A 62-year-old woman presents 48 hours after the onset of chest pain with dyspnea. ECG shows pathological Q waves in leads V1–V3. Troponin I is markedly elevated at 8.5 ng/mL (normal <0.04). Which investigation is most appropriate to assess the extent of myocardial necrosis and identify complications such as ventricular septal defect or papillary muscle rupture?