Example of measurement of main pulmonary artery (MPA) diameter in a patient with pulmonary hypertension. Diameter of the MPA on cross-sectional imaging was measured in the scan plane of the pulmonary artery bifurcation where right main pulmonary artery is visualized in its extent, at a right angle to its long axis. On this patient, the diameter was 5.5 cm. MPA/aorta diameter ratio was 1.4.