Pranav Bhagirath

94 Chapter 5 The standard definitions for paroxysmal (self-terminating episodes of AF lasting 7 days) and persistent AF (AF sustained 7 days and/or requiring electrical or pharmacological cardioversion) were used. Imaging protocol All patients had undergone an electrocardiogram (ECG) gated contrast enhanced computed tomography (CT) examination (Siemens Somatom Flash Definition) 24-48 hours prior to the index ablation procedure to identify pulmonary vein (PV) anatomy and to exclude left atrial appendage (LAA) thrombus. A second CT examwas performed 6 months following the index procedure to evaluate PV stenosis. Scanning was prospectively triggered at 25% of the R-R interval on ECG. Images were acquired from the supra-aortic region till upper abdomen. Seventy five ml of contrast agent (Xenetix 350, Guerbet, France) was injected in all patients. Image post-processing and analysis Volume The LA volume was obtained from the CT images using the volume and statistics module of ITK-Snap (www.itk-snap.org ) and indexed to the BSA, calculated according to the Mosteller formula (13) . Prior to using this volume module, a semi-automatic segmentation of the LA cavity was performed. Briefly, the CT images were loaded in ITK-Snap and the region of interest (ROI) was defined, ensuring that the PV’s and LAA were completely encompassed. Seed points were then selected within the LA and PV allowing a built-in region-growing algorithm to segment the LA cavity. The generated segmentation was sub-classified manually by labeling the right and left PV’s. The LAA was grouped as part of the left PV’s. The resulting LA cavity segmentation was exported for further analysis. Avisual representationof the segmentationprocess is provided in video 1 (supplemental material). Sphericity The LASP, represented as a percentage, was computed using custom-made software. The applied algorithm used for computing LASP is recently proposed and validated by Bisbal et al (12) . Briefly, the LA cavity segmentation for volumetric analysis was represented as a triangular mesh. The center-of-mass of LA was then computed from