Non-invasive imaging of electrical activation requires high-density body surface potential mapping. The nine electrodes of the 12-lead electrocardiogram (ECG) are insufficient for a reliable reconstruction with standard inverse methods. Patient-specific modelling may offer an alternative route to physiologically constraint the reconstruction. The aim of the study was to assess the feasibility of reconstructing the fully 3D electrical activation map of the ventricles from the 12-lead ECG and cardiovascular magnetic resonance (CMR).Ventricular activation was estimated by iteratively optimizing the parameters (conduction velocity and sites of earliestactivation) of a patient-specific model to fit the simulated to the recorded ECG. Chest and cardiac anatomy of11 patients (QRS duration 126–180 ms, documented scar in two) were segmented from CMR images. Scar presencewas assessed by magnetic resonance (MR) contrast enhancement. Activation sequences were modelled witha physiologically based propagation model and ECGs with lead field theory. Validation was performed by comparingreconstructed activation maps with those acquired by invasive electroanatomical mapping of coronary sinus/veins (CS) and right ventricular (RV) and left ventricular (LV) endocardium. The QRS complex was correctlyreproduced by the model (Pearson’s correlation r = 0.923). Reconstructions accurately located the earliest and latestactivated LV regions (median barycentre distance 8.2 mm, IQR 8.8 mm). Correlation of simulated with recordedactivation time was very good at LV endocardium (r = 0.83) and good at CS (r = 0.68) and RV endocardium(r = 0.58).Non-invasive assessment of biventricular 3D activation using the 12-lead ECG and MR imaging is feasible. Potentialapplications include patient-specific modelling and pre-/per-procedural evaluation of ventricular activation.< Réduire