Myocardial infarction (MI) or heart attack, is localized based on the specific area of the heart muscle deprived of blood flow and oxygen, typically due to a blockage in a coronary artery. The location of the MI is determined using the 12-lead electrocardiogram (ECG), which reveals characteristic changes in ST segments and Q waves, and by analyzing the patient's symptoms, such as chest pain, and other diagnostic tests like blood tests and echocardiograms.