What causes the spine sign on the lateral chest radiograph?
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Answer:
The spine sign is loss of the normal progressive increase in lucency of the thoracic vertebral bodies from above downwards on a lateral chest radiograph. Normally the upper thoracic vertebrae are relatively dense because the shoulder girdles and arm musculature overlie them, and each successive vertebra appears darker as that soft tissue is left behind. When posterior lung is consolidated, collapsed or covered by pleural fluid, the added attenuation cancels this gradient, so the lower vertebrae appear as dense as or denser than those above. It indicates disease in a lower lobe and is a useful check whenever the frontal film looks normal but clinical suspicion persists, since retrocardiac and posterobasal disease is easily hidden on the PA view. It is non-specific as to lobe and cause, with reported sensitivity of roughly 60-70% but a high positive predictive value of about 91-97%. A separate thoracic spine sign exists on lung ultrasound, where visibility of the vertebrae above the diaphragm indicates pleural effusion.
Why is it called so?
The sign is read off the appearance of the spine itself rather than of the lung.
Pathophysiology
Radiographic density on a lateral projection is determined by the total attenuating tissue traversed by the beam. In the upper thorax the beam passes through both shoulders and the arm musculature, so the vertebrae there are relatively opaque; caudally, that soft tissue falls away and progressively less tissue overlies each vertebra, producing the normal downward gradient of increasing lucency. Consolidated or collapsed lower lobe lung, or a layer of pleural fluid, restores attenuation over the lower vertebrae and abolishes or reverses the gradient. Because air-filled lung contributes almost nothing to attenuation, only its replacement by fluid or tissue produces the change.
Alternative names: Spine sign of Felson, vertebral fade-off sign
Other associated named signs: Silhouette sign, hilum overlay sign, flat waist sign, and the ultrasound thoracic spine sign of pleural effusion
References
Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/
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