We evaluated 44 patients who underwent catheter ablation for AF retrospectively. All patients received an MRI prior to the intervention which included 4 chamber cine sequences covering the whole heart with a slice thickness of 6 mm each and a temporal resolution of 40 ms.

The freely-available Segment Software^A was used for segmentation of the left atrium: the contours of the left atrium were manually drawn on each slice and at every time point (Fig. 1). The pulmonary veins and the left atrial appendage were excluded. Also, we defined the plane of the mitral valve as border line of the LA.

Once the whole LA was fully segmented, we fitted an ellipsoid into the contours of the LA using a custom-written algorithm^B (Fig. 2). Thereby, main axis diameter, volume, surface area, sphericity and their respective rates (delta over time) could be extracted for further evaluation.

The beginning of the left atrial contraction phase was defined as pre-atrial contraction (V_pre, Fig. 3).

Minimum follow-up for AF recurrence was 12 months after last procedure (max. 2 procedures per patient).