719 Three-Dimensional Anatomy of the Left Atrium by Magnetic Resonance Angiography: Implications for Catheter Ablation for Atrial Fibrillation MOUSSA MANSOUR, M.D., ∗ MARWAN REFAAT, M.D.,‡ EDWIN KEVIN HEIST, M.D., PH.D., ∗ THEOFANIE MELA, M.D., ∗ RICARDO CURY, M.D.,† GODTFRED HOLMVANG, M.D., ∗ and JEREMY N. RUSKIN, M.D. ∗ From the ∗ Division of Cardiology, †Departments of Radiology and ‡Internal Medicine, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts, USA 3D Anatomy of the Left Atrium. Background: Pulmonary vein isolation (PVI) has become one of the primary treatments for symptomatic drug-refractory atrial fibrillation (AF). During this procedure, delivery of ablation lesions to certain regions of the left atrium can be technically challenging. Among the most challenging regions are the ridges separating the left pulmonary veins (LPV) from the left atrial appendage (LAA), and the right middle pulmonary vein (RMPV) from the right superior (RSPV) and right inferior (RIPV) pulmonary veins. A detailed anatomical characterization of these regions has not been previously reported. Methods: Magnetic resonance angiography (MRA) was performed in patients prior to undergoing PVI. Fifty consecutive patients with a RMPV identified by MRA were included in this study. Ridges associated with the left pulmonary veins were examined in an additional 30 patients who did not have a RMPV. En- doluminal views were reconstructed from the gadolinium-enhanced, breath-hold three-dimensional MRA data sets. Measurements were performed using electronic calipers. Results: The width of the ridge separating the LPV from the LAA was found to be 3.7 ± 1.1 mm at its narrowest point. The segment of this ridge with a width of 5 mm or less was 16.6 ± 6.4 mm long. The width of the ridges separating the RMPV from the RSPV and the RIPV was found to be 3.0 ±1.5 mm and 3.1 ±1.8 mm, respectively. There were no significant differences between LPV ridges for patients with versus without a RMPV. Conclusion: The width of the ridges of atrial tissue separating LPV from the LAA and the RMPV from its neighboring veins may explain the technical challenge in obtaining stable catheter positions in these areas. A detailed assessment of the anatomy of these regions may improve the safety and efficacy of catheter ablation at these sites. (J Cardiovasc Electrophysiol, Vol. 17, pp. 719-723, July 2006) atrial fibrillation, pulmonary veins, left atrial appendage, catheter ablation Introduction Catheter ablation has become one of the primary treat- ments for patients with symptomatic, drug-refractory atrial fibrillation (AF). 1 The most commonly performed ablation technique is circumferential left atrial ablation, with 2 or with- out 3,4 pulmonary vein isolation. During this procedure, ra- diofrequency energy lesions are applied in a circular fash- ion around the pulmonary vein ostia. The pulmonary veins exhibit a complex anatomy with significant inter- and intra- patient variability in size, shape, bifurcation, and branching pattern. 5-10 Application of ablation lesions in certain areas of the left atrium can be technically challenging. Among Dr. Heist was supported, in part, by a Clinical Investigation Training Grant, awarded by Harvard/MIT, with funding from Pfizer and Merck. Address for correspondence: Moussa Mansour, M.D., Cardiac Arrhythmia Unit, Massachusetts General Hospital, Gray 109, 55 Fruit Street, Boston, MA 02114. Fax: 617-724-1241; E-mail: mmansour@partners.org Manuscript received 19 December 2005; Revised manuscript received 16 January 2006; Accepted for publication 8 February 2006. doi: 10.1111/j.1540-8167.2006.00491.x the most challenging regions are the ridge between the left pulmonary veins (LPV) and the left atrial appendage as well as the ridges separating the right middle pulmonary vein (RMPV) from the right superior (RSPV) and the right in- ferior (RIPV) pulmonary veins. The inability to obtain stable catheter positions along these ridges may result in both re- duced procedural success due to incomplete ablation lines and in greater procedural risk due to inadvertent application of ablation lesions within the pulmonary veins, potentially resulting in pulmonary vein stenosis. Many imaging modalities are currently used to delineate the anatomy of the left atrium and the pulmonary veins prior to and during the ablation procedure. These include three dimensional (3D) magnetic resonance angiography (MRA), 6,8-10 computed tomography (CT), 5,7 and intracar- diac ultrasound. 11 MRA has the advantage of not only being free of ionizing radiation and nephro-toxic contrast, but has also been shown to be particularly effective in providing de- tailed and complete imaging of the complex anatomy of the left atrium. 12 The diameter, angulation, and branching pat- terns of the pulmonary veins have been previously described in studies from our group 8 and others. 5-7,9,10 The objective of this study was to analyze by 3D MRA the anatomy of the left atrium and the pulmonary veins in patients undergoing