A 58-year-old man with a history of hypertension and type 2 diabetes presents with sudden onset severe tearing chest pain radiating to his back. On examination, his blood pressure is 190/110 mmHg in the right arm and 160/90 mmHg in the left arm. A diastolic murmur is heard. An ECG shows sinus tachycardia with left ventricular hypertrophy. What is the most important initial investigation to confirm the suspected diagnosis?