Abnormalities of the pulmonary arteries can be classified into acquired and congenital. 

Acquired abnormalities are usually symptomatic and include different entities, among which pulmonary embolism is by far the most frequent. 

Congenital alterations in adults include left pulmonary artery sling, unilateral proximal interruption of the pulmonary artery and iIdiopathic dilatation of the pulmonary trunk. Although these abnormalities can be symptomatic, they most often present as an incidental finding on imaging. 

Contrast-enhanced CT is the technique of choice to evaluate most of these entities. In some cases however, DECT, PET-CT and MR may be of help.

ACQUIRED PULMONARY ARTERY ABNORMALITIES 

Acute pulmonary thromboembolism 

Acute PE is the third leading cardiovascular cause of death after coronary artery disease and cerebrovascular accident. Characteristic though infrequent signs of PE on chest radiography include: focal or general lucency within the lung, representing oligemia due to vasoconstriction distal to the PE (Westermark sign); subpleural consolidations related to pulmonary infarction (Hampton hump sign); and focal enlargement and abrupt tapering of a central pulmonary artery (Fleischner sign).

Fig 1: Acute pulmonary embolism and related findings. Subpleural consolidations related to pulmonary infarction; Hampton hump sign (A and B). Opacification defects in main pulmonary arteries on axial contrast-enhanced CT (C). Lung infarction (D).

Fig 2: Signs of right ventricular overload in the context of pulmonary embolism. Leftward bowing of the interventricular septum with increased right to left ventricle diameter (A). Contrast reflux into the inferior vena cava and suprahepatic veins (B).

Chronic pulmonary thromboembolism 

In a small amount of cases, PE may not completely resolve and become chronic. Characteristic CT findings are: eccentrical filling defects that form obtuse angles with the vessel wall; lines of decreased opacity that traverse the lumen of contrast-filled pulmonary arteries known as webs or bands; narrowing or complete occlusion of a vessel; thickened and scalloped appearance of arterial walls and calcification of the filling defects.

Fig 3: Chronic pulmonary embolism. Eccentric filling defects (A and B). Dotlike filling defect which is a transversal view of a web(C). Linear filling defects (D, E and F).

Pulmonary Hypertension 

Pulmonary hypertension (PH) is defined as mean pulmonary artery pressure greater than 25 mm Hg at rest or greater than 30 mm Hg during exercise, determined by right heart catheterization. It is most often secondary to cardiac, pulmonary, or hepatic disease, and it is considered primary when there is no identifiable cause. Findings on imaging (CECT) include: dilatation of the pulmonary trunk greater than 29 mm, main pulmonary artery diameter to aorta diameter greater than 1:1, segmental artery-to-bronchus ratio greater than one in three lobes, pruning of the peripheral pulmonary arteries, calcifications in the arterial wall, mosaic lung attenuation and indirect signs of right sided heart failure.

Fig 4: Pulmonary hypertension. Severe dilatation (55 mm) of the pulmonary trunk, showing greater diameter than the aorta (A). Lung parenchyma showing mosaic attenuation pattern (B).

Pulmonary Artery Narrowing 

Several important disease processes may narrow the pulmonary arteries.

Fig 5: Pulmonary artery stenosis in a 65 year old man with chronic renal failure. Calcified atheromatous plaque causing stenosis of the pulmonary trunk (A). Ectasia of both main pulmonary arteries (B).

Fig 6: Severe stenosis of the proximal segment of the right (A) and the left (B) pulmonary arteries in a 50 year old female with a history of Takayasu arteritis.

Fig 7: Pulmonary artery stenosis in a young female patient with active Takayasu arteritis. Axial contrast-enhanced CT showing stenosis of the proximal segment of both pulmonary arteries (A). Same findings on axial MR angiography (B).

Tumors of the Pulmonary Arteries 

Primary pulmonary artery sarcomas are rare tumors that most commonly arise from the pulmonary trunk or proximal pulmonary arteries. The clinical manifestations are similar to those of PE. Chest radiography may be normal if tumors are entirely intraluminal and do not cause artery dilatation, otherwise they can manifest as hilar masses, simulating lung cancer or metastasis. On CT, pulmonary artery sarcoma can be difficult to distinguish from PE, as both manifest as intraluminal arterial filling defects.

Fig 8: Pulmonary artery angiosarcoma. Contrast-enhanced axial CT showing large low attenuating filling defects in the main pulmonary arteries (A), occupying the entire lumen (B), with extraluminal extension into adjacent mediastinum. PET-CT showing marked FDG uptake (C).

Multiple malignancies can cause metastatic emboli to the pulmonary arteries, such as carcinoma of the breast, stomach, liver, kidney, or lung, usually involving subsegmental arteries and arterioles. CT findings vary depending on the size of the vessels affected. Large emboli in the main, lobar, and segmental pulmonary arteries cause filling defects similar to PE.

Fig 9: Intravascular metastasis of endometrial carcinoma in right pulmonary artery (A) and subsegmental branches in the upper right lobe (B, C, D).

Lung tumors originating near the pulmonary hila can invade the pulmonary arteries. CT and MRI signs of pulmonary artery invasion include thickening of the pulmonary artery wall, luminal narrowing, and perivascular fat stranding.

Fig 10: Lung squamous cell carcinoma infiltrating the interlobar pulmonary artery.

Pulmonary artery aneurysm and pseudoaneurysm

Aneurysms of the pulmonary artery are focal dilatations involving all three layers of the vessel. Pseudoaneurysms however do not involve all the layers of the arterial wall and have therefore  a higher risk of rupture. Common causes of both alterations include congenital heart diseases, vasculitides, such as Behçet disease, infections such as tuberculosis and endocarditis, malignancies, and direct injury to the lung vasculature, either traumatic or iatrogenic. Contrast-enhanced CT is the non-invasive imaging modality of choice for their identification and characterization. Aneurysms appear as saccular or fusiform areas of dilatation, with homogeneous contrast filling simultaneously to the pulmonary artery.

Fig 11: Pseudoaneurysm associated with active tuberculosis (A) and necrotizing pneumonia (B). Pseudoaneurysms associated with tuberculosis or pneumonia are caused by destruction of the vessel wall from outer wall to inner lumen, and therefore will be found adjacent to areas of cavitation or consolidation. Mycotic pseudoaneurysm associated with endocarditis, are caused by seeding of septic emboli that erode the artery wall from within the inner lumen and progress to the outer wall. Therefore, they are not necessarily adjacent to lung consolidations. Primary lung cancer and pulmonary metastases can invade and erode into the pulmonary arteries causing pseudoaneurysms, which most often occur where the tumor encases the pulmonary arteries.

Pulmonary arteriovenous malformations

Pulmonary arteriovenous malformations (PAVMs) are abnormal direct communications between pulmonary arteries and pulmonary veins without an intervening capillary bed. Most cases are congenital and related to hereditary hemorrhagic telangiectasia, but they can also be acquired due to thoracic trauma, surgery, infection or malignancy. PAVMs present as well-defined nodular opacities on chest radiography, usually within the lung periphery. CT is the diagnostic imaging modality of choice, where they appear as homogenous, well-circumscribed nodules or a serpiginous mass connected with blood vessels, showing contrast enhancement.

Fig 12: Arteriovenous malformation of unknown origin in a 47 year old female: axial CT with lung (A) and soft tissue window (B) and coronal view with MIP (C), showing homogenous, contrast enhancing and well circumscribed nodules in the left lower lobe, connected with blood vessels (arterial and venous system).

CONGENITAL PULMONARY ARTERY ABNORMALITIES 

Left pulmonary artery sling is a rare vascular abnormality wherein the left pulmonary artery arises from the posterior aspect of the right pulmonary artery, passing between the trachea and esophagus towards the left hilum. CT establishes the diagnosis.

Fig 13: Pulmonary artery sling. Case 1: asymptomatic young female with left pulmonary artery arising from the right pulmonary artery, passing between the trachea and esophagus towards the left hilum, forming a sling around the trachea (A). Case 2: 91 year old woman with dysphagia due to left pulmonary artery sling (B) causing stenosis of the esophagus (C, D).

Unilateral proximal interruption of the pulmonary artery is an abnormal termination of a pulmonary artery at the level of the hilum, with blood supply to the lung through collateral systemic vessels, mainly bronchial arteries, and with normal bronchial branching. On CT the abnormal vessel may be completely absent or end within 1 cm of its origin and the collateral arteries that supply the lung can be identified.

Fig 14: Interruption of the pulmonary artery: proximal abnormal termination of the left pulmonary artery at the level of the hilum with thrombosed aneurysm at its ending.

Idiopathic dilatation of the pulmonary trunk is a diagnosis of exclusion characterized by enlargement of the pulmonary trunk, with greater diameter than 29 mm on CT.

Fig 15: Idiopathic dilatation of the pulmonary trunk and main pulmonary arteries.