A 32-year-old male presents to the emergency department 20 minutes after a motor vehicle collision with blunt abdominal trauma. He is haemodynamically stable (BP 128/82, HR 92), alert, and complaining of left-sided abdominal pain. Physical examination reveals tenderness in the left upper quadrant with mild guarding. Which is the investigation of choice to assess for intra-abdominal injury in this haemodynamically stable patient?