A 52-year-old man presents with progressive dyspnea and right-sided heart failure 8 months after an acute pulmonary embolism treated with anticoagulation. Echocardiography shows elevated right ventricular systolic pressure. The structure marked **D** in the diagram represents the category of pulmonary hypertension most likely responsible for his clinical presentation. Which of the following is the most appropriate FIRST-LINE imaging modality to confirm this diagnosis and guide further management?