Silhouette sign (x-rays)
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Silhouette sign is somewhat of a misnomer and in the true sense actually denotes the loss of a silhouette, thus, it is sometimes also known as loss of silhouette sign or loss of outline sign 4.
The differential attenuation of x-ray photons by two adjacent structures defines the silhouette, e.g. heart borders against the adjacent lung segments, and it is the pathological loss of this differentiation, which the silhouette sign refers to. In short, it denotes that a mediastinal border can only be obscured by pathology which is in direct anatomical contact.
Recognition of this sign is useful in localizing areas of airspace opacities, atelectasis or mass within the lung, with the loss of these normal silhouettes on frontal chest radiographs being generally indicative of the site of pathology 3,4:
- right paratracheal stripe: right upper lobe
- right heart border: right middle lobe or medial right lower lobe
- right hemidiaphragm: right lower lobe
- aortic knuckle: left upper lobe
- left heart border: lingular segments of the left upper lobe
- left hemidiaphragm or descending aorta: left lower lobe
Sites of silhouette sign on the lateral chest radiograph include 3:
- posterior border of the heart +/- posterior left hemidiaphragm: left lower lobe
- anterior right hemidiaphragm: right middle lobe
- posterior right hemidiaphragm: right lower lobe
A caveat however is when pulmonary or mediastinal anatomy has been altered by treatment (such as surgery or radiotherapy) or disease, volume loss may affect these specific associations 3.
History and etymology
Dr Benjamin Felson (1913-1988) and, his brother Dr Henry Felson (1907-1998), were American radiologists who popularized this sign 1. However, it was the Felsons themselves who state that Dr H (Henry) Kennon Dunham (1872-1944), an American chest physician and radiologist, was the first to describe the sign 15 years earlier (late 1920s) 1,5.
The presence of a silhouette sign may not be due to intrapulmonary disease. For example 3,4:
- 1. Felson B, Felson H. Localization of intrathoracic lesions by means of the postero-anterior roentgenogram; the silhouette sign. Radiology. 1950;55 (3): 363-74. doi:10.1148/55.3.363 - Pubmed citation
- 2. Marshall GB, Farnquist BA, MacGregor JH et-al. Signs in thoracic imaging. J Thorac Imaging. 2006;21 (1): 76-90. doi:10.1097/01.rti.0000189192.70442.7a - Pubmed citation
- 3. Webb WR, Higgins CB. Thoracic Imaging. Lippincott Williams & Wilkins. (2010) ISBN:1605479764. Read it at Google Books - Find it at Amazon
- 4. Wright FW. Radiology of the chest and related conditions. CRC Press. ISBN:0415281415. Read it at Google Books - Find it at Amazon
- 5. H. Kennon Dunham, M.D. 1872–1944. (1944). Radiology, 43(2), pp.192-193.