Chest Imaging

Bronchogenic cyst

cyst arising from an abnormality in the primitive foregut development. It is lined with respiratory epithelium and contains mucoid material. Its walls contain cartilage, smooth muscle or mucous glands. Most bronchogenic cysts arise in the mediastinum or hila. Although they occur in all three mediastinal compartments, the middle mediastinum is the most common site, adjacent to the major airways, particularly the subcarinal and right paratracheal areas. Less than 15% of bronchogenic cysts are located within the lung parenchyma.

Most bronchogenic cysts are asymptomatic, and discovered incidentally. Occasionally they may become manifest clinically as a result of compression of adjacent structures, infection or haemorrhage. They are stable in size except when complicated by infection or haemorrhage.

On the chest radiograph, bronchogenic cysts typically appear as smooth, sharply marginated mediastinal masses. On CT scans they appear as round or oval homogeneous masses with well-defined margins with barely or no perceptible walls. They have a certain plasticity and mould around normal anatomical structures (Fig.1). Half of them show an attenuation similar to that of water and the remainder appear of soft tissue attenuation. Occasionally they show a very high attenuation related to a milk of calcium content. Curvilinear calcification of the wall is very rare. Absence of enhancement after administration of iodinated contrast medium is the rule.

On MR scans, bronchogenic cysts frequently show a signal intensity higher than that of muscle on T1-weighted images due to their high proteinaceous content (Fig. 1b). Uncommonly a fluidfluid level may be present. The signal intensity on T2-weighted images is very high suggesting a cystic lesion (Fig. 1c). The absence of enhancement after intravenous injection of gadolinium allows differentiation of the cysts from solid tumours.

The bronchogenic cysts, particularly those located within the lung parenchyma, may become infected, resulting in communication with the tracheobronchial tree. As a result cavitation may occur and lead to the development of an airfluid level or a surrounding consolidation in the lung parenchyma that obscures the wall of the cyst.

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Fig.1

Patient with a bronchogenic cyst. a. CT scan showing a round mass of relatively low density developed from the middle mediastinum. Streak artefacts due to heart beats impaired any accurate analysis of attenuation and homogeneity of the mass. On MR scan (b and c), the mass has well-defined margins without any perceptible walls, and moulds around the descending aorta. It appears with a homogeneous signal that is higher than that of muscle on T1-weighted (b) and very bright on T2-weighted (c) MR scans, suggesting a cystic lesion.
Bronchogenic cyst, Fig.1 (a)
Bronchogenic cyst, Fig.1 (b)
Bronchogenic cyst, Fig.1 (c)