A 67-year-old woman presents complaining of dark urine. Over several months she has had increased fatigue and weight loss. There has not been any fever or night sweats. On physical examination, she has scleral icterus and mild jaundice. Her abdomen is soft, non-distended with a palpable mass in the right upper quadrant that is non-tender. Her laboratory values are notable for a total bilirubin of 6.4 mg/dl and a direct bilirubin of 5.0 mg/dl. What is your imaging study of choice