A 52-year-old man with a history of papillary thyroid carcinoma (PTC) underwent total thyroidectomy and radioactive iodine (RAI) ablation 6 months ago. He now presents with a rising serum thyroglobulin level (3.2 ng/mL, normal <0.5) despite negative whole-body iodine-131 scintigraphy and negative neck ultrasound. Which of the following is the most appropriate next investigation to localize the site of recurrence?