You are seeing an 80-year-old male in A & E who has presented with mild flank pain. His observations are: BP 120/80, HR 78, O2 saturation 99% on room air, temperature 37.2 degrees and RR 13. He has several risk factors for an AAA including COPD and a family history on his father’s side. You notice on examination that he has a large pulsatile mass in the centre of his abdomen when you completed your palpation. What is the first line imaging modality you would request in order to rule the given diagnosis?