Cardiac disease is the most common cause of natural death in the Western hemisphere. Half of cardiac deaths are sudden, arrhythmic. Myocardial infarction (MI) survivors have a four to five-fold higher risk of SCD compared to persons without a history of MI. Current risk assessment approaches fail to identify the majority of patients at risk of sudden, arrhythmic death. Given these facts, this patient population represents a relevant proportion of the total population at risk of developing sudden death and is currently not indicated for preventive therapy. The post-MI population with moderate LV function, specifically with a Left Ventricular Ejection Fraction (LVEF) between 35% and 50%, is therefore an appropriate group in which to assess SCD prevention strategies. It has been shown that an increased QRS fragmentation assessed by Magnetic Field Imaging (MFI) correlates with a higher incidence of arrhythmic deaths. MAGNITUDE is a prospective, multicenter, non-interventional, observational cohort study, which will provide detailed information about event rates and MFI threshold for the prediction of SCD/life threatening cardiac arrhythmic events. Target enrollment is 1000 patients, but enrollment may be extended until minimally 24 events are accrued (with a maximum of 1500 patients). All patients will be followed until at least 12 months after enrollment closure, but follow-up duration may be extended until minimally 40 events are accrued. Interim analysis will be performed after 10, 20 and 30 events, and also after 40 events if this milestone is reached 6 months or more before end of follow-up. Final analysis will be done at end of follow-up.