Anatomical variations of the pulmonary veins. Accessory right middle lobe (RML) and conjoined ostia are common and are seen in one fourth of studies. Accessory branches on the left side are not common. Supernumerary branch is usually a small branch found in 5–10% of chest studies. It usually drains the posterior segment of the right upper lobe and consistently travels behind the right main bronchus before entering the left atrium. Rarely, supernumerary branch arises from the superior segment of the right lower lobe and directly connects to the posteroinferior margin of the left atrium. Note close relationship of the right pulmonary artery (RPA) and the superior wall of the left atrium.
