ELECTROPHYSIOLOGY
Determination of the spatial orientation and shape of pulmonary vein ostia by contrast-enhanced magnetic resonance angiography
Department of Cardiology, Catharina Hospital PO Box 1350, 5602 ZA Eindhoven The Netherlands ; Department of Radiology, Catharina Hospital Eindhoven The Netherlands
Manuscript submitted 11 January 2005. Accepted after revision 9 September 2005.
Corresponding author. Tel: +31 40 2397004; fax: +31 40 2447885. E-mail address: pepijn.vd.voort{at}catharina-ziekenhuis.nl
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Abstract |
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Aims For catheter ablation of atrial fibrillation (AF), proper catheter positioning is crucial and depends on knowledge of pulmonary vein (PV) anatomy. The aim of this study was to assess PV spatial orientation and ostial shape by contrast-enhanced magnetic resonance angiography (CE-MRA).
Methods and results In 30 consecutive AF patients, CE-MRA was performed prior to ostial ablation. Using a centre-line technique, the PV ostium was defined perpendicular to this centre-line. Minimal and maximal ostial diameters, ostial perimeter, and angles in the anatomical frontal and transverse planes were measured. Twenty-one patients had four separate PVs. In four patients, there was a distinct right-middle PV and in five a common left common PV was found. Left-sided PV ostia were smaller and more elliptical than right-sided PVs. In the transverse plane, the ostia of both superior PVs were directed anteriorly (LS –15±13°, RS –13±11°) and both inferior PV ostia were directed posteriorly (LI 23±15°, RI 39±15°). In the frontal plane, both superior PV ostia pointed upwards (LS –27±14°, RS –33±12°) while the inferior ostia were directed horizontally (LI 2±11°, RI 3±13°).
Conclusion PV ostial shape and spatial orientation are variable and can be visualized adequately by CE-MRA.
Key Words: Magnetic resonance imaging, Atrial fibrillation, Catheter ablation, Pulmonary veins
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Introduction |
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Catheter ablation of the pulmonary veins (PVs) is a relatively new and successful treatment for atrial fibrillation (AF).1
–3 Electrical isolation of a PV is achieved by circumferential or segmental application of RF lesions guided by a multipolar circular catheter.2 Proper positioning of the ablation catheter and a multipolar circular catheter are required to allow optimal interpretation of signals and to prevent PV narrowing.4,5
Contrast-enhanced magnetic resonance angiography (CE-MRA) can be used to assess PV anatomy.6–8 During the ablation procedure, positioning of the catheters is usually guided by PV contrast angiography9,10 or intracardiac echocardiography.11 However, for optimal positioning of a multipolar circular catheter, not only the PV ostial shape and insertion site should be known but also knowledge of the spatial orientation in both the transverse and frontal planes can be used for selecting the most appropriate angulation for fluoroscopy to prevent oblique positioning of the catheter.
The aim of this study was to describe the spatial orientation and dimensions of the PV ostia as measured during CE-MRA using a ‘centre-line’ technique.
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Methods |
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Patients
In 30 consecutive patients with paroxysmal or persistent drug refractory AF who were scheduled for segmental ostial ablation of the PVs, Gd-enhanced contrast MRA of the left atrium (LA) and PVs was performed prior to PV ablation. Patient characteristics are shown in Table 1.
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Magnetic resonance angiography
All examinations were performed on a Gyroscan Intera 1.5 T scanner (software release 9.1; Philips Medical Systems, Best, The Netherlands) using a five-element cardiac surface coil. Patients were placed in a supine position. After the acquisition of a bolus timing scan, a 3D CE-MRA of the LA and PVs was obtained. The following parameters in the 3D T1-weighted fast field echo sequence (3D FFE) were used: repetition time (TR) 4.4 ms, echo time (TE) 1.42 ms, and flip angle 25°. The acquisition voxel size was 1.12×1.12×3.0 mm3, the calculated voxel size was 0.84×0.84×1.50 m3. The 3D data set was obtained in 17 s. Maximum intensity projections were calculated from the 3D data sets on a remote, dedicated workstation (Easy Vision; Philips Medical Systems, Best, The Netherlands).
Anatomy was assessed in transverse, frontal, and sagittal planes. A centre-line was constructed by selecting a number of points exactly in the middle of a PV in each of the three orthogonal planes (Figure 1). After construction of this centre-line, slices perpendicular to this centre-line were reviewed, from distal to proximal in steps of 1 mm, and the ostial plane was defined as the most proximal image in which the PV was still separate from the LA. In case this was not clear, we took the intersection between lines through the PV wall and LA posterior wall for determination of the ostium. In this ostial slice, the maximal (Dmax) and minimal (Dmin) diameters were measured by calipers and the perimeter of the ostium was tracked manually (Figure 1). In the transverse and frontal view, the angle of the ostial plane, respectively, to the anterior–posterior axis and the left–right axis were measured (Figure 2). All measurements were accepted after reaching consensus by two of the investigators.
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Statistical analysis
Data are expressed as mean±SD. One-way ANOVA was used for group analyses, with Bonferroni's post-test for comparison between the groups. A P-value <0.05 was considered significant.
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Results |
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Magnetic resonance angiography
In the majority of patients (n=21, 70%), there were two separate PVs at both left and right sides of the LA. In four patients (13%), a distinct right-middle (RM) PV was present. In five patients (17%), the left PVs joined before entering the LA, resulting in a left common (LC) PV; however, in three of these five patients, an additional 3D reconstruction was necessary to distinguish between a common ostium and two separate veins. No other anatomical variations were seen. Using CE-MRA, 114 of 119 PVs (96%) could adequately be visualized; in five PVs, the image quality was poor and could not be analysed. An example of the construction of the centre-line and measurements of the PV dimensions is shown in Figure 1. Figure 2 shows the determination of the angles in the transverse and frontal planes.
Ostial size and shape
Data are given in Table 2 and shown in Figure 3. In general, the perimeters of the superior PV ostia were larger than the inferior PV ostia, although this difference was significant only between LS and LI (P<0.01). The ostial perimeters of the left PVs were significantly smaller than the right-sided PV ostia: left superior (LS) 61±7 mm vs. right superior (RS) 72±12 mm (P<0.001) and left inferior (LI) 50±7 mm vs. right inferior (RI) 68±9 mm (P<0.001) (Figure 3, left panel). The ostial size of a RM PV was significantly smaller (47±3 mm)compared with both RS and RI (P<0.001) while a common left PV was larger than both left-sided PVs (85±17 mm, P<0.01). In Table 2, we calculated the optimal diameter (OD) for a circular multipolar diagnostic catheter based on the ostial perimeter.
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Also the shape of the ostia, elliptical shape expressed as (Dmax/Dmin), was different between right and left-sided PVs (Table 2 and Figure 3, right panel). For RS and RI, values were 1.7±0.5 and 1.8±0.4, respectively (P=NS) and for LS and LI, 1.3±0.1 and 1.3±0.3 (P=NS) (P<0.001 for the differences between right and left-sided PVs). If present, a LC PV has a pronounced elliptical shape (Dmax/Dmin=2.3±0.6; P<0.01 vs. LI and LS).
Spatial orientation of the ostia
In the transverse plane, both superior PVs were directed slightly anteriorly (LS –15±13°, RS –13±11°) while both inferior ostia had a clear posterior direction (LI 23±15°, RI 39±15°) (Figure 4, left panel). In the frontal plane, both superior veins pointed upward (LS –27±14°, RS –33±12°) and the two inferior veins had a nearly horizontal direction (LI 2±11°, RI 3±13°) (Figure 4, right panel). However, for each PV there was a rather wide interindividual range of directions. This interindividual variation in spatial orientation was even larger for the RM and LC PVs (Table 3). In Table 3 we show the fluoroscopic angulations for catheter positioning in the ostial plane based on mean transverse orientation.
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Discussion |
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PV anatomy
In the majority of patients in our series (70%), four separate PVs are present, two on each side. However, in a considerable number of patients, the anatomy was different: either there was a separate RM PV or both left veins had a common ostium. These results are in concordance with prior studies, although the reported incidence of these anatomic variants has a very wide range. In some reports, both from a pathological12
or radiological10 background, the incidence of anatomic variations is zero, while in other series the incidence is as high as 38%.7 There may be several explanations for these differences. On one hand, the studied patient groups may be different, for instance normal patients vs. AF patients; also ethnic or racial differences may lead to divergent results. On the other hand, the observed anatomical patterns are not very distinctive and its judgement may be very subjective, leading to differences between investigators. In particular, interpretation of 2D cross-sections can be difficult. In three patients of our group, an additional 3D reconstruction was required to differentiate between a common left ostium and two separate left PVs.
Size and shape of the PV ostia
In our series, the perimeters of the left PV ostia are generally smaller than the right PV ostia. Instead of measuring PV ostia size in only one direction or taking the maximum ostial diameter, we chose to measure the ostial circumference. Because the shape of particularly the left PV ostia is not circular but elliptical,6,13 the choice of a circular multipolar catheter should be more dependent on this perimeter than just the maximum ostial size. In case of an elliptically shaped ostium and a circular multipolar catheter, several possible interactions might occur. First, and most likely, if the compliance of the PV ostium is higher than the catheter stiffness, the ostium will be distorted and will become circular. In this situation, measurement of perimeter is more important than just the maximal diameter. Second, when ostial compliance and catheter stiffness are equal, the catheter will not fit in the ostium and will turn obliquely. Indeed, in our experience, it is sometimes not possible to place the catheter in the preferred position. Third, the circular catheter might be flexible enough to adapt to the shape of the ostium; however, most presently available catheters are rather stiff. These findings are in accordance with previous studies6,13 and this may have implications for future catheter design.
Most prior studies have reported only one diameter of the PV ostia and these mean diameters range widely, from 10 mm14 to 22 mm.13 When compared with these previous studies,6–10,12–17 maximal diameters are larger in our study. Differences between reports probably reflect various definitions of the level of the ostium and differences between imaging techniques and post-mortem analyses. As the veno-atrial junction is never a right angle, there is always a transition zone, which may be short and symmetrical, but can be large and asymmetrical. Angiographically, the level of the ostium probably will be determined at the border of PV lumen and transition zone where contrast density is maximal. In tomographic studies, generally, the intersection of PV line and LA line is used for definition of the ostium,7,8,10,13,15 and this level may be more proximal. Definition of an ostium is even more difficult and subjective in case a PV enters the LA obliquely. It is not known which level of LA–PV junction will correspond to an electrophysiological or histological border between the PV and the posterior wall of the LA, if any distinction can be made at all. For measurements of the ostium, we used the most proximal perpendicular image where the PV was distinct from the LA. In some cases, we might have judged the veno-atrial junction too proximal and this may have lead to some overestimation of the ostial size, compared with prior reports.6–10,12,14–17
Spatial orientation
The spatial pattern of orientation of the PV ostia is rather constant, although a substantial interindividual variation is observed.17 Knowledge of this spatial orientation is helpful for positioning of a multipolar circular catheter in the PV ostium. In case the circular catheter is placed obliquely in the PV, radiofrequency energy will be applied inside the PV, increasing the probability of PV stenosis.4,5 On the contrary, an obliquely placed catheter that is partly in the LA may display LA signals masquerading as PV potentials, and in this case isolation of the vein will be difficult.
There is only one prior study describing the spatial orientation of PV ostia.17 In this report, however, only the orientation in the longitudinal axis of the LA was measured. The relation of this longitudinal LA axis to the body axes was not determined; translation of these angles into practical values is difficult. We believe that for daily clinical practice, with the use of fluoroscopy, our measurements in both frontal and transverse planes may be more useful.
Limitations
We did not compare between measurements from CE-MRA and other techniques such as fluoroscopy or intracardiac echocardiography. These imaging techniques may have resulted in slightly different results. Also, measurements from 3D reconstructions might have given different data. Alternatively, CT might be used, leading to increased radiation exposure. Therefore, in our opinion, CE-MRA is a first-line imaging technique that allows optimal planning of the procedure.
Clinical implications
For PV isolation by catheter ablation, CE-MRA of the LA and the PVs provides indispensable information regarding pulmonary venous anatomy, ostial dimensions and shape, and spatial orientation of the PV ostia. This can be of great help for catheter selection and optimal catheter positioning, facilitating the verification of PV isolation and thereby improving the procedural results.
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