LA Image Segmentation:
Left atrial wall was manually segmented by expert observers from the LGE‐MRI images using the Corview image processing software (MARREK, Salt Lake City, UT). Briefly, the protocol for segmentation proceeded as follows: first, the endocardial border of the left atrium was defined, including the PV extensions, by manually tracing the left atrium‐PV blood pool in each slice of the LGE‐MRI volume. Next, the endocardial segmentation was morphologically dilated and then manually adjusted to create an assessment of the boundary of the epicardial left atrium surface. Finally, the endocardial segmentation was subtracted from the epicardial segmentation to define a wall segmentation, which was manually edited to exclude the mitral valve and PVs. The resulting left atrial wall segmentation included the 3D extent of both the left atrial wall and the antral regions of the PVs. Images that could not be segmented or quantified due to poor contrast, blurred left atrial boundaries, and presence of artifacts in left atrial region were defined as poor quality.