A 56-year-old man with long-standing hypertension and hyperlipidaemia presents with sudden onset of severe tearing chest pain radiating to the back. Blood pressure is 190/110 mmHg and there is a difference of 20 mmHg between arms. ECG is non-specific. According to Australian emergency guidelines, what is the most appropriate initial imaging modality to confirm the suspected diagnosis of aortic dissection in a haemodynamically stable patient?