A 62-year-old man with a history of long-standing poorly controlled hypertension presents to the emergency department with sudden onset of severe “tearing” chest pain that radiates to his back. He is diaphoretic and anxious. Vital signs: T 36.8°C (98.2°F), BP 198/112 mm Hg in the right arm and 172/96 mm Hg in the left arm, HR 112/min, RR 22/min, SpO2 97% on room air. Physical examination reveals cool extremities and a new early diastolic murmur at the left sternal border. Chest X-ray shows a widened mediastinum. While being evaluated, he becomes acutely hypotensive and confused. He has a known severe iodinated contrast allergy and his creatinine is 3.1 mg/dL (baseline 2.8 mg/dL). Which of the following is the most appropriate diagnostic test to confirm the suspected diagnosis at this time?