This study is being done with a radioisotope, 123I-mIBG (Adreview), to develop a nuclear diagnostic imaging test for patients with decreased heart function which can be used to predict the progression of the heart disease and provide the appropriate clinical treatment. The types of patients to be studied include patients who have had a heart attack where heart muscle may be damaged and patients diagnosed with heart failure who have enlarged hearts. Both conditions may cause poor muscle contraction and disturbances in electrical signal conduction. There will also be a control group of participants with no evidence of heart disease.