Pictorial review of the pulmonary vasculature: from arteries to veins

In rare circumstances, the pulmonary artery fails to fully develop. This condition, called pulmonary artery atresia, is also known as proximal interruption of the pulmonary artery as the distal portion of the pulmonary artery located within the lung is typically intact and perfused secondary to collateral flow from aortopulmonary collateral (APC) vessels. Right pulmonary artery atresia usually occurs in the absence of other congenital abnormalities, while left pulmonary artery atresia often occurs in conjunction with other congenital cardiac conditions, such as tetralogy of Fallot [6]. Most patients with this condition are symptomatic with a combination of dyspnoea, recurrent pulmonary infections and haemorrhage. CT or MRI will show an absence or hypoplasia of the affected artery proximally (Fig. 2). Additional findings include hypoplasia of the affected lung, serration and thickening of the pleura, and APC vessel formation [1].

Developmental failure of the left sixth aortic arch may lead to a pulmonary artery sling, an aberrant origin of the left pulmonary artery arising from the right pulmonary artery, which courses between the trachea/right mainstem bronchus and oesophagus (Fig. 3). This can lead to compression/focal stenosis of the airway and subsequent air trapping and atelectasis, although many patients may be asymptomatic when there is minimal airway compression [1, 7]. While this finding is easily seen on CT and MRI, it may be first detected on plain film imaging as a left-sided deviation of the trachea and mediastinum and on fluoroscopic oesophageal imaging as anterior indentation of the oesophagus.

This is a diagnosis of exclusion in which there is a congenitally dilated pulmonary trunk with normal pressures in the absence of other cardiopulmonary disease. Patients with this condition are, by definition, asymptomatic although close follow-up is recommended [1, 8]. The most accurate method to obtain the main pulmonary artery diameter is to use reformatting software to perform a double oblique measurement of the pulmonary artery at its largest diameter; alternatively, if this technique is not available, the transverse axial diameter of the main pulmonary artery can be measured at the level of the bifurcation of the right pulmonary artery (Fig. 4) [9].

Pulmonary arterial pressures greater than 25 mmHg at rest (as assessed by right-heart catheterisation) are diagnostic of pulmonary hypertension, with normal pressure being less than 20 mmHg [10, 11]. Rarely, pulmonary hypertension is idiopathic, commonly presenting in young women between 20 and 30 years of age. More often pulmonary hypertension is secondary to other pathology, with causes including heart failure (right and/or left), pulmonary parenchymal disease (including fibrosis and emphysema) and chronic pulmonary emboli. The current classification scheme of pulmonary hypertension (last updated in Nice, France) groups the causes of pulmonary hypertension into categories based on similar pathophysiology (Table 1) [12]. This framework is important as the aetiologies resulting in pulmonary hypertension often necessitate different therapeutic management. Imaging findings suggestive of pulmonary hypertension include an enlarged pulmonary trunk (> 29 mm), a pulmonary artery diameter to ascending aorta diameter of > 1, and a segmental artery-to-bronchus ratio > 1 in at least three of four lobes; additional imaging findings present may provide clues as to the underlying cause of the pulmonary hypertension (Figs. 5 and 6) [11, 13].

Aneurysms/pseudoaneurysms of the pulmonary artery may arise secondary to pulmonary hypertension, collagen/vascular conditions (e.g. Marfan syndrome, Takayasu arteritis and Behçet’s disease), iatrogenic catheter misplacement, trauma and infection. They may occur in isolation or may also be associated with other congenital cardiac disease. The normal pulmonary artery should gently taper in calibre as the vessel branches and extends peripherally. Imaging will reveal a focal dilated segment of the pulmonary artery, often saccular in appearance, with the phase of contrast following the pulmonary artery enhancement (Fig. 7) [1].

Obstruction of the pulmonary artery can occur secondary to malignant processes including both primary sarcoma of the pulmonary artery and other pulmonary neoplasms, inflammatory conditions including mediastinal fibrosis and Takayasu arteritis, or pulmonary embolism both acute and chronic. Primary sarcoma of the pulmonary arteries is a rare condition and is often confused for acute pulmonary embolism. CT imaging will show an enlarged artery with a filling defect that may encompass the entire lumen (Fig. 8a); findings that should raise the suspicious for sarcoma over bland thrombus include entire involvement of the proximal or main pulmonary artery, enlargement of the involved pulmonary arteries, and extension of the lesion into the surrounding tissues [14]. Occasionally, small enhancing vessels can also be seen within the tumour itself. Malignant pulmonary neoplasms can cause external compression or directly invade the pulmonary artery resulting in obstruction (Fig. 8b) [1]. Fibrosing mediastinitis is due to the pathological proliferation of fibrous tissue in the mediastinum and may also cause obstruction or narrowing when the pulmonary arteries are involved (Fig. 8c) [15]. While more commonly affecting the aorta, Takayasu arteritis also can affect the pulmonary arteries with CT angiography typically revealing circumferential wall thickening and enhancement [16].

Pulmonary embolism is a very common source of pulmonary artery obstruction. Acute pulmonary embolism is best evaluated with CT pulmonary angiography which demonstrates a central focal filling defect within the pulmonary artery, often rimmed by contrast, typically forming an acute angle within the vessel. Enlargement of the affected branch may also be seen (Fig. 9) [17, 18]. In some patients (especially those with repeat episodes or particularly large emboli) chronic pulmonary thromboembolism can develop with the prior embolus becoming incorporated into the luminal wall rendering anticoagulation less effective. This leads to stenosis of the pulmonary artery and may cause serious pulmonary hypertension. CT findings in chronic pulmonary thromboembolism include eccentric thrombus resulting in partial or complete obstruction, often making an obtuse angle with the vessel wall, post stenotic dilatation, partial calcification of the thrombus, and web-like bands within the pulmonary arteries. Indirect signs include mosaic attenuation of the pulmonary parenchyma due to perfusion defects and APC formation (Fig. 10) [19].

Anatomically, the normal configuration of the pulmonary venous territory involves four distinct pulmonary veins draining into four separate ostia on the left atrium (two right and two left); however, anatomic variants are common. A particularly common variant involves a common left pulmonary vein, where the superior and inferior left pulmonary veins fuse before draining into the left atrium (Fig. 11) [20]. Please note that the merging of the pulmonary veins should be distinguished from an anomalous unilateral single pulmonary vein, a condition where there is only a single pulmonary vein without merger (more common on the left side) [21]. A second common variant is a separate right middle lobe pulmonary vein draining independently from the superior and inferior right pulmonary veins. Other anatomical variants have been described such as the presence of more than two pulmonary veins on the same side [22].

Congenital pulmonary venous drainage anomalies refer to conditions in which pulmonary vein flow does not return to the left atrium but rather drains into the systemic system producing a left-to-right shunt. This pathology exists on a wide spectrum with tremendous variability. Total anomalous pulmonary venous return (TAPVR) occurs when all pulmonary venous return drains into the systemic system. TAPVR is classified into supracardiac, cardiac, infracardiac or mixed types depending on the location of the drainage. This is a condition incompatible with life and requires an accompanying right-to-left shunt for survival, along with immediate corrective surgery [23, 24]. TAPVR has several associations including heterotaxy syndrome and other congenital heart defects [24]. Partial anomalous pulmonary venous return (PAPVR) refers to conditions in which only some of the pulmonary veins drain aberrantly (Fig. 12) [2, 24, 25]. The severity of PAPVR depends on the degree of anomalous return, and these patients are often asymptomatic. A special subtype of PAPVR occurs when there is aberrant, typically right sided, pulmonary venous drainage into the inferior vena cava (or other veins below the diaphragm) with associated hypoplasia of the right lung. This condition is known as hypogenetic lung syndrome due to the associated small right lung or scimitar syndrome due to the scimitar sword-like appearance of the aberrant venous return extending below the diaphragm on frontal radiograph. The degree of PAPVR is a function of the severity of the disease process, with larger defects (such as aberrant entire return on one side) often identified in infancy and smaller defects (such as aberrant lobar or segmental return) often remaining asymptomatic, identified incidentally in adulthood (Fig. 13) [23, 26]. Treatment of all these conditions range from observation in mild cases to emergent surgery in more severe manifestations.

An isolated segment of dilated pulmonary vein is known as a varix or pulmonary venous aneurysm [27]. This dilatation is readily apparent on CT imaging and occurs most frequently near the ostium (Fig. 14). On chest X-ray, the varix may assume the appearance of a pulmonary nodule or hilar adenopathy [28]. Although we have grouped the pulmonary vein varix with congenital conditions, it can also be acquired secondary to other cardiopulmonary disease such as chronic pulmonary hypertension [2]. Pulmonary vein varices are typically asymptomatic; however, rupture and subsequent haemorrhage are known complications requiring treatment that may involve surgery or coiling [28].

Failure of the pulmonary vein to properly develop may result in atresia or stenosis. Patients will classically present with recurrent pneumonia and/or haemoptysis in the first few years of life, and there is an association with other congenital cardiac anomalies [29]. CT imaging may show ipsilateral mediastinal deviation towards the atretic pulmonary vein, a small ipsilateral hemithorax and a small ipsilateral pulmonary artery [2].

There are many conditions which may lead to the obstruction of pulmonary venous flow, including both intrinsic and extrinsic processes. The majority of cases relate to neoplastic processes or complications of radiofrequency ablation; other causes include surgical complications, fibrosing mediastinitis, tuberculosis and sarcoidosis [2, 15, 30‐32]. As with pulmonary arteries, neoplastic processes can result in either mechanical extrinsic compression or direct invasion of the pulmonary venous vasculature causing obstruction. While this typically results in stenosis or occlusion of the vessel, it can also lead to pulmonary vein thrombosis, with bland thrombus developing secondary to disrupted flow, hypercoagulable state or tumour thrombus from direct extension of tumour or metastasis. Other causes of stenosis/obstruction, such as pulmonary surgery or radiofrequency ablation, can also cause bland thrombus [33, 34]. Clinical presentation of thrombus or stenosis/obstruction in pulmonary veins is non-specific. Obstruction of the pulmonary veins can be detected on CT imaging with contrast; however, this is a commonly overlooked finding as the pulmonary veins are not typically analysed in detail during a general search pattern for CT chest imaging (Fig. 15) [35].

It is important to note that the pulmonary veins can be involved in a variety of collateral pathways, both congenital and acquired [2]. In cases of superior vena cava obstruction, collateral flow to the pulmonary veins can form, resulting in a right-to-left shunt (Fig. 16) [36, 37].

Pulmonary venous hypertension (defined as a pressure of 15 mmHg or higher) occurs most often in the background of left ventricular failure or other cardiac causes. A rare idiopathic cause of pulmonary venous hypertension is known as pulmonary veno-occlusive disease, which develops secondary to the occlusion or constriction of the pulmonary veins and venules, with the majority of patients affected presenting under the age of 50 years old [2, 38]. Patients with pulmonary venous hypertension show evidence of fluid overload on imaging, including pulmonary oedema and pleural effusions, mosaic attenuation of the pulmonary parenchyma and enlargement of the central pulmonary arteries [13].

Pulmonary vein calcifications are typically associated with mitral valve disease and chronic renal failure; however, they may also be seen in patients with atrial fibrillation [2, 39]. In addition to pulmonary vein calcifications, imaging often shows a dilated left atrium, which may also be calcified, described as a “mould-like” pattern [40].