A 54-year-old man has 1-hour history of chest pain and shortness of breath. He had been hospitalized one week ago for a colectomy for colon cancer. On examination BP is 110/60 mmHg, heart rate is 115/min, respiratory rate 24/min, and oxygen saturation is 89% on room air and 97% on oxygen. The lungs are clear. Chest X ray is negative for infiltrates, ECG shows sinus tachycardia Which of the following is the most appropriate study to confirm the diagnosis in this patient?