A 75.year old woman with chronic atrial fibrillation presented to the hospital with a 2-day of colicky abdominal pain. The physical examination revealed hypoactive bowel sounds and diffuse abdominal tenderness. Laboratory tests showed a white cell count of 19,400 per cubic millimeter with 92% neutrophils, a blood urea nitrogen level of 42 mg per deciliter (15 gmol per liter) and a serum creatinine level of 3.0 mg per deciliter (267 gmol per liter). The investigation of choice would be