Figures 1 and 2: Axial and coronal images demonstrate a 3.7 x 1.5 cm oval-shaped mass with high T2-signal intensity in the posterior right lower lobe.

Figure 3: Post-contrast image demonstrates that there is no significant enhancement with in the mass. There are, however, enlarged vessels adjacent to and surrounding this mass.

Figure 4: Reconstructed MRA sequence demonstrates that there is a large aberrant vessel that arises from the celiac axis and terminates in the area of the RLL mass.

Figure 5: Reconstructed MRA delayed phase sequence images again demonstrate the aberrant vessel feeding the RLL mass. In addition, this mass has venous drainage into the right inferior pulmonary vein.

Movie 1: MRA sequence follows the large aberrant vessel (Figures 4 and 5) arising from the celiac axis, ascending into the thorax and feeding the RLL mass.

Intralobar pulmonary sequestration.

Pulmonary sequestration is defined as an aberrant lung tissue mass that has no normal connection with the bronchial tree or with the pulmonary arteries. The arterial supply for these lesions arises from the systemic arteries, usually thoracic or abdominal aorta, and venous drainage is via the azygos system, the pulmonary veins, or the inferior vena cava. Pulmonary sequestration is divided into two types, extralobar and intralobar, based primarily upon venous drainage and the presence or absence of an independent pleural lining.

Intralobar sequestrations (ILS) represent approximately 75-85% of all sequestrations. An intralobar sequestration is contained within the lung and has a visceral pleural covering which is continuous with that of the rest of the lung. Although some intralobar sequestrations are clearly congenital anomalies developing from an accessory lung bud, there is evidence to support an acquired etiology for the majority of these lesions secondary to recurrent infections and bronchial obstruction. ILS has a low incidence of associated congenital anomalies with only 10 % of patients demonstrating associated skeletal, foregut or diaphragmantic anomalies. This type of sequestration is usually unilateral, located above the diaphragm, almost exclusively in the lower lobes, and predominantly on the left side. In ILS, the arterial supply is usually from the descending thoracic aorta (most common), abdominal aorta, celiac axis or splenic artery, and intercostal artery. The venous drainage of an ILS is most commonly via the pulmonary veins. Other mechanisms of drainage include the azygos, hemiazygos, or intercostals veins, or the inferior or superior vena cava.

Intralobar sequestrations usually affect older children, adolescents and adults. Clinically patients can present with recurrent pneumonias, chronic cough, dyspnea, hemopytsis, or intrathoracic hemorrhage, or may be completely asymptomatic. Radiologically, ILS can manifest as an area of increased opacity simulating pneumonia, as a mass with or without air-fluid levels, or as multiple cysts in the posterior basal segment of the left lower lobe. Air fluid levels and cavitation are commonly seen. Cross sectional imaging usually demonstrates a heterogeneous lesion with cyst formation, cavitation, and rarely calcification. Bronchiectasis may be demonstrated by contrast-enhanced CT or by MRI.

Extralobar sequestrations (ELS) account for approximately 14-25% of all sequestrations and affect males 4-6 times more often than females. It is defined as a mass of abnormal lung tissue that is surrounded by its own separate pleura such that there is complete anatomic and physiologic separation from adjacent lung tissue. These lesions are congenital anomalies that arise from an accessory lung bud that develops abnormally from the ventral aspect of the primitive foregut. This accessory lung bud forms after the lung pleura has developed and, thus, the ELS is not incorporated within the lung visceral pleural and has its own separate pleural covering. Up to 65% of patients with ELS have other associated congenital anomalies including, diaphragmatic defects, pulmonary hypoplasia, bronchogenic cysts, and cardiac anomalies. ELS is seen predominantly on the left side but it has been described in both the thorax and abdomen. The most common site is between the lower lobe and the diaphragm, but lesions have also been seen in the upper and middle thorax. Most ELS lesions receive arterial supply from the thoracic or abdominal aorta. Venous drainage is via the systemic system (azygos, hemiazygos, or inferior venal cava) in a majority of cases.

Extralobar sequestrations typically present at birth or in infancy with patients presenting with respiratory distress, cyanosis, feeding difficulties, failure to thrive, and recurrent pneumonias. ELS may manifest in utero with polyhydramnios with or without fetal hydrops. Radiographically, ELS manifests as a sharply defined, triangular-shaped opacity in the posterior costophrenic angle, usually adjacent to the left hemidiaphragm. Cross-sectional imaging demonstrates a homogeneous, sharply marginated soft tissue mass. Cystic changes may be present within the lesion.

References:

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2. Berrocal T, Madrid C, Novo S, et al. Congenital Anomalies of the Tracheobronchial Tree, Lung, and Mediastinum: Embryology, Radiology, and Pathology. Radiographics. 2003.
3. Felker RE, Tonkin ILD. Imaging of pulmonary sequestration. AJR. 1990; 154: 241-245.
4. Naidich DP, Rumanck WM, Ettenger NA et al. Congenital anomalies of the lungs in adults: MR diagnosis. AJR . 1988; 151: 13-19.