Abstract Background Late-gadolinium enhanced cardiac magnetic resonance imaging (LGE-CMR) is currently used to identify non-ischemic scar. However, the distribution of non-ischemic fibrosis related to ventricular arrhythmia (VA) may vary between patients and cannot be accurately delineated by LGE-CMR. Myocardial extracellular volume fraction (ECV) is a promising tool to estimate the amount of diffuse fibrosis in non-ischemic cardiomyopathies (NICM). Novel intra-cardiac mapping derived tools, such as volume-weighted unipolar voltage (vwUV), may allow intraprocedural quantification of diffuse fibrosis. Purpose To evaluate the performance of vwUV to determine the segmental amount of non-ischemic fibrosis defined by whole-heart T1 mapping. Methods Consecutive patients with a left dominant NICM referred for ablation of VA, who underwent 3D LGE-CMR, whole-heart pre- and post-contrast T1 mapping and high density electroanatomical voltage mapping (EAVM) were included. CMR images were segmented and merged with the 3D EAVM data. The 16-segment AHA model was applied and vwUV and ECVs were determined per segment. Results Twenty-one patients (age 54 years [49-60], 18 males [87%], LVEF 47% [37-57%], LGE 7.7% (2.8-18.2%) of LV myocardium) were included and 305 segments analysed. The median ECV per patient was 30.3% (28.8-35.2%). The median segmental vwUV was 1.8mV (1.2-2.3mV). The basal septal and basal inferolateral segments tended to have higher ECV. However there was important variation between patients (figure 1). Of note, the segmental vwUV correlated well with segmental ECV (r = -0.53, p < 0.001) (figure 2). Conclusions For the first time, a segmental comparison has been performed between ECV and vwUV. There are important segmental variations of ECV between patients with a NICM referred for VA ablation. The novel intra-cardiac mapping-derived tool of vwUV can localize non-ischemic fibrosis and may facilitate substrate based ablation approaches in this challenging patient population.Figure 1.Segmental ECV valuesFigure 2.Segmental ECV and vwUV