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Introduction: Catheter-myocardium contact force (CF) is a major determinant of ablation lesion generation, with higher CF associated with greater energy transfer. The effect of contact force on atrial lesion formation has not been characterized in an in vivo model. Acute and chronic atrial injury can be visualized with T2-weighted (T2W) and late gadolinium enhancement (LGE) cardiac magnetic resonance (CMR) and in combination with voltage mapping, histology and ex vivo MR provides the necessary tools for detailed ablation lesion characterization. We investigated the effect of contact force on lesion characteristics, including transmurality, acutely and 2 months after atrial ablation. Methods: Under general anaesthesia, femoral venous access was obtained in eight Gottingen minipigs. In two animals, pre-ablation CMR imaging was performed. Using an electro-anatomical mapping system (Carto3, Biosense Webster) the geometry of the right atrium was obtained with a 20-pole circular mapping catheter (Lasso Nav, D curve, Biosense Webster). An intercaval linear ablation lesion was created using a CF-sensing catheter (SmartTouch, Biosense Webster, 30W, 48C, 20ml/min irrigation). Target contact forces of >20g (high force) or

Introduction: Catheter-myocardium contact force (CF) is an important determinant of radiofrequency (RF) lesion generation. Novel RF-catheters enable real-time monitoring of CF, but evidence that CF is the primary determinant of lesion quality is lacking in humans. We used late gadolinium enhancement (LGE) cardiac magnetic resonance (CMR) to assess the relationship between applied contact force and chronic scar formation. Method: Twelve patients underwent pulmonary vein isolation using a CF-sensing catheter (SmartTouch Thermocool, Biosense-Webster, CA, USA). Ablation position was recorded using the CARTO-3 electroanatomical mapping system. LGE CMR was performed at a median of 7.2 (range 3-11) months post- ablation using an Achieva 3.0T MRI scanner (Philips, Amsterdam, The Netherlands) with a 3D ECG triggered, free-breathing inversion recovery turbo field echo scan sequence. The LGE-CMR images were segmented using Seg3D by an experienced observer. Signal intensities (SI) were projected onto the segmented shell, using a maximum intensity projection and fused to the CARTO-3 shell using an Iterative Closest Point (ICP) algorithm. The two shells (Contact Force and LGE signal) were then compared using custom built software. The shells were thresholded to blind the observer to local variations in LGE SI and CF before paired paths along the ablation lines were manually drawn on the surface of each shell. Results: For 1710 ablation points across 12 patients, there is a small but significant relationship between applied CF and chronic scar formation as indicated by CMR LGE SI (p