A 48-year-old woman with a history of Hashimoto's thyroiditis presents with progressive hypothyroidism despite increasing levothyroxine doses. She reports joint pain and fatigue. TSH is elevated (18 mIU/L), free T4 is low (6 pg/dL), and TPO antibodies are markedly elevated (>1000 IU/mL). Which investigation is most appropriate to assess for Riedel's thyroiditis (fibrous thyroiditis) as a complication of chronic autoimmune thyroiditis?