Planning of the pulmonary flow imaging planes. An axial slice showing the main, right, and left pulmonary arteries (MPA, RPA and LPA, respectively). Two planes are prescribed perpendicular to the flow direction in the pulmonary artery. Plane 1 is perpendicular to MPA, while plane 2 is perpendicular to LPA or RPA. The distance along the pulmonary artery between the two measuring sites (Δx) is used in calculating PWV.