What is the normal appearance of the pulmonary vessels on computed tomography (CT) and magnetic resonance imaging (MRI)?
The main pulmonary artery originates from the right ventricular outflow tract, anterior and to the left of the aortic root, and serves as a conduit for blood from the right ventricle to the pulmonary circulation. It bifurcates into the right and left pulmonary arteries. The right pulmonary artery passes horizontally through the mediastinum anterior to the major bronchi before bifurcating into the truncus arteriosus and right interlobar arteries, whereas the left pulmonary artery arches more superiorly to pass over the left mainstem bronchus before descending posterior to the major bronchi as the left interlobar artery. The pulmonary arteries then branch into lobar, segmental, and subsegmental pulmonary arterial branches.
The pulmonary veins, typically numbering four in total (right superior, right inferior, left superior, and left inferior), drain into the left atrium. However, common pulmonary venous trunks and accessory pulmonary veins are not uncommonly encountered.
On contrast-enhanced CT and MRI, the pulmonary arteries and veins will generally enhance to a degree similar to that of the right ventricle and left atrium, respectively, which will vary depending on the timing of image acquisition relative to the start of intravenous contrast administration. Other than the pulmonary valve leaflets at the origin of the main pulmonary artery, no filling defects are normally seen within the lumina of these vessels. The segmental pulmonary arteries are always located adjacent to bronchi, which are on average of similar caliber, whereas the pulmonary veins are located separately.