A 52-year-old man with a 15-year history of poorly controlled hypertension (BP 160/100 mmHg) presents with dyspnea on exertion. Physical examination reveals a sustained apical impulse and an S4 gallop. Echocardiography shows left ventricular wall thickness of 16 mm (normal <11 mm) with normal cavity size and preserved ejection fraction. The clinician suspects concentric left ventricular hypertrophy secondary to chronic pressure overload. Which investigation would be most appropriate to assess the degree of myocardial fibrosis and confirm the adaptive response to sustained hypertension?