A 52-year-old woman presents with bone pain, fatigue, and polyuria. Serum calcium is 11.2 mg/dL (normal 8.5–10.5), serum phosphate is 2.8 mg/dL (normal 2.5–4.5), and alkaline phosphatase is elevated. PTH level is 85 pg/mL (normal 15–65). Which investigation is most appropriate to confirm the diagnosis of primary hyperparathyroidism?