Signet Ring Sign

Definition

The signet ring sign is a combination resembling a signet (or pearl) ring, which is composed of a ring-shaped opacity representing a dilated bronchus in cross section and a smaller adjacent opacity representing its pulmonary artery [1] (Fig. 15.1). Normally, the diameter of a bronchus is equal to the diameter of the adjacent pulmonary artery. The signet ring sign occurs when the bronchoarterial ratio is increased. It is the basic CT sign of bronchiectasis.

Diseases Causing the Sign

Although the signet ring sign is the basic CT sign of bronchiectasis, it can also be seen in diseases characterized by abnormal reduced pulmonary arterial flow (e.g., proximal interruption of pulmonary artery or chronic thromboembolism) (Fig. 15.2) (Table 15.1).

Distribution

Refer to section “Airway Disease (Bronchiectasis and Bronchiolectasis)” in Chap. 13.

Clinical Considerations

Clinical considerations of diseases associated with bronchiectasis also refer to section “Airway Disease (Bronchiectasis and Bronchiolectasis)” in Chap. 13.

Proximal interruption of the right pulmonary artery (Fig. 15.2) is usually associated with other congenital cardiac malformations such as ventricular septal defects, the tetralogy of Fallot, coarctation of the aorta, subvalvular aortic stenosis, transposition of the great vessels, scimitar syndrome, and aortopulmonary fistula.

Key Points for Differential Diagnosis

1.It is an adjunct finding that can help in differentiating bronchiectasis from other cystic lung lesions [2]. Accompanying findings such as peribronchial thickening, lack of bronchial tapering, and visualization of bronchi within 1 cm of the pleura are all contributing findings to confirming the diagnosis of bronchiectasis.

2.In proximal interruption of the pulmonary artery, cylindrical dilatation of the proximal segmental or subsegmental bronchi is frequently seen, which is completely different from the postinfectious bronchiectasis (peripheral in distribution).

Proximal Interruption of the Right

Pulmonary Artery

Pathology and Pathogenesis

Proximal interruption of the right pulmonary artery is an uncommon developmental anomaly. The term interruption is used in preference to absence of a pulmonary artery, since the portion of the vessel that is in the lung is usually intact and patent. In proximal interruption, the pulmonary artery ends blindly at the hilum, and blood is supplied to the lung through collateral systemic vessels, mainly bronchial arteries but also transpleural branches of the intercostal, internal mammary, subclavian, and innominate arteries [3].

Symptoms and Signs

Recurrent pulmonary infection, hemorrhage, and mild dyspnea on exertion are the most common symptoms of this abnormality [4]. Few patients remain asymptomatic.

Hemoptysis attributable to the rupture of hypertrophied collateral vessels occurs in about 10 % of cases.

CT Findings

Vascular CT findings of proximal interruption of the right pulmonary artery include complete absence of the mediastinal portion of the right main pulmonary artery and enlarged collateral vessels [5] (Fig. 15.2). Parenchymal CT findings include reticular lesions, septal thickening, subpleural consolidation, cystic lung change, bronchial dilatation, and accompanying small systemic artery to constitute signet ring sign (Fig. 15.2), bronchial wall thickening, bronchiectasis, and pleural thickening associated with hypertrophied systemic collateral vessels [6].

CT–Pathology Comparisons

Smooth septal thickening is usually associated with engorged veins, lymphatics, or interstitial edema [7]. Cystic lung changes postulate that a pressure gradient between the systemic and the pulmonary arteries or high oxygen saturation level may induce lung injury such as ischemia, infarction or bleeding, and inflammatory change resulting in cystic lung change. Subpleural consolidation may be related to small pulmonary infarcts associated with decreased pulmonary circulation and pleural thickening is associated with the development of systemic vessels crossing the pleura. Similar to chronic thromboembolism, decreased pulmonary artery size may allow a reciprocal dilatation of the airways, because they are enclosed within the same bronchovascular connective tissue sheath [6].

Fig. 15.2 Proximal interruption of pulmonary artery showing signet ring sign in a 10-year-old girl. (a) Mediastinal window image of enhanced CT scan (5.0-mm section thickness) obtained at level of the main bronchi shows interruption of right proximal pulmonary artery (arrow) within pericardium. Also note hypertrophied right bronchial artery and branches (arrowheads). (b) Conventional pulmonary angio-

graph demonstrates interrupted right proximal pulmonary artery (arrow). (c) Lung window image obtained at ventricular level displays signet ring sign (arrows) constituted by dilated bronchus and accompanying small systemic artery in the right lung. Also note smooth interlobular septal thickening (arrowheads) and centrilobular ground-glass opacity nodules (due to aspirated blood with hemoptysis) in the right lung