A 52-year-old man with a history of hypertension presents to the emergency department with acute onset chest pain and dyspnea. Troponin I is elevated at 2.8 ng/mL (normal <0.04). ECG shows ST elevation in leads II, III, and aVF. Echocardiography reveals hypokinesis of the inferior wall with an ejection fraction of 48%. Which investigation would be most specific for determining whether the myocardial injury is reversible or has progressed to irreversible necrosis?