Inclusion Criteria:  Heart failure with LV ejection fraction ≤ 35%; New York Heart Association class II, III, or IV (ambulatory); Optimal medical treatment that is tolerable; Left bundle branch block (LBBB) and QRS ≥ 130 ms, OR non-LBBB and QRS ≥ 150 ms.  Exclusion Criteria:  Pregnancy or lactation; Subjects with impaired renal function (severe renal insufficiency, GFR < 30 ml/min/1.73m2); Atrial fibrillation or atrial fibrillation during MRI Documented allergic reaction to gadolinium; Impossibility to undergo an MRI scan; Participation in another clinical study that prohibits any procedures other than standard.