Embryology and Anatomy
The development of the vascular precursors of the pulmonary circulation begins when
the endodermal lung bud invaginates into the precursor cardiac mesoderm which on subsequent
specialization results in the anatomical and physiological relationship between the
heart and the lungs via the specialized pulmonary circulation. The development of
the pulmonary arteries begins with the formation truncus arteriosus and the sixth
pharyngeal arch. The truncus arteriosus forms during the development of the heart
as a successor to the conus arteriosus, which undergoes a spiral split via the conotruncal
septum to form main pulmonary trunk and the ascending aorta. Right and the left pulmonary
arteries develop from the ventral sixth arches. Dorsal portion of the right sixth
arch involutes, however, dorsal left sixth arch forms the ductus arteriosus in intrauterine
life.[1]
The pulmonary trunk arises from the outflow tract of right ventricle bifurcating into
right and left pulmonary arteries with the root of the main pulmonary artery (MPA)
guarded by the tricuspid pulmonic valve. Branching pattern of bilateral pulmonary
arteries is dichotomous, forming up to 17 divisions from the level of hilar branches.
In terms of relationship with the bronchi in the mediastinum, the right pulmonary
artery (RPA) is located anteriorly and the left pulmonary artery (LPA) is positioned
superiorly to the respective right and left main bronchi. At the hilum, the RPA bifurcates
into the truncus anterior and the interlobar arteries. The truncus anterior supplies
the right upper lobe, whereas the interlobar artery supplies the right middle and
right lower lobes. The LPA has a shorter course before branching and usually gives
apicoposterior branch, a separate branch to lingula and then terminate in branches
to basal segments.[2]
[3] Histologically, there are three types of pulmonary arterial branches including the
elastic, muscular, and transitional types. The large elastic arteries (0.5 to >1 mm
in diameter) accompany adjacent bronchi up to the subsegmental level. Muscular arteries
are smaller and are located distally accompanying the smaller subsegmental airways
up to the level of terminal bronchioles. They are characterized by the presence of
media of smooth muscle fibers with distinct internal and external elastic laminae.[3]
[4] Transitional arteries are constituted by the pulmonary arterioles (usually smaller
than 0.15 mm) which accompany the respiratory bronchioles and the alveolar ducts and
are distinguished by the absence of a distinct external elastic lamina.[3]
[4]
Technique of Measurement of Pulmonary Arteries on CT Pulmonary Angiography along with
Reference Measurements
The pulmonary arteries are assessed in the reconstructed slice thickness of 1 to 1.5
mm in the pulmonary embolism and mediastinal windows. The MPA is measured perpendicular
to its long axis at its bifurcation in the true axial plane across its maximum dimensions
([Fig. 3]). In healthy adults, the upper limits of diameters of the MPA, LPA, and RPA are
29.5 mm, 22.1 mm, and 19.8 mm, respectively.[6]
[7]
[8] Moreover, sex-specific upper limit of the diameter of MPA (26.9 mm in women; 28.9
mm in men) has also been described.[8] Apart from the absolute diameters of the MPA, the ratio of the diameter of MPA to
the ascending aorta at the same corresponding level, up to 0.9 during the same phase
of the cardiac cycle is considered the norm.[8] However in the pediatric population, a slightly higher ratio of 1.085 may be considered
normal.[9] In addition, normal cut-off values for the pulmonary artery diameter and ratio of
MPA to ascending aorta (MPA:AAo) can also be determined by the height of child, derived
using following formulas, i.e., MPA = 1.05635 × [height], and MPA: AAo = −0.002134
× height + 1.343391.[10]
Fig. 3 Technique of measurement of main pulmonary artery (MPA). MPA measured 35.3 mm while
the ascending aorta (AA) measured 20 mm.
Potential sources of measurement errors occur due to partial volume averaging in thick
slice of >3 mm, poor window settings, and inadequate contrast opacification of the
pulmonary arteries either due to early or delayed triggered acquisition leading to
low contrast detail between the wall of the pulmonary artery and adjacent mediastinal
tissues, inaccurate method of measurement in the oblique long axis proximal to the
MPA bifurcation, measurement of the diameter in diastolic phase of the cardiac cycle,
and inclusion of the wall of the arteries to consider for the upper limit calculation.
All errors need to be avoided so as to provide a definite and objective assessment
of the pulmonary artery dimensions.
Dilative Lesions of the Pulmonary Arteries
There are numerous etiologies for dilative lesions of the pulmonary arteries which
are enumerated in [Table 1]:
Table 1
Etiology of dilative lesions of the pulmonary arteries
|
Abbreviations: ASD, atrial septal defect; PDA, patent ductus arteriosus; VSD, ventricular
septal defect.
|
|
1
|
Pulmonary arterial hypertension
|
|
2
|
Due to turbulent blood flow
a. Congenital valvular pulmonary stenosis
b. Absent pulmonary valve syndrome
|
|
3
|
Due to increased blood flow
a. Intracardiac left to right shunts including VSD and ASD and extracardiac left to
right shunts including PDA
b. Cyanotic lesions like truncus arteriosus
|
|
4
|
Vasculitis: Behcet’s and Hughes-Stovin syndrome
|
|
5
|
Connective tissue disorder: Marfan’s syndrome, vascular Ehler-Danlos syndrome, Loeys-Dietz
syndrome
|
|
6
|
Infectious:
a. Mycotic
b. Tubercular
c. Fungal
|
|
7
|
Trauma
|
|
8
|
Iatrogenic
|
|
9
|
Neoplasm: pulmonary sarcoma
|
|
10
|
Pulmonary artery dissection
|
|
11
|
Idiopathic
|
Pulmonary Arterial Hypertension (PAH)
PAH is characterized by elevated (greater than 25 mm Hg) mean pulmonary artery pressure.[11] In 2013, based on the etiological factors, WHO (World Health Organization) has classified
PAH into five distinct groups.[12] Group 1 constitutes pathology at precapillary and post-capillary pulmonary vessels
with largely unknown underlying cause and may also include other disease entities
such as pulmonary capillary hemangiomatosis and pulmonary veno-occlusive disease.
Group 2 includes valvular and obstructive lesions of the left heart and is the most
common type. Group 3 comprises of primary lung parenchymal pathologies including emphysema,
pulmonary fibrosis which are associated with pulmonary vasoconstriction. Group 4 is
characterized by chronic thromboembolic pulmonary hypertension.[13] Group 5 comprises of the disease entities with unknown mechanisms not inclusive
of either of the previous group entities.
On CT pulmonary angiogram, the dilated MPA with diameter ≥29 mm has shown 87% sensitivity,
89% specificity, and positive predictive value of 97% in diagnosing PAH.[14] Apart from the absolute diameters, MPA to ascending aorta diameter ratio (MPA: AAo)
of ≥1.0 is suggestive of PAH, particularly in patients aged <50 years in the absence
of ectasia of the ascending aorta.[15] Also in the presence of MPA dilatation, the ratio of diameter of segmental artery
to accompanying bronchus >1:1 in three or four lobes, has 100% specificity in diagnosing
of PAH.[12] In addition to pulmonary arterial dimensions, the presence of mural calcifications,
tortuous pulmonary artery, and pruning of the peripheral pulmonary arterial branches
has been seen in long standing cases of PAH. An additional marker evaluated on ECG-gated
CT angiography is the reduced distensibility of the MPA (<16.5%) which is an accurate
marker of PAH with sensitivity and specificity of 86 and 96%, respectively.[16] Special attention should be paid to the adjacent structures including left main
coronary artery and the tracheobronchial tree as they can be compressed by the dilated
MPA, with higher risk when MPA diameter is >4 cm and MPA:AAo ratio of >1.21.[17]
Pulmonary thromboembolism constitutes an important cause of pulmonary artery dilatation.
Acute pulmonary thrombus is seen as a hypodense filling defect forming an acute angle
with the wall of the artery ([Fig. 4A],[B]). The presence of eccentric bland thrombus forming an obtuse angle with the wall
of the artery, hypodense webs ([Fig. 4C]), luminal stenosis or abrupt cut-off, post-stenotic dilatations, and outpouching
are suggestive of chronic pulmonary thromboembolism.[18] Chronic pulmonary arterial thrombus contracts the lumen in contradistinction to
acute embolus which distends the lumen.[11] Valuable indirect imaging signs of chronic thromboembolic pulmonary hypertension
(CTEPH) to distinguish from other causes of PAH include mosaic attenuation of lung
parenchyma due to alternating areas of hypoperfusion and hyperperfusion, tortuous
pulmonary vessels, peripheral infarcts seen as wedge-shaped consolidation with apex
truncation and a feeding vessel, hypertrophied systemic arterial collaterals including
bronchial arteries (associated with up to 73% of the CTEPH cases vs. 14% in primary
pulmonary hypertension), pleural and intercostal arteries and presence of pulmonary
artery webs/bands with focal stenotic segments.[19]
[20] The therapeutic options in CTEPH can be derived based on the location and extent
of thrombus visualized on CT angiography. Pulmonary endarterectomy is beneficial if
predominant thrombus is seen in the main, lobar, or proximal segmental arteries, whereas
medical therapy is preferred in distal disease.
Fig. 4 (A) Acute embolus (white arrow) in the left descending pulmonary artery forming acute angle with the wall of the
artery. (B) Acute thrombus in the right pulmonary artery (RPA) and left pulmonary artery (LPA)
placed eccentrically along the arterial wall. (C) Hypodense web (black arrowhead) in the right descending pulmonary artery. MPA, main pulmonary artery.
Congenital Valvular Pulmonic Stenosis
In general, congenital pulmonary stenosis in seen in up to 10% of the patients with
congenital heart diseases of which the valvular type of pulmonary stenosis comprises
nearly 90% of the cases apart from the subvalvular and supravalvular varieties.[2]
[21]
[22] The primary pathology is commissural fusion of the tricuspid pulmonic valve leaflets
resulting in inadequate opening of the valve leaflets during the ventricular systolic
phase. Clinical presentation is primarily determined by the gradient across the stenotic
valve, varying from asymptomatic patients to the systemic venous congestion secondary
to right heart failure. On retrospective-gated CT angiography, thickening of the pulmonary
valve leaflets with restricted mobility is seen in systolic and diastolic phases of
the cardiac cycle ([Fig. 5A]). There is also significant post-stenotic dilatation of the MPA and the LPA due
to the preferential leftward jet of the flow across the stenosed valve ([Fig. 5B]). Moreover, doming of the pulmonary valve, leaflet calcification, and hypertrophy
of the right ventricle may also be seen.[2]
Fig. 5 (A) Axial section at the level of pulmonary valve and the aortic sinus showing thickening
of the pulmonary valve leaflets (black arrowhead). (B) Axial section at the main pulmonary artery (MPA) bifurcation showing disproportionate
dilatation of the MPA and left pulmonary artery (LPA). RPA, right pulmonary artery.
Absent Pulmonary Valve Syndrome
The primary pathology is characterized by the absence or dysplasia of pulmonary valvular
leaflets resulting in the dilatation of the main and the branch pulmonary arteries
secondary to regurgitation physiology. In approximately 25% of the cases, the syndrome
is seen associated with 22q11 chromosome microdeletion.[23]
[24] On CT angiography thickening of the hypoplastic pulmonary valve leaflets is seen
with dilated main and branch pulmonary arteries ([Fig. 6]). Ventricular septal defect is the most common association. In cases with an intact
ventricular septum, the pulmonary arteries are relatively small in size with patent
ductus arteriosus and tricuspid atresia as their common associations. The regurgitation
physiology and the presence of ventricular septal defect result in cardiomegaly secondary
to raised end diastolic pressures in the right heart chambers. In addition to cardiomegaly,
the presence of bronchomalacia is associated with poor prognosis in this subset of
patients. In utero, secondary to compression of the esophagus and the trachea-bronchial
tree by the dilated pulmonary arteries, there is obstruction to the physiological
amniotic fluid circulation leading to polyhydramnios.
Fig. 6 Multiplanar reconstruction (MPR) images (A, C and D) and volume-rendered image (B) show constriction (black arrow) at the level of the annulus of the main pulmonary artery (MPA) with dilatation of
the MPA, left pulmonary artery (LPA), and right pulmonary artery (RPA) with hypoplastic
pulmonary valve leaflets.
Secondary to Underlying Intra- and Extracardiac Shunts
Common causes of left to right shunts include atrial septal defect ([Fig. 7A]), ventricular septal defect ([Fig. 7B],[C]), and patent ductus arteriosus ([Fig. 8A],[B]). Significant shunts result in volume and pressure overload of pulmonary artery
resulting in its dilatation. Usually no sex predilection is observed, except in atrial
septal defect where females are mostly affected. Cyanotic lesions like truncus arteriosus
can also result in pulmonary artery dilatation ([Fig. 9]). Elevated hemodynamic shear stress and increased flow due to the underlying defect
lead to vascular remodeling including wall thickening, adventitial fibroblast proliferation
increasing the wall stiffness, eventually increasing the pulmonary vascular resistance,
leading to dilatation of central pulmonary arteries.[25]
[26] Mural calcification can also result from the accelerated atherosclerotic changes
secondary to long standing severe pulmonary hypertension. The dilated pulmonary arteries
can cause extrinsic compression of the coronary arteries, and the adjacent bronchi
leading to atelectasis. The ECG-gated CT angiography will reveal both the underlying
cardiac shunts and features of pulmonary dilatation, which would guide in the treatment
and prognosis of the patient.
Fig. 7 Four chamber view of the heart (A) showing atrial septal defect (black arrow). Oblique axial image (B) and virtual dissection image (C) of a different patient shows a ventricular septal defect (white arrow) and dilated main pulmonary artery (MPA), respectively.
Fig. 8 Multiplanar reconstruction image (A) and volume-rendered image (B) showing patent ductus arteriosus (black arrow) with dilated main pulmonary artery (MPA).
Fig. 9 Multiplanar reconstruction image (A) and volume-rendered image (B) showing type I truncus arteriosus with common truncus channel (black arrow). Multiplanar reconstruction image (C) and volume rendered image (D) show Type B interrupted aortic arch with patent ductus arteriosus (white arrow) reforming the descending thoracic aorta (DTA). AA, ascending aorta; MPA, main pulmonary
artery.
Vasculitis
Behcet’s syndrome and Hughes–Stovin syndrome are the most common vasculitic disorders
associated with pulmonary artery dilatation. Both the disease entities are considered
to be part of the spectrum of similar pathological expression, with Hughes–Stovin
syndrome lacking classical clinical features of Behcet’s syndrome which include oral
ulcers, genital ulcers, and uveitis. In Behcet’s syndrome, CT angiography reveals
focal dilatation/aneurysms of bilateral pulmonary vasculature with right lower lobe
arteries being most commonly involved with associated mural inflammation and thrombosis
of the aneurysm sac ([Fig. 10]).[27] Although the immune-suppressive therapy may lead to regression of the dilatation,
the patients presenting with life threatening hemoptysis often need embolization.
CT angiography depicts the spectrum of the imaging findings including the pulmonary
hemorrhage, infarction, pericardial, and pleural effusions in addition to pulmonary
vascular involvement. Similar to Behcet’s, Hughes–Stovin syndrome leads to pulmonary
arterial dilatation, secondary to mural inflammation and weakening the arterial wall.[28] CT angiography shows enhancing pulmonary arterial wall (secondary to vasculitis),
pulmonary thromboembolism, and dysplastic tortuous bronchial arteries. The ongoing
vasculitis and accompanying thrombus form a nidus for the development of pulmonary
aneurysms which may rupture in severe cases.
Fig. 10 Volume-rendered images (A and B) showing pseudoaneurysms of the left descending pulmonary artery (black arrow) and the right descending pulmonary artery (white arrow), respectively.
Connective Tissue Disorders
Congenital deficiency of the intercellular matrix in the wall of the arteries seen
in the connective tissue disorders such as Marfan’s syndrome, Ehler–Danlos Syndrome,
Loeys–Dietz syndrome, and cystic medial necrosis typically causes the aorta ectasia
and dilatation.[29]
[30] Marfan’s syndrome is autosomal dominant disease characterized by abnormal microfibrils
due to Fibrillin-1 gene mutation. The Ghnet 2 criteria considers the various musculoskeletal
manifestation and aortic dilatation as the typical clinical features of the disease.
However, dilatation of the MPA as well as the root of the pulmonary trunk is also
a relatively common finding in Marfan’s syndrome (seen in up to 74–76% of the cases),
and is usually not related to the high pressures and is clinically asymptomatic ([Fig. 11]).[29]
Fig. 11 (A) Sagittal oblique images show dilated aortic sinus (*) and ascending aorta. (B) Oblique axial section showing dilated main pulmonary artery (MPA) and dilated ascending
aorta.
Loeys–Dietz syndrome is autosomal dominant condition due to mutation in TGRBR2 gene
leading to the loss of elastin content in the tunica media of the arterial walls.
Although the exact prevalence is unknown, pulmonary artery dilatation is usually seen
in Loeys–Dietz syndrome in patients with additional congenital cardiac shunts. Vascular
Ehler–Danlos syndrome is a rare genetic disease characterized by procollagen III deficiency
secondary to COLA3A1 gene mutation; pulmonary artery aneurysm is seen rarely due to
the deficiency of the vessel wall.
Infectious Etiology
Virulent infections produce necrosis of the arterial wall secondary to the occlusion
of the vasa vasorum by the septic emboli leading to the formation of pseudoaneurysms
of the pulmonary artery. Most common organisms include Staphylococcus, Streptococcus, Mycobacteria, Aspergillus, and Candida Albicans.[31]
[32]
[33]
[34] Pseudoaneurysms secondary to infective etiology are usually associated with hemoptysis
and pulmonary parenchymal hemorrhage leading to abrupt clinical deterioration. The
diagnosis of mycotic aneurysms is usually made on the background of spectrum of CT
imaging findings suggestive of infective pathology such as pulmonary parenchymal consolidation,
ground glass opacities, pneumonic effusion, and enlarged enhancing mediastinal and
hilar lymph nodes. The mycotic pulmonary artery aneurysms are well visualized in the
subacute and chronic stages of infection as areas of fusiform or saccular dilatations
with the enhancement similar to adjacent artery and often the feeding vessel can also
be identified to differentiate it from bronchial artery aneurysms.[32]
[33]Rasmussen aneurysms are seen in up to 5% cases of the reactivation of pulmonary tuberculosis
adjacent to or within the chronic tubercular cavitary lesions, and predominantly involve
the branches of the right upper lobe pulmonary artery.[34] Rarely pulmonary artery pseudoaneurysm has also been described in hyper-IgE syndrome
which by itself is a rare multisystem disorder.[35] CT angiography remains the imaging modality of choice to diagnose mycotic and Rasmussen
aneurysms and also provides a guide for excellent localization of the aneurysm and
the involved vessels for appropriate endovascular or surgical management.
Neoplastic—Pulmonary Artery Sarcoma
Pulmonary artery sarcoma is fatal neoplasm with rare incidence of 0.001 to 0.03%.
The lesion is seen to arise from intimal mesenchymal cells and is often misdiagnosed
as thrombus.[36] MDCT findings suggestive of diagnosis include a hypodense persistent filling defect
despite adequate anticoagulation, completely filling the lumen of main or proximal
pulmonary arteries with associated expansion of the affect arteries and extraluminal
extension.[36]
[37] Moreover, a slower onset with associated systemic symptoms and no history of venous
thromboembolism or positive hypercoagulability in patients who are nonresponsive to
anticoagulation helps in differentiation from pulmonary thromboembolism. Since FDG-PET
has superior sensitivity in detecting pulmonary sarcoma due to high FDG uptake, MDCT
is usually complemented with FDG-PET in complete noninvasive imaging evaluation of
pulmonary sarcomas.[38]
Trauma
Pulmonary artery pseudoaneurysms are rare in penetrating chest trauma and occur due
to injury to layers of the arterial wall, contained by the extravascular tissue and
clot with absence of true endothelial lining.[39] It is frequently associated with hemothorax, pulmonary hemorrhage, chest wall injuries,
and rib fractures. Multiple treatment options include transcatheter embolization,
thrombin injection, emergency surgical ligation, and lobectomy. CT angiography depicts
the pseudoaneurysm as circumscribed enhancing mass, isodense to the adjacent pulmonary
artery.[38] CT angiography has also been used in follow-up of conservative management of small
PA pseudoaneurysm in clinically stable, asymptomatic patients where spontaneous resolution
has been reported in literature, although very rare. [40]
[41]
Iatrogenic
Iatrogenic pulmonary artery pseudoaneurysm is a rare complication seen in 0.001 to
0.47% of the patients who undergo catheterization and is most commonly caused by a
malpositioned Swan-Ganz catheter causing erosion and weakening of the arterial wall.[27]
[39] Other possible mechanisms include retraction of inflated balloon and balloon inflations
at too high pressures. The clinical presentation is usually seen shortly after the
catheter insertion with the immediate onset of cough, hemoptysis, and hypoxemia, although
pseudoaneurysm has been reported up to 7 months after the interventional procedure.[42] It is seen commonly on the right side, involving the right middle lobe and right
lower lobe branches, which may be related to common location of Swan-Ganz catheter.[43]
[44] CT pulmonary angiography usually reveals a saccular aneurysm isodense to the adjacent
involved vessel with an occasional partial thrombus in the aneurysmal sac. Endovascular
embolization is a treatment option in catheter-induced pseudoaneurysms; in this regard
CT angiography guides in the therapeutic planning of embolization by delineating the
feeding vessel size and the aneurysm sac dimension.[42]
[43]
[44]
[45]
[46] Other causes of iatrogenic pulmonary artery pseudoaneurysms include chest tube insertion,
conventional pulmonary angiography, and thoracic surgery; hence the clinical history
and presenting complaints are of utmost importance when considering this disease entity.
Pulmonary Artery Dissection
Dissection of the pulmonary artery presents as one of the rare and dreaded complication
of chronic pulmonary arterial hypertension. Trauma, infective endocarditis involving
the right heart chambers, amyloidosis, chronic pulmonary artery inflammation, and
severe atherosclerosis are other proposed causes of pulmonary artery dissection.[47]
[48] This clinical condition is to be considered in PAH patients who present with chest
pain, central cyanosis, and cardiogenic shock. Dissection is usually seen at the point
of maximal arterial dilatation and is usually associated with medial degeneration
of the arterial wall. It has an increased propensity to rupture rather than developing
a reentry tear, unlike the aortic dissection.[49] Though echocardiography is the first line of investigation, CT angiography has the
ability to define the complete extent of the dissection flap, false lumen thrombosis,
and provide a clue to underlying etiology of the dissection ([Fig. 12]). CT angiography detects hemopericardium secondary to PA dissection extending into
the pericardium.[50] CT angiography can also confidently exclude pulmonary arterial thromboembolism,
which can also have similar clinical presentation.[51]
Fig. 12 Hypodense dissection flap (black arrow) in the right lower descending pulmonary artery with dilated right pulmonary artery
(RPA).
Idiopathic Dilatation of the Pulmonary Trunk
It is a diagnosis of exclusion when all the cardiac and pulmonary causes for the pulmonary
arterial dilatation have been ruled out. It is a rare congenital anomaly with an incidence
of up to 0.6%.[52] Clinical symptoms are highly variable with patient being asymptomatic to the development
of exertional dyspnea. Although the initial postulated mechanism for the development
of idiopathic dilatation was unequal division of the truncus arteriosus communis,
currently the most accepted theory is the presence of congenital weakness in the wall
of the pulmonary artery.[53]
[54]
[55] MDCT angiography defines the location, extent of dilatation of the pulmonary artery,
and also rules out imaging features of other etiologies that would lead to pulmonary
artery dilatation as described previously. Even though there are no established guidelines,
surgical repair is advised if the aneurysm is >6 cm in size or having symptomatic
disease or the aneurysm is at risk of rupture, dissection, or thromboembolism.[55]
[56]
[57]
