A 42-year-old male has experienced recurrent substernal pain for 1 year. The episodes are unrelated to exertion but often occur during brisk walking in cold wind or at 5 AM. Pain is relieved by nitroglycerin. Baseline ECG shows horizontal ST-segment depression of 0.75 mm in leads II, III, and aVF. During episodes, the ECG is normal. On the second day of hospitalization, the patient develops acute myocardial infarction (AMI). Which of the following is most valuable for assessing infarct size?