A 38-year-old woman presents to the emergency department with acute onset epigastric pain radiating to the back, associated with nausea and vomiting for 6 hours. She has a history of gallstones. On examination, she is febrile (38.5°C) with epigastric tenderness. Serum amylase is elevated at 450 U/L (normal <100). What is the investigation of choice to confirm acute pancreatitis and assess severity?