A 32-year-old woman presents with acute onset severe right upper quadrant pain and vomiting for 6 hours. She has a history of multiple episodes of similar pain over the past 2 years. On examination, she is febrile (38.2°C), tachycardic, and has positive Murphy's sign. Serum bilirubin is 1.8 mg/dL, ALT 120 IU/L, and WBC 14,500/μL. What is the imaging modality of choice to confirm the diagnosis?