© Turkish Society of Radiology 2011 18 R adiological practice includes classification of illnesses with similar characteristics through recognizable signs. Knowledge of and abil- ity to recognize these signs can aid the physician in shortening the differential diagnosis list and deciding on the ultimate diagnosis for a patient. In this report, 23 important and frequently seen radiological signs are presented and described using chest X-rays, computed tomog- raphy (CT) images, illustrations and photographs. Plain films Air bronchogram sign Bronchi, which are not normally seen, become visible as a result of opacification of the lung parenchyma. Branching, tubular lucencies of bronchi are seen in an opacified lung (Fig. 1a). This sign shows that the pathology is in the lung parenchyma itself (1). This sign is most frequently encountered in pneumonia and pulmonary edema. Its gen- eralized form can be seen in respiratory distress syndrome (2). The air bronchogram sign shows that the central bronchi are not obstructed; however, it can also be seen when a mass causes half-obstruction. Bron- chioalveolar carcinoma, lymphoma, interstitial fibrosis, alveolar hemor- rhage, fibrosis due to radiation and sarcoidosis can also present with this sign (1–3). This sign can also be seen on CT images (Fig. 1b). Silhouette sign In a chest x-ray, non-visualization of the border of an anatomical structure that is normally visualized shows that the area neighboring this margin is filled with tissue or material of the same density (Fig. 2). The silhouette sign is an important sign indicating the localization of a lesion (4). A well-known example is obliteration of the right heart border due to middle lobe atelectasis. This rule can also be applied to the arch of the aorta, the hemidiaphragms and the left border of the heart. A silhouette sign of the hila is called the “hilum overlay sign”. It is used to determine the localization of a lesion in the hilar region in chest X-rays. If hilar vessels can clearly be seen inside the lesion, the lesion is either anterior or posterior to the hilus. If the hilar vessels cannot be discriminated from the lesion, the lesion is at the hilus (Fig. 3) (5–7). Deep sulcus sign The deep sulcus sign describes the radiolucency extending from the lateral costophrenic angle to the hypochondrium (Fig. 4). It is an important clue indicating possible pneumothorax in chest x-rays ob- tained in the supine position. When plain films are taken with the sub- ject in an upright position, the free air in the pleural space gathers at the apicolateral space. In the supine position, the air accumulating at the anterior space forms a triangular radiolucency that makes the infe- CHEST IMAGING PICTORIAL ESSAY Signs in chest imaging Oktay Algın, Gökhan Gökalp, Uğur Topal From the Department of Radiology (O.A.  droktayalgin@gmail. com), Uludağ University School of Medicine, Bursa, Turkey. Received 12 June 2009; revision requested 4 October 2009; revision received 24 October 2009; accepted 1 November 2009. Published online 28 July 2010 DOI 10.4261/1305-3825.DIR.2901-09.1 ABSTRACT A radiological sign can sometimes resemble a particular object or pattern and is often highly suggestive of a group of similar pathologies. Awareness of such similarities can shorten the dif- ferential diagnosis list. Many such signs have been described for X-ray and computed tomography (CT) images. In this ar- ticle, we present the most frequently encountered plain film and CT signs in chest imaging. These signs include for plain films the air bronchogram sign, silhouette sign, deep sulcus sign, Continuous diaphragm sign, air crescent (“meniscus”) sign, Golden S sign, cervicothoracic sign, Luftsichel sign, scim- itar sign, doughnut sign, Hampton hump sign, Westermark sign, and juxtaphrenic peak sign, and for CT the gloved finger sign, CT halo sign, signet ring sign, comet tail sign, CT an- giogram sign, crazy paving pattern, tree-in-bud sign, feeding vessel sign, split pleura sign, and reversed halo sign. Key words:  X-ray  computed tomography, X-ray  thorax Diagn Interv Radiol 2011; 17:18–29