Pranav Bhagirath

80 Chapter 4 Image analysis Volume The LA volume was calculated using 2 methods: 1. Using the biplane area-length method [6] as given in equation 1: 2. By segmenting the LA cavity from the CMR images using ITKSnap (www.itk- snap.org ) and calculating the volume of the shell. Thrombus The MRA was used to create 3-D reconstructions of the LAA to evaluate the presence of thrombus. Sphericity and scar-map The LA sphericity, a measure of geometrical remodeling, was computed using custom- made software. The algorithmused for computing LA sphericity was recently introduced and validated by Bisbal et al [11] . The LA cavity segmentation for volumetric analysis was represented as a triangular mesh. The center-of-mass of LA was then computed from the area-weighted centers of each triangle in the mesh. The best-fit sphere for the LA could then be computed using the distances between the center-of-mass and each triangle in the mesh. The mean of these individual distances provided the radius (µ) of the sphere of best-fit. The standard deviation (σ) of these distances was also calculated. LA sphericity was then simply calculated as a ratio: 1 - σ/ µ. A LA scar-map was generated in patients undergoing redo-ablation, using a semi- automated 3-D method which has been described in detail previously [12] . The LA reconstruction created for the volumetric analysiswas fusedwith the LGE images.Vectors at a normal to the cardiac surface were created using the visual-toolkit libraries (VTK, Kitware, Inc., NY). A maximum intensity projection of the LGE images was performed at ±3 mm along each normal vector. The electrophysiologists were blinded to the sphericity and scar-map results.