139K.S. Lee et al., Radiology Illustrated: Chest Radiology, Radiology Illustrated,DOI 10.1007/978-3-642-37096-0_15, © Springer-Verlag Berlin Heidelberg 2014

Defi nition

The signet ring sign is a combination resembling a signet (or pearl) ring, which is composed of a ring-shaped opacity rep-resenting 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 bronchi-ectasis, it can also be seen in diseases characterized by abnormal reduced pulmonary arterial fl ow (e.g., proximal interruption of pulmonary artery or chronic thromboembo-lism) (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 bronchi-ectasis 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 car-diac malformations such as ventricular septal defects, the tetralogy of Fallot, coarctation of the aorta, subvalvular aor-tic stenosis, transposition of the great vessels, scimitar syn-drome, and aortopulmonary fi stula.

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 arter-ies 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.

Signet Ring Sign 15

Key Points for Differential Diagnosis

1. It is an adjunct fi nding that can help in differentiating bronchiectasis from other cystic lung lesions [ 2 ]. Accompanying fi ndings such as peribronchial thickening, lack of bronchial tapering, and visual-ization of bronchi within 1 cm of the pleura are all contributing fi ndings to confi rming 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).

140

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

CT Findings

Vascular CT fi ndings of proximal interruption of the right pulmonary artery include complete absence of the mediasti-nal portion of the right main pulmonary artery and enlarged collateral vessels [ 5 ] (Fig. 15.2 ). Parenchymal CT fi ndings include reticular lesions, septal thickening, subpleural con-solidation, 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 sys-temic 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 satura-tion level may induce lung injury such as ischemia, infarc-tion or bleeding, and infl ammatory change resulting in cystic lung change. Subpleural consolidation may be related to small pulmonary infarcts associated with decreased pulmo-nary 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 ].

a

b

Fig. 15.1 Bronchiectasis showing signet ring sign in a 44-year-old man. ( a ) Lung window image of CT scan (2.5-mm section thickness) obtained at level of the right upper lobar bronchus shows dilated bronchi showing signet ring sign ( arrows ) in right upper lobe. Also note mucus plugging ( arrowhead ) in dilated bronchi. ( b ) Gross pathologic specimen obtained with right upper lobectomy discloses cylindrical bronchiectasis and distal cystic changes ( arrows , cystic bronchiectasis). Also note thickened bronchial wall ( arrowheads ) with active infl ammation

15 Signet Ring Sign

141

a b

C

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

Proximal Interruption of the Right Pulmonary Artery

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Patient Prognosis

Pulmonary hypertension affects 19–25 % of patients with pulmonary artery interruption and is the most important determinant of the prognosis [ 4 ].

References

1. Hansell DM, Bankier AA, MacMahon H, McLoud TC, Muller NL, Remy J. Fleischner Society: glossary of terms for thoracic imaging. Radiology. 2008;246:697–722.

2. Marshall GB, Farnquist BA, MacGregor JH, Burrowes PW. Signs in thoracic imaging. J Thorac Imaging. 2006;21:76–90.

3. Castaner E, Gallardo X, Rimola J, et al. Congenital and acquired pulmonary artery anomalies in the adult: radiologic overview. Radiographics. 2006;26:349–71.

4. Bouros D, Pare P, Panagou P, Tsintiris K, Siafakas N. The varied manifestation of pulmonary artery agenesis in adulthood. Chest. 1995;108:670–6.

5. Morgan PW, Foley DW, Erickson SJ. Proximal interruption of a main pulmonary artery with transpleural collateral vessels: CT and MR appearances. J Comput Assist Tomogr. 1991;15:311–3.

6. Ryu DS, Spirn PW, Trotman-Dickenson B, et al. HRCT fi ndings of proximal interruption of the right pulmonary artery. J Thorac Imaging. 2004;19:171–5.

7. Aquino SL, Kee ST, Warnock ML, Gamsu G. Pulmonary aspergil-losis: imaging fi ndings with pathologic correlation. AJR Am J Roentgenol. 1994;163:811–5.

Table 15.1 Common diseases manifesting as signet ring sign

Disease Key points for differential diagnosis

Bronchiectasis Bronchial dilatation with peribronchial thickening

Proximal interruptionof pulmonary artery

Complete absence of the mediastinal portion of the right main pulmonary artery, septal thickening

15 Signet Ring Sign