signet ring sign
The signet ring sign is seen in bronchiectasis when the dilated bronchus and accompanying pulmonary artery branch are seen in cross-section. The bronchus and artery should be the same size, whereas in bronchiectasis, the bronchus is markedly dilated.
The signet ring analogy has also been applied to the scaphoid when it demonstrates a cortical ring as result of foreshortening on a PA projection, suggestive of volar rotation of the scaphoid secondary to scapholunate dissociation. This may also be a normal projectional finding, seen without dissociation as well .
An obscure use of the signet ring appearance has also been described in intravenous urography of renal papillary necrosis, where contrast fills the circumferential fornix of the necrotic papillae .
History and etymology
Signet rings were worn by nobles to enable them to place their seal in wax.