A 52-year-old man with known myasthenia gravis presents with acute worsening of generalized weakness, difficulty swallowing, and respiratory distress. He has been on pyridostigmine monotherapy for 2 years. Before initiating immunosuppression, the clinician needs to exclude a thymoma, which is present in 30–50% of generalized MG cases. Which is the most appropriate investigation of choice for detecting thymoma in this patient?