A 62-year-old man with a 20-year history of poorly controlled hypertension presents to the emergency department with sudden-onset, tearing chest pain that radiates to his back. He is diaphoretic and anxious. BP is 198/112 mm Hg in the right arm and 176/98 mm Hg in the left arm; pulse is 112/min. On exam, the right radial pulse is weaker than the left. A chest X-ray demonstrates a widened mediastinum. He is hemodynamically stable and has normal renal function (creatinine 0.9 mg/dL). After initial analgesia and BP control, which of the following is the most appropriate diagnostic study to confirm the suspected diagnosis?