A 60-year-old man presents with sudden severe tearing chest pain radiating to the back between the scapulae. He is hypertensive, with BP 200/110 mmHg in the right arm and 160/90 mmHg in the left. There is an early diastolic murmur at the left sternal edge. ECG shows left ventricular hypertrophy only. What is the most appropriate initial imaging investigation?