A 32-year-old man presents to the emergency department 45 minutes after a motor vehicle collision with blunt abdominal trauma. He is haemodynamically stable (BP 128/82 mmHg, HR 92/min). On examination, he has mild left-sided abdominal tenderness but no peritoneal signs. His haemoglobin is 13.2 g/dL. Which is the investigation of choice to evaluate for intra-abdominal injury?