A 62-year-old man with a history of colorectal cancer presents to the emergency department with acute onset abdominal distension, vomiting, and absolute constipation for 36 hours. On examination, he has a distended, tympanic abdomen with reduced bowel sounds. Plain abdominal radiograph shows dilated small bowel loops with air-fluid levels and a transition zone in the left lower abdomen. What is the investigation of choice to confirm the diagnosis and guide further management?