequency current was targeted to form separate circles surrounding the pulmonary venous ostia. If the distance between the border of the left atrium and the bifurcation of the pulmonary veins was greater than 0.5 cm, the ostium was defined as a common ostium because the ablation of these veins would be performed by encircling the common ostium rather than by approaching each vein independently. If a common ostium was observed, ablation points were targeted in a large circle surrounding the common ostium. All additional pulmonary veins that were observed were targeted to electrically isolate these veins from the left atrium as well.
During the procedure, the multi–detector row CT image was placed on a light panel opposite the operator and served as a road map for ablation during the procedure. Results of measurements of the pulmonary vein ostia and indexes were available during the procedure to guide the operator in targeting the pulmonary veins. A three-dimensional nonfluoroscopic mapping system (CARTO; Biosense Webster, Diamond Bar, Calif) was used to tag pulmonary veins, acquire electrophysiologic information, and mark ablation points. Tagging of the pulmonary veins was guided by using the information from the multi–detector row CT scan regarding number and insertion of pulmonary veins. All patients received heparin (activated clotting time of two to three times baseline value, checked hourly) to reduce the risk of thromboembolic complications. Electrical isolation of the pulmonary veins was confirmed by pacing from the left atrium and the pulmonary veins. The procedure was designated as successful when there was no capture after pacing in the left atrium or pulmonary veins and when the patient’s heart was in sinus rhythm after the procedure. Radiofrequency catheter ablation was performed by M.J.S., who was assisted by M.R.M.J.
Statistical Analysis
All statistical analysis was performed by E.B. Continuous data are expressed as mean values and corresponding standard deviations, and dichotomous data are expressed as numbers and percentages. Differences in baseline demographic and clinical characteristics between patient subgroups were analyzed by using the unpaired Student t test or the Fisher exact test as appropriate. The paired Student t test was applied to evaluate differences in the venous ostium indexes between right and left pulmonary veins. Finally, the McNemar test was used to evaluate the occurrence of common ostia and early branching between left and right veins. All tests were two tailed, and the threshold for statistical significance was stated at the conventional .05 probability level. Analyses were performed by using statistical software (SPSS 11.0; SPSS, Chicago, Ill).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Pulmonary Vein Anatomy
Among the 23 study patients, a total of 100 pulmonary veins were detected (per patient, 4.4 ± 0.6), whereas among the 11 control subjects, a total of 47 pulmonary veins were detected (per patient, 4.3 ± 0.5). Right pulmonary veins additional to the four main pulmonary veins were observed in six (26%) of 23 study patients and in three (27%) of 11 control subjects. One patient (of the six patients in whom additional veins were observed) had two additional right pulmonary veins. Thus, seven additional right pulmonary veins were observed in the study patient group. Five of these veins were situated between the two native right pulmonary veins and were referred to as right middle pulmonary veins. One additional right pulmonary vein was situated below the native lower right pulmonary vein. In the patient with two additional pulmonary veins, there was a large common ostium on the right side, from which two native and two additional veins originated. In one patient (4%), an additional left pulmonary vein was observed. Additional left pulmonary veins were not observed in the control subjects. Figure 2 demonstrates multi–detector row CT images obtained in a patient with normal insertion of the pulmonary veins. Multi–detector row CT images obtained in a patient with an additional right pulmonary vein are shown in Figure 3.
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Radiofrequency Catheter Ablation
Eventually, 21 patients were treated by means of radiofrequency catheter ablation for pulmonary vein isolation. Two patients were excluded because of the presence of intracardiac thrombus detected at echocardiography prior to radiofrequency catheter ablation. Targeting of radiofrequency ablation points was performed according to the information obtained with multi–detector row CT. In patients with common ostia, the radiofrequency ablation lesions were targeted around the common ostium of the pulmonary veins to achieve a circumferential line of blockage around the veins. In patients with additional pulmonary veins, ablation was aimed at isolation of these veins as well. Procedural success was achieved in 20 (95%) of 21 patients. Mean procedure time was 309 minutes ± 83, and mean fluoroscopy time was 56 minutes ± 16. No complications were observed during the first 48 hours after the procedure. After a mean of 14.3 months ± 3.8 of follow up, 14 (67%) of 21 patients had sinus rhythm.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Pulmonary Vein Anatomy
Information on the prevalence of variations in pulmonary vein anatomy is scarce. In the present study, interindividual variations in anatomy of pulmonary veins were demonstrated in both the study population and the control group. Published results from a recent study provided a useful classification system of pulmonary venous drainage patterns (15). However, lack of definitions concerning the border between the left atrium and the pulmonary veins complicates the description of the anatomy of pulmonary veins and their entrance in the left atrium. We have used extrapolation of the left atrial contour in the orthogonal two-dimensional planes as the boundary of the left atrium. The presence of a common ostium was determined according to the distance between this virtual border of the left atrium and the bifurcation of the two pulmonary veins in the transverse and coronal planes. In case of a common ostium, both veins could also be observed on the sagittal plane to unite before entering the left atrium. According to this definition, the pulmonary veins could be demonstrated to unite and enter the left atrium by means of the same ostium in 83% of patients. In 39% of patients, a common ostium of the right pulmonary veins was observed. On three-dimensional reconstructions, a large funnel-shaped entrance could often be observed.
A total of seven additional right pulmonary veins were observed in the patient group; five of these veins were positioned between the two native veins, and they were thus considered right middle pulmonary veins. An additional left pulmonary vein was observed in only one patient.
The exact prevalence of pulmonary vein anomalies has not been thoroughly investigated. Nathan and Eliakim (16) reported variations in numbers of pulmonary veins and insertion into the left atrium, although exact details were not provided. On the basis of observations in human hearts at autopsy, Ho et al (7) and Cabrera et al (17) observed common ostia in 25% of hearts, and in an additional 40%, the veins were separated only by muscle tissue less than 3 mm wide (7). In another study, an anomalous number of veins was present in 23% of examined hearts (6). Moubarak et al (18) described confluent superior and inferior veins in 25% of examined hearts, most commonly on the left side.
High-spatial-resolution imaging, such as multi–detector row CT and MR imaging, may allow more precise visualization of anatomy and may therefore depict more anomalies. This is precisely the information needed prior to radiofrequency catheter ablation to completely isolate all veins.
In particular, the right middle pulmonary vein has been associated with the occurrence of atrial fibrillation (8). This anatomic variant can be adequately delineated with multi–detector row CT, as was observed in five patients (22%) in our study.
In addition, information regarding the diameter and shape of the ostia is important for performing radiofrequency catheter ablation. For this purpose, three-dimensional imaging is needed since the diameter of the pulmonary venous wall is often oval, and two-dimensional assessment at angiography will not be sufficient. In the present study, the diameter of the pulmonary veins was measured at the ostia, and the venous ostium index was calculated from measurements performed in two perpendicular planes. Results indicated that the shape of the left pulmonary veins was more asymmetric or oval, whereas venous ostium index in the right pulmonary veins suggested that these veins were more round. These findings are in line with observations by Wittkampf et al (12), who used three-dimensional depiction of pulmonary veins with MR imaging. As also demonstrated by Wittkampf et al, three-dimensional evaluation of ostia is very important prior to radiofrequency catheter ablation since ostia can be very narrow (because of the oval shape), and this information cannot be derived with two-dimensional fluoroscopy.
Multi–Detector Row CT for Three-dimensional Depiction of Pulmonary Veins
Cardiac multi–detector row CT has been performed mostly for the evaluation of coronary artery stenoses (19,20). Multi–detector row CT for evaluation of pulmonary anatomy has recently been applied (15,21,22). Acquisition of multi–detector row CT data is performed in only 20 seconds, and all patients (including those with pacemakers, metallic objects [intracranial clips], or claustrophobia) can potentially undergo multi–detector row CT. This imaging technique allows thin sections to be obtained within a reasonable breath-hold duration. Because of the thin section thickness, advanced postprocessing techniques such as multiplanar reformatting, three-dimensional volume rendering, and maximum intensity projection can be applied for diagnostic evaluation. With the recent introduction of 16–detector row systems, the z-axis resolution can be reduced to 0.5 mm, which allows true isotropic imaging in any anatomic plane with preservation of in-plane resolution. This may result in improved diagnosis of small pulmonary vein branches, which may facilitate radiofrequency catheter ablation procedures. Finally, real-time catheter navigation in the heart performed by using three-dimensional CT images has recently been described (23).
A limitation of the use of multi–detector row CT is the radiation dose. In the current study, radiation exposure from multi–detector row CT was approximately 8 mSv. New acquisition techniques, such as electrocardiograph-dependent dose modulation or acquisition protocols that employ prospective cardiac triggering, will allow substantial dose reduction. A disadvantage of prospective triggering is that images from only one cardiac phase can be constructed, and this may cause inaccuracy of measurements of pulmonary veins in patients with irregular heart rhythms, such as in patients with atrial fibrillation.
Study Limitations
This was a descriptive study with an emphasis on the methods used to determine the atriovenous junction in order to describe variations in pulmonary venous anatomy prior to radiofrequency catheter ablation. To describe the size and shape of ostia, measurements of the diameters of the pulmonary veins were performed. Ours was a pilot study to evaluate the occurrence of different anatomic variations in a group of patients with and those without atrial fibrillation. Inferential statistics were performed with the marginal note that sample sizes may not be sufficient to draw positive conclusions. Although our results demonstrate a trend toward larger ostial diameters in the group of patients with atrial fibrillation, the small size of the studied group did not allow for a valid comparison between groups. Likewise, to determine whether our findings reflect anomalies or mere variations of normal anatomy, examination of larger patient groups both with and without atrial fibrillation is mandatory.
Electrocardiographic triggering may not be feasible in patients who have atrial fibrillation during scanning. However, the multi–detector row CT system we used allows adaptive segmentation and adaptive temporal resolution, which correct irregularities in cardiac rhythm to a certain extent by adjusting the segmentation for every heartbeat. Furthermore, retrospective electrocardiographic gating has an advantage in that reconstructions during different phases of the cardiac cycle can be obtained, which allows reconstruction of the cardiac phase that shows the minimum amount of motion artifacts.
In conclusion, multi–detector row CT allows visualization of pulmonary vein anatomy, providing a road map prior to radiofrequency catheter ablation that may be used to guide the application of radiofrequency current around pulmonary vein ostia in patients with atrial fibrillation.
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FOOTNOTES |
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Author contributions: Guarantors of integrity of entire study, M.R.M.J., J.J.B., M.J.S.; study concepts and design, all authors; literature research, M.R.M.J., M.S.D., J.J.B.; clinical studies, M.R.M.J., M.S.D., J.J.B., M.J.S.; data acquisition, M.R.M.J., M.S.D., J.J.B., M.J.S., A.d.R.; data analysis/interpretation, all authors; statistical analysis, E.B.; manuscript editing, M.R.M.J., M.S.D., J.J.B., M.J.S., E.B.; manuscript preparation, definition of intellectual content, revision/review, and final version approval, all authors
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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