Abstract Background The decision to perform a high-risk endo-epicardial approach for VT ablation is controversial and mostly based on previous endocardial failure or very specific etiologies. We systematically evaluated the 3D distribution of the automatically-detected conducting channels by CMR with the aim of assessing whether tissue thickness in those areas could allow for an endocardial-only approach independently of the etiology of the cardiomyopathy. This could have an important impact in terms of procedural planning. Purpose To study CMR channel depth as a predictor of VT recurrence after endocardial VT ablation. Methods Fifty-one consecutive patients with left scar-related VT undergoing ablation after CMR (October 2018 to June 2022) were included (median age 65.56±11.46 years, 94.1% male, median ejection fraction 31.82±8.76%, ischemic cardiomyopathy 72.5%). CMR channels were calculated with ADAS3D software. The depth of CMR channels was assessed based on the involved layers and the total wall thickness (WT) of the affected segment. Patients were prospectively followed for VT recurrence for one year. Results One-year VT recurrence was 13.7%. Overall, 159 CMR channels were analyzed (3.10±2.01 channels per patient). Univariate analysis showed that both the maximal depth of channel and the presence of an epicardial channel were the only predictors of VT recurrence during follow-up (OR 1.85 (1.11, 3.13), p=0.02 and OR 1.22 (1.05, 1.41), p=0.04 respectively). The presence of a channel with a maximal depth of more than 7.2mm from the endocardium had a sensitivity= 100%, specificity= 61.36%, negative predictive value (NPV)= 100% and positive predictive value (PPV)=29.0% with an area under the curve of 0.81 for VT recurrence. The presence of an epicardial channel showed, as well, sensitivity=100% and NPV=100% but a worse specificity (27.27%) and PPV (17.95%) compared with maximal depth of the channel. Non-ischemic cardiomyopathy was not significantly associated with a higher risk of VT recurrence after endocardial ablation in our study (OR 2.25 (0.43, 11.66), p 0.34). Conclusions The maximal depth of CMR channels is a predictor of VT recurrence in the follow up in patients with left scar-related VT who underwent endocardial ablation. A cutoff of maximal CMR channel depth of 7.2mm could be a better tool to select VT ablation approach than considering the type of cardiomyopathy or the presence of epicardial substrate.Examples of CMR channel depthProbability of VT recurrence