Forty eight hours after sustaining a blunt abdominal injury, a 15 year old boy presents with hematuria and pain in the left side of abdomen. On examination, he has a pulse rate of 96/ minute with a BP of 110/70 mmHg. His Hb is 10-8 gm% with a PCV of 31%. Abdominal examination revealed tenderness in left lumbar region but no palpable mass. The most appropriate investigation to diagnose and find the extent of renal injury would be-