A 37-year-old woman presents with a 2-year history of progressive enlargement of her hands and feet, coarse facial features, and joint pain. She also reports increased sweating and occasional headaches. Physical exam confirms widened fingers, prognathism, and soft tissue swelling. Fasting blood work shows an elevated insulin-like growth factor 1 (IGF-1) level. Which of the following is the most appropriate next step to confirm the diagnosis?