Findings typically include the presence of a well-defined mass with predominant localization in the lower lobes in 98% of cases. Among the lower lobes, the medial or posterior segment of the left lower lobe is the most common.

Fig 3: The most frequent location of pulmonary sequestration. A CT lung window scan on axial, sagittal, and coronal views shows a heterogeneous mass located in the posterior segment of the left inferior lobe.

Fig 4: Axial CT lung window scan shows a well-defined mass in the posterior segment of the right lower lobe, associated with minimal air bronchogram.

Fig 5: Axial CT angiography of the same patient shows an aberrant arterial supply with a branch originating from the aorta that supplies the right lower lobe mass.

Depending on the type of sequestration, different key findings can be observed: intralobar sequestration is associated with bronchial and vascular anomalies. Additionally, CT can help evaluate associated conditions, such as concomitant infections, and assist in ruling out differential diagnoses.

Intralobar Sequestration:

Due to its distribution, it is more frequently diagnosed in adult patients, of whom 75% are asymptomatic and are diagnosed in sectional studies performed for other reasons. The remaining 15% present with symptoms at the time of diagnosis, with the most frequent cause being symptoms associated with concomitant infections (dyspnea, dry or productive cough, hemoptysis).

In addition to the embryological etiology of this type of sequestration, another theory has been developed in which the intralobar type usually has an acquired origin caused by chronic pulmonary infection, leading to the proliferation of aberrant arterial vessels. This hypothesis is supported by the fact that it lacks its own pleural envelope because it is formed within normal lung parenchyma. In either case, the chronic inflammation can result in cystic degeneration.

Fig 6: Axial contrast-enhanced CT scan shows multiple cystic images showing peripheral post-contrast enhancement in a right intralobar pulmonary sequestration in association with cystic degeneration.

Extralobar Sequestration:

This type is most frequently found in male patients. It is commonly diagnosed via prenatal ultrasound, and approximately 60% present in the first six months of life with respiratory distress, malabsorption and high-output congestive heart failure due to right-to-left shunt.

77% of cases occur in the posterior costodiaphragmatic sulcus between the lower lobe and left hemidiaphragm, and only 10–15% occur below the diaphragm. Concomitant infections are rare due to the pleural separation between the normal lung tissue and tracheobronchial tree.

Fig 7: Sagittal lung window CT, and axial and coronal contrast-enhanced CT scans show extralobar pulmonary sequestration (white arrow), where the pleura is visualized surrounding the mass in the right lower lobe (blue dotted line).

CT and CT-Angiography Findings

Findings on CT include nodular lesions, solid heterogeneous or homogeneous masses, ground-glass opacities, and cavitation. In the extralobar type, pleural effusion can occur due to poor lymphatic drainage, while in the intrapulmonary type, air trapping may be present.

Fig 8: Axial CT lung window scan shows a well-defined nodular lesion in the posterior segment of the right lower lobe.

Fig 9: Axial, sagittal and coronal CT lung window scan shows a solid heterogeneous mass with peripheral ground-glass opacities located in the posterior segment of the right lower lobe.

Fig 10: CT mediastinal window scan on the axial plane shows left pulmonary sequestration associated with bilateral pleural effusion.

Concomitant infections are the most frequent complication. The spectrum of findings is broad; adjacent consolidations, air bronchograms, and in chronic or repetitive processes, the presence of calcifications can be observed.

Ground-glass opacities or cavitation with or without air/fluid levels may also be present. Particularly, the presence of an air/fluid level or air alone inside cysts is suggestive of communication with the tracheobronchial tree following recurrent infections and can mimic other conditions (necrotizing pneumonia, fungal or mycobacterial infections, cavitating tumors, or empyema).

Fig 11: A 44-year-old patient with a 5-month history of hemoptysis. Axial CT lung window scan shows a well-defined mass in the left lower lobe, associated with air bronchogram. The surrounding and contralateral lung parenchyma shows ground-glass opacities and atelectasis, indicative of an associated infectious process.

• Arterial Supply and Venous Drainage

The arterial supply for both types arises from the aorta in the majority of cases. In the intralobar type, 73% originates from the descending thoracic aorta, but it may also arise from the abdominal aorta, splenic artery, celiac axis, and intercostal arteries.

Venous drainage commonly occurs through the pulmonary veins to the left atrium in 95% of cases but may also be to the systemic circulations through the azygos or hemiazygos system, or the intercostal veins.

In the extralobar type, the arterial supply originates from the thoracic or abdominal aorta in 80% of cases, but it may also arise from the celiac, splenic, gastric, or intercostal arteries.

The venous drainage, unlike the intralobar type, is systemic in the majority of cases, occurring through the azygos and hemiazygos systems, and 20% through the pulmonary veins.

Fig 12: Coronal reconstruction CT angiography with maximum intensity projection (MIP) shows findings consistent with intralobar pulmonary sequestration, demonstrating arterial supply originating from the thoracic aorta.

Fig 13: Axial and sagittal reconstruction CT angiography with maximum intensity projection (MIP) demonstrates arterial supply originating from the abdominal aorta. The findings are consistent with intralobar pulmonary sequestration.

Fig 14: Axial, sagittal, and coronal contrast-enhanced CT scans in the venous phase show pulmonary sequestration in the right lower lobe with venous drainage from the pulmonary veins (orange arrow).

Fig 15: Axial contrast-enhanced CT scan shows left lower lobe pulmonary sequestration with mixed venous drainage from the pulmonary veins (green star) and through the hemiazygos system (yellow star).

State-of-the-art multidetector CT allows for a fast and comprehensive evaluation of the airways, lung parenchyma, and blood vessels. Modern dual-energy scanners optimize tissue and vessel contrast and assess functional parameters like pulmonary perfusion.

This is achieved through high-quality 2D and 3D image processing techniques.

Fig 16: Sagittal 3D reconstruction shows the arterial supply with a branch originating from the abdominal aorta.

Fig 17: 3D rendering of pulmonary sequestration shows arterial supply from a branch originating from the abdominal aorta.

X-ray

A chest radiograph is a very non-specific imaging study for pulmonary sequestration but may help distinguish normal lung tissue from a lesion and facilitate a sectional study for intentional search. It may appear as a homogeneous opacity with smooth or lobulated contours or as a patchy consolidation with irregular margins, typically located in the lower lobes.

Fig 18: A 42-year-old patient with a history of recurrent pulmonary infections. PA and lateral chest radiography show an opacity at the right lung base.

Ultrasound

Is the main imaging modality for prenatal imaging. Its appearance is that of a solid, homogeneous, and well-circumscribed echogenic mass. Polyhydramnios should be assessed, as it may result from esophageal compression or excessive fluid secretion by the sequestration.

Doppler color ultrasound demonstrates the aberrant arterial or venous flow, but it can be difficult to discern its origin.

Fig 19: Gray scale ultrasound shows a 34-week gestation pregnancy with a unilateral solid hyperechoic and heterogeneous lesion in the left lung.

Fig 20: Color Doppler ultrasound of the same patient shows an aberrant nutrient vessel; however, its origin cannot be clearly defined.

MRI

Fetal MRI is a complementary diagnostic tool for patients undergoing prenatal ultrasound, where the diagnosis is made, but the origin of the arterial supply and/or venous drainage cannot be determined. It usually appears as a well-defined mass with homogeneous, high signal intensity on T2-weighted images. MRI can provide a simultaneous depiction of both the morphology and vascular supply of the lesion, without subjecting the patient to ionizing radiation.

Fig 21: MRI sagittal and axial T2-weighted images show a triangular image with increased signal and solid appearance, involving the middle and lower thirds of the left hemithorax, in relation to pulmonary sequestration (orange arrow). The anomalous vessel comes from the descending aorta (green arrow).

Treatment 

Endovascular occlusion of the arterial supply can be a useful pre-surgical approach to minimize the risk of bleeding, as well as an alternative to surgery in selected cases. This is achieved by reducing blood flow, leading to necrosis and progressive regression. 

Surgical resection is the treatment of choice. Although endovascular occlusion is a valid treatment option, the likelihood of recurrent infections is a reason to opt for surgical treatment instead. Additionally, there is a risk of recurrence ranging. For these reasons, a combined approach is preferred, optimizing both techniques to achieve the best possible outcome.

Fig 22: Digital subtraction angiography (DSA) image shows the branch originating from the abdominal aorta, with multiple ramifications towards the aberrant lung tissue.

Fig 23: Digital subtraction angiography (DSA) image of the same patient post-embolization shows how the blood supply from the aberrant branch to its ramifications has stopped.